Types of Phase Inversion of Silica Particle Stabilized Emulsions

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Types of Phase Inversion of Silica Particle Stabilized Emulsions Containing Triglyceride Oil B. P. Binks* and J. A. Rodrigues Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, U.K. Received December 11, 2002 We report on a detailed study of the inversion of triglyceride oil-water emulsions stabilized solely by nanoparticles of silica. The majority of the data refers to pure tricaprylin, although similar findings are observed with the commercial analogue Miglyol 812. The effects of initial particle location, oil:water ratio, emulsification time, and particle concentration have been investigated for emulsions prepared in either a batch or a continuous manner. For particles initially dispersed in water, emulsions invert from simple water-in-oil (w/o) to oil-in-water (o/w) at low water content. For particles originating in oil, inversion occurs from w/o to multiple water-in-oil-in-water (w/o/w) at high water content. On the basis of measurements of oil-water contact angles on treated glass substrates, the difference between preferred emulsions is argued to be due to the hysteresis in contact angle at the three-phase line. For o/w emulsions, increasing the volume fraction of oil at constant particle concentration in water causes an increase in the average drop size and a concomitant decrease in polydispersity; interestingly, monodisperse emulsions form in a short time at high oil content. The appearance of multiple emulsions is shown to be linked to the coalescence of oil drops with inclusion of the continuous water phase during the process of emulsification. In other cases, fragmentation seems to dominate during emulsion formation.

Introduction It is now well established that solid particles of colloidal dimensions can act as excellent emulsifiers alone for both oil-in-water (o/w) and water-in-oil (w/o) emulsions.1 Particles of intermediate hydrophobicity, obtained by coating hydrophilic particles to different extents, are attached to interfaces irreversibly and provide a rigid layer around drops impeding coalescence.2 Much of the recent work in this area has involved nanosized silica particles and nonpolar hydrocarbon oils.3 Such oils are presumed to be inert and do not modify the surface and hence wettability of the particles in situ. In this paper, we describe the behavior of emulsions stabilized by the same particles but prepared using a polar triglyceride oil. In this case, adsorption of oil molecules onto particle surfaces is potentially possible, thus altering the inherent wettability of the particles. Indeed, it has been shown using infrared spectroscopy that triglyceride soy oil adsorbs onto both silicic acid4 and silica hydrogels5 via hydrogen bonding between the H atom in silanol groups and the ester carbonyl group. Medium chain triglycerides like Miglyol 812 are neutral, clear, low-viscosity oils used both in the pharmaceutical field, e.g., in capsules, aerosols, and parenteral emulsions, and in cosmetics, e.g., those in skin care, decorative, and sunscreen products. It is of interest to investigate whether surfactant-free emulsions containing such oils are stable and offer any advantages compared with existing formulations. Since the commercial oil contains a mixture of esters of different chain length, we carried out the initial work with it but have concentrated * Corresponding author: e-mail [email protected]. (1) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (2) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622. (3) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 2000, 2, 2959. (4) Adhikari, C.; Proctor, A.; Blyholder, G. D. J. Am. Oil Chem. Soc. 1994, 71, 589. (5) Proctor, A.; Adhikari, C.; Blyholder, G. D. J. Am. Oil Chem. Soc. 1996, 73, 693.

our studies on a pure sample of the tri-C8 ester, the major component in Miglyol 812, for a better understanding. We have shown earlier6 that certain emulsions stabilized by silica particles can be phase inverted from w/o to o/w simply by increasing the volume fraction of water, φw. The origin of this catastrophic inversion remains unclear, but its existence emphasizes the flexibility of these systems compared with those containing a single pure surfactant for which this is not possible. In our study with toluene as oil, we found no evidence of multiple emulsions around conditions of inversion, a common occurrence in surfactant emulsions. The understanding of phase inversion, in (impure) oil-water mixtures containing no added surfactant or particles, has advanced recently due to the systematic work of two groups. Pacek et al.7 describe the inversion of emulsions from w/o to o/w upon increasing φw via the formation of multiple oil-in-water-in-oil (o/w/o) emulsions in chlorobenzene-water mixtures. Groeneweg et al.8 similarly show evidence for the appearance of waterin-oil-in-water (w/o/w) multiple emulsions during the inversion from o/w to w/o of triglyceride-water mixtures with increased time of agitation. In both cases, however, multiple emulsions were not formed during inversion the other way around. It was argued that, by multiple emulsion formation, the effective volume fraction of dispersed phase required to induce inversion can be obtained by enclosing the continuous phase into drops of the dispersed phase. This occurs as a result of the coalescence of drops of the dispersed phase. The effective volume fraction of dispersed phase (made up of droplets inside globules) thus increases as long as this inclusion dominates over loss of such enclosed droplets by, say, dissolution back into the continuous phase. Inversion is thus governed by the balance between the breakup of bulk liquid/drops and the coalescence of drops. In probing the (6) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 2539. (7) Pacek, A. W.; Nienow, A. W.; Moore, I. P. T. Chem. Eng. Sci. 1994, 49, 3485. (8) Groeneweg, F.; Agterof, W. G. M.; Jaeger, P.; Janssen, J. J. M.; Wieringa, J. A.; Klahn, J. K. Trans. I. Chem. E 1998, 76, 55.

10.1021/la020960u CCC: $25.00 © 2003 American Chemical Society Published on Web 04/22/2003

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Table 1. Preferred Emulsion Type (Ow ) 0.5) and Stability of Triglyceride-Water Batch Emulsions Stabilized by Fumed Silica Particles of Different Hydrophobicity (% SiOH)a Miglyol 812

c

tricaprylin

% SiOH

typeb

stabilityc

type

stability

14.1 24.1 40.4 50.9 65.7 79.9 87.0 100.0

o/w o/w o/w w/o w/o w/o w/o o/w

creamed, no coalescence creamed, no coalescence creamed, no coalescence coalescence sedimented and coalesced no sedimentation/coalescence no sedimentation/coalescence creamed and coalesced

o/w o/w o/w o/w w/o w/o o/w o/w

complete coalescence complete coalescence creamed and coalesced creamed and coalesced sedimented, no coalescence sedimented, no coalescence creamed, no coalescence complete coalescence

a All emulsions prepared from 2 wt % particles in oil and 0.01 M aqueous NaCl. b Determined from conductivity and drop test measurements. Stability refers to 3 months since preparation.

mechanism of phase inversion of particle-stabilized emulsions, we report here the results of a series of experiments in which emulsions are prepared in different ways (batch vs continuous), and inversion is effected either by addition of more dispersed phase or by increasing the time of mixing. As seen earlier,3 emulsion type and subsequent inversion behavior are crucially dependent on the initial location of particles (oil or water), and we determine the oil-water contact angles on suitably treated planar substrates in order to offer an explanation. Experimental Section Materials. Water was passed through a reverse osmosis unit and then a Milli-Q reagent water system, with a resistivity of 16 MΩ cm and pH ) 5.8. Sodium chloride, used as a background electrolyte, was from BDH (purity 99.5%). Miglyol 812 is a relatively pure triglyceride oil supplied by Sasol (formerly Condea Chemie). It contains between 50 and 65 wt % of tri-C8 chains and between 30 and 45 wt % of tri-C10 chains (both saturated) and has a density at 20 °C of 0.945 g cm-3. Tricaprylin (1,2,3trioctanoylglycerol, g99%), of density 0.954 g cm-3 at 20 °C, was from Sigma and is the pure tri-C8 ester. Both oils were columned through chromatographic alumina before use. The amorphous fumed silica particles were from Wacker-Chemie (Munich), with an average primary particle diameter of 20 nm and a specific surface area of 200 m2 g-1. Starting with raw hydrophilic silica possessing 100% silanol (SiOH) groups on their surfaces, the particles are made progressively more hydrophobic by reaction with dichlorodimethylsilane (DCDMS). In this study, we have investigated emulsions stabilized by particles of eight different hydrophobicities, in which the SiOH content varies from 100 to 14%. DCDMS, used to coat glass microscope slides (Chance Propper), was supplied by Fluka of purity 99.5%. The other reagents required for this procedure (NaOH, P2O5, chloroform, cyclohexane) were of analytical grade from various sources. Methods. Dispersions of silica particles in either water or oil were prepared by dispersing a known mass of powder into the liquid using a high-intensity ultrasonic vibracell processor (Sonics & Materials, tip diameter 0.3 cm) operating at 20 kHz and up to 10 W for 2 min. In preparing emulsions, the total volumes of oil and water (10 cm3) were mixed using a Janke and Kunkel Ultra-Turrax T25 homogenizer with a 1.8 cm head operating at 13 000 rpm. For emulsions made in a single step (batch), 2 min was sufficient. For emulsions made in a number of steps (continuous), the first emulsion made as above was rehomogenized several times at 13 000 rpm for 1 min. 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 inferred by observing what happened when a drop of each emulsion was added to a volume of either pure oil or pure water. The stability at 25 °C of o/w emulsions to creaming or coalescence was assessed by monitoring the change with time of the position of the clear water-emulsion or oil-emulsion interfaces, respectively. 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. Drop size distributions of watercontinuous emulsions (o/w or w/o/w) were determined using a Malvern MasterSizer 2000 light scattering instrument, calibrated using a monodisperse sample of latex particles in water. Approximately 0.1 cm3 of emulsion was diluted to 100 cm3 with pure water. A Nikon Labophot microscope equipped with a DIC-U camera was used to observe emulsions also. A drop of emulsion was diluted with its own continuous phase on a microscope slide and gently covered with a coverslip. Images were processed using Adobe Photoshop 5.0 LE software. To simulate the coating around the silica particles, we have hydrophobized glass slides to different extents using the following procedure.9 The slides were made completely hydrophilic (θaw < 5°) by immersing in hot aqueous 30% NaOH and rinsing in pure water followed by drying at 110 °C for 20 min and storing in a desiccator. Silanization was carried out in an atmosphere of dry nitrogen using a glovebag containing Petri dishes of P2O5 drying agent. The hydrophilic plates were removed from the desiccator and placed in solutions of different concentrations of DCDMS in cyclohexane for 30 min, after which they were rinsed in cyclohexane and then chloroform in order to remove any unreacted silane reagent. They were dried at 110 °C overnight, desiccator stored, and used within 24 h. Contact angles of water drops (20 µL) in air or under oil were measured using a Kru¨ss G1 contact angle meter and glass chamber at ambient temperature. For oil-water interfaces, two procedures were adopted. In one a water drop was first formed on the substrate, and the cell was filled with oil (θow1). In the other, oil covered the substrate first and a water drop was formed subsequently (θow2).

Results and Discussion (a) Effect of Particle Hydrophobicity on Preferred Emulsions. To select the most suitable emulsion system for further study, we investigated the effect of silica particle hydrophobicity on the type and stability of preferred batch emulsions, i.e., those containing equal volumes of oil and water. The results are summarized in Table 1 for both commercial Miglyol 812 and pure tricaprylin, in which 2 wt % particles were initially dispersed in the oil phase. Contrary to the findings obtained with nonpolar oils like alkanes, we find in triglyceride systems that emulsion type is o/w for the more hydrophobic particles (low % SiOH) and w/o for the more hydrophilic ones (high % SiOH). The exception to this is in the case of the most hydrophilic particles (100% SiOH), but o/w emulsions, if formed at all, are completely unstable to coalescence. Tricaprylin behaves very similarly to Miglyol 812, and the most stable emulsions, here to both sedimentation and coalescence, are those with particles possessing 79.9% SiOH. The remainder of this paper is concerned only with these particular particles and pure tricaprylin, but comparisons will be made where possible with Miglyol 812. The (9) Newcombe, G.; Ralston, J. Langmuir 1992, 8, 190.

Silica Particle Stabilized Emulsions

Langmuir, Vol. 19, No. 12, 2003 4907 Table 2. Contact Angles in Air, θaw, and under Miglyol 812, θow, of Water Drops on Glass Slides Treated to Different Extents with Dichlorodimethylsilanea [DCDMS]/M

θaw/deg

θow1/deg

θow2/deg

0 10-5 10-4 6 × 10-4 3 × 10-3 10-2 10-1

5 44 47 68 74 85 91

10 65 66 87 130 140 138

10 87 89 117 155 157 156

a θ 1 refer to experiments in which water contacted the solid ow first; θow2 are for when oil contacted it first. [DCDMS] refers to its concentration in cyclohexane solution.

influence of the polarity of the oil on the ability of particles to stabilize emulsions is very noticeable, since in toluene systems2 maximum emulsion stability was achieved with particles containing 50% SiOH whereas more hydrophilic particles are required in the triester case. We investigate the effect of the mode of preparation and system variables on the inversion characteristics of such emulsions. (b) Emulsions Prepared in a Batch Process. (i) Influence of Oil:Water Ratio, Initial Particle Location, and Agitation Time. Figure 1 shows the change in both the emulsion conductivity and type with φw for batch emulsions prepared from 2 wt % particle dispersions in either water (open circles) or oil (filled circles). Since in this kind of scan the particle concentration within each emulsion is different, we have verified that the inversion remains unchanged for emulsions in which the overall particle concentration is held constant (triangles, initially in oil). The designation of emulsion type follows from microscopy (later), but the main finding is that the position of catastrophic inversion depends on the initial location of the particles, being at φw ) 0.225 if in water and at φw ) 0.625 if in oil. Preferred emulsions at φw ) 0.5 are thus o/w from water-borne dispersions but w/o from oil-borne ones. We thus see that the conclusion arrived at earlier (Table 1) is incomplete and must be treated with caution. One possible explanation for the marked influence of particle location on emulsion type is that the same particles exhibit a different contact angle with the oil-water interface in the two situations and is linked to the welldocumented phenomenon of contact angle hysteresis. If particles originate in water, it is likely that the equivalent of the receding contact angle (θr), measured into water, is adopted upon contact with oil, in which water recedes over the particle surface. By contrast, for particles originally in oil, the equivalent of the advancing angle (θa) is preferred upon contact with water, in which water advances over the particle surface. For many liquid/liquid/ solid systems,10 θr is normally smaller than θa, in line with our prediction that particles are more hydrophilic (smaller angle) and prefer o/w emulsions when initially

in water but are more hydrophobic (larger angle) and prefer w/o emulsions when initially in oil. To test these ideas, we have determined the contact angles of water drops under Miglyol 812 on planar glass substrates treated to different extents with the same silanizing agent as used for the particles (DCDMS). As seen in the second column of Table 2, the hydrophobicity of the substrate increases with the concentration of silanizing agent as evidenced from the air-water contact angles, θaw. Whatever the initial hydrophobicity of the glass substrate, we find that the contact angles at the oil-water interface are substantially lower when water contacts the solid first (θow1) than when oil contacts it first (θow2). Interestingly, at a [DCDMS] of 6 × 10-4 M, the two angles are below and above 90°, respectively, consistent with preferred emulsions being o/w in the first case and w/o in the second. A second important difference which emerges is the change in emulsion type accompanying inversion in the two cases. For particles initially in water, simple w/o emulsions invert to simple o/w emulsions as seen in the optical micrographs in Figure 2, in which the upper image is for conditions prior to inversion and the middle and lower images are for after inversion. However, for particles initially in oil, although simple w/o emulsions are formed before inversion, we observe multiple w/o/w emulsions at all water contents beyond inversion. This is evident from the representative images in Figure 3 for tricaprylin (upper and middle) with the same behavior occurring for Miglyol 812 systems at high φw (lower); inner water drops are clearly visible within oil globules. This is the first time we have observed the appearance of multiple emulsions in systems of pure oil + water containing a single particle type. We have shown earlier11 that both w/o/w and o/w/o multiple emulsions can be prepared using mixtures of silica particle types differing only in their hydrophobicity, in a similar way that mixtures of hydrophilic and hydrophobic surfactants are employed in conventional multiple emulsions. Photographs of the emulsions 6 months after preparation are given in Figure 4. For the series in which particles are located in water initially (upper), the w/o emulsions at low φw although thin and grayish sedimented but were stable to coalescence. The o/w emulsions after inversion were white and viscous stable to both creaming and coalescence. For the series in which particles are initially dispersed in oil (lower), w/o emulsions at low φw were white and viscous and did not sediment while the w/o/w emulsions although stable to coalescence showed signs of creaming. We have determined the initial mean drop size and polydispersity in size of the o/w emulsions in the upper series as a function of oil volume fraction, φo. The

(10) Contact Angle, Wettability and Adhesion; Advances in Chemistry Series 43; American Chemical Society: Washington, DC, 1964; Chapter 8.

(11) Binks, B. P.; Dyab, A. K. F.; Fletcher, P. D. I.; Barthel, H. Multiple emulsions, patent assigned to Wacker-Chemie (Munich), DE10211313 filed March 2002.

Figure 1. Conductivity and type of water-tricaprylin batch emulsions stabilized by silica particles with 79.9% SiOH as a function of water volume fraction. Open circles, 2 wt % particles initially in water; filled circles, 2 wt % particles initially in oil; triangles, 1 wt % particles in emulsion originating in oil. Points A-E are referred to later in the text.

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Figure 2. Optical microscopy images of batch tricaprylin emulsions with 2 wt % particles (79.9% SiOH) initially dispersed in water. Scale bar ) 50 µm in all: (upper) w/o with φw ) 0.20; (middle) o/w with φw ) 0.25 (note the low polydispersity); (lower) o/w with φw ) 0.6.

distribution in drop diameter is described by the uniformity defined as

U)

Nidi3|d h - di| ∑ 1 i d h

∑i Nidi

(1)

3

h is the where Ni is the number of drops of diameter i and d median diameter of the distribution, i.e., the diameter for which the cumulative undersized volume fraction is 50%. Relatively monodisperse surfactant-stabilized emulsions have uniformities of approximately 0.2.12 The data, shown in Figure 5, reveal an interesting phenomenon. Upon increasing the dispersed phase volume fraction over 7-fold, the mean diameter increases gradually from 7 to 64 µm while the uniformity decreases steadily to a limiting value (12) Mabille, C.; Schmitt, V.; Gorria, Ph.; Leal-Calderon, F.; Faye, V.; Deminie`re, B.; Bibette, J. Langmuir 2000, 16, 422.

Figure 3. Optical microscopy images of batch emulsions with 2 wt % particles (79.9% SiOH) initially dispersed in oil. (upper) w/o of tricaprylin with φw ) 0.60, scale bar ) 50 µm; (middle) w/o/w of tricaprylin with φw ) 0.65, scale bar ) 25 µm; (lower) w/o/w of Miglyol 812 with φw ) 0.65, scale bar ) 25 µm.

of approximately 0.23. The image in Figure 2 (middle) is that of an emulsion with φo ) 0.75 where the monodispersity is evident. By fixing the particle concentration in water and adding increasing amounts of oil, we discover a new way of preparing monodisperse solid-stabilized emulsions. The system adjusts by an increase in the drop size such that the particles present can saturate the new lowered interfacial area. In marked contrast to emulsions with surfactant stabilizers, the most monodisperse ones here are those of largest drop size. We recover the same result for emulsions prepared in a continuous way (later). It has been shown that inversion can be effected by increasing the agitation time.8 For o/w emulsions at an initial oil volume fraction of 0.78, continuous stirring caused a gradual increase in the effective oil fraction since inclusion occurred to create w/o/w globules. After a certain time, the effective oil volume fraction had increased so much that it was above that required for inversion and inversion to a w/o emulsion ensued. By contrast, starting with an o/w emulsion of lower φo ) 0.74, although inclusion occurred with time the effective oil volume fraction reached a steady-state value which was too low for inversion. The

Silica Particle Stabilized Emulsions

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Figure 4. Photograph of tricaprylin-water batch emulsions stabilized by 79.9% SiOH particles at different oil:water ratios: (upper) 2 wt % initially in water with φw from left to right being 0.1, 0.2 (w/o), 0.25, 0.4, 0.6, and 0.8 (o/w); (lower) 2 wt % initially in oil with φw ) 0.1, 0.4, 0.6 (w/o), 0.65, 0.7, and 0.8 (w/o/w).

Figure 5. Variation of mean diameter (left ordinate, filled points) and uniformity (right ordinate, open points) with oil volume fraction of o/w batch emulsions of tricaprylin from 2 wt % particles (79.9% SiOH) in water.

increased inclusion required in this case for inversion brings with it a higher rate of escape of inner water drops, and a balance is set up between inclusion and escape. We tested whether similar trends were observed in our systems. Starting with a batch o/w emulsion from waterborne particles at φo ) 0.7, for which inversion occurs at φo ) 0.775, we rehomogenized at 13 000 rpm in steps of 1 min. The emulsion remained o/w after a total of 12 min additional mixing (point A, Figure 1). By contrast, commencing with an o/w emulsion with φo ) 0.75 nearer to inversion (point B, Figure 1), we observed the appearance of all four emulsion types with time (Figure 6). A stable multiple w/o/w emulsion formed after an additional 6 min of mixing (middle), with a very unstable multiple o/w/o emulsion being visible after a further 5 min (lower). After a total of 15 min mixing, a simple w/o emulsion of high stability was formed, indicating complete inversion

Figure 6. Micrographs of emulsions prepared from 2 wt % particles in water, φo ) 0.75 at different times of agitation: (upper) original o/w after 2 min, (middle) multiple w/o/w after a further 10 min, (lower) mixture of multiple o/w/o and simple w/o after a total of 13 min. Scale bar is 50 µm in upper and lower and 25 µm in middle.

due to the inclusion mechanism. Particle-stabilized emulsions thus behave similarly to those containing surfactant in this way. For batch emulsions from oil-borne dispersions, inversion induced by additional mixing was comparatively easier, and the initial w/o emulsions at φw ) 0.5 (point C, Figure 1) or 0.6 (point D, Figure 1) inverted to a mixture of o/w drops and w/o/w globules after 3 and 1 min of further mixing, respectively. (ii) Effect of Particle Concentration. The existence of multiple emulsions has been associated with the coalescence of drops with inclusion of the continuous phase during the mixing process as a result of insufficient coverage of freshly formed interfaces. We questioned whether multiple emulsions only form when particles originate in oil (as implied from Figure 1) and whether it was possible to prepare them from aqueous dispersions at low particle concentrations prone to coalescence. Batch emulsions were prepared at φw ) 0.5 with particles dispersed in water and observed microscopically and using

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Figure 7. Effect of silica particle (79.9% SiOH) concentration initially dispersed in water on the mean diameter and type of water-continuous batch emulsions of tricaprylin, φw ) 0.5. In w/o/w emulsions, scattering is dominated by oil globules whose size is reported.

light diffraction. It can be seen from the plot in Figure 7 that multiple w/o/w emulsions are indeed formed at low particle concentrations, but simple o/w emulsions are preferred at and above 1 wt % in water, in line with the findings in Figure 1. The arithmetic mean diameter, D[4,3], decreases gradually from 220 µm for oil globules to a limiting value of 10 µm for oil drops at high particle concentration. For the same amount of shear, at low particle concentrations there are not enough particles present to fully cover the oil-water interfaces produced, and so oil drops coalesce easily with inclusion of water, thus growing and reducing the interfacial area. Since particles remain adsorbed, the interfacial particle density increases until eventually drops reach a particular size that are stable to further coalescence. At higher particle concentrations, although limited coalescence probably occurs to reach the final drop diameter,13,14 coalescing drops possess a higher coverage of particles initially and inclusion of water pools is much less likely. The drops remain simple o/w. Given the above result, the possibility existed that the other kind of multiple emulsion, o/w/o, may form during the coalescence of w/o drops from low particle concentration dispersions in oil. Batch emulsions at φw ) 0.5 were made at different particle concentrations initially in tricaprylin. Contrary to expectations, however, emulsions were multiple but of the w/o/w type, phase inverting to simple w/o ones by 0.75 wt % particles in oil. Both types were stable to coalescence after formation, and we thus find a new way to effect inversion achieved by increasing the particle concentration. Although we do not rule out the possibility that o/w/o emulsions do form, what is apparent is that they are certainly much less stable than those of w/o/w which therefore persist. (iii) Effect of Homogenization Time. To detect the first appearance of multiple w/o/w emulsions prepared from particle dispersions in oil, we have investigated the influence of the time of homogenization (at 13 000 rpm) on batch emulsification for φw ) 0.8 (point E, Figure 1). The homogenization time was varied from 1 s to 4 min, and the type, stability, and drop size distribution was (13) Whitesides, T. H.; Ross, D. S. J. Colloid Interface Sci. 1995, 169, 48. (14) Arditty, S.; Whitby, C. P.; Binks, B. P.; Schmitt, V.; Leal-Calderon, F. Eur. Phys. J. B, submitted.

Binks and Rodrigues

Figure 8. Effect of homogenization time on the mean diameter (filled points, left ordinate) and uniformity (open points, right ordinate) of oil globules in multiple w/o/w emulsions of tricaprylin-water, batch prepared at φw ) 0.8 with 2 wt % particles (79.9% SiOH) initially in oil.

determined for each emulsion made. At this high water content, we find that emulsions are of the w/o/w type immediately after applying shear to the oil-water mixture, i.e., after 1 s, and remain this type for all of the longer mixing times. For these conditions above the inversion boundary of Figure 1, a multiple emulsion is formed instantaneously and does not originate from the partial coalescence of oil drops in o/w emulsions as before. An increase in the time of homogenization serves to reduce the oil globule size (in w/o/w emulsions) from over 300 to 50 µm, with a concomitant increase in the uniformity of the size distribution (Figure 8). We observe for a second time that emulsions are more monodisperse the larger the average drop diameter, in this case obtained following a short period of homogenization. Fragmentation is the dominant process at all φw above inversion. For all of the w/o/w emulsions prepared, no visible coalescence instability occurred although they were prone to creaming under gravity. The stability to creaming is measured by monitoring the increase in volume of the resolved water phase below the emulsion cream and is defined by the fraction fw, being the volume of water released relative to the total volume of oil and water in the system. A value of fw ) 0.8 indicates complete coalescence of the emulsion, whereas values close to zero indicate good stability to creaming. The time course of the creaming of oil globules is shown in Figure 9 for emulsions prepared for different homogenization times. For short times (1 and 5 s), fw reaches values of between 0.7 and 0.75 within 1 min following preparation, indicating rapid creaming. At intermediate times (15-90 s) the kinetics and extent of creaming are gradually reduced, while for longer homogenization times (g120 s) the volume of resolved water is quite small. The increase in stability to creaming as a function of homogenization time mirrors the decrease in the average globule size reported above (Figure 8) since, from Stokes’ law, smaller drops are less prone to rise within an emulsion. It is worth noting that even at low times (1 and 5 s) the concentrated globules within the emulsion cream are stable to coalescence due to an adsorbed layer of particles. (c) Emulsions Prepared in a Continuous Process. (i) Particles Initially in Water. Emulsion inversion can also be effected by the stepwise addition of dispersed phase in a continuous process. For silica particles initially located

Silica Particle Stabilized Emulsions

Figure 9. Effect of homogenization time on the creaming kinetics of oil globules in multiple w/o/w emulsions of tricaprylin-water, batch prepared at φw ) 0.8 with 2 wt % particles (79.9% SiOH) initially in oil. The homogenization times (s) from top to bottom are 1, 5, 15, 30, 60, 90, 120, 180, and 240.

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Figure 11. Variation of mean oil drop diameter (filled points, left ordinate) and uniformity (open points, right ordinate) with oil volume fraction for tricaprylin-in-water emulsions prepared in a continuous process from 2 wt % particles (79.9% SiOH) in water. Table 3. Type of Batch Emulsion Formed in Tricaprylin-Water Systems Stabilized by 79.9 % SiOH Silica Particles Originating in Either Water or Oil Alone or in Both Phases

Figure 10. Tricaprylin-in-water emulsion drop size distributions for different φo values (given) from continuous experiments starting with 2 wt % particles (79.9% SiOH) in water. Note the narrowing of the distribution at high dispersed phase volume fractions.

in water, an o/w emulsion was prepared in the usual way (13 000 rpm, 2 min) at φo ) 0.1, and oil was then added sequentially, rehomogenizing for 1 min after each addition. We observed phase inversion from simple o/w to w/o emulsions at an oil volume fraction between 0.7 and 0.8 with the appearance of, first, multiple w/o/w and then multiple o/w/o emulsions within this volume fraction range. Since this is very close to that observed in batch experiments (φo ) 0.775), we conclude that catastrophic phase inversion does not depend on the experimental protocol. Selected drop size distributions for o/w emulsions prior to inversion are shown in Figure 10 where it can be seen that, in addition to an increase in average diameter, the width of the distribution decreases upon increasing the oil volume fraction. The mean diameter and uniformity of these emulsions at all oil contents are plotted in Figure 11. The drop diameter (filled points) increases gradually from 17 to 145 µm with a concomitant decrease in the uniformity (open points) from values over unity to around 0.2. As for the batch method (Figure 5), relatively monodisperse emulsions are formed in a continuous manner at high droplet volume fraction (g0.6). (ii) Particles Initially in Oil. Similarly, a starting w/o emulsion with φw ) 0.1 and with particles dispersed in oil was rehomogenized in steps following addition of increas-

φw

[particles]water/wt %

[particles]oil/wt %

type

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.2 0.7

2.0 1.7 1.3 1.0 0.7 0.3 0 1.0 1.0

0 0.3 0.7 1.0 1.3 1.7 2.0 1.0 1.0

o/w o/w o/w + w/o/w o/w + w/o/w o/w + w/o/w o/w + w/o/w w/o w/oa o/w + w/o/wb

a At this φ , w/o formed when particles start in either water w alone or oil alone. b At this φw, o/w formed from particles in water alone and w/o/w from particles in oil alone.

ing amounts of water. Inversion from w/o to multiple w/o/w emulsions took place at a water volume fraction close to 0.6, compared with 0.625 when prepared in a batch process. At conditions above inversion, we also noticed the appearance of empty oil globules, i.e., simple oil drops, in coexistence with w/o/w globules. (d) Batch Emulsions with Particles in Both Water and Oil Initially. Since the initial location of particles is crucial in dictating the type of emulsion ultimately formed, it is of interest to investigate what emulsion is stabilized when both the aqueous and oil phases contain dispersed particles. We summarize the findings in Table 3 for preferred emulsions mainly (φw ) 0.5) but also for extremes of water volume fraction (0.2 and 0.7). At a fixed particle concentration in emulsions of 1 wt % and for φw ) 0.5, increasing the concentration of particles in the oil phase with a concomitant decrease in the aqueous phase results in a change from o/w emulsions to emulsions containing a mixture of o/w drops and w/o/w globules and finally inversion to w/o emulsions at 2 wt % in oil. The competition for emulsion type (o/w vs w/o) in such mixtures is thus dominated by the behavior of the aqueous-based dispersion, but the appearance of multiple w/o/w emulsions also suggests an influence from the oil-based dispersion. At lower water content, φw ) 0.2, dispersing equal amounts of particles in both oil and water initially results in the formation of a w/o emulsion, the same as what is formed if particles originate solely in either water or oil at the

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Langmuir, Vol. 19, No. 12, 2003

outset. At higher water content, φw ) 0.7, the same experiment yields a mixture of o/w drops, as formed from aqueous dispersions alone, and w/o/w globules, as formed from oil dispersions alone; i.e., the influence of both types of behavior is evidenced. Conclusions We have studied various aspects relating to the inversion of emulsions stabilized solely by small silica particles in water + triglyceride oil systems. Inversion of emulsion type from w/o to either o/w or multiple w/o/w can be effected by changing the initial location of particles, by varying the oil:water ratio, and by continued agitation. We compare the findings for emulsions prepared in a batch vs a continuous process. For preferred emulsions (equal oil:

Binks and Rodrigues

water ratio), the continuous phase is that in which the particles are first dispersed. An explanation based on advancing and receding contact angles is consistent with the data. We discover a new method for preparing relatively monodisperse o/w emulsions which involves addition of high volume fractions of dispersed phase at constant particle concentration in the continuous phase. Surprisingly, the polydispersity in drop size decreases as the average drop size increases for both o/w and w/o/w emulsions. Acknowledgment. The authors thank GlaxoSmithKline (Weybridge) for a fully funded studentship to J.A.R. and Mr. P. J. Frost for his support. LA020960U