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Enhanced Stabilization of Emulsions Due to Surfactant-Induced Nanoparticle Flocculation Bernard P. Binks* and Jhonny A. Rodrigues Surfactant and Colloid Group, Department of Chemistry, UniVersity of Hull, Hull, HU6 7RX, U.K. ReceiVed March 1, 2007. In Final Form: April 26, 2007 We have shown recently (Binks, B. P.; Rodrigues, J. A.; Frith, W. J. Langmuir 2007, 23, 3626) that, for mixtures of negatively charged silica nanoparticles and cationic surfactant, oil-in-water emulsions are most stable to creaming and coalescence at conditions of maximum flocculation of particles by surfactant in aqueous dispersions alone. Here, we extend the idea using positively charged silica particles in mixtures with anionic surfactant.
Introduction Emulsions of oil and water can be made stable to coalescence using surfactant alone1 or particles alone.2 In the former case, the hydrophile-lipophile balance number of the surfactant is crucial in selecting the best emulsifier for the particular oil, whereas, for particles, their wettability at the oil-water interface is paramount. In some cases involving this mixture, surfactant adsorption onto particles is followed by particle agglomeration, and such agglomerates adsorb at the oil-water interface yielding fine emulsions of excellent long-term stability.3-6 There are various indications in the literature that good emulsion stabilization may be achieved when the particles are weakly flocculated, either by surfactant3 or salt7-10 in the case of oil-in-water (o/w) emulsions, or by surfactant in the case of water-in-oil (w/o) ones.11 The reason for this remains unclear. In addition, in connection with the liquid-liquid extraction of fine particles from aqueous suspensions, Lai and Fuerstenau12 showed how the adsorption of alkyl sulfonate anionic surfactants onto positively charged alumina particles in water increased their hydrophobicity, allowing the transfer of particles from water to oil and subsequent emulsion stabilization and phase inversion. Despite some activity in this area, systematic studies of the behavior and properties of emulsions in mixtures of surfactant and surface-active particles are lacking. Recently, we investigated the mixed system comprising negatively charged silica particles and positively charged single-chain cationic surfactant.13 Preferred emulsions were o/w at all surfactant concentrations and were most stable at conditions where particles were most flocculated, induced by surfactant monolayer adsorption onto particle surfaces. In order to determine if this finding is generic to mixtures of oppositely charged components, here we report the enhanced stabilization of o/w emulsions using a combination of positively * Corresponding author. E-mail:
[email protected]. (1) Binks, B. P., Ed. Modern Aspects of Emulsion Science; The Royal Society of Chemistry: Cambridge, U.K., 1998. (2) Binks, B. P., Horozov, T. S., Eds. Colloidal Particles at Liquid Interfaces; Cambridge University Press: Cambridge, U.K., 2006. (3) Hassander, H.; Johansson, B.; To¨rnell, B. Colloids Surf. 1989, 40, 93. (4) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1993, 157, 244. (5) Lagaly, G.; Reese, M.; Abend, S. Appl. Clay Sci. 1999, 14, 83. (6) Binks, B. P.; Desforges, A.; Duff, D. G. Langmuir 2007, 23, 1098. (7) Briggs, T. R. J. Ind. Eng. Chem. 1921, 13, 1008. (8) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 1999, 1, 3007. (9) Ashby, N. P.; Binks, B. P. Phys. Chem. Chem. Phys. 2000, 2, 5640. (10) Yang, F.; Liu, S.; Xu, J.; Lan, Q.; Wei, F.; Sun, D. J. Colloid Interface Sci. 2006, 302, 159. (11) Lucassen-Reynders, E. H.; van den Tempel, M. J. Phys. Chem. 1963, 67, 731. (12) Lai, R. W. M.; Fuerstenau, D. W. Trans. Soc. Min. Eng. 1968, 241, 549. (13) Binks, B. P.; Rodrigues, J. A.; Frith, W. J. Langmuir 2007, 23, 3626.
charged alumina-coated silica particles and negatively charged anionic surfactant. Experimental Materials. Water was first passed through an Elga reverse osmosis unit and then a Milli-Q reagent water system. The oil used was n-dodecane (Aldrich, >99%). It was passed through a column of chromatographic alumina twice before use. Pure anionic sodium dodecyl sulfate (SDS) surfactant was purchased from Aldrich with a purity >99%. The critical micelle concentration in water at 25 °C was 8 mM. The particles used were those of Ludox CL (Grace Davison), received as a 30 wt % aqueous dispersion at pH ) 3.5. This is a colloidal silica in which each particle is coated with a layer of alumina. The isoelectric point of such particles is ∼8.5,14 causing them to possess a positive charge at low pH. Using transmission electron microscopy (not shown), the particles are seen to be spherical and relatively monodisperse with an average diameter of 15 nm and a specific surface area of 230 m2 g-1. Hydrochloric acid (Fluka, 99%) was used to maintain the pH at 3.5. Methods. (A) Aqueous Dispersions. We investigated aluminacoated silica particle dispersions in water (2 wt %) in the presence of SDS surfactant at pH ) 3.5. Aliquots of aqueous SDS were added with stirring to a volume of a diluted, as-received dispersion at this pH. The extent of sedimentation of dispersions was determined by measuring the height of the sediment as a function of time. The ζ potentials of 1 cm3 of a 2 wt % particle dispersion at the same pH in the presence of increasing amounts of SDS were determined using a Malvern Zetasizer HS-3000 instrument employing a dip cell, using the Smoluchowski equation for converting measured mobilities. At least two separate measurements were made for each surfactant concentration. (B) Emulsions. For systems containing either surfactant or particles alone, emulsions of equal volumes of oil and water (total 10 cm3) were prepared in glass vessels using a Janke and Kunkel UltraTurrax T25 homogenizer with a 1 cm head operating at 11 000 rpm for 2 min. Both particles and surfactant originate in water. For systems containing a mixture of surfactant and particles, surfactant was added to a particle dispersion in water and left 30 min to equilibrate. Emulsification was the same as described above. Immediately after homogenization, the conductivity of the emulsions was determined using a Jenway 4310 digital conductivity meter with Pt/Pt black electrodes. Emulsion type was also confirmed using the “drop” test, in which a drop of emulsion is added separately to a small volume of pure oil and pure water. The stability of the emulsions to creaming and coalescence was assessed by monitoring the positions of the water-emulsion and emulsion-oil interfaces, respectively, with time. Photographs of the vessels were taken with a Canon IXUS 55 digital camera. A Nikon Labophot microscope equipped with a (14) van der Meeren, P.; Saveyn, H.; Bogale Kassa, S.; Doyen, W.; Leysen, R. Phys. Chem. Chem. Phys. 2004, 6, 1408.
10.1021/la700597k CCC: $37.00 © 2007 American Chemical Society Published on Web 05/31/2007
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QICAM 12-bit Mono Fast 1394 camera (QImaging) was used to observe emulsion samples. A drop of emulsion was diluted in its continuous phase on a microscope slide and gently covered with a cover slip. The images were processed using Image-Pro Plus 5.1 software (Media Cybernetics). Drop size distributions for selected emulsions were obtained by light diffraction of a diluted emulsion using a Malvern Mastersizer 2000 instrument. Freeze-fracture scanning electron microscopy (SEM) measurements were carried out on selected emulsion samples in the following way: A small volume of fresh emulsion was mounted onto a 0.1 cm diameter brass rivet and plunged into nitrogen slush. The frozen sample was transferred to a Gatan Alto 2500 cryo preparation system, fractured with a scalpel blade at -97 °C, etched for 2 min, cooled to -110 °C, and coated with 2 nm of Pt/Pd. Examination was done at -150 °C using a Jeol 6301F field-emission SEM as described elsewhere.15 All measurements were carried out at room temperature, that is, 20 ( 2 °C.
Results and Discussion In order to rationalize the properties of emulsions in mixtures of the two emulsifiers, we first investigated emulsions stabilized by each emulsifier separately. (A) Emulsions Stabilized by Each Emulsifier Alone. At pH ) 3.5, Ludox CL particles are positively charged and hydrophilic. Attempts to prepare emulsions from equal volumes of aqueous dispersion and dodecane yielded mixtures that phase separated completely within several minutes of emulsification. This occurred for all particle concentrations in water between 0.1 and 5 wt %. The particles are too hydrophilic to be held at the oilwater interface, and coalescence ensues readily. By contrast, emulsions are o/w when stabilized by SDS alone at this pH. Their stability to coalescence increases progressively with surfactant concentration in water, such that, by 10-2 M, no coalescence is visible, although emulsions cream to liberate a serum below. (B) Emulsions Stabilized by a Mixture of Surfactant and Particles. In our earlier work with cationic surfactant and negatively charged nanoparticles, we investigated the stability of aqueous dispersions containing surfactant prior to emulsification with oil.13 Starting from stable dispersions in the absence of surfactant, the dispersions first became unstable to sedimentation and then restabilized upon increasing the surfactant concentration, as particles were first rendered hydrophobic and then hydrophilic following surfactant adsorption. Emulsions prepared from these dispersions were most stable to both creaming and coalescence at conditions of optimum flocculation of the particles by surfactant. In the same way, we have monitored the stability of aqueous dispersions of positively charged particles (2 wt %) upon adding negatively charged surfactant. Figure 1a is a photograph of the vessels containing the aqueous dispersions 1 day after mixing with SDS surfactant. Without surfactant, the dispersion is bluish and stable. Close to the critical micelle concentration of SDS (8 mM), the dispersion becomes unstable, and flocculated particles sediment quickly. The extent of sedimentation increases with further increase in SDS concentration up to a maximum around 20 mM and then decreases progressively at higher concentrations. The extent of sedimentation displays a maximum with respect to SDS concentration (Figure 1b), and is reproducible after reshaking the dispersions a number of times. The origin of the destabilization followed by restabilization of the dispersions can be sought by measuring the ζ potential of the particles as a function of SDS concentration. The results are given in Figure 2. The ζ potential of the cationic particles (15) Binks, B. P.; Kirkland, M. Phys. Chem. Chem. Phys. 2002, 4, 3727.
Figure 1. (a) Photograph of vessels at 20 °C containing 2 wt % aqueous Ludox CL particles at pH ) 3.5 and increasing concentration of SDS surfactant (given) 24 h after mixing. (b) Sediment height after 1 day versus SDS concentration for systems in panel a. Circles ) original; squares ) after reshaking once; triangles ) after reshaking twice. The foam volume is not accounted for in the sediment height determination.
Figure 2. ζ potential of 2 wt % aqueous Ludox CL dispersions at pH ) 3.5 as a function of SDS concentration. The value in the absence of SDS is +43 ( 1 mV.
alone is approximately +43 mV at pH ) 3.5, which is relatively high since this pH is far from the isoelectric point of 8.5.14 Upon adding SDS, the ζ potential decreases in magnitude, changes sign, and becomes increasingly negative up to a point. This charge reversal is directly linked to the adsorption of surfactant, initially as a monolayer with chains exposed to solution and then as a bilayer with sulfate headgroups exposed to solution. We note that the concentration of SDS required to coat particles yielding
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Figure 3. (a) Photograph of vessels containing equal volumes of 2 wt % aqueous Ludox CL particles (pH ) 3.5) and dodecane as a function of SDS concentration in water (given) 24 h after emulsification. (b) Percentage of water resolved due to creaming 1 week after preparing the emulsions in panel a. Circles ) original; squares ) after redispersing the cream once; triangles ) after redispersing twice.
zero charge (20-30 mM) is very close to that required to destabilize the aqueous dispersions the most (20 mM), as expected. Particles of low charge are also more hydrophobic than those at SDS concentrations on either side of this, and so particle flocculation leading to sedimentation occurs readily. The formation of a surfactant monolayer and then a bilayer with concomitant charge reversal of the particles has been discussed for a system similar to that discussed here of positively charged alumina particles with anionic sodium dodecylsulfonate surfactant.16 Emulsions of equal volumes of mixed particle-surfactant aqueous dispersion and dodecane were prepared in a standard protocol. At a fixed particle concentration (2 wt % in water), all emulsions were o/w as a function of SDS concentration (Figure 3a), and, interestingly, they were all gel-like. Their stability to coalescence was excellent with no oil released within 6 months, and their average drop diameter also remained unchanged. This is particularly apparent for the 10-3 M SDS mixed emulsion, which displays at least 50% coalescence in the absence of particles at the same concentration. A substantial synergy thus exists between particles and surfactant to stabilize the emulsion. The emulsions composed of mixed emulsifiers cream, but, interestingly, the stability to creaming passes through a shallow maximum at intermediate concentrations of SDS (Figure 3b), which is reproducible after shaking and redispersing the cream. We notice that the separated aqueous phase below the cream is clear and probably devoid of particles around conditions of minimum creaming (30 mM). The apparent median drop diameter obtained (16) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90.
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from light diffraction analysis assuming spherical drops increases progressively from 37 µm at 10-3 M SDS to 77 µm at 5 × 10-2 M. At higher concentrations, the drop size distributions for the viscous emulsions were bimodal, and many drops were extremely nonspherical and flocculated (see below). We thus find that emulsions are most stable at surfactant concentrations corresponding to least stable and most flocculated dispersions, as observed previously with anionic particles and cationic surfactant.13 It appears therefore that the main determinant of such behavior is the difference in charge between the two emulsifiers and not the chemical details of the system. Since a method does not exist for the determination of the contact angles of nanoparticles at oil-water interfaces, it is common to find such indications for the same system on planar, well-characterized substrates. Using a system similar to that studied here, Shergold and Mellgren17 reported the contact angles of captive iso-octane oil drops under water on hematite (Fe2O3) at a pH below the isoelectric point (positively charged) as a function of SDS concentration. These angles, measured through the water phase, increase with the adsorbed amount of SDS (measured separately) on the solid up to monolayer coverage, reaching values around 100°. This increase in the hydrophobicity of the solid surface enhances the adsorption of the coated particles to the oil-water interface to stabilize the emulsions. Although the experiments were not done in ref 17, we anticipate that, at higher SDS concentrations, the contact angle would decrease following the formation of an adsorbed bilayer on particle surfaces. This has been demonstrated in the cetyltrimethylammonium bromide/silica system by us recently.13 We used microscopy to investigate how the drops behave in the emulsions as a whole and the arrangement of particles at the interfaces of individual drops also. For emulsions stabilized by low molar mass surfactant alone, drops are nearly always spherical since their interfaces are fluid-like and can relax following deformation.1 In contrast, nonspherical emulsion drops are commonly observed in particle-only systems since relaxation to spherical shape is prevented due to a more solid-like film at the interfaces.18 In our mixtures, at low SDS concentration at which drop interfaces are most likely saturated mainly with surfactant, the oil drops are all spherical (Figure 4a). In contrast, at high SDS concentration at which all the particles possess an adsorbed layer of surfactant, rendering them more surface-active, the emulsions are very viscous, and the drops are of a variety of shapes (Figure 4b), as expected. The surfaces of the drops have been visualized using freeze-fracture cryo-SEM. For a stable emulsion with SDS alone, Figure 4c shows that the surface is featureless as expected for a molecular monolayer. At a low [SDS] in the presence of particles at which no sedimentation occurs, Figure 4d shows a portion of a relatively large drop (oil is the upper half) with a thin interface and where many particles are present as small flocs in the continuous aqueous phase (lower half). It appears that these particles approach the drop interface but are not adsorbed to it. At intermediate [SDS] at which the sedimentation of particles in water alone is maximum, we see that the interfaces of drops are covered with a dense layer of flocculated particles too (Figure 4e). In addition, no excess particles are observed in the continuous aqueous phase (upper right and other images not shown), implying that all the particles become bound to drop interfaces, ensuring stabilization. Finally, at high [SDS] at which particle flocs are partially redispersed in bulk, Figure 4f reveals the external surface of a number of drops in close contact where individual particles can be seen well (17) Shergold, H. L.; Mellgren, O. Trans. Inst. Min. Metall. 1969, 78, 121. (18) Ramsden, W. Proc. R. Soc. 1903, 72, 156.
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Figure 4. Optical micrographs of o/w emulsions stabilized by 2 wt % aqueous Ludox CL particles and (a) 10-3 and (b) 10-1 M SDS. Cryo-freeze-fracture SEM images of o/w emulsions stabilized by (c) 3 × 10-2 M SDS alone, and mixtures containing 2 wt % aqueous Ludox CL particles and (d) 10-3, (e) 3 × 10-2, and (f) 5 × 10-1 M SDS.
separated from each other. This is to be expected since bilayercoated particles are negatively charged. It is possible that monomeric surfactant is adsorbed on drops between such particles, although this is difficult to prove. The synergistic stabilization of emulsion drops caused by mixing oppositely charged particles and surfactant shown here has also very recently been evidenced in the stabilization of air bubbles in water.19 In that work, no stable bubbles could be formed from aqueous dispersions of hydrophilic silica alone, whereas very stable bubbles occurred upon increasing the cationic surfactant concentration. In addition, we have recently shown that emulsions can be inverted from o/w to w/o upon increasing the cationic surfactant concentration in mixtures with negatively charged silica particles.20 A dichain surfactant is required for (19) Kostakis, T.; Ettelaie, R.; Murray, B. S. In Food Colloids: Self-Assembly and Material Science; Dickinson, E., Leser, M. E., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 2007; p 357. (20) Binks, B. P.; Rodrigues, J. A. Angew. Chem., Int. Ed., in press.
this purpose, however, since it renders particles more hydrophobic than the analogous single-chain surfactant.
Conclusion Synergistic stabilization of o/w emulsions occurs when positively charged alumina-coated silica nanoparticles are mixed with the anionic surfactant SDS in water. At conditions of maximum particle flocculation in water, for which particles have zero charge, emulsions are most stable to both creaming and coalescence. Particle flocs are shown to adsorb to drop interfaces. All emulsions are of gel-like consistency. Acknowledgment. We thank Unilever Corporate Research, Colworth, U.K. for a postdoctoral grant to J.A.R. and Mr. M. Kirkland, Unilever Colworth, for carrying out the freeze-fracture SEM measurements. LA700597K
