Ind. Eng. Chem. Res. 2002, 41, 6033-6041
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Phase Inversion in Abnormal O/W/O Emulsions: I. Effect of Surfactant Concentration Shahriar Sajjadi, Fatemeh Jahanzad, and Brian W. Brooks* Department of Chemical Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom
The catastrophic phase inversion of a model abnormal O/W/O emulsion of a system composed of polyisobutylene, water, and a water-soluble grade surfactant was studied. The experiments were started with water being fed to the mixing vessel containing polyisobutylene to form a W/O emulsion. As the water volume fraction (fw) increased, multiple oil-in-water-in-oil drops formed by inclusion of the oil from the continuous phase. An increase in the surfactant concentration reduced the size of the oil-in-water-in-oil drops but did not significantly affect the size of the water-in-oil droplets. It also increased the inclusion of the continuous phase into the dispersed phase and advanced phase inversion. The mechanism of inclusion transformed from drop deformation to drop coalescence with increasing surfactant concentration and fw. After an inversion, the internal oil-in-water-in-oil droplets were released into the water-continuous phase, forming a substantial number of the drops in the inverted emulsion. 1. Introduction Conventional emulsions are simple two-phase systems in which one phase is dispersed in another, continuous phase as droplets of microscopic or colloidal size with the aid of mixing. Depending on which phase makes up the droplets, there are two kinds of simple emulsions: oil-in-water (O/W) and water-in-oil (W/O). A two-component emulsion is extremely unstable in nature and separates into two phases. To make a stable emulsion, a surface-active material must be present to protect the newly formed droplets against immediate recoalescence. However, according to Bancroft,1 a stable emulsion can be obtained if the phase in which the surfactant is predominately dissolved becomes the continuous phase. If an emulsion is formed so that the phase dissolving the surfactant becomes the dispersed phase, an unstable emulsion is formed. These unstable emulsions, or so-called abnormal emulsions,2 are not new in the emulsion industry. In fact, these emulsions are the intermediates for preparation of stable emulsions. In a method called the agent-in-oil or continental method,3 the water-soluble surfactant is dispersed in the oil phase. Then, water is directly added to the mixture until the initially formed unstable W/O emulsion inverts to a stable O/W emulsion. This has been reported to be the best method for the preparation of O/W emulsions.3 Lin4 reported that such a procedure will only give finer and more stable emulsions when the surfactant is not highly soluble in either the oil or the water phase. Some researchers have studied the volume fraction of the water (fw) required to induce phase inversion when the surfactant is dissolved in the oil phase. Becher5 used different types and concentrations of surfactants. In one set of experiments, he started from the oil phase containing water-soluble surfactant as the continuous phase and added water. He found that the value of fw at which phase inversion occurred (W/O to O/W) decreased with increasing surfactant concentration. Marszall6 studied phase inversion by the addition * To whom correspondence should be addressed.
of the water phase to the oil phase containing a surfactant. He also concluded that fw at the inversion point decreases with the concentration of hydrophilic surfactant. Silva et al. investigated the dynamic inversion hysteresis of emulsions containing anionic surfactants. They reported that the hysteresis zone widens as the surfactant concentration increases.7 Despite the appearance of a few reports in the literature regarding the application of this method to the preparation of O/W emulsions via phase inversion of abnormal W/O emulsions, to the knowledge of the authors, very little attention has been paid to date to the structural variation of the abnormal emulsions before inversion. In a recent paper, Salager et al. reviewed the transitional and catastrophic phase inversions that bound normal and abnormal emulsion regions on a phase behavior map.8 The formation of abnormal emulsions is usually associated with the formation of multiple drops. In multiple emulsions, a dispersed drop is itself an emulsion. Because the external phase of abnormal multiple emulsions is very unstable, it cannot accommodate a large quantity of dispersed phase, so that abnormal emulsions can be prepared only for a low and medium range of dispersed-phase ratio. An increasing dispersed-phase ratio will result in a catastrophic phase inversion to a normal emulsion. In our previous publications, the morphological change and dynamic behavior of drops in the polyisobutylene (PIB)/water/polyoxyethylene nonylphenyl ether (NPE) system were investigated.9,10 The aim of this paper is to investigate the variation in drop sizes and structures with surfactant concentration and dispersed-phase volume fraction during the phase inversion process. The mechanism of phase inversion is also discussed. 2. Experimental Procedure 2.1. Preparation. The experimental setup is similar to that described elsewhere.9,10 The experiments were performed using a baffled 1-L jacketed glass vessel and a conventional four-flat-blade turbine agitator. The stirrer speed was set at 500 rpm. All experiments were
10.1021/ie0202266 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/26/2002
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carried out at a temperature of 60.0 ( 0.5 °C. Drop size measurements were carried out using an optical microscope connected to a video camera. The oil phase was low-molecular-weight polyisobutylene (PIB), supplied by British Petroleum, with the trade name Hyvis07, having a number-average molecular weight of 440, a density of 0.871 (at 15 °C), and a viscosity of 65 cSt at 60 °C. A water-soluble grade of polyoxyethylene nonylphenyl ether (NPE) with a polyoxyethylene chain length of 12 and a corresponding hydrophilic/lipophilic balance (HLB) of 14.2 (Igepal 720 supplied by Aldrich) was used as the surfactant. The cloud point of NPE12 in aqueous surfactant solutions was found to be 82.0 °C (reduced to 78 °C in water containing 0.5 wt % KCl), a much higher temperature than the experimental temperature (60 °C). No phase inversion temperature was obtained for the PIB/water/NPE12 system up to 96 °C, where water started to evaporate. The transitional phase inversion for the PIB/water emulsion with NPE surfactants at 60 °C occurs at HLB ) 10.50.11 These results indicate that the formulation applied in this study was in the region of Winsor I, which tends to form normal O/W emulsions. The experiments were started with 200 cm3 of PIB, and the phase inversion was brought about by the addition of water (increasing fw) in aliquots of 10 cm3. Such a procedure results in an induced inversion of type W/O to O/W. The phase inversions were indicated by measuring changes in electrical conductivity. The time interval for water addition was 15.0 min (equivalent to the rate of addition of 40.0 cm3/h). This time interval assured that the steady-state drop size, or a nearsteady-state drop size, was reached for the experiments with low surfactant loading (e1 wt %).10 For the higher surfactant loading (5.0 wt %), the steady-state drop size was not reached within the experimental time. To avoid variations in the surfactant concentration during water addition, we placed the same concentration of the surfactant in both the water and oil phases. 2.2. Drop Size Measurements and Analysis. The sizes of water-in-oil (W/O), oil-in-water-in-oil (O/W/O), and water-in-oil-in-water-in-oil (W/O/W/O) drops were measured according to the procedures described elsewhere.9,10 The drop size distributions for all drop types were found so that reliable surface-average diameters (Sauter mean diameters) could be calculated. The surface average diameters of drops (d32) were calculated using the equation
d32 )
∑nidi3/∑nidi2
(1)
where ni is the number of drops with diameter di. dwo, dowo, and dwowo are used in this paper as the Sauter mean average diameters of W/O drops and internal O/W/O and W/O/W/O droplets, respectively. The techniques for measuring the internal volume fraction of multiple drops of diameter di (φi) have been explained elsewhere.9 2.3. Formulations. The effects of surfactant concentration were studied with five different weight percentages of NPE12 surfactant, namely, 5.0, 1.0, 0.20, 0.04, and 0.03 wt % of the total emulsion. The attempt made to carry out drop measurements for the run with no surfactant failed because of instantaneous phase separation. 3. Results and Discussion A catastrophic inversion is a process by which, for example, a W/O emulsion can be inverted to an O/W
Figure 1. Variation in internal phase ratio of the dispersed W/O drops (φi) with diameter at different values of fw for dispersions using 0.2 wt % NPE12.
emulsion by increasing the water phase ratio. The effects of surfactant concentration on the (phase) behavior of the three regions of preinversion, catastrophic inversion, and postinversion of the abnormal PIB/water/ NPE emulsions are discussed in the following sections. 3.1. Preinversion Region. 3.1.1. Inclusion. All experiments were started with a predetermined amount of oil in the mixing vessel. Small water drops were formed after the addition of the first aliquot of water to the oil; some of those drops contained a few small oil droplets. As more water was added (increasing fw), the water drops grew in size. The growth of drops was associated with inclusion of the continuous oil phase as small internal oil droplets, which increased the effective volume fraction of the dispersed phase, fd. Eventually, with increasing fw, a point was reached where the oil phase could not remain as the continuous phase, and phase inversion occurred. The internal structure of the drops in emulsions, as well as the size of internal droplets, is a complex function of the dispersed-phase ratio, surfactant type and concentration, and hydrodynamics of the mixing system. To study the internal structure of drops, we classified the water drops into bins with a size range of 10 µm. Thus, the average diameter of 15 µm, for example, represents drops within the size range of 1020 µm. For any fixed fw, drops within the range of 10100 µm were analyzed for the internal phase ratio (φi), which is defined as the ratio of the volume of the internal oil droplets contained in a multiple water drop with diameter di to the volume of the multiple water drop (πdi3/6). The average values for φi were assigned to the representative bin sizes. The results of the statistical analysis of the drop size intervals for φi are shown in Figures 1 and 2 for different values of fw and for the surfactant loadings of 0.20 and 5.0 wt %, respectively. The two imminent conclusions emerging from the cross examination of these figures are as follows: (1) At a constant surfactant loading and fw, drops with a larger size have a higher internal phase ratio (φi). (2) At a constant size, drops with a higher content of surfactant have a higher internal phase ratio. (This conclusion could be verified only for drops smaller than 40 µm because of a rather low and measurable internal phase ratio compared with the larger drops.)
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Figure 3. Variation in the Sauter mean diameter of multiple water drops (dwo) with fw for different surfactant loadings. (The phase inversion points are shown by arrows.) Figure 2. Variation in internal phase ratio of the dispersed W/O drops (φi) with diameter at different values of fw for dispersions using 5.0 wt % NPE12.
The mechanism of inclusion is usually attributed to either drop deformation or drop coalescence. According to the former mechanism, inclusion can occur by the large deformation of the dispersed water phase under the high shear rate applied by stirring. Drop deformation seems to be more effective for large drops because pressure fluctuations, or shear forces for laminar regions, across the drop surface can more easily result in the breakup of an emerging internal droplet from the continuous oil phase and the formation of a new internal droplet (first conclusion).9 The latter mechanism indicates that internal droplets might also be formed from continuous oil phase trapped between coalescing water drops.10,12,13 Two different mechanisms can contribute to this type of inclusion: (a) multibody collision of drops, which mostly occurs at a higher dispersed-phase fraction when a close-packed arrangement between the drops is reached, and (b) two-body collisions of drops, which occurs if some collisions are of such high intensity that flattened drops are formed.14 For the system under study, it was shown that drop flattening cannot be responsible for the formation of large internal droplets in the water drops. Thus, the drop deformation mechanism was suggested to be the main mechanism for inclusion at low fw.10 A closer look at Figures 1 and 2 reveals some crucial indications of the mechanism of inclusion. According to Figure 1, for the lower surfactant loading, the internal phase ratio of drops of a fixed size interval essentially did not change with fw, within the experimental error. Furthermore, the extent of inclusion was very low. For the higher surfactant loading, as shown in Figure 2, an appreciable increase in φi was observed with increasing fw (fd . fw). As fd increases with fw, the rate of collision of drops rises. This will progressively lead to a greater contribution of the coalescence-dependent mechanism to the inclusion. It should be noted that the flow regime is not appreciably altered with increasing fw because the viscosity of the dispersion is mainly controlled by the viscosity of the continuous polymer phase. A conclusion might emerge as follows: The mechanism of inclusion is transformed from the deformation mechanism to the coalescence mechanism with increasing surfactant concentration and fw. It is noteworthy that, with increasing inventory of surfactant in
the dispersed phase, the suppression of the interfacial gradients is enhanced. This leads to a more deformable interface (Maragoni effect), and as a result, more inclusion might occur. Whatever the source of deformation, either drop encounter with an eddy or drop encounter with another drop, drops with a low surfactant concentration are less susceptible to inclusion following a deformation. As a result, drops containing a low surfactant concentration have similar φi values with increasing fw value up to 0.40, as revealed in Figure 1. It is also inferred from Figures 1 and 2 that there is a critical size for drops below which they cannot entrain a substantial amount of internal droplets. The critical drop size seems to decrease with increasing surfactant concentration. 3.1.2. Variation in Drop Sizes and Drop Size Distributions. (a) Multiple Water Drops (W/O). It is well-known that the application of a large amount of surfactant to conventional emulsions will result in a substantial decrease in the size of emulsion drops. Figure 3 shows the variation in the Sauter mean diameter of multiple water drops, dwo, with fw for the four surfactant loadings used. Interestingly, the plot of drop sizes versus fw from all runs can be approximated by a single line. This indicates that, for the abnormal emulsions under study, the Sauter mean diameters of multiple water drops are insensitive to the surfactant loading for any fixed value of fw within experimental error. Whereas the pattern of drop size variation with fw remained unchanged for all surfactant concentrations used, a larger drop size could be reached before phase inversion for the lower surfactant loading. We reported previously that, for the system under study, an agitated dispersion at high continuous-phase viscosity, the flow is in a state of transition. Thus, the flow is expected to be laminar around the vessel wall and turbulent in the impeller region, where drop breakup will occur by inertial forces. For conventional dispersions, most researchers have reported typical Weber number correlations with a linear function, F(fd), to allow for the change in drop sizes with fd, i.e., F(fd) ) (1 + afd). a is a constant for any system, which allows for a rise in dwo with fd.15-18 For the O/W/O emulsion under study, in which inclusion of the continuous oil phase into the dispersed water phase leads to an increase in the effective volume fraction of the dispersed
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Figure 4. Variation in the size distribution of multiple water drops with fw for dispersions containing 5.0 wt % surfactant.
phase, fd and fw are not equivalent. They are correlated through the correlation9
fd ) fw/(1 - φo)
Figure 5. Variation in the Sauter mean diameter of inter oil droplets (dowo) with fw for different surfactant loadings. The last point for the surfactant loading of 5.0 wt % is the size of oil droplets in the inverted emulsion.
(2)
where φo, the overall internal phase ratio of the multiple drops or dispersed phase, is calculated by taking an average of φi over all ranges of multiple water drops in the dispersion. To obtain fd, φo must be known. The measurement of φo is quite a tedious task and cannot be done accurately for drops with a high internal phase ratio.7 Thus, the drop size analysis in terms of fd could not be carried out in the context of this study. Instead, we attempted to correlate the drop sizes in terms of fw. A closer look at Figure 3 reveals that the linear functionality between dwo and fw is more significant for surfactant loadings equal to or smaller than 1 wt %. For these systems, and particularly for those containing 0.2 and 0.04 wt % surfactant, not only do all data points fall on a representative linear curve but there is also a low value for inclusion. Thus, it seems that the typical function of drop size in terms of volume fraction of the dispersed phase (1 + afd) can adequately predict the variation in dwo with fw for the lower range of surfactant loadings where inclusion is not important so that fd ≈ fw. The steady-state multiple water drops showed approximately a unimodal drop size distribution that gradually transformed to a positively skewed distribution with increasing fw for all surfactant loadings used. A typical evolution of drop size distribution with fw is shown in Figure 4 for the surfactant loading of 5.0 wt %. This is in accordance with the generalization of the conclusion reached by Chatzi and Kiparissides,19 who reported that, for emulsions with a high rate of drop coalescence, an essentially unimodal distribution can be obtained. (b) Internal Oil Droplets (O/W/O). Contrary to the results obtained for multiple water drops, the size of internal oil droplets is influenced by the surfactant concentration in the same way as occurs in normal emulsions.3 Figure 5 represents the variation in size of internal oil droplets with fw and surfactant loading. The average diameter of droplets increased with fw for all surfactant concentrations. At a constant fw, the droplet size increased with decreasing surfactant concentration. Although a semiempirical linear relationship between the diameter of internal oil droplets (dowo) and fw, such
Figure 6. Variation in the size distribution of internal oil droplets with fw for dispersions containing (a) 5.0 and (b) 1.0 wt % of NPE12. The size distribution for fw ) 0.21 from part a is for the oil drops in the inverted emulsion. The data points of part b were fitted with a polynominal of order 5.
as dowo ) (1 + bfw), can be inferred from Figure 5, precaution should be taken in the interpretation of the results, in comparison with those reported in the literature for single emulsions. The typical evolution of the size distribution of internal oil droplets with fw is shown in Figure 6 for the surfactant loadings of 5.0 and 1.0 wt %. Figure 6a shows that, for the surfactant loading of 5.0 wt %, the
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size distributions in the preinversion region are all bimodal. The distributions almost maintained their bimodality with increasing fw, while the first peak shrank and the second peak dilated until the inversion occurred.10 A mechanism described previously for inclusion together with simultaneous breakup of internal droplets can generate a bimodal drop size distribution.10 Figure 6b shows that, for the surfactant loading of 1.0 wt %, a bimodal distribution was obtained at low fw that transformed into a unimodal distribution with progressive skewness with increasing fw. When the number of internal oil droplets present in a water drop increases with fw, the oil droplets might undergo coalescence. In the presence of a sufficient amount of surfactant (5.0 wt %), the internal droplets are quite stable and do not undergo coalescence extensively. Therefore, drop formation is governed by drop breakup, resulting in a bimodal distribution. At a low surfactant loading (e1.0 wt %), internal droplets are not sufficiently stable and coalesce quickly. The coalescence process hinders the formation of a bimodal size distribution by the collision of drops with different sizes.19 (c) Water Droplets in the Internal Oil Droplets (W/O/W/O). The internal oil droplets contained some very small internal water droplets. The diameter of W/O/W/O droplets (dwowo) did not vary appreciably with fw or with the surfactant concentration. For the surfactant loading of 5.0%, dwowo was around 15 µm. The mechanism of inclusion of water droplets into the internal oil droplets is believed not to be different from that of oil droplets into water drops.10 When a part of the continuous oil phase is pulled into a water drop through an inclusion event, the small water droplets dispersed in the oil phase (W/O) might be carried over into the large water drop (W/O), forming W/O/W/O droplets. The possibility of carrying water droplets surrounded by oil phase into the water drops increases when the volume of inclusion is large (simply because the possibility of the presence of small water droplets in oil droplets is higher when a large domain of the oil phase is pulled into the large water drops). In fact, W/O/ W/O droplets constitute the lower end of the drop size distribution of W/O drops, as shown in Figure 4. The small W/O drops (from the lower range of the distribution, I) dispersed in the continuous phase are pulled into large W/O drops (from the upper range of the distribution, II), and as a result, W/O/W/O droplets are formed. 3.1.3. Drop Structures. The structure of the drops is subject to variations with increasing fw and surfactant loading. Figure 7 shows the variations in drop size with fw for the surfactant loading of 1 wt %, from which the variations in drop structure (in terms of the ratios dowo/ dwo and dwowo/dowo) with fw can be easily calculated. It has been shown previously that, for dispersions of the type studied here, the size ratio of internal oil droplets to that of multiple water drops, dowo/dwo, decreases with agitation time, whereas the internal phase ratio of the dispersed phase, φo, increases with time until a steady state is reached.9 At a constant surfactant loading and under steady-state conditions, dowo/dwo decreases with increasing fw, indicating that multiple drops of a larger size contain relatively smaller-sized internal droplets. The unstable multiple water drops grow in size more severely through coalescence compared to the stabilized internal oil droplets, as is clear from the slopes of the d32-fw lines shown in Figure 7. For any constant fw value, the ratio dowo/dwo decreases with increasing
Figure 7. Variation in the Sauter mean diameter of W/O multiple drops (dwo), O/W/O internal droplets (dowo), and W/O/W/O internal droplets (dwowo) with fw for a dispersion containing 1.0 wt % loading of NPE12.
surfactant concentration. This can easily be deduced from a cross examination of Figures 3 and 5. Figure 7 shows that, for the surfactant loading of 1 wt %, the sizes of W/O, O/W/O, and W/O/W/O drops all converge at a low value of fw. This indicates that multiple drops can form above a critical volume fraction of the dispersed phase (about fw ) 0.03 for the conditions given in Figure 7) at which the size of the drops has reached a certain critical value, as stated previously. The W/O/W/O droplets are formed at a higher fw (>0.10) where W/O drops have sufficiently enlarged to accommodate small W/O droplets. The drop structure is highly influenced by the properties of the medium in which the drops are dispersed. Florence and Whitehill20 have discussed the breakdown of multiple emulsions and suggested several possible mechanisms. At lower surfactant loadings (0.04 and 0.03 wt %), the drop stability was so weakened that it was not possible to quantify the internal structure of the multiple drops, because of the rapid coalescence of internal oil droplets during measurements. It has been shown that escape occurs more frequently when dowo/ dwo is very large.20 The expulsion (escape) of the internal oil droplets, grown by successive coalescence, following the rupture of thin water films during the interaction of the internal and external oil phase results in the breakdown of the multiple O/W/O emulsions to the ordinary W/O emulsions. Microscopic examination of the samples at the surfactant concentration of 0.2 wt % when exposed to external disturbances revealed some coalescence of internal oil droplets and also escape of oil droplets from the multiple water drops. It is likely that the lifetime of internal oil droplets at the lower surfactant concentrations is so short that droplets cannot be observed experimentally, before their coalescence and eventual expulsion to the continuous phase. 3.2. Catastrophic Phase Inversion. 3.2.1 Phase Inversion. Catastrophic phase inversion is the result of a rapid increase in the rate of drop coalescence compared to the rate of drop breakup. As the volume fraction of the dispersed phase increases, drop coalescence rate increases because of the increased frequency of collision, and as a result, larger dispersed drops are formed. This will continue until a point is reached where the drop coalescence rate largely exceeds the drop breakup rate so that a new balance cannot be reached. This will result in an inversion. Figure 3 clearly shows
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lescence efficiency has been defined as21,22
cf ) e-td/ti
Figure 8. Variation in the volume fraction of water (fw) at the phase inversion points with the surfactant loadings.
that catastrophic inversions are proceeded by a gradual increase in drop diameter with fw. The formation of multiple drops, which is due to inclusion of a part of the continuous phase in the dispersed phase, results in increasing fd. This will intensify the rate of drop coalescence and, hence, cause the inversion to occur at a lower fw. The results obtained in this research clearly indicate that the delay in phase inversion with increasing fw correlates well with complex drop formation. Figure 8 shows the variation in fw at the inversion point with surfactant loading. The data can be fitted with an exponential correlation. For the run using a surfactant loading of 5.0 wt % in which substantial inclusion occurred, phase inversion was brought about at fw ) 0.21. For the runs using surfactant loadings of 1.0, 0.20, and 0.04 wt %, phase inversions were delayed till fw ) 0.30, 0.41, and 0.66, respectively. This is in accord with the data of Salva et al., who showed that catastrophic phase inversion of W/O emulsions containing water-soluble anionic surfactant(s) is advanced, in terms of water volume fraction, with increasing surfactant concentration.7 For the latter cases, less inclusion occurred compared to that for the surfactant loading of 5.0 wt %. Finally, for the surfactant loading of 0.03 wt %, where only very a low internal phase ratio was obtained, phase inversion occurred at fw ) 0.71, which is close to the closest-packing arrangement. For the dispersion with no surfactant, phase inversion occurred around fw ) 0.75 ( 0.01, which is in the vicinity of the closest-packing arrangement. (The detection of the inversion point for this case was difficult because of the very low stability of the dispersion formed.) It is apparent from the results that phase inversion occurs at the closest-packing arrangement when a low amount of surfactant that does not allow for inclusion is used. It is depicted in Figures 1 and 2 that the higher surfactant loadings allow for a larger internal phase ratio for any drops of a fixed size. This indicates that, despite a low fw at the inversion point for the higher surfactant loadings, a large fd must have been attained in the system. 3.2.2. Coalescence Efficiency. The drop coalescence rate is influenced by the collision rate between the drops, which is a function of the drop size and hydrodynamics of the dispersion and the coalescence efficiency between the colliding drops, cf. For coalescence to occur, the drops must first collide and remain in contact long enough for the intervening liquid film to drain to its critical thickness, whereupon film rupture occurs. Coa-
(3)
where td and ti are drainage time and interaction time, respectively. A coalescence efficiency equal to 1 indicates that the drainage time is nonexistent. The coalescence efficiency depends on interfacial properties, such as interfacial tension and van der Waals attraction and double-layer forces, and bulk properties such as the viscosities of the continuous and dispersed phases.22 The interfacial properties are mainly determined by surfactant type and concentration at the interface and by surfactant interactions with both phases. The concentration of the nonionic surfactants at the oil/water interface can drastically change with the total surfactant concentration in the emulsion, oil type, and oil/ water ratio because of selective partitioning.23 The HLB at the interface stays almost constant in higher surfactant concentration ranges, but it is raised at lower concentrations when the oil phase is nonpolar, e.g., for saturated hydrocarbons.24 For a PIB/water/NPE system, where PIB represents a nonpolar oil such as saturated hydrocarbons, shorter oxyethylene chain homologues of surfactant preferentially dissolve in the PIB phase, and the average HLB of the surfactant at the oil/water interface becomes larger, that is, more hydrophilic. This indicates that the formulation for an abnormal emulsion that is to be hydrophilic over the entire surfactant concentration range has been achieved in this study. For normal emulsions, the coalescence efficiency decreases with increasing emulsifier concentration, resulting in an emulsion with smaller-sized drops. When surfaces are developed contrary to the natural tendency of the emulsifier, surfactants do not behave very efficiently. It is known that emulsifiers can even act as destabilizers. In such a case, the film drainage of the surfactant at the interface occurs quickly and might not be the rate-determining step for coalescence.25 Vaessen et al.25 developed a predictive correlation for phase inversion using an idea initially proposed by Arashmid and Jeffrey.26 They showed theoretically that the catastrophic phase inversion for abnormal emulsions in which drop coalescence occurs after each collision, cf ) 1, can occur at fd ) 0.26. In support of their theoretical finding, Vaessen et al. reported an fw value of 0.37 for phase inversion in the abnormal water-inhexane dispersion.25 However, this result is not conclusive, as they did not report on the structure of the drops (extent of inclusion and its effect on fd). The results from other investigators indicate that the dispersed-phase ratio at the inversion point varies widely with emulsification conditions such as oil type, surfactant type, rate of addition of the dispersed phase, and mixing.5,7,8 In their reports, the fraction of dispersed phase at the inversion point had small- or medium-sized values; however, no data regarding the actual effective volume fraction of the dispersed phase were provided. For abnormal emulsions, inclusion plays an important role. The results of this study indicate that phase inversions occur at the high fd values and in the vicinity of closestpacking arrangement (fd ≈ 0.74). This implies that cf ) 1.0 is not a necessary condition for inversion of abnormal emulsions and that inversions can occur at a much higher fd than 0.26. It should be noted, however, that, for the system under study, which has a high continuous-phase viscosity, a low film drainage rate and thus
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Figure 9. Schematic presentation of drop formation in the inverted emulsion by the internal mechanism. (The gray and dark regions represent the oil phase.)
a low coalescence rate is expected. (The intervening liquid film between colliding drops expands in the continuous phase.) This could be one of the effects that contributed to the stability of drops by reducing cf and causing the inversion to occur at a high fd, which otherwise could have happened at a lower fd value. 3.3. Postinversion Region. When a W/O emulsion breaks into an O/W emulsion, the oil droplets in the inverted emulsion are formed by (1) a coalescence mechanism and (2) a breakup (disruption) mechanism.12 The former is dominant when phase inversion occurs at the closest-packing configuration where the new dispersed phase is formed from the continuous phase caught between a number of colliding drops. The latter occurs when the dispersion is far from closest packing (dispersed-phase volume fraction is small) where the simultaneous collision of few drops is unlikely. The new oil drops are formed by transformation of the continuous oil phase by a large domain of water phase (or by disruption of the oil-continuous phase into the inverted continuous phase by a simple breakup mechanism). For the intermediate dispersed-phase hold-up, both mechanisms can contribute to the inversion. For the conditions of this study, in which phase inversions occurred in the vicinity of the closest-packing arrangement, the coalescence mechanism of inversion is expected to contribute mostly to drop formation in the inverted emulsion. For multiple O/W/O emulsions, however, a third mechanism contributes to the formation of O/W drops in inverted (O/W) emulsions. This mechanism can be called the “internal mechanism”. When an O/W/O
dispersion is inverted to an O/W dispersion, the internal oil droplets are released to the water-continuous phase and constitute a part of the dispersed drops if suitable conditions exist. Figure 9 schematically demonstrates such a release of internal droplets to the continuous phase after inversion. If the size data given for the surfactant loading of 5.0 wt % in Figure 5 are studied, a continuity is observed in the size data of the internal oil droplets in the preinversion region with that immediately after inversion. (The last point on the curve is for the postinversion region.) This continuity indicates that, at a high surfactant loading, most of the oil drops in the inverted O/W emulsion keep their sizes when they are released from the water drops in the preinversion structure into the continuous water phase of the postinversion dispersion. A simple calculation shows that, if we assume fd at the inversion point is 0.74, then, for the dispersion with fw ) 0.21 and with 5.0 wt % surfactant loading, φo ) 0.72 is obtained using eq 2. Thus, the volume fraction of the oil phase entrained in the dispersed phase [) fdφo/(1 - fw)] is obtained as 0.67. This means that about two-thirds of the drops in the inverted O/W emulsion are released from the interior of multiple water drops in the preinversion structure, and only one-third emerges from the other inversion mechanisms. This occurs despite the fact that the flow regime changes from a transitional one in the preinversion region to a turbulent one in the postinversion region, where water becomes the continuous phase. Further evidence for the internal mechanism can be obtained from Figure 6a, which compares the size distribution of oil droplets in the inverted O/W disper-
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Figure 10. Variations in the Sauter mean diameter of internal oil droplets (O/W/O) before inversion and oil drops (O/W) in the inverted emulsion with the surfactant loading.
sion with that from the preinversion stage (at 5.0 wt % surfactant). The distribution of O/W drops in the inverted emulsion was very similar to that of O/W/O droplets at the inversion point, except that a small peak at the higher size, which can be attributed to the drop formation by other inversion mechanisms, appeared after the inversion. It should be noted that a part of the difference in the drop size distributions might be the result of increased fw over the sampling interval (fw ) 0.17 and 0.21 for the preinversion and postinversion points, respectively). The contribution of internal oil droplets entrained in the multiple water drops to the oil drops in the inverted O/W emulsions progressively increases with surfactant concentration. Figure 10 shows the variation in the size of O/W/O drops at the inversion point (one aliquot before inversion) and the variation in size of O/W drops in the inverted emulsion with the surfactant concentration. For the surfactant loading of 5.0 wt %, similar diameters were obtained for the both kinds of drops. However, the gap widened with decreasing surfactant loading. For the surfactant loadings lower than 0.20 wt %, the diameter of the O/W drops in the inverted emulsion became even greater than that for multiple water drops (W/O) before the inversion. For the surfactant loading of 0.04 wt %, for example, the diameter of oil drops in the inverted emulsion was around 140 µm (from Figure 10), which is higher than the 120 µm found for the corresponding multiple drops at the inversion point (see Figure 3). This implies that such drops definitely cannot be the results of the internal mechanism. It is also concluded that, at a high surfactant concentration, the internal drops in the preinversion structures are so stable that they do not change in size upon entering the postinversion structure. Conclusions When water is dispersed in an oil phase containing a highly water-soluble surfactant, an abnormal emulsion is formed. Although the water-soluble surfactant molecules migrate into the interfacial area, they cannot provide sufficient stabilization for the drops, and the rate of drop coalescence is high. An emulsion of the type studied here includes both normal and abnormal emulsions. The multiple water drops are quite unstable, because they are dispersed in an oil-continuous phase containing a surfactant with a low oil solubility, whereas
the internal oil droplets are quite stable because the dispersed phase contains water-soluble surfactant. As a result, any variation in the surfactant concentration strongly affects the internal structure of the abnormal emulsion, whereas the external phase was insensitive to the surfactant concentration. Catastrophic phase inversion was brought about by inclusion of the oil from the continuous phase into the dispersed water drops, which continuously increased the effective volume fraction of the dispersed phase. The higher surfactant loading in the dispersed phase increased the affinity of the dispersed phase to entrain the larger amount of oil from the continuous phase and to induce the phase inversion at a lower water volume fraction. The drop coalescence mechanism was responsible for inclusion at a higher surfactant loading and fw, but a drop deformation mechanism was responsible for inclusion at a lower surfactant loading. Some indications were found that catastrophic phase inversion for the system under study occurred in the vicinity of a closest-packing arrangement. At a high surfactant concentration, most of the oil drops in the inverted emulsion originated from the internal structure of water drops at the inversion point. Acknowledgment The authors thank the EPSRC for financial support (Ref GR/K 78249). Literature Cited (1) Bancroft, W. D. The Theory of Emulsification. J. Phys. Chem. 1913, 17, 501. (2) Salager, J. L.; Minanaperez, M.; Perezsanchez, M.; Ramirezgouveia, M.; Rojas, C. I. Surfactant-Oil-Water Systems Near the Affinity Inversion Part III: Two Kinds of Emulsion Inversion. J. Dispersion Sci. Technol. 1983, 4, 313. (3) Becher, P. Emulsions: Theory and Practice; American Chemical Society: Washington, DC, 2001. (4) Lin, T. J.; Kurihara, H.; Ohta, H. Effects of Phase Inversion and Surfactant Location on the Formation of O/W Emulsions. J. Soc. Cosmet. 1975, 26, 121. (5) Becher, P. J. Soc. Cosmet. Chem. 1958, 9, 141. (6) Marszall, L. Study on the Required HLB of Oil-in-Water Emulsions by a Simple Phase-Inversion Titration. Cosmet. Perfum. 1975, 90, 37. (7) Silva, F.; Pena, A.; Minana-Perez, M.; Salager, J. L. Dynamic inversion hysteresis of emulsions containing anionic surfactants. Colloids Surf., A 1998, 132, 221. (8) Salager, J. L.; Marquez, L.; Pena, A.; Rondon, M.; Silva, F.; Tyrode, E. Current Phenomenological Know-How and Modeling of Emulsion Inversion. Ind. Eng. Chem. Res. 2000, 39, 2665. (9) Sajjadi, S.; Zerfa, M.; Brooks, B. W. Morphological Change in Drop Structure with Time for Abnormal Polymer/Water Surfactant Dispersions. Langmuir 2000, 16, 10015. (10) Sajjadi, S.; Zerfa, M.; Brooks, B. W. Dynamic Behaviour of Drops in Oil/Water/Oil Dispersions. Chem. Eng. Sci. 2002, 57, 663. (11) Zerfa, M.; Sajjadi, S.; Brooks, B. W. Phase behaviour of polymeric emulsions during the phase inversion process in the presence of nonionic surfactants. Colloids Surf. A 2001, 178, 1. (12) Brooks, B. W.; Richmond, H. N. Phase Inversion in NonIonic Surfactant-Oil-Water Systems II. Drop Size Studies in Catastrophic Inversion with Turbulent Mixing. Chem. Eng. Sci. 1994, 49, 1065. (13) Kumar, S. On Phase Inversion Characteristic of Stirred Dispersions. Chem. Eng. Sci. 1996. 51, 831. (14) Davies, G. A.; Jeffreys, G. V.; Smith, D. V.; Ali, F. A. The Formation of Secondary Droplets in a Dispersion at a Phase Boundary. Can. J. Chem. Eng. 1970, 48, 328. (15) Calderbank, P. H. Physical Rate Processes in Industrial Fermentation. Part 1. The Interfacial Area of Gas-Liquid Contacting with Mechanical Agitation. Trans. Inst. Chem. Eng. 1958, 36, 443.
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Received for review March 22, 2002 Revised manuscript received August 29, 2002 Accepted September 3, 2002 IE0202266
