Langmuir 2000, 16, 2539-2547
2539
Catastrophic Phase Inversion of Water-in-Oil Emulsions Stabilized by Hydrophobic Silica B. P. Binks* and S. O. Lumsdon Surfactant Science Group, Department of Chemistry, University of Hull, Hull, HU6 7RX, United Kingdom Received August 10, 1999. In Final Form: November 24, 1999 A short review of the experimental findings concerning the stabilization of emulsions by solid particles is given. We then describe the preparation and properties of water-in-oil (w/o) emulsions stabilized by nanometer-sized hydrophobic silica particles alone. Emulsions of median diameter equal to 0.6 µm are completely stable to coalescence as a result of an adsorbed layer of particles at the oil-water interface. Their stability to sedimentation increases with particle concentration due to network formation of the particles in the continuous oil phase. The w/o emulsions catastrophically invert, without hysteresis, to oil-in-water (o/w) at volume fractions of water around 0.7, i.e., as soon as the drops begin to deform. The drops in o/w emulsions are larger (100 µm) and cream rapidly but remain stable to coalescence. We demonstrate that for emulsions stabilized by hydrophilic silica particles, phase inversion from o/w to w/o occurs at the same dispersed phase volume fraction as above. It is therefore suggested that the system hydrophile-lipophile balance is determined by the particle wettability. Comparisons with the behavior of surfactant-stabilized emulsions are given throughout.
Introduction The majority of reports describing the preparation, stability, and properties of macroemulsions involve the use of low molecular weight or polymeric surfactants, either alone or in mixtures.1 There are relatively few studies dealing with emulsions whose interfaces are stabilized by solid particles.2-16 Further, most of these involve particle-surfactant combinations necessary for the required stability and very few relate to emulsions prepared from oil, water, and solid alone.2,4,5,12,15 In the early 1900s Pickering2 recognized the role of finely divided solid particles in stabilizing emulsions, but it was not until the work of Finkle et al.3 that the relationship between the type of solid and emulsion type (oil-in-water, o/w, or water-in-oil, w/o) was recognized. They stated that in an emulsion containing solid, one of the liquids will probably wet the solid more than the other liquid, with the more poorly wetting liquid becoming the dispersed phase. The importance of the wettability of the particles at the oilwater interface, quantified by the contact angle θ that the particle makes with it, was thus noted. If θ (measured (1) See chapters in Modern Aspects of Emulsion Science; Binks, B. P., Ed.; Royal Society of Chemistry: Cambridge, 1998. (2) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001. (3) Finkle, P.; Draper, H. D.; Hildebrand, J. H. J. Am. Chem. Soc. 1923, 45, 2780. (4) Moore, W. C. J. Am. Chem. Soc. 1919, 41, 940. (5) Briggs, T. R. J. Ind. Eng. Chem. 1921, 13, 1008. (6) Schulman, J. H.; Leja, J. Trans. Faraday Soc. 1954, 50, 598. (7) Kruglyakov, P. M.; Koretskii, A. F. Izv. Sib. Otd. AN SSSR, Ser. Khim. Nauk, 1971, 9, 16. (8) Tsugita, A.; Takemoto, S.; Mori, K.; Yoneya, T.; Otani, Y. J. Colloid Interface Sci. 1983, 95, 551. (9) Gelot, A.; Friesen, W.; Hamza, H. A. Colloids Surf. 1984, 12, 271. (10) Hassander, H.; Johansson, B.; To¨rnell, B. Colloids Surf. 1989, 40, 93. (11) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1993, 157, 244. (12) Abend, S.; Bonnke, N.; Gutschner, U.; Lagaly, G. Colloid Polym. Sci. 1998, 276, 730. (13) Midmore, B. R. Colloids Surf. A 1998, 132, 257. (14) Midmore, B. R. J. Colloid Interface Sci. 1999, 213, 352. (15) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 1999, 1, 3007. (16) Lagaly, G.; Reese, M.; Abend, S. Appl. Clay Sci. 1999, 14, 83.
through the water phase) is slightly less than 90° the particles will be held at the interface and tend to stabilize o/w emulsions. For conditions such that θ is slightly greater than 90° the particles will still be held at the interface but will now stabilize w/o emulsions. However, if the particles are either too hydrophilic (low θ) or too hydrophobic (high θ), they will tend to remain dispersed in either the aqueous or oil phase respectively, giving rise to unstable emulsions. A wide variety of solid materials has been used as stabilizers of either o/w or w/o emulsions including iron oxide, hydroxides, metal sulfates, silica, clays, and carbon.2,4-15 The effectiveness of the solid in stabilizing emulsions depends, inter alia, on particle size, particle shape, particle concentration, particle wettability, and the interactions between particles. Table 1 summarizes the main experimental findings. It can be seen that the particle wettability may be altered by adsorption of suitable surfactants,6-11 in some cases leading to emulsion phase inversion.7,9,11 In addition, several studies conclude that stable emulsions can only be formed if the particles are flocculated to some extent.5,10,13,15 Depending on the exact system, there are at least two mechanisms by which colloidal particles stabilize emulsions. In the first, the particles are required to adsorb at the oil-water interface and remain there forming a dense film (monolayer or multilayer) around the dispersed drops impeding coalescence. In the second, additional stabilization arises when the particle-particle interactions are such that a threedimensional network of particles forms in the continuous phase surrounding the drops. This has been invoked particularly in clay-containing systems in which the emulsion oil drops become captured and largely immobilized in the array of clay platelets in water.12,16 The enhanced viscosity of the continuous phase reduces the rate and extent of creaming. Figure 1a shows quite clearly the relationship between the contact angle at the solid-oil-water interface and emulsion behavior, recast from data given in ref 11. The angles refer to those of a water drop on a calcite crystal surface under decane containing different concentrations of stearic acid. The initially hydrophilic surface becomes
10.1021/la991081j CCC: $19.00 © 2000 American Chemical Society Published on Web 01/31/2000
2540
Langmuir, Vol. 16, No. 6, 2000
Binks and Lumsdon
Table 1. Brief Summary of Main Experimental Work with Solid-Stabilized Emulsions solid
oil
type
basic CuSO4 or Fe(OH)2 lamp black hydrous Fe2O3
solar distillate o/w kerosene w/o benzene o/w
silica
kerosene
carbon black BaSO4 glass powder Na-montmorillonite
kerosene benzene heptane paraffin
Ca-bentonite carbon black Ludox silica
toluene toluene paraffin
CaCO3 or BaSO4 or silica
decane
graphite Na-montmorillonite + dihydroxide paraffin Ludox silica hydrophobic silica Aerosil silica
paraffin paraffin toluene
o/w w/o
}
main findings
ref.
no emulsions when solid was dried and fine structure destroyed drop size falls with solid content no emulsion with pure water; stable emulsion with weak flocculant (NaCl); no emulsion with strong flocculant (Na2SO4)
2 4 5
silica and carbon together, no emulsion (antagonistic)
o/w or w/o SDSa alone is o/w; add solid gives w/o; oleic acid is w/o; add solid gives o/w o/w or w/o low [OTACb] give o/w; high [OTAC] give w/o o/w stable o/w formed when glycerol monopalmitate added; complex formed at interface and network of platelets exist in water o/w or w/o solid alone is o/w; adsorb DoTABc gives w/o w/o w/o with or without surfactant o/w stable emulsions in presence of either CTABd or PVPe which cause agglomeration of particles o/w or w/o o/w emulsions with solid alone invert to w/o with stearic acid; angles pass through 90° w/o w/o formed at all [stearic acid] o/w heterocoagulation of negative clay with positive hydroxide gives very stable emulsions; combination of particle layer around oil drops and network of platelets in water o/w stable emulsions if silica flocculated by HPCf w/o stable emulsions if HPC present; flocculated at extremes of pH o/w stable emulsions if particles flocculated by salt
a SDS, sodium dodecyl sulfate. b OTAC, octadecyltrimethylammonium chloride. c DoTAB, dodecyltrimethylammonium bromide. cetyltrimethylammonium bromide. e PVP, polyvinylpyrrolidone. f HPC, hydroxypropylcellulose.
Figure 1. (a) Variation of contact angle (through water) of a water drop on a calcite crystal under decane (left-hand ordinate) and emulsion stability (right-hand ordinate) as a function of stearic acid concentration in decane. Emulsions contained equal volumes (15 cm3) of oil (containing stearic acid) and water (containing calcium carbonate particles, 2.5 wt %). Volume of emulsion remaining after 6 h is plotted, with o/w emulsions shown above the zero level and w/o emulsions below it. Data from ref 11. (b) Variation of contact angle (through water) of a benzene drop on a barium sulfate crystal under water as a function of pH. SDS (10-3 M in water) emulsions were w/o at all pH. Oleic acid (10-3 M in oil) emulsions were o/w at pH < 8 and w/o above this. Data from ref 6.
hydrophobic as the carboxylic acid adsorbs. Emulsions containing calcium carbonate particles invert from o/w to w/o on increasing the stearic acid concentration as
d
6 7 8 9 10 11 12 13 14 15
CTAB,
predicted. The stability of both types of emulsion (expressed as the volume of emulsion remaining after 6 h) decreases substantially at concentrations where the contact angle approaches 90°, i.e., around phase inversion, as in surfactant-stabilized emulsions. At first sight this may appear contradictory to adsorption energy considerations where the energy of attachment of a spherical particle is greatest at 90° and decreases either side of this value.17 However, as discussed by Denkov et al.,18 the liquid meniscuses surrounding the particles in a thin film are most stable at 0° and 180° and most unstable at 90°. It turns out that the maximum capillary pressure which can be resisted by liquid meniscuses formed between the adsorbed particles is least at 90°. In a second example, Figure 1b shows the variation of θ with aqueous phase pH for benzene drops on BaSO4 surfaces under water.6 In the absence of solid particles, SDS forms o/w emulsions but these become w/o on adding BaSO4. θ lies above 90° and is independent of pH as expected. On the contrary, oleic acid stabilizes w/o emulsions without added solid but phase inverts at low pH in the presence of BaSO4 with a concomitant reduction of θ to below 90°. It is believed that the surface density of oleic acid adsorbed on the particles increases with pH rendering them hydrophobic as a result of two dissociated carboxylate headgroups binding to each barium cation. The present paper describes the effects of particle concentration, type of oil, and oil/water ratio on the type and stability of emulsions stabilized by silica particles alone. Catastrophic phase inversion, induced by increasing the volume fraction of dispersed phase, occurs from w/o to o/w using hydrophobic silica and from o/w to w/o using hydrophilic silica. Inversion is accompanied by dramatic changes in the stability and size distribution of the emulsions. Experimental Section Materials. Water was passed through a reverse osmosis unit and then a Milli-Q reagent water system. Toluene (Fisher, >99.9%), tetradecane (Sigma, 99%), perfluorohexane (Fluoro(17) Clint, J. H.; Taylor, S. E. Colloids Surf. 1992, 65, 61. (18) Denkov, N. D.; Ivanov, I. B.; Kralchevsky, P. A.; Wasan, D. T. J. Colloid Interface Sci. 1992, 150, 589.
Catastrophic Phase Inversion of W/O Emulsions chem, 99%), and diiodomethane (Lancaster, 99%) were columned twice through chromatographic alumina in order to remove polar impurities. A sample of poly(dimethylsiloxane) (PDMS) oil of viscosity 20 cS at 25 °C from Dow Corning was used as received. The background electrolyte used for conductivity measurements was NaCl (Prolabo, 99.5%). The amorphous fumed silica powders were a gift from Wacker-Chemie (Burghausen) with primary particle diameters quoted of between 5 and 30 nm. Hydrophilic silica (HDK N20), prepared by flame hydrolysis, has a BET surface area of 200 m2 g-1 and contains around 2 OH groups/ nm2. Hydrophobic silica (HDK H30), of surface area 250 m2 g-1, is prepared by reacting hydrophilic silica with dichlorodimethylsilane in the presence of water giving a surface density of silanol groups of around 1/nm2, i.e., half of the surface contains silanol (Si-OH) groups and half contains Si-O-Si(CH3)2 groups. The primary particles can aggregate into larger units of about 100 nm in diameter.19 Methods. (a) Preparation and Characterization of Silica Dispersions in Water or Oil. Dispersions of hydrophilic silica in water (of pH around 6) or of hydrophobic silica in oil were prepared by dispersing a known mass of powder into 10 cm3 of liquid using a high-intensity ultrasonic vibracell processor (Sonics & Materials, tip diameter 3 mm), operating at 20 kHz and up to 10 W for 2 min. During sonication, it was necessary to cool the vessel in an ice bath and the resulting solutions were colorless or bluish in appearance. Transmission electron microscopy (TEM) of the colloids was performed using a JEOL 100C 80 kV electron microscope. Ten microliters of dispersion were placed on a copper grid containing a very thin layer of carbon which acts as a support for the solution. The solutions were freeze-dried by immersing the grid in liquid nitrogen for 2 min. Excess nitrogen was removed and the cell was loaded into an Edwards freeze-drier under vacuum for 3 h before being observed in the microscope. Dynamic light scattering was performed on H30 in toluene solutions using a Malvern 4700 PCS instrument employing an Ar-ion laser (488 nm). (b) Preparation, Stability, and Characterization of Emulsions. Two routes were used in preparing the emulsions. In the direct route using hydrophobic silica, the total volumes of oil (containing particles) and water were emulsified using a Janke and Kunkel Ultra Turrax T25 homogenizer (rotor-stator) with an 18 mm head operating at 13 500 rpm for 2 min. The other route involved adding the dispersed phase sequentially to the continuous phase and vice versa, rehomogenizing for 1 min between additions. The particles were intially in oil (hydrophobic) or water (hydrophilic). The emulsions were transferred into stoppered, graduated glass vessels of inner diameter 1.3 cm and length 10 cm and thermostated at 25 °C. Immediately after homogenization, the conductivity of the emulsions was determined using a PTI58 digital conductivity meter with Pt/Pt black electrodes. Emulsion type was also inferred by observing what happened when a drop of each emulsion was added to a volume of either pure oil or pure water. Water continuous (oil continuous) emulsions dispersed in water (oil) and remained as drops in oil (water). The stability of o/w emulsions to creaming was assessed by monitoring the increase with time of the position of the clear water (serum)-emulsion interface. For w/o emulsions, the downward movement of the oil-emulsion boundary was used as a measure of the stability to sedimentation, and the position of the water-emulsion interface was used as an indicator of coalescence. Emulsion drop size distributions were determined using a Malvern MasterSizer MS20 particle sizer employing Fraunhofer diffraction. By use of three different lenses, the instrument covers the range from 0.1 to 600 µm and was calibrated using a monodisperse sample of latex particles in water. Approximately 0.5 cm3 of emulsion was diluted to 50 cm3 with its own continuous phase (either water or oil). This was to ensure that, as far as is possible, dilution occurred without affecting the size distribution. Care was taken to clean the flow cell between samples by rinsing in isopropyl alcohol. Optical microscopy of the emulsions involved adding a small sample to an haemocytometer cell (Weber Scientific) and viewing it with a Nikon Labophot microscope fitted with a DIC-U camera (World Precision (19) Wacker HDK Fumed Silica brochure Nr. 4955e., Wacker-Chemie GmbH, Mu¨nchen, 1998 (December).
Langmuir, Vol. 16, No. 6, 2000 2541
Figure 2. Stability after 30 min, expressed as fraction of emulsion appearing as separated oil or aqueous phase, of waterin-toluene emulsions (φw ) 0.5) stabilized by hydrophobic H30 particles as a function of initial concentration of particles in oil. Open points refer to sedimentation and filled points to coalescence. Instruments). Images were processed using Aldus Photostyler 2.0 software.
Results and Discussion (i) Effect of Particle Concentration and Homogenization Speed. Wacker HDK silica particles are used commercially to thicken a variety of fluids providing thixotropic behavior. The primary particles can fuse into larger aggregates of between 100 and 500 nm in diameter which may arrange further into agglomerates (≈10 µm) until a three-dimensional network is formed. As this network forms, the viscosity of the liquid increases greatly. For hydrophilic N20, the interactions between the particles are due to hydrogen bonds between silanol (Si-OH) groups on the surfaces of adjacent particles. For hydrophobic H30, additional attractive interactions occur between the silylated surfaces. Dispersing H30 particles in toluene using ultrasound produces colorless solutions of similar viscosity to the oil up to around 1 wt %. Between 2 and 5.5 wt %, the solutions acquire a clear blue color of increasing viscosity such that by 6 wt % a gellike sample occurs which does not flow on inverting the vessel. Light scattering from solutions containing between 0.1 and 1 wt % particles indicated the presence of aggregates of particles as opposed to the primary particles themselves, i.e., sample times were in excess of 100 µs before slight correlations in the scattering were observed. Filtering the 0.05 wt % solution through a 200 nm Whatman Anotop 10 membrane resulted in no change to the scattering indicating that aggregate sizes were at least as large as this value. We reported earlier on the stabilization of o/w emulsions using hydrophilic silica particles.15 In contrast, at a volume fraction of water φw equal to 0.5, hydrophobic silica stabilizes w/o emulsions. Figure 2 shows how the emulsion stability varies with initial particle concentration in toluene 30 min after preparation. The ordinate is the fraction of emulsion which forms either a separate oil phase (via sedimentation) or a separate water phase (via coalescence). The stability to sedimentation increases progressively with particle concentration such that by 3 wt % no oil is released. Since sedimentation velocity depends on both the emulsion drop size and the continuous (oil) phase viscosity, and since the average drop size remains unchanged in this range (later), the increasing
2542
Langmuir, Vol. 16, No. 6, 2000
Binks and Lumsdon
Figure 3. Drop diameter volume distributions for water-intoluene emulsions (φw ) 0.5) stabilized by 2 wt % H30 for various concentrations (in wt %, given) of particles in toluene as the diluting phase.
stability is most likely a result of the increase in the viscosity of the oil phase mentioned earlier. Below 0.25 wt % coalescence is substantial but becomes zero for all particle concentrations above this. In addition, these emulsions remained completely stable for at least 12 months. In determining emulsion drop size distributions by a variety of techniques, it is usually necessary to dilute the emulsion to low dispersed phase volume fractions. In surfactant-containing systems, the diluting phase should be of the same composition as the continuous phase of the emulsion so that desorption of surfactant from emulsion drop interfaces does not occur. If, for o/w emulsions, pure water is used instead of an aqueous surfactant solution then drop growth via coalescence can occur during the dilution since surfactant molecules are very labile species. To obtain a protocol for diluting emulsions containing solid particles, we investigated the effect of changing the composition of the diluting phase on the subsequent size distributions. Figure 3 depicts the drop diameter distributions for w/o emulsions prepared from 2 wt % of particles initially in oil. To a first approximation, the distributions remain unchanged when diluting with pure toluene (0%) or with particle/toluene solutions and we thus use pure toluene for all subsequent determinations. We suggest that since these emulsions are very stable to coalescence, particles once adsorbed to the oil-water interface are strongly held there and dilution results in reducing the drop volume fraction at constant drop size without desorption. The emulsion drop size distributions are shown in Figure 4 for different particle concentrations. At low concentrations we observe two populations of drops whose diameters are centered around 0.6 µm and 50 µm. It is important to note that over 80 vol % of the drops are in the range of diameters between 0.1 and 1 µm. For this modest energy input during homogenization the fact that these stable, fine emulsions can be prepared implies that such solid particles are both very good emulsifiers and stabilizers compared with conventional surfactants. Upon increasing the particle concentration, the larger drop size fraction disappears and the two populations merge into one. The improved emulsification efficiency is in line with the increase in emulsion stability shown in Figure 2. During emulsification, an increase of the energy consumption by the mixture invariably leads to a reduction
Figure 4. Drop diameter volume distributions for water-intoluene emulsions (φw ) 0.5) stabilized by H30 for different initial concentrations of particles (in wt %, given) in toluene.
Figure 5. Effect of homogenization speed for 2 min on the various volume drop diameters for water-in-toluene emulsions (φw ) 0.1) stabilized by 2 wt % H30. Here v(x) is the drop diameter below which 100x % of drops are present.
in the average drop diameter for surfactant-stabilized systems.20 The data in Figure 5 shows that this is not the case for the w/o emulsions stabilized by H30. The average drop diameter (0.6 µm) and the span of the distribution (given as v(0.9) - v(0.1)/v(0.5) ) 2.5) do not vary significantly between 8000 and 24 000 rpm of the Ultra Turrax. The reason for this is that even at the lowest speed emulsification is so effective in reducing the drop size, which reaches a limit for this particular system. (ii) Effect of Dispersed Phase Volume Fraction. In equilibrium oil-water-surfactant systems, multiphases coexist which can be of several types. Winsor I systems are two phase comprising an o/w microemulsion plus an excess oil phase. Winsor II systems are also two (20) Walstra, P.; Smulders, P. E. A. In Modern Aspects of Emulsion Science; Binks, B. P., Ed.; Royal Society of Chemistry: Cambridge, 1998; pp. 56-99.
Catastrophic Phase Inversion of W/O Emulsions
Langmuir, Vol. 16, No. 6, 2000 2543
Figure 6. Schematic representation of both transitional (by changing system HLB) and catastrophic (by changing φw) phase inversion of emulsions and, in the case of surfactant-containing systems, the transitions between Winsor systems with system HLB. For the solid-stabilized emulsions studied here, path B to A upon increasing φw is followed for hydrophobic silica, and path B′ to A′ upon decreasing φw is followed for hydrophilic silica.
phase but a w/o microemulsion coexists with an excess water phase. Three phase systems (Winsor III) are made up of a surfactant-rich phase (containing oil and water) plus excess oil and water phases. The locus of formation of the surfactant aggregates changes from water to oil as the system hydrophile-lipophile balance (HLB) is decreased in some way. For ionic surfactants this can be effected by increasing the electrolyte concentration leading to the progression Winsor I-III-II.21 These findings have been rationalized in terms of changes in the spontaneous curvature of the monolayer coating the microemulsion droplets, driven by variations in the effective geometry in situ of the surfactant adsorbed at the oil-water interface.22 It is frequently observed that the type of emulsion formed by homogenization of the Winsor system is the same as that of the equilibrium microemulsion,23 e.g., emulsification of a Winsor I system generally gives an o/w emulsion, the continuous phase of which is itself an o/w microemulsion. Converting one type of emulsion into another involves phase inversion which can be one of two kinds (Figure 6).24 Transitional inversion is induced by changing factors which affect the system HLB like temperature or salt concentration. Catastrophic inversion is induced by increasing the fraction of the dispersed phase and results in a sudden change in behavior of the system following a gradual change in conditions. Figure 6 is a schematic plot linking the system HLB with the Winsor system and the volume fraction of water, φw, the thick line being the inversion locus. The preferred emulsion type is o/w for high HLB and w/o for low HLB systems. Their stabilities increase on approaching the inversion locus, i.e., B and B′ emulsions are not too stable. Catastrophic inversion occurs from o/w to w/o around φw equal to 0.3 for high HLB systems (B′ to A′), and from w/o to o/w around φw equal to 0.7 for low HLB systems (B to A). Both occur when the volume fraction of dispersed phase reaches the close-packed condition for spheres of around 0.7. The abnormal emulsions A and A′ are frequently very unstable and can be multiple emulsions,25 e.g., A may be (21) Binks, B. P.; Dong, J.; Rebolj, N. Phys. Chem. Chem. Phys. 1999, 1, 2335. (22) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. In The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloor, D. M., Wyn-Jones, E., Eds.; Kluwer: Amsterdam, 1990; p 557. (23) Binks, B. P. Langmuir 1993, 9, 25. (24) Salager, J.-L. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1988; Vol. 3, p 79. (25) Brooks, B. W.; Richmond, H. N. Chem. Eng. Sci. 1994, 49, 1065.
Figure 7. Conductivity of water-toluene emulsions stabilized by hydrophobic H30 silica particles as a function of the volume fraction of water. (a) Oil contained 2 wt % particles; water added sequentially to oil (open points) or oil added sequentially to water (filled points). (b) Emulsions prepared at each φw by direct mixing of components. Open points: 1 wt % particles in emulsion. Filled points: 2 wt % particles in oil initially.
a water-in-oil-in-water (w/o/w) emulsion. By analogy, for solid-stabilized emulsions the system HLB which determines the preferred emulsion type may be varied by changing the wettability of the particles. We hypothesise that hydrophobic particles give rise to low HLB systems and hydrophilic particles form high HLB systems. In addition, in one and the same system, it may be possible to phase invert the emulsion via catastrophic inversion without altering the particle wettability. In this way, both types of emulsion may be prepared from only one kind of solid. In agreement with our predictions, we will show that emulsions stabilized by hydrophobic silica can be inverted from w/o to o/w upon increasing φw, and emulsions stabilized by hydrophilic silica can be inverted from o/w to w/o upon increasing the volume fraction of oil, φo. Conductivity, light diffraction, and optical microscopy are used to characterize the type and stability of emulsions before and after inversion. Starting with hydrophobic H30 particles, Figure 7 shows the variation of the emulsion conductivity with φw. The experiment has been performed in four ways and the results are virtually identical. In (a), emulsions were prepared either by sequential addition of water to toluene or by sequential addition of toluene to water, the oil containing 2 wt % particles in both cases. As seen the conductivity is low and close to that of pure toluene (0.1 µS cm-1) at low φw and increases by 4 orders of magnitude at high φw. Below φw ) 0.7 the emulsions dispersed into toluene but not into water, whereas above this value they dispersed into water and not toluene. Phase
2544
Langmuir, Vol. 16, No. 6, 2000
Binks and Lumsdon
Figure 9. Stability after 1 h of toluene-water emulsions stabilized by H30 particles as a function of φw. Triangles refer to coalescence; circles refer to sedimentation for w/o and creaming for o/w emulsions.
Figure 8. Fraction of continuous phase resolved versus time for toluene-water emulsions stabilized by hydrophobic H30 particles (2 wt % in oil). (a) Sedimentation of w/o emulsions for different φw given. (b) Creaming of o/w emulsions for different φw given.
inversion from w/o to o/w emulsions thus occurs at this volume fraction and there is no sign of any hysteresis. This is to be contrasted with surfactant-stabilized emulsions where the two curves may be separated by as much as 0.3 in volume fraction. Hysteresis is linked with irreversible instability phenomena sometimes referred to as catastrophes. In (b), emulsions were prepared separately at each φw by direct mixing of the components. The two data sets refer to a fixed concentration of particles in oil ([particles] in different emulsions varies) and a constant particle concentration in all the emulsions. The agreement between the data sets is encouraging and confirms that, irrespective of the route or particle concentration, emulsion phase inversion can be achieved without altering the particle wettability. We also note that the emulsion viscosity passes through a maximum near the inversion condition. The stabilities of the above emulsions are very sensitive to the volume fraction of water. Importantly however, both w/o and o/w emulsions at any φw are completely stable to coalescence with no sign of the dispersed phase separating in over 6 months. This is quite remarkable and in marked contrast to surfactant systems in which coalescence is appreciable around phase inversion.26 Figure 8a shows the time course of the sedimentation of the w/o emulsions, while Figure 8b relates to the creaming of the o/w emulsions. The ordinates indicate the fraction of continuous phase which is resolved. For w/o emulsions, stability increases with φw, with a noticeable increase between 0.2 (26) Binks, B. P.; Cho, W-G.; Fletcher, P. D. I.; Petsev, D. N. Langmuir 2000, 16, 1025.
Figure 10. Effect of φw (given) on the drop diameter volume distributions for toluene-water emulsions stabilized by H30 particles prepared by sequential addition of water to oil (containing 2 wt % particles). Emulsions are w/o for φw e 0.65 and o/w above this.
and 0.3 (cf. vertical dotted line in Figure 6). For o/w emulsions, stability decreases with φw with creaming virtually complete after 1 h. Figure 9 summarizes the effect of φw on emulsion stability after 1 h where a maximum can be seen. The most stable emulsions which are w/o are close to inversion and the stabilities of the o/w emulsions are significantly lower than any of the w/o emulsions. We will see below that this large difference in the relative stabilities is due in part to very different drop sizes in the two types of emulsion. At a constant initial particle concentration in oil of 2 wt %, Figure 10 presents the drop diameter distributions as a function of φw for emulsions prepared by sequential addition of water to oil. Up to φw ) 0.6 (w/o emulsions),
Catastrophic Phase Inversion of W/O Emulsions
Langmuir, Vol. 16, No. 6, 2000 2545
Figure 11. Drop diameters versus φw for toluene-water emulsions stabilized by H30 particles. d (4,3) is the arithmetic mean diameter, d (v, 0.5) is the diameter below which 50 vol % of drops lie. Circles: emulsions prepared by sequential addition; diamonds: emulsions prepared by direct addition.
the distribution changes from a single log-normal one at low volume fraction to a bimodal distribution at higher volume fractions, although the majority of drops are very small and less than 1 µm. Since the particle concentration in the system falls progressively, it appears that some of the additional water added creates a new interface which may not be covered sufficiently by adsorbed particles, so that drop growth by coalescence becomes possible during emulsification. For the two o/w emulsions shown at high φw, a single distribution is found since the diameters are now much larger (around 100 µm) with a concomitant reduction in interface area. The variation of the emulsion drop diameters through phase inversion is shown in Figure 11, where both the arithmetic mean of the distribution (d (4,3)) and the diameter below which 50% of drops exist (d (v, 0.5)) are plotted. The mean diameter, which is more representative of the whole distribution of drop sizes in these cases, increases progressively to 10 µm up to inversion and exhibits a jump by a factor of 10 once the o/w emulsions are formed. The d (v, 0.5) is more biased to the small size drop fraction and a discontinuous change occurs between w/o and o/w emulsions. Included in Figure 11 are sizes of emulsions prepared separately at each φw by direct mixing (diamonds). The fact that the two data sets overlap indicates that phase inversion does not depend on the previous history of the sample. The slow sedimentation rate observed for w/o emulsions compared with the fast creaming rate for o/w emulsions is in line with differences in the average drop sizes of around 100 µm between the two types. During phase inversion, coalescence of water (or oil) drops must occur as the originally continuous oil (or water) phase forms drops. Since the volume fraction of the dispersed phase at inversion is either 0.7 upon increasing the water content or 0.3 upon increasing the oil content (Figure 7), the mechanism of inversion is presumably different in the two cases. Inversion occurs from a state of close-packed, small drops to a more dilute state of large drops or vice versa. The volume fraction of 0.7 is near to the limit of close-packing of spherical drops implying that coalescence sets in as soon as the drops are forced into closer proximity such that they begin to deform. Above inversion, the o/w emulsions cream to an extent which depends on the initial φw. We have verified that the continuous phase of the cream is water and Figure 12a
Figure 12. Toluene-in-water emulsions stabilized by hydrophobic H30 particles. (a) Volume fraction of oil in creamed emulsion versus initial volume fraction of water. Micrographs of o/w emulsion from initial φw ) 0.85 sampled from bottom of cream (b) and top of cream (c). The bar represents 50 µm for both images.
shows how the volume fraction of oil in the cream varies with φw. During creaming, φo increases with a larger change occurring at higher φw. It is worth pointing out that even for the most concentrated cream (φw ) 0.95), the value of φo is, within error, just less than that at the critical inversion condition and hence it itself does not invert but remains o/w. In forming the cream, there is a fractionation in size of the drops as the larger ones rise first. Figure 12b and c shows images of the emulsions taken from the bottom and top of the cream layer, respectively. Near the base of the cream, the smaller drops are nearly all spherical and slightly flocculated. The larger drops present at the top of the cream are nonspherical exhibiting peculiar shapes. These drops move very slowly across the field of view. As two or three oil domains come into mutual contact, coalescence does not occur but the interfaces are seen to deform followed by drop separation. Unlike surfactant systems, there was no evidence of multiple w/o/w drops. Since demonstrating phase inversion in hydrophobic particle-containing emulsions, we were intrigued to know how general this phenomenon was and therefore briefly investigated a second system. We find that hydrophilic N20 silica particles stabilize o/w emulsions in equal volume
2546
Langmuir, Vol. 16, No. 6, 2000
Binks and Lumsdon
Figure 14. Conductivity of oil-water emulsions stabilized by H30 particles versus φw. Oils are perfluorohexane (triangles, 2 wt %), tetradecane (circles, 2 wt %), PDMS (squares, 2 wt %), and diiodomethane (crosses, 0.5 wt %). Tetradecane and water at pH ) 1 (diamonds, 2 wt %) also shown.
oil and water systems, which can be phase inverted on addition of further oil. In Figure 13a we show how the emulsion conductivity varies with φw. Starting with the aqueous particle dispersion (open points), the conductivity falls gradually on increasing the volume fraction of oil since the oil drops formed act as obstructing entities in the conduction of the aqueous phase. Between φo equal to 0.6 and 0.7, the conductivity falls precipitously to very low values indicative of inversion to w/o emulsions. Although the latter emulsions are very unstable, catastrophic phase inversion is also possible in these solidstabilized emulsions. Again, we observe no hysteresis in the inversion (filled points). In contrast to w/o emulsions stabilized by hydrophobic particles, o/w emulsions stabilized by N20 are of average diameter around 100 µm (Figure 13b) and therefore cream rapidly. Since the N20 particles are very hydrophilic this lowers the tendency for them to collect at the oil-water interface. The H30 particles are only partially hydrophobic and adsorb more effectively. (iii) Effect of Oil Type and Aqueous Phase Composition. In emulsions stabilized by a particular surfactant, the type of oil is important in dictating the emulsion type for equal volumes of oil and water. Short chain alkanes and aromatic oils favor w/o emulsions whereas long chain alkanes and polymeric silicone oils favor o/w emulsions.27 The reasons for this difference are related to the extent of oil penetration into the surfactant chain region and hence to the preferred curvature of the
oil-water interface. In an attempt to see if the type of oil is important in controlling the wettability of the hydrophobic silica particles, we have looked at a fluorinated oil, a linear alkane, a silicone oil, and an iodine-containing oil. These were chosen so as to vary the cohesive energy density (ced) of the oil, defined as the enthalpy required to vaporize a unit volume of liquid, from a low (fluorinated) to a high extreme (iodine-containing). The ced is reflected in, for example, the liquid-vapor surface tension, γ, and the contact angle the liquid makes with a hydrophobic solid surface in air, θa. Values of γ (in mN m-1) and θa (in °) at 25 °C respectively are 11.4 and 0 for perfluorohexane, 27.9 and 50 for toluene and 51.3 and 102 for diiodomethane, i.e., the cohesion between molecules in the pure liquid is lowest for fluorocarbons, is intermediate for hydrocarbons (silicone oils behaving similarly), and is highest for diiodomethane (due mainly to its high refractive index of 1.7425 and hence polarizability). The conductivity of emulsions in these various oil-water mixtures is shown in Figure 14 where it can be seen that inversion from w/o to o/w occurs for perfluorohexane, tetradecane, and diiodomethane around φw equal to 0.65 as for toluene. For PDMS, emulsions remained w/o up to φw ) 0.9 and the conductivity increases slightly. Since it is known that PDMS adsorbs onto silylated silica,28 this may increase the hydrophobicity of the particles further so that the system cannot be tuned to form o/w emulsions even at high volume fractions. From these results it can be concluded that the preferred emulsion type in the case of these hydrophobic particles is w/o and that the type of oil has no influence on this. Since part of the surface of the H30 particles is composed of silanol groups which are capable of becoming ionized and hence more water-liking, we wondered whether the preferred emulsion type for φw ) 0.5 could be switched from w/o to o/w at extremes of aqueous phase pH. The isoelectric point for untreated silica is around 2, with surfaces being positively charged below this and negatively charged above it. Emulsions of tetradecane and water containing 2 wt % particles were w/o at pH ) 1 (adding HCl) and pH ) 10 (adding NaOH), and those at pH 1 inverted in the usual way (Figure 14) again indicating that changing the wettability of these particles in these ways is not so straightforward.
(27) Aveyard, R.; Binks, B. P.; Mead, J. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1755.
(28) Barthel, H.; Ro¨sch, L.; Weis, J.; Khalfi, A.; Balard, H.; Papirer, E. Composite Interfaces 1999, 6, 27.
Figure 13. Toluene-water emulsions stabilized by hydrophilic N20 silica particles. (a) In situ conductivity as a function of φw. Oil added sequentially to 10 wt % silica in water (open points) or water added to oil (filled points) homogenizing at 24 000 rpm for 1 min between additions. (b) Drop diameter distribution for o/w emulsion at φw ) 0.5 (2 wt % silica).
Catastrophic Phase Inversion of W/O Emulsions
Conclusions The following conclusions can be drawn concerning the properties of emulsions containing silica particles alone as studied here: •Water-in-toluene emulsions stabilized by hydrophobic silica can be prepared with a small drop diameter (0.6 µm) which are stable to coalescence indefinitely. •Their stability to sedimentation increases with an increase in particle concentration, due to an increase in viscosity of the continuous oil phase. •Catastrophic phase inversion of the emulsions from w/o to o/w occurs at a volume fraction of water close to 0.7. No hysteresis is observed during the transition.
Langmuir, Vol. 16, No. 6, 2000 2547
•Dramatic changes in the emulsion stability and drop diameter occur on inversion. •Preferred emulsion type is w/o irrespective of the nature of the oil or the aqueous phase pH. •Oil-in-water emulsions stabilized by hydrophilic silica invert to w/o at oil volume fractions of around 0.7. The preferred emulsions are also very stable to coalescence. Acknowledgment. We would like to thank the EPSRC and ICI Paints (Slough) for a CASE award to fund S.O.L. LA991081J
