Contact angle of micro- and nanoparticles at fluid interfaces

Current Opinion in Colloid & Interface Science 19 (2014) 355–367

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Current Opinion in Colloid & Interface Science journal homepage: www.elsevier.com/locate/cocis

Contact angle of micro- and nanoparticles at fluid interfaces Armando Maestro a, Eduardo Guzmán b,⁎, Francisco Ortega c, Ramón G. Rubio c,⁎ a b c

Department of Physics-Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, CB3 0HE- Cambridge, United Kingdom Istituto per l’Energetica e le Interfasi-U.O.S. Genova, Consiglio Nazionale delle Ricerche, Via De Marini 6, 16149-Genova, Italy Departamento de Química Física I-Facultad de Ciencias Química, Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040-Madrid, Spain

a r t i c l e

i n f o

Article history: Received 30 January 2014 Received in revised form 21 April 2014 Accepted 23 April 2014 Available online 2 May 2014 Keywords: Particles Fluid interfaces Contact angle Wettability Particle laden films

a b s t r a c t The contact angle of particles attached to fluid interfaces plays a key role in many scientific and technological aspects of particle-laden layers. In spite of the recognized importance, the laws that govern this property are still poorly understood. The main problem associated with the study of this property is that multiple variables are involved in the wetting process of particles by fluid interfaces. Such variables are associated with the chemical nature of both the particles and the fluid phases, and with the particle’s size. Understanding of the different aspects controlling the contact angle of particles is a physico-chemical challenge, and is very important because of the many technological aspects in which particle laden interfaces are involved. This review discusses the current status and the aspects to be dealt with in the near future in the study of the contact angle of particles attached to fluid interfaces. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Particles at fluid interfaces are ubiquitous in nature and in technological processes [1••,2•,3•], from food science to biomedicine, and from materials science to renewable energies [1••,4,5]. Furthermore, confinement of particles at fluid interfaces has lead to an important research focused on fundamental problems of soft matter, aiming to develop new products and processes based on a better knowledge of the interaction of particles at interfaces. Some specific questions still not well understood are the particle adsorption process, and the structure– properties relationship of the particle-laden layers [6,7]. The energies associated with the attachment of particles to fluid interfaces depend on the chemical nature, size, roughness and wettability of the particles. The latter plays a central role and is reflected by the three-phase contact angle, θ. However, θ is neglected in many studies of particle–laden interfaces. To shed light on this description, Fig. 1a shows the geometrical definition of the contact angle of a particle attached to a fluid interface. The contact angle of particles plays a role analogous to the hydrophilic–lipophilic balance (HLB) of common surfactants [3•,8], thus the partition of the particles between the two fluids phase is correlated to θ. There are two limit cases for θ = 0–10° and θ = 170–180° where the particles remain preferentially in one of the two fluid phases. For a water/oil interface particles are considered hydrophilic when they remain mainly immersed in the water (θ b 90°), otherwise they are ⁎ Corresponding authors. E-mail addresses: [email protected] (E. Guzmán), [email protected] (R.G. Rubio).

http://dx.doi.org/10.1016/j.cocis.2014.04.008 1359-0294/© 2014 Elsevier Ltd. All rights reserved.

hydrophobic (θ N 90°). Fig. 1b qualitatively shows the effect of θ on the relative position of the particles at the interfacial plane. The position of the particles at the interfacial plane can led to an asymmetric distribution of the interactions between particles [9] that modifies their performance in stabilizing the interfaces. For instance, Zhang et al. recently pointed out the influence of the contact angle on the mechanical properties of interfaces containing silica nanoparticles [10], which determines their performance as foam stabilizers [11]. Furthermore, θ plays a key role in determining the friction coefficients in surface microrheology experiments [12••,13••]. Despite all the above, most of papers published in the field of particle laden interfaces have assumed a constant contact angle value, frequently 90°. The absence of a reliable evaluation of θ has been mainly due to the difficulties associated with monitoring a single particle adsorbed at the fluid interface. The first systematic studies trying to measure the contact angle of microparticles were the ones by Paunov, who introduced the so-called Gel Trapping Technique (GTT) [14••]. Later, this method was extended to nanoparticles [15•]. In the following, we will review the recent developments in the study of the contact angles of particles attached to fluid interfaces, emphasizing the physicochemical background underneath this property. The main scope of this comprehensive manuscript is then to emphasize the importance of this poorly understood property. 2. Contact angle and energies of particles trapped at fluid interfaces Wettability and adhesion control the contact angle of particles, and therefore their attachment and their flotation height, h (see Fig. 1.a). Since θ and h are related, and the perimeter of the three-phase contact

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Fig. 1. (a) Sketch of the contact angle for a particle at a fluid interface. γP f 1 is the interfacial tension particle-bottom fluid, γPf2 the interfacial tension particle-upper phase and γ P f 1 f 2 the interfacial tension fluid-fluid. (b) Sketch of the position of a particle, with increasingly hydrophobicity at the interface. (c) Attachment energy of the particles to the interface, ΔEp, dependence on θ for particles of R = 10 nm (left) and R = 1 μm (right) at 25 °C (γ = 50 mN/m).

line depends on h, θ strongly depends on the line tension τ [16••]. Although the effect of line tension is negligible in the case of microparticles, it can be significant for nanoparticles in which the line tension may lead to their detachment from fluid interfaces [17••]. The most frequently used definition of θ for microparticles attached to fluid interface – neglecting τ and any adsorption of components from the fluid phases to the interface – is given by Young’s equation [18,19]: 0 ¼ γ P f 2 −γ P f 1 −γ f 1 f 2 cos θ

ð1Þ

where γP f 2 is the interfacial tension particle-upper phase, γP f 1 is the interfacial tension particle-bottom phase, and γ f 1 f 2 is the interfacial tension between the fluid phases. WhenγP f 2 N γ P f 1 particles are hydrophilic (0 ≤ θ° b 90), whereas they are hydrophobic (90 b θ° ≤ 180) for γ P f 2 b γ P f 1 . The main difficulty for obtaining reliable values θ comes from the measurement of the fluid–solid interfacial tension. Despite those tensions might be measured for flat surfaces using techniques based on drop-shape analysis [20], they cannot be used for colloidal particles with sizes ranging from the μm to nm scale. This fact has triggered the design of new experimental methods [14••,21••,22••,23••]. The energy of attachment of the particles to the fluid interface is a balance between the energy of the particle at the interface and in the bulk suspension. For spherical particles, small enough for the gravity effects being negligible, the attachment energy, ΔEp, is given by 2

2

ΔEp ¼ −πR γ f 1 f 2 ð1  cos θÞ

ð2Þ

where R is the radius of the particle. The ± signs correspond to the cases where the particle center is above (+) or below (−) the interfacial plane. The particles are attached to the interface if ΔEp N N kBT [24••], kB being the Boltzmann constant and T the temperature. Eq. (2) points out that R and θ control the reversibility or irreversibility of the particle attachment to the interface. In general, microparticles are usually irreversibly attached to the interface, whereas for nanoparticles the interface/bulk exchange can be tuned by changing θ. Fig. 1c shows the effects of R and θ on ΔEp emphasizing the huge differences between nanoparticles and microparticles. The reversibility or irreversibility of the attachment of particles to fluid interfaces plays a major role on the

equilibrium structure of particles assemblies at interfaces because they affect the particle interactions (e.g. capillary attractions) [25••]. Eq. (1) frequently fails for nanoparticle systems because it does not include contributions such as roughness or line tension which are important at the nanoscale [26]. Including the effect of τ leads to a modified Young’s equation
 γ P f 2 −γP f 1 cos θ

−γ f 1 f 2 ¼ −

τ R sin θ

ð3Þ

from which the trapping energy of a particle at the interface 2

2

2

ΔEp ¼ γ f 1 f 2 cos θ∞ 2πR ð1− cos θÞ þ 2τπR sin θ−πR γ f 1 f 2 cos θ

ð4Þ

where θ∞ is the macroscopic contact angle defined by the Young’s equation (Eq. (1)). If the particles at the interface are in mechanical equilibrium (dΔEp/dθ) = 0, therefore one can write " cos θ ¼ cos θ∞ 1−

τ Rγ f 1 f 2

#−1 ð5Þ

that shows the dependence of θ on τ. For θ ≈ 90° the contact angles obtained by Eqs. (1) and (5) are quite similar because the line tension is almost perpendicular to the interface, thus the tangential component of τ is very small and does not modify h. Contact angles different from 90° lead to larger tangential component of τ and of the curvature (1/R sin θ). The maximum contribution of the line is found near 0° and 180°. Zeng et al. had shown that the line tension plays a relevant role for practical purposes when the contact angle takes values lower than 60° or higher than 120° [27]. 3. Effect of the contact angle on the interaction between colloidal particles at fluid interfaces Long range dipolar interactions are very important in particle laden interfaces [24••]. A dipole normal to the surface arises from the asymmetric distribution of counterions between the parts of the particle immersed in the two phases. It is still today a matter of debate whether

A. Maestro et al. / Current Opinion in Colloid & Interface Science 19 (2014) 355–367

some charges remain on the particle cap immersed in the low dielectric constant phase. Although a low charge density in the non-polar fluid phase might seem negligible for inter-particle interactions, the effect of charge renormalization on the polar fluid phase (mostly water) remarkably reduces the electrostatic interactions in such a way that the presence of charges in the non-polar phase plays an important role on interparticle interactions [9]. Obviously the dipole depends on h, and therefore on the wettability of the particles. When charges are on the water phase, the total electrostatic force acting vertically on a particle can be obtained analytically from the Poisson-Boltzmann equation assuming a contact angle of 90° [9]. For other values of θ numerical solutions have been obtained [28]. For a homogeneous distribution of charges along the non-polar side of the colloid, the total electrostatic force acting vertically has been initially solved numerically by Danov and Krachelvsky [29]. Later on Oettel and Dietrich [9] proposed a simple expression that depends on θ, R, and the charge distribution on the nonpolar side. After the pioneering work of Pieranski [24••] many interesting experimental studies on the determination of the electrostatic dipolar potential have been published, either from the analysis of the pair correlation function [30], or by measuring it directly with optical tweezers [31]. Nevertheless, studies correlating particle interaction and θ at fluid interfaces are very scarce. 4. Experimental methods to measure the contact angle of particles at fluid interfaces The accurate evaluation of the contact angle of particles attached at fluid interfaces is complex, being sometimes limited by the accuracy of the experimental techniques and by the need of using an interface model for calculating θ from the experimental variables [15•,22••,32••]. The most appropriate techniques may be different for micro- and nanoparticles. Hereafter, we describe the most significant techniques currently used. 4.1. Contact angle measurements using drop shape techniques Some authors have assumed that for particles θ is the same than for a flat surface made of the same material, or for a close-packed monolayer of particles deposited onto a flat substrate [33••]. However, this approach neglects the effects of surface roughness and line tension. In fact, McBride and Law [17••] pointed out that this is a good approach only for micrometric powder. They found that θ ≈ 65° for silanized silica flat surfaces whereas θ ≈ 40° for particles of the same chemical nature and R = 60 nm. Also, in view of the significance of the particle’s roughness, Guo et al. [33••] measured the contact angle of a close packed array of particles deposited onto a glass surface and described the effect of roughness using Wenzel’s model [34], which lead to θ ≈ 125° for CTAB decorated silica particles in the range 140 nm to 550 nm of diameter. 4.2. Contact angle evaluation from the collapse pressure of surface pressure-area isotherms Clint and Taylor proposed to calculate θ from the collapse pressure of a compression monolayer of particles attached to a fluid interface assuming that the particles form a hexagonal close-packed structure at the interface [21••]. This method has been used both for microparticles and nanoparticles [21••,35–37•]. Hórvölgyi et al. [37•] generalized this method for any kind of packing considering that before the collapse of the monolayer, a percolation regime is achieved, with the particles being probably expelled from the interfacial layer for higher surface densities. The above method was developed for monolayers of microparticles at fluid interfaces [21••,37•], both at air-water and at water– oil interfaces, though recently Santini et al. [38] had shown the possibility to apply it to monolayers obtained by spontaneous accumulation of CTAB decorated silica nanoparticles at the air-water interface. An

357

important limitation of this method is that any possible change of the contact angle with the compression is neglected. The possible modification of the contact angle of particles at the interface under compression was recently analysed by Garbin et al. [39]. They pointed out the existence of different interfacial relaxation processes once the mechanical energy associated to compression overcomes the attachment energy of the particles to the interface. Such relaxation processes might be due to the bucking or crumpling of the interfacial layer, as well as to the expulsion of particles from the interface. In both cases compression can eventually modify the contact angle of the particles, and consequently it is worth to consider these relaxation processes in the study of the contact angle of particles at the fluid interface using the compression isotherms. Furthermore, in some cases the Π − A isotherms do not show a defined collapse pressure due to reorganization mechanisms of the particles in three dimensional layers. This prevents the calculation of the contact angle of the particles from the collapse pressure [40,41]. 4.3. Washburn capillary rise method The revised Washburn capillary rise method is one of the most successful methods used for measuring θ of particulate matter [42••–44]. This method consists in the evaluation of the rate of capillary rising of a liquid through a packed bed of the particles. In this condition, the contact angle of the particles is related to the rising rate of the liquid into the packed bed of the powder, which is assumed to be a bundle of capillary tubes with circular section and an equivalent radius, Req. The driving force of this process is considered to be only the capillary pressure, leading to 2

hL ¼

γ f 1 f 2 Req cos θ 2η

t

ð6Þ

where hL is the height of liquid penetrating in the bed in time t and η is the viscosity of the wetting liquid. The Req values were found to be strongly dependent on the type, shapes and diameter of the particles [44]. 4.4. Atomic force microscopy (AFM) coupled to a colloidal probe This method is based on the analysis of the interaction between the colloidal probe and an air bubble and is limited to microparticles [45••, 46]. This technique has made possible to measure the contact angle of silica microparticles (size higher than 2 μm) at an air-water interface as well as the effect of the surfactant addition in the contact angle of these particles [45••,46]. 4.5. Gel trapping technique (GTT) The gel trapping technique (GTT) was originally developed for microparticles by direct observation using Scanning Electronic Microscopy (SEM) [14••], but it has been recently extended to nanoparticles using AFM [15•,17••]. The subphase is a solution of a non-adsorbing gellant polymer that does not adsorb at the interface. After gellation, the particles are trapped at the gel surface using a polydimethylsiloxane (PDMS) matrix that can finally be observed by microscopy (SEM or AFM) thus providing θ [14••,15•]. This method has been applied to the determination of the contact angle of different types of nanoparticles attached at both air-water interface and water–oil one [14••,15•]. For monolayers of poly(methylmethacrylate) particles, the values of θ obtained by the GTT and the analysis of the collapse pressure are in good agreement [35]. Paunov [14••] pointed out that polystyrene sulphate latex particles do not show any size dependence of the contact angle neither at the airwater interface (θ–75°) nor at the decane-water one (θ–105°). These values are different from the one for a flat surface, 128°, in the case of

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the water/decane interface, which is a strong evidence of the inaccuracy of the evaluation of θ of particles by the measurement of the wetting of flat surfaces. Using nanoparticles of carboxylic polystyrene Arnaudov et al. [15•] pointed out that the surface charge density of the particles plays also an important role in the contact angle. Increasing the surface charge density of the particles by a factor of 2 decreases the contact angle by a factor 2/3. 4.6. Freeze Fracture Shadow Casting (FreSCa) An alternative to GTT that overcomes some of its problems is the freeze fracture shadow-casting (FreSCa) coupled to cryo-SEM imaging [32••,47]. This technique allows one to reduce the size of the particles studied down to 10 nm. A particle monolayer at the fluid interface is frozen and then fractured to obtain a solid sample with immobilized particles that are then unidirectionally metal-coated. In this way Isa et al. [32••] have evaluated the contact angle of nanoparticles with a size below 102 nm at an oil–water (water/n-decane) interface. A strong advantage of this method is the possibility to study particles at interfaces that are difficult to be assessed by the GTT technique, such as the water/hexane. The system studied in more detail by this method was amidine polystyrene latex at water/n-decane interface in a size range between 20 nm to 500 nm. In this case θ = 110° independently of the particle size. For microparticles the values obtained by FreSCa were 20–30° higher than those obtained from the GTT. The combination of FreSCa and Cryo-SEM has allowed to evaluate the contact angle of hydrophilic gold particles (100 nm) stabilized by citrate at the water/ n-decane interface (θ–82°). Recently, Sander et al. [48] have used this method to study the effect of the addition of different surfactants (Tween 20, sodium dodecyl sulphate, trimethoxy(octadecyl)silane) and polymers (poly(vinyl alcohol), Pluronic F-127), on the contact angle of silica nanoparticles (100 nm in diameter) at the water/toluene interface. The main aim of this work was to evaluate the influence of surfactant and polymers on the ability of the particles to stabilize emulsions. The authors found that the nature of the additives does not have a big effect on contact angle of the silica nanoparticles at the interface that is always within 130–150° range. Geisel et al. [49] have pointed out that the pH does not affect the attachment of poly(N-isopropylacrylamide) microgels at the water/n-heptane interface, obtaining θ ≈ 30°. An additional advantage of the FreSCa method is that it allows one to perform an average analysis for large regions of the monolayer, thus increasing the accuracy of the measurements. Moreover, this also provides information about the heterogeneity of the wettability of the particles along the monolayer. 4.7. Contact angle by the excluded area method For particles irreversibly adsorbed at fluid interfaces Grigoriev et al. applied the excluded area formalism [23••,50•]. It is based on measuring the area shifts of a surface pressure vs. area (Π–A) isotherm of a Langmuir monolayer of an insoluble surfactant after adding particles to the monolayer. The shift is due to the fact that the surface occupied by the particles is inaccessible to the surfactant, however this method is only valid if there are no interactions between particles and the surfactant. This allows one to calculate the area occupied by the particles from which θ can be obtained. A more realistic analysis makes it necessary to take into account the dependence of the contact angle of the particle on the surface tension via the Young’s equation [50•]. 4.8. Film-calliper method (FCM) Horozov et al. [51••] developed the so-called film-calliper method (FCM), which is based on interferometric measurements, and is suitable for both micro- and nanoparticles. The main requirement is to have particles attached to both surfaces of a free-standing liquid film, thus forming a particle bridge. Once the particles formed stable bridges, the

dark and bright interference pattern can be observed using an optical microscope, which allows the calculation of the contact angle. By using this FCM technique, it is possible to achieve the measurement of θ of many particles simultaneously, thus improving its accuracy, and it can be used for non-spherical particles. 4.9. Contact angle measurements by ellipsometry Hunter et al. developed a non invasive method to determine the contact angle of sub-micrometer particle at the air-water interface using ellipsometry [22••,52]. It has quickly become a reference method for nanoparticles [53,54•]. It considers that a particle-laden interface is analogous to a bilayer, with the first layer below the interfacial plane formed by mixture of particles and water and the upper layer composed by the particles and air. It is necessary to assume that the two layers behave as two independent media with homogeneous reflectance, thus neglecting the influence of light scattered by the particles. The model allows one to establish a relationship between the contact angle of the particles at the fluid interface and the thickness of the upper layer. In order to obtain reliable values of θ it is necessary to model the response by the Effective Medium Approximation (EMA) [55]. Details on the application of the EMA to obtain the thickness and contact angle of particle-laden interface can be found in the literature [22••,52–54•]. 5. Variables affecting the contact angle of particles at fluid interfaces Different variables can alter the wettability of particles attached to fluid interfaces. Among them, the most important is the HLB of the particles, which mainly depends on the chemical nature of the particle and on the chemical nature of the phases adjacent to the interface. This balance can be modified either by physisorption or chemisorption of molecules on the particle’s surface [35,53,54•]. In the first case, the adsorption of surface active modifiers (e.g. long-chain surfactant [54•] or even short-chain alcohols [35]) is due to electrostatics, hydrogen bonds and/or VDW interactions, whereas in the second one a chemical functionalization of the particles surface is performed (e.g. silanization of silica surfaces [53]). The contact angle can be also tuned by using Janus-type particles that have assymetrical wettability [39,40,56]. The use of these particles has attracted much interest in recent years because of their particular physico-chemical properties and the structure of the layers formed at fluid interfaces. Other parameters such as the particle nature or size can also play an important role [1••]. Table 1 summarizes the contact angle values reported in the literature for particles of different chemical nature and size trapped at several fluid/fluid interfaces, and measured by different experimental methods. The results evidence that most of the studies in literature have paid attention to two types of particles: polystyrene latex and silica [15•,35,53]. Some differences can be found between θ for the same particles depending on the method used, which can be due to the different assumptions involved in relating the raw data. In most cases the authors do not clear indicate if the value reported corresponds to advancing or receding conditions [32••,51••]. In the following, some relevant results will be discussed in more detail. The effect of the particles size on θ has probably been the most studied. For most particles θ increase with the particle sizes till a plateau values (see Table 1). This dependence was explained by Horozov et al. [51••] as a consequence of the different surface chemistry of the particles obtained during the synthesis of spheres with different sizes, thus conferring the particles different hydrophobicity. It was mentioned in Section 4 that McBride and Law [17••] pointed out that the growth of θ with the particle size was due to the effect of the line tension that decreased as the diameter increased. The plateau corresponds to the values of contact angles measured for the flat surface and the particles of the same nature when they are equal. In any case, the trend of the available experimental θ with the particle radius shows a rather large

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359

Table 1 Contact angle of particles attached to fluid interfaces. Particles

Interface

Diameter

θ/deg

Notes

Ref.

Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex Polystyrene sulfate latex

Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water

9.6 μm 9.6 μm 5.7 μm 5.7 μm 3.6 μm 2.9 μm 2.9 μm 2.0 μm 1.6 μm 1.6 μm 1.0 μm 1.0 μm 810 nm 800 nm

74 40 30 37 73 59 76 58 88 89 121 63 60 73

[14••] [51••] [35] [35] [14••] [35] [35] [51••] [35] [35] [35] [35] [51••] [57]

Polystyrene sulfate latex Polystyrene latex

Air/water Air/water

630 nm 122 nm

50 43

Polystyrene carboxilate latex White aldehyde/sulphate latex Polystyrene carboxilate latex Polystyrene carboxilate latex Poly(methyl methacrylate) Poly(methyl methacrylate) Poly(methyl methacrylate) Poly(methyl methacrylate) Poly(methyl methacrylate) Poly(methyl methacrylate) Poly(methyl methacrylate) Poly(methyl methacrylate) Silanized silica Silanized silica Silanized silica Silanized silica Silica Silica Silica Silica

Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water Air/water

120 nm 83 nm 53 nm 37 nm 2.0 μm 2.0 μm 2.0 μm 2.0 μm 1.0 μm 1.0 μm 1.0 μm 1.0 μm 3.0 μm 3.0 μm 3.0 μm 2.2 μm 1.0 μm 1.0 μm 265 nm 30 nm

67 89 92 90 56 56 29 26 36 31 18 15 40 56 67 94 70 41 37 59

Silica

Air/water

30 nm

79

Silica

Air/water

30 nm

64

Silica

Air/water

30 nm

50

Silica

Air/water

30 nm

34

Silica

Air/water

30 nm

54

Silica

Air/water

30 nm

74

Silica

Air/water

30 nm

58

Silica

Air/water

30 nm

45

Silica

Air/water

30 nm

26

Fumed silica Fumed silica Fumed silica Fumed silica Amidine polystyrene latex Amidine polystyrene latex Amidine polystyrene latex Polystyrene Sulfate latex Polystyrene Sulfate latex Polystyrene Sulfate latex Polystyrene Sulfate latex Polystyrene Sulfate latex Polystyrene Sulfate latex Polystyrene Sulfate latex Polystyrene Sulfate latex Polystyrene Sulfate latex Polystyrene Sulfate latex

Air/water Air/water Air/water Air/water Water/n-hexane Water/n-hexane Water/n-hexane Water/Octane Water/Octane Water/Octane Water/Octane Water/Octane Water/Octane Water/Octane Water/Octane Water/Octane Water/Octane

20-30 nm 20-30 nm 20 nm 20 nm 200 nm 100 nm 20 nm 5.7 μm 5.7 μm 2.9 μm 2.9 μm 1.6 μm 1.6 μm 1.0 μm 1.0 μm 0.5 μm 0.5 μm

15 146 55 120 108 116 103 120 119 135 120 120 56 140 28 74 77

Measured by GTT Measured by FCM Spread using isopropanol measured by GTT Spread using methanol measured by GTT Measured by GTT Spread using isopropanol measured by GTT Spread using methanol measured by GTT Measured by FCM Spread using isopropanol measured by GTT Spread using methanol measured by GTT Spread using isopropanol measured by GTT Spread using methanol measured by GTT Measured by FCM Sterically stabilized by poly(glycerol-monomethacrylate), independently on chain length measured by GTT Measured by FCM Poly(ethyleneglycol monomethacrylate) grafted in a weight fraction of 2.2 wt% Measured by ellipsometry Measured by GTT Measured by GTT Measured by GTT Measured by GTT Spread using isopropanol measured by GTT Spread using isopropanol measured by the excluded area method Spread using methanol measured by GTT Spread using methanol measured by the excluded area method Spread using isopropanol measured by GTT Spread using isopropanol measured by the excluded area method Spread using methanol measured by GTT Spread using methanol measured by the excluded area method Silanizing agent hexamethyldisilazane, c = 0.1 mM) measured by FCM Silanizing agent hexamethyldisilazane, c = 1 mM measured by FCM Silanizing agent hexamethyldisilazane, c = 10 mM measured by FCM Silanizing agent octadecylsilane measured by GTT Spread using isopropanol measured by GTT Spread using methanol measured by GTT Measured by ellipsometry Modified by cationic surfactant dodecyltrimethylammonium bromide, c = 0.01 mM measured by ellipsometry Modified by cationic surfactant dodecyltrimethylammonium bromide, c = 0.1 mM measured by ellipsometry Modified by cationic surfactant dodecyltrimethylammonium bromide, c = 0.2 mM measured by ellipsometry Modified by cationic surfactant dodecyltrimethylammonium bromide, c = 0.5 mM measured by ellipsometry Modified by cationic surfactant dodecyltrimethylammonium bromide, c = 0.8 mM measured by ellipsometry Modified by cationic surfactant cetryltrimethylammonium bromide, c = 0.01 mM measured by ellipsometry Modified by cationic surfactant cetryltrimethylammonium bromide, c = 0.1 mM measured by ellipsometry Modified by cationic surfactant cetryltrimethylammonium bromide, c = 0.2 mM measured by ellipsometry Modified by cationic surfactant cetryltrimethylammonium bromide, c = 0.5 mM measured by ellipsometry Modified by cationic surfactant cetryltrimethylammonium bromide, c = 0.8 mM measured by ellipsometry 100% of free silanol groups on the particle surface measured by ellipsometry 14% of free silanol groups on the particle surface measured by ellipsometry 34% of free silanol groups on the particle surface measured by ellipsometry 20% of free silanol groups on the particle surface measured by ellipsometry Measured by FreSCa Measured by FreSCa Measured by FreSCa Spread using isopropanol measured by GTT Spread using methanol measured by GTT Spread using isopropanol measured by GTT Spread using methanol measured by GTT Spread using isopropanol measured by GTT Spread using methanol measured by GTT Spread using isopropanol measured by GTT Spread using methanol measured by GTT Spread using isopropanol measured by GTT Spread using isopropanol Calculated from the collapse pressure of the compression isotherm

[51••] [52] [15•] [15•] [15•] [15•] [35] [35] [35] [35] [35] [35] [35] [35] [51••] [51••] [51••] [14••] [35] [35] [22••] [54•] [54•] [54•] [54•] [54•] [54•] [54•] [54•] [54•] [54•] [58] [58] [53] [53] [32••] [32••] [32••] [35] [35] [35] [35] [35] [35] [35] [35] [35] [35]

(continued on next page)

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Table 1 (continued) Particles

Interface

Diameter

θ/deg

Notes

Ref.

Silica Silica Polystyrene Sulfate latex Polystyrene Sulfate latex Polystyrene Polystyrene Amidine Polystyrene latex Amidine Polystyrene latex Amidine Polystyrene latex Polystyrene Carboxilate latex Amidine Polystyrene latex Polystyrene Sulfate latex White aldehyde/sulphate latex Polystyrene Carboxilate latex Amidine Polystyrene latex Polystyrene Carboxilate latex Amidine Polystyrene latex Poly(methyl methacrylate) Poly(methyl methacrylate) Gold Silanized Silica Silica

Water/Octane Water/Octane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane Water/n-decane

1.0 μm 1.0 μm 9.6 μm 3.6 μm 2.8 μm 2.8 μm 500 nm 500 nm 200 nm 120 nm 100 nm 100 μm 83 nm 53 nm 40 nm 37 nm 20 nm 2.2 μm 2.2 μm 100 μm 2.2 μm 404 nm

148 68 111 101 85 122 103 127 102 72 124 116 123 102 116 99 113 130 157 82 136 42

0.01 mM

[35] [35] [14••] [14••] [32••] [32••] [32••] [32••] [32••] [15•] [32••] [32••] [15•] [15•] [32••] [15•] [32••] [32••] [32••] [32••] [14••] [47]

Silica

Water/n-decane

404 nm

52

0.03 mM

[47]

Silica

Water/n-decane

404 nm

75

0.05 mM

[47]

Silica

Water/n-decane

404 nm

105

0.1 mM

[47]

Silica

Water/n-decane

404 nm

91

0.3 mM

[47]

Silica

Water/n-decane

404 nm

92

0.5 mM

[47]

Silica

Water/n-decane

404 nm

75

1 mM

[47]

Silica

Water/n-decane

404 nm

50

4 mM

[47]

Silica

Water/n-decane

404 nm

40

10 mM

[47]

Polystyrene Sulfate latex

Water/n-dodecane

800 nm

78

Spread using isopropanol measured by GTT Spread using methanol measured by GTT Measured by GTT Measured by GTT Measured by FreSCa Measured by GTT Measured by FreSCa Measured by GTT Measured by FreSCa Measured by GTT Measured by FreSCa Measured by FreSCa Measured by GTT Measured by GTT Measured by FreSCa Measured by GTT Measured by FreSCa Measured by FreSCa Measured by GTT Measured by FreSCa Silanizing agent octadecylsilane measured by GTT Modified by cationic surfactant didecyldimethylammonium bromide, c = measured by FreSCa Modified by cationic surfactant didecyldimethylammonium bromide, c = measured by FreSCa Modified by cationic surfactant didecyldimethylammonium bromide, c = measured by FreSCa Modified by cationic surfactant didecyldimethylammonium bromide, c = measured by FreSCa Modified by cationic surfactant didecyldimethylammonium bromide, c = measured by FreSCa Modified by cationic surfactant didecyldimethylammonium bromide, c = measured by FreSCa Modified by cationic surfactant didecyldimethylammonium bromide, c = measured by FreSCa Modified by cationic surfactant didecyldimethylammonium bromide, c = measured by FreSCa Modified by cationic surfactant didecyldimethylammonium bromide, c = measured by FreSCa Sterically stabilized by Poly(glycerol-monomethacrylate), independently on chain length measured by GTT

scattering due both the possible differences of the particle-surface nature and the different experimental techniques used. Another critical parameter affecting the contact angle of particles at fluid interfaces is their surface charge density. As already said, this is due to the importance of the charge of the particles in the intricate balance of interactions (electric-field-induced-capillary attraction vs. electrostatic repulsion) that governs the stabilization of the particle laden interface, leading to the particles to reach their equilibrium contact angle at the interface [59,60]. Arnaudov et al. [15•] observed that the contact angle of carboxylic polystyrene latex, at both air/water and water/n-decane interface, decreases with the charge density of the particles. Park et al. [60] showed that the screening of the electrostatic repulsion by the adsorption of SDS to a water/decane interface lead to an increase of the particle contact angle from 118° to 142° because of the partial hindering of the electrostatic barrier imposed by the repulsion between particles which controls their adsorption. On the other side, the addition of salts does not alter significantly the contact angle of the particles at the fluid interface that pointed out clearly the central role of the non polar phase on the modulation of the particles contact angle at fluid interfaces. Furthermore, as the attractive van der Waals interactions increase their impact, the contact angle of the particles at the fluid interface decreases. The importance of the screening effects of the electrostatic repulsions on the contact angle of particles has been also observed by Maestro et al. [35] for polystyrene sulphate latex. They found that the contact angle at the water/octane interface of particles with quite different surface charge density (− 5.7 and −9.7 μC · cm−2) but the same diameter (2.9 μm) is within the experimental error the same (around 125°). This behaviour was ascribed to

[57]

the formation of a layer of the solvent used for the particles spreading around the particles that hinders the effect of the inter-particles repulsion. The difference of dielectric constants between the two fluid phases, and therefore the nature of the fluid interface plays a critical role in the contact angle of particles at the interface. This is because particles close to the two phase boundary experience image-particle interactions, hence electrodipping force acts on each particle towards the phase with higher dielectric constant [61]. As a consequence, increasing the difference between the dielectric constant of the two fluids, being water one of them, reduces the contact angle as the particles are pushed by the electrodipping force to the water phase. This explains why the contact angle of a particle at water-air is smaller than at water–oil interfaces [15•,55]. An additional aspect that plays an important role in the wettability of the particles is their chemical surface nature. In general this can be controlled by the chemical modification of the particle surface (e.g. silanization of silica particles) [54•] or by the addition of surface active modifiers such as polymers, surfactant or short chain alcohols. Zang et al. [54•] have pointed out that increasing the silanization of silica nanoparticles degree from 66% to 80% increases the contact angle of the particles at the air-water interface for a factor almost of two; Binks and Tyowua [58] have evidenced that highly hydrophobic particles can be obtained by fluorination of the particles surfaces, increasing significantly their contact angle at the air-water interface, and that in the case of interfaces containing oils this leads to a preferential partition of the particles to the most hydrophobic phase. Reed et al. [57] have shown that grafting poly(glycerol-monomethacrylate) chains to the latex particles surfaces decreases θ with the increase of the length of

A. Maestro et al. / Current Opinion in Colloid & Interface Science 19 (2014) 355–367

grafted polymer chains observed at both air/water and n-dodecane/ water interfaces. Similar effects can be obtained by physical adsorption of surfactants on the particles surface. Maestro et al. [54•] have recently shown that the addition of hexadecyl-trimethylammonium bromide (CTAB) and dodecyl-trimethylammonium bromide (DTAB) allows one to change the wettability of silica nanoparticles by the air-water interface. The wettability of single-chain surfactant decorated silica nanoparticles adsorbed at air/water interfaces can be ascribed to an intricate balance of electrostatic and hydrophobic interactions between the particle’s surface and the surfactant molecules, thus depending on the surfactant concentration [54•]. By using ellipsometry on planar air/water interfaces, they measured the contact angle of the silica nanoparticle– surfactant composite layers adsorbed as a function of the surfactant concentration. They observed an increase in the wettability until a certain value of the surfactant concentration corresponding to the neutralization of the average charge of the particles-decorated by surfactant. Further increase of the surfactant concentration lead to the increase of the hydrophobic interactions between the hydrocarbon tails of surfactant molecules attached to the particle surface. This interaction could change the structure of the adsorbed surfactant layer onto the silica nanoparticles, inducing the formation of a more hydrophilic interfacial layer, thus rendering lower values of θ. Another peculiar aspect evidenced in this work is the lack of a synergic effect between particles and surfactants in lowering the surface tension, which lead to the conclusion that only the change of wettability is affected by increasing the amount of surfactant molecules. Similar effects have been recently reported by Binks et al. [47] at the water/n-decane interface using di-decyldimethylammonium bromide. Small molecules such as alcohols also can modify the contact angle of particles at fluid interfaces, Maestro et al. [35] found that the spreading of particle with different nature (polystyrene sulphate latex, poly(methyl-methacrylate), silica) using isopropanol lead to a contact angles of the particles at the water/octane interface higher than when methanol was used as spreading solvent. The opposite was found true for the particles spread at the air/water interface (see Fig. 2). These effects were explained in terms of the adsorption of alcohol molecules on the surface of the particles that modified the surface nature of the particles. The role of the solvent in the attachment of particles to fluid interfaces has been recently deepened by Fernandez-Rodriguez et al. [62]. They found that the spreading of silver Janus-like particles at the air-water interface leads to different types of interfacial layers depending on the nature of the spreading solvent (pure methanol vs. mixture 1 methanol:1 isopropanol) as was evidenced from the changes in both the surface pressure and rheological behaviour of the monolayers. This behaviour is related to changes of the particles contact angle due to the formation of solvent layers onto the particles. Other variables that can play an important role in the contact angle of particles at fluid interfaces are the roughness and porosity of the particles which are especially critical as the particle size decreases [35,63]. Higher roughness and porosity of the particles increases the contact angle at the fluid interface in a way that can be qualitatively rationalized with the classical Cassie-Baxter model [64]. This presents critical implications in technological applications of particles such as flotation processes. Furthermore, the increase in the porosity and roughness can facilitate the attachment to the particle surface of surface active modifiers. However, the effect of these parameters on the contact angle of particles is far from being completely understood. Finally, the experimental protocol followed for making the particle laden interface plays a non-negligible role in the value of the contact angle obtained. [39]. The adsorption of particles from the bulk to the interface always implies a subtle energy balance (interactions fluid-fluid and fluid-particles) that defines the equilibrium contact angle of the particles at the fluid interface. Whereas for spread particles monolayers, it is possible to deposit the particles at the interface even when the contact angle is not well defined. An important problem associated

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Fig. 2. Contact angle y as a function of the diameter s of the particles attached to the interface (a) air–water and (b) oil–water. The nature of the particles and the spreading solvent as follows: Polystyrene (PS) particles spread with methanol (□), and isopropanol (Δ). Poly(methyl methacrylate) (PMMA) particles in methanol (■), ( ) and isopropanol (▲), ( ) using GTT and the excluded area formalism, respectively. In (a) the general θ–σ dependency is denoted by a solid line and in (b) the θ–σ behaviour for methanol and isopropanol is denoted by a dashed and dotted line, respectively. Reproduced from Ref. [35] with permission from the PCCP Owner Societies.

with the determination of the contact angle of particles attached to the fluid interface is related to the type of contact angle measured (advancing or receding). 6. Dynamic of particle wetting at fluid interfaces Although the equilibrium contact angle is used in most papers, many questions remain open about the dynamic aspects of the wetting [25••]. The formation of a three-phase contact line of the particles at fluid interfaces presents a discontinuity determined by the rupture of the interface due to the particle contact. The process presents different relaxation mechanisms since the particle spreading till θ reaches the equilibrium value, and the dynamics of wetting has been recently studied using digital holographic microscopy [25••,65]. Fig. 3 shows the wetting dynamics during the attachment of latex at the water/n-decane interface. At low ionic strengths (strong electrostatic interactions) the relaxation processes are hindered, thus once the particles reach the interface they remain trapped at equilibrium conditions. The relaxation processes are logarithmic and much slower than expected from hydrodynamics considerations. This behaviour is similar to those observed for physical aging processes in other non-equilibrium systems [66], and is driven by fluctuations of the three phase contact line over defects with an associated activation energy. According to ref. [67] the velocity of the particles slows down because the interfacial force imposed on the particles decreases with the motion, and therefore a high energy is necessary for overcoming the activation barrier. Obviously the surface interactions of the particles with the interface plays key role in the wetting dynamics. Fig. 3b shows that particles of similar sizes (around 1.8 μm) with different chemical nature show different relaxations. The differences on the dynamics due to the particle nature allow one to rule out the bulk effects as the driving force of the contact angle equilibration. This indicates that the nanoscale surface features can affect strongly the wetting dynamics, and consequently the attachment of soft colloids to fluid interfaces. These processes are governed mostly by molecular dissipation mechanisms, with the hydrodynamic dissipation playing a

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Fig. 3. (a) Trajectories of polystyrene particles during their attachment at the water/n-decane interface under different ionic strength conditions. (b) Relaxation trajectories for particles with different chemical nature. Note that a logarithmic relaxation explains accurately all the relaxation dynamics. Reprinted by permission from Macmillan Publishers Ltd.: Nature Materials [25••], Copyright (2012).

negligible role in contrast to the macroscopic wetting where the opposite behaviour is true [67]. This picture is in good agreement with the molecular dynamics simulations by Colosqui et al. [65], who showed that the relaxation process that leads to the contact angle till the equilibrium is reminiscent of physical aging processes of glassy systems. The dynamics of the attachment of the particles to fluid interfaces is strongly influenced by the type of systems considered: adsorption or spreading [39]. The adsorption of particles from a fluid 1 to the interface requires that fluid 1 partially dewets the particle with the subsequent creation of a contact line. In general, this process can be longer than the time-scale of the experiments due to the influence of the thermally activated depinning of the advancing contact line from the particle surface. On the other side, the temporal scale is not an issue in spread monolayer where the deposition process of the particle at the interface is fast, and frequently implies a turbulent flow of solvent at the interface. Similar considerations could be made for particle-stabilized emulsions where the mechanical energy is enough to overcome the depinning energy [68]. However, the depinning effects are quite important in adsorbed monolayers.

7. Effect of the particle contact angle on the mechanical properties of particle-laden interfaces In spite of the important practical implications in the stabilization of dispersed systems such as thin films, foams and emulsions [69], few studies have paid attention to the effect of the contact angle of the particle attached to the interface on the rheological aspects of particleladen systems [7,10]. A first theoretical model was presented by Miller et al. [70], who revisited the thermodynamic models developed for proteins and proteins + surfactant interfacial layers. They introduced the so called cohesion pressure correction that accounts for the change on the mechanical properties due to the contact angle of the particles attached to the fluid interface [6]. However, this model often fails in describing accurately the experimental data.

Zang et al. [10] studied the effect of the contact angle of silica nanoparticles on the dilational and shear properties of the particle laden interface. They found a significant densification of the layer with the hydrophobicity of the particles that modifies the mechanical response of the particles layers. The dilational modulus shows a maximum for particles of intermediate hydrophobicity (34% of silanization over the total content of silanol groups on the particles surface). Also the shear modulus, as well as the yield and melt strains, show a maximum for θ ≈ 90°. The effects observed were explained considering that compact layers have a strong facility to fracture because they form clusters that alter the layer cohesion [71]. These phenomena play an important role in the stabilization of foams by these silica nanoparticles [72], affecting also drop coalescence and coarsening of emulsions. Similar effects were described for silica particles decorated by CTAB and DTAB [73]. 8. Influence of the wettability of particles in stabilizing foams and emulsions It has been well established that partially hydrophobic micro- and nanoparticles stabilize air bubbles or droplets – creating the well known Ramsden-Pickering emulsions [74,75] – in surfactant-free diluted suspensions [3,76]. In general, foams and emulsions are thermodynamically metastable systems, and their unstable nature is a critical issue in all these applications. Instabilities due mainly to film rupture – coalescence – and coarsening due to fluid diffusion between bubbles or droplets arise from the high energy associated to the air/liquid and liquid/liquid interfaces involved respectively [77]. The high energy of attachment of particles at fluid interfaces – hundreds of kBT – makes a difference with respect to standard amphiphiles, lipids or proteins that usually adsorb reversibly. It is in this context where the wetting behaviour of solid particles, measured through the water phase, is considered as key rule for distinguishing the specific nature of a particle dispersion [78]. For particles in an arbitrary fluid/fluid interface one can consider two different situations: If the particle is relatively hydrophilic, θ b 90°, an oil(air)/

A. Maestro et al. / Current Opinion in Colloid & Interface Science 19 (2014) 355–367

water dispersion is preferred in mixtures of equal volumes of the two fluids. On the other hand, if the particle is relatively hydrophobic, θ N 90°, a water/oil(air) dispersion is favoured (see Fig. 4a). θ = 90° is considered as the point of inversion, for which an arbitrary particle is equally wetted by both phases [2•]. As in the case of surfactants, the particles minimise the interfacial energy by bending the interface imbibing the dispersed phase — the preferentially wetting phase. However, unlike the surfactants the particles do not affect dispersion stability by significantly reducing the surface tension [54•]. Actually, it is the particle interfacial network which indeed is the responsible for the dispersion stability through the formation of a steric barrier against the instability processes. A sketch representing the stabilizing effect of particles in foams and emulsions is presented in Fig. 4a. A transitional inversion of the curvature of an oil/water interfaces containing adsorbed particles can be achieved by modifying θ at a fixed oil/water ratio [2•]. Binks et al. [47] have studied a double phase

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inversion of an emulsion stabilised by silica nanoparticles – bicatenary cationic surfactant mixed interfacial layers. For a fixed amount of particles, increasing the surfactant concentration lead to stable emulsions that showed an inversion from oil/water to water/oil and recovering oil/water at high enough concentrations (see Fig. 4b) [79]. Very recently, a study on the influence of the contact angle of the nanoparticles versus the surfactant concentration confirms the suggested mechanism for the double emulsion phase inversion. The contact angle of the particles at a planar oil/water interface is measured by the method of FreSCa (see Fig. 4c). At low concentrations θ b 90° leading to the formation of an oil/water emulsion. At intermediate concentrations θ reaches its maximum value around 105°, which means that the particles become hydrophobic and then a water/oil emulsion is formed. Finally, at higher concentrations, θ decreases to values less than 90°, for which the particles becomes more hydrophilic and as result an oil/water emulsion is formed [47].

Fig. 4. (a) Sketch of the stabilizing effect of particles with different hydrophobicity in emulsions (top panel) and foams (bottom panel). (b) Changes in the nature of emulsions stabilized by nanoparticles as function of the surfactant concentration. Reprinted with permission from Ref. [79], Copyright (2013) Wiley and Sons Co. (c) Contact angles at the n-decane/water interface obtained from FreSCa measurements for silica particles as a function of surfactant concentration (in absence of surfactant θ = 33°). Reprinted with permission from Ref. [47], Copyright (2013) American Chemical Society. (d) Images relatives to the transitional inversion of the curvature of the air/water surfaces with respect to particle wettability. Reprinted by permission from Macmillan Publishers Ltd.: Nature Materials [78], Copyright (2006). (e) Different regions observed in the contact angle of silica particles as function the silanization degree. Reprinted with permission from Ref. [86], Copyright (2006) American Chemical Society.

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adhesion force is a function of the particle wettability and edge geometry, which in turn may change the behaviour of the particles at the air–liquid interface. 9. Effect of the particle contact angle on the microrheological characterization of monolayers The dilational and shear viscoelastic moduli of monolayers of surface active molecules characterize the mechanical properties of fluid interfaces, which determine the stability of emulsions and foams [90]. Nevertheless, measurements of the viscoelasticity of fluid interfaces by standard rheological techniques can be challenging [13••,91]. Very recently, interfacial micro-rheology has emerged as a powerful method to obtain information on the viscoelastic character of such interfaces by tracking the trajectories of particles adsorbed at the interface. The method can be used even at low shear viscosities, and allows one to detect mesoscale spatial heterogeneities in the interfacial layer. However, the friction coefficient f for particle trapped at fluid interfaces cannot be obtained from the Stokes’ law, but it is a complex function of the viscosities of the phases η, the geometry of the particle, and the immersion depth of the particle measured [92]. Indeed, the translational 2D trajectories of the particles adsorbed at such interfaces can be governed by the wetting behaviour of such particles. Recently, Maestro et al. [12••] have shown how particles adsorbed at the air/water interface with a dissimilar contact angle can lead to different values of short-time diffusion coefficient D. D was obtained from the time dependence of the measured mean square displacement of the particles, and particles with roughly the same diameter and different chemical nature were used in the low surface particle density to avoid interparticle interactions (see Fig. 5). In that work, the surface shear viscoelastic moduli of poly(t-butylacrylate), PtBA, interfacial layers have been extracted from the particles mean square displacements, using different particles. In Ref. [12••] f was calculated using the Fischer’s theory [92]. Fischer’s theory points out that f, and therefore the interfacial shear viscosity, ηs, depends on θ. Fig. 5 shows the power law dependence of the interfacial shear viscosity with the PtBa density. The authors used particles of difference chemical nature and hence wettability, and found that particles with different values of θ lead to the same values of the shear viscosity. Lee et al. [93] studied the interfacial viscosity of β-lactoglobulin (BLG), BLG layers adsorbed at the n-decane/water interface using polystyrene particles with a well-defined contact angle. They studied the time dependence of the shear viscosity as BLG adsorbed at the interface.

a)

0.8

D0 (μm2/s)

0.6

1.0

b)

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0.2

0.0 20

40

60

80

100

Contact angle θ (ο)

0.6

109·η (N·s/m)

This change of wettability is not only a singular situation for emulsions, this is also valid to explain changes in the stability of aqueous foams stabilised by nanoparticles–surfactant systems [80]. Gonzenbach et al. [81•,82] showed the hierarchical structure of a foam stabilized by a mixture of silica particles and surfactant molecules, pointing out that the surface hydrophobicity achieved by an optimum wettability of the stabilizing particles can be controlled by the adsorption of small surfactant molecules such as hexylamine. The fine tuning of the wettability of silica particles by surfactant can be very interesting in order to achieve the stability of foams and emulsions by changing the amount of surfactant [54•,83]. This idea has been also used by Zabiegaj et al. [84] for the fabrication of porous cellular materials stabilizing foams with carbonaceous nanoparticles. In contrast to the behaviour of emulsions, the inversion of the curvature of particles adsorbed at air/water interfaces has not been observed so far by varying the surfactant concentration. Nevertheless, this inversion scheme can be achieved exploring the transition from air bubbles dispersed in water – aqueous foam – by using partially hydrophilic particles (silica particles) to water drops dispersed in air by means of hydrophobic particles. The latter are able to encapsulate isolated water drops in air creating the so-called liquid marbles as well as dry water — a free-flowing powder which contains this particle coated water drops [85]. Binks et al. [78] showed for the first time the inversion scheme in particles adsorbed at air/water interfaces. They used silica nanoparticles chemically coated with dichlorodimethylsilane to yield particles with varying percent grafting of alkyl chains and thus different wettability. Fig. 4d shows the transitional inversion of the curvature of the air/ water surfaces with respect to particle wettability. The figure shows a sequence of photographs of samples containing the same amount of particles. The silanol (SiOH) content, in percent particle surface, decreases from right to left rendering more hydrophobic particles. It can be observed that the mixtures changes from stable aqueous dispersions (for relatively hydrophilic particles, SiOH N 66%) to air-in-water foams (for particles at intermediate hydrophobicity, 32% b SiOH b 62%). In this case, the volume of the foam increases with the particle hydrophobicity. Finally, for very hydrophobic particles SiOH b 20%, all the water is confined in stable macroscopic drops in air forming a free-flowing powder. To explain this behaviour, in an independent work, Kostakis et al. [86] measured θ between an aqueous drop an a silica plate for a wide range of silanization degree (SiOR) – in this case, these authors measured SiOR (%) = 100 − SiOH (%). Fig. 4e shows that in the range studied, where is possible to distinguish three regions: a first one for values less than 30-35%, where the particles remain with a hydrophilic character. A sharp transition in θ in between 30-55% over which the values found passes from 30° to more than 90°, roughly 110°, which means that the particles progressively increases their hydrophobicity. Finally, for values larger than 60% the particles, for which the particles have a further hydrophobic behaviour, the contact angle values define a pseudo-plateau. A comparison of both works allows one to conclude that the wetting behaviour of the particles is the governing factor for controlling not only the stability of aqueous foams, but also to achieve an inversion scheme for which air or water can be encapsulated. San-Miguel et al. [87] studied how the particle wettability depends on roughness, and demonstrated that the stability of Pickering emulsions increases with roughness. The effect of roughness on particle– particle interactions at air–liquid interfaces was first investigated by Stamou et al. [88], who proposed a mechanism for the long-range attraction based on non-uniform wetting that causes an irregular shape of the particle meniscus. Recently, Maestro et al. [35] reported the effect of surface roughness on the contact angles, particularly as particle size decreases. Little work has been done so far on the effect of edge pinning on particle-interface interactions. Ally et al. [89] studied modified spherical particles addressing the effect of edge pinning on particle adhesion to air–liquid interfaces by AFM. They concluded that the

D / D0

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0.4

0.2 0.0

0.2

0.4

0.6

0.8

Γ (mg/m2)

1.0

1.2

c) 1

0.1 η∼Γ1.98

0.01

0.1

1 Γ (mg/m2)

Fig. 5. (a) D/D0 versus polymer concentration considering particles of different particles, with D0 being the diffusion coefficient of the particles in the low dilution limit. (b) Relation of D versus contact angle for particles with different nature (silica–SiO− 2 , polymethyl– methacryle [PMMA] and Poly(t-butylacrylate [PtBA]) with roughly the same size (≈ 1 μm). (c) The shear viscosity was obtained applying the Fischer theory. Adapted from Ref. [54•] with permission from The Royal Society of Chemistry.

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A contact angle of 90° for the polystyrene particles was measured at the bare decane/water interface, which was remarkable insensitive to the variation of surface tension once the BLG starts to be adsorbed on it. This fact means that the adsorption of BLG layers (whose thickness is roughly few nm, and therefore it is negligible in comparison to the particle’s immersion depth) decreases the tension of the decane/water interface, though such decreases does not affect at all the value of the contact angle. Using a different theoretical approach, Sickert et al. [94] studied the shear viscosity of surfactant Langmuir monolayers in an attempt to characterize the fluid membranes by using the Brownian diffusion method defined by Sickert and Rondelez [95], and combining the theories of Danov and Fischer [92,96]. 10. Concluding remarks and future perspectives The interest in the study of the contact angle of particles attached to fluid interfaces has undergone a spectacular growth in the last decade due to their recognized implication in many technological aspects involving the interaction of particles and fluid interfaces such as the stabilization of dispersed systems. However, the complete understanding of the control mechanisms that govern this key property is hardly attainable due to the complexity of the theoretical and experimental tools associated with its study. Furthermore, the number of available contact angle data is in general scarce and limited to a small number of types of particles and dimensions that makes difficult to establish general rules related to the physico-chemical aspects, associated to both the particles (size, chemical nature, roughness, charge, shape, etc.) and the two fluid phases (dielectric constant, etc.), involved in the control of the contact angle of particles at the fluid interface. This review has presented a comprehensive analysis of the current knowledge on the contact angle of particles at fluid interface considering both the static and dynamic aspects as well as the different theoretical and experimental approaches developed to determine this important property. The problems associated with the development of an integrative theory that allows one to establish the role of the contact angle in the attachment of particles to fluid interface require the development of complex physico-chemical models considering both the interaction of the particles with the interface and the interaction between particles as well as the physico-chemical properties of particles and interface. The main difficulty arrives from the multiple variables to consider that make necessary the use of standard protocols to evaluate the contact angle of the particles attached to the fluid interface, thus eliminating the discrepancies due to the methodological approach. Another aspect to consider is that most of the current studies are devoted to the investigation of the contact angle of spherical particles. However, non-spherical particles have become in an interesting alternative to replace spherical ones in recent years due to their unique properties that has raised new features in the study of the contact angle at fluid interfaces. The extension of the theoretical and experimental approaches used in the study of spherical particles is an interesting new challenge on the understanding of the parameters governing the contact angle at fluid interface. The complete understanding of the physico-chemical aspects related to the contact angle of particles at fluid interfaces will open a new perspective to the study of interfaces and to the application of these intriguing systems for research and/or technological purposes. Acknowledgements This work was supported in part by MINECO under Grant FIS201238231-C02-01, by ESA under Grants MAP AO-00-052 (FASES) and PASTA, and by EU under Grant Marie Curie ITN-COWET, and carried out in the framework of the ERF COST actions CM1101 “Colloidal Aspects of Nanoscience for Innovative Processes and Materials”.

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References and recommended reading•,•• [1] Binks BP, Horozov TS. Colloidal particles at liquid interfaces. Cambridge: Cambridge •• University Press; 2006. Comprehensive overview on the state of the art of the study of the attachment of particles at fluid interfaces. [2] Aveyard R, Binks BP, Clint JH. Emulsions stabilised solely by colloidal particles. Adv • Colloid Interface Sci 2003;100–102:503–46. Comprehensive review of the effect of particles in the stabilization of dispersed systems such as emulsions and foams. [3] Binks BP. Particles as surfactants – similarities and differences. Curr Opin Colloid • Interface Sci 2002;7:21–41. Comprehensive review that analyze the importance of the attachment of particles at fluid interface in many soft matter problems. [4] Dinsmore AD, Hsu MF, Nikolaides MG, Marquez M, Bausch AR, Weitz DA. Colloidosomes: selectively permeable capsules composed of colloidal particles. Science 2002;298:1006–9. [5] Velev OD, Gupta S. 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