Adsorption of Hydrophobic Silica Nanoparticles at the PDMS Droplet

In contrast to the behavior at the air−water interface, less hydrophobic beads are in a ... It has been reported recently15 that charged colloidal p...
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Langmuir 2003, 19, 8364-8370

Adsorption of Hydrophobic Silica Nanoparticles at the PDMS Droplet-Water Interface Spomenka Simovic and Clive A. Prestidge* Ian Wark Research Institute, The ARC Special Research Centre for Particle and Material Interfaces, University of South Australia, Mawson Lakes, SA 5095, Australia Received April 29, 2003. In Final Form: June 29, 2003 The adsorption of hydrophobic silica nanoparticles at the poly(dimethylsiloxane) (PDMS) dropletwater interface has been investigated through particle adsorption isotherms, with complementary studies of the adsorbed layer structure by freeze-fracture SEM. The influences of pH and electrolyte concentration as well as droplet cross-linking (deformability) are reported. Adsorption isotherms for liquid droplets are sigmoidal, whereas those for cross-linked droplets are of “low affinity” or “affinity increase” type. The surface coverage reaches values that correspond to multiple close-packed layers and are significantly influenced by droplet cross-linking, conferring extensive interfacial penetration, as confirmed by SEM. Densely packed adsorbed particle layers with interfacial aggregation are observed over a wide range of solution conditions. Both liquid and cross-linked PDMS droplets show pH-dependent adsorption, in agreement with DLVO theory; this is in contrast to hydrophilic silica adsorption [Langmuir 2003, 19, 3785]. Interfacial particle saturation occurred at a salt concentration 2 orders of magnitude less than the critical coagulation concentration (ccc) for hydrophobic silica in water. This phenomenon was independent of droplet cross-linking and indicates that particle interaction through the water phase plays a decisive role in particle packing at the interface. SEM indicated the presence of a rigid interfacial crust layer at salt concentrations corresponding to interfacial saturation and a multilayered interfacial particle wall at salt concentrations g ccc.

Introduction Particle stabilized emulsions are important in many areas such as enhanced oil recovery, food, pharmaceutics, and cosmetics.1-3 Emulsion stabilization by solid particles depends largely on sufficient particle adsorption and the formation of a “densely packed” layer at the oil-water interface, which sterically inhibits droplet coalescence.1-5 If charged, particles may give rise to electrostatic repulsion, which further enhances emulsion stability.1,2 The structure of interfacial particle layers is governed by particle-droplet and particle-particle interactions and controlled through colloidal forces; these are in turn dependent upon particle wettability, i.e., the particle immersion depth at the oil-water interface.6-8 Levine et al.9,10 reported a theoretical investigation of interparticle interactions in a close-packed interfacial particle film at the planar oil-water interface. Upon entering the oil phase from the water phase, electrical double-layer interaction between two spherical particles at the interface is critically dependent on the contact angle * To whom correspondence should be addressed: e-mail [email protected]; telephone +61-8083023569; Fax +61-8-83023683. (1) Tadros, Th. F.; Vincent, B. In Encyclopedia of Emulsion Technology, Basic Theory; Marcel Dekker: New York, 1983; Vol. 1, p 129. (2) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1993, 15, 244. (3) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 2539. (4) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 3748. (5) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622. (6) Horvolgyi, Z.; Mate, M.; Zrinyi, M. Colloids Surf., A 1994, 84, 207. (7) Horvolgyi, Z.; Nemeth, S.; Fendler, J. H. Collods Surf., A 1993, 71, 327. (8) Horvolgyi, Z.; Medveczky, G.; Zrinyi, M. Colloid Polym. Sci. 1993, 271, 396. (9) Levine, S.; Bowen, B.; Partridge, S. Colloids Surf., A 1989, 38, 325. (10) Levine, S.; Bowen, B.; Partridge, S. Colloids Surf., A 1989, 38, 345.

(θ): for θ up to 60° the interaction energy is unchanged, and from 60° to 120° it is significantly reduced. Repulsive dipole-dipole forces appear due to the asymmetric charge distribution that acts between particles, which are only charged on their surfaces immersed in the water phase.9,10 Dipole-dipole forces are ∼0.8kT and significantly less than electric double-layer forces.9,10 It was thus concluded10 that the dipole repulsion was negligible even for 1 µm polystyrene particles with a ζ potential of -80 mV. However, Pieranski11 noted that for charged particles at an interface between an aqueous phase and air, or another medium of low dielectric constant, electrostatic repulsion is enhanced. At the interface a particle (which is partially immersed in water) has an asymmetric counterion distribution, which results in a dipole moment normal to the water interface. Repulsive interactions between the effective dipoles due to neighboring particles occur through the phase of low dielectric constant, and repulsion through the aqueous phase is screened by the free ions in solution.11 Aveyard et al.12 reported that latex particle monolayers at the octane-water interface remain highly ordered as a result of long-ranged repulsion, even in concentrated electrolyte solution. Enhanced lateral repulsion between the latex particles was attributed to the existence of residual surface charges at the particle-octane interface. Levine et al.9 calculated that van der Waals forces are not significantly dependent on the contact angle at the oilwater interface in comparison to that operating within the bulk aqueous phase, particularly at larger particle separations. Williams and Berg13 investigated the adsorption and subsequent aggregation into small clusters of colloidal particles (advancing θ ) 102°, i.e., ∼40% immersion into (11) Pieranski, P. Phys. Rev. Lett. 1980, 45, 569. (12) Aveyard, R.; Clint, J.; Nees, D.; Paunov, V. Langmuir 2000, 16, 1969. (13) Williams, D. F.; Berg, J. C. J. Colloid Interface Sci. 1992, 152, 218.

10.1021/la0347197 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/16/2003

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water phase) at the air-aqueous electrolyte interface. Interestingly, although in principle dipole repulsion can enhance the aggregation stability of particles at interfaces, the surface aggregation commenced at salt concentrations up to 2 orders of magnitude less than those effective in the bulk. Strong dipole repulsion would have inhibited the observed clustering at the surface.12 These authors13 argue that enhanced interfacial aggregation is due to (i) an increased concentration of particles in the surface relative to the bulk and (ii) a more favorable interaction potential for aggregation between particles at the interface than in the bulk; i.e., electrostatic repulsion between particles decreases, and attractive dispersion forces are greater through the air phase due to the increased Hamaker constant.14 Horvologyi et al.6-8 have reported the effect of particle hydrophobicity and electrolyte concentration on the interaction and aggregation of surface-modified glass beads at the boundary layer of water/octane and air/octane phases. Increasing particle hydrophobicity increased aggregation at the water-air interface,6-7 whereas the opposite trend was observed for the water-octane interface.8 Screening of electrical double-layer repulsion in the case of a hydrophilic particle monolayer increases the structural strength, which is not necessarily the case for a hydrophobic particle monolayer. Weaker attraction between the less hydrophobic particles at the air-water interface has been attributed to the presence of a stable, thin water layer (repulsive solvation interaction). In contrast to the behavior at the air-water interface, less hydrophobic beads are in a primary minimum at the octane-water interface due to hydrophobic interaction through the water phase, whereas more hydrophobic beads are in a secondary minimum because weaker hydrophobic interactions act through the octane phase than through the water phase.8 In addition to the conventional interparticle forces encountered in 3-dimensional dispersions,12 capillary forces (for particles < 10 µm, these forces arise as a consequence of the interfacial deformations due to the particle wetting properties) are also important in interfacial particle arrangement.14 When particles are partially immersed into a liquid film at planar interfaces, the energy of capillary interaction can be significantly greater than kT, even for nanometer-sized particles. A general conclusion is that lateral immersion forces are strong enough to produce aggregation and ordering of submicrometer particles14 at planar interfaces. In Pickering emulsions, however, the force balance is fulfilled without any deformation. For a spherical interface, the force that pushes the particle outside the drop is in equilibrium with the force that pushes the particle inside the drop, so that a lateral capillary force between particles is not evident.14 It has been reported recently15 that charged colloidal particles trapped at the air-water interface experience long-ranged attractive forces, not accounted for by the standard theories of colloidal interactions. The proposed mechanism for this attraction is based on nonuniform wetting causing an irregularly shaped particle meniscus. For 1 µm diameter spheres at an interparticle distance of 2 µm, a 50 nm deviation from the ideal contact line results in an interaction energy of the order of 104kT.15 In addition to particle adsorption thermodynamics, the mobility of a particle-coated droplet interface will also play an important role in controlling the coalescence of (14) Kralchevsky, P.; Nagayama, K. Adv. Colloid Inerface Sci. 2000, 85, 145. (15) Stamou, D.; Duschl, C. Phys. Rev. E 2000, 62, 5263.

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Pickering emulsions. With this in mind, interfacial rheology is strongly dependent on colloidal particle loading,16-18 with a dramatic increase in interfacial rigidity observed as the solid domain fraction increases to 5060%.16 Furthermore, the structure and dynamics of colloidal particles at liquid-liquid interfaces is interdependently controlled by the shear flow and particle concentration.17,18 From the above considerations it is clear that there are many unanswered questions regarding the influence of colloidal forces on interfacial particle arrangements. Furthermore, relatively little is known concerning particle adsorption and adsorbed layer microstructure at the emulsion droplet-water interface, especially for submicron particles. Emulsion systems that are commonly available have poorly described interfacial areas, and as such, the interfacial adsorbed amount and adsorbed particle layer structure are difficult to quantify. Previously,19 we reported investigations of the adsorption of hydrophilic silica nanoparticles at poly(dimethylsiloxane) (PDMS) emulsion droplets and electron microscopy images of the corresponding particle layer microstructure. In this case, the range of the electrical double-layer force governed the interfacial packing, and densely packed adsorbed monolayers are only observed when the double-layer thickness of both particles and droplets is a few nanometers. In the current investigation we investigate the adsorption behavior and layer structure of hydrophobic silica nanoparticles at the surface of PDMS emulsion droplets. The influence of solution conditions was assessed to determine the role of the magnitude and range of particledroplet and particle-particle interactions, and the influence of PDMS cross-linking was investigated to address the influence of droplet deformability and interfacial penetrability. Experimental Section Materials. High-purity water (Milli-Q) was used throughout the study. Diethoxydimethylsilane (DEDMS) and triethoxymethylsilane (TEMS) were supplied by Aldrich (Milwaukee, WI) and redistilled under nitrogen prior to use. Ammonia (Aldrich), KNO3, NaCl (Merck, Darmstadt, Germany), and other reagents used were analytical grade. Fumed silica particles (Aerosil R 974) were kindly supplied from Degussa and are reported20 to have a BET surface area of 170 ( 20 m2 g-1, 0.39 Si-OH groups nm-2, and an average primary particle size of 12 nm. Particle contact angles estimated from enthalpy of immersion data are reported21 to be 117 ( 4° (water/air) and 75° (toluene/water). Methods. a. Preparation and Characterization of Silica Dispersions. The hydrophobic silica particles were dispersed in aqueous solution using an ultrasonic bath (300 W) for ∼5 h. The pH was controlled by small additions of nitric acid or sodium hydroxide solutions. Dynamic light scattering (Brookhaven Instruments) showed the presence of particles in the range of 20-120 nm with a mean diameter of 52 ( 0.5 nm. That is, the particles are stable, porous clusters and retain their organic coating in aqueous solution. Freeze-fracture scanning electron microscopy (SEM) confirmed the particle size and the high level of particle dispersion in water (see Figure 1). A small degree of distortion in the cryo-SEM image is considered to be due to the (16) Ding, J.; Warriner, H. E.; Zasadzinski, J. A. Phys. Rev. Lett. 2002, 88, 168102-1. (17) Stancik, E. J.; Widenbrant, M. J. O.; Laschitsch, A. T.; Vermant, J.; Fuller, G. G. Langmuir 2002, 18, 4372. (18) Stancik, E. J.; Gavranovic, G. T.; Widenbrant, M. J. O.; Laschitsch, A. T.; Vermant, J.; Fuller, G. G. Faraday Discuss. 2003, 123, 145. (19) Simovic, S.; Prestidge, C. A. Langmuir 2003, 19, 3785. (20) Technical Bulletin Pigments. Degussa-Huls 1994, 18, 5. (21) Yan, N.; Maham, Y.; Masliyah, J. H.; Gray, M. R.; Mather, A. E. J. Colloid Interface Sci. 2000, 228, 1.

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Figure 1. Freeze-fracture SEM image of an aqueous hydrophobic silica nanoparticle dispersion in water (0.25 wt %).

Figure 3. Adsorption kinetics for hydrophobic silica nanoparticles at (O) liquid and (0) cross-linked PDMS droplets (pH ) 9, 10-5 M NaCl) at (a) sub-monolayer coverage and (b) multilayer coverage. An equivalent hexagonally close-packed monolayer of 50 nm diameter silica hard spheres corresponds to ∼200 mg m-2. Figure 2. ζ potential vs pH data (0.25 vol % emulsion droplets in water) for liquid PDMS droplets [(O) 10-4 and (b) 10-3 M NaCl], cross-linked PDMS droplets [(0) 10-4 and (9) 10-3 M NaCl], and hydrophobic silica nanoparticles (0.25 wt % in water) [(]) 10-4 and ([) 10-3 M NaCl]. flow of cold air, causing the specimen block to vibrate, and the thermal movement of the sample during imaging. Further studies have confirmed that particle size distributions are independent of particle concentration and pH over the range of conditions employed in this investigation. Electrophoretic mobilities were determined using phase analysis light scattering (PALS) and converted to ζ potentials using the Smoluchowski equation, which is reasonable for fumed silica oxides if ζ < 50 mV and κa .122 (κ is the reciprocal Debye length and a is the particle diameter). ζ potential vs pH data are given in Figure 2. Hydrophobization has minimal influence on the ζ potential of the silica nanoparticles, which is in accord with previous reports.22,23 Critical coagulation concentrations (ccc) for 0.25 wt % aqueous dispersions of hydrophobic silica, determined using a turbidimetric method,24 are as follows: 5 × 10-4 M NaCl at pH 4, 10-3 M NaCl at pH 5, 10-2 M NaCl at pH 7, and 10-1 M NaCl at pH 9 (the standard error was estimated to be (2 × 10-5 M NaCl). These values are significantly lower than those determined for equivalent hydrophilic silica.19 b. Preparation and Characterization of PDMS Droplets. PDMS in water emulsions were prepared through the basecatalyzed polymerization of DEDMS using a modified method of that reported by Obey and Vincent25 and Goller et al.26 A more detailed preparation procedure and characterization are given elsewhere.19 Liquid PDMS droplets were prepared in the absence of TEMS, and cross-linked droplets prepared by the inclusion of 50% TEMS. Drop size distributions were characterized by laser diffraction (Malvern Mastersizer X). Average drop sizes and the size span [defined as (d(v,0.9) - d(v,0.1))/d(v,0.5)] are 1.25 µm and 0.56 for the liquid droplets and 1.05 µm and 1.2 for the (22) Gun’ko, V. M.; Zarko, V. I.; Leboda, R.; Chibowski, E. Adv. Colloid Interface Sci. 2001, 91 1. (23) Xu, Z.; Yoon, R. J. Colloid Interface Sci. 1989, 132, 532. (24) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 1999, 1, 3007. (25) Obey, T. M.; Vincent, B. J. Colloid Interface Sci. 1994, 163, 454. (26) Goller, M. I.; Obey, T. M.; Declan, O. H. T.; Vincent, B.; Wegener, M. R. Colloids Surf., A 1997, 123, 183.

cross-linked droplets. These emulsion samples are considerably more monodisperse than typical o/w or w/o emulsions prepared by homogenization and are stabilized exclusively by charge development due to the dissociation of surface silanol groups. The ζ potentials reported in Figure 2 are in agreement with previous reports on similar droplets.27 c. Adsorption Isotherms. Known volumes and concentrations of aqueous silica sols and PDMS droplets (of known interfacial area) were combined under mixing in 20 mL reaction vials. The vials were sealed and equilibrated by tumbling at 30 rpm for 20 h at 22 °C. For a surface coverage less than an equivalent close-packed monolayer, full adsorption equilibrium is established within ∼3 h (Figure 3a), whereas for multilayer surface coverages pseudo-equilibrium is reached after ∼15 h (Figure 3b). (N.B.: An equivalent hexagonally close-packed monolayer of 50 nm diameter silica hard spheres corresponds to ∼200 mg m-2 for both liquid and cross-linked PDMS droplets; hence, surface coverage can be estimated.) These differences in adsorption kinetics reflect structural constraints at the dropletwater interface, which is also influenced by droplet cross-linking. Upon attaining adsorption equilibrium, 5 mL aliquots were removed and filtered through a hydrophilic membrane filter with pore size 0.45 µm (MF-Millipore membrane filter) and analyzed spectrophotometrically at λ ) 250 nm for silica particle content. This filtration method removes PDMS droplets, but not the silica particles. Equilibrium silica particle concentrations were determined from previously constructed calibration curves (absorbance at λ ) 250 nm vs concentration). Silica particle adsorbed amounts (determined in triplicate) were calculated from the ratio of the adsorbed concentration (initial particle concentration equilibrium particle concentration) and the PDMS interfacial area. Errors in the adsorbed amount values were less than (10%. d. Scanning Electron Microscopy (SEM). A freeze-fracture SEM technique (Philips XL 30 FEG scanning electron microscope with Oxford CT 1500 cryotransfer system) was used to image PDMS droplets with adsorbed particles according to the method previously described.19 The methodology consisted of emulsion cryofixation, fracturing, etching, platinum coating, and imaging. It should be noted that the precise method required to effectively image a droplet is highly dependent on the sample’s properties, e.g., particle type and coverage. In general, emulsion samples (50 µL) were deposited on a flat copper substrate holder and (27) Barnes, T. J.; Prestidge, C. A. Langmuir 2000, 16, 4116.

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cryofixed by rapid cooling with liquid nitrogen (-196 °C) in order to reach the vitreous state. Frozen samples were then mounted on a cold table still under liquid nitrogen and then inserted into the freeze-fracture equipment at -150 °C and 10-6 Torr. A singleedge scalpel blade precooled to -150 °C is then used to induce the fracture. Surface ice is then removed during a sublimation step, i.e., by increasing the sample temperature to either -92 or -100 °C for a period of 2-5 min. Great care is necessary during this step in order to avoid droplet evaporation and disintegration. The fractured and etched sample was then sputter-coated with platinum (∼2 nm), prior to SEM imaging.

Results and Discussion Particle Adsorption Isotherms. a. Influence of pH. Adsorption isotherms for hydrophobic silica particles at liquid and cross-linked PDMS droplets from aqueous solution containing 10-5 and 10-4 M electrolyte are presented in parts a and b of Figure 4, respectively. For liquid PDMS droplets, the isotherms are sigmoidal,28,29 whereas low-affinity-type isotherms28,29 are observed for cross-linked droplets. Low-affinity isotherms are rare for adsorption from solution and typical when adsorption affinity increases during adsorption;28,29 in this case it is considered to be a consequence of increased hydrophobic driving force during the adsorption process. It has also been reported30 that strong lateral attraction between particles may result in this isotherm type, and hydrophobic attraction through the water phase is the driving force in the current system. Given that an equivalent hexagonally close-packed monolayer of 50 nm diameter silica hard spheres corresponds to ∼200 mg m-2, the hydrophobic nanoparticles adsorb at both liquid and cross-linked droplets to in excess of an equivalent monolayer. Colloidal clay particles (with varying contact angles) have also been reported31-33 to adsorb at the o/w emulsion interface with equivalent sigmoidal isotherms to those observed here for the liquidlike PDMS droplet interface. In these previous studies31-33 adsorption increased with particle hydrophobicity, and the minimum oil droplet diameter and most efficient barrier for droplet coalescence were exhibited for a clay particle θ of ∼65°. Adsorption isotherms for liquidlike PDMS droplets were fitted with a confidence level > 0.95 by the BET adsorption model:28,33

Cs/Cm ) K1Cw/(1 - KmCw)(1 - KmCw + K1Cw) (1) where Cs and Cw are the equilibrium concentrations of silica particles at the oil droplet surface and in the aqueous phase, respectively, Cm is the equivalent monolayer coverage of silica particles at the PDMS droplet surface, and K1 and Km are the equilibrium constants for the first layer and subsequent layers, respectively. Linearized BET plots are given in Figure 4c, and the main fitting parameters are listed in Table 1. When Km equals zero, only a monolayer is formed, and eq 1 is equivalent to the Langmuir isotherm, which describes adsorption of hydrophilic nanoparticles at the PDMS-water interface.19 In contrast, the BET adsorption model was unable to describe the adsorption isotherms for the cross-linked droplets with acceptable confidence levels. (28) Shaw, J. D. In Colloid and Surface Chemistry; ButterworthHeinemann, Ltd.: Woburn, MA, 1992; p 128. (29) Dabrowski, A. Adv. Colloid Interface Sci. 2001, 93, 135. (30) Harley, S.; Thompson, D. W.; Vincent, B. Colloids Surf., A 1992, 62, 163. (31) Yan, N.; Gray, M. R.; Masliyah, J. H. Colloids Surf., A 2001, 193, 97. (32) Yan, Y.; Masliyah, J. H. Colloids Surf., A 1995, 96, 229. (33) Yan, Y.; Masliyah J. H. J. Colloid Interface Sci. 1994, 168, 386.

Figure 4. Adsorption isotherms for hydrophobic silica and (a) liquid (b) cross-linked PDMS droplets at pH: ], 9; 4, 7; 0, 5 (10-5 M NaCl); O, 4 (10-4 M NaCl); (c) data from (a) in linearized BET isotherm (see eq 1) form.

The adsorption parameters can be rationalized in terms of particle-droplet and particle-particle lateral interactions. Previously,19 we confirmed that particle-droplet interactions do not control the surface coverage of hydrophilic silica nanoparticles at PDMS droplets; i.e., the energy barrier for particle-droplet coagulation was only 3kT-5kT. For the hydrophobic nanoparticles under investigation here, an additional hydrophobic dropletparticle attractive force is anticipated to further reduce the barrier for adsorption; hence, lateral particle-particle interactions govern the adsorption process and influence the adsorption parameters. Over a wide range of pH and salt concentrations the values of Cm (see Table 1) correspond to adsorbed sizes greater than the effective particle size (hard sphere + double layer thickness). This confirms that lateral particle-particle hydrophobic interactions play an important role in particle packing. Free energies of adsorption (∆Gads) are in the range -18 to -25 kJ mol-1 and concordant with a physical adsorption mechanism.28 For liquidlike PDMS droplets, formation of the initial adsorbed monolayer is thermodynamically more favorable than for multilayer adsorption. The adsorption behavior of hydrophobic silica nanoparticles is highly contrasting to that for hydrophilic silica nanoparticles at the same PDMS droplets.19 Pseudoplateau surface coverage for hydrophobic silica corre-

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Table 1. Parameters for Adsorption of Hydrophobic Silica Nanoparticles at the Liquid PDMS Droplet-Water Interface (BET Model) pH

NaCl (M)

Cm (mg m-2)a

area per particle (×10-15 m2)b

K1a

Kma

∆G°ads (kJ mol-1), first layer

∆G°ads (kJ mol-1), multilayer

4 ( 0.1 5 ( 0.1 7 ( 0.1 9 ( 0.1 9 ( 0.1 9 ( 0.1 9 ( 0.1

10-4 10-5 10-5 10-5 10-4 10-3 10-2

147 118 90 82 115 199 198

3.55 4.43 5.81 6.37 4.55 2.64 2.64

1789 1706 650 617 1460 29630 29550

223 202 141 123 140 550 553

-18.4 -18.4 -16 -15.9 -17.9 -25.5 -25.5

-13.4 -13.1 -12.2 -11.9 -12.2 -15.6 -15.6

a Coefficient of variation was estimated 100 nm) hydrophobic forces may become apparent but are unlikely for particles with contact angles in the range 40°-70°.34 Cm decreases with pH, and ∆Gads values for both the initial monolayer and subsequent adsorption are significantly influenced by pH, i.e., on reduction in the magnitude of particle-droplet and particle-particle electrostatic repulsion (see Table 1). Under the same conditions, hydrophilic silica particle adsorption showed “non-DLVO” adsorption behavior, i.e., pH-independent adsorption, which was attributed to repulsive hydration forces that are more pronounced at low pH.19 Hydrophobization of the silica surfaces eliminated the hydration forces;35 they cannot therefore play a role in the adsorption of hydrophobic nanoparticles and introduced a hydrophobic attraction that reduced the overall repulsion. The pHdependent (“DLVO-like”) adsorption behavior can thus be rationalized. In a similar manner, Menon et al.36 reported dramatic variations in the particle interaction energy at a planar oil-water interface with ζ potential variation, and Yan and Maslyah37 reported pH-dependent particle adsorption at oil droplet surfaces. b. Influence of Salt Addition and Droplet CrossLinking Level. Upon increasing the salt concentration from 10-5 or 10-4 to 10-3 M (see Figure 5), we observe a dramatic increase in particle interfacial packing (with a corresponding decrease in the adsorbed interfacial area per particle) and in the other adsorption parameters for liquid droplets (see Table 1). At 10-3 M NaCl the Cm value corresponds to hexagonally close-packed hard spheres. ∆Gads for both the initial adsorbed particle monolayer and subsequent adsorption are more negative on increasing the salt concentration to 10-3 M, but little influenced by further salt addition. This confirms that interfacial packing reaches a saturation level at 10-3 M NaCl, which is significantly lower than the ccc for hydrophobic silica (0.1 M NaCl at pH ) 9). The fact that interfacial saturation (34) Christensoson, H. K.; Claesson, P. M.; Berg, J.; Herder P. C. J. Phys. Chem. 1989, 93, 1472. (35) Grabbe, A. Langmuir 1993, 9, 797. (36) Menon, V. B.; Nikolov, A. D.; Wasan, D. T. J. Colloid Interface Sci. 1988, 124, 317. (37) Yan, Y.; Masliyah, J. H. J. Colloid Interface Sci. 1996, 181, 20.

Figure 5. Adsorption isotherms for hydrophobic silica and (a) liquid and (b) cross-linked PDMS droplets at pH ) 9 and in the presence of ([)10-4, (9) 10-3, and (2) 10-2 M NaCl.

is reached at the same salt concentration for both liquid (penetrable) (Figure 5a) and cross-linked (nonpenetrable) (Figure 5b) droplets is indicative that hydrophobic particle attraction plays a decisive role in interfacial particle packing and occurs through the water phase, not through the oil phase. In further considering the influence of droplet cross-linking on hydrophobic silica adsorption (see Figures 4 and 5), we can identify a clear difference in adsorption isotherm shape, with significantly greater adsorbed amounts for liquid droplets. This is considered to be a consequence of deeper particle penetration and closer packing for the liquid droplets, i.e., geometrical packing restriction.38 That is, the number of particles that can be accommodated at the interface is largely dependent upon their contact angle, and closer packing is facilitated for deeper penetration into the droplets.38 The hydrophobic attraction through the water phase may also play an important role in describing particle interfacial arrangements. Given the irregular shape of the particle clusters and their considerable surface roughness, nonuniform wetting may occur and cause irregularly shaped particle menisci. In similar situations, (38) Thompson, A. P.; Corti, D. S.; Myers, A. L.; Glandt, E. D. Proc. IVth Int. Conf. Fundam. Adsorpt. 1992, 671.

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Figure 6. Freeze-fracture SEM images of hydrophobic silica at liquid PDMS droplets (pH ) 9; 10-4 M NaCl) at multilayer coverage.

Figure 7. Freeze-fracture SEM images of liquid PDMS droplets with a multilayer coverage of hydrophobic silica adsorbed at pH ) 9 and in the presence of (a, b) 10-3 M NaCl and (c, d) 10-2 M NaCl.

long-ranged lateral attraction has been suggested,15 the strength of which scales with the fourth power of the particle radius, and calculations have shown them to be important down to the nanoscopic particle size range.15 Further work is clearly required to address the importance of such forces in nanoparticle-stabilized emulsions. Adsorbed Particle Layer Microstructure. In the absence of adsorbed particles, PDMS droplets tend to evaporate during freeze-fracture SEM and are observed as imprints (holes) in the vitrified ice surface. However, in the presence of adsorbed silica nanoparticles, the evaporation of PDMS droplets is significantly reduced. The sublimation step is especially critical for the production of representative images of high quality and needs careful control in terms of time and temperature. We have demonstrated that PDMS droplets undergo deformation at low surface coverages of hydrophilic silica nanoparticles

but are resistant to evaporation (sublimation at 100 °C for 5 min), whereas at high particle coverages the droplets are resistant to both deformation and evaporation (sublimation at 92 °C for 2 min).19 In the presence of adsorbed hydrophobic silica nanoparticles, the evaporation and disintegration of PDMS droplets during freeze fracture SEM are also prevented, particularly at multilayer coverages. Hence, useful information regarding the adsorbed particle layer structure may be obtained. Freezefracture SEM images of liquid PDMS droplets with hydrophobic nanoparticles adsorbed from a 10-4 M NaCl solution are presented in Figure 6. It is apparent that the nanoparticles are highly wetted by the PDMS phase and penetrate into the droplets. Some evidence of particle spacing can be observed in the high-magnification image, and this is presumed to reflect double-layer repulsion. (It is important to note that nonadsorbed particles in the

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Figure 8. Freeze-fracture SEM images of liquid PDMS droplets with hydrophobic silica adsorbed at pH ) 9 and in the presence 10-1 M NaCl.

Figure 9. Freeze-fracture SEM images of liquid PDMS droplets with hydrophobic silica adsorbed at pH ) 5 and in the presence 10-3 M NaCl.

bulk water phase are highly dispersed.) These droplet images are significantly different from those in the presence of adsorbed hydrophilic particles,19 where limited particle wetting occurred, and the nanoparticles were clearly visible at the droplet surface. Hydrophobic nanoparticle-coated droplets prepared from both 10-3 and 10-2 M NaCl solutions (see Figure 7) are highly rigidified with some evidence of close particle packing at the interface. The interfacial structure is indicative of particle-particle attraction and extensive particle penetrated into the droplets. Droplet images are independent of salt concentration in the 10-3-10-2 M range and in accord with the adsorption isotherms (see Figure 5a); i.e., the interface is saturated at 10-3 M NaCl. The tendency of hydrophobic particles to “flocculate” at the droplet interface was not observed for hydrophilic silica particles.19 In the presence of 10-1 M NaCl at pH ) 9 (see Figure 8) and 10-3 M NaCl at pH ) 5 (see Figure 9), i.e., at the ccc for hydrophobic silica particles in bulk solution, PDMS droplets are enclosed with a rigid multilayer of particles. Under these solution conditions a range of aggregated silica particle structures are observed in the bulk solution, and these may directly adsorb at the droplet surfaces to produce thick particle encapsulating walls. The level of interfacial penetration by silica aggregates and adsorption from solution are impossible to quantify under such conditions; however, the incidence of multilayer coatings suggests relatively high adsorbed amounts. From previous reports,39-42 the onset of particle coagulation in solution is regarded as an enhancer for emulsion stability, whereas intensive flocculation destabilizes emulsions. This phenomenon is attributed to an increase in the attachment

energy for larger particles. Further work is clearly required to determine how different interfacial arrangements of adsorbed nanoparticles influence the flocculation and coalescence of droplets.

(39) Ashby, N. P.; Binks, B. P. Phys. Chem. Chem. Phys. 2000, 2, 5640.

Conclusions The adsorption behavior and adsorbed layer microstructure of hydrophobic silica particles (diameter ∼50 nm) at the PDMS droplet-water interface have been determined. Over a wide range of pH and salt concentrations, a balance of hydrophobic and electrostatic forces controls hydrophobic particle adsorption. Sigmoidal isotherms are observed for liquid droplets and “low-affinity” isotherms for cross-linked droplets, and in both cases multilayer surface coverages may form. Decreasing the magnitude of droplet-particle and particle-particle electrostatic repulsion (i.e., pH decrease) enhances particle adsorption. Salt addition dramatically increases particle adsorption. Interfacial saturation of both liquid and crosslinked droplets occurred at salt concentrations 2 orders of magnitude less than the ccc of silica in bulk aqueous solution; i.e., particle layer microstructuring is governed by particle attraction through the water phase. Freezefracture SEM confirmed interfacial rigidification below the ccc and visualized thick interfacial particle walls above the ccc. Acknowledgment. The Australian Research Council’s Special Research Centre for Particle and Material Interfaces is acknowledged for funding. Dr. Peter Self is thanked for assistance with the SEM investigations and analysis. LA0347197 (40) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 1999, 1, 3007. (41) Midmore, B. R. Colloids Surf., A 1998, 132, 257. (42) Midmore, B. R. Colloids Surf., A 1998, 145, 133.