4540
Langmuir 2001, 17, 4540-4547
Pickering Emulsions Stabilized by Monodisperse Latex Particles: Effects of Particle Size B. P. Binks* and S. O. Lumsdon Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, U.K. Received March 13, 2001. In Final Form: May 4, 2001 The preparation, type, and stability of emulsions of oil and water stabilized solely by spherical, monodisperse polystyrene latex particles of different size is described. Two types of behavior occur depending on whether particles remain intact (in the case of cyclohexane) or dissolve to give free polymer chains (in the case of toluene). Emulsions formed with cyclohexane and either “hydrophilic” aldehyde/sulfate particles or “hydrophobic” sulfate particles are water-in-oil (w/o) over a wide range of salt concentrations and water volume fractions. Average emulsion drop diameters initially increase from 35 to 75 µm with increasing particle diameter and then remain constant. Although such emulsions sediment, there is no sign of coalescence for over 6 months. We show evidence of the transition from nonflocculated to flocculated emulsions upon increasing the water volume fraction, as predicted theoretically for charged drops in oil. By use of toluene and “hydrophilic” particles however, emulsions can be inverted from oil-in-water (o/w) to w/o with increasing salt concentration. The concentration of salt required to screen the repulsions between negatively charged adsorbed polymers increases with initial particle size as the average molecular weight also increases. Water-in-oil emulsions, of around 1 µm diameter, are stable to coalescence for long periods.
Introduction We have recently described the preparation, stability, and inversion characteristics of oil-water emulsions stabilized by solid particles alone.1-6 These so-called Pickering emulsions can be remarkably stable to coalescence due, in part, to the very high energies of attachment for particles held at liquid-liquid interfaces. The solids employed were either submicrometer, approximately spherical silica particles or 30 nm diameter disk-shaped Laponite clay particles. In both cases, the samples contained a distribution of particle sizes, although the clay sample was less polydisperse. It was found that emulsion type (oil-in-water, o/w, or water-in-oil, w/o), preferred drop sizes, and emulsion stability (with respect to both creaming/sedimentation and coalescence) were all crucially dependent on the hydrophobicity of the particles themselves. For silica, the wettability of the particles was varied systematically by coating them to different extents with a silane reagent. Those of intermediate hydrophobicity yielded the most stable emulsions with the smallest drop sizes (less than 1 µm).4 For clay, stable o/w emulsions were only formed at conditions where the particles were flocculated.6 This was achieved by addition of electrolyte, and it was suggested that the presence of salt caused an increase in the hydrophobicity of the particles promoting their attachment to the oil-water interface. The present paper reports the findings of a study aimed at investigating the effectiveness of utilizing spherical, polystyrene latex particles in order to stabilize emulsions. Latex particles offer the advantage of being relatively monodisperse (99%, included toluene, cyclohexane, and heptane (Fisher), cineole (1-isopropyl-4-methyl-7-oxabicyclo[2.2.1]heptane), eugenol (4-allyl-2-methoxyphenol), isopropyl myristate, and methyl myristate (Fluka), undecan-1-ol (Aldrich), dodecane (Avocado), and 0.65 and 50 cS PDMS (Dow Corning). They were columned twice through chromatographic alumina in order to remove polar impurities. The electrolyte used was NaCl (Prolabo, 99.5%). The charged polystyrene surfactant-free latex particles were supplied by the Interfacial Dynamics Corporation (Oregon). Two sets were employed, one of hydrophilic aldehyde/sulfate latexes (type ID:12) and the other of hydrophobic sulfate latexes (type ID:1). The relevant properties given by the manufacturer are summarized in Table 1. Methods. All emulsions were made from aqueous latex dispersions and oil phases (10 cm3 total) using a Janke and Kunkel Ultra Turrax T25 homogenizer (rotor-stator) with a 1.8 cm head operating at 13 500 rpm for 2 min. The emulsions were transferred into stoppered, graduated glass vessels of internal 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 PTI-58 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 position of the clear water (serum)-emulsion interface, whereas the coalescence extent was estimated from the movement of the oil-emulsion boundary. 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 (9) Binks, B. P. Colloids Surf., A 1993, 71, 167.
Figure 1. Optical microscope images of systems containing polystyrene latex particles on glass substrates: (a) aqueous dispersion of aldehyde/sulfate particles of diameter 3.2 µm after drying; (b) as (a) after addition of cyclohexane; (c) as (a) after addition of toluene; (d) dried aqueous dispersion of sulfate particles of diameter 2.7 µm; (e) as (d) after addition of cyclohexane; (f) as (d) after addition of toluene. 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 sample of the latex particles in water. Approximately 0.5 cm3 of emulsion was diluted to 50 cm3 with either pure water or pure oil. 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 a haemocytometer cell (Weber Scientific) and viewing it with a Nikon Labophot microscope fitted with a DIC-U camera (World Precision Instruments). Images were processed using Aldus Photostyler 3.0 software. Gel permeation chromatography (GPC) of selected latex particles was carried out using refractive index detection (Gilson 133). A sample of aqueous stock dispersion was allowed to dry in a desiccator overnight. The dried powder was dissolved in tetrahydrofuran (Sigma, >99%) as the mobile phase with the stationary column phase being composed of mixed-D Polymer Laboratories (PL) gel. The calibration employed polystyrene particles of known narrow molecular weight distribution. The analysis was performed using PL Caliber software.
Results and Discussion (i) Emulsions Stabilized by “Hydrophilic” Latex Particles. Since polystyrene latex particles are soluble in certain organic solvents, we performed the following experiment in order to establish whether the particles remain intact or dissolve in the presence of various oils. Using the hydrophilic aldehyde/sulfate particles (diameter ) 3.2 µm), we placed a small drop of aqueous dispersion on a clean glass microscope slide and allowed the water to evaporate in air. Figure 1a shows the formation of
4542
Langmuir, Vol. 17, No. 15, 2001
Binks and Lumsdon
Figure 2. Conductivity and type of water-cyclohexane emulsions (φw ) 0.5) stabilized by 2 wt % hydrophilic latex particles in water as a function of salt concentration. Particle diameters (in µm) are (×) 0.029, (0) 0.16, (2) 0.52, (O) 1.1, (9) 3.2, and (b) 6.1.
aggregates of particles, observed using optical microscopy, for the dried state. A small volume of either cyclohexane or toluene was added to this surface, and it was reexamined after 24 h of contact. It can be seen that the particles remain solidlike with minimal swelling for cyclohexane (1b) but “disappear” leaving a continuous film on the substrate in the case of toluene (1c). This difference in emulsifier type (particle versus free polymer) has important consequences on the properties of emulsions stabilized by them. We also verified that no change occurs to the hydrophobic sulfate latex particles (diameter ) 2.7 µm) in the presence of cyclohexane (1d and e), but a similar dissolution of the particles in toluene occurs as before (1f). For cyclohexane as oil and using so-called hydrophilic aldehyde/sulfate particles, preferred emulsions, i.e., those prepared at a volume fraction of water φw equal to 0.5, have been prepared from aqueous phases containing 2 wt % of particles. The conductivity and type are given in Figure 2 where it can be seen that emulsions are w/o for all particle sizes chosen and at all salt concentrations (between 10-3 and 1 M). Clearly contact with this oil results in them stabilizing water drops and hence behaving reasonably hydrophobic. It is therefore not possible to effect emulsion phase inversion from o/w to w/o by adding salt with these particles. Despite this, all emulsions were stable to coalescence for over 6 months but sedimented leaving a clear oil phase above the emulsion. The stability to sedimentation increased with salt concentration as the initial aqueous latex dispersion became flocculated (>0.1 M NaCl). In such cases, gentle inversion of vessels containing equal volumes of milky aqueous latex and clear oil resulted in the clearing of the aqueous phase and the oil phase becoming turbid as particles transfer from water to oil. An optical microscope image of a single, relatively large water drop from this system is shown in Figure 3, upper. The particles are clearly visible adsorbed over the entire surface, with the occasional larger particle also being present. The lower picture represents a magnified portion of the same drop surface (taken from near the center). Although there are regions of hexagonally closepacked particles, e.g., bottom left, there are also regions where this ordering is disturbed and small gaps appear between particle arrays, e.g., center right. This nonuniform packing around a curved interface is in contrast to
Figure 3. (upper) Optical microscope image of a single waterin-cyclohexane emulsion drop coated with 3.2 µm diameter hydrophilic latex particles (φw ) 0.6, 1 wt % in water). Scale bar ) 50 µm. (lower) Enlarged portion of the water drop surface showing packing of individual latex particles. Scale bar ) 15 µm.
hexagonal packing over large distances for planar monolayers.8 In contrast to the above, in the presence of toluene these particles dissolve giving rise to polymer chains of various overall lengths. If such polymers adsorb at the oil-water interface, one can also expect a different mode of emulsion stabilization compared with solid particles. In addition to changing the mechanical barrier affecting coalescence stability, the polymer-stabilized interfaces may have different interfacial tensions than those coated with particles. If we assume that each polymer chain possesses a charged sulfate group at both ends, we can estimate the number average molecular weight (M h n) of the polymer from a knowledge of the particle diameter and the number of sulfate groups per unit area of particle surface. The calculations yield values of around 60 000 and 113 000 g mol-1 for particles of diameter 0.81 and 2.7 µm, respectively. GPC yields experimental values for M h n of 97 500 and 122 500 g mol-1, respectively, values which are sufficiently close to confirm that polymer chains possess charged groups at both ends and that the average chain length increases with initial particle size. The effect of salt concentration on the conductivity and type of preferred emulsions with toluene has been investigated starting with 1 wt % of particles in the aqueous phase. Figure 4 reveals new features in these systems. For the smallest particles (0.029 and 0.16 µm diameter), emulsions are of low conductivity and disperse in oil at all salt concentra-
Stabilized Emulsions
Figure 4. Effect of salt concentration on the conductivity and type of water-toluene emulsions (φw ) 0.5) stabilized by 1 wt % hydrophilic latex particles in water. Particle diameters (in µm) are (×) 0.029, (0) 0.16, (2) 0.52, (O) 1.1, (9) 3.2, and (b) 6.1.
Figure 5. Median (O) and arithmetic mean (b) initial emulsion drop diameters as a function of salt concentration for emulsions of water-toluene (φw ) 0.5) stabilized by 1 wt % hydrophilic latex particles (diameter 0.52 µm) in water.
tions between 10-3 and 1 M. For particle diameters g0.52 µm, emulsions of high conductivity and which disperse in water are formed at low [NaCl], inverting to w/o emulsions of low conductivity which disperse in oil at high [NaCl]. The salt concentration required for inversion increases progressively with particle size from 0.075 to 0.4 M. If the effect of salt is simply to screen the negative charges between adjacent adsorbed polymers, then this trend with particle size can be understood since inspection of Table 1 shows a rough correlation with surface charge density in that the smaller particles have lower values (around 4 µC cm-2) than the larger ones (>7 µC cm-2). The smaller particle-size-containing emulsions are already above phase inversion in 10-3 M NaCl and remain so even in systems containing pure water (i.e., no added salt). The initial median and arithmetic mean drop diameters are shown in Figure 5 as a function of salt concentration for emulsions stabilized by particles of 0.52 µm diameter. The oil drops at low [salt] are relatively large (>15 µm) compared with the water drops at higher salt concentration (around 1-2 µm) and decrease in size approaching inversion.
Langmuir, Vol. 17, No. 15, 2001 4543
Figure 6. Median (O) and arithmetic mean (b) emulsion drop diameters as a function of initial particle diameter for emulsions of water-toluene (φw ) 0.5) stabilized by 1 wt % hydrophilic latex particles in pure water.
In the absence of salt, the influence of the initial particle diameter on the preferred emulsion drop size is given in Figure 6. Since phase inversion occurs for these conditions with respect to particle size, it can be regarded as a type of transitional inversion in which the system hydrophilelipophile balance increases from left to right. Emulsions of smallest drop size are close to inversion, and the sizes increase on both sides away from the inversion region. We note however that, apart from emulsions stabilized by particles of diameter 0.16 µm, the drop diameters are above 25 µm, reflecting a relatively high interfacial tension in these systems without salt. Coalescence was extensive within 30 min for emulsions containing the smallest (w/o) and largest (o/w) particles and zero for intermediate sizes. The most stable emulsions to creaming and sedimentation were those immediately either side of inversion (dotted line), in line with them having the smallest drop diameters. In contrast, at a salt concentration of 0.5 M, emulsions formed initially by latex particles of all sizes are w/o and the drop diameters are shown in Figure 7. The median diameters are all around 1 µm, although the mean diameters increase to over 5 µm with increasing particle size. The difference between the two sets is a consequence of an increasing low population of drops greater than 10 µm in diameter appearing in the distributions. Importantly, emulsion drop sizes are much smaller than those of systems in Figure 6, which is a consequence of a lower interfacial tension in the presence of salt aiding emulsification. All the emulsions of Figure 7 were completely stable to coalescence, their stability to sedimentation decreasing slightly with (initial) particle size as a second population of large drops is formed. (ii) Emulsions Stabilized by “Hydrophobic” Latex Particles. For cyclohexane as oil, we have determined the minimum concentration of latex particles initially dispersed in water required to stabilize emulsions for a significant period of time. Figure 8 shows a typical set of data for the smallest particles (0.21 µm diameter) in the presence of 1 mM NaCl in the aqueous phase, φw ) 0.5. For these w/o emulsions, the fraction of oil resolved due to sedimentation is rapid and complete, indicating coalescence, at 0.25 and 0.5 wt % particles, decreasing in both rate and extent at 1 and 1.5 wt % until by 2 wt %
4544
Langmuir, Vol. 17, No. 15, 2001
Figure 7. Median (O) and arithmetic mean (b) emulsion drop diameters as a function of particle diameter for water-in-toluene emulsions (φw ) 0.5) stabilized by 1 wt % hydrophilic latex particles in 0.5 M aqueous NaCl.
Figure 8. Stability to sedimentation of 10-3 M aqueous NaClin-cyclohexane emulsions (φw ) 0.5) stabilized by 0.21 µm diameter hydrophobic latex particles. Particle concentrations in water (wt %) are 0.25 (×), 0.5 (O), 1 (2), 1.5 (]), 1.75 (b), and 2 (4).
virtually no sedimentation occurs within 30 min and the emulsions remain completely stable to coalescence for months. A concentration of 2 wt % particles in water was taken for all subsequent experiments involving different latex particle sizes. The conductivity and “drop test” results for preferred emulsions stabilized by particles of increasing diameter are given in Figure 9 in the presence of either 1 mM or 1 M NaCl chosen such that they are below and above the critical coagulation concentration, respectively. The conductivities are all low and similar to that of pure cyclohexane (0.2 µS cm-1) indicating oil external emulsions, and all emulsions disperse in pure cyclohexane and remain as drops on surfaces of pure water. Since the particles are charged and residual charge remains on the oil side of the interface, inspection of Table 1 shows that the surface charge density (in water) increases with particle diameter in line with the detectable increase in conductivity seen in Figure 9. Prior to emulsification, it
Binks and Lumsdon
Figure 9. Conductivity and type of water-cyclohexane emulsions (φw ) 0.5) stabilized by 2 wt % hydrophobic latex particles in water for different particle sizes. Results are shown in the presence of 10-3 (b) and 1 M NaCl (O).
Figure 10. Stability to sedimentation of water-in-cyclohexane emulsions, φw ) 0.5, stabilized by 2 wt % hydrophobic latex particles in 10-3 M NaCl for different particle diameters. From top to bottom, particle diameters (µm) are 2.7, 1.5, 0.81, 0.63, 0.40, and 0.21.
was noticed that the smaller particles flocculated to a greater extent than the larger ones in 1 M NaCl. For both salt concentrations, the water-in-oil emulsions were completely stable to coalescence for over 6 months, but their stability to sedimentation depends on particle diameter. In Figure 10, the fraction of oil phase resolved as a function of time is shown for emulsions stabilized by different particle sizes in 1 mM NaCl. Since the ordinate represents the ratio of separated oil to total oil present (a value of 0.5 indicates half the oil volume appears), an increase in particle diameter leads to a marked decrease in sedimentation stability. However, it can be seen that sedimentation reaches a limit in almost all cases, and although the volume fraction of water in the sedimented emulsion becomes close to 0.9 for the largest particles, they remain stable to coalescence. This is a common feature
Stabilized Emulsions
Langmuir, Vol. 17, No. 15, 2001 4545
re ) 4φwrp/φp
Figure 11. (upper) Initial drop diameter volume distribution (right-hand ordinate) for water-in-cyclohexane emulsion stabilized by 1.5 µm diameter hydrophobic latex particles (2 wt % in 10-3 M NaCl). The solid curve shows the cumulative distribution (left-hand ordinate). (lower) Variation of initial median emulsion drop diameter with hydrophobic latex particle diameter for water-in-cyclohexane emulsions, φw ) 0.5, containing 2 wt % particles in water. Results are shown for 10-3 (b) and 1 M NaCl (O).
of Pickering emulsions. By use of light diffraction, the initial drop size distribution of these w/o emulsions has been determined. Figure 11 (upper) shows a typical result where it can be seen that a polydisperse emulsion comprising a log-normal distribution of drop diameters is obtained. The lower part of Figure 11 displays the variation of the median emulsion drop diameter (diameter below which 50 vol % of drops exist) with latex particle diameter. Finer emulsions are formed with the smallest latex particles and addition of salt also causes a decrease in average drop diameter. Electrolyte addition is anticipated to result in screening the charges between adjacent particles promoting closer packing at drop interfaces and a concomitant reduction in emulsion drop size. For water drops of radius re stabilized by latex particles of radius rp, assuming that re . rp and that the particles are arranged in a hexagonal close-packed arrangement on drop surfaces, the volume fraction of water drops, φw, is given by
φw ) 4πre3ne/3
(1)
where ne is the number of emulsion drops per unit volume. Taking the contact angle that the particles make with the oil-water interface equal to 90°, the total number of particles surrounding all drops is
np ) 4πre2ne/2(31/2)rp2
(2)
and the volume fraction of particles in the emulsion is
φp ) 4πrp3np/3
(3)
Combination of eqs 1-3 leads to the following expression for the radius of an emulsion drop in terms of the volume fraction of water, the radius of the particles, and their volume fraction
(4)
We see from eq 4 that the emulsion drop radius should be a linear function of particle radius at constant φw and φp as long as the particle wettability is unchanged with respect to particle size. This relation is borne out by the results in Figure 11 for the first four particle sizes, after which the drop size becomes independent of particle diameter. This departure may be due to a change in the contact angle with particle size (affecting the number of particles residing at drop interfaces) or due to the fact that not all particles are located at drop interfaces. Determination of the partitioning of particles between both bulk phases and the interface is required before resolving this behavior. In emulsions stabilized by silica particles of intermediate wettability, we have shown that phase inversion can be achieved simply by increasing the volume fraction of the dispersed phase.2 Such catastrophic inversion, occurring at constant contact angle, is not possible for systems containing very hydrophobic silica particles4 or very hydrophilic clay particles.6 For the smallest latex sulfate particles (0.21 µm), the conductivity and continuous phase type of emulsions with cyclohexane have been determined at low and high salt concentrations for different volume fractions of water. The data are shown in Figure 12. In 10-3 M NaCl, emulsions prepared from a fixed concentration of 2 wt % particles in the aqueous phase (filled circles) are of low conductivity for φw between 0.3 and 0.75 but exhibit apparently high conductivities above this volume fraction. All these emulsions however are oil continuous resting as drops when placed on pure water. Those at φw > 0.78 are unstable with a large fraction of the water resolving quickly below the emulsion, whereas those below this value are completely stable. Since the concentration of particles in the system changes in this kind of scan, we have redone the measurements at a fixed concentration of particles in the emulsions equal to 1 wt %. The data are indicated by the open circles where, with sufficient particles to coat all drops, all emulsions are of low conductivity, disperse in oil, and are very stable up to a φw value of 0.9. Similar behavior is observed for emulsions at 1 M NaCl, in which the unstable emulsion of high conductivity at high φw (open triangle) becomes stable and of low conductivity at high enough particle concentration (cross). We thus see that inversion of the emulsions from w/o to o/w by increasing the water content is not possible with such particles. In an attempt to see if the type of preferred emulsion stabilized by these particles is dependent on the oil type, we prepared emulsions from 2 wt % particles of diameter 0.4 µm in pure water and for φw ) 0.5 using the various oils listed in the Experimental Section. They were all w/o, of conductivity 1 ( 0.5 µS cm-1, even for more polar oils such as silicones, esters, and alcohols. For toluene, in which the particles dissolve entirely, emulsions were also w/o but of very low stability. We have no ready explanation for the difference in emulsion stability between the aldehyde/sulfate latex and the sulfate latex systems with this oil. Water-in-oil emulsions stabilized by either ionic or nonionic surfactants can be flocculated depending on conditions.10-13 In some cases flocculation is induced by the presence of depleting entities such as w/o microemulsion droplets11 or nonadsorbing polymer, in other cases by changing the quality of the oil solvent with respect to the surfactant chains.12,13 Flocculation of water drops has also been reported for the simple case of a charged surfactant at relatively low concentration. Schulman and Cockbain10 argued that a w/o emulsion cannot be stabilized
4546
Langmuir, Vol. 17, No. 15, 2001
Binks and Lumsdon
Figure 12. Conductivity and type of water-cyclohexane emulsions stabilized by hydrophobic latex particles of diameter 0.21 µm as a function of water volume fraction: (b) 2 wt % particles in 10-3 M NaCl; (O) 1 wt % particles in emulsion for 10-3 M NaCl; (4) 2 wt % particles in 1 M NaCl; (+) 1 wt % particles in emulsion for 1 M NaCl.
against flocculation by a charge on the drops since, because oil is a nonionizing medium, an electrical diffuse layer cannot be built up. However, it is known that even in nonpolar oils a distinct though very low degree of ionization may occur so that, despite the low dielectric constant of the medium, the electrical double layer is very diffuse and the corresponding Debye length can be as high as several micrometers. This is of the same order of magnitude as the distance between drops in a moderately concentrated emulsion. As pointed out by Albers and Overbeek,14 very little charge is then needed to obtain appreciable surface potentials. The repulsive energy of interaction at the low ionic concentrations involved between two drops of radius re is described by Coulomb’s law 2
Vr )
2
ψo re 2re + d
(5)
in which ψo is the surface potential, is the dielectric constant of the continuous oil phase, and d is the shortest distance between the surfaces of the drops. When this repulsion is combined with the van der Waals attractive energy between the same two drops
Va ) - Are/12d
(6)
where A is the Hamaker constant and d , re, an energy barrier of around 15 kT is obtained for the case where re ) 1 µm, ψo ) -25 mV, ) 2, and A ) 0.5 × 10-20 J. This relatively high maximum, which should be sufficient to prevent flocculation, is due to the very slow decay with distance of the electrical repulsion between the drops. The apparent contradiction between this result and experimental findings that w/o emulsions of moderate dispersed phase volume fraction are flocculated lies in (10) Schulman, J. H.; Cockbain, E. G. Trans. Faraday Soc. 1940, 36, 661. (11) Binks, B. P.; Fletcher, P. D. I.; Horsup, D. I. Colloids Surf. 1991, 61, 291. (12) Leermakers, F. A. M.; Sdranis, Y. S.; Lyklema, J.; Groot, R. D. Colloids Surf., A 1994, 85, 135. (13) Leal-Calderon, F.; Mondain-Monval, O.; Pays, K.; Royer, N.; Bibette, J. Langmuir 1997, 13, 7008. (14) Albers, W.; Overbeek, J. Th. G. J. Colloid Sci. 1959, 14, 501.
Figure 13. Effect of φw on the flocculation of aqueous 10-3 M NaCl-in-cyclohexane emulsions stabilized by 2 wt % hydrophobic latex particles (diameter 0.21 µm) in each emulsion. Volume fraction of water in prepared emulsions is given, although emulsions were diluted by a factor of 2 for the optical microscopy. Scale bar ) 100 µm.
the fact that the calculations for a pair of drops are not relevant. In a follow-up paper, Albers and Overbeek15 present a model which takes into account the interactions between large numbers of water drops. In summary, due to the long range of the repulsion in oil continuous emulsions, the drops possess a potential energy with respect to separation at infinite distance. The energy barrier is lowered as a consequence, and the stability against flocculation is reduced. A further lowering of the energy barrier occurs as a result of the combined interaction of more than two drops. Such considerations lead to the prediction that, for a fixed Debye length, w/o emulsions of low water volume fraction are stable to flocculation and flocculation is enhanced continuously with increasing volume fraction as the energy barrier is reduced. This is borne out experimentally in a study by Bedenko et al.16 for water-in-heptane emulsions stabilized by a commercial nonionic surfactant (diester of oleic acid and pentaerythritol) in which the degree of flocculation is shown to increase with φw. (15) Albers, W.; Overbeek, J. Th. G. J. Colloid Sci. 1959, 14, 510. (16) Bedenko, V. G.; Chernin, V. N.; Chistyakov, B. E.; Pertsov, A. V. Colloid J. U S.S.R. 1983, 45, 263.
Stabilized Emulsions
Langmuir, Vol. 17, No. 15, 2001 4547
We have tested the prediction mentioned above for one of the particle-stabilized emulsion systems for which a negative charge is thought to exist at the particle-oil interface8 around water drops. Emulsions of water-incyclohexane containing overall 2 wt % hydrophobic latex particles of diameter 0.21 µm were prepared at different volume fractions of water and observed microscopically immediately after formation. The images are given in Figure 13. For φw e 0.2, emulsion drops are discrete. For φw > 0.3, nearly all the drops observed are contained in flocs, the degree of interconnecting increasing with φw. For φw ) 0.3, the emulsion contains a mixture of isolated and flocculated drops. The prediction from theory is therefore closely followed in these emulsions stabilized by charged latex particles. Conclusions We have investigated emulsions stabilized by polystyrene latex particles alone. Recent freeze fracture scanning electron microscopy measurements on these emulsions17 reassuringly confirm the picture put forward here, namely that the adsorbed emulsifier is solid particles in the case
of cyclohexane but not particulate and most likely free polymer chains in the case of toluene. For cyclohexane as oil, emulsions are w/o over a wide range of salt concentration and water volume fraction for both hydrophilic and hydrophobic particles. The average emulsion drop diameter (35-75 µm) increases initially with increasing particle size and then remains constant, in line with an increase in the sedimentation rate. For toluene and hydrophilic particles, emulsions can be inverted from o/w to w/o with increasing salt concentration. The amount of salt required increases with an increase in (initial) particle diameter. Very stable w/o emulsions of average diameter around 1 µm can be prepared. Acknowledgment. We thank the EPSRC and ICI Paints (Slough) for funding a CASE award to S.O.L. and Dr. R. Ettelaie formerly of ICI (Wilton Centre) for useful discussions. LA0103822 (17) Binks, B. P.; Kirkland, M. Submitted for publication in Langmuir.
