Rheological Behavior of Water-in-Oil Emulsions Stabilized by

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Rheological Behavior of Water-in-Oil Emulsions Stabilized by Hydrophobic Bentonite Particles Bernard P. Binks, John H. Clint, and Catherine P. Whitby* Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull, HU6 7RX, United Kingdom Received January 28, 2005. In Final Form: March 24, 2005 A study of the rheological behavior of water-in-oil emulsions stabilized by hydrophobic bentonite particles is described. Concentrated emulsions were prepared and diluted at constant particle concentration to investigate the effect of drop volume fraction on the viscosity and viscoelastic response of the emulsions. The influence of the structure of the hydrophobic clay particles in the oil has also been studied by using oils in which the clay swells to very different extents. Emulsions prepared from isopropyl myristate, in which the particles do not swell, are increasingly flocculated as the drop volume fraction increases and the viscosity of the emulsions increases accordingly. The concentrated emulsions are viscoelastic and the elastic storage and viscous loss moduli also increase with increasing drop volume fraction. Emulsions prepared from toluene, in which the clay particles swell to form tactoids, are highly structured due to the formation of an integrated network of clay tactoids and drops, and the moduli of the emulsions are significantly larger than those of the emulsions prepared from isopropyl myristate.

Introduction Macroemulsions are mixtures of two immiscible liquids, one of which is dispersed as micrometer-sized drops within the other liquid. The flow behavior of these systems, which may combine liquids with very different viscosities and responses to shear, can vary from Newtonian to highly viscoelastic, depending on factors such as the drop size and the relative proportions of the two liquids. Due to the large interfacial area between the dispersed and continuous phases, emulsions are thermodynamically unstable. Emulsions can be kinetically stabilized by the adsorption of surfactant or low molecular weight polymers at the oil-water interface, and the rheology of surfactant- and polymer-stabilized emulsions has been extensively studied.1,2 Colloidal particles can also be used to kinetically stabilize emulsions.3 Particles adsorb at the oil-water interface and sterically hinder the close approach of drops, thus reducing the extent of coalescence. Tambe and Sharma4,5 have shown that, at sufficiently high concentrations of adsorbed particles, the oil-water interface is viscoelastic, which should reduce the extent of film drainage between drops. The interfacial elasticity, in particular, increases dramatically with increasing particle concentration. Particles in emulsions may also form networks in the continuous phase that trap drops and impede the drainage of liquid films between droplets, thus hindering coalescence.6,7 The literature describing rheological studies of emulsions containing solid particles can be divided into studies * To whom correspondence should be addressed: e-mail [email protected]. (1) Dickinson, E. In Modern Aspects of Emulsion Science; Binks, B. P., Ed.; Royal Society of Chemistry: Cambridge, U.K., 1998; p 145. (2) Mason, T. G. Curr. Opin. Colloid Interface Sci. 1999, 4, 231. (3) Aveyard, R.; Binks, B. P.; Clint, J. H. Adv. Colloid Interface Sci. 2003, 100-102, 503. (4) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1994, 162, 1. (5) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1995, 171, 456. (6) Abend, S.; Bonnke, N.; Gutschner, U.; Lagaly, G. Colloid Polym. Sci. 1998, 276, 730. (7) Lagaly, G.; Reese, M.; Abend, S. Appl. Clay Sci. 1999, 14, 83.

of mixtures of surfactant-stabilized emulsions and micrometer-sized particles (suspoemulsions) and studies of nanoparticle-stabilized emulsions. Masliyah and coworkers8-10 observed that the addition of solid particles (with sizes typically ranging from 10 to 40 µm) to surfactant-stabilized emulsions (with average drop diameters of about 10 µm) enhanced the viscosity of the emulsions. The magnitude of the change increased as the size of the added particles decreased or where the particles were irregularly shaped compared to spherical particles.9 Where there was no great affinity between the particles and drops and the particles were much larger (3-4 times) than the drops, it was found possible to predict the viscosity of the mixtures from the viscosities of the emulsion, particle suspension, and pure continuous phase.8,10 Pal11 found that where there was significant flocculation between drops and particles (heteroflocculation), the viscosity of the mixtures was much higher than expected. Lagaly et al.12 linked the rheological properties of paraffin-in-water emulsions stabilized by smectite clay particles to the flow behavior of the aqueous smectite dispersions. Dispersions of montmorillonites in water were observed to show Newtonian flow behavior, and emulsions stabilized by the montmorillonites showed slightly pseudoplastic behavior, with shear stresses and viscosities higher than those of the dispersions, due to the presence of the oil drops. For emulsions containing a co-emulsifier in the oil phase that adsorbed strongly to the clay, the pseudoplastic behavior was pronounced and concentrated emulsions showed linear viscoelastic behavior with large elastic storage moduli. This was attributed to the strong attachment of the particles to the oil-water interface. Midmore13 found that the elastic storage moduli of oil-in-water (o/w) emulsions stabilized by mixtures of silica particles and nonionic surfactants increased as the drop size decreased and showed little correlation with the moduli of the dispersions of particles in surfactant solutions. It was (8) Pal, R.; Masliyah, J. Can. J. Chem. Eng. 1990, 68, 24. (9) Yan, Y.; Pal, R.; Masliyah, J. Chem. Eng. Sci. 1991, 46, 985. (10) Yan, Y.; Pal, R.; Masliyah, J. Chem. Eng. Sci. 1991, 46, 1823. (11) Pal, R. Chem. Eng. Commun. 1993, 121, 81. (12) Lagaly, G.; Reese, M.; Abend, S. Appl. Clay Sci. 1999, 14, 279. (13) Midmore, B. R. Colloids Surf. 1998, 145, 133.

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proposed that the nonionic surfactants competitively adsorbed at the oil-water interface, thus reducing the surfactant concentration available to form the flocculated particle structures formed in the dispersions. Abend et al.6 studied the rheological properties of paraffin-in-water emulsions stabilized by mixtures of a layered double hydroxide particle and montmorillonite. In the absence of montmorillonite, the emulsions showed Newtonian flow behavior and it was argued that the hydroxide particles adsorb at the oil-water interface to stabilize the drops. Over a wide range of fractions of montmorillonite in the stabilizing mixture, the emulsions formed were highly elastic, which was attributed to the positively charged hydroxide particles forming a network with the negatively charged montmorillonite particles in the water phase, which stabilized the drops. The viscosity and yield stress of the emulsions passed through a maximum at relatively high fractions of montmorillonite in the stabilizing mixture, which was thought to correspond to the ratio of solids where the magnitude of the surface charges on the two types of particles was the same. The flow and deformation behavior of particle-stabilized emulsions will depend on factors that affect the emulsion structure: drop size, dispersed phase volume fraction, particle concentration, and the structure of the interfacial region, as well as the nature of the interactions between the particles and drops in the continuous phase. We report here a series of experiments that examine both the flow viscometry and viscoelastic behavior of waterin-oil (w/o) emulsions stabilized by hydrophobic clay particles as the drop volume fraction is varied by dilution, while the drop size and overall concentration of particles in the emulsion is held constant. The hydrophobic clay particles swell very differently in the two oils used, isopropyl myristate (IPM) and toluene, and thus the influence of the structure of the oil dispersions of clay on the emulsion rheology is also investigated. Experimental Section (i) Materials. All water used was first passed through an Elga reverse osmosis unit and then a Milli-Q reagent water system. The oils used, isopropyl myristate (IPM, Aldrich) and toluene (Fisher), were of purity greater than 99%. Prior to use, they were passed twice through columns of chromatographic alumina to remove polar impurities. The hydrophobic clay, bentonite for organic systems, was obtained from Fluka, which acts as a distributor for hydrophobic clay particles called Tixogel VP manufactured by Su¨d-Chemie (see later). (ii) Methods: (a) Preparation of Dispersions and Emulsions. The dispersions were prepared by dispersing a known mass of hydrophobic clay in oil by first stirring (to ensure the solid was wet by the oil) and then using a high-intensity ultrasonic vibracell processor (Sonics & Materials, tip diameter 3 mm) operating at about 20 kHz and up to 10 W for 4 min. In preparing concentrated w/o emulsions, dispersions of hydrophobic clay in oil were emulsified with water using a Janke and Kunkel Ultra-Turrax rotor stator device with an 18 mm head operating at 13 000 rpm for 2 min. The emulsion type was inferred by observing whether a drop of the emulsion dispersed when added to a small volume of water or oil. The drop volume fraction of the emulsions was reduced at constant particle concentration by gently mixing together appropriate volumes of the concentrated emulsion and dispersions of hydrophobic clay in oil in stoppered measuring cylinders (to prepare between 30 and 50 mL of emulsion). Some batch emulsions at various water volume fractions were prepared by homogenizing together the required volumes of water and dispersions of hydrophobic clay in oil for different times to obtain emulsions with different average drop diameters. The emulsions were thermostated at 25 °C. The stability over time of the emulsions was determined by monitoring the appearance and movement of the oil-emulsion and water-emulsion interfaces. The emulsion drop size distributions were determined by both

Binks et al. optical microscopy and light scattering techniques. Small, dilute samples of the emulsions were added to a hemocytometer cell (Weber Scientific) and viewed with a Nikon Labophot microscope fitted with a DIC-U (World Precision Instruments) camera. The images were processed with Adobe Photoshop 5.0 software. The volume weighted droplet diameter distribution of the emulsions (and the particle size distributions in the dispersions) was determined using a Malvern Mastersizer 2000. The emulsion samples were diluted in oil and circulated through the dispersion unit. The optical unit was cleaned between samples by rinsing with isopropyl alcohol. Photographic images of emulsions and planar oil-water interfaces were recorded by use of a Kodak DX 4330 digital camera. The images were processed with Adobe Photoshop 5.0 software. The structure of the adsorbed particle layer at the oil-water interface of the emulsions was imaged by low-temperature field emission scanning electron microscopy (LTFESEM) following the methodology previously described.14 A drop of emulsion was mounted in an aluminum low-temperature SEM stub and frozen in nitrogen slush. The stub was then transferred into a lowtemperature chamber (Gatan Alto 2500) maintained under ultrahigh vacuum where the emulsions were fractured at -150 °C. The exposed emulsion surface was coated with a thin layer of a mixture of gold and palladium. The stub was then transferred onto a cold stage, operating at -150 °C, fitted in the FE-SEM (JEOL 6301F). Images were captured with Analysis and Soft Imaging ADAII system software. (b) Rheological Measurements. The rheological measurements were made using a Bohlin CVO 120 High-Resolution rheometer in the controlled rate mode with a truncated cone (4° cone angle, 40 mm cone diameter) and plate geometry (150 µm gap width). Geometries with either smooth or roughened (by sand blasting) surfaces were used. A few measurements were also made with a (smooth) double-gap geometry (40 mm inner diameter, 50 mm outer diameter). The geometries were cleaned by rinsing or sonicating in isopropyl alcohol for about 5 min in a Decon FS3006 ultrasonic bath. Prior to sampling for measurements, the emulsions were gently shaken to mix the sedimented emulsion drops back into the released oil. A solvent trap was used to prevent evaporation during measurements. The emulsions were typically presheared at a rate of 2 s-1 for 30 s and then left to rest for 30 s. For flow viscometry measurements, after preconditioning, the applied shear rate was scanned up and then down in a series of logarithmic steps. The lowest shear rate applied (0.0716 s-1) was the lowest achievable by the rheometer, while the upper rate was just below that at which the emulsions were ejected from the geometry. For each step, shear was applied for 20 s, with the shear stress being measured and the viscosity of the material calculated during the latter 10 s. Measurements made where the deviation between the achieved shear rate and the target shear rate was greater than 5% were rejected. For oscillatory measurements, after preconditioning, the amplitude (stress) of the oscillation was scanned between a lower and an upper limit in a series of logarithmic steps at a constant frequency (typically 0.1 Hz). The lowest amplitude applied (0.005 96 Pa) was the lowest achievable by the rheometer, while the upper amplitude was below that at which the emulsions were ejected. For each step, the stress was applied for 30 s, with the difference in the oscillating stress and strain response being analyzed over two periods of the waveforms and the shear modulus of the material calculated during the latter 20 s. After a fresh sample was preconditioned, the frequency of the oscillation was then scanned from a higher to a lower limit at a constant stress chosen from the stress range where the maximum strain response was low ( 0.5, due to problems in incorporating large volumes of water into the increasingly viscous dispersions. Examining the drops visually by microscopy, we find that the drops are typically spherical in shape and polydisperse in size, with the average drop diameter decreasing as the concentration of particles in the emulsion increases, as can be seen in Figure 8. For particle concentrations of 1%, 3%, and 5% (w/v) in the emulsion, the volume weighted average drop diameters determined by light scattering are approximately 41, 22, and 10 µm, respectively. The extensive flocculation of the drops is likely to have affected these measurements. LTFESEM images of the drop surfaces reveal adsorbed particle layers with structures similar to those for clay-stabilized waterin-IPM emulsions. The effect of the water volume fraction on the rheology of these emulsions was investigated by diluting the emulsions at constant particle concentration. Dispersions of clay in toluene show an apparent yield stress at low shear rates, while at high shear rates the shear stress increases as a nonlinear function of the shear rate, as shown for 3% (w/v) hydrophobic clay in toluene in Figure 9a. The minimum shear stress that must be applied for flow to occur increases as the particle concentration increases. Throughout these measurements, the truncated cone in the measuring geometry turns freely. In contrast, the flow behavior of emulsions of water in these clay dispersions is unusual, with the shear stress passing through a minimum with increasing shear rate at intermediate shear rates. During these measurements, the motion of the cone is discontinuous at shear rates coinciding with the shear rate regime where the shear stress remains constant or decreases with increasing shear rate. At some higher shear rate, coinciding with the regime where the shear stress increases with increasing shear

Figure 8. Optical micrographs of 50 vol % water-in-toluene emulsions stabilized by 1% (w/v) (upper), 3% (w/v) (middle), and 5% (w/v) (lower) hydrophobic clay in the emulsion. The scale bars represent 50 µm.

rate, the cone begins to turn freely. Unlike for the waterin-IPM emulsions, overall the viscosity of the water-intoluene emulsions does not decrease as the water volume fraction decreases. This behavior is observed to a lesser extent for emulsions stabilized by lower concentrations of hydrophobic clay (which, it should be noted, also have a larger average drop size). There is no evidence of a yield stress, and the minimum in the shear stress occurs at lower shear rates and is less pronounced. Emulsions stabilized by higher concentrations of particles (and with smaller average drop sizes) are so viscous that viscometry measurements cannot be reliably performed. Anomalous flow behavior, where the shear stress decreases with increasing shear rate, has been observed for controlled shear rate measurements of dense suspensions of strongly repulsive colloids undergoing a transition from a polycrystalline to a sliding layer microstructure (see ref 41 and references therein). In controlled stress measurements, a discontinuous jump in the shear rate was observed when the suspension microstructure changed. It has been suggested that, in the anomalous flow regime, the flow field is unstable and regions of different local shear rates and viscosities separated by sharp interfaces develop. Evidence for the occurrence of (41) Gray, J. J.; Bonnecaze, R. T. J. Rheol. 1998, 42, 1121.

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Figure 10. Variation in the shear strain with oscillating stress for water-in-toluene emulsions stabilized by hydrophobic clay at concentrations of 1% (w/v) (circles) and 5% (w/v) (triangles) in the emulsion. The water volume fraction in the emulsions is 0.1 (half-filled symbols) and 0.5 (solid symbols). Also shown is the variation in the shear strain with oscillating stress for dispersions of hydrophobic clay in toluene at concentrations of 1% (w/v) (O) and 5% (w/v) (4).

Figure 9. (a) Variation in shear stress with applied shear rate of a water-in-toluene emulsion stabilized by 3% (w/v) hydrophobic clay in the emulsion at a water volume fraction of 0.5 (2). Also shown are the results for measurements with the same emulsion diluted to φw of 0.4 (9), 0.2 (b), and 0.1 ([) at constant particle concentration. (Results are not shown for φw of 0.3 simply for clarity.) For comparison, the variation in the shear stress with applied shear rate for a dispersion of 3% (w/v) hydrophobic clay in toluene is shown (O). (b) Variation in shear stress with applied shear rate of a water-in-toluene emulsion stabilized by 3% (w/v) hydrophobic clay in the emulsion at a water volume fraction of 0.4. The data were measured as the applied shear rate was increased (9) and then decreased (0). This is an example of the hysteresis observed in the flow behavior of water-in-toluene emulsions stabilized by hydrophobic clay.

shear banding in solutions of entangled wormlike surfactant micelles (the shear stress minimum has been linked to the micelles becoming aligned in the flow) was obtained from NMR microscopy measurements of the velocity distribution in solutions in a cone and plate rheometer during shear.42 Flocculated suspensions have also been found to show anomalous flow behavior, and this was related to structural changes in the dispersions (see ref 43 and references therein). For the system studied here, it is noted that the clay tactoids form networks in toluene that are disrupted as the applied shear rate increases (thus the dispersions show (42) Britton, M. M.; Callaghan, P. T. Phys. Rev. Lett. 1997, 78, 4930. (43) Uriev, N. B.; Trofimova, L. E. Colloid J. 2003, 65, 378.

shear-thinning behavior) but recover as the applied shear rate decreases. In contrast, the shear stress (and hence viscosity) of the emulsions measured at low shear rates as the applied shear rate increases is higher than that measured as the shear rate decreases, as shown by the example in Figure 9b. With the emulsification of water into the dispersions, a proportion of the particle population adsorbs at the oil-water interface created to stabilize the drops. It is likely that some of the particles adsorbed on the drop surfaces may remain part of the tactoid network in the oil phase, which would strengthen the network. Thus for flow to occur, the adsorbed particle layers on the drops may have to be disrupted. It should be noted that similar flow behavior is observed for measurements made with a double-gap geometry, indicating that sedimentation of the water drops during measurements is unlikely to be the cause of the anomalous flow. The shear rate at which the transition occurs in the flow behavior increases with increasing particle concentration and drop volume fraction, as would be expected. Similar phenomena are not observed for the shearing of water-in-IPM emulsions stabilized by hydrophobic clay due to the very different structure of the clay dispersions in IPM. It is noted that anomalous flow behavior was observed for 25 vol % oilin-water (at pH 6) emulsions stabilized by Ludox silica, where NaCl was added after emulsification to give an ionic strength of 1 M in the aqueous phase.44 Prior to emulsification, the silica particles were flocculated by the addition of hydroxypropyl cellulose, and it was thought that the emulsions were stabilized by adsorption of the flocs onto the drop surfaces. Since the anomalous flow behavior was not observed in the absence of salt, it was proposed that the electrolyte caused the flocculation of silica particles adsorbed on different drops. Thus the small shear stress minimum observed was attributed to the disruption of the drop aggregates. The larger shear stress minima observed here probably reflect the greater extent of networking between drops and tactoids in the oil phase. In Figure 10, examples of the variation in shear strain with applied oscillating stress for dispersions of clay in (44) Midmore, B. R. Colloids Surf. A 1998, 132, 257.

Water-in-Oil Emulsions Stabilized by Bentonite

Figure 11. Variation in the elastic (solid symbols) and viscous (open symbols) moduli with the water volume fraction of waterin-toluene emulsions stabilized by hydrophobic clay at concentrations of 1% (w/v) (b, O), 3% (w/v) (9, 0), and 5% (w/v) (2, 4) in the emulsion. Since both types of moduli are frequencyindependent, the average values of the moduli measured over the frequency range studied are shown for each volume fraction.

toluene at different particle concentrations and emulsions at different water volume fractions are shown. For dispersions at the lowest particle concentration (1% w/v, O), the shear strain at low stresses is relatively high and increases to values greater than 1 with increasing shear stress, indicating that the displacement of the dispersion under these stresses is equal to or greater than the gap between the cone and plate and thus the dispersion is permanently deformed by the oscillating stress. For emulsions of water in the oil dispersion at water volume fractions ranging from 0.1 to 0.6, the overall shear strain is significantly lower at low applied stress and is a linear function of shear stress over a wide range of applied stresses. Thus the emulsions show a linear viscoleastic response at low stresses but are permanently deformed at very high stresses. Linear viscoelastic behavior is observed for dispersions of hydrophobic clay in toluene at higher concentrations. With the emulsification of water into these dispersions, the modulus becomes so high that, at small stresses, the resulting small strains are close to the resolution limits of the instrument and inertia effects dominate. Over the stress range that could be studied, the emulsions show linear viscoelastic behavior. In the linear viscoelastic behavior regime, the elastic and viscous moduli of the emulsions and the dispersions of clay in toluene are roughly independent of frequency. The elastic moduli are typically an order of magnitude larger than the viscous moduli, indicating that elastic storage dominates the behavior of these emulsions. For emulsions stabilized by 1% (w/v) hydrophobic clay in the emulsion, the moduli increase by 2 orders of magnitude as the water volume fraction increases by a factor of 6 as shown in Figure 11. It should be noted that, for these emulsions, the oil dispersion of particles does not show linear viscoelastic behavior. In contrast, for emulsions prepared from dispersions at higher concentrations, which do show linear viscoelastic behavior, there is a significant increase between the moduli of the emulsions and the moduli of the dispersions, while the increase in the moduli with increasing water volume fraction is less dramatic. It should be noted that the average drop diameter of the emulsions decreased as the particle concentration increased (shown previously).

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Figure 12. Variation in the elastic moduli (G′) normalized by the ratio of the oil-water interfacial tension (γow) to the average drop radius (r) of water-in-toluene emulsions stabilized by hydrophobic clay, at concentrations of 1% (w/v) (O), 3% (w/v) (0), and 5% (w/v) (4) hydrophobic clay in the emulsion, with the water volume fraction. The average drop radii of the emulsions were 41, 22, and 10 µm, respectively. Also shown is the variation in the normalized elastic moduli of water-in-IPM emulsions stabilized by 5% (w/v) (2) hydrophobic clay in the emulsion with the water volume fraction. The average drop diameter of these emulsions was 49 µm. The extra data points show the normalized elastic moduli for batch water-in-IPM emulsions stabilized by 5% (w/v) hydrophobic clay in the emulsion, with mean drop diameters of 18 µm (9), 24 µm (1), and 38 µm (b).

Given that the emulsions behave predominantly as elastic solids, the volume fraction dependence of the elastic modulus may provide information about the source of the elasticity in these emulsions. Mason et al.45 found that the elastic moduli of monodisperse, concentrated o/w emulsions stabilized by sodium dodecyl sulfate could be scaled by the ratio of the oil-water interfacial tension (γow) to the drop radius (r), as predicted by Princen,39,40 and obtained a universal master curve for the increase in elastic modulus with increasing drop volume fraction. Thus the elastic moduli of concentrated surfactantstabilized emulsions were shown to be governed by the Laplace pressure. Arditty et al.46 observed that although the elastic moduli of silica particle-stabilized oil-in-water emulsions scaled with the ratio γow/r, the normalized moduli were significantly larger than the elastic moduli of surfactant-stabilized emulsions. They proposed that the source of the interfacial elasticity was adhesive interactions between the adsorbed particles due to interpenetration of the grafted layers of octyltriethoxysilane on the particle surfaces. In Figure 12, the elastic moduli of the water-in-toluene and water-in-IPM emulsions stabilized by hydrophobic clay normalized by the ratio γow/r are shown as a function of the water volume fraction. (The interfacial tensions used were those for bare oil-water interfaces.) Of course, here the drop volume fractions are lower than that required for close packing of the drops, so the source of the elasticity under these conditions should be drop flocculation. The normalized elastic moduli for the waterin-IPM emulsions stabilized by 5% (w/v) hydrophobic clay (45) Mason, T. G.; Lacasse, M.-D.; Grest, G. S.; Levine, D.; Bibette, J.; Weitz, D. A. Phys. Rev. E 1997, 56, 3150. (46) Arditty, S.; Schmitt, V.; Giermanska-Kahn, J.; Leal-Calderon, F. J. Colloid Interface Sci. 2004, 275, 659.

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(2) are remarkably similar to those observed by Arditty et al.46 for silica particle-stabilized o/w emulsions at the same drop volume fractions (not shown). These emulsions have an average drop diameter of 49 µm (shown previously), and to further investigate this behavior, batch water-in-IPM emulsions stabilized by 5% (w/v) hydrophobic clay were prepared with different mean drop diameters. The normalized elastic moduli for the batch emulsions fall on the same curve, indicating that the elastic moduli of the clay-stabilized water-in-IPM emulsions scale with the ratio γow/r. In contrast, the normalized elastic moduli for the hydrophobic clay-stabilized water-intoluene emulsions do not fall on a single curve and are significantly higher. This is presumably due to both drop flocculation and the network of clay tactoids in the oil contributing to the emulsion elasticity. Conclusion We have investigated the rheological behavior of w/o emulsions prepared from dispersions of hydrophobic clay in oils where the particles either sediment or swell to form tactoids which remain suspended. Dispersions of hydrophobic clay in IPM consist of large, micrometer-sized particles that sediment rapidly. Emulsions of water-inIPM stabilized by hydrophobic clay are somewhat flocculated, with the extent of flocculation increasing as the drop volume fraction increases. Thus these emulsions exhibit small yield stresses and shear-thinning flow behavior, with the emulsion viscosity increasing as the

Binks et al.

drop volume fraction increases. The more concentrated emulsions are also viscoelastic, with the elastic storage and viscous loss moduli increasing with drop volume fraction. In contrast, the hydrophobic clay particles swell in toluene to form micrometer-sized tactoids that remain suspended in the oil due to the formation of networks. Emulsions of water-in-toluene stabilized by hydrophobic clay are highly viscous. The emulsion viscosity does not, however, increase with the drop volume fraction in the same way as for the water-in-IPM emulsions, since unusual minima in the shear stress are observed as the applied shear rate increases. Also, the elastic storage and viscous moduli of these emulsions are affected more by the particle concentration in the emulsions than by the drop volume fraction. It is proposed that some of the adsorbed particles on the drop surfaces remain linked to the tactoid network in the continuous phase and thus an integrated network of drops and tactoids forms, which is highly elastic but can be broken under high shear. Acknowledgment. We thank M. Kirkland (Unilever Research Colworth) for performing the LTFESEM measurements and R. Stocker (University of Hull) for performing some of the rheological measurements in Figure 12. This research was funded by Halliburton Energy Services (Duncan, USA), and we thank Dr. I. D. Robb for useful discussions. LA050255W