Emulsions stabilized by inorganic nanoclays and surfactants: Stability, viscosity, and implications for applications

Journal Pre-proofs Research paper Emulsions Stabilized by Inorganic Nanoclays and Surfactants: Stability, Vis‐ cosity, and Implications for Applications Bingqian Zheng, Bingjing Zheng, Amanda J. Carr, Xiaoxi Yu, D. Julian McClements, Surita R. Bhatia PII: DOI: Reference:

S0020-1693(19)31803-1 https://doi.org/10.1016/j.ica.2020.119566 ICA 119566

To appear in:

Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

21 November 2019 28 February 2020 2 March 2020

Please cite this article as: B. Zheng, B. Zheng, A.J. Carr, X. Yu, D. Julian McClements, S.R. Bhatia, Emulsions Stabilized by Inorganic Nanoclays and Surfactants: Stability, Viscosity, and Implications for Applications, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119566

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Emulsions Stabilized by Inorganic Nanoclays and Surfactants: Stability, Viscosity, and Implications for Applications

Bingqian Zheng1, Bingjing Zheng2, Amanda J. Carr1, Xiaoxi Yu1, D. Julian McClements2 and Surita R. Bhatia1,*

1 Department 2 Department

of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA

of Food Science, University of Massachusetts, Amherst, MA 01002, USA *Corresponding author: [email protected]

1

Abstract Pickering emulsions, or emulsions with solid particles at the interface, have attracted significant interest in Enhanced Oil Recovery (EOR) processes, cosmetics, and drug delivery systems due to their ability to resist coalescence. Here, a synthetic clay nanoparticle, laponite®, is utilized to create oil-in-water (o/w) emulsions, and the addition of small-molecule surfactants induces a more stable emulsion. In this study, the stability of laponite® Pickering emulsions with and without the surfactants (dodecyltrimethylammonium bromide (DTAB), Pluronic F68 (F68), and sodium dodecyl sulfate (SDS) is investigated using dynamic light scattering (DLS), ζ-potential, optical microscopy, and rheology. With laponite® and no added surfactants, the DLS and ζpotential results show formation of emulsion droplets with a diameter of 3 µm and a ζ-potential of -90 mV. With the addition of surfactants, both the droplet diameter and ζ-potential increase, suggesting adsorption of surfactants on the surface of laponite® particle. Optical microscopy suggests that the Pickering emulsion without surfactant undergoes flocculation, while the emulsion becomes stable to coalescence and creaming with addition of surfactants due to formation of a network structure. Regardless of the formation of network structure, the laponite®-F68 emulsion rheologically behaves as a Newtonian fluid, while the laponite®-SDS and laponite®-DTAB emulsions display shear thinning behavior. The difference in the rheological behavior can be attributed to the weak adsorption of F68 on laponite® and electrostatic interactions between laponite® and charged surfactants at oil-water interface.

Keywords: emulsion, laponite, colloid, droplet, interfacial rheology, rheology

1

Introduction Emulsions stabilized by surfactants and solid particles have been extensively studied for a variety of applications in EOR, drug delivery, cosmetics, and the food industry.1-3 Emulsions stabilized by solid particles, known as Pickering emulsions, have attracted a great of interest due to their stability to coalescence, low toxicity, and easily modified surface.2, 4 The principle behind Pickering emulsions is that solid particles which adsorb at an oil-water interface create a kinetic barrier to coalescence.5-7 The stabilization energy required to remove particles from the interface is directly proportional to the radius of solid particle, interfacial tension between oil and water, and wettability of the solid particle.3, 8, 9 The particles have to be partially wetted by both aqueous phase and oil phase to form stable emulsions. If particle is too hydrophilic or hydrophobic, the particles may remain in aqueous phase or oil phase. Therefore, tuning the particle size, interfacial tension, and wettability of the solid particle at the water and oil interface are crucial for the formulation of stable Pickering emulsions. Research has been conducted to study the stability of Pickering emulsions composed of several types of solid particles, including hydroxyapatite10, silica3, 11, carbon black12, clays13, 14, barium sulfate and calcium carbonate15. The stability of Pickering emulsions can be controlled by modifying the particle hydrophobicity. Binks and Lumsdon reported that hydrophobic silica particles could be used to tailor the wettability and adsorption of particles around emulsion droplets, thus enhancing the stability of Pickering emulsions.16 They also observed a phase inversion between oil-in-water (o/w) emulsion to water-in-oil (w/o) emulsion by both hydrophilic and hydrophobic silica particles at high volume of continuous phase. An enhancement in the stability of Pickering emulsion was found when mixing two oppositely charged particles.17,

18

The

electrostatic attraction between oppositely charged particles resulted in formation of

2

heteroaggregates and increasing of the viscosity of continuous phase. Kawaguchi discovered that the addition of hydroxypropyl methyl cellulose (HPMC) to silica Pickering emulsions enhanced the stability of emulsions due to the adsorption of HPMC on the silica particle, which increased the viscoelastic response of continuous phase.19, 20 Additionally, addition of surfactant is often found to promote the stability of Pickering emulsions. Hassander reported the enhanced stability of Pickering emulsions with adsorption of cetryltrimethylammonium bromide (CTAB) onto Ludox silica particle, which caused particle agglomeration and adsorbed at the oil-water interface.21 Addition of silica particles to emulsion stabilized by tetradecyl trimethylammonium bromide (TTAB) induced coalescence under shear.22 Binks and Desforges discovered that addition of nonionic surfactant to hydrophilic silica particle led to an increase in the hydrophobicity of silica particle and formation of stable emulsions.23 They also claimed that the most stable emulsion was formed by flocculated dispersions, which led to an increase in the viscosity and elasticity of the emulsions. Together, these previous studies suggest several general strategies to promote stability of Pickering emulsions: i) inducing flocculation between particles, ii) tailoring the hydrophobicity of particle, and iii) increasing the viscosity and viscoelastic properties of continuous phase. Clay particles have also been explored to stabilize Pickering emulsions. Laponite®, which is an inorganic synthetic clay nanoparticle, is desirable for this application and offers the possibility of tuning interparticle interactions through pH and ionic strength. Laponite® particles are in a family of phyllosilicates with empirical formula of Na+ 0.7[(Si8Mg5.5Li0.3)O20(OH)4]-0.7. Laponite® nanparticles are disk-shaped with approximate dimensions of 25 nm in diameter and 1 nm in thickness, comprising a sandwich structure with two layers of tetrahedral silica sheets and one Mg2+ octahedral sheet. The Mg2+ is randomly substituted by Li+, resulting in a net negative charged on the nanoparticle face which is balanced by Na+ counterions. Upon dispersion of the particles in

3

water, Na+ is released to form a negatively charged face. However, the hydroxyl on edge of the laponite® particle can yield either a positive charge or weak negative charge, depending on the pH of the environment.24 At pH < 11, the hydroxyl groups on the edges are protonated and possess a positive charge, while at pH > 11, the edge remains negatively charged.25 Binks discovered that o/w emulsions could be stabilized by laponite® particles when the particles flocculated at intermediate particle concentration and in the presence of salt.13 At low laponite® concentration (0.5 wt%) and high concentration (4.5 wt%), emulsions were unstable to creaming and coalescence due to the formation of discrete particles dispersions and gels. At intermediate concentration (13.5 wt%), the stability of emulsions increased with particle concentration, owing to the viscoelastic properties of the dispersion. Studies further reported the stability of particles could be tuned by the addition of surfactant to control the hydrophobicity of the particle surface and adsorption of surfactants onto the particle surface.9, 23, 26 In this study, laponite® Pickering o/w emulsions were formulated with three different surfactants, and the stability, structure, and rheological properties were examined. The surfactants mediate interactions between the laponite® particles via electrostatic attractions and hydrophobic attractions. We expected that the adsorption of surfactant onto the particle surface tailored the wettability of particle and modified the stability of Pickering emulsions. The charged surfactants (dodecyltrimethylammonium bromide (DTAB) and sodium dodecyl sulfate (SDS) and the nonionic surfactant Pluronic® F68 prior to formulation of emulsions were studied. Then, the Pickering emulsions were prepared by homogenizing mineral oil and laponite® dispersions with and without surfactants. The microstructure and stability of emulsions were examined using visualization, dynamic light scattering, 𝜁-potential, and confocal and optical microscopic techniques. The long-term stability of emulsion was characterized using bulk rheology. The

4

results suggested that the stability of Pickering emulsions were enhanced by forming threedimensional interparticle network. We also proposed possible mechanisms for the formation of stable laponite® Pickering emulsion with and without surfactants.

Materials and methods Materials Laponite® (Na+ 70.[(Si8Mg5.5Li0.3)O20(OH)4]−0.7) was obtained from Southern Clay Products (Gonzales, TX). Pluronic F68 and dodecyltrimethylammonium bromide (DTAB) were purchased from Sigma, and sodium dodecyl sulfate was purchased from Fisher Chemical. Mineral oil was purchased from Sigma-Aldrich. All chemicals were used as received. Emulsion preparation Laponite® dispersions were prepared by adding 2 wt% of laponite® into 200 mL of deionized distilled water (DI water) at neutral pH, and homogenizing using a T25 Ultra Turrax homogenizer at the rate of 11000 rpm for 5 min to assure that laponite® was fully dispersed. This concentration was selected based on literature studies of laponite dispersions performed at similar concentrations. The dispersions were then stirred on a stirring plate for 20 min. Aqueous solutions containing 2 wt% of surfactant were prepared and added to an equal amount of the laponite® dispersion. The emulsion was prepared by homogenizing 90 vol% of the mixture of laponite® and surfactant and 10 vol% mineral oil for 5 min at 11000 rmp using the T25 Ultra Turrax homogenizer. All emulsions were capped and stored at room temperature. Characterization of laponite® and surfactants in aqueous solution Measurements of the 𝜁-potential of aqueous solutions were carried out using Nanobrook Omni particle size and 𝜁-potential analyzer (Brookhaven Instruments Co., Holtsville, NY). The 𝜁potential was determined by measuring the electrophoretic mobility of particles under an applied

5

electric field. The 𝜁-potential was estimated as the average of 5 repeat measurements. Measurements of stability After formation of emulsions, some creaming was observed over time, and samples separated into a milky phase and an aqueous phase. As described in the Results and Discussion, optical microscopy verified that the milky phase consisted of an oil-in-water (o/w) emulsion (Figure 1). The total height (𝐻tot) of the formulation and the height of o/w phase (𝐻o/w) over a course of 28 days were measured using a ruler. The stability of emulsions was determined by the fraction of the aqueous phase (𝑓aq) which was recorded as: 𝑓𝑎𝑞 = 1 ―

𝐻𝑜/𝑤 𝐻𝑡𝑜𝑡

Size determination of emulsions A Malvern Mastersizer 2000 was employed to measure the average diameter of emulsions. Samples were extracted from the o/w phase, diluted with distilled water to prevent multiple scattering, and immediately measured. All samples were measured three times, and average particle diameter was reported. 𝜁-potential measurements of emulsions 𝜁-potential measurements were conducted on the o/w emulsion by Malvern Zetamaster, which determined the 𝜁-potential of emulsion droplets by measuring the direction and velocity of emulsion in an applied electric field. The refractive index of mineral oil, 1.47, was applied to calculate the 𝜁-potential of emulsion droplets. The average 𝜁 -potential was calculated from three measurements. Microstructure analysis of emulsions by optical microscopy and very small angle neutron scattering (vSANS) The microstructure of samples was characterized using optical microscopy (Nikon D6

Eclipse C1 80i, Nikon, Melville, NY). A drop of the o/w emulsion phase was placed on a microscope slide. The microstructure of emulsion was determined using optical microscopy. The images were acquired using a CCD camera connected to a Digital Image Process system. All images were captured with a 10× eyepiece and a 60× objective lens. Samples for vSANS measurements were prepared as described above, except using D2O instead of DI water, and measurements were conducted on the o/w emulsion phase. vSANS measurements were conducted on the NG-0 beamline at the National Institution of Standard and technology (NIST) center for neutron Research, Gaithersburg, MD. The data reduction was performed using Igor macros developed at NIST.27 The reduced data was analyzed and fitted by the core-shell model using SasView.28, 29 The diameters of oil droplet and thickness of a interfacial film were reported. Bulk rheology measurements An AR-G2 stress-controlled rheometer was used to study the rheological properties of the o/w emulsion phase. Measurements were performed using a cone-and-plate geometry with a diameter of 40 mm and 2o of cone at 25 oC. The sample was placed on the plate after mixing and waited for 300 seconds before measurements. The steady shear measurements were carried out in the shear rate ranging from 1 s-1 to 100 s-1. Although the measurements were performed on a stresscontrolled rheometer, results are plotted as a function of shear rate, as this is the more conventional way of presenting rheological data. Three repeats were typically performed, and typical relative variability within the repeats was < 10%. The shear viscosity (𝜂) and shear stress were reported, and shear stress versus shear rate was fitted by Herschel-Bulkley model using OriginPro. Solvent traps were used to prevent solvent evaporation.

Results and Discussion

7

Interactions between laponite® and surfactants in aqueous dispersions Prior to addition of mineral oil and formation of emulsions, we probed the interactions between laponite® and surfactants in aqueous dispersions. In this study, 𝜁-potential measurements of laponite®, surfactants, and mixtures of laponite® and surfactant were used to determine the interactions between laponite® and surfactants (shown in table 1). Previous studies reported that laponite® particles carry an overall negative charge at neutral pH. The 𝜁 -potential of laponite® particles in the dispersions was -69.10 mV. The 𝜁 -potential of SDS, F68, and DTAB were found to be -24.51 mV, -2.55 mV, and 47.67 mV, respectively. A change of 𝜁-potential of the dispersions was observed after adding surfactant to laponite® dispersions. In the presence of SDS, the 𝜁 potential of the dispersion becomes more negative, -85.70 mV. The increase of the magnitude of 𝜁potential can be attributed to repulsions between negatively-charged laponite® particles and SDS. The 𝜁 - potential of laponite® with F68 was -26.71 mV, suggesting that the adsorption of F68 onto the laponite® particle screens the charged surface of laponite® particle. F68 is a short triblock copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), and previous studies of other PEO-PPO-PEO copolymers have shown adsorption of the copolymers onto laponite® surfaces via H-bonding interaction between ethylene oxide groups of triblock copolymer and hydroxyl groups on laponite®. The layer of adsorbed polymer chains reduces mobility of charged counterions near the particle surface, reducing the 𝜁 -potential of the particles.30 By contrast, addition of DTAB increased the 𝜁 -potential of the laponite® particle and changed the sign to 45.24 mV. One possible explanation of this dramatic increase is that the addition of DTAB neutralized the negatively charged particle via electrostatic attractions, and a significant amount of free DTAB remained in the sample. The interactions between laponite® particles and surfactants can directly affect the microstructure and stability of emulsions. The 𝜁 -

8

potential results suggest that the DTAB and F68 were adsorbed onto the particle surface. Therefore, we expected the structure and stability of Pickering emulsions would be altered with interactions between surfactants and laponite®.

-potential (mV)

50

0

-50

-100 Laponite

SDS

F68

DTAB

L/SDS

L/F68

L/DTAB

Figure 1. 𝜁 -potential of freshly prepared laponite® and two series of samples:SDS, F68, DTAB; and laponite®/SDS, laponite®/F68, and laponite®/DTAB, at 1 wt%.

Stability and microstructure of Pickering emulsions by laponite® particle and surfactants The emulsions were prepared by mechanically homogenizing mineral oil with dispersions of laponite® and laponite® /surfactant. The initial concentrations of laponite® and surfactants were kept constant at 1 wt% in the aqueous phase, where wt% = (mass of each component)/(mass of aqueous phase); and mineral oil concentration was 10 vol%. These concentrations were chosen in part based on previous studies of related systems.1 The formation of laponite® Pickering emulsions with and without surfactant was visually examined. On day 0 immediately after

9

preparation, all samples were visually cloudy. By day 1, creaming (e.g. formation of a milky layer at the top of the samples) was observed in all samples. Optical microscopy (Figure 2) was used to verify that this milky phase consisted of an o/w emulsion and that the observed phase separation did not represent a bulk separation of oil and water. Although stability to creaming over long periods of time is preferable for applications requiring long shelf life, there are situations where shelf-stability is not the primary design criteria where rheological properties of the creamed phase will be important.

Figure 2. Optical microscopy images of samples without and with surfactant before and after storage under incubation at room temperature for 28 days. The day 28 images are taken off the o/w emulsion phase after creaming. The microstructure image was used a length of 10 μm for scale bars.

The lower aqueous phases of laponite® Pickering emulsion and laponite®/F68 emulsion were turbid, while the lower aqueous phases of laponite® /DTAB and laponite®/SDS were clear. After 28 days of storage, only the lower phase of laponite® was still turbid. The lower aqueous phases for laponite® and laponite®/F68 were turbid owing to the presence of excess laponite® particles, which we attribute to the weak adsorption of particles at interface. Upon aging, due to

10

the attraction between F68 and laponite, laponite particles slowly adsorb onto the interface, resulting in a clear aqueous phase. The stability of all emulsions to creaming was assessed by monitoring the movement of emulsion-water interface with time. The fraction of water released (faq) over 28 days reduced with addition of surfactant (Figure 3). The faq of laponite® Pickering emulsion and laponite®/F68 emulsion increased rapidly overnight and remained constant. Addition of DTAB and SDS reduced faq and slowed down the creaming process, laponite®/DTAB and laponite®/SDS required 2 days to complete the creaming process. The increase of faq over time means that water was expelled from the o/w emulsion phase.

1.0

0.8

faq

0.6

0.4

laponite laponite/SDS laponite/F68 lapnite/DTAB

0.2

0.0 0

5

10

15

20

25

time (day)

Figure 3. The fraction of aqueous phase after 28 days of storage.

11

The average diameter and 𝜁 -potential of emulsion droplets in the o/w emulsion phase were measured over a course of 28 days using dynamic light scattering in Figure 4. The average diameter and 𝜁 -potential for emulsion droplets formed by laponite® were about 3 µm and -88 mV. With addition of surfactants, both average diameter and 𝜁-potential of droplets increased. The average diameters of laponite®/F68 emulsion droplets and laponite®/SDS emulsion were approximately 10 µm and remained stable over the course of 28 days. The 𝜁 -potential of laponite®/F68 and laponite®/SDS emulsion droplets were approximately -70 mV and -80 mV, respectively. Within the uncertainty of the measurement, the value for laponite®/SDS emulsions was not appreciably different than laponite® emulsions. We speculate that highly negatively charged droplets in the laponite®/F68 emulsion and laponite®/SDS emulsion promoted the stability of droplets to coalescence owing to electrostatic repulsions. However, the droplet diameter of laponite®/DTAB emulsion increased to 50 µm, and we observed a change in the sign of 𝜁 -potential after 20 days of preparation. We believe this is consistent with the droplet flocculation observed in this sample. Both DLS and 𝜁 -potential measurements illustrated addition of surfactants altered the characteristics of the emulsions. Therefore, optical microscopy was employed to monitor the microstructure of laponite® Pickering emulsion and laponite®/surfactants emulsion.

12

a.

70

Laponite Laponite/SDS Laponite/F68 Laponite/DTAB

Size (m)

60

50

10

0 0

5

10

15

20

25

30

time (day)

b.

100

Laponite Laponite/SDS Laponite/F68 Laponite/DTAB

-potential (mV)

50

0

-50

-100 0

5

10

15

20

25

30

time (day)

Figure 4. The average diameter (a) and 𝜁-potential (b) of laponite® Pickering emulsion (∎), laponite®/SDS emulsion (●), laponite®/F68 emulsion (▲), and laponite®/DTAB emulsion (▼) over a course of 28 days. 13

Optical microscopy images revealed that the microstructure of emulsion depended on the surfactant added. Figure 2 shows the microscopy images of laponite® Pickering emulsion with and without surfactant at day 0 and day 28. Day 28 images are taken of the upper milky phase, the o/w emulsion phase. At day 0, laponite® Pickering emulsions formed small and discrete oil droplets. The addition of F68 caused flocculation of droplets with diameter greater than laponite® Pickering emulsions. On the other hand, in the presence of SDS and DTAB, emulsions composed of extensively flocculated droplets which were closely packed and formed an interparticle connected network structure. After 28 days, the structure remained unchanged, excepted for the laponite®/F68 emulsion which formed a bridging structure between flocculated oil droplets. The microscopy results suggest that the microstructure of laponite and laponite/surfactants emulsions remained stable after 28 days of preparation. The reduced vSANS data (Figure 5) was utilized to elucidate the interfacial film properties of emulsions containing laponite® and surfactant. The scattering for laponite® o/w emulsion had a low scattering intensity and high signal-to-noise, which caused difficulties in fitting the data. However, the laponite®/surfactant o/w emulsions showed stronger scattering and could be fit. We deduced the diameter of droplets and the thickness of the interfacial film by fitting the reduced vSANS data with a polydisperse core-shell model (Table 1). The diameters of laponite®/surfactant emulsions coincide with DLS and microscopy results, which indicates that the model is a promising model for characterizing Pickering emulsions. The shell thickness of the emulsion droplets suggests formation of multilayers surrounding droplets. Laponite® is a disk-shaped particle with a diameter of 30 nm and a thickness of 1 nm. The shell thicknesses of laponite®/F68 emulsion and laponite®/DTAB emulsion are 3.18 nm and 5.29 nm, respectively. The large shell thickness of the laponite®/SDS emulsion droplets, 27 nm, implies formation of a film of loosely packed SDS-laponite® aggregates, which 14

could result from the electrostatic interactions between residual positive charges on laponite® edges and negatively-charged SDS molecules. Note that o/w emulsions of SDS, DTAB, and F68 typically have much larger sizes than those seen there, although the size is dependent upon the preparation method. For example, mean diameters of about 120 nm have been reported for SDS and DTAB,31 and a mean diameter of 281 nm for F68 o/w emulsion droplets.32

100000

laponite laponite/SDS laponite/F68 laponite/DTAB

I(Q) (cm-1)

1000

10

0.1 0.001

0.01

0.1

-1

Q (A )

Figure 5. SANS data (filled symbols) and fitting results (solid lines) for o/w emulsions containing laponite®, laponite®/SDS, laponite®/F68, laponite®/DTAB after 2 days of storage. Fits utilize a core-shell model where the polydispersity of the core size and shell thickness were both set to 0.2. Table 1: The diameter of oil droplets and the thickness of the interfacial film by fitting reduced SANS with a polydisperse core-shell model. Polydispersity of diameter and thickness were set to 0.2.

laponite®/SDS Diameter (μm) Thickness (nm)

laponite®/F68

laponite®/DTAB

9.01

10.00

50.00

27.72 ± 0.98

3.18 ± 0.38

5.29 ± 0.06

15

Laponite® and laponite®/surfactant were used as stabilizers to form stable o/w emulsions. Laponite® Pickering emulsion formed small size of droplets which were unstable and underwent oil phase separation over time. Laponite®/surfactant aggregates adsorbed at the oil and water interface and formed interfacial films to avoid coalescence, as suggested by the size, 𝜁-potential, and SANS measurements of droplets. The creaming behaviors reflected the stability of emulsion to aggregation. Optical microscopy images displayed larger volume fraction of droplets with addition of surfactants. The droplets of laponite®/SDS and laponite®/DTAB tend to form aggregates, while the droplets of laponite®/F68 flocculate and form clusters in the continuous phase. Owing to their highly charged surfactant, we visually observed that these emulsions remained stable over a few months. The aggregation of droplets in laponite®/SDS emulsion and laponite®/DTAB emulsion may lead to formation of interparticle network structure, resulting to higher viscosity which could further enhance creaming process. Rheological properties of Pickering emulsions Bulk rheology of emulsions was also investigated using steady shear measurements to study long-term physical stability.33 Bulk rheology measurements were performed on the o/w emulsion phase. The viscosity of emulsions increased with the addition of SDS and DTAB, while the viscosity of laponite®/F68 emulsions decreased compared to laponite® Pickering emulsions. Two distinct rheological behaviors were also observed (Figure 6). Figure 6a shows that viscosity of laponite®/DTAB emulsion and laponite®/SDS emulsion decreased with an increase in shear rate, displaying characteristic shear-thinning behavior; whereas viscosity of emulsions of laponite® and laponite®/F68 was insensitive to the shear rate, exhibiting Newtonian fluid behavior. The shearthinning behavior of laponite®/DTAB emulsion and laponite®/SDS emulsion may be due to aggregation of oil droplets and the rupture of network structure during shearing.34 After 3 months, the flow behavior for laponite®/surfactants emulsions doesn’t change significantly with an slightly 16

increase of viscosity (Figure 6b). Interestingly, the viscosity of laponite® Pickering emulsion increased by 2 orders of magnitude after 3 months, and the flow behavior transitions from Newtonian fluid behavior to shear-thinning behavior. This increase of viscosity in the laponite® emulsion over time has also been observed in aqueous laponite® dispersions35 and may correspond to formation of gel-like structure due to the long-range repulsion between clusters. a. 10

Laponite Laponite/SDS Laponite/F68 Laponite/DTAB

Viscosity (Pa.s)

1

0.1

0.01

0.001

1E-4 1

10

100

Shear Rate (1/s)

b. 10

Laponite Laponite/SDS Laponite/F68 Laponite/DTAB

Viscosity (Pa.s)

1

0.1

0.01

0.001

1E-4 1

10

100

Shear rate (1/s)

Figure 6. Flow curves of laponite® Pickering emulsion ( ∎ ), laponite®/SDS emulsion (●), laponite®/ F68 emulsion (▲), laponite®/DTAB emulsion (▼) after (a) 28 days and (b) 3 months. 17

Shear stress (𝜏) as a function of shear rate (𝛾) in the range of 5 s-1 to 100 s-1 was plotted (Figure 7) and fitted by Herschel- Bulkley model to study the stability of emulsions: 𝜏 = 𝜏𝐻𝐵 + 𝐾 ∗ 𝛾𝑝 where 𝜏HB, K, and p are fitting parameters. The yield stress is given by 𝜏HB . For 𝜏 < 𝜏HB, the system behaves elastically, whereas for 𝜏 > 𝜏HB the system flows and behaves as a fluid. The parameter c is a flow coefficient in Pa.s, and p is a flow index. If p < 1, the system exhibits shearthinning behavior. If p >1, the system has shear-thickening behavior. If p = 1, the system shows Bingham behavior.36-38 The fitting parameters are listed in Table 2. The shear stress versus shear rate is in a good agreement with flow curves. Emulsions of laponite®/SDS and laponite®/DTAB exhibited shear thinning behaviors with 𝑝 < 1. The addition of SDS and DTAB resulted in the increase of the yield stresses of emulsions to 0.386 Pa and 0.115 Pa, indicating laponite®/SDS emulsions have more elastic character. In comparison to laponite®/SDS emulsions and laponite®/DTAB emulsions, the laponite® Pickering emulsions and laponite®/F68 emulsions behaved rheologically as Newtonian fluids with 𝑝 ≈ 1 at 28 days with 𝜏 𝐻𝐵 ≈ 0. We ascribe the distinct rheological behaviors of Laponite®/F68 in bulk rheology to formation of oil droplets in the continuous phase, resulting to the formation of stable creamed and aqueous interface. However, the oil droplets are too dilute to form a network, thus exhibiting a Newtonian fluid behavior. The rheological results agreed with optical microscopy observation, where the emulsions of laponite®/SDS and laponite®/DTAB formed a closely-packed structure, and emulsions of laponite® and laponite®/F68 formed discrete oil droplets in the continuous phase. The 𝜏HB of laponite® Pickering emulsions, laponite®/SDS emulsions, and laponite®/DTAB emulsion increased after 3 months, which could be attributed to the aging behavior of laponite® particles in the continuous phase, and which may be impacted by the presence of counterions from the SDS and DTAB surfactants. 18

a. 10

Stress (Pa)

1

0.1

0.01

Laponite Laponite/SDS Laponite/F68 Laponite/DTAB

0.001 10

100

Shear Rate (1/s)

b. 10

Stress (Pa)

1

0.1

0.01

laponite laponite/SDS laponite/F68 laponite/DTAB

0.001 10

100

Shear rate (1/s)

Figure 7. Shear stress as a function of shear rate of laponite® Pickering emulsion ( ∎ ), laponite®/SDS emulsion (●), laponite®/ F68 emulsion (▲), laponite®/DTAB emulsion (▼) after (a) 28 days and (b) 3 months. Lines were fitting by the Herschel-Bulkley model. 19

Table 2. the Herschel-Bulkley parameters for emulsions of laponite® and laponite®/surfactant after storage of 28 days and 3 months obtaining from figure 7. Laponite® 𝝉𝑯𝑩 (Pa) 28 Days

L/F68

0.386 ± 0.026

L/DTAB

0.007± 0.002 0.115± 0.050

K (Pa.s) 0.004± 0.0001

0.033± 0.007

0.002± 0.001

0.022± 0.016

0.942± 0.008

0.785± 0.042

0.995± 0.096

0.711± 0.145

𝒑 3 months

0± 0.001

L/SDS

𝝉𝑯𝑩 (Pa)

1.460± 0.086

0.457± 0.029

0± 0.004

0.131± 0.007

K (Pa.s)

0.136± 0.036

0.054± 0.007

0.002± 0.001

0.014± 0.002

𝒑

0.518± 0.044

0.685± 0.025

0.959± 0.053

0.778± 0.020

Mechanism for the formation of laponite® Pickering emulsions Based on the above results, we propose a possible mechanism for the formation of laponite® o/w Pickering emulsions with and without surfactant (Figure 8). To form stable o/w Pickering emulsions, the particles must adsorb onto the surface of oil droplets, forming a dense film at the interface that resists coalescence, and the stabilization energy must typically be much larger than thermal energy, kBT.9 In this study, the microscopic images of all emulsions showed the formation of oil droplets in the presence of laponite® and laponite®/surfactant. However, our results showed laponite Pickering emulsions suffered from destabilization and formed oil phase separation over time. The addition of 1 wt% surfactants significantly improved the stability of laponite® Pickering emulsions evidenced by SANS and rheological studies. At 1 wt%, both SDS and DTAB formed ellipsoidal micelles with minor and major radii a = 1.20 nm and b = 2.03 nm for systems with SDS, and a = 1.24 nm and b = 2.16 nm for systems with DTAB.39 Despite their similar micellar and emulsion

structure,

laponite®/SDS

and

laponite®/DTAB 20

displayed

distinct

interfacial

characteristics and rheology. Laponite®/SDS emulsions not only formed a thick interface with thickness of 27 nm, but also had a higher viscosity. We expect that laponite® and SDS pack around oil droplets, with an excess of SDS forming micelles in continuous phase, leading to the formation of a network structure and an increase in the viscosity of continuous phase, which subsequently enhanced the stability of laponite® Pickering emulsions against coalescence and shearing (𝜏HB=0.386 Pa). Alternatively, the increase of viscosity can be also due to the screening effects of free SDS. The presence of SDS screens the electrostatic repulsion between laponite particles and induce formation of laponite aggregates. The same observation was reported for laponite dispersions in the presence of salt.40 By contrast, the formation of stable laponite®/DTAB emulsions i s micelles and highly negatively charged laponite® as evidenced by 𝜁 -potential, SANS, and rheology measurements. Laponite®/DTAB emulsions possess positively charged droplets with interfacial thickness of 5 nm (the diameter of DTAB micelle is approximate 4 nm and the thickness of laponite® is 1 nm), and yield stress of continuous phase (𝜏HB = 0.115 𝑃𝑎). Similar to laponite®/DTAB, the stability of laponite®/F68 emulsion is enhanced by adsorbing surfactant onto the surface of laponite® particle. The diameter of F68 spherical micelles has been reported to be 9 nm.41 One possible explanation is that F68 copolymer chain is adsorbing onto the laponite® surface, rather than F68 micelles. Due to the loosely attached F68 onto laponite®, the laponite®/F68 emulsion behave as a Newtonian fluid.

21

a.

b.

Figure 8. Schematic diagram of formation mechanism of (a) laponite® Pickering emulsion, and (a) laponite®/SDS, laponite®/F68, and laponite®/DTAB from top to bottom.

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Conclusions In this study, we investigated the effects of surfactants on the stability of laponite® o/w Pickering emulsions. The adsorption of surfactants onto the laponite® particle surface via electrostatic interactions and hydrophobicity was first studied using 𝜁 -potential measurements in water prior to the formation of emulsion. While formulation of o/w emulsions were obtained by laponite® alone, stability to coalescence and creaming was enhanced with addition of surfactants. The enhancement of emulsion stability in the presence of SDS corresponded to the formation of a dense network structures and increase of viscosity in the continuous phase, while enhanced stability of the laponite®/DTAB and laponite®/F68 emulsion could be due to formation of more compacted interfaces around the oil droplets. Rheology measurements were performed to study the long-term physical stability of emulsions. The emulsions of laponite®/SDS and laponite®/DTAB exhibited a yield stress and shear-thinning, while laponite® Pickering emulsion and laponite®/F68 emulsion behaved rheologically as Newtonian fluids over the course of 28 days.

Acknowledgment The authors gratefully acknowledge financial support from NSF CBET1335787, ACS PRF grant 55729-ND9, Department of Education Award P200A160163 fellowships for AJC and BZ, NSF CHE1609494 for XY, and NIH-NIGMS GM097971 for XY. The sponsors had no role in the study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the article for publication. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work. Access to the vSANS instrument was provided by the Center for High Resolution Neutron Scattering, a partnership between the 23

National Institute of Standards and Technology and the National Science Foundation under Agreement No. DMR-1508249.We thank Grethe Jensen at NIST for her help with SANS measurements.

24

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Bingqian Zheng: Conceptualization, Investigation, Formal analysis, Writing – original draft, Writing – review and editing. Bingjing Zheng: Investigation, Methodology. Amanda J. Carr: Investigation. Xiaoxi Yu: Investigation. D. Julian McClements: Supervision, Resources, Writing – review and editing. Surita R. Bhatia: Conceptualization, Funding acquisition, Supervision, Writing – review and editing. Highlights 

Pickering emulsions (emulsions with solid particles at the interface) were created with a synthetic clay nanoparticle and a variety of surfactants, F68, DTAB, and SDS.



Pickering emulsions without surfactant undergo flocculation, while the emulsion becomes more stable to coalescence and creaming with addition of surfactants due to formation of a network of droplets.



The laponite®-F68 emulsion rheologically behaves as a Newtonian fluid, while the laponite®-SDS and laponite®-DTAB emulsions display shear thinning behavior.

28