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Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion
Journal Pre-proof Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion
Liang Ee Low, Sangeetaprivya P. Siva, Yong Kuen Ho, Eng Seng Chan, Beng Ti Tey PII:
S0001-8686(19)30433-6
DOI:
https://doi.org/10.1016/j.cis.2020.102117
Reference:
CIS 102117
To appear in:
Advances in Colloid and Interface Science
Revised date:
20 January 2020
Please cite this article as: L.E. Low, S.P. Siva, Y.K. Ho, et al., Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion, Advances in Colloid and Interface Science(2019), https://doi.org/10.1016/ j.cis.2020.102117
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© 2019 Published by Elsevier.
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Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion
Liang Ee Lowa,b,c,d,1 , Sangeetaprivya P. Sivaa,1 , Yong Kuen Hoa, Eng Seng Chana,e, Beng Ti
Chemical Engineering Discipline, School of Engineering, Monash University Malaysia, Jalan
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a
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Teya,b*
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Lagoon Selatan, 47500 Bandar Sunway, Selangor, Malaysia. b
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Advanced Engineering Platform, Monash University Malaysia, Jalan Lagoon Selatan, 47500
Bandar Sunway, Selangor, Malaysia.
Institute of Pharmaceutics, College of Pharmaceutical Sciences, Zhejiang University, Hangzhou
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310058, P. R. China. d
Key Laboratory of Biomedical Engineering of the Ministry of Education, College of Biomedical
Monash-Industry Palm Oil Education and Research Platform (MIPO), Monash University
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e
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Engineering & Instrument Science, Zhejiang University, Hangzhou 310058, P. R. China.
Malaysia, Jalan Lagoon Selatan, 47500 Bandar Sunway, Selangor, Malaysia.
Author information *Corresponding author E-mail: [email protected] (B.T. Tey) 1
These authors have contributed equally to this work
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ABSTRACT Recently, there have been increasing demand for the application of Pickering emulsions in various industries due to its combined advantage in terms of cost, quality and sustainability. This review aims to provide a complete overview of the available methodology for the physical characterization of emulsions that are stabilized by solid particles (known as Pickering emulsion). Current approaches and techniques for the analysis of the formation and properties of the
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Pickering emulsion were outlined along with the expected results of these methods on the
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emulsions. Besides, the application of modelling techniques has also been elaborated for the
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effective characterization of Pickering emulsions. Additionally, approaches to assess the stability
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of Pickering emulsions against physical deformation such as coalescence and gravitational
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separation were reviewed. Potential future developments of these characterization techniques were also briefly discussed. This review can act as a guide to researchers to better understand the
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standard procedures of Pickering emulsion assessment and the advanced methods available to
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date to study these emulsions, down to the minute details.
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Contents
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correlation
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Keywords: Pickering emulsion, Formation, Properties, Stability, Characterization, Mathematical
ABSTRACT.................................................................................................................................... 2 1. Introduction ................................................................................................................................. 6 2. Formation of Pickering emulsion................................................................................................ 8 2.1 Surface wettability .............................................................................................................. 10 2.2 Surface charge..................................................................................................................... 15 2.3 Dimensions of Pickering particles ...................................................................................... 16 2.4 Influence of dispersed and continuous phase of emulsion.................................................. 18
Journal Pre-proof 3. Properties of Pickering emulsion .............................................................................................. 20 3.1 Size and distribution of emulsion droplets.......................................................................... 20 3.1.1 Laser diffraction measurement..................................................................................... 23 3.1.2 Microscopic visualization ............................................................................................ 25 3.1.3 Mathematical characterization methods....................................................................... 25
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3.2 Morphology and interfacial properties................................................................................ 30
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3.2.1 Optical microscopy ...................................................................................................... 34
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3.2.2 Electron microscopy .................................................................................................... 38
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3.2.3 Atomic force microscopy............................................................................................. 43
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3.3 Surface coverage ................................................................................................................. 45
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3.4 Droplet charge..................................................................................................................... 47 3.5 Colors of emulsions ............................................................................................................ 48
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4. Physical stability of Pickering emulsion ................................................................................... 50
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4.1 Coalescence stability........................................................................................................... 52 4.1.1 Emulsion storage properties analysis ........................................................................... 55 4.1.2 Emulsion micromanipulation technique ...................................................................... 56 4.1.3 Accelerated coalescence test ........................................................................................ 58 4.1.4 Light scattering measurement ...................................................................................... 60 4.1.5 Analytical centrifugation method................................................................................. 61 4.2 Gravitational separation ...................................................................................................... 63
Journal Pre-proof 4.2.1 Creaming index analysis .............................................................................................. 65 4.2.2 Analytical centrifugation method................................................................................. 66 4.2.3 Mathematical relationship............................................................................................ 67 5. Concluding remarks .................................................................................................................. 68 Acknowledgements ....................................................................................................................... 69
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Abbreviations ................................................................................................................................ 70
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Notes ............................................................................................................................................. 72
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References ..................................................................................................................................... 72
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1. Introduction An emulsion is a system composed of two immiscible liquids where one of the liquid is dispersed in the other. Since an emulsion system is normally considered thermodynamically unstable due to the high surface energy between the two immiscible phases, the formation of an emulsion is generally carried out in the presence of a surface-active agent or stabilizer such as chemical surfactant or solid particles. Among these two, the emulsion stabilized by solid
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particles (commonly known as Pickering emulsion) has been the choice of many researchers
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recently due to its reduced toxicity, lower cost and simple recovery properties as compared to
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that of conventional surfactants [1-3].
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The development of Pickering emulsions started as early as the last century since the
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pioneering work of Ramsden [4] and Pickering [5]. The highlight of a Pickering emulsion is the fact that this system possesses considerably higher resistance to deformation as compared to the
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conventional surfactant-stabilized emulsions due to the irreversible adsorption of solid particles at the interfaces of two immiscible liquids [6-8] which is made possible due to partial wetting
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properties of the solid colloidal particles [9]. Initially, the use of Pickering emulsions did not
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receive much attention due to the limited choices of material available [10]. However, with recent advances in the development of material sc ience and technology, many interesting and innovative particles with tunable surface wettability have since been designed for the stabilization of Pickering emulsion [10]. This has allowed a more wide-spread application of Pickering emulsions in various fields, which includes food, cosmetics, oil recovery, drug delivery and others [2-3,11-23]. In all these applications, the quality and features of the resulting Pickering emulsions are of utmost importance as this directly affects the outcome of the applied process. These features
Journal Pre-proof of Pickering emulsions are strongly influenced by the characteristics of the resulting droplets such as concentration, size, interactions and charge [24-25]. Thus, various instrumental analyses and experimental methods are used to obtain information regarding emulsion droplet characteristics [26-27]. These methods are not only needed for research and development of Pickering emulsion-based products but also acts as an essential and useful approach to ensure the quality of the formulated Pickering emulsions before, during and after production, as well as
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during storage [27-28].
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This review aims to provide an up-to-date overview of the commonly used techniques to
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evaluate the formation, properties and stability of various Pickering-based products. In brief,
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section 2.0 describes the various governing properties behind the formation of a Pickering emulsion along with methods to characterize them. Section 3.0 illustrates the latest available
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measurement techniques for determining the characteristics of a Pickering emulsion. Finally,
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section 4.0 shows various ways to quantify primary destabilization mechanisms of a Pickering
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emulsion. The structural outline of the current review is shown in Figure 1.
Formation
Characterization of Pickering emulsion
Properties
Physical stability
Figure 1. Schematic outline of the review on the characterization of Pickering emulsion.
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2. Formation of Pickering emulsion In general, the energy required for a solid particle to be attached/detached to/from the interfaces between continuous and dispersed phase (also known as detachment energy, ∆E) can be expressed as [9,20]: 2 E OW Rsphere 1 cos
2
(1)
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where, 𝛾𝑜𝑤 is the interfacial tension between the oil and water (disperse and continuous phase
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respectively), Rsphere is the radius of the spherical particle and θ is the three-phase contact angle (also known as the wetting contact angle). The three-phase contact angle is the intersection
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region between the disperse phase, continuous phase and the solid particles. However, it should
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be noted that the Equation (1) is only meant for estimating the detachment energy for a spherical
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particle, especially since there are various Pickering particles with diverse shapes and anisotropy available out there, as demonstrated by the examples in Figure 2.
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Recently, Vis et al. [29] showed that detachment energy for a disc- like particle with
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comparable thickness to the interfacial thickness is as shown in Equation (2):
(2)
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2 E OW Rdisc 1 cos
where Rdisc is the radius of disc- like particles. Pedireddy et al. [30] took this as inspiration later to deduce the detachment energy for rod-like particles as follows:
E OW lq 1 cos
(3)
where l and q are the length and width of the rod- like particles. Looking at these different methods to calculate detachment energy, insightful information regarding the stabilizing properties of the different Pickering particles can be obtained. It can be deduced that the detachment energy varies quadratically with 1-|cos θ| for spheres and varies linearly with 1-|cos
Journal Pre-proof θ| for discs and rods, showing that more energy would be required to desorb disc and rod- like Pickering particles from a liquid- liquid interface when compared to spherical particles [29], implicative of the fact that non-spherical particles are potentially better emulsion stabilizers than spherical particles. This is corroborated by previous observations of some anistropical particles that have attractive forces between them. They were able to form volume filling networks at concentrations much lower than the spherical particles of similar hydrodynamic volume, creating
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a more stable emulsion as compared to that stabilized by spherical particles [31]. Not to mention,
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as particles deviates from sphericity, their orientation becomes altered to maximize interfacial
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area occupied [32]. For an instance in the case of rod-shaped particles such as cellulose
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nanocrystals (CNC), the resulting emulsions exhibit as individual droplets when short nanocrystals are used, and entangled networking systems are observed when long nanocrystals
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are applied [33]. Disc-shaped particles are established to be able to cover a larger area of oil-
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water interface without resulting in an increase in mass of emulsion that may destabilize the emulsion droplets [29]. The combination of all these features allows the resulting emulsion to
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exhibit higher creaming index and viscosity, smaller size, and therefore, higher stability [34].
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It can be seen that for a solid particle to be used as a Pickering stabilizer for emulsion, it must possess the following properties: (i) the particles must be partially wettable by both dispersed and continuous phases of system while ensuring the particles are insoluble in either phase; (ii) the surface charge of the particles must not be too high that it repels each other instead of adsorbing firmly to the interfaces between the two immiscible liquids; (iii) the size of the particles should be much smaller than the desired emulsion size. From Equations (1) to (3), it is evident that the main governing parameters of the formation of Pickering emulsion are (i) interfacial tension of emulsion, (ii) dimension of
Journal Pre-proof Pickering particles, (iii) surface wetting properties (as represented by θ). Thus, the importance of each governing properties of Pickering stabilization along with the respective analytical and
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experimental characterization techniques has been reviewed as in the sub-sections that follow.
Figure 2. Graphical illustration of solid particles with varies shapes, with (a) sphere-shaped, (b)
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cubic-shaped, (c) disc-shaped and (d) rod-shaped.
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2.1 Surface wettability
Similar to how the Hydrophilic- Lipophilic Balance (HLB) value that plays a vital role in
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conventional surfactant emulsifier, the surface wettability of solid particle is the crucial property
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that governs the resulting stabilization mechanisms and the eventual types of Pickering emulsions formed is indicative from the magnitude of wetting contact angle (see Figure 3) [35]. From the survey of the literature, Kaptay [36] mentioned that for mono- layered solid particles stabilization, o/w emulsions would be produced when the contact angle at 15𝑜 < 𝜃 < 90𝑜 while for w/o emulsions, the desired contact angle falls within 90𝑜 < 𝜃 < 165𝑜 . On the other hand, if the Pickering emulsion is stabilized by multiple layers of particles, the contact angle ranging from 15𝑜 < 𝜃 < 129.3𝑜 for the formation of o/w emulsion and the contact angle ranging from 50.7𝑜 < 𝜃 < 165𝑜 will result in w/o emulsion [36]. Additionally, according to Xiao, Li and
Journal Pre-proof Huang [35], irreversible adsorption properties of Pickering emulsion will take place if the wetting contact angle falls within 30o to 150o where the desorption energy of the particles is
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several orders of magnitude larger than the thermal energy of Brownian motion.
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Figure 3. Graphical illustration of solid particles with varies wetting contact angle around the
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interface between two immiscible liquids, with (a) 𝜃 < 90o and (b) 𝜃 > 90o .
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One of the most commonly practiced methods to obtain the three-phase contact angle data is the estimation through the classical Young’s equation (Equations (4)-(6)) [1]:
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cosaw as ws / aw
(4) (5)
cosow os ws / ow
(6)
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cosao as os / ao
where 𝜃𝑖𝑗 (𝑖, 𝑗 = 𝑎, 𝑜, 𝑤) is the three-phase contact angle of air, oil and water respectively, and 𝛾𝑖𝑗 (𝑖, 𝑗 = 𝑎, 𝑠, 𝑜, 𝑤) refers to the interfacial tension between the air, solid, oil or water phase respectively. Zhou et al. [37] further derived the three equations and resulted in Equation (7) as follow:
cosaw
aw cos aw ao cos ao ow ow
(7)
Journal Pre-proof where the contact angles measurement for the equation can be done via the sessile drop method (directly measuring the contact angle of either water or oil droplet on a particles-coated glass substrate) (Figure 4a). The Young’s equation approach has, in fact, been employed by many researchers to estimate the three-phase contact angle of various developed particles [35,37-40]. Besides the tactic mentioned above for wettability estimation, another popular method is through the direct measurement of the three-phase contact angle. In a recent report, de Folter et al. [41]
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showed that the contact angle measurement of zein colloidal particles could be done via the
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captive drop method where homogeneous zein films were first deposited onto glass substrates,
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and this was placed on top of a water subphase. Next, an oil droplet was formed and attached to
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the film surfaces using a bent needle, and the contact angle was then analyzed by approximating the contour of the imaged droplet with the Laplace-Young fit [41] (Figure 4b). Although the
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proposed method could not provide a clear visual of the exact three-phase contact angle of a
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single particle in the o/w interface, such estimation is without a doubt, a rapid and straightforward measurement method that can provide reliable information on the surface
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wettability of solid particles. This approach has been practiced in many other research works
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since its introduction [13, 42-45].
Recently, Cayre and Paunov [46] developed a new technique to measure the three-phase contact angle based on the gel trapping technique (GTT) where first, particles were spread around the o/w interfaces followed by the gelling of the aqueous subphase using a gelling agent to trap the particles at the gel surfaces. Next, the oil or air phase was replaced by poly(dimethylsiloxane) (PDMS). After curing the PDMS gel entrapping the particles, the gel is peeled off and the particles entrapped-PDMS is then visualized by scanning electron microscopy (SEM) or atomic force microscopy (AFM) and the contact angle is determined from the obtained
Journal Pre-proof images (see Figure 4c and d) [46-47]. Although this method is time-consuming due to the tedious sample preparation steps and electron microscopic analysis procedure, it allowed researchers to study and analyze surface wettability of a solid particle down to the submicron scale. Several recent reports have also shown the feasibility of the aforementioned approach in characterizing the contact angles of various particles [47-50]. The GTT may be suitable to act as an in-depth surface wettability analysis to verify the quick results obtained from both Young’s equations and
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captive drop method.
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The surface wettability of a particle may be the most important characteristic of a
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Pickering emulsifier as it directly influences the Pickering stabilizing potential of the particles.
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Thus such information is vital to be presented as the primary support for the solid particles if it’s
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potential in stabilizing Pickering emulsions is to be demonstrated.
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Figure 4. (a) Contact angle of water droplet on the surface of chitosan coated thin film. (b)
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oil/water contact angle determined from the photograph of Soy bean oil droplet attached to zein films immersed in an aqueous subphase. (c) SEM images of C 18-modified silica particles obtained from decane-water interface by the GTT method ((c’) showed the higher magnification photograph). (d) AFM height scan of PDMS replica of carboxylic polystyrene latex at air-water interface, including the cross-section image across one latex particle and a high resolution image showing the carboxylic latex particle partially embedded in the PDMS surface. Adapted with permission from ref. [51] (Copyright (2019) Elsevier) (a), ref. [41] (Copyright (2019) Royal society of Chemistry) (b), ref. [46] (Copyright (2019) American Chemical Society) (c and c’) and ref. [48] (Copyright (2019) The Royal Society of Chemistry) (d).
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2.2 Surface charge The magnitude of surface charge is vitally important to determine the stability of a colloidal dispersion which can be analyzed by measuring the zeta potential (Zp) of particles suspension. Generally, when a suspension has the Zp value of Zp > ±30 mV, its inter-particle interactions are dominant by the electrostatic repulsion due to high surface charge and agglomeration of particles
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can be inhibited. On the other hand, dispersions of Zp range of -30 mV < Zp < 30 mV is of low
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surface charge region where the attractive van der Waals force dominates the suspension system
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and thus results in a small degree of aggregation of the dispersed particles [52].
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For Pickering emulsion formation, the surface charge plays an important role not only for the colloidal properties of solid particles but also for the adsorption of solid particles onto the
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interfaces between two immiscible liquids. When the solid particles possess a high Zp, the
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particles tend to repel each other instead of adsorbing firmly onto the o/w interfaces. Reducing the Zp to low-charged region, on the other hand, causes aggregation of the solid particles, which
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further strengthen the particles network in the continuous phase and improves the stabilization of
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the emulsion. Binks and Rodrigues [53] showed that the most stable fresh emulsion was obtained at the maximum flocculation of silica particles (at 0 mV Zp). Besides that, Fuma and Kawaguchi [54] also demonstrated the enhanced stabilization of Pickering emulsion by hydrophilic silica particles as the NaCl content increased. Cellulose nanocrystals (CNC) is another famous type of Pickering emulsifier was previously claimed to be unable to stabilize emulsions before its surface charge is reduced [55] and this condition was improved upon the addition of salt to the system for charge screening [55-56]. Additionally, the further decrease in Zp value does not result in the disruption of the Pickering emulsion [56]. In fact, recent reports have also
Journal Pre-proof demonstrated the possible formation of Pickering emulsions using CNC-based particles with very low net negative Zp [56-59]. Besides the above- mentioned examples, similar outcomes were also observed by many other researchers [60-64]. From these works, it can be said that particles with lower surface charge tend to be more suitable for the stabilization of Pickering emulsions. However, one must carefully monitor the surface charge of particles during Pickering emulsion preparation since the low Zp may also
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induce droplets aggregation after the emulsion is formed. The effects of Zp on emulsion droplets
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will be reviewed in Section 3.4.
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2.3 Dimensions of Pickering particles
The dimensions of the Pickering particles essentially control two major properties of the final
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emulsion that will be produced: (i) the stability of the emulsion and (ii) the size of emulsion
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droplets. From Equations (1) to (3), one can see that the detachment energy of the particles from a liquid-liquid interface increases with a measured dimension of Pickering particles (i.e., radius
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of sphere and disc, length and width of rods). Despite that, Li et al. [65] observed that no
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emulsion forms when extensively large potato starch particles are used as Pickering particles which implies that there are other forces at play that more distinctively affects the stabilization mechanism of Pickering emulsions. This irregularity in observation led to another theory that is repetitively mentioned by many researchers regarding Pickering stabilization which is that smaller particles produce more stable emulsions because smaller particles have faster adsorption kinetics, assuming there is no barrier to this adsorption which leads to more efficient packing at the liquid-liquid interface [10,65-67]. However, beyond a critical particle dimension, the stability of emulsion decreases
Journal Pre-proof with particle size since Brownian effects become significant enough to affect the partitioning of particles at the liquid-liquid interface [66]. The size of particles also affects the size of droplets formed upon emulsification, whereby droplet size decreases with the dimensions of the particles used for stabilization [6,10,68-69]. Binks and Lumsdon [6] in fact, found the following relationship between the diameter of emulsion droplet and Pickering particles as follows:
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4d rp
p
(8)
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Where re and rp are the radius of emulsion droplets and Pickering particles respectively whereas ϕ w and ϕp are the volume fraction of dispersed phase and particles respectively.
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According to this relationship, for constant volume fraction of dispersed phase and Pickering
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particles, the emulsion droplets should grow in size proportionally to the radius of Pickering
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particles (Figure 5). However, Binks found that this relationship becomes void after some time which is most probably due to the change in contact angle with particle which directly affects the
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number of particles occupying the liquid- liquid interface [6]. In spite of that, the linear decrease
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in emulsion drop size with particle size encouraged a general rule of thumb where-by the chosen particle size for Pickering stabilization should at least be one order of magnitude smaller than the desired droplet size to prepare a stable emulsion [67]. Thus, there is a general compromise between the size of emulsion droplets and the general stability of the emulsion that would lead to the most suitable size of Pickering particles to be selected for the formation of the desired Pickering emulsion. The thorough characterization methods of the dimension of Pickering particles were described elsewhere and will not be ventured further in this paper [70-71].
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Figure 5. Effect of particle size on emulsion droplet size with emulsions stabilized by silica nanoparticles of size (a) 5nm; (b) 12nm; (c) 25nm and (d) 80nm (scale bar represents 100μm)
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Adapted with permission from ref. [72] (Copyright (2019) Springer). A graphical representation
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of changes in emulsion droplet size is shown in (e).
2.4 Influence of dispersed and continuous phase of emulsion
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The two phases that make up the emulsions play an important role on the resulting emulsion microstructure. As such, the interfacial tension is one of the most important parameters that describes the interaction between two liquids. Although it is a general notion that an emulsifier should be able to reduce interfacial tension of the emulsion [25,73], particle residence at the liquid- liquid interface does not bring about significant changes to the interfacial tension, indicative of the fact that reduction of interfacial tension is not the governing mechanism behind Pickering stabilization [13,74]. On a side note, different types of oil considerably affect emulsions whereby the increase in polarity of dispersed phase used in the emulsion was able to
Journal Pre-proof reduce the interfacial tension of the oil-water interface [75-76]. This will reduce the detachment energy of the particles from the oil-water interface. Thus, the type of dispersed and continuous phase affects the strength of adsorption of the Pickering particles. While the contents of Equations (1), (2), and (3) represent the various parameters of Pickering emulsification that affect stability of emulsion in terms of the strength of adsorption of Pickering particles to the interface between the dispersed phase and continuous phase, another
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important parameter that these equations do not account for and should be taken into
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consideration is the influence of the volume fractions of the two liquids forming the emulsion
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which will also dictate the final emulsion microstructure [77-78]. This is especially important
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since the amount of dispersed phase present determines the interfacial area available to be stabilized by the Pickering particles. Therefore, three basic regimes of emulsification can be
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formed based on the availability of the particles in the emulsion. The first describes emulsion
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broth of high dispersed phase fraction when emulsification fails due to insufficient concentration of Pickering particles, the second defines emulsions of moderate dispersed phase concentration
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where the emulsion microstructure (i.e. droplet size and polydispersity) is dictated by availability
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of Pickering particles to stabilize the interfacial area of emulsion droplets (better known as the “limited coalescence phenomena”) and the final regime describes a condition where there is an excess of Pickering particles as compared to dispersed phase amount where the emulsion microstructure is controlled by the hydrodynamics of the process [13]. Thus, a correct balance in terms of the formulation of the emulsion is highly crucial since this highly affects e ven the formation of emulsion in the first place.
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3. Properties of Pickering emulsion An emulsion can be stabilized by particles as long as these particles obey the governing conditions in terms of the characteristics mentioned in Section 2.0 previously. The many different types of particles that obey these conditions can be utilized to result in corresponding various types of Pickering emulsions that have a multitude range of physical properties, both macroscopic and microscopically. Therefore, a proper set of methodology into the investigation
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of these different properties are required to screen these different emulsions to determine the
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exact quality of emulsion produced. In the following subsections, the methods to characterize
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some primary features of a Pickering emulsion will be discussed along with published results
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from the application of these methods as well as minor thoughts on further improving these
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techniques in the future.
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3.1 Size and distribution of emulsion droplets
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The size of Pickering emulsion droplet is a vital parameter to be measured as it has a strong impact on the characteristics, stability and applications of the formulated emulsions [27]. To date,
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a variety of commercially available analytical instruments can be employed for the measurement of the size and how the Pickering emulsion droplets are distributed according to size. Since Pickering emulsion is often observed to have a size ≥ 1 µm, the measuring equipment should possess the ability to accurately measure or estimate the size of an emulsion system at the range of 10 nm to 1 mm. Representative research practices in the characterization of the size and distribution of Pickering emulsion are summarized in Table 1.
Journal Pre-proof Table 1 Advantages and limitations of methodologies for determining the size and distribution of Pickering emulsions. Methodology
Advantages
Laser diffraction
Quantitative results on average size and droplet size
measurement [27-28,78-81]
Limitations
visualization [27-28,98]*
estimation [85-94]*
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distribution of Pickering emulsion.
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Visualization of the Pickering emulsion droplets.
unmeasurable
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emulsion turbidity. The observed microscopic images only represent a
Size distribution of emulsion cannot be obtained accurately without the aids from other software.
about
the
Dilution may be needed for desired images.
Emulsion size estimation may be dependent o n
emulsification process Assist fundamental understanding of emulsification process.
Amount of sample required dependent on the
small portion of the Pickering emulsions.
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Abstract
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Qualitative results on average size and droplet size
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results consistency.
Simple and rapid measuring procedure.
Simple sample preparation.
Mathematical
Frequent instrument flushing is required to ensure
distribution of Pickering emulsion.
Sample can be measured directly after synthesis. Microscopic
governing assumptions.
Lack of information about distributive properties o f droplets from these versions of models.
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Journal Pre-proof 3.1.1 Laser diffraction measurement One of the most commonly used equipment is one that measures emulsion droplet size based on the laser diffraction technique which is a well-established method that estimates emulsion drop size distribution through the measurement of the angular variation in the intensity of light scattered as the laser passes through an emulsion medium [79]. The difference in size of emulsion droplet directly affects the angles of diffraction where larger sized emulsion will result
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in lower diffraction angle and vice versa [27,79-80]. The diffraction pattern is then detected and
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analyzed to calculate the size and size distribution of the measured samples in various forms (e.g.,
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as surface-weighted mean diameter, d3,2 ni di3 / ni di2 , volume-weighted mean diameter,
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d4,3 ni di4 / ni di3 , etc.) [27,79], where ni is the number of emulsion droplets with the
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diameter di. The d4,3 is often used as the average diameter that describes the Pickering emulsion as this diameter is more sensitive to presence of larger droplets and therefore is unaffected by
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excess Pickering particles that are may be present in the emulsion [77,81].The number of
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emulsion droplets is then categorized into several classes that eventually build up the size distribution of the emulsions. The entire process of detection of particle size distribution using
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the laser diffraction technique is as demonstrated in Figure 6.
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Figure 6. The summary of measurement of size distribution of emulsion droplets using the laser
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diffraction technique.
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The presence of particles on the interface of the droplets may pose some interesting
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effects on the light scattering property of the Pickering emulsions that should be tackled carefully by experimenters. Some works acknowledged the changes in the optical properties of the
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emulsion due to the presence of particles by assuming that the droplets are completely covered
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by the Pickering particles and therefore, the refractive index value used for measurement is that of the particle stabilizer [82]. Others chose to assume that the light scattering pattern of the emulsion is independent of the solid particles adsorbed on the emulsion droplets as the thickness of the stabilizing layer is only of the order of few nanometers [78]. Thus, these experimenters supplied the value of the refractive index of the pure dispersed phase to the particle size analyzer [78,83]. As such, the impact of the particles on the light scattering properties of emulsion have not reach a unanimous agreement amongst experimenters currently and should be a subject for future research efforts in standardizing how light scattering and laser diffraction methods are used to characterize emulsions. A temporal way to avoid misleading data from this analysis
Journal Pre-proof would be performing the qualitative optical/electron microscopic analysis for the verification of the quantitative results obtained from the laser diffraction measurement.
3.1.2 Microscopic visualization Besides the quantitative method via laser diffraction or light scattering, the visualization of Pickering emulsions via optical microscopy appears to be another rapid and simple method used
of
by researchers to analyze the emulsion droplet sizes qualitatively. Optical microscopy utilizes
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visible light and several lenses to obtain images magnified up to 1000 times. After an image is
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captured, researchers can analyze the image using imaging software to get the average size as
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well as the size distribution (histogram) of the Pickering emulsions captured in the image. The obtained values can then be compared to the quantitative results recorded using laser diffraction
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method. Besides optical microscopy, electron microscopy and atomic force microscopy are also
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widely used to characterize the size of Pickering emulsions. However, these approaches require tedious sample preparation and instrument handling skills, making them less suitable to be used
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to determine merely the size of a Pickering emulsion. Instead, the electron microscopy and
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atomic force microscopy will be more useful to generate valuable information regarding the surface morphology and interfacial properties of the Pickering emulsion. This will be discussed in the next section.
3.1.3 Mathematical characterization methods Beyond direct experimental measurements, Pickering emulsions can also be characterized via mathematical methods whereby measured changes in the properties of emulsion droplets (i.e., droplet size) are correlated to the various emulsification parameters. The main advantage of
Journal Pre-proof mathematical methods is the ability to abstract insights about the emulsification process that are not readily inferable via experimental techniques. In this context, the mathematical techniques that are discussed here are those which characterize the size of Pickering emulsion droplets as well as their role in facilitating fundamental understanding of the different aspects of Pickering emulsification. Tsabet and Fradette [84] introduced a semi-empirical approach to estimate the surface-
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weighted mean diameter of a Pickering emulsion stabilized by high concentration of particles.
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Using a series of semi-empirical equations, the following procedure is introduced to predict the
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final droplet mean size: i) Calculation of interface generation and interface coverage potential, ii)
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Deduction of theoretically covered interface, iii) Determination of stabilization efficiencies, iv) Calculation of the effectively covered surface, and v) Deduction of the final mean droplet size.
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By this approach, the primary mechanism which dominates the stabilization of emulsion, i.e.
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interface generation vs. interface coverage, can be determined. In addition, quantification of the droplet coverage efficiency, particle-droplet collision efficiency, three-phase contact line
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formation efficiency and particle attachment efficiency as part of the evaluation procedure also
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yield information about the stabilization efficiency of the Pickering particles. The droplet size predicted by this method was validated by experimental droplet size obtained from laser diffraction methods. The authors concluded that this approach is potentially able to assist future scale-up efforts as it was found that due to the similarity in emulsion stabilization mechanism, slight variations in geometry can be accounted for by appropriately adjusting model coefficients. Nevertheless, this approach negates the first principles and therefore is highly system-specific, i.e. similar physicochemical properties of emulsion.
Journal Pre-proof Another mathematical characterization approach found in the Pickering emulsification literature is based on the limited coalescence theory. The limited coalescence phenomenon is one of the unique features that sets Pickering emulsions apart from their conventional counterparts stabilized by surfactants, which, unlike Pickering particles, cannot be irreversibly adsorbed [85]. In emulsions where there is no excess supply of particles, emulsion drops are formed initially with only partial coverage of irreversibly adsorbed Pickering particles. These partially covered
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droplets then coalesce until a complete particle coverage is attained. This is the essential
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mechanism behind the limited coalescence phenomenon. Initially, it was observed that the
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stability of the emulsion increases with increasing size of droplet [86] which was later found to
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be due to the increased stability of the coalesced emulsion droplets that achieve higher particle coverage and hence, higher stability.
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The limited coalescence theory has been applied in many instances to effectively describe
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the changes in equilibrium Pickering emulsion droplet size. In Wiley’s model [86], this theory was adopted for the first time to capture the effects of particle concentration on the droplet size.
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It was, later on, discovered that the limited coalescence phenomenon is not only a function of the
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particle concentration but it is also dependent on the type of emulsion produced. From the works of Golemanov et al. [87], two types of emulsions can be formed based on the solubility of the particles in the dispersed or continuous phase. When the emulsifier is more soluble in the continuous phase of a stable emulsion, this emulsion is known as the Bancroft emulsion whereas when the opposite is true, the emulsion is known as the anti-Bancroft emulsion. The equation describing the variation of the smallest stable droplet size, ds in an anti- Bancroft emulsion is shown below:
Journal Pre-proof ds
8rpa pd
(9)
p
Where ρpd is the ratio between the density of particle and the density of dispersed phase, φa is the mass fraction of the surface area that is covered by particles, φp is the mass fraction of solid particles in the drop of a specific radius and rp is the radius of a particle. The equation describing the variation of smallest stable droplet size, ds in a Bancroft emulsion is shown below:
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8rpa pd d p 1 d
(10)
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ds
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Where ϕd is the volume fraction of the dispersed phase in an emulsion. The smallest stable droplet size, ds from Equation (9)and Equation (10) was compared to experimental droplet size
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data obtained from dynamic light scattering techniques and acceptable agreement was observed
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between the experimental and predicted data. Characterization of droplet size based on the
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limited coalescence theory, although useful in certain cases, is limited to the major assumption that all available particles are adsorbed to the interface of the dispersed and continuous phase
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[85-88]. This, in reality, may not be true as particles can be present in the continuous or
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dispersed phase but not adsorbed on the emulsion droplets [85]. Apart from the above, recently, Siva et al. [89] introduced a transient scaling law for the prediction of droplet size based on the existence of sub-universal self- similar traits. The scaling law is able to describe the trend of average droplet diameter with time according to the following equation: 1
d 2
1 1 d t2 3
0
(11)
Journal Pre-proof Where ε is the measured power density, t is time of emulsification, d0 and d are the average diameters at t = 0 and t respectively, α is the characteristic value of an emulsion system and η is the dynamic break-up potential. Using this approach, good transient predictions of the droplet size can be obtained via laser diffraction procedures with minimal experimental dataset - a distinct advantage of this approach over the other two mathematical methods above. These transient scaling predictions
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may come in handy for scale-up of the emulsification process where the shortest time required
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for the achievement of the desired droplet size can be determined. In addition, this approach is an
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innovative way to understand the effects of different hydrodynamic and physicochemical
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properties on the emulsification process. The parameter η is a dimensionless group that describes the “dynamic potential” for overall droplet size reduction due to the combined effect of droplet
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break-up and coalescence of droplets during emulsification that can be calculated via a minimum
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pool of experimental dataset as long as the value of α is known. Via this equation, the combined effect of different process parameters can be analyzed by studying the values of the
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dimensionless group η where smaller values indicate a more efficient emulsion formation
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process. Doing so can help practitioners and researchers screen the emulsification process for appropriate combination of process parameters. Not only that, the characteristic value α is also indicative of the relative ability of a specific combination of dispersed and continuous phases and emulsifier to attain a minimum droplet size. This scaling law is currently limited to processes that allow measurable overall changes in temperature of emulsion broth. All the mathematical methods mentioned above only describe the changes in average properties of emulsion as a function of the processing parameters of the emulsification process. Future efforts should be directed towards the prediction of droplet size distribution transient in
Journal Pre-proof order to obtain deeper insights into the process. A potential approach to study drop size distribution in emulsification would be the population balance approach that allows an explicit account of breakage and coalescence mechanism in terms of measurable physical parameters and operational conditions [90]. This approach has been used many times to study surfactantstabilized emulsion systems [90-93], but has yet to be tested on the formation of Pickering
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3.2 Morphology and interfacial properties
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emulsions.
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Since the surface morphology and colloidal properties of a Picke ring emulsion are directly
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influenced by the particles adsorbed at its interfaces, the structural features of Pickering emulsion should be carefully monitored throughout the synthesis and development of Pickering emulsion-
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based materials. Besides knowing the size, the visualization of Pickering emulsion appears to be
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an effective method to identify the morphological and structural properties of the Pickering emulsion. It is known that the unaided human eye can only resolve object down to the size of
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around 0.1 mm (100 µm) [28]. However, the structural component of the Pickering emulsion is
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far lower than the mentioned limit. Thus a variety of microscopic techniques including optical, confocal, electron and atomic force microscopy have been developed for the visualization of these features [94-98]. Each of these microscopic techniques works on different physicochemical principles and can be employed for the examination of the emulsion at different levels [25]. The advantages and limitations of various techniques to be described are summarized in Table 2.
Journal Pre-proof Table 2 Advantages and limitations of different methodologies for determining the morphology and interfacial properties of Pickering emulsions. Methodology Bright- field
Advantages Optical
microscopy
Quick view on the morphology and integrity
contrast
f o
May be hard to differentiate between the oil and
o r p
of the Pickering emulsion.
[27-28,97,107,111-114]
Phase
Limitations
aqueous phase of the emulsion system.
e
Simple sample preparation procedure.
l a
r P
Allowed one to distinguish between the
microscopy
disperse and continuous phase of an emulsion
[25,97,107,111]
system.
n r u
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Enables the visualization on the locations of particles around the Pickering emulsion. Dual staining can be performed.
In-depth analysis of the morphology of Pickering emulsions is not possible.
Presence of bright light/UV may degrade the color stain and results in poor images. Oil/aqueous phase staining may be difficult depending on the staining agent. Modification
or
enhancement
of
optical
microscope may be required if staining is not applicable.
Confocal laser scanning
Revealing the structure and location of solid
Dark room is preferred during the analysis to
Journal Pre-proof microscopy
(CLSM)
[99-109,114]
particles adsorbed on the
interface of
avoid the degradation of the dye. Staining procedure may be tedious depending on
Pickering emulsion. Visual on the cross-sectional layer of a
the targeted particles and its staining agent.
Pickering emulsion can be done.
f o
3D imaging is possible. Raman
microscopy
[110]
No extra staining steps needed to show the
l a
3D imaging is possible Scanning microscopy [115-122]
e
surface morphology of the adsorbed particles at emulsion interface.
o r p
Sample must be in stagnant during the Raman
r P
electron
Analysis of the structure and adsorption
(SEM)
behavior of solid particles on emulsion
n r u
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interfaces in nanoscale ranges. In-depth observation of the structure and morphology of the Pickering emulsions. Able to view and measure the thickness of Pickering shell formed by the solid particles.
measurement.
Special sample holder is required for liquid-type samples. Polymerization/freeze drying of emulsion may be needed prior to sample visualization. Gold/platinum coating may be required to provide Pickering emulsions of higher magnification and clarity. Tedious operating procedure as compared to the optical microscopy techniques.
Journal Pre-proof Transmitting
electron
microscopy
(TEM)
[76,122-130]
High
definition
emulsion
droplet
imaging
of
Pickering
with/without
Special sample holder (e.g. copper grid) is needed
staining
for sample loading prior to analysis. Sample dilution must be done to provide a clear
procedure. High-resolution detection of the particles
image of a single or multiple Pickering emulsions.
f o
Tedious operating procedure as compared to the
adsorbed on the emulsion interfaces.
o r p
Nanoscale surveillance of the cross-sectional
e
image of Pickering emulsion. Atomic microscopy [109,131-135]
force (AFM)
r P
High-resolution detection of the particles
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adsorbed on the emulsion interfaces.
Allowed measurement of the stiffness and
n r u
strength of the interfacial shell of Pickering
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emulsion.
Online morphological and stiffness analysis is possible if integrated with CLSM.
optical microscopy techniques.
Not suitable for liquid sample. Special modification on the probe tip may be necessary for stiffness and interfacial shell strength measurement Poor quality of image without incorporation with other microscopic technique (such as CLSM)
Journal Pre-proof 3.2.1 Optical microscopy Among the developed techniques, optical microscopy is of the most commonly used approach for the emulsion viewing due to its simplicity in instrument handling and sample preparation. However, as regarded by its magnification limits (down to approximately 1 µm), the image obtained from optical microscopy could not provide clear information on the structural features of a Pickering emulsion (e.g. particles aggregates, particles configuration, interfacial layer
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thickness and others) that generally falls in the size ranges that is considerably lower than the
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specified limit of optical microscopy [27-28]. In certain conditions where large particles of
-p
irregular shapes are used as stabilizer (such as Janus particles with anisotropic morphology), the
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use of bright-field microscopy may be feasible to observe the Pickering particles adsorbed to the oil/water interfaces [112]. However, one should always remember that the major components
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(location of oil and water phase) of the Pickering emulsion droplet captured by optical
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microscopy is often hard to be distinguished due to the similarity in the refractive indices or colors when the bright-field mode is used [27].
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Although the Pickering emulsion stabilized by Janus particles can be viewed quite easily
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using bright- field microscopy, efforts are therefore put forward to enhance the contrast between the major components of the emulsion along with the image quality and resulting interfacial information of the optical microscopy image [97]. There are many types of chemical stain that can tag the adsorbed components (Pickering type stabilizer) on the emulsion interfaces or selectively partitioned into either the disperse phase or the continuous phase [97]. Upon binding/partitioning, these dyes can be detected by (1) bright- field microscopy if they absorb light in the visible region (water/oil color dye), (2) fluorescent microscopy if they are fluorescent. As an example, the photograph obtained for bright-field and fluorescent microscopy has been
Journal Pre-proof presented in Figure 7a and b. The staining of oil phase (red halo) and solid particles (blue halo) allowed one to clearly distinguish between the location of the particles as well as the disperse and continuous phases (Figure 7a and b). Besides that, the contrast between different components of emulsion can also be enhanced by modifying the design of the optical microscope through the installation of special lenses that convert small differences in refractive index into differences in light intensity [25] which is known as the phase contrast or differential
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interference contrast microscopy [25,97].
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Confocal laser scanning microscopy (CLSM) is one of the most important innovations in
-p
microscopy techniques for studying the interfacial structures of a Pickering emulsion which
re
focuses an extremely narrow laser beam at a particular point in the analyzed sample and the resulting laser signal is then passed through an adjustable pinhole set before arriving the detector
lP
to prevent the out-of- focus blurring [99]. Similar to fluorescent microscopy, the CLSM approach
na
also requires the staining of emulsion/components of emulsion before analysis. It enables one to obtain the cross-sectional images of Pickering emulsions that hold essential morphological,
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structural and interfacial information. Many literature reports have, in fact, showed the
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effectivity of the CLSM in identifying the location and morphology of solid particles at the Pickering emulsion interface [51,55,100-104]. Recently, the use of CLSM has been further extended to three dimensional Pickering emulsion imaging, which has allowed researchers to obtain a more in-depth understanding and visualization of the surface morphology, coverage and interfacial orientation of solid particles across the entire Pickering emulsion droplets [105-109]. The differences between the optical images obtained for bright- field, fluorescent, CLSM and 3D CLSM visualization are available in Figure 7c to f. The 3D CLSM imaging is expected to receive growth interest as it may provide online information on the testified samples (even for a
Journal Pre-proof single Pickering droplet), which thus easing the monitoring and understanding of the Pickering emulsions for various characterization and applications. The incorporation of Raman spectroscopy to optical imaging (generally known as Raman microscopy) appears to be another method that may gain considerable attention in the future due to its ability to show the chemical contrast of materials in a label- free condition. The Raman microscopy utilized the differences in several characteristics spectra of various chemical bonds
of
to differentiate between particles, and other molecules [110]. However, it should be noted that
ro
the sample used for Raman microscopy is the same as that for optical microscopy, which is in the
-p
form of emulsion suspension. This causes the continuous motion of emulsions and thus the
re
retardation of the Raman measurement accuracy. This remained as a major issue to be solved
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ur
na
lP
prior to the employment of Raman microscopy in Pickering emulsion imaging.
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Figure 7. (a) Bright-field and (b) fluorescent micrograph of Pickering emulsions stabilized by Fe3 O4 @CNC nanocomposites (particles stained with calcofluor white while oil phase stained with nile red). (c) Bright- field microscopic image of particle-stabilized ionic liquid droplets in oil. (d) Fluorescent, (e) 2D CLSM and (f) 3D CLSM microscopy ima ge for the ionic liquid in oil Pickering emulsions (ionic liquid labelled by fluorescein isothiocyanate isomer (FITC-I)). Adapted with permission from ref. [111] (Copyright (2019) Elsevier) for (a) and (b), ref. [107] (Copyright (2019) The Royal Society of Chemistry) for (c) to (f) respectively.
Journal Pre-proof 3.2.2 Electron microscopy Electron microscopy is an alternative approach that has been widely utilized to examine the structure of nanoparticles, nanoemulsions and Pickering emulsions. It uses e lectron beams that have much smaller wavelengths than light to help to visualize the structure of materials that is way smaller than the limits of visible light [25,97]. Two types of electron microscopy approach, namely the scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
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are normally employed for the examination of Pickering emulsions.
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SEM allows one to see the surface topography of specimens through the measurement of
-p
the secondary electrons generated when the sample is bombarded by the electron beams [25]. It
re
should be noted that the electron microscopy is normally performed under high vacuum conditions because electrons are easily scattered by atoms or molecules in a gas. Thus, extensive
lP
preparation of sample is required to ensure that the analyzed materials are free of all volatile
na
components (water and organic molecules) that may evaporate [25,115]. Unfortunately, the general sample preparation procedure (fixation and dehydration) for electron microscopy is not
ur
suitable for Pickering emulsion as the dehydration of Pickering emulsion often leads to droplets
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disruption and oil leakage [116-117]. Polymerizations of Pickering emulsions have been practiced by researchers to ensure the emulsion remained its integrity during SEM analysis [55,57,115,118]. Such sample preparation procedure thus allowed one to clearly identify the morphological structure, orientation and thickness of solid particles on the emulsion surfaces (Figure 8a and b) [55,57,115,118]. The cryogenic (Cryo) SEM (Cryo-SEM) is another famous electron microscopy method that has been used to characterize the interfacial thickness and properties of Pickering emulsions in-situ [49,61,106,119-121]. It involved, first the rapid freezing of emulsion by liquid nitrogen to
Journal Pre-proof retain the structure and morphology of the specimen, followed by the freeze- fracture of sample by cold knife to expose the internal microstructure [35]. The fractured frozen emulsions then undergo controlled sublimation to remove a designated amount of water or oil (to show the voids between droplets) and coated with a thin layer of gold prior to observing it under SEM chamber [35]. These preparation steps not only lead to visualization of the interfacial layer of the Pickering emulsion droplet under high resolution but also allowed the discovery of packing
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structure and orientation of solid particles at the surfaces of a Pickering emulsion (Figure 8c to f).
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Figure 8. (a) SEM image of eicosane@SiO 2 capsule and (b) the thickness of its shell. (c) CryoSEM image of WPM particles stabilized heptane in water emulsion. The respective heptane in water emulsion prepared at (d) pH 3 (discrete organization), (e) pH 4.8 (aggregated particles) and (f) pH 7 (flocs of particles). Adapted with permission from ref. [118] Copyright (2019) American Chemical Society (a and b) and ref. [120] (Copyright (2019) The Royal Society of Chemistry) (c to f).
Journal Pre-proof
Besides SEM, TEM has also been widely used to characterize a Pickering emulsion. In TEM, the electron beam would travel directly through the analyzed sample where the electron beams that are transmitted to the specimen are magnified and captured as an image [25]. TEM was found to be able to observe Pickering emulsion with/without chemically stained prior to the microscopy. For instance, Lan et al. [40] showed the direct visualization of Fe3 O4 @oleic acid-
of
stabilized Pickering emulsion via TEM and suggested the presence of the particles and particle
ro
flocs around the interface. Qian et al. [122] also showed the aggregation of lignin nanoparticles
-p
around the shell of Pickering emulsion via TEM imaging without staining the Pickering
re
emulsions (Figure 9a and b). It should be noticed that these emulsion samples were subjected to TEM visualization without the extra sample preparation steps. Thus, the distinction between
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dispersed and continuous phases of Pickering emulsion in the obtained images remained unclear.
na
The staining of emulsion for TEM analysis appears to be another interesting technique to observe the Pickering emulsion. As an example, the clear TEM images of peanut oil- in- water Pickering
ur
emulsions stabilized by bacterial cellulose has been successfully acquired upon staining of
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cellulose particles and oil phase with phosphotungstic acid (Figure 9c) [123]. Similar sample preparation procedure via emulsion polymerization and cryo-electron microscopy has been reported to provide TEM images with clearer contrast between the particles, dispersed and continuous phases [121,123-129]. As compared to the staining method, the polymerization of Pickering emulsion seems to be able to favor the TEM images with higher resolution [124]. This can be evidenced by the TEM images of Pickering emulsions stabilized by PMMA@silica nanoparticles that have been successfully captured after the polymerization of Pickering emulsions [125]. The obtained image not only showed clear distribution of PMMA@silica
Journal Pre-proof nanoparticles around the emulsions but also revealed a clear contrast between the dispersed and continuous phases of the specimen (Figure 9d) [124]. The Cryo-TEM method was also able to provide high- resolution images that are comparable to those prepared via polymerization approach [128-129]. For example, the Cryo- TEM images presented by Persson and co-workers [128] has demonstrated the densely packed particles around the silica-stabilized Pickering emulsion with clearly distinguishable squalene and water phases (Figure 9e and f). Although the
of
polymerization and Cryo-TEM methods favored higher resolution TEM images, it should be
ro
reminded that both methods involved tedious sample preparation steps as compared to the simple
-p
and direct phosphotungstic staining approach. Thus, the choice of imaging method is highly
re
dependent on the information required from the analysis (e.g., size, morphology, interfacial
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ur
na
lP
thickness and orientation, etc.).
Figure 9. TEM images of (a) lignin- g-DEAEMA stabilized Pickering emulsions with (b) its higher magnification photograph. (c) TEM images of peanut oil in water Pickering emulsions stained with phosphotungstic acid. (d) TEM images of Pickering emulsion polymerized poly(methyl methacrylate) latex armored silica nanoparticles. Inset showed a soft shell of poly(n-
Journal Pre-proof butyl acrylate). Cryo-TEM images of (e) a single and (f) multiple squalene in water emulsions. Adapted with permission from ref. [122] (Copyright (2019) The Royal Society of Chemistry) (a and b), ref. [123] (Copyright (2019) Elsevier) (c), ref. [124] Copyright (2019) American Chemical Society (d) and ref. [128] (Copyright (2019) Elsevier) (e and f), correspondingly.
3.2.3 Atomic force microscopy
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Although atomic force microscopy (AFM) can be used to visualize Pickering emulsion
ro
droplets (see Figure 10a), its usage for the characterization of Pickering emulsions often includes
-p
the measurement of the stiffness and strength of the interfacial shell as well as the forces needed
re
to disrupt the Pickering shell [130]. The AFM produces images by moving a tiny probe (a few micrometers in size or smaller) across the surface of the targeted sample. Upon contact with the
lP
surface of the sample, the cantilever attached to the probe bends, and the deflection of the stylus
na
is measured by a detector and is converted into images [25]. From the working principle, one can envision that the resolution of the AFM depends strongly on the size and shape of the probe as
ur
well as its possible positioning relative to the sample. Additionally, it should be noted that the
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cantilever and probe of the AFM are highly fragile, thus to avoid possible damage to the stylus of the AFM probe, the force acted on the sample is measured so that the deflection of the stylus is always at constant [25]. In recent studies, the AFM analysis for emulsions is more often presented as in-depth analysis with/without coupling with other techniques to show the strength of the interfacial barrier of Pickering emulsions (Figure 10b) [109,130-134]. For example, in the works of Xie et al. [134], AFM is employed to measure the surface forces and interactions between emulsion drops and gas bubbles. Besides that, Mettu et al. [130] integrated CLSM to AFM to show the stiffness and the force required to rupture the chitosan microcapsules prepared
Journal Pre-proof via Pickering emulsion cross-linking (Figure 10c). The force measurement was performed simultaneously while the microcapsule was monitored in-situ using 2D and 3D AFM-CLSM analysis (see Figure 10d and e for example images) [130]. In another study, Jamieson et al. [132] evaluated the forces between oil droplets using AFM and further compared the drops interactions across a microfluidic device. The reported studies showed the possibility of coupling AFM with other microscopic and microfluidic analysis to not only reveal the surface structure and
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morphology of the Pickering emulsions, but also examine the strength of its interfacial barrier,
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ur
na
lP
re
-p
ro
thus achieving an advanced, in-depth characterization of Pickering emulsions.
Figure 10. (a) AFM image of dried Pickering emulsions in pectin gel. (b) Deformation force measurement on a clay-armoured Pickering droplet. Insets images showing CLSM images of the droplet during the AFM compression measurements. (c) Schematic of the CLSM-AFM experiment. (d) 2D and (e) 3D CLSM images of a microcapsule during the indentation experiment with AFM. Adapted with permission from ref. [135] (Copyright (2019) American
Journal Pre-proof Chemical Society) (a), ref. [109] (Copyright (2019) The Royal Society of Chemistry) (b) and ref. [130] (Copyright (2019) The Royal Society of Chemistry) (c to e), respectively.
3.3 Surface coverage Typically for a surfactant-stabilized emulsion, droplets are required to be completely covered by surfactants without which optimal stabilization of liquid-liquid interface cannot be achieved
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[136]. However, for Pickering emulsions, it was observed that stability was present even when
ro
droplets are only partially covered with Pickering particles [74,137]. In fact, Vignati et al. [74]
-p
claim that there is no straightforward observable relationship between the degree of droplet
re
coverage and emulsion stability, which further complicates the understanding of the influence of surface coverage on droplet stability. This shows the importance of proper characterization of
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droplet surface coverage with Pickering particles to as scrutinize-able detail as possible.
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One of the ways to determine the degree of surface coverage is from the knowledge of
p re 4d rp
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[138]:
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the size of droplets, re and size of Pickering particles, rp according to the following equation
(12)
This equation however can only be used for spherical particles which led to the works of Kalashnikova et al. [55] who proposed a new equation for rod-shaped particles based on the ratio of maximum surface area that can be covered by Pickering particles to the total emulsion droplets surface area to be covered [55,139] which is as follows:
m p re
12h pVoil
(13)
Journal Pre-proof Where m p , h and ρ are the mass, thickness and density of the rod-shaped Pickering particles and Voil is the volume of dispersed phase. According to Equations (12) and (13), surface coverage of droplets is identical irrespective of the type of particles used for emulsification as long as the dimensions of droplets and particles are maintained constant. This may not be the case all the time because the interfacial coverage of particles also depends on the surface charge of particles where-by highly charged particles pack less tightly at the liquid-liquid interface [83]. This
of
dependence on the surface charge of particles is not taken into account in this equation. Another
ro
way the surface coverage can be measured is through the visualization of droplet covered by
-p
particles itself to qualitatively characterize the surface coverage of droplets. Binks and Whitby
re
[140] qualitatively viewed the structure of adsorbed silica particles layer at the oil-water interface using low-temperature field emission scanning electron microscopy (LTFESEM)
lP
(Figure 11a). On the other hand, Vignati et al. [74] visualized the coverage of fluorescent silica
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colloids for oil droplets in water via epifluorescence microscopy technique where the resulting images were processed by a video elaboration software to further enhance the coverage
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properties of the particles (Figure 11b). Though excellent images of surface coverage were
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obtained using these methods as shown in Figure 11, only average quantitative coverage information can be obtained. More elaborate characterization techniques for surface coverage should be enacted to understand the surface coverage properties of Pickering emulsions better. This is especially important to better comprehend the enhanced stability of emulsion droplets under “particle-poor” conditions which is a more practical process condition under which Pickering emulsion is prepared [74]. Besides that, the surface coverage of Pickering emulsion also be observed via simple bright- field microscopy. In a recent study, Haney et al. [112] showed the optical images of Janus
Journal Pre-proof particles adsorbing firmly at the interface between water and toluene. The success in such visualization could be due to the anisotropic shape of the Janus particles, making it differentiable to the emulsion droplets (see Figure 11c). With this feature, the Janus particle-stabilized emulsion is expected to be more easily observed under a higher magnification microscopy (AFM,
re
-p
ro
of
SEM, TEM, etc.) as compared to that of normal Pickering emulsion.
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Figure 11. (a) Low-temperature field emission scanning electron microscopy image of
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polydimethylsiloxane-in-water emulsions stabilized by hydrophobic silica particles (Bar represents 500nm). (b) epifluorescence microscopy image of isooctane- in-water emulsions
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stabilized by hydrophilic silica particles. (c) Bright- field images of Janus particles adsorbing at
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water/toluene interface. Adapted with permission from ref. [140] (Copyright (2019) American Chemical Society) (a), ref. [74] (Copyright (2019) American Chemical Society) (b) and ref. [112] (Copyright (2019) American Chemical Society) (c).
3.4 Droplet charge The origin of droplet charge of Pickering emulsions is normally due to the adsorption of charged particles on the emulsion interfaces. The surface charge of these particles may be influenced by various environmental factors including pH, ionic strength and che mical reactions [25]. These
Journal Pre-proof changes may eventually lead to the variation of surface charges of the Pickering emulsions and thus the surface properties and stability of the Pickering drops. Similar to the colloidal suspension of particles, the magnitude of the surface charge of emulsion droplets is also measured by recording its Zp. As discussed earlier, the Pickering emulsions with higher stability can be formed using the colloidal particles with low Zp values as compared to those form under high Zp [53,55,58,141-142]. However, it should be noticed that the low Zp may lead to a great
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extent of the aggregation of Pickering emulsions and so, the eventual emulsion droplets
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coalescence and destabilization if the attractive force surpasses the desorption energy of the
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particles. For Pickering emulsions that normally hold supreme resistance against coalescence and
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deformation, the droplet instability owing to agglomeration incident may be difficult to be observed in a short period. The problem may be tackled by recording the transmission extinction
3.5 Colors of emulsions
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profiles of samples through analytical centrifugation which will be disclosed in Section 4.1.5.
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The perceived quality of a variety of commercial emulsion-based products from various
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industries that include food, pharmaceutical and cosmetics depends on the appearance of these emulsions, especially in terms of the color of these emulsions [143-145]. Thus, it is highly important for the proper characterization of this basic property via available analytical and experimentation techniques. An emulsion color depends primarily on the scattering and absorption efficiency in the emulsion broth where-by the latter is controlled by the coloring agent properties (i.e., absorption spectra and concentration) and the former is influenced by the droplet characteristics (i.e., droplet size, droplet concentration, etc.) [145-147]. Typically, the color of an emulsion can be measured
Journal Pre-proof in a colorimeter and can be presented in the form of L,a,b color system, a set of mathematical variable system where L indicates the “lightness” of an emulsion where-as the a- and b- value indicate the actual color of the emulsion [147]. From past works involving surfactant-stabilized emulsions, it was observed that as droplet size increases, there would be an imminent decrease in scattering efficiency of droplets and therefore an emulsion of lower lightness and the enhanced color is resultant as shown in [144,147]. McClements [145], on the other hand, observed an
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increase in the brightness of the emulsion as droplet concentration increases. Based on these
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changes, McClements developed a theoretical protocol to predict emulsion color based on their
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composition and microstructure that uses a range of mechanistic theories (i.e. Mie theory,
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radiative transfer theory, Kubelka-Munk theory) to relate the scattering, adsorption and reflectance characteristics of the emulsion and to finally translate this information to the L,a,b
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coordinates, representing the color of the emulsion in question. This innovative way to control
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emulsion characteristics based on its color allows emulsion developers to more efficiently arrive at the desired emulsion quality with a lesser reliance on timely and costly experimental methods.
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Although the above methods are derived primarily based on surfactant-stabilized
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emulsions, the theory could still be applied on Pickering emulsions because as stated earlier, the color of an emulsion is determined by the composition and microstructure of an emulsion which is no way is related to the type of emulsifier used in the formulation. In fact, emulsions of higher level of lightness can be expected when Pickering particles are used as emulsifiers due to the fact that Pickering particles are typically larger than surfactant particles in size resulting in larger sized emulsion droplets as well [1] which is typically in micrometer range that is trackable by visible light. In fact, it is possible to estimate the size of emulsion droplets according to the color of the emulsions if the particles stabilizer used possess certain color. Recent reports on Pickering
Journal Pre-proof emulsions stabilized by black-colored particles (e.g., magnetite nanoparticles, graphite) revealed that the color of the emulsions faded (from dark light) as the size of the emulsions decreased [37,111,141,148-149,151-152]. Additionally, recent works by Binks and Olusanya [143] introduced emulsion stabilized by colored pigments where-by emulsions produced changes in color from blue to indigo through dark brown to orange as the hydrophobic pigment used as Pickering emulsifier increases in
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concentration. This suggested the possibility of rapid-prediction of the size of Pickering
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emulsions if a related model can be produced in the future for the size estimation application. As
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a matter of fact, the Pseudo-coloring simulation method has been recently demonstrated to be
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reliable in determining the power intensity of an ultrasound generator based on the light intensity obtained from sonochemiluminescence [153]. Several fatal challenges to be solved are the
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relationship between the emulsion color and (1) their respective greyscale intensity and (2) the
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particles coverage at the emulsion surfaces. The model for Pickering emulsion size prediction
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will be highly feasible upon addressing of the mentioned problems.
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4. Physical stability of Pickering emulsion The term “emulsion stability” normally refers to the ability of an emulsion to resist the changes in its physicochemical properties over time [27]. The physical stability of emulsion refers to the proficiency of the emulsion to remain its physical characteristics (size, shape, rheology, morphology and other non-chemical related properties) over time. For an emulsion system, several most commonly noticeable instability mechanisms include gravitational separation (creaming/sedimentation), coalescence, flocculation, Ostwald ripening and phase separation. Although gravitational separation could sometimes happen merely due to the difference in the
Journal Pre-proof density of the two immiscible phases, the coalescence that involved the collision of two or more emulsion droplets to form a single larger droplet [154], can be considered as the parent instability mechanism that can lead to all the aforementioned instability mechanism. Concisely, Ostwald ripening occurs as an outcome of the steady growth of larger droplets by expensing the smaller droplets, where those tiny droplets formed initially would disappear in time [155]. This is highly similar to that of coalescence as well as the flocculation where the emulsion droplets collided
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and clump together into a floc [156]. After the emulsion coalesced, the flocs/larger droplets then
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either settled down (sedimentation) or floated (creaming) due to gravitational separation and
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eventually results in phase separation (see Figure 12 for graphical illustration) [157].
Figure 12. The different Pickering emulsion instability mechanisms
Journal Pre-proof Therefore, the characterization technique for determining the coalescence stability of the Pickering emulsion will be focused in this paper. On the other hand, the method for examining the gravitational separation (or creaming) behavior of the Pickering emulsion system will also be reviewed since it may occur even without the presence of droplets coalescence.
4.1 Coalescence stability
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Coalescence is a process involving the merging of two or more emulsion droplets a larger one
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[154]. As mentioned above, the presence of droplet coalescence will cause the emulsion system
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to undergo creaming or sedimentation, followed by phase separation more rapidly due to the
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increase in the droplet size. Understanding the most precise and reliable methods available to date for recording the coalescence stability of a Pickering emulsion system is, therefore of
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utmost importance for the achievement of desired quality of the emulsions which will be further
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discussed in Table 3.
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outlined in the sub-sections that follow. The pros and cons of the several techniques are briefly
Journal Pre-proof Table 3 Advantages and limitations of various techniques for monitoring the coalescence stability of Pickering emulsions. Methodology
Advantages
Limitations
Storage analysis
Simple and most commonly used.
Long experimental period (at least one month).
[22,63,111,152,158-160]
Result obtained directly shows the storage
Unable to clearly illustrate the threshold force
stability of the Pickering emulsion.
Micromanipulation [161-167]
e r P
l a
that cause two emulsion droplets to coalesce. Provide
n r u
feasibility for
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-p
Ability to reveal the critical Laplace pressure
the synthesis of
Pickering emulsions with different shapes.
f o
needed
to
induce/prevent coalescence on
Pickering emulsions.
Special experimental setup is necessary. Limited to droplets of macro-sized. CLSM integrated AFM method is required for the analysis in nanoscale.
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Examination of droplet coalescence stability under applied pressure conditions Accelerated coalescence
Rapid analysis.
[59,112,114,141,159,168-
Facile way to disclose the effect of work on the
172]
coalescence stability of Pickering emulsions.
Driving force (stirring/centrifuging speed) must be determine manually to prevent the disastrous disruption of the Pickering emulsion.
Journal Pre-proof May be correlated with storage analysis.
Online monitoring of coalescence properties is
Evaluation of droplet coalescence stability
not possible without the aid of other techniques.
under applied flow conditions Light scattering
Simple and rapid analysis.
Preliminary level analysis
f o
Indirect measurement on the coalescence
[145]
o r p
stability of Pickering emulsion.
Analytical centrifugation [173-175]
l a
e
r P
Can measure the intensity of the transmitted
n r u
light as function of time and position over the
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entire sample length simultaneously. Data obtained can be related to Pickering emulsion size. Evaluation of droplet coalescence stability under applied flow conditions.
Unable to show the essential threshold force to induce/prevent
coalescence
on
Pickering
emulsions. Not suitable for evaluating the coalescence stability of a single Pickering emulsion. Data obtained is mainly for information in bulk.
Journal Pre-proof 4.1.1 Emulsion storage properties analysis The evaluation of the change of emulsion droplet size over time can be considered as one of the simplest and most commonly used methods for the evaluation of emulsion coalescence stability. The size and distribution of the emulsion droplets can be measured using the emulsion droplet sizing techniques mentioned earlier periodically up to a designated storage period (7 days or more) to carefully monitor the presence of coalescence after they have been in contact for
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extended periods (e.g., after creaming, flocculation) (Figure 13a). The obtained results can then
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be supported by the qualitative results from microscopic analysis (Figure 13b and c). The
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employments of the above- mentioned methods for determining the Pickering emulsions stability
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have been widely practiced by researchers [22,63,111,152,158-160]. It should be noticed that the aid of microscopic visualization in determining the coalescence stability may sometimes be
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useful in the case where Pickering emulsions flocs are present. This is because the flocculation of
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emulsion droplets may happen if the Pickering emulsions were stabilized by particles with low zeta potential. However, the droplets flocculation did not result in droplets coalescence due to its
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enhanced resistance. The presence of flocculation of emulsion droplets with no emulsion
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coalescence has been observed in a recent study for the preparation of pharmaceutical Pickering emulsions (Figure 13b) [111].
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Figure 13. Change in the droplet diameter of MCNC-stabilized Pickering emulsions at pH 6 and
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8 after up to 336 hours of production (or 14 days). Optical micrograph of the Pickering
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emulsions at (b) pH 6 and (b) pH 8 upon 0, 7 and 14 days storage. Adapted with permission from
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ref. [111] (Copyright (2019) Elsevier).
4.1.2 Emulsion micromanipulation technique
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The use of microscopic analysis for detecting coalescence can also be related to the film trapping
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technique (FTT) [161]. In FTT, oil droplets are trapped between an oil phase held in a capillary tube connected to a pressure control system and the water phase. Then, the pressure in the
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capillary tube is increased until the oil droplets coalesced with the oil-water interface to determine the critical capillary pressure required for inducing coalescence (Figure 14a) [161]. More recently, the measures of Laplace pressure between two Pickering emulsion droplets for coalescence or partial coalescence have been established in-situ via the micromanipulation technique consisting of both capillary tube and optical microscopy [162]. The experimental setup of the technique consists of two capillary fashioned micropipettes where each of them are connected to two separate water reservoir. The Pickering emulsion droplet held on both micropipette were pushed together into contact and their coalescence behavior was monitored via
Journal Pre-proof the microscope (Figure 14b) [162]. From the micromanipulation coalescence experiment, the Laplace pressure, ΔP that drives the coalescence of droplets can be estimated according to the following relationship: P
2 ow re
(14)
The developed method allowed researchers to determine the exact pressure required for partial
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and total coalescence of two Pickering emulsion droplets (Figure 14c). The fundamental
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knowledge resulted from the development not only able to provide clearer understanding on the
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coalescence behavior of different Pickering emulsion systems but also showed promising potential for the development of Pickering emulsions of different size and shape for various
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industrial usage [162-167]. In fact, the Laplace pressure required for the coalescence Pickering
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emulsion of various sizes and shapes has been reported [162-167]. This information will be useful for the design and utilization of Pickering emulsion-based materials for applications that
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require the temporal stability with the subsequent destabilization of the Pickering emulsions.
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Besides the as- mentioned capillary-based techniques, the utilization of CLSM and AFM in
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micromanipulation appear to be another promising method for analyzing the stiffness and strength of the Pickering emulsions. The possible instrumental set-up and several interesting results reported in the literature studies have been captured in section 3.2.3 (see Figure 10b to e). Overall, emulsion micromanipulation techniques allow researchers to disclose the effective pressure needed for the deformation of Pickering emulsion. However, the technique developed so far is restricted to the controlled coalescence between two droplets at any time. This may differ from the actual environment where multiple emulsion droplets collide at the same time. Addressing this issue would definitely result in a big leap in evaluating the coalescence stability of Pickering emulsions.
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Figure 14. (a) Schematic illustration of film trapping apparatus. (b) Coalescence behaviour of Pickering emulsion droplet at different surface coverage. Scale bars = 50 m. (c) Different
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coalescence regimes (total coalescence, arrested coalescence and total stability) as a function of
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droplet surface coverage. Adapted with permission from ref. [161] Copyright (2019) American
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Chemical Society (a) and ref. [162] (Copyright (2019) The Royal Society of Chemistry) (b and
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c).
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4.1.3 Accelerated coalescence test
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The coalescence of emulsion droplets may be accelerated in the presence of mechanical agitation because of the increased collision frequency of emulsion droplets. The accelerated coalescence tests are therefore used to disclose the coalescence stability data of the Pickering emulsion systems under flow conditions. The accelerated coalescence tests can be categorized into mechanical agitation and centrifugation method. The mechanical agitation method increases the rate of collision between droplets through stirring of the Pickering emulsions (Figure 15a) [27]. The properties of the Pickering emulsions (sizes, morphology, appearance, phase separation) are measured over time according to the methods mentio ned earlier. Several examples of emulsion coalescence due to stirring- induced collision under different experimental conditions have been
Journal Pre-proof previously demonstrated in the literature [59,141,159,168-170]. On the other hand, the centrifugation method involved the centrifugation of emulsions to force the droplets to bump into each other as a result of the centrifugal force (Figure 15b). The centrifugation method allowed researchers to obtained both qualitative and quantitative information about the stability of emulsions to coalescence [27]. According to works by Tcholakova, after subjecting the proteinstabilized emulsion to centrifugation in an optically transparent centrifuge tube for a designated
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speed and time, the oil globules with lower density than the continuous aqueous phase tend to
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move towards the axis of rotation of the centrifuge [161,171-172]. This forced the Pickering
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emulsions to closely aggregate with each other while retaining their original size [27], and in
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some cases, the droplets coalesced when the centrifugal force overwhelmed the Pickering barrier formed by solid particles [112]. Upon droplet coalescence, there will be formation of layers of
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Pickering emulsion droplets according to their respective sizes, with the larger Pickering
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emulsion droplets located nearer to the axis of rotation than the smaller ones. Pertaining to the possibility of increased coalescence due to the increasing centrifugal force, it can be expected
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that the oiling off will occur at higher centrifugation speed [161,171-172]. From the accelerated
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cr centrifugation test, the critical osmotic pressure ( Posm ) for drops disruption can be measured
experimentally as follow: cr Posm
c gca Vtotal Vreleased Across
(15)
where c c 2 c1 is the difference in the density of continuous and dispersed phases, gca is the centrifugal acceleration, Vtotal is the total added volume of the dispersed phase for emulsification, Vreleased is the centrifuged- induced released volume of the dispersed phase and Across is the cross-sectional area of the centrifuge tube.
Journal Pre-proof Mechanical agitation and centrifugation methods provide researchers the ability to examine the pressure required to induce coalescence to Pickering droplets under any flow conditions. In contrast to the micromanipulation techniques that record the Laplace pressure between two individual droplets, the data obtained by accelerated coalescence test were measured in bulk (multiple drops coalescence). Both characterization techniques can be utilized together to compensate each other to provide a more in-depth analysis on the coalescence
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stability of a Pickering emulsion system.
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Figure 15. Schematic illustration the accelerated coalescence test via (a) mechanical agitation and (b) centrifugation.
4.1.4 Light scattering measurement It is well known that the coalescence of emulsion droplets leads to the change in the appearance of the system since the light scattering capability of a large droplet is weaker as compared to that of a smaller droplet [145]. As a result, the emulsion may become less turbid and more intensely colored (Figure 16a and b) [145]. In that sense, the measurement of the turbidity of a Pickering emulsion can be considered for determining the coalescence stability of the Pickering emulsion
Journal Pre-proof system. It should also be noted that the gravitational separation can also cause the emulsion drops to concentrate in a specific region (either top (for creaming) (Figure 16c) or bottom section (for sedimentation)) (Figure 16d). Thus the turbidity measurement may be more suitable to act as a supplementary or a preliminary analysis to characterize the coalescence stability of a Pickering
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emulsion system.
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Figure 16. Schematic illustration of the changes in turbidity of emulsions upon light scattering/transmission test, with (a) stable, (b) coalesced (larger), (c) Creamed and (d) Sediment
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Pickering emulsions.
4.1.5 Analytical centrifugation method As an extended analysis for the centrifugation and light scattering method, Shimoni et al. [173] employed an analytical centrifugation technique that involved the integration of light transmission with centrifugation to determine the coalescence stability of Pickering emulsions. The as-mentioned approach utilizes the Space and Time resolved Extinction Profiles (STEP T M) Technology and can be used to measure the intensity of the transmitted light as function of time and position over the entire sample length simultaneously (Figure 17a and b), and the data produced can be used to calculate the creaming rate and the mean droplet size of emulsions [173-
Journal Pre-proof 174]. Although several recent reported findings suggested that the analytical centrifugation method may be an advanced method that can provide a much higher resolution of droplet size and distribution as compared to the laser diffraction because of the droplet fractionation during the measurement [173,175], the analytical centrifugation method is more often used for determining the gravitational separation profile of emulsions to date. The analytical centrifugation method is worth exploring into, not only because it provides higher resolution of
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the characteristics of Pickering emulsions, but the data obtained may hold potential to be used for
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developing mathematical models that correlate the relationship between centrifugation force,
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emulsion properties as well as its stabilization/destabilization behavior under different storage
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conditions.
Figure 17. (a) Schematic illustration of the analytical centrifugation, with (1) indicating the light source that (2) transmit NIR- light (3) through the samples (4) lying on the rotor base. The transmission data is then recorded over the entire sample length by the (5) detector. (b) The STEPT M graph produced from (5) over certain centrifuging time. Adapted with permission from ref. [174] (Copyright (2019) Wiley) (a) and ref. [173] (Copyright (2019) Elsevier) (b) correspondingly.
Journal Pre-proof 4.2 Gravitational separation Since an emulsion is a system consisting of the homogeneous mixture of two immiscible liquids, these immiscible liquids will also have different densities. Thus, upon emulsion formation, the dispersed and continuous phases will tend to separate into two la yers due to the gravitational forces. If the dispersed phase is of lower density, they will have the tendency to move upwards (creaming) and vice versa (moving downwards, sedimentation). In this section, the gravitational
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separation will be focused on the non-coalescence- induced creaming or sedimentation. Table 4
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summarized the advantage and drawback of several reported techniques.
Journal Pre-proof Table 4 Advantages and limitations of various techniques for monitoring the creaming stability of Pickering emulsions. Methodology Creaming
index
calculation [27,51,149]
Advantages
Limitations
Easy and most commonly used.
Long experimental period (at least one month).
Result obtained directly shows the creaming
Preliminary findings.
e
Creaming rate can be calculated using the
Analytical centrifugation [173-179]
o r p
Reason for creaming (either coalescence- or
properties of the Pickering emulsion.
recorded creaming index data.
l a
f o
r P
Can measure the intensity of the transmitted
n r u
light as function of time and position over the
gravity- induced) cannot be identified without the emulsion size measurement.
The obtained creaming results only represent information in bulk.
entire sample length simultaneously.
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Data can be related to emulsion creaming rate. No extra sample preparation steps. Mathematical relationship [180-181]
Quick estimation of the creaming profile of a Pickering emulsion system.
Experimental evaluation may be required to verify the estimated creaming behavior.
Journal Pre-proof 4.2.1 Creaming index analysis The simplest and most direct way to monitor the creaming or sedimentation of a Pickering emulsion system is through the measurement of the height of stable Pickering emulsions over time to calculate the creaming index (CI). The CI results can also be further supported by calculating the gravitational separation rate of the Pickering emulsions based on the Stokes’ Law [27]: (16)
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of
gd 2 c d v 18c
-p
where v is the gravitational separation rate, g is the gravitational acceleration, d is the emulsion droplet diameter, 𝜌𝑐 and 𝜌𝑑 are the densities of continuous (water) and dispersed (oil) phases,
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and 𝜂𝑐 is the viscosity of the continuous phase. An example of CI and v calculation is as shown
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in Figure 18 [149]. However, the application of Stokes’ Law has several weaknesses whereby
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this theory is found to be applicable only for systems of mono-dispersed, non-interacting spherical droplets where predictions of gravitational separation rate in concentrated emulsions
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can become complicated due to influences other than hydrodynamic factors [150]. For these
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concentrated emulsions, it is recommended to use analytical centrifugation methods as described in the subsequent section.
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Figure 18. Photograph of Pickering emulsions stabilized by various content of MCNC at (a) Day
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0 and (b) day 14. The (c) creaming index and (d) creaming rate of the respective Pickering
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emulsion. Adapted with permission from ref. [149].
4.2.2 Analytical centrifugation method Besides that, the utilization of light scattering for optical profiling is also one of the most commonly used methods to obtain gravitational separation information on the Pickering emulsion [176-177]. For the light scattering approach, the emulsion is placed in a transparent glass tube and a monochromatic beam of near infrared (NIR) light is passed through it. The transmitted/backscattered light is then recorded as a function of emulsion height accord ing to the setting in Figure 16. More recently, Detloff, Sobisch and Lerche [174] integrate the light scattering with the centrifugal method to perform advance characterization of gravitational
Journal Pre-proof separation for particles suspension. As mentioned earlier, the as-developed multisample analytical centrifuge machine (known as LUMiSizer® ) utilizes the STEPT M Technology and allows the measurement of the intensity of the transmitted light as function of time and position over the entire sample length simultaneously (see Figure 17a). The separation behavior of the particles can then be compared and analyzed in detail by tracing the variation of transmission at any location of the sample [174]. The LUMiSizer® analysis has been recently extended to the
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Pickering emulsion system (Figure 17b) [173,175,178-179]. The utilization of the centrifugal
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analysis provided the separation velocity and the creaming/sedimentation profiles of the
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Pickering emulsions over the relative centrifugal force [173,175,178-179]. These results can be
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correlated with the creaming index and the gravitational separation data obtained from Stokes’
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4.2.3 Mathematical relationship
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Law, acting as the in-depth phase separation analysis for Pickering emulsions.
Another method to access gravitational separation in emulsions would be via correlation of
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governing parameters of the phenomenon by taking into advantage of the fact that this process is
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governed by density difference between dispersed and continuous phases. A pioneering study of the separation process was conducted by Stokes for of sedimentation process whereby the settling velocity of a single hard sphere, U s in a motionless fluid is given by a universal expression given below [180]: Us
2 d set g
18c
(17)
Where ∆𝜌 = 𝜌𝑑 − 𝜌𝑐 is the density difference between the fluids, g is the gravitational acceleration, dset is the diameter of the settling particles and 𝜂𝑐 is the dynamic viscosity of the continuous phase.
Journal Pre-proof The works of Stokes was taken as an inspiration for a similar mathematical correlation that describes gravitational separation for particle-stabilized emulsions in the works of Yan and Masliyah [181]. The highlight of this method is where the adsorption of particles to the emulsion droplets is taken into account of where-by the dispersed phase droplets should have larger effective diameter due to the adsorbed particles and therefore, higher effective density. The combined effect of these changes in governing parameters of gravitational separation is taken
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into account in the modified equation for gravitational separation for particle-stabilized
18c
1 Ko
2.93
where K 1 776Cs
-p
Us
2 d set c eff gK 2/3
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emulsions which is as follows:
(18)
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Where ϕo is the oil volume fraction in the emulsion and Cs is the equilibrium concentration of
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particles at the dispersed phase surface, Voil is the volume of dispersed phase where-as ρ c and ρ eff
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are the continuous phase density and effective density of emulsion. From this model, it is observed that the creaming velocity decreased with the particle concentration at the droplet
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surface. The proper understanding of the gravitational separation behavior of a Pickering
emulsions further.
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emulsion that can be obtained from this model is crucial to comprehend the physical stability of
5. Concluding remarks Despite the increase in the investigation and scientific papers of Pickering emulsions, the standard and advance characterization techniques for Pickering emulsion were not compiled and updated since the last decade. In this work, we tried to provide an overview of the latest and commonly- used techniques to evaluate the formation, properties and stability of Pickering
Journal Pre-proof emulsions. An evaluation was provided on the proper characterization techniques for measuring and controlling the formation and features of the Pickering emulsion. The elementary mechanism of these techniques was discussed, and the respective characterization results were illustrated. The thorough knowledge of these techniques is essentially important for any researchers venturing into the general field of colloidal and Pickering emulsion engineering since the structure of these emulsions are fundamentally different than the conventional surfactant-
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stabilized emulsions. Besides that, characterization studies via modelling emulsion droplet size
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in terms of various operating parameters were also reviewed in this paper. A variety of models
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were introduced, through which the effects of various parameters to the properties of the
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emulsion can be studied more effectively, complementing the results of the different existing experimental and analytical characterization techniques. Nevertheless, there is still room for
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further development of these models, which primarily includes the modelling of the distributio n
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of Pickering emulsion droplet. Finally, the paper also reviewed the various methods to analyze the coalescence and gravitational separation of a Pickering emulsion and suggested several
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potential innovations on these techniques. This review can act as a template to researchers to
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understand the standard characterization of Pickering emulsion as well as the advanced and useful approaches developed to study these emulsions.
Acknowledgements This work was supported by the School of Engineering of Monash University Malaysia. The authors also gratefully acknowledge Advanced Engineering Platform (AEP), Monash University Malaysia for the financial support.
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Abbreviations Air
AFM
Atomic force microscopy
Across
Cross-sectional area of the centrifuge tube
CI
Creaming index
CLSM
Confocal laser scanning microscopy
Cryo-SEM
Cryogenic scanning electron microscopy
Cs
Equilibrium concentration of particles at the dispersed phase surface
d
Diameter of emulsion droplets (m)
d3,2
Surface-weighted mean diameter (m)
d3,2
Volume-weighted mean diameter (m)
ds
Diameter of smallest stable droplet (m)
dset
Diameter of the settling particles (m)
ΔE
Detachment energy (J)
FFT
Film trapping technique
g
Acceleration due to gravity (ms-2 )
gca
Centrifugal acceleration (ms-2 )
GTT
Gell trapping technique
h
Thickness of rod-like particle (m)
HLB
Hydrophilic-Lipophilic Balance
l
Length of rod-like particle (m)
LTFESEM
Low-temperature field emission scanning electron microscopy
mp
Mass of Pickering particles (kg)
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a
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Number of emulsion droplets
NIR
Near infrared
o/w
Oil-in-water
PDMS
Poly(dimethylsiloxane)
cr Posm
Critical osmotic pressure (Pa) Width of rod-like particle (m)
s
Solid
SEM
Scanning electron microscopy
STEPT M
Space and Time resolved Extinction Profiles
re
Radius of emulsion droplets (m)
rp
Radius of Pickering particles (m)
Rsphere
Radius of the spherical particle (m)
Rdisc
Radius of the disc-like particle (m)
Us
Setting velocity of a single hard sphere (ms-1 )
Voil
Volume of dispersed phase (m3 )
Vreleased
Centrifuge induced released volume of the dispersed phase (m3 )
Vtotal
Total added volume of dispersed phase for emulsion (m3 )
w
Water
w/o
Water-in-oil
Zp
Zeta potential (mV)
γOW
Interfacial tension between the oil and water (Nm-1 )
α
Characteristic value of an emulsion system
η
Dynamic break-up potential
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q
Journal Pre-proof Viscosity of the continuous phase (m2 s-1 )
ε
Power density (J kg-1 s-1 )
ϕd
Volume fraction of dispersed phase
ϕp
Volume fraction of Pickering particles
φa
Mass fraction of the surface area that is covered by particles
φp
Mass fraction of solid particles in drop of a specific radius
ρc
Density of continuous phase (kg m-3 )
ρeff
Effective density of emulsion (kg m-3 )
ρd
Density of dispersed phase (kg m-3 )
ρp
Density of Pickering particles (kg m-3 )
ρpd
Ratio between density of particle and the density of dispersed phase
θ
Three phase contact angle (°)
τ
Coverage of emulsion droplets
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ηc
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The authors declare no conflict of interest.
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Journal Pre-proof Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion
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Liang Ee Low, Sangeetaprivya P. Siva, Yong Kuen Ho, Eng Seng Chan, Beng Ti Tey*
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Graphical Abstract
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Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion
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Liang Ee Low, Sangeetaprivya P. Siva, Yong Kuen Ho, Eng Seng Chan, Beng Ti Tey*
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Highlights
Latest Pickering emulsion measurement (analytical) methods are reviewed.
Potential of innovative approaches for Pickering emulsion characterization are discussed
Mathematical correlations on Pickering emulsions analysis are described.
Challenges and opportunities for future research are addressed.
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Liang Ee Low, Sangeetaprivya P. Siva, Yong Kuen Ho, Eng Seng Chan, Beng Ti Tey PII:
S0001-8686(19)30433-6
DOI:
https://doi.org/10.1016/j.cis.2020.102117
Reference:
CIS 102117
To appear in:
Advances in Colloid and Interface Science
Revised date:
20 January 2020
Please cite this article as: L.E. Low, S.P. Siva, Y.K. Ho, et al., Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion, Advances in Colloid and Interface Science(2019), https://doi.org/10.1016/ j.cis.2020.102117
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© 2019 Published by Elsevier.
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Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion
Liang Ee Lowa,b,c,d,1 , Sangeetaprivya P. Sivaa,1 , Yong Kuen Hoa, Eng Seng Chana,e, Beng Ti
Chemical Engineering Discipline, School of Engineering, Monash University Malaysia, Jalan
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a
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Teya,b*
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Lagoon Selatan, 47500 Bandar Sunway, Selangor, Malaysia. b
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Advanced Engineering Platform, Monash University Malaysia, Jalan Lagoon Selatan, 47500
Bandar Sunway, Selangor, Malaysia.
Institute of Pharmaceutics, College of Pharmaceutical Sciences, Zhejiang University, Hangzhou
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310058, P. R. China. d
Key Laboratory of Biomedical Engineering of the Ministry of Education, College of Biomedical
Monash-Industry Palm Oil Education and Research Platform (MIPO), Monash University
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e
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Engineering & Instrument Science, Zhejiang University, Hangzhou 310058, P. R. China.
Malaysia, Jalan Lagoon Selatan, 47500 Bandar Sunway, Selangor, Malaysia.
Author information *Corresponding author E-mail: [email protected] (B.T. Tey) 1
These authors have contributed equally to this work
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ABSTRACT Recently, there have been increasing demand for the application of Pickering emulsions in various industries due to its combined advantage in terms of cost, quality and sustainability. This review aims to provide a complete overview of the available methodology for the physical characterization of emulsions that are stabilized by solid particles (known as Pickering emulsion). Current approaches and techniques for the analysis of the formation and properties of the
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Pickering emulsion were outlined along with the expected results of these methods on the
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emulsions. Besides, the application of modelling techniques has also been elaborated for the
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effective characterization of Pickering emulsions. Additionally, approaches to assess the stability
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of Pickering emulsions against physical deformation such as coalescence and gravitational
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separation were reviewed. Potential future developments of these characterization techniques were also briefly discussed. This review can act as a guide to researchers to better understand the
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standard procedures of Pickering emulsion assessment and the advanced methods available to
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date to study these emulsions, down to the minute details.
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Contents
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correlation
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Keywords: Pickering emulsion, Formation, Properties, Stability, Characterization, Mathematical
ABSTRACT.................................................................................................................................... 2 1. Introduction ................................................................................................................................. 6 2. Formation of Pickering emulsion................................................................................................ 8 2.1 Surface wettability .............................................................................................................. 10 2.2 Surface charge..................................................................................................................... 15 2.3 Dimensions of Pickering particles ...................................................................................... 16 2.4 Influence of dispersed and continuous phase of emulsion.................................................. 18
Journal Pre-proof 3. Properties of Pickering emulsion .............................................................................................. 20 3.1 Size and distribution of emulsion droplets.......................................................................... 20 3.1.1 Laser diffraction measurement..................................................................................... 23 3.1.2 Microscopic visualization ............................................................................................ 25 3.1.3 Mathematical characterization methods....................................................................... 25
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3.2 Morphology and interfacial properties................................................................................ 30
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3.2.1 Optical microscopy ...................................................................................................... 34
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3.2.2 Electron microscopy .................................................................................................... 38
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3.2.3 Atomic force microscopy............................................................................................. 43
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3.3 Surface coverage ................................................................................................................. 45
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3.4 Droplet charge..................................................................................................................... 47 3.5 Colors of emulsions ............................................................................................................ 48
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4. Physical stability of Pickering emulsion ................................................................................... 50
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4.1 Coalescence stability........................................................................................................... 52 4.1.1 Emulsion storage properties analysis ........................................................................... 55 4.1.2 Emulsion micromanipulation technique ...................................................................... 56 4.1.3 Accelerated coalescence test ........................................................................................ 58 4.1.4 Light scattering measurement ...................................................................................... 60 4.1.5 Analytical centrifugation method................................................................................. 61 4.2 Gravitational separation ...................................................................................................... 63
Journal Pre-proof 4.2.1 Creaming index analysis .............................................................................................. 65 4.2.2 Analytical centrifugation method................................................................................. 66 4.2.3 Mathematical relationship............................................................................................ 67 5. Concluding remarks .................................................................................................................. 68 Acknowledgements ....................................................................................................................... 69
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Abbreviations ................................................................................................................................ 70
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Notes ............................................................................................................................................. 72
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References ..................................................................................................................................... 72
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1. Introduction An emulsion is a system composed of two immiscible liquids where one of the liquid is dispersed in the other. Since an emulsion system is normally considered thermodynamically unstable due to the high surface energy between the two immiscible phases, the formation of an emulsion is generally carried out in the presence of a surface-active agent or stabilizer such as chemical surfactant or solid particles. Among these two, the emulsion stabilized by solid
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particles (commonly known as Pickering emulsion) has been the choice of many researchers
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recently due to its reduced toxicity, lower cost and simple recovery properties as compared to
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that of conventional surfactants [1-3].
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The development of Pickering emulsions started as early as the last century since the
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pioneering work of Ramsden [4] and Pickering [5]. The highlight of a Pickering emulsion is the fact that this system possesses considerably higher resistance to deformation as compared to the
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conventional surfactant-stabilized emulsions due to the irreversible adsorption of solid particles at the interfaces of two immiscible liquids [6-8] which is made possible due to partial wetting
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properties of the solid colloidal particles [9]. Initially, the use of Pickering emulsions did not
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receive much attention due to the limited choices of material available [10]. However, with recent advances in the development of material sc ience and technology, many interesting and innovative particles with tunable surface wettability have since been designed for the stabilization of Pickering emulsion [10]. This has allowed a more wide-spread application of Pickering emulsions in various fields, which includes food, cosmetics, oil recovery, drug delivery and others [2-3,11-23]. In all these applications, the quality and features of the resulting Pickering emulsions are of utmost importance as this directly affects the outcome of the applied process. These features
Journal Pre-proof of Pickering emulsions are strongly influenced by the characteristics of the resulting droplets such as concentration, size, interactions and charge [24-25]. Thus, various instrumental analyses and experimental methods are used to obtain information regarding emulsion droplet characteristics [26-27]. These methods are not only needed for research and development of Pickering emulsion-based products but also acts as an essential and useful approach to ensure the quality of the formulated Pickering emulsions before, during and after production, as well as
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during storage [27-28].
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This review aims to provide an up-to-date overview of the commonly used techniques to
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evaluate the formation, properties and stability of various Pickering-based products. In brief,
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section 2.0 describes the various governing properties behind the formation of a Pickering emulsion along with methods to characterize them. Section 3.0 illustrates the latest available
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measurement techniques for determining the characteristics of a Pickering emulsion. Finally,
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section 4.0 shows various ways to quantify primary destabilization mechanisms of a Pickering
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emulsion. The structural outline of the current review is shown in Figure 1.
Formation
Characterization of Pickering emulsion
Properties
Physical stability
Figure 1. Schematic outline of the review on the characterization of Pickering emulsion.
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2. Formation of Pickering emulsion In general, the energy required for a solid particle to be attached/detached to/from the interfaces between continuous and dispersed phase (also known as detachment energy, ∆E) can be expressed as [9,20]: 2 E OW Rsphere 1 cos
2
(1)
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where, 𝛾𝑜𝑤 is the interfacial tension between the oil and water (disperse and continuous phase
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respectively), Rsphere is the radius of the spherical particle and θ is the three-phase contact angle (also known as the wetting contact angle). The three-phase contact angle is the intersection
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region between the disperse phase, continuous phase and the solid particles. However, it should
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be noted that the Equation (1) is only meant for estimating the detachment energy for a spherical
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particle, especially since there are various Pickering particles with diverse shapes and anisotropy available out there, as demonstrated by the examples in Figure 2.
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Recently, Vis et al. [29] showed that detachment energy for a disc- like particle with
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comparable thickness to the interfacial thickness is as shown in Equation (2):
(2)
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2 E OW Rdisc 1 cos
where Rdisc is the radius of disc- like particles. Pedireddy et al. [30] took this as inspiration later to deduce the detachment energy for rod-like particles as follows:
E OW lq 1 cos
(3)
where l and q are the length and width of the rod- like particles. Looking at these different methods to calculate detachment energy, insightful information regarding the stabilizing properties of the different Pickering particles can be obtained. It can be deduced that the detachment energy varies quadratically with 1-|cos θ| for spheres and varies linearly with 1-|cos
Journal Pre-proof θ| for discs and rods, showing that more energy would be required to desorb disc and rod- like Pickering particles from a liquid- liquid interface when compared to spherical particles [29], implicative of the fact that non-spherical particles are potentially better emulsion stabilizers than spherical particles. This is corroborated by previous observations of some anistropical particles that have attractive forces between them. They were able to form volume filling networks at concentrations much lower than the spherical particles of similar hydrodynamic volume, creating
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a more stable emulsion as compared to that stabilized by spherical particles [31]. Not to mention,
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as particles deviates from sphericity, their orientation becomes altered to maximize interfacial
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area occupied [32]. For an instance in the case of rod-shaped particles such as cellulose
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nanocrystals (CNC), the resulting emulsions exhibit as individual droplets when short nanocrystals are used, and entangled networking systems are observed when long nanocrystals
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are applied [33]. Disc-shaped particles are established to be able to cover a larger area of oil-
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water interface without resulting in an increase in mass of emulsion that may destabilize the emulsion droplets [29]. The combination of all these features allows the resulting emulsion to
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exhibit higher creaming index and viscosity, smaller size, and therefore, higher stability [34].
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It can be seen that for a solid particle to be used as a Pickering stabilizer for emulsion, it must possess the following properties: (i) the particles must be partially wettable by both dispersed and continuous phases of system while ensuring the particles are insoluble in either phase; (ii) the surface charge of the particles must not be too high that it repels each other instead of adsorbing firmly to the interfaces between the two immiscible liquids; (iii) the size of the particles should be much smaller than the desired emulsion size. From Equations (1) to (3), it is evident that the main governing parameters of the formation of Pickering emulsion are (i) interfacial tension of emulsion, (ii) dimension of
Journal Pre-proof Pickering particles, (iii) surface wetting properties (as represented by θ). Thus, the importance of each governing properties of Pickering stabilization along with the respective analytical and
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experimental characterization techniques has been reviewed as in the sub-sections that follow.
Figure 2. Graphical illustration of solid particles with varies shapes, with (a) sphere-shaped, (b)
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cubic-shaped, (c) disc-shaped and (d) rod-shaped.
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2.1 Surface wettability
Similar to how the Hydrophilic- Lipophilic Balance (HLB) value that plays a vital role in
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conventional surfactant emulsifier, the surface wettability of solid particle is the crucial property
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that governs the resulting stabilization mechanisms and the eventual types of Pickering emulsions formed is indicative from the magnitude of wetting contact angle (see Figure 3) [35]. From the survey of the literature, Kaptay [36] mentioned that for mono- layered solid particles stabilization, o/w emulsions would be produced when the contact angle at 15𝑜 < 𝜃 < 90𝑜 while for w/o emulsions, the desired contact angle falls within 90𝑜 < 𝜃 < 165𝑜 . On the other hand, if the Pickering emulsion is stabilized by multiple layers of particles, the contact angle ranging from 15𝑜 < 𝜃 < 129.3𝑜 for the formation of o/w emulsion and the contact angle ranging from 50.7𝑜 < 𝜃 < 165𝑜 will result in w/o emulsion [36]. Additionally, according to Xiao, Li and
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several orders of magnitude larger than the thermal energy of Brownian motion.
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Figure 3. Graphical illustration of solid particles with varies wetting contact angle around the
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interface between two immiscible liquids, with (a) 𝜃 < 90o and (b) 𝜃 > 90o .
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One of the most commonly practiced methods to obtain the three-phase contact angle data is the estimation through the classical Young’s equation (Equations (4)-(6)) [1]:
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cosaw as ws / aw
(4) (5)
cosow os ws / ow
(6)
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cosao as os / ao
where 𝜃𝑖𝑗 (𝑖, 𝑗 = 𝑎, 𝑜, 𝑤) is the three-phase contact angle of air, oil and water respectively, and 𝛾𝑖𝑗 (𝑖, 𝑗 = 𝑎, 𝑠, 𝑜, 𝑤) refers to the interfacial tension between the air, solid, oil or water phase respectively. Zhou et al. [37] further derived the three equations and resulted in Equation (7) as follow:
cosaw
aw cos aw ao cos ao ow ow
(7)
Journal Pre-proof where the contact angles measurement for the equation can be done via the sessile drop method (directly measuring the contact angle of either water or oil droplet on a particles-coated glass substrate) (Figure 4a). The Young’s equation approach has, in fact, been employed by many researchers to estimate the three-phase contact angle of various developed particles [35,37-40]. Besides the tactic mentioned above for wettability estimation, another popular method is through the direct measurement of the three-phase contact angle. In a recent report, de Folter et al. [41]
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showed that the contact angle measurement of zein colloidal particles could be done via the
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captive drop method where homogeneous zein films were first deposited onto glass substrates,
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and this was placed on top of a water subphase. Next, an oil droplet was formed and attached to
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the film surfaces using a bent needle, and the contact angle was then analyzed by approximating the contour of the imaged droplet with the Laplace-Young fit [41] (Figure 4b). Although the
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proposed method could not provide a clear visual of the exact three-phase contact angle of a
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single particle in the o/w interface, such estimation is without a doubt, a rapid and straightforward measurement method that can provide reliable information on the surface
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wettability of solid particles. This approach has been practiced in many other research works
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since its introduction [13, 42-45].
Recently, Cayre and Paunov [46] developed a new technique to measure the three-phase contact angle based on the gel trapping technique (GTT) where first, particles were spread around the o/w interfaces followed by the gelling of the aqueous subphase using a gelling agent to trap the particles at the gel surfaces. Next, the oil or air phase was replaced by poly(dimethylsiloxane) (PDMS). After curing the PDMS gel entrapping the particles, the gel is peeled off and the particles entrapped-PDMS is then visualized by scanning electron microscopy (SEM) or atomic force microscopy (AFM) and the contact angle is determined from the obtained
Journal Pre-proof images (see Figure 4c and d) [46-47]. Although this method is time-consuming due to the tedious sample preparation steps and electron microscopic analysis procedure, it allowed researchers to study and analyze surface wettability of a solid particle down to the submicron scale. Several recent reports have also shown the feasibility of the aforementioned approach in characterizing the contact angles of various particles [47-50]. The GTT may be suitable to act as an in-depth surface wettability analysis to verify the quick results obtained from both Young’s equations and
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captive drop method.
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The surface wettability of a particle may be the most important characteristic of a
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Pickering emulsifier as it directly influences the Pickering stabilizing potential of the particles.
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Thus such information is vital to be presented as the primary support for the solid particles if it’s
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potential in stabilizing Pickering emulsions is to be demonstrated.
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Figure 4. (a) Contact angle of water droplet on the surface of chitosan coated thin film. (b)
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oil/water contact angle determined from the photograph of Soy bean oil droplet attached to zein films immersed in an aqueous subphase. (c) SEM images of C 18-modified silica particles obtained from decane-water interface by the GTT method ((c’) showed the higher magnification photograph). (d) AFM height scan of PDMS replica of carboxylic polystyrene latex at air-water interface, including the cross-section image across one latex particle and a high resolution image showing the carboxylic latex particle partially embedded in the PDMS surface. Adapted with permission from ref. [51] (Copyright (2019) Elsevier) (a), ref. [41] (Copyright (2019) Royal society of Chemistry) (b), ref. [46] (Copyright (2019) American Chemical Society) (c and c’) and ref. [48] (Copyright (2019) The Royal Society of Chemistry) (d).
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2.2 Surface charge The magnitude of surface charge is vitally important to determine the stability of a colloidal dispersion which can be analyzed by measuring the zeta potential (Zp) of particles suspension. Generally, when a suspension has the Zp value of Zp > ±30 mV, its inter-particle interactions are dominant by the electrostatic repulsion due to high surface charge and agglomeration of particles
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can be inhibited. On the other hand, dispersions of Zp range of -30 mV < Zp < 30 mV is of low
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surface charge region where the attractive van der Waals force dominates the suspension system
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and thus results in a small degree of aggregation of the dispersed particles [52].
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For Pickering emulsion formation, the surface charge plays an important role not only for the colloidal properties of solid particles but also for the adsorption of solid particles onto the
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interfaces between two immiscible liquids. When the solid particles possess a high Zp, the
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particles tend to repel each other instead of adsorbing firmly onto the o/w interfaces. Reducing the Zp to low-charged region, on the other hand, causes aggregation of the solid particles, which
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further strengthen the particles network in the continuous phase and improves the stabilization of
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the emulsion. Binks and Rodrigues [53] showed that the most stable fresh emulsion was obtained at the maximum flocculation of silica particles (at 0 mV Zp). Besides that, Fuma and Kawaguchi [54] also demonstrated the enhanced stabilization of Pickering emulsion by hydrophilic silica particles as the NaCl content increased. Cellulose nanocrystals (CNC) is another famous type of Pickering emulsifier was previously claimed to be unable to stabilize emulsions before its surface charge is reduced [55] and this condition was improved upon the addition of salt to the system for charge screening [55-56]. Additionally, the further decrease in Zp value does not result in the disruption of the Pickering emulsion [56]. In fact, recent reports have also
Journal Pre-proof demonstrated the possible formation of Pickering emulsions using CNC-based particles with very low net negative Zp [56-59]. Besides the above- mentioned examples, similar outcomes were also observed by many other researchers [60-64]. From these works, it can be said that particles with lower surface charge tend to be more suitable for the stabilization of Pickering emulsions. However, one must carefully monitor the surface charge of particles during Pickering emulsion preparation since the low Zp may also
of
induce droplets aggregation after the emulsion is formed. The effects of Zp on emulsion droplets
-p
ro
will be reviewed in Section 3.4.
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2.3 Dimensions of Pickering particles
The dimensions of the Pickering particles essentially control two major properties of the final
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emulsion that will be produced: (i) the stability of the emulsion and (ii) the size of emulsion
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droplets. From Equations (1) to (3), one can see that the detachment energy of the particles from a liquid-liquid interface increases with a measured dimension of Pickering particles (i.e., radius
ur
of sphere and disc, length and width of rods). Despite that, Li et al. [65] observed that no
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emulsion forms when extensively large potato starch particles are used as Pickering particles which implies that there are other forces at play that more distinctively affects the stabilization mechanism of Pickering emulsions. This irregularity in observation led to another theory that is repetitively mentioned by many researchers regarding Pickering stabilization which is that smaller particles produce more stable emulsions because smaller particles have faster adsorption kinetics, assuming there is no barrier to this adsorption which leads to more efficient packing at the liquid-liquid interface [10,65-67]. However, beyond a critical particle dimension, the stability of emulsion decreases
Journal Pre-proof with particle size since Brownian effects become significant enough to affect the partitioning of particles at the liquid-liquid interface [66]. The size of particles also affects the size of droplets formed upon emulsification, whereby droplet size decreases with the dimensions of the particles used for stabilization [6,10,68-69]. Binks and Lumsdon [6] in fact, found the following relationship between the diameter of emulsion droplet and Pickering particles as follows:
of
4d rp
p
(8)
ro
re
-p
Where re and rp are the radius of emulsion droplets and Pickering particles respectively whereas ϕ w and ϕp are the volume fraction of dispersed phase and particles respectively.
re
According to this relationship, for constant volume fraction of dispersed phase and Pickering
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particles, the emulsion droplets should grow in size proportionally to the radius of Pickering
na
particles (Figure 5). However, Binks found that this relationship becomes void after some time which is most probably due to the change in contact angle with particle which directly affects the
ur
number of particles occupying the liquid- liquid interface [6]. In spite of that, the linear decrease
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in emulsion drop size with particle size encouraged a general rule of thumb where-by the chosen particle size for Pickering stabilization should at least be one order of magnitude smaller than the desired droplet size to prepare a stable emulsion [67]. Thus, there is a general compromise between the size of emulsion droplets and the general stability of the emulsion that would lead to the most suitable size of Pickering particles to be selected for the formation of the desired Pickering emulsion. The thorough characterization methods of the dimension of Pickering particles were described elsewhere and will not be ventured further in this paper [70-71].
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Figure 5. Effect of particle size on emulsion droplet size with emulsions stabilized by silica nanoparticles of size (a) 5nm; (b) 12nm; (c) 25nm and (d) 80nm (scale bar represents 100μm)
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Adapted with permission from ref. [72] (Copyright (2019) Springer). A graphical representation
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of changes in emulsion droplet size is shown in (e).
2.4 Influence of dispersed and continuous phase of emulsion
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The two phases that make up the emulsions play an important role on the resulting emulsion microstructure. As such, the interfacial tension is one of the most important parameters that describes the interaction between two liquids. Although it is a general notion that an emulsifier should be able to reduce interfacial tension of the emulsion [25,73], particle residence at the liquid- liquid interface does not bring about significant changes to the interfacial tension, indicative of the fact that reduction of interfacial tension is not the governing mechanism behind Pickering stabilization [13,74]. On a side note, different types of oil considerably affect emulsions whereby the increase in polarity of dispersed phase used in the emulsion was able to
Journal Pre-proof reduce the interfacial tension of the oil-water interface [75-76]. This will reduce the detachment energy of the particles from the oil-water interface. Thus, the type of dispersed and continuous phase affects the strength of adsorption of the Pickering particles. While the contents of Equations (1), (2), and (3) represent the various parameters of Pickering emulsification that affect stability of emulsion in terms of the strength of adsorption of Pickering particles to the interface between the dispersed phase and continuous phase, another
of
important parameter that these equations do not account for and should be taken into
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consideration is the influence of the volume fractions of the two liquids forming the emulsion
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which will also dictate the final emulsion microstructure [77-78]. This is especially important
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since the amount of dispersed phase present determines the interfacial area available to be stabilized by the Pickering particles. Therefore, three basic regimes of emulsification can be
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formed based on the availability of the particles in the emulsion. The first describes emulsion
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broth of high dispersed phase fraction when emulsification fails due to insufficient concentration of Pickering particles, the second defines emulsions of moderate dispersed phase concentration
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where the emulsion microstructure (i.e. droplet size and polydispersity) is dictated by availability
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of Pickering particles to stabilize the interfacial area of emulsion droplets (better known as the “limited coalescence phenomena”) and the final regime describes a condition where there is an excess of Pickering particles as compared to dispersed phase amount where the emulsion microstructure is controlled by the hydrodynamics of the process [13]. Thus, a correct balance in terms of the formulation of the emulsion is highly crucial since this highly affects e ven the formation of emulsion in the first place.
Journal Pre-proof
3. Properties of Pickering emulsion An emulsion can be stabilized by particles as long as these particles obey the governing conditions in terms of the characteristics mentioned in Section 2.0 previously. The many different types of particles that obey these conditions can be utilized to result in corresponding various types of Pickering emulsions that have a multitude range of physical properties, both macroscopic and microscopically. Therefore, a proper set of methodology into the investigation
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of these different properties are required to screen these different emulsions to determine the
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exact quality of emulsion produced. In the following subsections, the methods to characterize
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some primary features of a Pickering emulsion will be discussed along with published results
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from the application of these methods as well as minor thoughts on further improving these
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techniques in the future.
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3.1 Size and distribution of emulsion droplets
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The size of Pickering emulsion droplet is a vital parameter to be measured as it has a strong impact on the characteristics, stability and applications of the formulated emulsions [27]. To date,
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a variety of commercially available analytical instruments can be employed for the measurement of the size and how the Pickering emulsion droplets are distributed according to size. Since Pickering emulsion is often observed to have a size ≥ 1 µm, the measuring equipment should possess the ability to accurately measure or estimate the size of an emulsion system at the range of 10 nm to 1 mm. Representative research practices in the characterization of the size and distribution of Pickering emulsion are summarized in Table 1.
Journal Pre-proof Table 1 Advantages and limitations of methodologies for determining the size and distribution of Pickering emulsions. Methodology
Advantages
Laser diffraction
Quantitative results on average size and droplet size
measurement [27-28,78-81]
Limitations
visualization [27-28,98]*
estimation [85-94]*
l a
distribution of Pickering emulsion.
e
r P
Visualization of the Pickering emulsion droplets.
unmeasurable
details
emulsion turbidity. The observed microscopic images only represent a
Size distribution of emulsion cannot be obtained accurately without the aids from other software.
about
the
Dilution may be needed for desired images.
Emulsion size estimation may be dependent o n
emulsification process Assist fundamental understanding of emulsification process.
Amount of sample required dependent on the
small portion of the Pickering emulsions.
n r u
Abstract
o r p
Qualitative results on average size and droplet size
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f o
results consistency.
Simple and rapid measuring procedure.
Simple sample preparation.
Mathematical
Frequent instrument flushing is required to ensure
distribution of Pickering emulsion.
Sample can be measured directly after synthesis. Microscopic
governing assumptions.
Lack of information about distributive properties o f droplets from these versions of models.
Journal Pre-proof Time saving since experimental work can be reduced.
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e
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Journal Pre-proof 3.1.1 Laser diffraction measurement One of the most commonly used equipment is one that measures emulsion droplet size based on the laser diffraction technique which is a well-established method that estimates emulsion drop size distribution through the measurement of the angular variation in the intensity of light scattered as the laser passes through an emulsion medium [79]. The difference in size of emulsion droplet directly affects the angles of diffraction where larger sized emulsion will result
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in lower diffraction angle and vice versa [27,79-80]. The diffraction pattern is then detected and
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analyzed to calculate the size and size distribution of the measured samples in various forms (e.g.,
-p
as surface-weighted mean diameter, d3,2 ni di3 / ni di2 , volume-weighted mean diameter,
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d4,3 ni di4 / ni di3 , etc.) [27,79], where ni is the number of emulsion droplets with the
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diameter di. The d4,3 is often used as the average diameter that describes the Pickering emulsion as this diameter is more sensitive to presence of larger droplets and therefore is unaffected by
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excess Pickering particles that are may be present in the emulsion [77,81].The number of
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emulsion droplets is then categorized into several classes that eventually build up the size distribution of the emulsions. The entire process of detection of particle size distribution using
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the laser diffraction technique is as demonstrated in Figure 6.
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Figure 6. The summary of measurement of size distribution of emulsion droplets using the laser
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diffraction technique.
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The presence of particles on the interface of the droplets may pose some interesting
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effects on the light scattering property of the Pickering emulsions that should be tackled carefully by experimenters. Some works acknowledged the changes in the optical properties of the
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emulsion due to the presence of particles by assuming that the droplets are completely covered
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by the Pickering particles and therefore, the refractive index value used for measurement is that of the particle stabilizer [82]. Others chose to assume that the light scattering pattern of the emulsion is independent of the solid particles adsorbed on the emulsion droplets as the thickness of the stabilizing layer is only of the order of few nanometers [78]. Thus, these experimenters supplied the value of the refractive index of the pure dispersed phase to the particle size analyzer [78,83]. As such, the impact of the particles on the light scattering properties of emulsion have not reach a unanimous agreement amongst experimenters currently and should be a subject for future research efforts in standardizing how light scattering and laser diffraction methods are used to characterize emulsions. A temporal way to avoid misleading data from this analysis
Journal Pre-proof would be performing the qualitative optical/electron microscopic analysis for the verification of the quantitative results obtained from the laser diffraction measurement.
3.1.2 Microscopic visualization Besides the quantitative method via laser diffraction or light scattering, the visualization of Pickering emulsions via optical microscopy appears to be another rapid and simple method used
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by researchers to analyze the emulsion droplet sizes qualitatively. Optical microscopy utilizes
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visible light and several lenses to obtain images magnified up to 1000 times. After an image is
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captured, researchers can analyze the image using imaging software to get the average size as
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well as the size distribution (histogram) of the Pickering emulsions captured in the image. The obtained values can then be compared to the quantitative results recorded using laser diffraction
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method. Besides optical microscopy, electron microscopy and atomic force microscopy are also
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widely used to characterize the size of Pickering emulsions. However, these approaches require tedious sample preparation and instrument handling skills, making them less suitable to be used
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to determine merely the size of a Pickering emulsion. Instead, the electron microscopy and
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atomic force microscopy will be more useful to generate valuable information regarding the surface morphology and interfacial properties of the Pickering emulsion. This will be discussed in the next section.
3.1.3 Mathematical characterization methods Beyond direct experimental measurements, Pickering emulsions can also be characterized via mathematical methods whereby measured changes in the properties of emulsion droplets (i.e., droplet size) are correlated to the various emulsification parameters. The main advantage of
Journal Pre-proof mathematical methods is the ability to abstract insights about the emulsification process that are not readily inferable via experimental techniques. In this context, the mathematical techniques that are discussed here are those which characterize the size of Pickering emulsion droplets as well as their role in facilitating fundamental understanding of the different aspects of Pickering emulsification. Tsabet and Fradette [84] introduced a semi-empirical approach to estimate the surface-
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weighted mean diameter of a Pickering emulsion stabilized by high concentration of particles.
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Using a series of semi-empirical equations, the following procedure is introduced to predict the
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final droplet mean size: i) Calculation of interface generation and interface coverage potential, ii)
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Deduction of theoretically covered interface, iii) Determination of stabilization efficiencies, iv) Calculation of the effectively covered surface, and v) Deduction of the final mean droplet size.
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By this approach, the primary mechanism which dominates the stabilization of emulsion, i.e.
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interface generation vs. interface coverage, can be determined. In addition, quantification of the droplet coverage efficiency, particle-droplet collision efficiency, three-phase contact line
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formation efficiency and particle attachment efficiency as part of the evaluation procedure also
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yield information about the stabilization efficiency of the Pickering particles. The droplet size predicted by this method was validated by experimental droplet size obtained from laser diffraction methods. The authors concluded that this approach is potentially able to assist future scale-up efforts as it was found that due to the similarity in emulsion stabilization mechanism, slight variations in geometry can be accounted for by appropriately adjusting model coefficients. Nevertheless, this approach negates the first principles and therefore is highly system-specific, i.e. similar physicochemical properties of emulsion.
Journal Pre-proof Another mathematical characterization approach found in the Pickering emulsification literature is based on the limited coalescence theory. The limited coalescence phenomenon is one of the unique features that sets Pickering emulsions apart from their conventional counterparts stabilized by surfactants, which, unlike Pickering particles, cannot be irreversibly adsorbed [85]. In emulsions where there is no excess supply of particles, emulsion drops are formed initially with only partial coverage of irreversibly adsorbed Pickering particles. These partially covered
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droplets then coalesce until a complete particle coverage is attained. This is the essential
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mechanism behind the limited coalescence phenomenon. Initially, it was observed that the
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stability of the emulsion increases with increasing size of droplet [86] which was later found to
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be due to the increased stability of the coalesced emulsion droplets that achieve higher particle coverage and hence, higher stability.
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The limited coalescence theory has been applied in many instances to effectively describe
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the changes in equilibrium Pickering emulsion droplet size. In Wiley’s model [86], this theory was adopted for the first time to capture the effects of particle concentration on the droplet size.
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It was, later on, discovered that the limited coalescence phenomenon is not only a function of the
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particle concentration but it is also dependent on the type of emulsion produced. From the works of Golemanov et al. [87], two types of emulsions can be formed based on the solubility of the particles in the dispersed or continuous phase. When the emulsifier is more soluble in the continuous phase of a stable emulsion, this emulsion is known as the Bancroft emulsion whereas when the opposite is true, the emulsion is known as the anti-Bancroft emulsion. The equation describing the variation of the smallest stable droplet size, ds in an anti- Bancroft emulsion is shown below:
Journal Pre-proof ds
8rpa pd
(9)
p
Where ρpd is the ratio between the density of particle and the density of dispersed phase, φa is the mass fraction of the surface area that is covered by particles, φp is the mass fraction of solid particles in the drop of a specific radius and rp is the radius of a particle. The equation describing the variation of smallest stable droplet size, ds in a Bancroft emulsion is shown below:
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8rpa pd d p 1 d
(10)
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ds
-p
Where ϕd is the volume fraction of the dispersed phase in an emulsion. The smallest stable droplet size, ds from Equation (9)and Equation (10) was compared to experimental droplet size
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data obtained from dynamic light scattering techniques and acceptable agreement was observed
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between the experimental and predicted data. Characterization of droplet size based on the
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limited coalescence theory, although useful in certain cases, is limited to the major assumption that all available particles are adsorbed to the interface of the dispersed and continuous phase
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[85-88]. This, in reality, may not be true as particles can be present in the continuous or
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dispersed phase but not adsorbed on the emulsion droplets [85]. Apart from the above, recently, Siva et al. [89] introduced a transient scaling law for the prediction of droplet size based on the existence of sub-universal self- similar traits. The scaling law is able to describe the trend of average droplet diameter with time according to the following equation: 1
d 2
1 1 d t2 3
0
(11)
Journal Pre-proof Where ε is the measured power density, t is time of emulsification, d0 and d are the average diameters at t = 0 and t respectively, α is the characteristic value of an emulsion system and η is the dynamic break-up potential. Using this approach, good transient predictions of the droplet size can be obtained via laser diffraction procedures with minimal experimental dataset - a distinct advantage of this approach over the other two mathematical methods above. These transient scaling predictions
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may come in handy for scale-up of the emulsification process where the shortest time required
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for the achievement of the desired droplet size can be determined. In addition, this approach is an
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innovative way to understand the effects of different hydrodynamic and physicochemical
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properties on the emulsification process. The parameter η is a dimensionless group that describes the “dynamic potential” for overall droplet size reduction due to the combined effect of droplet
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break-up and coalescence of droplets during emulsification that can be calculated via a minimum
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pool of experimental dataset as long as the value of α is known. Via this equation, the combined effect of different process parameters can be analyzed by studying the values of the
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dimensionless group η where smaller values indicate a more efficient emulsion formation
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process. Doing so can help practitioners and researchers screen the emulsification process for appropriate combination of process parameters. Not only that, the characteristic value α is also indicative of the relative ability of a specific combination of dispersed and continuous phases and emulsifier to attain a minimum droplet size. This scaling law is currently limited to processes that allow measurable overall changes in temperature of emulsion broth. All the mathematical methods mentioned above only describe the changes in average properties of emulsion as a function of the processing parameters of the emulsification process. Future efforts should be directed towards the prediction of droplet size distribution transient in
Journal Pre-proof order to obtain deeper insights into the process. A potential approach to study drop size distribution in emulsification would be the population balance approach that allows an explicit account of breakage and coalescence mechanism in terms of measurable physical parameters and operational conditions [90]. This approach has been used many times to study surfactantstabilized emulsion systems [90-93], but has yet to be tested on the formation of Pickering
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3.2 Morphology and interfacial properties
of
emulsions.
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Since the surface morphology and colloidal properties of a Picke ring emulsion are directly
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influenced by the particles adsorbed at its interfaces, the structural features of Pickering emulsion should be carefully monitored throughout the synthesis and development of Pickering emulsion-
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based materials. Besides knowing the size, the visualization of Pickering emulsion appears to be
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an effective method to identify the morphological and structural properties of the Pickering emulsion. It is known that the unaided human eye can only resolve object down to the size of
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around 0.1 mm (100 µm) [28]. However, the structural component of the Pickering emulsion is
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far lower than the mentioned limit. Thus a variety of microscopic techniques including optical, confocal, electron and atomic force microscopy have been developed for the visualization of these features [94-98]. Each of these microscopic techniques works on different physicochemical principles and can be employed for the examination of the emulsion at different levels [25]. The advantages and limitations of various techniques to be described are summarized in Table 2.
Journal Pre-proof Table 2 Advantages and limitations of different methodologies for determining the morphology and interfacial properties of Pickering emulsions. Methodology Bright- field
Advantages Optical
microscopy
Quick view on the morphology and integrity
contrast
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May be hard to differentiate between the oil and
o r p
of the Pickering emulsion.
[27-28,97,107,111-114]
Phase
Limitations
aqueous phase of the emulsion system.
e
Simple sample preparation procedure.
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Allowed one to distinguish between the
microscopy
disperse and continuous phase of an emulsion
[25,97,107,111]
system.
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Enables the visualization on the locations of particles around the Pickering emulsion. Dual staining can be performed.
In-depth analysis of the morphology of Pickering emulsions is not possible.
Presence of bright light/UV may degrade the color stain and results in poor images. Oil/aqueous phase staining may be difficult depending on the staining agent. Modification
or
enhancement
of
optical
microscope may be required if staining is not applicable.
Confocal laser scanning
Revealing the structure and location of solid
Dark room is preferred during the analysis to
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(CLSM)
[99-109,114]
particles adsorbed on the
interface of
avoid the degradation of the dye. Staining procedure may be tedious depending on
Pickering emulsion. Visual on the cross-sectional layer of a
the targeted particles and its staining agent.
Pickering emulsion can be done.
f o
3D imaging is possible. Raman
microscopy
[110]
No extra staining steps needed to show the
l a
3D imaging is possible Scanning microscopy [115-122]
e
surface morphology of the adsorbed particles at emulsion interface.
o r p
Sample must be in stagnant during the Raman
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electron
Analysis of the structure and adsorption
(SEM)
behavior of solid particles on emulsion
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interfaces in nanoscale ranges. In-depth observation of the structure and morphology of the Pickering emulsions. Able to view and measure the thickness of Pickering shell formed by the solid particles.
measurement.
Special sample holder is required for liquid-type samples. Polymerization/freeze drying of emulsion may be needed prior to sample visualization. Gold/platinum coating may be required to provide Pickering emulsions of higher magnification and clarity. Tedious operating procedure as compared to the optical microscopy techniques.
Journal Pre-proof Transmitting
electron
microscopy
(TEM)
[76,122-130]
High
definition
emulsion
droplet
imaging
of
Pickering
with/without
Special sample holder (e.g. copper grid) is needed
staining
for sample loading prior to analysis. Sample dilution must be done to provide a clear
procedure. High-resolution detection of the particles
image of a single or multiple Pickering emulsions.
f o
Tedious operating procedure as compared to the
adsorbed on the emulsion interfaces.
o r p
Nanoscale surveillance of the cross-sectional
e
image of Pickering emulsion. Atomic microscopy [109,131-135]
force (AFM)
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High-resolution detection of the particles
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adsorbed on the emulsion interfaces.
Allowed measurement of the stiffness and
n r u
strength of the interfacial shell of Pickering
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emulsion.
Online morphological and stiffness analysis is possible if integrated with CLSM.
optical microscopy techniques.
Not suitable for liquid sample. Special modification on the probe tip may be necessary for stiffness and interfacial shell strength measurement Poor quality of image without incorporation with other microscopic technique (such as CLSM)
Journal Pre-proof 3.2.1 Optical microscopy Among the developed techniques, optical microscopy is of the most commonly used approach for the emulsion viewing due to its simplicity in instrument handling and sample preparation. However, as regarded by its magnification limits (down to approximately 1 µm), the image obtained from optical microscopy could not provide clear information on the structural features of a Pickering emulsion (e.g. particles aggregates, particles configuration, interfacial layer
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thickness and others) that generally falls in the size ranges that is considerably lower than the
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specified limit of optical microscopy [27-28]. In certain conditions where large particles of
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irregular shapes are used as stabilizer (such as Janus particles with anisotropic morphology), the
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use of bright-field microscopy may be feasible to observe the Pickering particles adsorbed to the oil/water interfaces [112]. However, one should always remember that the major components
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(location of oil and water phase) of the Pickering emulsion droplet captured by optical
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microscopy is often hard to be distinguished due to the similarity in the refractive indices or colors when the bright-field mode is used [27].
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Although the Pickering emulsion stabilized by Janus particles can be viewed quite easily
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using bright- field microscopy, efforts are therefore put forward to enhance the contrast between the major components of the emulsion along with the image quality and resulting interfacial information of the optical microscopy image [97]. There are many types of chemical stain that can tag the adsorbed components (Pickering type stabilizer) on the emulsion interfaces or selectively partitioned into either the disperse phase or the continuous phase [97]. Upon binding/partitioning, these dyes can be detected by (1) bright- field microscopy if they absorb light in the visible region (water/oil color dye), (2) fluorescent microscopy if they are fluorescent. As an example, the photograph obtained for bright-field and fluorescent microscopy has been
Journal Pre-proof presented in Figure 7a and b. The staining of oil phase (red halo) and solid particles (blue halo) allowed one to clearly distinguish between the location of the particles as well as the disperse and continuous phases (Figure 7a and b). Besides that, the contrast between different components of emulsion can also be enhanced by modifying the design of the optical microscope through the installation of special lenses that convert small differences in refractive index into differences in light intensity [25] which is known as the phase contrast or differential
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interference contrast microscopy [25,97].
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Confocal laser scanning microscopy (CLSM) is one of the most important innovations in
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microscopy techniques for studying the interfacial structures of a Pickering emulsion which
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focuses an extremely narrow laser beam at a particular point in the analyzed sample and the resulting laser signal is then passed through an adjustable pinhole set before arriving the detector
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to prevent the out-of- focus blurring [99]. Similar to fluorescent microscopy, the CLSM approach
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also requires the staining of emulsion/components of emulsion before analysis. It enables one to obtain the cross-sectional images of Pickering emulsions that hold essential morphological,
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structural and interfacial information. Many literature reports have, in fact, showed the
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effectivity of the CLSM in identifying the location and morphology of solid particles at the Pickering emulsion interface [51,55,100-104]. Recently, the use of CLSM has been further extended to three dimensional Pickering emulsion imaging, which has allowed researchers to obtain a more in-depth understanding and visualization of the surface morphology, coverage and interfacial orientation of solid particles across the entire Pickering emulsion droplets [105-109]. The differences between the optical images obtained for bright- field, fluorescent, CLSM and 3D CLSM visualization are available in Figure 7c to f. The 3D CLSM imaging is expected to receive growth interest as it may provide online information on the testified samples (even for a
Journal Pre-proof single Pickering droplet), which thus easing the monitoring and understanding of the Pickering emulsions for various characterization and applications. The incorporation of Raman spectroscopy to optical imaging (generally known as Raman microscopy) appears to be another method that may gain considerable attention in the future due to its ability to show the chemical contrast of materials in a label- free condition. The Raman microscopy utilized the differences in several characteristics spectra of various chemical bonds
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to differentiate between particles, and other molecules [110]. However, it should be noted that
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the sample used for Raman microscopy is the same as that for optical microscopy, which is in the
-p
form of emulsion suspension. This causes the continuous motion of emulsions and thus the
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retardation of the Raman measurement accuracy. This remained as a major issue to be solved
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prior to the employment of Raman microscopy in Pickering emulsion imaging.
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Figure 7. (a) Bright-field and (b) fluorescent micrograph of Pickering emulsions stabilized by Fe3 O4 @CNC nanocomposites (particles stained with calcofluor white while oil phase stained with nile red). (c) Bright- field microscopic image of particle-stabilized ionic liquid droplets in oil. (d) Fluorescent, (e) 2D CLSM and (f) 3D CLSM microscopy ima ge for the ionic liquid in oil Pickering emulsions (ionic liquid labelled by fluorescein isothiocyanate isomer (FITC-I)). Adapted with permission from ref. [111] (Copyright (2019) Elsevier) for (a) and (b), ref. [107] (Copyright (2019) The Royal Society of Chemistry) for (c) to (f) respectively.
Journal Pre-proof 3.2.2 Electron microscopy Electron microscopy is an alternative approach that has been widely utilized to examine the structure of nanoparticles, nanoemulsions and Pickering emulsions. It uses e lectron beams that have much smaller wavelengths than light to help to visualize the structure of materials that is way smaller than the limits of visible light [25,97]. Two types of electron microscopy approach, namely the scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
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are normally employed for the examination of Pickering emulsions.
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SEM allows one to see the surface topography of specimens through the measurement of
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the secondary electrons generated when the sample is bombarded by the electron beams [25]. It
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should be noted that the electron microscopy is normally performed under high vacuum conditions because electrons are easily scattered by atoms or molecules in a gas. Thus, extensive
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preparation of sample is required to ensure that the analyzed materials are free of all volatile
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components (water and organic molecules) that may evaporate [25,115]. Unfortunately, the general sample preparation procedure (fixation and dehydration) for electron microscopy is not
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suitable for Pickering emulsion as the dehydration of Pickering emulsion often leads to droplets
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disruption and oil leakage [116-117]. Polymerizations of Pickering emulsions have been practiced by researchers to ensure the emulsion remained its integrity during SEM analysis [55,57,115,118]. Such sample preparation procedure thus allowed one to clearly identify the morphological structure, orientation and thickness of solid particles on the emulsion surfaces (Figure 8a and b) [55,57,115,118]. The cryogenic (Cryo) SEM (Cryo-SEM) is another famous electron microscopy method that has been used to characterize the interfacial thickness and properties of Pickering emulsions in-situ [49,61,106,119-121]. It involved, first the rapid freezing of emulsion by liquid nitrogen to
Journal Pre-proof retain the structure and morphology of the specimen, followed by the freeze- fracture of sample by cold knife to expose the internal microstructure [35]. The fractured frozen emulsions then undergo controlled sublimation to remove a designated amount of water or oil (to show the voids between droplets) and coated with a thin layer of gold prior to observing it under SEM chamber [35]. These preparation steps not only lead to visualization of the interfacial layer of the Pickering emulsion droplet under high resolution but also allowed the discovery of packing
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structure and orientation of solid particles at the surfaces of a Pickering emulsion (Figure 8c to f).
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Figure 8. (a) SEM image of eicosane@SiO 2 capsule and (b) the thickness of its shell. (c) CryoSEM image of WPM particles stabilized heptane in water emulsion. The respective heptane in water emulsion prepared at (d) pH 3 (discrete organization), (e) pH 4.8 (aggregated particles) and (f) pH 7 (flocs of particles). Adapted with permission from ref. [118] Copyright (2019) American Chemical Society (a and b) and ref. [120] (Copyright (2019) The Royal Society of Chemistry) (c to f).
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Besides SEM, TEM has also been widely used to characterize a Pickering emulsion. In TEM, the electron beam would travel directly through the analyzed sample where the electron beams that are transmitted to the specimen are magnified and captured as an image [25]. TEM was found to be able to observe Pickering emulsion with/without chemically stained prior to the microscopy. For instance, Lan et al. [40] showed the direct visualization of Fe3 O4 @oleic acid-
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stabilized Pickering emulsion via TEM and suggested the presence of the particles and particle
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flocs around the interface. Qian et al. [122] also showed the aggregation of lignin nanoparticles
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around the shell of Pickering emulsion via TEM imaging without staining the Pickering
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emulsions (Figure 9a and b). It should be noticed that these emulsion samples were subjected to TEM visualization without the extra sample preparation steps. Thus, the distinction between
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dispersed and continuous phases of Pickering emulsion in the obtained images remained unclear.
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The staining of emulsion for TEM analysis appears to be another interesting technique to observe the Pickering emulsion. As an example, the clear TEM images of peanut oil- in- water Pickering
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emulsions stabilized by bacterial cellulose has been successfully acquired upon staining of
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cellulose particles and oil phase with phosphotungstic acid (Figure 9c) [123]. Similar sample preparation procedure via emulsion polymerization and cryo-electron microscopy has been reported to provide TEM images with clearer contrast between the particles, dispersed and continuous phases [121,123-129]. As compared to the staining method, the polymerization of Pickering emulsion seems to be able to favor the TEM images with higher resolution [124]. This can be evidenced by the TEM images of Pickering emulsions stabilized by PMMA@silica nanoparticles that have been successfully captured after the polymerization of Pickering emulsions [125]. The obtained image not only showed clear distribution of PMMA@silica
Journal Pre-proof nanoparticles around the emulsions but also revealed a clear contrast between the dispersed and continuous phases of the specimen (Figure 9d) [124]. The Cryo-TEM method was also able to provide high- resolution images that are comparable to those prepared via polymerization approach [128-129]. For example, the Cryo- TEM images presented by Persson and co-workers [128] has demonstrated the densely packed particles around the silica-stabilized Pickering emulsion with clearly distinguishable squalene and water phases (Figure 9e and f). Although the
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polymerization and Cryo-TEM methods favored higher resolution TEM images, it should be
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reminded that both methods involved tedious sample preparation steps as compared to the simple
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and direct phosphotungstic staining approach. Thus, the choice of imaging method is highly
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dependent on the information required from the analysis (e.g., size, morphology, interfacial
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thickness and orientation, etc.).
Figure 9. TEM images of (a) lignin- g-DEAEMA stabilized Pickering emulsions with (b) its higher magnification photograph. (c) TEM images of peanut oil in water Pickering emulsions stained with phosphotungstic acid. (d) TEM images of Pickering emulsion polymerized poly(methyl methacrylate) latex armored silica nanoparticles. Inset showed a soft shell of poly(n-
Journal Pre-proof butyl acrylate). Cryo-TEM images of (e) a single and (f) multiple squalene in water emulsions. Adapted with permission from ref. [122] (Copyright (2019) The Royal Society of Chemistry) (a and b), ref. [123] (Copyright (2019) Elsevier) (c), ref. [124] Copyright (2019) American Chemical Society (d) and ref. [128] (Copyright (2019) Elsevier) (e and f), correspondingly.
3.2.3 Atomic force microscopy
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Although atomic force microscopy (AFM) can be used to visualize Pickering emulsion
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droplets (see Figure 10a), its usage for the characterization of Pickering emulsions often includes
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the measurement of the stiffness and strength of the interfacial shell as well as the forces needed
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to disrupt the Pickering shell [130]. The AFM produces images by moving a tiny probe (a few micrometers in size or smaller) across the surface of the targeted sample. Upon contact with the
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surface of the sample, the cantilever attached to the probe bends, and the deflection of the stylus
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is measured by a detector and is converted into images [25]. From the working principle, one can envision that the resolution of the AFM depends strongly on the size and shape of the probe as
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well as its possible positioning relative to the sample. Additionally, it should be noted that the
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cantilever and probe of the AFM are highly fragile, thus to avoid possible damage to the stylus of the AFM probe, the force acted on the sample is measured so that the deflection of the stylus is always at constant [25]. In recent studies, the AFM analysis for emulsions is more often presented as in-depth analysis with/without coupling with other techniques to show the strength of the interfacial barrier of Pickering emulsions (Figure 10b) [109,130-134]. For example, in the works of Xie et al. [134], AFM is employed to measure the surface forces and interactions between emulsion drops and gas bubbles. Besides that, Mettu et al. [130] integrated CLSM to AFM to show the stiffness and the force required to rupture the chitosan microcapsules prepared
Journal Pre-proof via Pickering emulsion cross-linking (Figure 10c). The force measurement was performed simultaneously while the microcapsule was monitored in-situ using 2D and 3D AFM-CLSM analysis (see Figure 10d and e for example images) [130]. In another study, Jamieson et al. [132] evaluated the forces between oil droplets using AFM and further compared the drops interactions across a microfluidic device. The reported studies showed the possibility of coupling AFM with other microscopic and microfluidic analysis to not only reveal the surface structure and
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morphology of the Pickering emulsions, but also examine the strength of its interfacial barrier,
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thus achieving an advanced, in-depth characterization of Pickering emulsions.
Figure 10. (a) AFM image of dried Pickering emulsions in pectin gel. (b) Deformation force measurement on a clay-armoured Pickering droplet. Insets images showing CLSM images of the droplet during the AFM compression measurements. (c) Schematic of the CLSM-AFM experiment. (d) 2D and (e) 3D CLSM images of a microcapsule during the indentation experiment with AFM. Adapted with permission from ref. [135] (Copyright (2019) American
Journal Pre-proof Chemical Society) (a), ref. [109] (Copyright (2019) The Royal Society of Chemistry) (b) and ref. [130] (Copyright (2019) The Royal Society of Chemistry) (c to e), respectively.
3.3 Surface coverage Typically for a surfactant-stabilized emulsion, droplets are required to be completely covered by surfactants without which optimal stabilization of liquid-liquid interface cannot be achieved
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[136]. However, for Pickering emulsions, it was observed that stability was present even when
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droplets are only partially covered with Pickering particles [74,137]. In fact, Vignati et al. [74]
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claim that there is no straightforward observable relationship between the degree of droplet
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coverage and emulsion stability, which further complicates the understanding of the influence of surface coverage on droplet stability. This shows the importance of proper characterization of
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droplet surface coverage with Pickering particles to as scrutinize-able detail as possible.
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One of the ways to determine the degree of surface coverage is from the knowledge of
p re 4d rp
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[138]:
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the size of droplets, re and size of Pickering particles, rp according to the following equation
(12)
This equation however can only be used for spherical particles which led to the works of Kalashnikova et al. [55] who proposed a new equation for rod-shaped particles based on the ratio of maximum surface area that can be covered by Pickering particles to the total emulsion droplets surface area to be covered [55,139] which is as follows:
m p re
12h pVoil
(13)
Journal Pre-proof Where m p , h and ρ are the mass, thickness and density of the rod-shaped Pickering particles and Voil is the volume of dispersed phase. According to Equations (12) and (13), surface coverage of droplets is identical irrespective of the type of particles used for emulsification as long as the dimensions of droplets and particles are maintained constant. This may not be the case all the time because the interfacial coverage of particles also depends on the surface charge of particles where-by highly charged particles pack less tightly at the liquid-liquid interface [83]. This
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dependence on the surface charge of particles is not taken into account in this equation. Another
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way the surface coverage can be measured is through the visualization of droplet covered by
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particles itself to qualitatively characterize the surface coverage of droplets. Binks and Whitby
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[140] qualitatively viewed the structure of adsorbed silica particles layer at the oil-water interface using low-temperature field emission scanning electron microscopy (LTFESEM)
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(Figure 11a). On the other hand, Vignati et al. [74] visualized the coverage of fluorescent silica
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colloids for oil droplets in water via epifluorescence microscopy technique where the resulting images were processed by a video elaboration software to further enhance the coverage
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properties of the particles (Figure 11b). Though excellent images of surface coverage were
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obtained using these methods as shown in Figure 11, only average quantitative coverage information can be obtained. More elaborate characterization techniques for surface coverage should be enacted to understand the surface coverage properties of Pickering emulsions better. This is especially important to better comprehend the enhanced stability of emulsion droplets under “particle-poor” conditions which is a more practical process condition under which Pickering emulsion is prepared [74]. Besides that, the surface coverage of Pickering emulsion also be observed via simple bright- field microscopy. In a recent study, Haney et al. [112] showed the optical images of Janus
Journal Pre-proof particles adsorbing firmly at the interface between water and toluene. The success in such visualization could be due to the anisotropic shape of the Janus particles, making it differentiable to the emulsion droplets (see Figure 11c). With this feature, the Janus particle-stabilized emulsion is expected to be more easily observed under a higher magnification microscopy (AFM,
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SEM, TEM, etc.) as compared to that of normal Pickering emulsion.
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Figure 11. (a) Low-temperature field emission scanning electron microscopy image of
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polydimethylsiloxane-in-water emulsions stabilized by hydrophobic silica particles (Bar represents 500nm). (b) epifluorescence microscopy image of isooctane- in-water emulsions
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stabilized by hydrophilic silica particles. (c) Bright- field images of Janus particles adsorbing at
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water/toluene interface. Adapted with permission from ref. [140] (Copyright (2019) American Chemical Society) (a), ref. [74] (Copyright (2019) American Chemical Society) (b) and ref. [112] (Copyright (2019) American Chemical Society) (c).
3.4 Droplet charge The origin of droplet charge of Pickering emulsions is normally due to the adsorption of charged particles on the emulsion interfaces. The surface charge of these particles may be influenced by various environmental factors including pH, ionic strength and che mical reactions [25]. These
Journal Pre-proof changes may eventually lead to the variation of surface charges of the Pickering emulsions and thus the surface properties and stability of the Pickering drops. Similar to the colloidal suspension of particles, the magnitude of the surface charge of emulsion droplets is also measured by recording its Zp. As discussed earlier, the Pickering emulsions with higher stability can be formed using the colloidal particles with low Zp values as compared to those form under high Zp [53,55,58,141-142]. However, it should be noticed that the low Zp may lead to a great
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extent of the aggregation of Pickering emulsions and so, the eventual emulsion droplets
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coalescence and destabilization if the attractive force surpasses the desorption energy of the
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particles. For Pickering emulsions that normally hold supreme resistance against coalescence and
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deformation, the droplet instability owing to agglomeration incident may be difficult to be observed in a short period. The problem may be tackled by recording the transmission extinction
3.5 Colors of emulsions
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profiles of samples through analytical centrifugation which will be disclosed in Section 4.1.5.
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The perceived quality of a variety of commercial emulsion-based products from various
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industries that include food, pharmaceutical and cosmetics depends on the appearance of these emulsions, especially in terms of the color of these emulsions [143-145]. Thus, it is highly important for the proper characterization of this basic property via available analytical and experimentation techniques. An emulsion color depends primarily on the scattering and absorption efficiency in the emulsion broth where-by the latter is controlled by the coloring agent properties (i.e., absorption spectra and concentration) and the former is influenced by the droplet characteristics (i.e., droplet size, droplet concentration, etc.) [145-147]. Typically, the color of an emulsion can be measured
Journal Pre-proof in a colorimeter and can be presented in the form of L,a,b color system, a set of mathematical variable system where L indicates the “lightness” of an emulsion where-as the a- and b- value indicate the actual color of the emulsion [147]. From past works involving surfactant-stabilized emulsions, it was observed that as droplet size increases, there would be an imminent decrease in scattering efficiency of droplets and therefore an emulsion of lower lightness and the enhanced color is resultant as shown in [144,147]. McClements [145], on the other hand, observed an
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increase in the brightness of the emulsion as droplet concentration increases. Based on these
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changes, McClements developed a theoretical protocol to predict emulsion color based on their
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composition and microstructure that uses a range of mechanistic theories (i.e. Mie theory,
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radiative transfer theory, Kubelka-Munk theory) to relate the scattering, adsorption and reflectance characteristics of the emulsion and to finally translate this information to the L,a,b
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coordinates, representing the color of the emulsion in question. This innovative way to control
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emulsion characteristics based on its color allows emulsion developers to more efficiently arrive at the desired emulsion quality with a lesser reliance on timely and costly experimental methods.
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Although the above methods are derived primarily based on surfactant-stabilized
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emulsions, the theory could still be applied on Pickering emulsions because as stated earlier, the color of an emulsion is determined by the composition and microstructure of an emulsion which is no way is related to the type of emulsifier used in the formulation. In fact, emulsions of higher level of lightness can be expected when Pickering particles are used as emulsifiers due to the fact that Pickering particles are typically larger than surfactant particles in size resulting in larger sized emulsion droplets as well [1] which is typically in micrometer range that is trackable by visible light. In fact, it is possible to estimate the size of emulsion droplets according to the color of the emulsions if the particles stabilizer used possess certain color. Recent reports on Pickering
Journal Pre-proof emulsions stabilized by black-colored particles (e.g., magnetite nanoparticles, graphite) revealed that the color of the emulsions faded (from dark light) as the size of the emulsions decreased [37,111,141,148-149,151-152]. Additionally, recent works by Binks and Olusanya [143] introduced emulsion stabilized by colored pigments where-by emulsions produced changes in color from blue to indigo through dark brown to orange as the hydrophobic pigment used as Pickering emulsifier increases in
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concentration. This suggested the possibility of rapid-prediction of the size of Pickering
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emulsions if a related model can be produced in the future for the size estimation application. As
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a matter of fact, the Pseudo-coloring simulation method has been recently demonstrated to be
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reliable in determining the power intensity of an ultrasound generator based on the light intensity obtained from sonochemiluminescence [153]. Several fatal challenges to be solved are the
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relationship between the emulsion color and (1) their respective greyscale intensity and (2) the
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particles coverage at the emulsion surfaces. The model for Pickering emulsion size prediction
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will be highly feasible upon addressing of the mentioned problems.
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4. Physical stability of Pickering emulsion The term “emulsion stability” normally refers to the ability of an emulsion to resist the changes in its physicochemical properties over time [27]. The physical stability of emulsion refers to the proficiency of the emulsion to remain its physical characteristics (size, shape, rheology, morphology and other non-chemical related properties) over time. For an emulsion system, several most commonly noticeable instability mechanisms include gravitational separation (creaming/sedimentation), coalescence, flocculation, Ostwald ripening and phase separation. Although gravitational separation could sometimes happen merely due to the difference in the
Journal Pre-proof density of the two immiscible phases, the coalescence that involved the collision of two or more emulsion droplets to form a single larger droplet [154], can be considered as the parent instability mechanism that can lead to all the aforementioned instability mechanism. Concisely, Ostwald ripening occurs as an outcome of the steady growth of larger droplets by expensing the smaller droplets, where those tiny droplets formed initially would disappear in time [155]. This is highly similar to that of coalescence as well as the flocculation where the emulsion droplets collided
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and clump together into a floc [156]. After the emulsion coalesced, the flocs/larger droplets then
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either settled down (sedimentation) or floated (creaming) due to gravitational separation and
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eventually results in phase separation (see Figure 12 for graphical illustration) [157].
Figure 12. The different Pickering emulsion instability mechanisms
Journal Pre-proof Therefore, the characterization technique for determining the coalescence stability of the Pickering emulsion will be focused in this paper. On the other hand, the method for examining the gravitational separation (or creaming) behavior of the Pickering emulsion system will also be reviewed since it may occur even without the presence of droplets coalescence.
4.1 Coalescence stability
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Coalescence is a process involving the merging of two or more emulsion droplets a larger one
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[154]. As mentioned above, the presence of droplet coalescence will cause the emulsion system
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to undergo creaming or sedimentation, followed by phase separation more rapidly due to the
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increase in the droplet size. Understanding the most precise and reliable methods available to date for recording the coalescence stability of a Pickering emulsion system is, therefore of
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utmost importance for the achievement of desired quality of the emulsions which will be further
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discussed in Table 3.
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outlined in the sub-sections that follow. The pros and cons of the several techniques are briefly
Journal Pre-proof Table 3 Advantages and limitations of various techniques for monitoring the coalescence stability of Pickering emulsions. Methodology
Advantages
Limitations
Storage analysis
Simple and most commonly used.
Long experimental period (at least one month).
[22,63,111,152,158-160]
Result obtained directly shows the storage
Unable to clearly illustrate the threshold force
stability of the Pickering emulsion.
Micromanipulation [161-167]
e r P
l a
that cause two emulsion droplets to coalesce. Provide
n r u
feasibility for
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Ability to reveal the critical Laplace pressure
the synthesis of
Pickering emulsions with different shapes.
f o
needed
to
induce/prevent coalescence on
Pickering emulsions.
Special experimental setup is necessary. Limited to droplets of macro-sized. CLSM integrated AFM method is required for the analysis in nanoscale.
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Examination of droplet coalescence stability under applied pressure conditions Accelerated coalescence
Rapid analysis.
[59,112,114,141,159,168-
Facile way to disclose the effect of work on the
172]
coalescence stability of Pickering emulsions.
Driving force (stirring/centrifuging speed) must be determine manually to prevent the disastrous disruption of the Pickering emulsion.
Journal Pre-proof May be correlated with storage analysis.
Online monitoring of coalescence properties is
Evaluation of droplet coalescence stability
not possible without the aid of other techniques.
under applied flow conditions Light scattering
Simple and rapid analysis.
Preliminary level analysis
f o
Indirect measurement on the coalescence
[145]
o r p
stability of Pickering emulsion.
Analytical centrifugation [173-175]
l a
e
r P
Can measure the intensity of the transmitted
n r u
light as function of time and position over the
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entire sample length simultaneously. Data obtained can be related to Pickering emulsion size. Evaluation of droplet coalescence stability under applied flow conditions.
Unable to show the essential threshold force to induce/prevent
coalescence
on
Pickering
emulsions. Not suitable for evaluating the coalescence stability of a single Pickering emulsion. Data obtained is mainly for information in bulk.
Journal Pre-proof 4.1.1 Emulsion storage properties analysis The evaluation of the change of emulsion droplet size over time can be considered as one of the simplest and most commonly used methods for the evaluation of emulsion coalescence stability. The size and distribution of the emulsion droplets can be measured using the emulsion droplet sizing techniques mentioned earlier periodically up to a designated storage period (7 days or more) to carefully monitor the presence of coalescence after they have been in contact for
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extended periods (e.g., after creaming, flocculation) (Figure 13a). The obtained results can then
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be supported by the qualitative results from microscopic analysis (Figure 13b and c). The
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employments of the above- mentioned methods for determining the Pickering emulsions stability
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have been widely practiced by researchers [22,63,111,152,158-160]. It should be noticed that the aid of microscopic visualization in determining the coalescence stability may sometimes be
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useful in the case where Pickering emulsions flocs are present. This is because the flocculation of
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emulsion droplets may happen if the Pickering emulsions were stabilized by particles with low zeta potential. However, the droplets flocculation did not result in droplets coalescence due to its
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enhanced resistance. The presence of flocculation of emulsion droplets with no emulsion
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coalescence has been observed in a recent study for the preparation of pharmaceutical Pickering emulsions (Figure 13b) [111].
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Figure 13. Change in the droplet diameter of MCNC-stabilized Pickering emulsions at pH 6 and
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8 after up to 336 hours of production (or 14 days). Optical micrograph of the Pickering
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emulsions at (b) pH 6 and (b) pH 8 upon 0, 7 and 14 days storage. Adapted with permission from
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ref. [111] (Copyright (2019) Elsevier).
4.1.2 Emulsion micromanipulation technique
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The use of microscopic analysis for detecting coalescence can also be related to the film trapping
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technique (FTT) [161]. In FTT, oil droplets are trapped between an oil phase held in a capillary tube connected to a pressure control system and the water phase. Then, the pressure in the
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capillary tube is increased until the oil droplets coalesced with the oil-water interface to determine the critical capillary pressure required for inducing coalescence (Figure 14a) [161]. More recently, the measures of Laplace pressure between two Pickering emulsion droplets for coalescence or partial coalescence have been established in-situ via the micromanipulation technique consisting of both capillary tube and optical microscopy [162]. The experimental setup of the technique consists of two capillary fashioned micropipettes where each of them are connected to two separate water reservoir. The Pickering emulsion droplet held on both micropipette were pushed together into contact and their coalescence behavior was monitored via
Journal Pre-proof the microscope (Figure 14b) [162]. From the micromanipulation coalescence experiment, the Laplace pressure, ΔP that drives the coalescence of droplets can be estimated according to the following relationship: P
2 ow re
(14)
The developed method allowed researchers to determine the exact pressure required for partial
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and total coalescence of two Pickering emulsion droplets (Figure 14c). The fundamental
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knowledge resulted from the development not only able to provide clearer understanding on the
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coalescence behavior of different Pickering emulsion systems but also showed promising potential for the development of Pickering emulsions of different size and shape for various
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industrial usage [162-167]. In fact, the Laplace pressure required for the coalescence Pickering
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emulsion of various sizes and shapes has been reported [162-167]. This information will be useful for the design and utilization of Pickering emulsion-based materials for applications that
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require the temporal stability with the subsequent destabilization of the Pickering emulsions.
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Besides the as- mentioned capillary-based techniques, the utilization of CLSM and AFM in
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micromanipulation appear to be another promising method for analyzing the stiffness and strength of the Pickering emulsions. The possible instrumental set-up and several interesting results reported in the literature studies have been captured in section 3.2.3 (see Figure 10b to e). Overall, emulsion micromanipulation techniques allow researchers to disclose the effective pressure needed for the deformation of Pickering emulsion. However, the technique developed so far is restricted to the controlled coalescence between two droplets at any time. This may differ from the actual environment where multiple emulsion droplets collide at the same time. Addressing this issue would definitely result in a big leap in evaluating the coalescence stability of Pickering emulsions.
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Figure 14. (a) Schematic illustration of film trapping apparatus. (b) Coalescence behaviour of Pickering emulsion droplet at different surface coverage. Scale bars = 50 m. (c) Different
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coalescence regimes (total coalescence, arrested coalescence and total stability) as a function of
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droplet surface coverage. Adapted with permission from ref. [161] Copyright (2019) American
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Chemical Society (a) and ref. [162] (Copyright (2019) The Royal Society of Chemistry) (b and
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c).
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4.1.3 Accelerated coalescence test
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The coalescence of emulsion droplets may be accelerated in the presence of mechanical agitation because of the increased collision frequency of emulsion droplets. The accelerated coalescence tests are therefore used to disclose the coalescence stability data of the Pickering emulsion systems under flow conditions. The accelerated coalescence tests can be categorized into mechanical agitation and centrifugation method. The mechanical agitation method increases the rate of collision between droplets through stirring of the Pickering emulsions (Figure 15a) [27]. The properties of the Pickering emulsions (sizes, morphology, appearance, phase separation) are measured over time according to the methods mentio ned earlier. Several examples of emulsion coalescence due to stirring- induced collision under different experimental conditions have been
Journal Pre-proof previously demonstrated in the literature [59,141,159,168-170]. On the other hand, the centrifugation method involved the centrifugation of emulsions to force the droplets to bump into each other as a result of the centrifugal force (Figure 15b). The centrifugation method allowed researchers to obtained both qualitative and quantitative information about the stability of emulsions to coalescence [27]. According to works by Tcholakova, after subjecting the proteinstabilized emulsion to centrifugation in an optically transparent centrifuge tube for a designated
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speed and time, the oil globules with lower density than the continuous aqueous phase tend to
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move towards the axis of rotation of the centrifuge [161,171-172]. This forced the Pickering
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emulsions to closely aggregate with each other while retaining their original size [27], and in
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some cases, the droplets coalesced when the centrifugal force overwhelmed the Pickering barrier formed by solid particles [112]. Upon droplet coalescence, there will be formation of layers of
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Pickering emulsion droplets according to their respective sizes, with the larger Pickering
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emulsion droplets located nearer to the axis of rotation than the smaller ones. Pertaining to the possibility of increased coalescence due to the increasing centrifugal force, it can be expected
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that the oiling off will occur at higher centrifugation speed [161,171-172]. From the accelerated
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cr centrifugation test, the critical osmotic pressure ( Posm ) for drops disruption can be measured
experimentally as follow: cr Posm
c gca Vtotal Vreleased Across
(15)
where c c 2 c1 is the difference in the density of continuous and dispersed phases, gca is the centrifugal acceleration, Vtotal is the total added volume of the dispersed phase for emulsification, Vreleased is the centrifuged- induced released volume of the dispersed phase and Across is the cross-sectional area of the centrifuge tube.
Journal Pre-proof Mechanical agitation and centrifugation methods provide researchers the ability to examine the pressure required to induce coalescence to Pickering droplets under any flow conditions. In contrast to the micromanipulation techniques that record the Laplace pressure between two individual droplets, the data obtained by accelerated coalescence test were measured in bulk (multiple drops coalescence). Both characterization techniques can be utilized together to compensate each other to provide a more in-depth analysis on the coalescence
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stability of a Pickering emulsion system.
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Figure 15. Schematic illustration the accelerated coalescence test via (a) mechanical agitation and (b) centrifugation.
4.1.4 Light scattering measurement It is well known that the coalescence of emulsion droplets leads to the change in the appearance of the system since the light scattering capability of a large droplet is weaker as compared to that of a smaller droplet [145]. As a result, the emulsion may become less turbid and more intensely colored (Figure 16a and b) [145]. In that sense, the measurement of the turbidity of a Pickering emulsion can be considered for determining the coalescence stability of the Pickering emulsion
Journal Pre-proof system. It should also be noted that the gravitational separation can also cause the emulsion drops to concentrate in a specific region (either top (for creaming) (Figure 16c) or bottom section (for sedimentation)) (Figure 16d). Thus the turbidity measurement may be more suitable to act as a supplementary or a preliminary analysis to characterize the coalescence stability of a Pickering
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emulsion system.
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Figure 16. Schematic illustration of the changes in turbidity of emulsions upon light scattering/transmission test, with (a) stable, (b) coalesced (larger), (c) Creamed and (d) Sediment
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Pickering emulsions.
4.1.5 Analytical centrifugation method As an extended analysis for the centrifugation and light scattering method, Shimoni et al. [173] employed an analytical centrifugation technique that involved the integration of light transmission with centrifugation to determine the coalescence stability of Pickering emulsions. The as-mentioned approach utilizes the Space and Time resolved Extinction Profiles (STEP T M) Technology and can be used to measure the intensity of the transmitted light as function of time and position over the entire sample length simultaneously (Figure 17a and b), and the data produced can be used to calculate the creaming rate and the mean droplet size of emulsions [173-
Journal Pre-proof 174]. Although several recent reported findings suggested that the analytical centrifugation method may be an advanced method that can provide a much higher resolution of droplet size and distribution as compared to the laser diffraction because of the droplet fractionation during the measurement [173,175], the analytical centrifugation method is more often used for determining the gravitational separation profile of emulsions to date. The analytical centrifugation method is worth exploring into, not only because it provides higher resolution of
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the characteristics of Pickering emulsions, but the data obtained may hold potential to be used for
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developing mathematical models that correlate the relationship between centrifugation force,
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emulsion properties as well as its stabilization/destabilization behavior under different storage
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conditions.
Figure 17. (a) Schematic illustration of the analytical centrifugation, with (1) indicating the light source that (2) transmit NIR- light (3) through the samples (4) lying on the rotor base. The transmission data is then recorded over the entire sample length by the (5) detector. (b) The STEPT M graph produced from (5) over certain centrifuging time. Adapted with permission from ref. [174] (Copyright (2019) Wiley) (a) and ref. [173] (Copyright (2019) Elsevier) (b) correspondingly.
Journal Pre-proof 4.2 Gravitational separation Since an emulsion is a system consisting of the homogeneous mixture of two immiscible liquids, these immiscible liquids will also have different densities. Thus, upon emulsion formation, the dispersed and continuous phases will tend to separate into two la yers due to the gravitational forces. If the dispersed phase is of lower density, they will have the tendency to move upwards (creaming) and vice versa (moving downwards, sedimentation). In this section, the gravitational
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separation will be focused on the non-coalescence- induced creaming or sedimentation. Table 4
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summarized the advantage and drawback of several reported techniques.
Journal Pre-proof Table 4 Advantages and limitations of various techniques for monitoring the creaming stability of Pickering emulsions. Methodology Creaming
index
calculation [27,51,149]
Advantages
Limitations
Easy and most commonly used.
Long experimental period (at least one month).
Result obtained directly shows the creaming
Preliminary findings.
e
Creaming rate can be calculated using the
Analytical centrifugation [173-179]
o r p
Reason for creaming (either coalescence- or
properties of the Pickering emulsion.
recorded creaming index data.
l a
f o
r P
Can measure the intensity of the transmitted
n r u
light as function of time and position over the
gravity- induced) cannot be identified without the emulsion size measurement.
The obtained creaming results only represent information in bulk.
entire sample length simultaneously.
Jo
Data can be related to emulsion creaming rate. No extra sample preparation steps. Mathematical relationship [180-181]
Quick estimation of the creaming profile of a Pickering emulsion system.
Experimental evaluation may be required to verify the estimated creaming behavior.
Journal Pre-proof 4.2.1 Creaming index analysis The simplest and most direct way to monitor the creaming or sedimentation of a Pickering emulsion system is through the measurement of the height of stable Pickering emulsions over time to calculate the creaming index (CI). The CI results can also be further supported by calculating the gravitational separation rate of the Pickering emulsions based on the Stokes’ Law [27]: (16)
ro
of
gd 2 c d v 18c
-p
where v is the gravitational separation rate, g is the gravitational acceleration, d is the emulsion droplet diameter, 𝜌𝑐 and 𝜌𝑑 are the densities of continuous (water) and dispersed (oil) phases,
re
and 𝜂𝑐 is the viscosity of the continuous phase. An example of CI and v calculation is as shown
lP
in Figure 18 [149]. However, the application of Stokes’ Law has several weaknesses whereby
na
this theory is found to be applicable only for systems of mono-dispersed, non-interacting spherical droplets where predictions of gravitational separation rate in concentrated emulsions
ur
can become complicated due to influences other than hydrodynamic factors [150]. For these
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concentrated emulsions, it is recommended to use analytical centrifugation methods as described in the subsequent section.
lP
re
-p
ro
of
Journal Pre-proof
Figure 18. Photograph of Pickering emulsions stabilized by various content of MCNC at (a) Day
na
0 and (b) day 14. The (c) creaming index and (d) creaming rate of the respective Pickering
Jo
ur
emulsion. Adapted with permission from ref. [149].
4.2.2 Analytical centrifugation method Besides that, the utilization of light scattering for optical profiling is also one of the most commonly used methods to obtain gravitational separation information on the Pickering emulsion [176-177]. For the light scattering approach, the emulsion is placed in a transparent glass tube and a monochromatic beam of near infrared (NIR) light is passed through it. The transmitted/backscattered light is then recorded as a function of emulsion height accord ing to the setting in Figure 16. More recently, Detloff, Sobisch and Lerche [174] integrate the light scattering with the centrifugal method to perform advance characterization of gravitational
Journal Pre-proof separation for particles suspension. As mentioned earlier, the as-developed multisample analytical centrifuge machine (known as LUMiSizer® ) utilizes the STEPT M Technology and allows the measurement of the intensity of the transmitted light as function of time and position over the entire sample length simultaneously (see Figure 17a). The separation behavior of the particles can then be compared and analyzed in detail by tracing the variation of transmission at any location of the sample [174]. The LUMiSizer® analysis has been recently extended to the
of
Pickering emulsion system (Figure 17b) [173,175,178-179]. The utilization of the centrifugal
ro
analysis provided the separation velocity and the creaming/sedimentation profiles of the
-p
Pickering emulsions over the relative centrifugal force [173,175,178-179]. These results can be
re
correlated with the creaming index and the gravitational separation data obtained from Stokes’
na
4.2.3 Mathematical relationship
lP
Law, acting as the in-depth phase separation analysis for Pickering emulsions.
Another method to access gravitational separation in emulsions would be via correlation of
ur
governing parameters of the phenomenon by taking into advantage of the fact that this process is
Jo
governed by density difference between dispersed and continuous phases. A pioneering study of the separation process was conducted by Stokes for of sedimentation process whereby the settling velocity of a single hard sphere, U s in a motionless fluid is given by a universal expression given below [180]: Us
2 d set g
18c
(17)
Where ∆𝜌 = 𝜌𝑑 − 𝜌𝑐 is the density difference between the fluids, g is the gravitational acceleration, dset is the diameter of the settling particles and 𝜂𝑐 is the dynamic viscosity of the continuous phase.
Journal Pre-proof The works of Stokes was taken as an inspiration for a similar mathematical correlation that describes gravitational separation for particle-stabilized emulsions in the works of Yan and Masliyah [181]. The highlight of this method is where the adsorption of particles to the emulsion droplets is taken into account of where-by the dispersed phase droplets should have larger effective diameter due to the adsorbed particles and therefore, higher effective density. The combined effect of these changes in governing parameters of gravitational separation is taken
of
into account in the modified equation for gravitational separation for particle-stabilized
18c
1 Ko
2.93
where K 1 776Cs
-p
Us
2 d set c eff gK 2/3
ro
emulsions which is as follows:
(18)
re
Where ϕo is the oil volume fraction in the emulsion and Cs is the equilibrium concentration of
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particles at the dispersed phase surface, Voil is the volume of dispersed phase where-as ρ c and ρ eff
na
are the continuous phase density and effective density of emulsion. From this model, it is observed that the creaming velocity decreased with the particle concentration at the droplet
ur
surface. The proper understanding of the gravitational separation behavior of a Pickering
emulsions further.
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emulsion that can be obtained from this model is crucial to comprehend the physical stability of
5. Concluding remarks Despite the increase in the investigation and scientific papers of Pickering emulsions, the standard and advance characterization techniques for Pickering emulsion were not compiled and updated since the last decade. In this work, we tried to provide an overview of the latest and commonly- used techniques to evaluate the formation, properties and stability of Pickering
Journal Pre-proof emulsions. An evaluation was provided on the proper characterization techniques for measuring and controlling the formation and features of the Pickering emulsion. The elementary mechanism of these techniques was discussed, and the respective characterization results were illustrated. The thorough knowledge of these techniques is essentially important for any researchers venturing into the general field of colloidal and Pickering emulsion engineering since the structure of these emulsions are fundamentally different than the conventional surfactant-
of
stabilized emulsions. Besides that, characterization studies via modelling emulsion droplet size
ro
in terms of various operating parameters were also reviewed in this paper. A variety of models
-p
were introduced, through which the effects of various parameters to the properties of the
re
emulsion can be studied more effectively, complementing the results of the different existing experimental and analytical characterization techniques. Nevertheless, there is still room for
lP
further development of these models, which primarily includes the modelling of the distributio n
na
of Pickering emulsion droplet. Finally, the paper also reviewed the various methods to analyze the coalescence and gravitational separation of a Pickering emulsion and suggested several
ur
potential innovations on these techniques. This review can act as a template to researchers to
Jo
understand the standard characterization of Pickering emulsion as well as the advanced and useful approaches developed to study these emulsions.
Acknowledgements This work was supported by the School of Engineering of Monash University Malaysia. The authors also gratefully acknowledge Advanced Engineering Platform (AEP), Monash University Malaysia for the financial support.
Journal Pre-proof
Abbreviations Air
AFM
Atomic force microscopy
Across
Cross-sectional area of the centrifuge tube
CI
Creaming index
CLSM
Confocal laser scanning microscopy
Cryo-SEM
Cryogenic scanning electron microscopy
Cs
Equilibrium concentration of particles at the dispersed phase surface
d
Diameter of emulsion droplets (m)
d3,2
Surface-weighted mean diameter (m)
d3,2
Volume-weighted mean diameter (m)
ds
Diameter of smallest stable droplet (m)
dset
Diameter of the settling particles (m)
ΔE
Detachment energy (J)
FFT
Film trapping technique
g
Acceleration due to gravity (ms-2 )
gca
Centrifugal acceleration (ms-2 )
GTT
Gell trapping technique
h
Thickness of rod-like particle (m)
HLB
Hydrophilic-Lipophilic Balance
l
Length of rod-like particle (m)
LTFESEM
Low-temperature field emission scanning electron microscopy
mp
Mass of Pickering particles (kg)
Jo
ur
na
lP
re
-p
ro
of
a
Journal Pre-proof n
Number of emulsion droplets
NIR
Near infrared
o/w
Oil-in-water
PDMS
Poly(dimethylsiloxane)
cr Posm
Critical osmotic pressure (Pa) Width of rod-like particle (m)
s
Solid
SEM
Scanning electron microscopy
STEPT M
Space and Time resolved Extinction Profiles
re
Radius of emulsion droplets (m)
rp
Radius of Pickering particles (m)
Rsphere
Radius of the spherical particle (m)
Rdisc
Radius of the disc-like particle (m)
Us
Setting velocity of a single hard sphere (ms-1 )
Voil
Volume of dispersed phase (m3 )
Vreleased
Centrifuge induced released volume of the dispersed phase (m3 )
Vtotal
Total added volume of dispersed phase for emulsion (m3 )
w
Water
w/o
Water-in-oil
Zp
Zeta potential (mV)
γOW
Interfacial tension between the oil and water (Nm-1 )
α
Characteristic value of an emulsion system
η
Dynamic break-up potential
Jo
ur
na
lP
re
-p
ro
of
q
Journal Pre-proof Viscosity of the continuous phase (m2 s-1 )
ε
Power density (J kg-1 s-1 )
ϕd
Volume fraction of dispersed phase
ϕp
Volume fraction of Pickering particles
φa
Mass fraction of the surface area that is covered by particles
φp
Mass fraction of solid particles in drop of a specific radius
ρc
Density of continuous phase (kg m-3 )
ρeff
Effective density of emulsion (kg m-3 )
ρd
Density of dispersed phase (kg m-3 )
ρp
Density of Pickering particles (kg m-3 )
ρpd
Ratio between density of particle and the density of dispersed phase
θ
Three phase contact angle (°)
τ
Coverage of emulsion droplets
na
lP
re
-p
ro
of
ηc
ur
Notes
Jo
The authors declare no conflict of interest.
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Journal Pre-proof Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion
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Liang Ee Low, Sangeetaprivya P. Siva, Yong Kuen Ho, Eng Seng Chan, Beng Ti Tey*
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Graphical Abstract
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Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion
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Liang Ee Low, Sangeetaprivya P. Siva, Yong Kuen Ho, Eng Seng Chan, Beng Ti Tey*
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Highlights
Latest Pickering emulsion measurement (analytical) methods are reviewed.
Potential of innovative approaches for Pickering emulsion characterization are discussed
Mathematical correlations on Pickering emulsions analysis are described.
Challenges and opportunities for future research are addressed.
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