O Emulsions. 2. Effect of Surfactant

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Ind. Eng. Chem. Res. 2003, 42, 3571-3577

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Phase Inversion in Abnormal O/W/O Emulsions. 2. Effect of Surfactant Hydrophilic-Lipophilic Balance S. Sajjadi,† F. Jahanzad,‡ M. Yianneskis,† and B. W. Brooks*,‡ ECLAT, Department of Mechanical Engineering, King’s College London, London, WC2R 2LS, United Kingdom, and Department of Chemical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, United Kingdom

The effect of surfactant hydrophilic-lipophilic balance (HLB) on the catastrophic phase inversion of an emulsion, and the internal structure of the drops, was studied by using a model abnormal O/W/O emulsion. The system was composed of polyisobutylene, water, and a mixture of a watersoluble and an oil-soluble surfactant. The abnormal O/W/O emulsions were made by gradual addition of water to the oil in the presence of surfactant until the catastrophic phase inversion to the corresponding normal O/W emulsions occurred. The presence of an oil-soluble component in the surfactant, even in a low quantity, enhanced the stability of the abnormal W/O emulsion. The phase inversion was slightly advanced, in terms of water volume fraction, with decreasing HLB. Some indication was found that as the optimum HLB was approached, the inclusion of continuous phase into the dispersed phase increased. The size of internal oil droplets, which were entrained in the multiple water drops, was largely reduced with decreasing HLB as the optimum formulation was approached. The size of multiple water drops, which are dispersed in the continuous oil phase, however, showed a considerably smaller decrease with decreasing HLB. When, at a fixed oil/water ratio, the HLB of the abnormal emulsion approached that of the optimum, the catastrophic phase inversion to a corresponding normal emulsion occurred, indicating that abnormal emulsions cannot exist in the vicinity of the locus of transitional inversion. 1. Introduction Emulsions are dispersions of droplets of one liquid in another, immiscible, liquid in which the droplets are of colloidal or near-colloidal sizes. A surfactant is usually required to render stability to the emulsions. In recent years the application of nonionic surfactants is rapidly increasing in many fields such as cosmetics and paints, and correspondingly there has been an increasing interest in studying the aggregates of nonionic surfactants in the aqueous phase, particularly because of their potential uses in tertiary oil recovery. Nonionic emulsifiers offer certain advantages over conventional ionic emulsifier. The most important one is the ability to make controlled changes in the hydrophilicity of the emulsifier. This is particularly true for polyoxyethylene compounds in which there is a possibility of changing the length of the polyoxyethylene chain and, consequently, modifying the hydrophile-lipophile balance (HLB). As the hydrophilicity of a surfactant increases, it becomes more soluble in the water phase. Surfactants with high HLB values are expected to stabilize O/W emulsions, while those with low HLB values stabilize W/O emulsions.1,2 Thus, as the surfactant composition in an O/W emulsion is altered from hydrophilic to hydrophobic, phase inversion from O/W emulsion to W/O emulsion can occur.3 This type of inversion is called transitional phase inversion.4 In the transitional phase inversion, the affinity of surfactant toward oil-water is changed and the curvature of the oil-water interface * To whom correspondence should be addressed. E-mail: [email protected]. † King’s College London. ‡ Loughborough University.

gradually changes from positive to negative, passing through zero curvature at the inversion point. The variation in the curvature of the interface is associated with changes in interfacial tension (which decreases to an ultralow value and then subsequently increases5,6). The formulations at which transitional inversion occurs are usually referred to as optimum formulations.7 For an emulsion with a constant HLB, the increasing amount of the dispersed phase will eventually lead to an inversion to the opposite emulsion. This type of inversion is usually called catastrophic phase inversion.4,8 In normal emulsions, which are formed under equilibrated conditions so that the phase dissolving the surfactant becomes the continuous phase, the dispersed drops are kinetically stable. Normal emulsions with high dispersed phase ratio can be formed when an appropriate surfactant is present in the continuous phase. Under dynamic conditions, it is often possible to violate the morphology required by the nature of surfactant. If the phase dissolving surfactant becomes the dispersed phase, an abnormal emulsion is formed. This can be done, for example, by adding the dispersed phase to the continuous phase under continuous agitation. Abnormal emulsions are extremely unstable. Drop coalescence in abnormal emulsions is stimulated because the Marangoni effect is suppressed. It has been stated that for an abnormal emulsion the intervening film between colliding drops behaves as if surfactant is absent.9 For the same reason, the dispersed drops in abnormal emulsions are usually very large. The term abnormal dispersion thus can better describe this type of emulsion. Abnormal emulsions can usually contain a small amount of the dispersed phase. Increasing the

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amount of the dispersed phase will lead to a catastrophic phase inversion to the normal emulsion. Catastrophic phase inversion is the result of a rapid increase in the rate of drop coalescence compared to the rate of drop breakup. At the steady state, drop coalescence and drop breakup are in balance. As the volume fraction of the dispersed phase is increased, a point is reached where the rate of drop coalescence largely exceeds the rate of drop breakup, leading to a phase inversion. Abnormal emulsions are usually multiple emulsions. Multiple drops are formed because the dispersed phase contains a soluble surfactant which can emulsify the continuous phase. The inclusion of part of the continuous phase in the dispersed phase results in increasing the effective volume fraction of dispersed phase (fd) and causes the inversion to occur at a lower holdup. A boost of actual volume fraction of the dispersed phase due to the formation of multiple emulsions could produce a catastrophic phase inversion over a wide range of dispersed phase volume fractions, depending on the surfactant concentration and properties.10,11 This is perhaps the most important factor which advances the catastrophic phase inversion in abnormal emulsions. However, such an important effect has been almost ignored in the literature. The time evolution of drop morphology and of drop sizes for a model abnormal emulsion, polyisobutylene (PIB)/water/polyoxyethylene nonylphenylether system, has been previously reported.12,13 In the companion paper,11 we investigated the effect of variation in the concentration of the water-soluble surfactant on the drop structures and sizes and on the delayed catastrophic phase inversion of the model abnormal emulsion. The objective of this paper is to study the effect of HLB on the drop morphology and on catastrophic phase inversion of the model abnormal emulsion. We believe this is the first quantitative study on the variation of drop size and structures with HLB in abnormal emulsions. The use of a medium viscosity PIB, as the continuous phase of the model abnormal emulsion, lowers the rate drop coalescence and allows for precise size measurements to be made. 2. Experimental Procedure The experimental setup and procedure are similar to those described elsewhere.11,12 The experiments were performed using a baffled 1-L jacketed glass vessel and a conventional 6 flat-blade disc-turbine agitator (diameter ) 5.0 cm) connected to a digital variable speed motor. The stirring speed was kept constant at 500 rpm. The oil phase was PIB, supplied by British Petroleum with trade name Hyvis07, having a molecular weight number average of 440, a density of 0.871 (at 15 °C), and a viscosity of 65 cSt at 60 °C. Different grades of Igepal (polyoxyethylene nonylphenylether, NPE), supplied by Aldrich, were used as surfactants. The surfactant mixtures were made from surfactants containing Igepal co210 (NPE2; HLB ) 4.6), Igepal co520 (NPE5; HLB ) 10.0), and Igepal co720 (NPE12; HLB ) 14.2). The numbers 2, 5, and 12 indicate the polyoxyethylene chain length of the NPE2, NPE5, and NPE12 surfactants, respectively. Weight averages were used for making a surfactant mixture with a predetermined HLB. Most experiments were started with PIB oil and the phase inversion was brought about by addition of water at a constant HLB. Such a procedure results in an induced catastrophic phase inversion of type O/Wm/O

Figure 1. Typical micrograph of multiple O/W/O emulsions.

Figure 2. Phase transition map for PIB/water/(NPE5/NPE12) system. 1. Transitional phase inversion; 2. catastrophic phase inversion. The phase behavior map for the PIB/water/(NPE5/ NPE12) system (the routes of experiments are shown by 1 and 2).

to O/Wm. Subscript “m” refers to the micelle-containing phase (according to Bancroft’s rule,1 therefore, O/Wm and W/Om emulsions are normal or stable emulsions, whereas Wm/O and Om/W are unstable or abnormal emulsions). In one experiment, the catastrophic phase inversion was brought about at a constant water volume fraction (fw) with HLB as the variable. The phase inversions were indicated by measuring changes in electrical conductivity. The water and oil phases in all experiments contained 5.0 wt % surfactant. All experiments were carried out at 60 ( 0.50 °C. Drop size measurements were carried out using an optical microscope connected to a video camera. The Sauter mean (surface-average) diameter of drops (d32) was calculated using the following equation

d32 )

∑nidi3/∑nidi2

(1)

where ni is the number of drops with diameter di. In the following text, dwo and dowo indicate the Sauter mean diameter of multiple W/O drops and internal O/W/O droplets, respectively. The techniques for measurement of internal volume fraction of multiple drops of diameter di (φi ) have been explained elsewhere.12 A typical micrograph of the abnormal emulsions is shown in Figure 1. 3. Results and Discussion Figure 2 shows the phase inversion map for PIB/ water/NPE emulsions at 60 °C. The transitional phase

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Figure 3. Variation in the Sauter mean diameter of multiple water drops with fw for different HLB values using 5.0 wt % of NPE5/NPE12.

inversion occurs in the vicinity of HLBav ) 10.5.14 At HLBav < 10.5, Winsor-II W/Om emulsions are formed. At HLBav > 10.5, Winsor-I O/Wm emulsions are formed. The HLB value of the surfactant system was used as the main variable in the current research. We confined our study to the range of HLBav > 10.5 in which normal O/Wm emulsions are thermodynamically favored. Abnormal emulsions were formed by adding water to the oil phase. 3.1. Effect of HLB of the Emulsifier System. Effects of HLB of a surfactant mixture were investigated using four HLB values of 14.2, 13.0, 12.0, and 11.0. These HLB values were made by using a combination of NPE5 and NPE12. Most experiments were started with water being fed to the agitated vessel containing the PIB oil (they are specified by route 1 shown in Figure 2, unless otherwise stated). This results in increasing the volume fraction of the water phase (fw). After the addition of the first aliquots of water, complex water drops containing internal oil droplets were formed by inclusion of the oil phase from the continuous phase. This led to a concomitant increase in dispersed phase ratio, fd, so that fd > fw. The water addition was continued until catastrophic phase inversion of the abnormal O/Wm/O emulsion to the corresponding O/Wm emulsion occurred. 3.1.1. Multiple Water Drops. Figure 3 shows the variations in Sauter mean diameter of multiple water drops with fw for different HLB values used. A gradual increase in drop diameters, with increasing fw, precedes catastrophic phase inversions. The variations in the size of multiple water drops with the HLB values were not appreciable at low values of fw, but drop size increases became more apparent at higher values of fw. As the surfactant HLB approached the optimum, the affinity of the surfactant mixture toward the oil phase was increased.2 This is associated with a decrease in the interfacial tension.5,6 This favors more breakup and, as a result, the size of water drops decreased with decreasing HLB in the lower range of fw where the size of drops is mainly controlled by the rate of drop breakup. At a higher value of fw, where the drop coalescence is more important, the size of multiple water drops increased with fw with a rate depending on the surfactant HLB. A higher surfactant HLB resulted in more severe drop size increase with fw. This indicates that stability of the water drops against coalescence was improved with decreasing HLB. Thus, the overall effect of surfactant HLB on the size of multiple drops is minor

Figure 4. Variation in the Sauter mean diameter of the internal oil droplets with fw for different HLB values using 5.0 wt % of NPE5/NPE12 (the large symbols demonstrate the corresponding size of oil droplets in the inverted emulsion).

at a low fw and becomes progressively more important at a higher fw. In the companion paper,11 it was shown that the size of multiple water drops at a constant fw does not show a significant variation with the concentration of the water-soluble surfactant (NPE12). The results suggest that the size of multiple water drops may vary with the surfactant concentration, if a combination of a low surfactant and a high HLB surfactant is used. For conventional dispersions, most researchers have reported a typical linear function between d32 and fd. The experimental results obtained here for PIB/water/ NPE systems at different HLBs show that an exponential type of correlation between dwo and fw is obtained, as shown in Figure 3. Delichatios and Probstein,15 by taking into account the coalescence frequency of colliding drops at a high holdup fraction, derived a semiempirical exponential relation between d32 and fd. For the system under study, however, fd and fw are not identical because of inclusion. This indicates that the curvature of the exponential function could be less pronounced than those shown in Figure 3 (and Figure 6 as discussed later), if drawn in terms of fd. It might be possible that dwo - fw curves tend to be linear if converted to dwo - fd because fd > fw. 3.1.2. Internal Oil Droplets. Figure 4 depicts the variations in the Sauter mean diameter of internal oil droplets with fw for the different HLB values used. The size of internal droplets showed an appreciable increase with HLB, similar to what is generally expected for normal emulsions. As the size of a multiple drop increases with fw, inclusion, due to drop deformation and drop coalescence, results in a concomitant formation of larger internal oil droplets. A linear relationship between dowo and fw can be inferred from Figure 4. The slope seems to decrease with decreasing HLB, indicating that the size of internal oil droplets become less influenced by fw as the optimum formulation is approached. 3.1.3. Drop Structure and Stability. The internal phase ratios of dispersed drops with fixed sizes of 15 ( 5 and 25 ( 5 micrometers and at fixed fw of 0.17 are given in Table 1. The internal phase ratio of drops increased with decreasing HLB. The measurements of the internal phase ratio of drops for the lower HLB values were hampered by the small sizes of entrained droplets. For the HLB value of 11.0, for example, the interior of water drops was fully packed with droplets of very small size. The decrease in the interfacial

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Table 1. Internal Phase Ratio of Multiple Water Drops at a Fixed Water Volume Fraction of 0.17 for HLB Values of 14.2 and 13.0 internal phase ratio (φ) drop diameter (µm) 15 ( 5 25 ( 5

HLB )14.2

HLB ) 13.0

NPE12

NPE5/NPE12

NPE2/NPE12

0.16 ( 0.04 0.21 ( 0.04

0.19 ( 0.06 0.25 ( 0.07

0.17 ( 0.04 0.23 ( 0.06

Figure 6. Variation in dowo and dwo with HLB at a constant fw value of 0.12 (according to route 2 of Figure 2). T and C indicate the locus of transitional inversion and the locus of catastrophic inversion from abnormal O/Wm/O emulsion to normal W/Om emulsion, respectively.

Figure 5. Variation in dowo/dwo with fw for different HLB values.

tension, in association with the decrease in HLB, enhances drop deformation and results in more inclusion. It follows that a larger internal phase ratio was always obtained with decreasing size of internal oil droplets. This conclusion is consistent with the results previously published on the effect of surfactant concentration on inclusion.11 The comparative stability of the drops can be better judged by the ratio of the Sauter mean diameter of the internal oil droplets over that of external water drops (dowo/dwo). Figure 5 shows the variation in dowo/dwo with fw for different HLB values. In the absence of the oilsoluble surfactant (i.e., at HLB ) 14.2), dowo/dwo decreased with fw. This implies that the internal oil droplets are more stable against the variation in fw than the external water drops. At lower HLB values, made by a combination of the water-soluble surfactant NPE12 and the oil-soluble surfactant NPE5, dowo/dwo increased with fw, indicating that the internal droplets are more sensitive (i.e., less stable) to variations in fw than the external drops. This might be surprising because, for a Winsor-I O/Wm emulsion, the oil drops are expected to be more stable than the water drops in the corresponding abnormal Wm/O emulsion. If stirring ceases for a moment, multiple drops coalesce quickly and phase separation occurs, whereas the internal oil droplets released into the water continuous phase remain relatively stable. It can be stated that, however, the presence of NPE5 in the oil phase reduces the instability of the water drops in the abnormal Wm/O emulsions so that their variations with fw becomes less intensive. The decrease in HLB toward the optimum HLB is associated with a decrease in interfacial tension. The region of lowest interfacial tension is in fact at the transitional inversion point and is a point of low stability.16 The stability of the water drops might not be attained at lower HLB values if the vicinity of the transitional inversion point is approached.17 It has been shown that for normal emulsions the minimum drop size occurs at some distance from the optimum HLB. For the current study in which the optimum HLB is 10.50, the HLB values considered are sufficiently above

the optimum one. The HLB scan of a typical formulation is discussed later. 3.1.4. Water Volume Fraction at the Inversion Points. The variation of the volume fraction of the water phase at the inversion point, fw,inv, with HLB values is shown in Figure 2. The phase inversion is slightly delayed, in terms of fw, with increasing surfactant HLB. The variation in fw,inv with HLB is the result of interactions of two factors: drop stability and internal phase ratio of the dispersed phase. Both factors affect the rate of drop coalescence, but in different ways. As the HLB value is reduced, both the stability of water drops against coalescence and the effective volume fraction of the dispersed phase increase. The former delays phase inversion but the latter advances it. The balance will determine the variation in the phase inversion points with HLB. From Figure 4 it can be concluded that the effective volume fraction plays a slightly more important role in the overall rate of drop coalescence, as the phase inversion points advanced with decreasing HLB. This is consistent with the results of Silva et al.,18 which show that the hysteresis band narrows as the formulation approached optimum (HLB ) 10.5). It should be also explained that the difference in fw,inv obtained in this study from those previously reported for the similar system14 might originate from the difference in the rate of dispersed phase addition. In the current study, the rate of addition was kept at a low level to ensure that emulsions can reach near steady-state conditions. 3.1.5. HLB Scan of Abnormal Emulsion. One might think that when the HLB of an abnormal emulsion is continuously changed in favor of the continuous phase, the abnormal emulsion is gradually changed to a normal emulsion without going through a phase inversion. So far, we have described experiments carried out at constant HLB with fw as the variable. In one experiment, we scanned the formulation at a constant fw but with varying HLB, according to route 2 of Figure 2. Figure 6 shows the variations in the Sauter mean diameter of the internal oil droplets and multiple water drops of the O/Wm/O emulsion with HLB. An emulsion with fw ) 0.12 and HLB ) 14.2 (NPE12) was made. The HLB of the surfactant, then, was reduced by the addition of water and oil with the same ratio and containing 5.0 wt % NPE5 surfactant (HLB ) 10). The volume of the emulsion was kept constant by removal of some of the emulsion from the vessel. The comparison

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of the size data with those obtained with fw ) 0.12 but at constant HLB (route 1) values showed good agreement. The results indicate that as the transitional inversion line is approached, dwo and dowo are reduced. Interestingly, the size of multiple water drops did not vary much as the optimum HLB was approached. However, the diameter of internal oil droplets decreased to below 1 µm, the size which is usually observed at optimum conditions. At one point, the abnormal Wm/O emulsion underwent a catastrophic phase inversion to the corresponding O/Wm normal emulsion. As expected with normal emulsions, a further decrease in the HLB of the resulting O/Wm emulsion gave a transitional phase inversion to form a W/Om emulsion. This indicates that the stability of abnormal emulsions in the vicinity of a transitional inversion line is too low for them to continue existence in a nonpreferred morphology. The catastrophic inversion from an abnormal emulsion to the corresponding normal emulsion causes a reorientation of surfactant molecules at the interface according to their solubility and a subsequent transitional phase inversion will occur by a further decrease in HLB. 3.1.6. Oil Droplets in the Inverted O/W Emulsions. In the companion publication11 we referred to a mechanism named internal mechanism, which contributes to the formation of oil drops in the inverted emulsions. When the abnormal O/Wm/O dispersions are inverted to the O/Wm dispersions, the internal oil droplets are released to the water continuous phase and can constitute a part of the dispersed drops. The last points on all lines from Figure 4 were obtained from the inverted O/Wm emulsions (this can be taken as the inversion point). It can be seen from these figures that these points fall on the corresponding lines for the preinversion region.11 Such a continuity occurs only with a high surfactant loading (5.0 wt %) where the internal oil droplets are sufficiently protected against coalescence in the internal structure of the preinversion emulsion and in the inverted emulsion. The continuity in the variation of the size of oil droplets with fw for all surfactant HLBs across the regions of preinversion and postinversion can be better addressed if the flow regimes are also considered. Generally, an increase in the internal phase ratio to a value greater than 0.40-0.50 is associated with a drastic increase in the emulsion viscosity. Pal19 studied the viscous flow properties of PIB-thickened W/O emulsions. He showed that with an increase in the polymer content in the oil phase (i.e., the viscosity of the oil phase), the variations in relative viscosity, the ratio of the viscosity of the emulsion to the continuous phase viscosity, with fd is reduced. This implies that as the viscosity of the continuous oil phase increases, it has a greater effect on the viscosity of the emulsion. The flow behavior of a system can be best judged by the Reynolds number. When the viscosity of the continuous phases is used as the viscosity of the corresponding emulsions, Re numbers of around 3 × 102 and 104 are estimated for preinversion and postinversion regions, respectively.13 This indicates that the flow behavior changes from transitional in the preinversion region, in which drop size is influenced by both viscous and inertia forces, to near-turbulent in the postinversion region in which the size of drops is mainly determined by inertia forces. A large increase in the intensity of mixing of the dispersion in the stirred vessel was always observed when inversion occurred. From the discussion above, it

might be expected that when the internal oil droplets are released into the postinversion structure under turbulent conditions, their size is substantially reduced. But the size of the oil drops in the inverted O/W emulsion, even after a long agitation time, was never reduced to a size smaller than that in the preinversion region. For normal emulsions at a high internal phase ratio, drop interaction allows the transfer of shear through direct deformations, resulting in the formation of small and monodispersed drops.17,20 For abnormal emulsions, the variation in the viscosity of the emulsion with the dispersed phase ratio is not as significant as it is with normal emulsions because the size of most external drops is quite large. Furthermore, the size of multiple drops and internal droplets are both continuously increasing with fd. Therefore, for typical low-viscosity abnormal emulsions, the flow behavior in preinversion and postinversion regions could be similar. For the current study of the abnormal emulsions with PIB as the continuous phase, however, the viscosity of the emulsion is rather high, even for a low value of fd. There is some evidence that, with increasing viscosity of the continuous phase, the size of dispersed drops does not significantly vary, or may even be reduced.21,22 It is speculated that, with a high continuous phase viscosity, the shear stress exerted on the dispersed water drops is sufficiently high to allow for an intensive drop deformation with subsequent inclusions. The internal oil droplets formed in the preinversion region are so small that they could not be further reduced under the high-intensity turbulence of the postinversion region and they maintain their identity in the postinversion structures when they are released to the continuous water phase. 3.2. Effect of Chain Length Distribution of Surfactant. The effect of the distribution of the hydrophilic chain length of the surfactant system on the behavior of the abnormal emulsions was briefly studied. In this series of experiments, two different sets of NPE surfactant combinations, NPE2/NPE12 and NPE5/NPE12, were used to obtain mixtures which had the same HLB (of 13.0). NPE2 (chain length of ethylene oxide is 2) is more hydrophobic than NPE5 (with a chain length of 5) and is easily dissolved in PIB. Generally, the broadly distributed surfactants are considered to be more hydrophilic than the surfactants with sharp distribution because the shorter chain homologue preferentially dissolves into the oil phase and the average polyoxyethylene chain length of the adsorbed surfactants at the interface become more hydrophilic.23 However, in the context of the abnormal emulsion under study, NPE2/ NPE12 surfactants revealed mixed behavior in terms of hydrophilicity in comparison with NPE5/NPE12 for a fixed HLB value. The presence of NPE2 at the interface, even in low amounts, improved the stability of the water drops dispersed in the oil phase. Figure 7 shows that smaller multiple water drops were obtained with NPE2/NPE12 at the lower range of fw. This can be regarded as a sign of relative hydrophobicity of the surfactant set (Figure 3 indicates that the size of multiple drops decreases with HLB). The presence of NPE2 in the oil phase enhanced the stability of multiple drops so that eventually the phase inversion was delayed up to a higher value of fw compared with NPE5/NPE12. Table 1 presents some evidence, though not conclusive, that multiple drops with NPE2/NPE12 have a smaller internal phase ratio

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4. Conclusions

Figure 7. Variation in the Sauter mean diameter of multiple water drops with fw for the two sets of NPE5/NPE12 and NPE2/ NPE12 at the constant HLB value of 13.0.

Abnormal emulsions usually consist of an internal normal emulsion within an external abnormal emulsion. Any variation in the surfactant HLB greatly affects the size and structure of the dispersed droplets in the normal emulsion. The size of internal droplets significantly decreased as the optimum HLB was approached. The external water drops showed little sensitivity to HLB variations at a lower fw, but gradually showed a large increase in size with HLB at a higher fw. The decrease in HLB improved the stability of the dispersed water drops and enhanced inclusion. The former delayed the phase inversion, but the latter advanced it. The net result is that the phase inversion occurred earlier, in terms of volume fraction of the water, as HLB decreased. When a more hydrophobic surfactant was used at a constant HLB, the stability of the dispersed water drops was improved and inclusion was reduced, leading to a delay in phase inversion in terms of fw. In the vicinity of the optimum HLB, the abnormal emulsions became so unstable that they could not exist in a nonpreferred morphology and inverted to corresponding normal emulsions. Literature Cited

Figure 8. Variation in the Sauter mean diameter of the internal oil droplets with fw for the two sets of NPE5/NPE12 and NPE2/ NPE12 at the constant HLB value of 13.0.

than the multiple drops with NPE5/NPE12. It can also be inferred from Table 1 that a low internal phase ratio could be indicative of more hydrophilicity of the NPE2/ NPE12 surfactant set. The lower effective volume fraction obtained with NPE2/NPE12 allows the water drops to achieve a larger size before phase inversion can occur, a trend which can also be observed in Figure 3 with increasing surfactant HLB. Figure 8 shows the variation in the size of internal oil droplets with fw for the two NPE sets. A larger internal oil droplet size was obtained for NPE2/NPE12 than for NPE5/NPE12, at a constant fw. This observation, which suggests a relatively high hydrophilicity for NPE2/NPE12, is consistent with the trend observed in Figure 4. This behavior is due to the wide solubility difference of NPE2 and NPE12, and also to the distance of their average HLB from the corresponding optimum formulation. Note that the optimum formulation for the pair of NPE5/NPE12 occurs at the HLB value of 10.50, whereas for NPE2/NPE12 it is expected to occur at a lower HLB value (not measured), due to a high solubility of NPE2 in the oil phase. Furthermore, because of the low solubility of NPE2 in the water phase, its contribution to the interfacial tension lowering, compared to NPE5, is limited. This can lead to a larger size of internal oil droplets. Figure 8 also shows that the internal oil droplets formed with NPE5/NPE12 are more stable, in terms of fw, than those made with NPE5/ NPE12, as revealed by the comparison of the slopes of dowo-fw curves.

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Received for review December 19, 2002 Revised manuscript received May 7, 2003 Accepted May 15, 2003 IE021044E