Hydrophilic Silica Nanoparticles at the PDMS Droplet−Water Interface

Mar 29, 2003 - Particle-stabilized emulsions are encountered in many areas such as enhanced oil recovery, food, pharmaceutics, and cosmetics. ..... La...
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Langmuir 2003, 19, 3785-3792

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Hydrophilic 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 November 5, 2002. In Final Form: January 30, 2003 The adsorption behavior of hydrophilic silica nanoparticles (fumed Aerosil 380) at the polydimethylsiloxane (PDMS) droplet-water interface has been investigated through particle adsorption isotherms, with complementary studies of the adsorbed layer structure by freeze-fracture SEM. The influence of solution conditions (pH and electrolyte concentration) has indicated the magnitude of particle-droplet and particleparticle interactions, and the influence of droplet cross-linking (deformability) indicated the role of particle penetration at the droplet-water interface. The silica particles adsorb onto PDMS droplets with plateau surface coverages that correspond to their effective particle size (hard sphere radius + double-layer thickness), i.e., lateral silica-silica interaction controls particle packing. Free energies of adsorption (∆Gads) are in the range -15 to -23 kJ mol-1 and concordant with a physical adsorption mechanism. The plateau surface coverages ∆Gads and particle packing at the interface are only weakly influenced by pH, but significantly influenced by salt addition. Droplet cross-linking results in a reduction in particle adsorption, but only at higher salt concentrations: this was attributed to the increased likelihood of silica particles wetting PDMS and interfacial penetration. Freeze-fracture SEM revealed that in the low-salt regime individual silica particles are adsorbed at the droplet interface with negligible interfacial aggregation. Densely packed adsorbed particle layers are only observed when the double-layer thickness is reduced to a few nanometers, and even in the presence of a closely packed particle layer, droplets are not resistant to coalescence by excess salt. These findings lead to an improved understanding of particle adsorption to stabilize emulsion droplets.

Introduction Particle-stabilized emulsions are encountered in many areas such as enhanced oil recovery, food, pharmaceutics, and cosmetics. Various particle types have been used as emulsion stabilizers, e.g., BaSO4, crystalline ferric oxide, carbon black, bentonite, kaolinite clay, latex, and silica particles.1-11 The effectiveness of particles in stabilizing emulsions depends on their particle size, wettability, and initial location, as well as on the level of interparticle interactions.2-6,11,12 The stabilizing effectiveness is often rationalized in terms of the attachment energy E,7 i.e., the energy to expel the particles from the interface into one of the bulk phases. For a spherical particle of radius R, at a planar oil-water (o/w) interface of interfacial tension γow, with θ the contact angle the particle makes at the interface (measured through the water phase), the attachment energy is

E ) πR2γow(1 + cos θ)2

(1)

The sign within the bracket becomes negative for particle * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: +81 8 83023569. Fax: +61-8-8302368. (1) Abend, S.; Bonnke, N.; Gutschner, U. Lagaly, G. Colloid Polym. Sci. 1998, 276, 730. (2) Ashby, N. P.; Binks, B. P. Phys. Chem. Chem. Phys. 2000, 2, 5640. (3) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 1999, 1, 3007. (4) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 2000, 2, 2959. (5) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 2539. (6) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 3748. (7) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622. (8) Midmore, B. R. Colloids Surf., A 1998, 132, 257. (9) Midmore, B. R. J. Coll. Interface Sci. 1999, 213, 352. (10) Binks, B. P.; Lumsdon, S. O. Langmuir 2001, 17, 4540.

removal into the water phase. It follows that a particle is most strongly held at the interface for θ ) 90°, is more easily removed into oil for θ > 90°, and more easily removed into water for θ < 90°. It is generally accepted that particle wettability, which is often expressed as a contact angle at the three-phase boundary, determines both the stability and type of particle-stabilized emulsions. Hydrophilic solids with a contact angle slightly less than 90° stabilize o/w emulsions, while hydrophobic solids with a contact angle slightly larger than 90° stabilize water-oil (w/o) emulsions.7,11,12 For strongly hydrophilic particles, a large fraction of their volume resides in the water phase, so they cannot provide a sufficient barrier for droplet coalescence and emulsions are unstable. Conversely, for strongly hydrophobic particles, a large number of the particles will remain in the oil phase, resulting in less protection for water droplet coalescence and poor emulsion stability. Particles of intermediate hydrophobicity position themselves at the oil-water interface such that they provide maximum protection against coalescence. Thus, the concept of “contact angle for solid particles” is equivalent to “HLB for surfactants”.7,13 With these thoughts in mind, Binks and Lumsdon7 reported that the emulsion type, preferred drop sizes and emulsion stability (creaming and coalescence) are critically dependent on particle hydrophobicity. Particles of intermediate hydrophobicity yielded the most stable emulsions with the smallest drop sizes. In addition, the phase in which the particles reside prior to emulsi(11) Tadros, Th. F.; Vincent, B. In Encyclopedia of Emulsion Technology, Basic Theory; Marcel Dekker: New York, 1983; Vol.1, p 129. (12) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1993, 15, 244. (13) Yan, N.; Gray, M. R.; Masliyah, J. H. Colloids Surf., A 2001, 193, 97.

10.1021/la026803c CCC: $25.00 © 2003 American Chemical Society Published on Web 03/29/2003

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fication is important. To prepare w/o emulsions, hydrophobic particles have to be in the oil phase prior to emulsification.13 Yan and Masliyah14 have observed that clay particles partition at the oil-water interface and remain within the phase in which they are first dispersed. Many reports2,3,8,15 indicate that weak particle aggregation enhances emulsion stability, whereas intensive flocculation destabilizes emulsions. This effect was attributed to enhancement of the attachment energy with increasing particle size (eq 1) and was demonstrated for kaolinite clay particles flocculated by electrolytes.2 Furthermore, stable o/w emulsions based on hydrophilic silica particles can only be formed if silica is weakly flocculated (e.g., by polymers or surfactants).8,15,16 Binks and Lumsdon3 examined the effect of flocculating hydrophilic silica with electrolytes on emulsion stability. In the presence of NaCl, emulsions are less stable once the particles are flocculated and flocs either prevent the formation of emulsions or act to break them once formed. On the contrary, in the presence of lanthane chloride or tetraethylammonium bromide, both of which increase the hydrophobicity of particles, emulsion stability increases dramatically for conditions where silica is weakly flocculated. 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 prevents droplet coalescence, i.e., the ability of particles to form rigid structures that can sterically inhibit droplet coalescence. If charged, particles can give rise to electrostatic repulsion, which further enhances emulsion stability.11,12 Yan and Maslyah14,17,18 investigated the adsorption of colloidal clay particles with varying contact angles (from 40 to 143°) at the interface of o/w emulsions. Adsorption increased with particle hydrophobicity and adsorbed amounts equivalent to multilayers were observed. The minimum oil droplet diameter and most efficient barrier for droplet coalescence were exhibited at a clay particle θ of ∼65°. Of further note, Menon et al.19 developed an experimental technique to measure the film tension and particle interaction energy at a planar oil-water interface and reported dramatic variations in the particle interaction energy with zeta potential variation (pH controlled). It is clear from the above review that the thermodynamics of emulsion stability induced through adsorbed particles is well understood; however, relatively little is known concerning particle adsorption at the dropletwater interface or the adsorbed layer structure, especially for submicron particles. 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 oilwater interface.20,21 Further work in understanding the interfacial interactions in particle-stabilized emulsions is clearly warranted, in particular, quantitative assessment of the adsorbed particle amount, adsorbed layer microstructure, and influence of droplet penetrability on (14) Yan, Y.; Masliyah, J. H. Colloids Surf., A 1995, 96, 229. (15) Midmore, B. R. Colloids Surf., A 1998, 145, 133. (16) Hassander, H.; Johansson, B.; Tornell, B. Colloids Surf., A 1989, 40, 93. (17) Yan, Y.; Masliyah J. H. J. Colloid Interface Sci. 1994, 168, 386. (18) Yan, Y.; Masliyah, J. H. J. Colloid Interface Sci. 1996, 181, 20. (19) Menon, V. B.; Nikolov, A. D.; Wasan, D. T. J. Colloid Interface Sci. 1988, 124, 317. (20) Horvolgyi, Z.; Mate, M.; Zrinyi, M. Colloids Surf., A 1994, 84, 207. (21) Horvolgyi, Z.; Nemeth, S.; Fendler, J. H. Colloids Surf., A 1993, 71, 327.

Simovic and Prestidge

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

the structure of the adsorbed layers. Emulsion systems that are commonly available do not enable the interfacial adsorbed amount to be quantitatively determined, due to their poorly described interfacial area. Experimental techniques for characterizing particles at droplet interfaces are also lacking. Colloidal polydimethylsiloxane (PDMS) droplets22-24 may be synthesized that are highly monodisperse, stable in the absence of added stabilizers and therefore have well characterized interfacial areas: these are an appropriate model system for particle adsorption investigations. Furthermore, their internal cross-linking level can be controlled,22-24 producing either “liquidlike” or viscoelastic droplets. The present study is concerned with the adsorption behavior and layer structure (determined by freeze-fracture SEM) of hydrophilic silica particles at the surface of PDMS emulsion droplets. Variation of solution conditions (pH and salt concentration) has enabled the magnitude and range of particle-droplet and particle-particle interactions to be probed, and PDMS cross-linking level variation enabled droplet deformability and interfacial penetrability to be addressed. 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 380) were kindly supplied from Degussa and are reported25 to have a BET surface area of 380 ( 30 m2 g-1, 2.5 Si-OH groups nm-2 (determined from Li-Al-hydride method), and an average primary particle size of 7 nm. Contact angles estimated from enthalpy of immersion data are reported26 to be 14° (water/air) and 0° (toluene/water). Methods. Preparation and Characterization of Silica Dispersions. Silica particles were dispersed in aqueous solution using an ultrasonic bath (300W for 2 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 20-110 nm with a mean diameter of ∼50 nm. Freeze-fracture SEM (details follow) see Figure 1, confirmed the particle size and the high level of dispersion. (A small degree of distortion in the cryo-SEM image is considered to be due to the flow of cold air causing the specimen block to vibrate, and the thermal movement of the sample during imaging; our best attempts were made to minimize the distortion.) (22) Obey, T. M.; Vincent, B. J. Colloid Interface Sci. 1994, 163, 454. (23) Goller, M. I.; Obey, T. M.; Declan, O. H. T.; Vincent, B.; Wegener, M. R. Colloids Surf., A 1997, 123, 183. (24) Barnes, T. J.; Prestidge, C. A. Langmuir 2000, 16, 4116. (25) Technical Bulletin Pigments. Degussa-Huls 1994, 18, 5. (26) Yan, N.; Maham, Y.; Masliyah, J. H.; Gray, M. R.; Mather, A. E. J. Colloid Interface Sci. 2000, 228, 1.

Hydrophilic Silica Nanoparticles

Figure 2. Zeta potential vs pH for hydrophilic silica particles (0.25 wt % aqueous dispersions) determined by PALS at NaCl concentrations: ([)10-4 M, (b) 10-3 M, and (2) 10-2 M.

Figure 3. Zeta potentials of PDMS droplets as a function of pH (0.25 vol % emulsion droplets in water): (O) liquid droplets 10-4 M NaCl, (b) liquid droplets 10-3 M NaCl, (0) cross-linked 10-4 M NaCl, (9) cross-linked 10-3 M NaCl. As would be expected for fumed silica, the particle size is greater than the primary size quoted by the manufacturer.25 Further studies have confirmed that particle size distributions are independent of particle concentration, salt 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 if ζ < 50 mV and κa . 127 (κ is the reciprocal Debye length parameter, and a is the particle diameter). Zeta potential versus pH data are given in Figure 2 and are in accord with previous reports.27,28 Preparation and Characterization of PDMS Droplets. PDMS in water emulsions were prepared through the base catalized polymerization of DEDMS using a modified method of that reported by Obey and Vincent22 and Goller et al.23 Aqueous solutions containing 1% DEDMS and 10% ammonia were sealed in a 250 mL reaction vessel, shaken vigorously for 30s, and then tumbled at 30 rpm and 25 °C for 18 h. These droplets are composed of low-molecular-weight cyclic and linear PDMS, having a viscosity of ∼10 mN m-2 s, and are termed “liquid”. A further batch of cross-linked PDMS droplets were prepared using a 1:1 mixture of DEDMS and TEMS as the monomer and have been shown to be substantially viscoelastic.23,24,29 Drop-size distributions were characterized by laser diffraction (Malvern Mastersizer X). Average drop sizes and 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 cross-linked droplets. These emulsion samples are considerably more monodisperse than typical o/w or w/o emulsions prepared by homogenization. Electrophoretic mobilities and hence ζ potentials were determined using a combination of microelectrophoresis (Rank Bros, Mark H) and PALS; ζ potentials (Figure 3) are in agreement with previous reports on equivalent droplets.24 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 18 h at 22 °C. Upon attaining adsorption equilibrium, aliquots (5 mL) were removed using a syringe, filtered through a hydrophilic (27) Gun’ko, V. M.; Zarko, V. I.; Leboda, R.; Chibowski, E. Adv. Colloid Interface Sci. 2001, 91, 1. (28) Xu, Z.; Yoon, R. J. Colloid Interface Sci. 1989, 132, 532. (29) Gillies, G.; Prestidge, C. A.; Attard, P. Langmuir 2002, 18, 1674.

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Figure 4. Adsorption isotherms for hydrophilic silica and liquid PDMS droplets (10-4 M NaCl) at pH: ([) 9, (2) 7, (9) 5, (b) 4. membrane filter with pore size 0.45 µm (MF-Millipore Membrane Filter) and analyzed spectrophotometrically at λ ) 250 nm for silica particle content. We have shown that this filtration method removes PDMS droplets, but not the silica particles. Equilibrium silica (unadsorbed) 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 concentrated-equilibrium particle concentrated) and the PDMS droplet interfacial area. Errors in the adsorbed amount values were less than (10%. 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. The methodology consisted of emulsion cryofixation, fracturing, etching, platinum coating, and imaging.30,31 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 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 single-edge 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. Influence of pH. Adsorption isotherms for hydrophilic silica particles at liquid PDMS droplets from aqueous solution containing 10-4 M NaCl are presented in Figure 4 (in the interest of brevity, data for cross-linked droplets are not shown). Irrespective of droplet cross-linking, the isotherms are of intermediate affinity (typical for physical adsorption from solution) and the plateau adsorbed amount corresponds to ∼15% of an equivalent hexagonally close-packed monolayer of hard spheres (calculated to be 200 mg m-2 for 50 nm diameter silica particles). Fractional surface coverage is calculated from the ratio of the plateau adsorbed amount and the theoretical value for a hexagonally close packed monolayer. On increasing the NaCl concentration to 10-3 M the isotherm plateau values increase (see Figure 5) and correspond to ∼30% of an equivalent hexagonally close-packed monolayer of hard spheres. The isotherms are independent of pH and droplet cross-linking. Physical adsorption (physisorption) is a reversible process17 and involves intermolecular forces, such as van (30) Sawyer, L.; Grubb, D. In Polymer Microscopy: Chapman and Hall: London, 1987; p 76. (31) Robbe-Tomine, L.; Hen-Ferrenbach, C. In Multiple Emulsions: Structure, Properties and Applications; Editions De Sante: Paris, 1998; p 141.

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Simovic and Prestidge Table 1. Langmuir Model Parameters for Adsorption of Hydrophilic Silica Particles onto (A) Liquid PDMS Droplets and (B) Cross-Linked PDMS Droplets

Figure 5. Adsorption isotherms for hydrophilic silica and liquid PDMS droplets (10-3 M NaCl) at pH: (b) 9, ([) 7, (9) 5; no reproducible isotherm could be obtained at pH ) 4 due to the droplet flocculation and coalescence.

der Waals, hydrophobic forces, and hydrogen bonds. The adsorption of hydrophilic silica particles from aqueous solution onto PDMS droplets is regarded as a physisorption process and as such may be described by the Langmuir model:32

Ceq/Γ ) 1/KΓmax + Ceq/Γmax

(2)

where Ceq is the equilibrium solution concentration, Γ the adsorbed amount (mg m-2), Γmax the plateau adsorbed amount (mg m-2), and K the equilibrium constant for adsorption. The standard free energy of adsorption (∆G°ads) can be calculated from

∆G°ads ) -RT ln K

(3)

where R is the gas constant and T the absolute temperature. Experimental data from a wide variety of adsorbing systems have been fitted to the Langmuir model despite deviations from the model’s inherent assumptions, i.e., reversibility, isolated adsorbent sites and small adsorbate species.32,33 When considering colloidal particle adsorption, the characteristic particle dimensions are too large to enable identification of isolated adsorbent sites; rather, the interface is more appropriately treated as a continuum. Furthermore, depending on the nature of the adsorbent, adsorbate and solvent, a range of degrees of reversibility is possible. For instance, the adsorption of small particles on larger particles with opposite charge was essentially irreversible below a certain electrolyte concentration when electrostatic attractive forces are strong, but above that concentration adsorption is reversible with no hysteresis.34-36 For particles of increasing size, dispersion forces increase and so does the likelihood of irreversible adsorption, i.e., adsorption energy per particle may become greater than the thermal energy.33 For the silica particles under investigation, and considering only electrostatic and van der Waals forces, there is a weak energy barrier to adsorption; this is discussed further in the following sections. It is reasonable to consider that adsorption is weak and reversible, and therefore to employ the Langmuir adsorption model. Langmuir model fits to the adsorption data lead to correlation coefficients in the range 0.97-0.99, and though this does not imply that all assumptions of the Langmuir model are obeyed, it gives (32) Shaw, J. D. In Colloid and Surface Chemistry; ButterworthHeinemann, Ltd.: Woburn, MA, 1992; p 128. (33) Oberholzer, M. R.; Stankovich, J. M.; Carnie, S. L.; Chan, Y. C.; Lenhoff, A. M. J. Colloid Interface Sci 1997, 194, 138. (34) Vincent, B.; Young, C.; Tadros, Th. F. J. Chem. Soc., Faraday I 1980, 76, 665. (35) Vincent, B.; Jafelicci, M.; Luckham, P. F. J. Chem. Soc., Faraday I 1980, 76, 674. (36) Harley, S.; Thompson, D. W.; Vincent, B. Colloids Surf., A 1992, 62, 163.

pH

(M NaCl)

Γmax (mg m-2)a

Ka

∆Goads (kJ mol-1)

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

10-4 10-4 10-4 10-4 10-3 10-3 10-3 10-2 10-1

33.1 33.9 31.1 34.2 50.5 56.5 51.7 184.5 234.7

603.4 515.1 920.5 465.1 4750 4545 4500 6873 8306

-15.9 ( 0.5 -15.5 ( 0.5 -16.9 ( 0.5 -15.2 ( 0.5 -20.9 ( 0.5 -20.9 ( 0.6 -20.8 ( 0.6 -21.6 ( 0.5 -22.8 ( 0.5

pH

(M NaCl)

Γmax (mg m-2)a

Ka

∆Goads (kJ mol-1)

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

10-4 10-4 10-4 10-4 10-3 10-3 10-3 10-2 10-1

28.4 29.0 29.2 28.3 46.7 50.5 50.8 166.7 182.5

526 517 572 509 1786 1818 1818 3420 6810

-15.5 ( 0.5 -15.5 ( 0.5 -15.7 ( 0.5 -15.4 ( 0.5 -18.5 ( 0.6 -18.6 ( 0.7 -18.6 ( 0.7 -20.1 ( 0.5 -21.2 ( 0.5

a

Coefficient of variation was estimated at (10%.

some credence to the model’s applicability and enables a semiquantitative interpretation of the adsorption data. Adsorption parameters from the Langmuir model fits are listed in Table 1. Values of ∆Gads are in the range -15 to -23 kJ mol-1 and in good agreement with those reported for physisorption processes.32 In general, Γmax and ∆Gads are not significantly influenced by pH, i.e., the magnitude of particle-droplet and particle-particle electrostatic repulsion. (Liquid PDMS droplets at pH 7 and 10-4 M NaCl are an exception to this generalization and shows significantly greater silica adsorption; this is discussed further below.) The balance of normal particle-PDMS droplet and lateral particle-particle interactions34-36 governs adsorption. Silica particle-PDMS droplet interaction may be regarded as a heterocoagulation process between spherical colloidal particles of different radii and unequal surface potentials, and thus may be described by the extended DLVO theory of Hogg, Healy, and Fuerstenau (HHF).37 The HHF model is restricted to binary systems in which the two components are of similar chemical type and with common potential determining ions, and is applicable to surface potentials less than 60 mV, when the double-layer thickness is small in comparison with particle size and for large separation distances. The HHF model has been successfully used for solving numerous particle deposition problems,38-40 e.g., adsorption of Ludox silica onto PVC latex particles.40 For the silica-PDMS system under investigation, the Hamaker constant is estimated to be 0.456 kT.40 Potential energy vs distance curves calculated from the HHF theory are given in Figure 6. Under dilute salt conditions (10-410-3 M NaCl) an energy barrier for heterocoagulation of 3-5 kT can be identified for alkaline pH values; this disappeared at acidic pH values. The predicted energy barriers are significantly below 15 kT, which is considered sufficient to provide extensive colloid stability.39 It is clear (37) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday Soc. 1966, 62, 1638. (38) Adamzzyk, Z.; Weronski, P. Adv. Colloid Interface Sci. 1999, 83, 137. (39) Adamzcik, Z. Colloids Surf., A 1989, 39, 1. (40) Bleier, A.; Matijevic, E. Trans. Faraday Soc. 1977, 62, 1347.

Hydrophilic Silica Nanoparticles

Figure 6. HHF calculations of interaction energy vs distance for hydrophilic silica and PDMS droplets at pH: (b) 9, (9) 7, (2) 5, ([) 4; (a) 10-4 M NaCl and (b) 10-3 M NaCl.

that there is limited correlation between the energy barrier and the adsorption parameters (Table 1). Of particular interest, is the lack of pH influence on Γmax and ∆Gads, even though the magnitude of particle-particle lateral repulsion increases with increased pH. In contrast, the magnitude of lateral electrostatic interactions between small latex particles has been reported to significantly influence their plateau surface coverage at the solidliquid interface.41 When considering the adsorption behavior of silica particles it is important to recognize their “anomalous” (non-DLVO) colloid stability behavior.3 That is, in contrast to DLVO theory that predicts the critical coagulation concentration (ccc) of a sol is relatively low around the isoelectric point (iep) and increases progressively away from it, silica sols are remarkably stable to electrolytes at acidic pH (ccc > solubility limit of NaCl), and above pH ∼ 7 the ccc decreases with a further increase in pH.3 This behavior has been documented for Ludox and Aerosil 200 silica,3 both with a primary particle diameter of ∼15 nm, and has been confirmed for Aerosil 380 as used in this study, i.e., no coagulation is detectable for acidic pHs, at pH 7 the ccc is ∼1 M NaCl and at pH 9 the ccc is ∼0.5 M NaCl. Allen and Matijevic42,43 proposed that this “anomalous” stability of silica is caused by particle hydration, with the silanol groups being capable of hydrogen bonding with water, i.e., lyophilic rather than lyophobic colloidal behavior. With increasing pH, silanol protons are exchanged by cations of the electrolyte thus leading to sol destabilization by eliminating sites for hydrogen bonding (dehydration). Given the low Hamaker constant for silica, it appears that one monolayer of water is sufficient to screen the weak dispersion forces and prevent coagulation. Depasse44 proposed interparticle bridging as a coagulation mechanism for silica, i.e., at pH values > 6, coagulation may occur through acid-base bonding between silanol groups on one particle and dissociated silanol groups on another. Steric stabilization due to oligomeric or polymeric silicate species at the silica-water interface has also been considered.3 (41) Alince, B. Colloids Surf., A 1989, 39, 39. (42) Allen, L. H.; Matijevic, E. J. Colloid Interface Sci. 1969, 31, 153. (43) Allen, L. H.; Matijevic, E. J. Colloid Interface Sci. 1970, 33, 420. (44) Depasse, J. J. Colloid Interface Sci. 1997, 194, 260.

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The non-DLVO repulsive forces (hydration or steric) may reduce the dominating role of the magnitude of electrostatic forces in the silica-PDMS droplet system and be influential in controlling the pH-independent adsorption behavior. Similarly, for protein adsorption at hydrophobic surfaces a tightly bound water layer prevents strong adsorption.45 Clay particles with preadsorbed water have a lower affinity for adsorption of hydrophobic species.46 Furthermore, o/w emulsions based on hydrophilic silica have been shown3 to exhibit the highest stability to coalescence near pH 4; again, this may be considered to be due to the increased hydration force at low pH lowering particle adsorption at oil/water interface. The hypothesis that a non-DLVO force is responsible for the pH independent adsorption behavior is further supported by the fact that hydrophobic silica nanoparticles (where hydration/steric forces are eliminated) exhibit pronounced pH dependent adsorption at the PDMS droplet-water interface.47 The PDMS droplets under investigation resemble silica in that they have iep values in the vicinity of pH 2.5 and “silanol” (Si-OH) groups that regulate their surface charge. Cross-linked PDMS droplets contain (-O)2(CH3)Si-OH surface groups in addition to the less acidic (CH3)2(-O)Si-OH surface groups found on liquid PDMS droplets: these differences in composition and their surface concentrations are reflected in the zeta potential versus pH behavior in Figure 3. At intermediate pH values there is an optimal ratio of dissociated and un-dissociated silanol groups and acid-base (hydrogen) bonding is most likely.44 Bonding between undissociated silanol groups at PDMS droplets and dissociated silanol groups of silica is the most probable interaction mechanism,27 and since liquid droplets have a higher surface concentration of undissociated silanol groups at pH 7, more pronounced bonding would be expected. The Influence of Salt Addition and Droplet CrossLinking Level. Increasing the NaCl concentration to 10-2 M and 10-1 M further increased silica particle adsorption onto PDMS droplets, see Figure 7. Within this high salt concentration region, surface coverages at the adsorption plateau are now in excess of 75% of an equivalent hexagonally close-packed monolayer of hard spheres, increase with increasing salt concentration and are greater for liquid than cross-linked PDMS droplets. The adsorption isotherms at high salt are well described by the Langmuir model. Values of Γmax and ∆G°ads(Table 1) increase significantly with salt addition, i.e., upon a reduction in the range of particle-droplet and particle-particle lateral electrostatic repulsion. HHF calculations of the interaction energy between PDMS and silica confirm the reduction of the energy barrier as a function of salt addition. Irrespective of the driving force for adsorption, Γmax values are controlled by lateral particle-particle interactions at the droplet surface, which is in turn controlled by κa.34-36,38,39 The lateral separation h between neighboring spherical particles in a hexagonal array with plateau fractional coverage θmax can be rationalized:36

h ) 2a(1/θ max1/2 - 1)

(4)

Accounting for the particle double-layer thickness in determining maximum packing yields h ) 2 (a + 1/κ). On the basis of this approach, θmax for 50 nm diameter particles (45) Feder, J.; Giaever, I. J. Colloid Interface Sci. 1980, 78, 144. (46) Menon, V. B.; Wasan, D. T. Coll. Surf., A 1986, 19, 107. (47) Simovic, S.; Prestidge, C. A. Langmuir. Submitted for publication.

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Simovic and Prestidge

Figure 7. Adsorption isotherms for hydrophilic silica and PDMS droplets as a function of salt concentration (pH ) 9): ([) 10-3 M NaCl, (9) 10-2 M NaCl, (2) 10-1 M NaCl; (a) liquid droplets and (b) cross-linked droplets. Figure 9. Freeze-fracture SEM images of liquid PDMS droplets with plateau coverages of silica nanoparticle at pH ) 9: (a) 10-4 M NaCl and (b) 10-3 M NaCl.

Figure 8. Fractional surface coverage (θ) plotted against κa parameter for silica (κ-Debye reciprocal length parameter, (a) average silica particle size): for (b) liquid and for (9) crosslinked PDMS droplets and (2) based on calculations of effective size.

in different electrolyte (1:1) concentrations are as follows: 0.18 at 10-4 M, 0.35 at 10-3 M, 0.75 at 10-2 M, and 0.90 at 10-1 M, and are compared with experimental data in Figure 8. In general, over the entire salt and pH range investigated, the lateral separation and effective particle radius are in good agreement, and hence long-ranged double-layer forces govern particle packing. For liquid droplets at higher salt concentrations the plateau fractional coverage is slightly greater than predicted by lateral overlap of the silica double layers and suggest that weak attractive forces play a role. Factors that promote interfacial aggregation of particles relative to the bulk include increased particle concentration at the interface and more favorable interaction energy for coagulation,48 i.e., shortranged van der Waals force may become operative at the high-salt regime, inducing closer particle packing. At low salt concentrations (10-4 and 10-3 M NaCl) small differences in the silica particle adsorption parameters for liquid in comparison with cross-linked droplets (see Table 1) may be rationalized by the small increase in zeta potential upon cross-linking. At higher salt concentrations the adsorption parameters are significantly reduced for cross-linked droplets; this is hypothesised to be due to the reduced likelihood of interfacial penetration for cross(48) Williams, D. F.; Berg, J. C. J. Colloid Interface Sci. 1992, 152, 218.

linked droplets (this phenomenon is more pronounced for hydrophobic silica particles47). Electrolyte addition has been reported20,21 to increase the contact angle of micrometer-sized silica particles and may influence the wetting characteristics of hydrophilic silica particles. That is, the stable wetting film expected between a PDMS oil droplet and silica at low electrolyte may be ruptured at higher electrolyte concentrations hence closer dropletsilica surface approach. In the case where the attractive van der Waals forces overcome the electrostatic repulsion, a finite contact angle may form, resulting in an increased Γmax and a more negative ∆G°ads, and this would be more pronounced for liquid than cross-linked PDMS droplets. Adsorbed Particle Layer Microstructure. Freezefracture SEM has been used to characterize adsorbed silica particle layers at the PDMS droplet-water interface. It should be recognized that in the absence of adsorbed particles PDMS droplets tend to evaporate during SEM and are observed as imprints (holes) in the vitrified ice surface. However, in the presence of adsorbed silica particles the evaporation of PDMS droplets is significantly reduced. At low particle coverages the droplets undergo deformation, but are resistant to evaporation, whereas at high particle coverages the droplets are resistant to both deformation and evaporation. In both cases useful information concerning the adsorbed layer structure can be obtained. SEM images of the liquid PDMS droplet-water interface, at plateau surface coverage of silica and in the presence of low salt concentrations, are given in Figure 9. Droplet deformation is evident in these samples, however, individually adsorbed particles can be observed with surface coverages in agreement with the adsorption studies, i.e., θmax ∼ 0.15 and increasing to 0.3 as the salt concentration was increased from 10-4 M (Figure 9a) to 10-3 M (Figure 9b). More extensive studies have confirmed that pH variation had negligible influence on the adsorbed layer structure. Further studies are required to ascertain whether size dependent adsorption occurs. SEM images

Hydrophilic Silica Nanoparticles

Figure 10. Freeze-fracture SEM images of liquid PDMS droplets with a plateau coverage of silica particles (pH ) 9; 10-2 M NaCl), at two levels of magnification.

Figure 11. Freeze-fracture SEM images of liquid PDMS droplets with a plateau coverage of silica particles (pH ) 9; 10-1M NaCl), at two levels of magnification.

at higher salt concentrations (Figures 10 and 11) confirm the significantly increased surface coverage of silica particles. It is indeed pleasing that based on a 50 nm particle diameter the estimated θmax values of 0.7-0.9 from the adsorption isotherms are in good agreement with the SEM images. The higher magnification images show that the majority of adsorbed particles at the interface are in the 50-110 nm size range, with in many cases clear evidence of interparticle spacing, i.e., no evidence for

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extensive surface aggregation. These images are in contrast to equivalent images of hydrophobic silica particles at the PDMS droplet-water interface47 where extensive interfacial aggregation is evident, and individual particles and particle spacing cannot be identified. Given that silica particle suspensions contain particles in the range 20-110 nm and are not influenced by the salt concentrations experienced here, solution phase aggregation does not occur, but a certain degree of surface aggregation and size-dependent adsorption cannot be discounted. These findings improve our understanding of the adsorption mechanism for hydrophilic silica at the dropletwater interface and should be discussed in the context of previous observation regarding droplet stability in the presence of particles. On the basis of the observation that significant improvements in emulsion stability may be achieved if particles are flocculated, it has been proposed that the attachment of individual particles at the droplet interface is weak. Particle attachment requires that the kinetic energy of the impact of the particle with the interface exceeds the work of formation of the nucleating contact16 and is not expected to occur for particles with radii below 500 nm. Thus preflocculation was considered necessary for adsorption to occur. Freeze-fracture TEM images of emulsions stabilized by Ludox silica and surfactants confirmed the formation of a close-packed interfacial structure composed of monomeric and oligomeric silica particles. However, surfactants were present in this study and may adsorb at the interface and substantially alter particle wettability, particle layer structure, and emulsion stability;15 therefore, strong conclusions regarding adsorption of hydrophilic particles may not be justified. The current investigation indicates that individual silica particles can be attached at the oil droplet-water interface, and form a closely packed layer of incompletely aggregated particles, even in the presence of 0.1 M salt. With this in mind, the poor performance of hydrophilic silica as an emulsifier is not considered to be due to its limited adsorption. According to Menon and Wasan49 only for particles less than 1 nm does Brownian motion prevent attachment at the interface, whereas larger particles may attach at the interface. Previous studies have shown that adsorbed particle layers at the droplet-water interface provide protection against coalescence12 at a particle surface coverage greater than ∼0.7. Our preliminary optical microscopy and freezefracture SEM investigations on PDMS droplets in the presence of adsorbed silica particles (θ max near 1.0) have shown that significant coalescence occurs in the presence of excess salt, even though the particles are not displaced from the interface into the bulk water phase. This indicates instability toward thinning and rupturing of thin films in the presence of hydrophilic particles. During film thinning, the flow of the continuous phase fluid away from the contact region will tend to displace the particles along the interface. Tambe and Sharma12 proposed that the energetically more favorable mechanism of particle removal is lateral displacement of particles away from the dropdrop contact region. In addition, thin liquid films between attached particles with low contact angle are unstable because the Laplace pressure will force liquid away from the particle, hence enhancing film drainage.50 Further investigations of the influences of adsorbed particles on droplet interaction and coalescence are clearly warranted. (49) Menon, V. B.; Wasan, D. T. Sep. Sci. Technol. 1988, 23, 2131. (50) Aveyard, R.; Clint, J. J. Chem. Soc., Faraday Trans. 1995, 91, 2681.

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Conclusions Methods have been developed to determine adsorption isotherms and layer structure for hydrophilic fumed silica particles (diameter ∼ 50 nm) at the PDMS droplet-water interface. ∆Gads is in the range -15 to -23 kJ mol-1 and suggests physical adsorption. Adsorption is weakly influenced by pH, but significantly influenced by salt addition. At salt concentrations up to 10-1 M, and at a particle surface coverage up to close packing, particles adsorb without extensive surface aggregation. Interparticle double-layer forces govern particle packing at the droplet surface. Droplet cross-linking has negligible influence on particles adsorption at 20, but these do not provide protection against coalescence in excess salt. 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. LA026803C