Nanoemulsion Formation by Phase Inversion Emulsification: On the

May 19, 2006 - Division of Engineering, ECLAT, King's College London, London, WC2R 2LS. Langmuir ..... Jian Zhang. Journal of Dispersion Science and T...
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Langmuir 2006, 22, 5597-5603

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Nanoemulsion Formation by Phase Inversion Emulsification: On the Nature of Inversion Shahriar Sajjadi* DiVision of Engineering, ECLAT, King’s College London, London, WC2R 2LS ReceiVed January 5, 2006. In Final Form: April 3, 2006 Emulsification processes are usually characterized by the way they allow the surfactants, as well as the dispersed phase, to be incorporated into emulsions. A model cyclohexane-in-water emulsion using a pair of polyoxyethylene nonylphenyl ether surfactants, one oil-soluble and one water-soluble, was considered. Two surfactant mixing approaches consisting of mixed surfactants (agent-in-oil and agent-in-water) and segregated surfactants (agent in corresponding oil and water phases) were used to produce the model emulsion. Formation of oil-in-water nanodroplets could be only achieved if emulsification was associated with the formation of a three-phase microemulsion structure (transitional phase inversion) across the path. This occurred only if segregated surfactants were used in a process in which water was added to oil. With decreasing surfactant concentration, a point was reached below which the inversion mechanism transformed from transitional to catastrophic, leading to the formation of large droplets. The transformation was also accompanied by a shift in the evolution of the drop size. Drop size variations showed a minimum at the inversion point for the transitional phase inversion, whereas they showed a maximum for the catastrophic phase inversion. The agent-in-oil technique followed a catastrophic phase inversion mechanism and ranked second in terms of drop size.

1. Introduction Emulsions are the dispersion of a phase, called the dispersed phase, in a second phase, the continuous phase, by the help of a mechanical agitation. Dispersions are usually unstable, so phase separation may occur as soon as stirring ceases. To make a kinetically stable emulsion, a third component, surfactant, or emulsifier is required. There are two types of emulsions: waterin-oil (W/O) and oil-in-water (O/W). Both types have found wide applications in industry. There are different types of surfactants available for making emulsions. For any emulsion system, the choice of the right emulsifier is of very crucial importance. Several types of surfactants can be used. An important advantage of nonionic surfactants is the ability to make systematic and controlled changes in the polarity of the surfactant. This can be done only to a limited extent with ionic surfactants. Generally, a single surfactant cannot produce the desired stability, and the application of a mixture of surfactants can enhance the emulsion stability.1 The type of emulsion is usually determined by the nature of the surfactant used. Oil-soluble surfactants produce W/O emulsions, whereas water-soluble surfactants form O/W emulsions.2 The limitation of this rule has been discussed in the literature.3,4 The location of a surfactant can play an important role in the dynamic of emulsification.5,6 In a method called agent-in-oil (AO), the surfactant is dispersed in the oil phase. Then water is directly added to the mixture. Similarly, in the agent-in-water (AW) method, the surfactant mixture is dispersed in the water phase. The pure oil is then added to the mixture.1 Emulsifications are sometimes carried out by a direct emulsification method. In this technique, the dispersed phase is simply added to the continuous phase under intensive agitation. * E-mail: [email protected]. (1) Becher, P. Emulsions: Theory and Practice; American Chemical Society: Washington, DC, 2001. (2) Bancroft, W. D. J. Phys. Chem. 1913, 17, 501 (3) Smith, D. H.; Lim, K. H. J. Phys. Chem. 1990, 94, 3746. (4) Binks, B. P. Langmuir 1993, 9, 25. (5) Lin, T. J.; Kurihara H.; Ohta, H. J. Soc. Cosmet. Chem. 1975, 26, 121. (6) Sajjadi, S. Chem. Eng. Sci. 2006, 61, 3009.

This is called direct emulsification because the type of initial emulsion is that of the intended emulsion. In some literature, however, direct emulsification is also attributed to a simultaneous mixing of all of the emulsion components in the mixing chamber. In a phase inVersion method, the phase that is intended to be the continuous phase is added to the intended dispersed phase until phase inversion occurs and the desired emulsion type is formed. Despite the fact that, under normal conditions, the ultimate morphology of emulsions is dictated by the surfactant nature, the emulsification path can significantly affect the properties of the resulting emulsions. Note that the nature of the surfactant can cause some complexities in the labeling of emulsification methods. When the surfactant mixture is soluble in the water phase, for instance, the AO method may be categorized under phase inversion emulsification because the W/O emulsion initially formed will eventually invert to an O/W emulsion morphology dictated by the type of surfactant used. But when the surfactant is more oil soluble, the AO method is simply a direct emulsification method. There are at least two types of inversions: catastrophic and transitional.7 There has been a great deal of interest in catastrophic phase inversion (CPI) in recent years.8-13 The term catastrophic comes from the fact that emulsion inversion is a mimic of events described by catastrophe theory.14,15 In the literature, CPI has been attributed to an increase in the volume fraction of the dispersed phase. In a more general term, CPI can be defined as an inversion due to a large increase in the rate of drop coalescence (7) Salager, J. L.; Minanaperez, M.; Perezsanchez, M.; Ramirezgouveia, M.; Rojas, C. I. J. Dispersion Sci. Technol. 1983, 4, 313. (8) Smith, D. H.; Lim, K. H. Langmuir 1990, 6, 1071. (9) Brooks, B. W.; Richmond, H. N. Colloids Surf. 1991, 58, 131. (10) Sajjadi, S.; Jahanzad, F.; Brooks, B. W. Ind. Eng. Chem. Res. 2002, 41, 6033. (11) Sajjadi, S.; Jahanzad, F.; Yianneskis, M.; Brooks, B. W. Ind. Eng. Chem. Res. 2003, 42, 3571. (12) Zambrano, N.; Tyrode, E.; Mira, I.; Marquez, L.; Rodriguez, M. P.; Pizzino, A.; Salager, J. L. Ind. Eng. Chem. Res. 2003, 42, 50. (13) Mira, I.; Zambrano, N.; Tyrode, E.; Marquez, L.; Pena, A. A.; Pizzino, A.; Salager, J. L. Ind. Eng. Chem. Res. 2003, 42, 57. (14) Dickinson, E. J. Colloid Interface Sci. 1981, 84, 284. (15) Sallager, J. L. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; M. Dekker: New York, 1988; Vol. 3.

10.1021/la060043e CCC: $33.50 © 2006 American Chemical Society Published on Web 05/19/2006

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Figure 1. Schematic presentation of a simplified dynamic map.

so that the balance between the rate of drop coalescence and drop break up can no longer be maintained. CPI may be induced by the addition of a dispersed phase16 or any other factors that can increase the rate of drop coalescence. Transitional phase inversion (TPI) occurs when the affinity of the surfactant for the water phase equilibrates its affinity for the oil phase. The variation in the affinity or hydrophobiclypophilic balance (HLB) of the surfactant can be conducted by an alteration in temperature,17-19 the addition of a surfactant with a different HLB,8,9,16 as well as the addition of a phase or other additives as described in a recent review.20 The phase behavior for several ternary or quaternary systems under equilibrium conditions has been described in the literature.4,19,21-26 The formation of ultrafine droplets via TPI was first reported and documented by Shinoda and co-workers.18,19 They used temperature to alter the HLB of a surfactant. TPI emulsification has been widely used for preparation of ultrafine emulsions.27-33 The two types of phase inversions can be shown on dynamic phase maps. Figure 1 shows a schematic presentation of a simplified dynamic map prepared with a combination of a low HLB surfactant and a high HLB surfactant. The abscissa in this figure indicates the water volume fraction (fw). The transitional inVersion line, or the locus of optimum HLB value (HLBop), is shown by a solid line. This line is usually slanted because of the selective partitioning of nonionic surfactants in the oil phase. For the same reason, transitional inversion can also occur by (16) Sajjadi, S.; Jahanzad, F.; Yianneskis, M. Colloids Surf., A 2004, 240, 149. (17) Friberg, S.; Solans, C. J. Colloid Interface Sci. 1978, 66, 367. (18) Shinoda, K.; Hanrin, M.; Kunieda, H.; Saito, H. Colloids Surf. 1981, 2, 301. (19) Shinoda, K.; Friberg, S. Emulsion and Solubilization; John Wiley: New York, 1986. (20) Salager, J. L.; Forgiarini, A.; Marquez, L.; Pena, A.; Pizzino, A.; Rodriguez, M. P.; Rondon-Gonzales, M. AdV. Colloid Interface Sci. 2004, 108, 259. (21) Shah, D. O., Schechter, R. S., Eds. ImproVed Oil RecoVery by Surfactant and Polymer Flooding; Academic Press: New York, 1977. (22) Forster, W. V. R.; Von Rybinski, W.; Walde, A. AdV. Colloid Interface Sci. 1995, 58, 119. (23) Lim, K. H.; Lee, J. M.; Smith, D. H. Langmuir 2002, 18, 6003-6009. (24) Strey, R. Colloid Polym. Sci. 1994, 272, 1005. (25) Lopez-Montilla, J. C.; Herrera-Morales, P. E.; Pandey, S.; Shah, D. O. J. Dispersion Sci. Technol. 2002, 23, 219. (26) Izquierdo, P.; Esquena, J.; Tadros, T. F.; Dederen, J. C.; Feng, J.; GarciaCelma, M. J.; Azemar, N.; Solans, C. Langmuir 2004, 20, 6594. (27) Brooks, B. W.; Richmond, H. N. Chem. Eng. Sci. 1994, 49, 1053. (28) Zerfa, M.; Sajjadi, S.; Brooks, B. W. Colloids Surf., A 1999, 155, 323. (29) Forster, T.; Schambil, F.; Vonrybinski, W. J. Dispersion Sci. Technol. 1992, 13, 183. (30) Forgiarini, A.; Esquena, J.; Gonzalez, C.; Solans, C. Langmuir 2001, 17, 2076. (31) Morales, D.; Gutierrez, J. M.; Garcia-Celam, M. J.; Solans, Y. C. Langmuir 2003, 19, 7196. (32) Minana-Perez, M.; Gutron, C.; Zundel, C.; Anderez, J. M.; Salager, J. L. J. Dispersion Sci. Technol. 1999, 20, 893. (33) Fernandez, P.; Andre, V.; Rieger, J.; Kuhnle, A. Colloids Surf., A 2004, 251, 53.

Sajjadi

changing the oil-water ratio at a constant, but within a limited range of HLB.16 The transitional line divides the domains of normal O/Wm and W/Om emulsions; O/Wm emulsions below the transitional line (HLB > HLBop) and W/Om emulsions above the transitional line. The subscript “m” refers to the micelle-containing phase; therefore, O/Wm and W/Om emulsions are normal, whereas Wm/O and Om/W are abnormal emulsions. Normal emulsions are those that obey the Bancroft’s rule, while abnormal emulsions are those that deviate from the rule. An emulsion with the continuous phase containing a surfactant that is more soluble in the dispersed phase is an example of abnormal emulsion. Abnormal emulsions are unstable and can only be maintained under vigorous mixing for a rather short period of time. Their ultimate fate is usually an inversion to the opposite normal emulsion with time.20,34,35 According to Figure 1, abnormal Wm/O emulsions are formed when water is added to oil at a HLB value greater than the optimum one. The dashed lines in Figure 1 indicate the boundaries of the catastrophic inversion of abnormal to normal emulsions. We previously reported that abnormal emulsion cannot exist in the vicinity of the locus of transitional inversion because of extreme instability.16 The arrows show the types of starting emulsions and the transformation to the opposite emulsion after the phase inversion. According to the map, normal O/Wm emulsion is the sole possible morphology for high values of HLB and fw. Similarly, W/Om emulsion is the sole possible morphology for low values of HLB and fw (high oil phase ratio). In the other parts of the map, both types of morphologies (one normal and one abnormal) are possible for any single formulation, depending on the conditions. While emulsions are usually formulated to be far from the optimum formulation (transitional inversion point), they might pass through such a point during emulsification, at least locally, if appropriate conditions exist. Optimum formulation may be obtained by changing any formulation variable that alters the balance of affinity of the surfactant for the oil and water phase. Variables such as temperature, salinity, and alcohol content serve to provide such an effect.20 Formulations that contain a mixture of high and low HLB nonionic surfactants also have the potential to achieve optimum formulation and induce an ultralow interfacial tension, which is a prerequisite for TPI to occur. The wide range of process variables available for a fixed emulsion formulation (given that the type of oil, water, and surfactant mixture is known), including location of the surfactant mixture and modes of addition of the second phase, makes the screening of the emulsification processes very difficult. In this study, we show how these process variables can be incorporated to dynamic phase behavior maps of surfactant-water-oil emulsions and how the final properties of an emulsion can be predicted by tracking them along the routes on the maps. The boundary between TPIs and CPIs is sometimes quite vague.36 Before proceeding to the next section, it is important to note the characteristics of the two types of inversions.

2. Characteristics of Phase Inversions Marszall37 described a technique (AO in terms of the location of the surfactant) in which the surfactant mixture was initially placed in the oil phase, and then water was added until phase inversion occurred. He used a wide range of surfactant HLBs. Marszall reported that the minimum amount of water required (34) Zerfa, M.; Sajjadi, S.; Brooks, B. W. Colloids Surf., A 2001, 178, 41. (35) Sajjadi, S.; Zerfa, M.; Brooks, B. W. Chem. Eng. Sci. 2002, 57, 663. (36) Sajjadi, S.; Zerfa, M.; Brooks, B. W. Colloids Surf., A 2003, 218, 223. (37) Marszall, L. Cosmet. Perfum. 1975, 90, 37.

Nanoemulsion Formation by Phase InVersion

to cause phase inversion occurs at a specified HLB. He labeled his method as phase inversion emulsification, but did not explicitly specify the type of inversion involved. The AO method has been reported to be the best way to prepare O/W emulsions, if the surfactant is more water soluble. This requires that phase inversion be involved, but the type of inversion remains unspecified. Forgiarini et al.30 confirmed that the finest O/W emulsion was obtained when water was added to the oil dissolving the surfactant via TPI. One should notice that the type of formulation variable cannot determine the type of inversion. Alteration in dispersed phase ratio, for example, can lead to either type of inversion.16 However, only the phenomenological events occurring during an inversion will determine its types. The two types of inversions can be characterized by the following features: 1. As the label implies, the CPI is an instant or catastrophic event by which the dispersed phase displaces the continuous phase, whereas the transitional one is, in fact, a gradual process by which a shift in the continuous phase occurs by diffusion due to a continuous change in the affinity of the surfactant toward the dispersed phase.15,38 At the optimum formulation, where the affinity of the surfactant toward both phases is balanced, a microemulsion phase or lamellar liquid crystal with possible excess water and oil can form. The phase separation is associated with the thermodynamic instability of emulsions at an optimum point.36,39-43 The variation in the curvature of the interface is also associated with changes in interfacial tension, which decreases to an ultralow value.18,44-46 Nothing such as minimum interfacial tension occurs along a CPI route, although emulsions become kinetically less stable as the region of CPI is approached. 2. TPI is a process between two kinetically stable normal emulsions, whereas the catastrophic inversion is between one normal and one abnormal emulsion. As a result, the transitional inversion is a reversible process (the initial emulsion can be restored if the change in the variable is reversed), but the catastrophic one is not and exhibits a strong hysteresis.20,36 3. As a result of the continuity of process, most of the emulsion properties change slowly, but continuously, during a transitional inversion process, whereas the CPI, as the name implies, is usually associated with a sudden jump/fall in the emulsion properties such as conductivity. There is a different pattern of size variations, for example, for the inversions. For the TPI, the droplet size deceases as the inversion is approached and eventually reaches a minimum at the inversion point as a result of ultralow interfacial tension.27 For the CPI of an abnormal to normal emulsion, the drop size prior to inversion increases with fw because of the increased coalescence of the emulsion drops.10 Note that the size measurement of a drop in abnormal emulsions is a difficult task because of their significant instability. Figure 2 compares the pattern of variations in the Sauter mean diameter (d32) with fw across two paths for the two types of inversions in a typical polyisobutylene (PIB)/water/polyoxyethylene nonylphenyl ether (NPE) emulsion. The application of low molecular-weight PIB (38) Miller, C. A. Colloids Surf. 1988, 29, 89. (39) Bourrel, M.; Graciaa, A.; Shechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1979, 72, 161. (40) Salager, J. L.; Quintero, L.; Ramos, E.; Anderez, J. M. J. Colloid Interface Sci. 1980, 77, 288. (41) Kalbanov, A.; Weers, J. Langmuir 1996, 12, 1931. (42) Binks, B. P.; Cho, W. G.; Fletcher, P. D. I.; Petsey, D. N. Langmuir 2000, 16, 1025. (43) Ruckenstein, E. AdV. Colloid Interface Sci. 1999, 79, 59. (44) Salager, J. L.; Morgan, J.; Schechter, R. S.; Wade, W. H.; Vasquez, E. Soc. Pet. Eng. J. 1979, 19, 107. (45) Sottmann, T.; Strey, R. J. Chem. Phys. 1997, 106, 8606. (46) Miller, C. A.; Hwan, R. N.; Benton, W.; Fort, T., Jr. J. Colloid Interface Sci. 1997, 61, 554.

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Figure 2. The pattern of variations in the Sauter mean diameter (d32) of drops in typical catastrophic and TPIs of PIB/water/NPE5NPE12 system. The CPI data was obtained when water was added to oil while each phase contained NPE12 with HLB ) 14.2. The TPI data was obtained when water-dissolving NPE12 was added to oil-dissolving NPE5 with HLB ) 10. The concentration of surfactant in both phases was 5.0 wt % (data from references 10 and 34).

enhanced the stability of the drops so that size measurements of the unstable drops could be carried out. It would be quite interesting to verify whether the same principle applies to the two phase inversions across the same path. We later show how this finding can be used to identify the mechanism of inversion. 3. Experimental Section Chemical. Cyclohexane was used as the oil phase. Two grades of Igepal (NPE), supplied by Aldrich, were used as surfactants. They are Igepal co520, and Igepal co720, with nonylphenol ethoxylate chain lengths of 5 (NPE5) and 12 (NPE12), respectively. NPE5 has an HLB of 10 and is an oil-soluble grade. NPE12 has an HLB of 14.2 and is a water-soluble grade. Apparatus. Emulsifications were carried out in a 1-L jacketed glass reactor with a diameter of 10 cm equipped with four baffles, with a width of 1.0 cm, equally spaced at 90° intervals, and a fourblade turbine with a diameter of 5.0 cm. The stirring speed was controlled at 500 rpm during emulsifications. The emulsifications were carried out at room temperature (22 °C). Emulsion inversions were determined from measurements of emulsion conductivities and where a large change in conductivity occurred. Target Emulsions. The target emulsion was defined using a model cyclohexane-in-water (O/W) emulsion with a combination of NPE5 and NPE12. The HLB of the target emulsion was set at 12.3. Weight averages were used for making a surfactant mixture with the predetermined HLB. The individual amounts of the surfactants were assigned to achieve the target HLB. The target emulsion had fw ) 0.50. Volumes of around 200 mL of cyclohexane and 200 mL of deionized water were used. The water phase contained 0.50 wt % of potassium chloride. Emulsification Procedures. In all experiments, one phase was initially placed in the stirred vessel, and then the second phase was gradually added to the mixing vessel at a rate of 20 mL per 10 min. No equilibration was considered prior to mixing. Two major types of emulsifications were used: direct emulsification and phase inversion emulsification. The incorporation of the surfactants was carried out according to the following procedures: Segregated Surfactants. The surfactants were dissolved in the phase with the higher solubility: NPE5 in cyclohexane and NPE12 in water. This policy minimizes the surfactants’ diffusion through the interface during emulsification and allows equilibrium to be reached quickly. The addition of water to oil resulted in a phase inversion emulsification (P), whereas the addition of oil to water was a simple direct emulsification (D).

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Figure 3. Simplified phase map for cyclohexane/water/NPE system with different surfactant concentrations. Mixed Surfactants. The surfactant mixture was placed either in the oil (AO) or in the water phase (AW), followed by gradual addition of the second phase that had no surfactant. For the target O/W emulsion, the AO and AW methods represent phase inversion and direct emulsification, respectively. This policy involves diffusion of the surfactants to the phase in which they are more soluble during the emulsification, thus it can be considered a nonequilibrated emulsification. Application of other mixing approaches has been reported elsewhere.6 Drop Size Measurement. The Sauter mean diameter and the distribution of oil drops were obtained by a laser diffraction method. The average sizes of the oil and water drops were measured and checked by processing images obtained from emulsions by a video camera connected to an optical microscope and computer, when possible. The size of the external water drops across the CPI route (Wm/O f O/Wm) could not be measured by an off-line technique because of the instability of abnormal Wm/O emulsions. The average drop size of the water drops was thus estimated using an in situ online optical probe.47 Interfacial Tension Measurement. Measurements were carried out with a Du Nouy tensiometer. The interfacial tension between the water and cyclohexane phases was measured as a function of time. The surfactant mixture was placed in oil and water for the AO and AW routes, respectively. No mixing was used during the measurements. Several readings were made, and an average value was used. Emulsion Stability. Stability was measured by the traditional method of phase separation. Final emulsions were poured into measuring cylinders, and the percentage of phase separation (vol %) was measured as a function of time.

4. Results 4.1. Comparison of Different Routes. Before emulsions from different routes are compared, it is helpful to look along the routes on the dynamic phase map. Figure 3 shows the map for the locus of TPI for the cyclohexane/water/NPE5-NPE12 emulsion with different surfactant concentrations ([S]). The TPI lines locate within HLB values of 10.0 and 11.2, depending on the water volume fraction (fw) and the surfactant concentration. Note that the lines apply to emulsions with a constant surfactant concentration. The collection of data for the locus of CPI is a tedious task, as it depends on the emulsification path, location of surfactant, rate of mixing, and rate of addition, as well as the surfactant concentration and HLB. This was not shown on the map. However, it is understood that any inversion outside the locus of TPI is likely to be a catastrophic one. The formulation of the target emulsion is shown by a full circle in Figure 3. It (47) Alban, F. B.; Sajjadi, S.; Yianneskis, M. Chem. Eng. Res. Des. 2004, 82, 1054.

Figure 4. Variations in the Sauter mean diameter (d32) with (a) HLB and (b) fw for the emulsions obtained using different emulsification paths at [S] ) 3.0 wt % (the dashed line shows fw or the HLB of the target emulsion).

is evident that the target formulation corresponds to a Winsor-1 O/Wm emulsion. Panels a and b of Figure 4 show the evolution of the average drop sizes with HLB and fw, respectively, for the target emulsion with a surfactant concentration of 3.0 wt %. Route P produced the finest emulsion. The order of routes in terms of drop size is P < AO < D < AW. To highlight the underlining mechanisms of drop formation, the emulsification paths across the dynamic phase map are tracked from the starting emulsion to the target emulsion. Routes D and AW are simple direct emulsifications in which the oil phase was added to the water continuous phase. The difference between these two routes is only in the location of the surfactant. In route D, addition was ceased before the locus of transition inversion was crossed. Since the emulsion formed was normal, its stability was quite high, and it could not undergo any inversion unless a high dispersed phase ratio, such as 90% or even higher, was achieved (the locus of normal to abnormal inversion is not shown on the phase map in Figure 1). The target emulsion had a composition of fw ) 0.50, so the addition of the oil phase was stopped at this point. Routes P and AO represent phase inversion emulsification. It is obvious from the map shown in Figure 3 that route P represents the transitional inversion for [S] ) 3.0 wt % since it crossed the transitional locus. For this route, emulsification started with the oil phase, and, as more water was added, the resulting W/Om emulsion inverted to an O/Wm emulsion at fw = 0.10 (for [S] ) 3.0 wt %). The interfacial tension approached a very low value at this point.6 The viscosity of the emulsion increased significantly after inversion because of the formation of a concentrated O/Wm emulsion (90/10) as a result of the inversion of the initial dilute W/Om emulsion (10/90). Route AO represents CPI emulsification since this route did not cross the transitional line. The initial W/O emulsion was an abnormal one. However, the internal phase (W) behaved as a normal emulsion because it corresponded to the nature of the surfactant mixture used. Across route AO, multiple oil-in-waterin-oil (O/W/O) drops formed by inclusion of oil from the continuous phase into the water drops.9,10,35 The size of multiple

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Figure 6. Stability of the final emulsions in terms of the volume percent of phase separation as a function of time for [S] ) 3.0 wt %. Figure 5. Drop size distribution of final emulsions for different paths at [S] ) 3.0 wt %.

drops continuously increased with further addition of water until CPI to normal O/W emulsion occurred. The CPI occurred at fw ) 0.23 and 0.18 for surfactant concentrations of 3.0 and 5.0 wt %, respectively. For the lower surfactant concentrations, phase inversion did not occur before the addition of water was completed. After CPI, the internal O/W/O droplets were released into the water phase and formed O/W droplets. The size of the oil droplets formed after inversion are shown in Figure 4. Further addition of water, as the continuous phase, to the resulting O/W emulsion did not significantly affect the drop size. The results presented so far clearly indicate that the TPI emulsification is a far better option for making fine emulsions in comparison to the other routes studied at the surfactant concentration of 3.0 wt %. Furthermore, the results imply that TPI occurs when a full segregation of surfactant occurs (each surfactant is dissolved in a phase with the highest solubility) and water is added to oil. This may seem to be in contrast to the literature that reports that the AO method gives the finest O/W emulsion. Such procedures, however, should be considered as special cases. The AO method only gives finer emulsions when the surfactant is not highly soluble in either oil or the water phase.5 Figure 5 shows the final drop size distributions for different routes. The size distribution of drops obtained from route P was quite sharp in comparison to the rest. The AO method produced the second narrowest distribution. Generally, the size distribution of the drops broadened as the average drop size increased. The stability of emulsions will be affected by those factors that affect drop coalescence/flocculation. Furthermore, stability also depends on the rate of creaming/sedimentation of the drops as well as their size. According to Figure 6, the stability of the resulting emulsions followed the same trend as that for the size of the drops. Emulsions with smaller drops were more stable. Because emulsions prepared by phase inversion emulsification (routes P and AO) produced the smallest drops with the highest stability, these two routes were selected for further study. 4.2. Change in the Nature of the Inversion. Figure 7 shows the evolution of the Sauter mean diameter of drops with fw and surfactant concentrations for route P. Nanodroplets could be only produced above a critical surfactant concentration. The critical surfactant concentration is, in fact, the minimum concentration of surfactant that allows for TPI to occur over the whole range of the oil phase ratio. This concentration in fact reflects the critical micellar concentration (CMC) of the mixed surfactants in the oil phase. The transitional line versus phase ratio does not change strongly with the concentration of surfactant

Figure 7. The evolution of the Sauter mean diameter (d32) of postinversion oil droplets with fw obtained by route P at different [S]. (The closed symbols for the insets show the variations in d32 of water droplets in the W/O emulsion with [S] ) 2.0 and 3.0 wt %. The coordinates of all images are identical.

in concentrated solutions, but does change in dilute solutions of surfactants. The saturation concentration of nonionic surfactants in water is generally very small, but those in oil are much larger. The lypophilic surfactant dissolves much better in the oil phase than the hydrophilic one does in the oil phase. Therefore, there will be a favored adsorption of the hydrophilic surfactant at the oil/water interface with increasing oil ratio. To maintain a constant HLB at the interface, at which TPI occurs, more lipophilic surfactant is required. This leads to a slanted TPI with the oil phase ratio.48,49 Figure 3 clearly shows that the slope of the transitional line increases with decreasing surfactant concentration. The size of the drops was significantly increased when a surfactant concentration lower than 3 wt % was used. Figure 8 shows that the distribution of drops becomes narrower with increasing surfactant concentration. A sudden change in the size and breadth of the distribution of drops within the surfactant concentration range of 2.0-3.0 wt % implies a change in the mechanism of inversion. At a high surfactant concentration, route P produced nanodroplets associated with a low interfacial tension and the extensive formation of the bicontinuous phase.6 It is clear from the right inset of Figure 7 that droplet size with the surfactant concentration of 3.0 wt % showed a minimum during inversion, which is a characteristic of TPI. The inversion was found to be reversible. (48) Arai, H.; Shinoda, K. J. Colloid Interface Sci. 1967, 25, 396. (49) Graciaa, A.; Lachaise, J.; Sayous, J. G.; Grenier, P.; Yiv, S.; Schechter R. S.; Wade, W. H. J. Colloid Interface Sci. 1983, 93, 474.

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Figure 8. Drop size distribution of final O/W emulsions obtained by route P at different [S].

According to Figure 3, route P does not cross the transitional inversion line for surfactant concentrations lower than 3.0 wt %. The size measurement of the water droplets in the W/O emulsion formed with 2.0 wt % surfactant indicates that drop size increased with further addition of water prior to inversion (this is shown in the left inset of Figure 7). This is a characteristic of CPI, as shown in Figure 2. This finding was supported by the observation that the inversion was not reversible and that little surfactant phase formed during inversion. This indicates that, for the surfactant concentration range of e2.0 wt %, route P represents a catastrophic type of phase inversion in which the cause of phase inversion is mainly due to an increased rate of drop coalescence with an increasing dispersed phase ratio. This set of experimental results shows, for the first time, to the best knowledge of the author, how the evolution of drops varies in conjunction with the transformation of inversions across a very narrow range of surfactant concentration under a set of experimental conditions (note that data depicted in Figure 2 were obtained for different emulsification paths). For route P with [S] ) 0.50 wt %, and for route AO with [S] ) 0.50 and 2.0 wt %, CPI did not occur before fw ) 0.50 was reached. For such cases, the addition of water was completed at fw ) 0.50, and the W/O emulsions were maintained under agitation until phase inversion to O/W emulsions occurred with time as a result of further inclusion of the continuous phase into the dispersed drops.50 Thus, only the data of the final emulsion, after inversion, are shown as a point on Figure 7 for route P using a surfactant concentration of 0.50 wt %. Another interesting characteristic feature of the two types of inversions can be inferred from Figure 7. The CPI ([S] e 2.0 wt %) delayed in terms of fw with decreasing surfactant concentration, whereas the transitional inversion ([S] g 3.0 wt %) delayed with increasing surfactant concentration. The former is due to weakened inclusion with decreasing surfactant concentration, but the latter is due to favored partitioning of the surfactants in oil with increasing surfactant concentration. The minimum in fw occurred at the critical surfactant concentration of 3.0 wt %. Figure 9 compares the average drop size of final emulsions obtained from the phase inversion emulsification routes: P and AO. Route P always produced smaller droplets in comparison with route AO. However, the difference decayed with decreasing surfactant concentration. For the AO route, phase inversion always occurred through CPI, whereas different mechanisms were responsible for phase inversion in route P, as previously stated. Below a surfactant concentration of 3.0 wt %, the size of the drops increased significantly as a result of a shift in the mechanism (50) Sajjadi, S.; Zerfa, M.; Brooks, B. W. Langmuir 2000, 16, 10015.

Sajjadi

Figure 9. Sauter mean diameter (d32) versus surfactant concentration for routes P and AO. Route AO is always confined to CPI.

of inversion from a transitional to a catastrophic one. The reason route P produced drops smaller than those of route AO in the region of [S] < 3.0 wt %, while both routes underwent CPI, could be that its emulsification path is closer to the locus of minimum interfacial tension (TPI), as it can be inferred from the insets of Figure 9. This results in the formation of smaller droplets in comparison with those of route AO. The dynamic evolution of interfacial tension also plays an important role here, as described below. 4.3. Evolution of Interfacial Tension and Mass Transfer Limitation of the Surfactant Mixture. When the interfacial tension is very low, spontaneous emulsification may occur, leading to the formation of nanodroplets. Although there is a debate on how nanodroplets are formed and stabilized,22,25,30,51-53 there is no doubt that the interfacial tension plays a key role. The evolution of equilibrium interfacial tension with fw for routes P and D were reported elsewhere.6 In these routes, each phase dissolved the corresponding surfactant. The minimum equilibrated interfacial tension appeared at fw = 0.10, indicating that the formation of a cyclohexane/water emulsion with fw ) 0.50 could only be achieved if water was added to oil (route P) as a result of TPI.6 While the minimum equilibrated interfacial tension encountered in route P could justify the formation of nanodroplets in this route,6 no such phenomenon occurs for the other routes. However, for the other cases, dynamic interfacial tension in association with diffusion-controlled surfactant transport may play an important role. The surfactant transport during the emulsification processes can significantly affect the emulsion type and properties.5,6,9-13,16,35,36,54 A diffusion-controlled limitation may arise if the surfactant, or the surfactant mixture, is placed in the phase with a low solubility. This may lead to an inefficacy of the surfactant molecules at the interface. To obtain some insight into the mechanisms of emulsification and to simulate the interaction of interfacial tension and surfactant transport, the dynamic interfacial tension measurements were monitored. Figure 10 shows the time variations in the interfacial tension for the cyclohexane-water-(NPE5-NPE12) systems with a typical fw ) 0.50 and surfactant concentrations of 3.0 wt % with two types of surfactant partitioning. While the interfacial tension in both routes approached the equilibrium value (1.2 mN/m) with time, the history of interfacial tension evolution was quite different. (Note that, during the emulsification and under vigorous (51) Friberg, S. E.; Mandell, L.; Larson, M. J. Colloid Interface Sci. 1969, 29, 155. (52) Nishimi, T.; Miller, C. A. Langmuir 2000, 16, 9233. (53) Shahidzadeh, N.; Bonn, D.; Meunier, J. Langmuir 2000, 16, 9703 (54) Sallager, J. L.; Moreno, N. M.; Anton, R.; Marfisi, S. Langmuir 2002, 18, 607.

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larger than the formulation HLB (12.3) and thus much larger than the optimum HLB (= 10.0-11.2) at which the interfacial tension is minimum. The interfacial tension descended from a rather high value to the equilibrium value (1.2 mN/m) with time, as shown in Figure 10. Similar behavior may occur when both surfactants are dissolved in the oil phase, but probably to a lesser extent because of higher solubility of the surfactants in the oil phase. The distinctive difference is that the relative limitation in the diffusion of the water-soluble surfactant to the interface in route AO will cause the actual HLB of the emulsion at the water-oil interface to be smaller than the formulation HLB (12.3). Thus, the actual HLB approached the optimum HLB. This resulted in a subsequent decrease in the interfacial tension and drop size. Figure 10. Variations in interfacial tensions as a function of time for cyclohexane-water systems with a 3.0 wt % NPE5/NPE12 concentration (at HLB ) 12.3): (AO) surfactants are placed in oil phase; (AW) surfactants are placed in the water phase.

agitation, the surfactant transport rate is much higher than what is observed for two stagnant oil and water phases, and the equilibrium interfacial tension will probably be achieved in a shorter period of time.) For route AW, the dynamic interfacial tension is descending, whereas, for route AO, it is ascending. Drops formed across route AO experienced a lower interfacial tension and thus broke into smaller droplets. This difference in the interfacial tension can be the reason, at least partly, for a difference in the size of the drops. The evolution of dynamic interfacial tension may be explained as follows. In route AW, the surfactant mixture was initially dissolved in the water phase. The solubility of the low HLB surfactant, NPE5, in water is limited. Because NPE5 was not fully dissolved in the water phase, it formed a cloudy dispersion. When NPE12 was added to the dispersion, however, it became clear. The sudden change from a turbid emulsion to a transparent solution is a conspicuous feature. The formation of mixed micelles of NPE5 and NPE12 in the water phase is unlikely to be the main reason. Therefore, it can be stated that the low HLB surfactant, NPE5, was solubilized by the high HLB surfactant micelles in the water phase. This implies that, for the AW route, the diffusion of the oil-soluble surfactant (NPE5) from the interior of the watersoluble surfactant (NPE12) micelles to the oil phase, via the water phase, could be a slow process. As a result, the surfactant trapped inside the water-soluble surfactant micelles did not adsorb on the oil-water interface and did not contribute to the actual HLB and interfacial tension lowering at the interface. This caused the actual HLB of the emulsion at the water-oil interface to be

5. Conclusions The location of surfactants can play an important role in the dynamics of emulsification as well as the final properties of emulsions. While the formulation of emulsions demands a specified type of morphology, it is sometimes possible to obtain that morphology via phase inversion by application of an appropriate emulsification path. As a general statement, the finest O/W emulsion containing a pair of water-soluble and oil-soluble surfactants was obtained when the emulsification path proceeded via the addition of water dissolving the water-soluble surfactant to oil dissolving the oilsoluble surfactant, and was associated with the formation of the microemulsion phase. With decreasing concentration of surfactant, a point was reached below which the transitional inversion could not be achieved. As a result, the mechanism of inversion transformed from transitional to catastrophic. This was associated with a shift in the mechanism of drop formation and led to the formation of coarse emulsions. With decreasing [S], the evolution of the drop size showed a change from one having a minimum to one having a maximum. The AO method progressed according to the CPI route, within the range of surfactant concentration and HLB studied, and formed rather large droplets, but ranked second. The formation of nanodroplets, as seen in route P, was explained by the low interfacial tension obtained in the vicinity of the locus of TPI. It was discussed that the limitation in the diffusion of the less soluble surfactant through the phases favored the formation of fine droplets in the AO route, but formed coarse droplets in the AW route. The former is associated with a decrease in the dynamic interfacial tension, whereas the latter is associated with an increase in the dynamic interfacial tension. LA060043E