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Effect of Didodecyldimethylammonium Bromide on the Phase Behavior of Nonionic Surfactant-Silicone Oil Microemulsions James A. Silas and Eric W. Kaler* Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Randal M. Hill Central R&D, Dow Corning Corporation, 2200 West Salzburg, Midland, Michigan 48686-0994 Received March 8, 2001. In Final Form: May 9, 2001 The effects of adding didodecyldimethylammonium bromide (DDAB) to mixtures of n-alkyl polyglycol ether (CiEj), water, and silicone oil are systematically studied. In mixtures of C8E3 and C12E6 with water, small amounts of DDAB cause the upper miscibility gap to vanish and be replaced by a high-temperature lamellar phase. In mixtures of C12E5 with octamethylcyclotetrasiloxane (D4), the addition of DDAB makes the surfactant mixture more hydrophilic and expands the lower miscibility gap by increasing the (pseudo)critical temperature. In ternary mixtures of CiEj, D4, and water, adding DDAB increases the surfactant efficiency by up to a factor of three, expanding the single-phase microemulsion region to higher temperatures and lower surfactant concentrations. The temperature limits of the single-phase microemulsion correlate with changes in the pseudo-binary phase diagrams upon addition of DDAB. Additionally, the surfactant mixture stabilizes liquid crystalline regions, but at temperatures that do not obscure the gains in surfactant efficiency offered by the ionic-nonionic surfactant mixture. Similar results are reported for two linear silicone oils, hexamethyldisiloxane and decamethyltetrasiloxane.
Introduction Microemulsions are thermodynamically stable, isotropic, microstructured solutions of water, oil, and surfactant. There is a large and growing body of research about the thermodynamic, structural, and dynamic properties of these solutions,1,2 yet many unanswered questions remain. There are also many practical applications that make use of the different forms of microstructure available within microemulsions,3 but economic factors nearly always require the product or process to use the least surfactant possible. Obtaining efficient microemulsions with a particular oil usually involves the addition of cosurfactant, such as a copolymer4 or an ionic surfactant,5 or matching the surfactant structure to the oil in question.6,7 However, the addition of ionic cosurfactant usually introduces unwanted liquid crystalline regions that dominate the phase behavior and can obscure any enhanced efficiency. Silicone oils are an industrially important class of materials that show excellent chemical stability and are used in cosmetics and other personal care products, in textile manufacture, and as lubricants, foam control agents, and mold release agents.8,9 Incorporating the (1) Strey, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 402-410. (2) Schubert, K.-V.; Kaler, E. W. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 190-205. (3) Solans, C.; Kuneida, H. Industrial Applications of Microemulsions; Surfactant Science Series Vol. 66; Marcel Dekker: New York, 1997. (4) Jakobs, B.; Sottman, T.; Strey, R.; Allgaier, J.; Willner, L.; Richter, D. Langmuir 1999, 15, 6707. (5) Kahlweit, M.; Faulhaber, B.; Busse, G. Langmuir 1994, 10, 25282532. (6) Li, X.; Washenberger, R. M.; Scriven, L. E.; Davis, H. T.; Hill, R. M. Langmuir 1999, 15, 2267-2277. (7) Li, X.; Washenberger, R. M.; Scriven, L. E.; Davis, H. T.; Hill, R. M. Langmuir 1999, 15, 2278-2289. (8) O’Lenick, A. J., Jr. J. Surfactants Deterg. 2000, 3, 229-236. (9) Auner, N.; Weis, J. Organosilicon Chemistry IV; Auner, N., Weis, J., Eds.; Wiley-VCH: New York, 2000.
physical properties of silicone oils into efficient microemulsions is a first step to new products, and the unique features of silicone oils open new areas for scientific study. While there are empirical rules to guide the formation of efficient microemulsions with hydrocarbon alkane oils,10,11 there has been relatively little examination of the ternary phase behavior of silicone oils in solutions of either siloxane or carbon-based surfactants. Ternary siloxane surfactantsilicone oil-water systems show phase behavior results that are similar to those of mixtures of hydrocarbons and carbon-based nonionic surfactants.6,7 However, siloxane surfactants are not available with the range of molecular architectures found in hydrocarbon surfactants. Generally, the studies of hydrocarbon surfactants with silicone oils mostly report the inability to solubilize much silicone oil in water. Mixtures of hydrocarbon surfactants with modified silicone oil (where the modification is an amine12 or a phenyl derivative13) and water show somewhat improved solubilization over that achieved with unmodified silicone oils,14-16 but the level of solubilization is still not comparable to that found in hydrocarbon systems. Similarly, a co-oil can increase solubilization of the silicone oil and lead to the formation of microemulsions and interesting liquid crystalline phases.17 (10) Kahlweit, M. J. Phys. Chem. 1995, 99, 1281-1284. (11) Kunieda, H.; Nakano, A.; Akimaru, M. J. Colloid Interface Sci. 1995, 170, 78-84. (12) Katayama, H.; Tagawa, T.; Kuneida, H. J. Colloid Interface Sci. 1992, 153, 429-436. (13) Steytler, D. C.; Dowding, P. J.; Robinson, B. H.; Hague, J. D.; Rennie, J. H. S.; Leng, C. A.; Eastoe, J.; Heenan, R. K. Langmuir 1998, 14, 3517-3523. (14) Binks, B. P.; Dong, J. Colloids Surf., A 1998, 132, 289-301. (15) Dowding, P. Ph.D. Thesis, University of East Anglia, Norwich, U.K., 1995. (16) Messier, A.; Schorsch, G.; Rouviere, J.; Tenebre, L. Prog. Colloid Polym. Sci. 1989, 79, 249-256.
10.1021/la010359g CCC: $20.00 © 2001 American Chemical Society Published on Web 06/27/2001
Effect of DDAB on Surfactant-Oil Microemulsions
The most effective way to examine ternary systems experimentally is by constructing an upright ternary Gibbs phase prism and making planar cuts through the prism along different lines of constant composition. The resulting vertical sections have temperature as the ordinate and a composition variable as abscissa. All or part of the body of heterogeneous phases is contained in this Gibbs prism. Mapping the changes in phase behavior as a function of temperature and composition provides essential thermodynamic information. Studies of mixtures of the n-alkyl polyglycol ethers (CiEj’s), alkanes, and water show how the critical points on the surfactant-water and surfactant-oil binaries influence the behavior of ternary mixtures.18-21 A similar approach can show how the addition of a cosurfactant can affect the critical points and formation of heterogeneous phases.5,22-24 As explained by Kahlweit and Strey,19 the lower critical point on the surfactant-water binary, Tβ, is connected by a critical line to the lower critical temperature of the three-phase body, Tl. Similarly, the upper critical point on the surfactant-oil binary is connected by a critical line to the upper critical temperature of the three-phase body, Tu. Therefore, movement of the critical points TR and Tβ directly affect the extent of the three-phase body in temperature by moving Tu and Tl, respectively. To make efficient microemulsions, surfactant mixtures have been employed to tailor the properties of the surfactant film to the particular application. For example, the addition of ionic surfactant to nonionic surfactant changes the phase progression and temperature dependence of surfactant-water mixtures.25-29 Ionic-nonionic surfactant mixtures are known to increase the surfactant efficiency in systems of CiEj-alkane oil-water. Phenomenologically, these observations can be explained in terms of the relative location of the tricritical point5 or by the combined hydrophilic-lipophilic balance (HLB) of the surfactant mixture.30-32 Structurally, the addition of charge to nonionic membranes causes an increase in the bending moduli of the nonionic membrane33 and can change the mechanism of stabilization in lyotropic lamellar phases.34 In this paper, we present the results of phase behavior studies on the system CiEj-D4-water. In addition, we report how addition of a cationic cosurfactant changes (17) John, A. C.; Uchiyama, H.; Nakamura, K.; Kuneida, H. J. Colloid Interface Sci. 1997, 186, 294-299. (18) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1983, 87, 5032-5040. (19) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24, 654-668. (20) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1984, 88, 1937-1944. (21) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.; Jen, J.; Schoma¨cker, R. Langmuir 1988, 4, 499-511. (22) Ryan, L. D.; Schubert, K.-V.; Kaler, E. W. Langmuir 1997, 13, 1510-1518. (23) Ryan, L. D.; Kaler, E. W. Langmuir 1997, 13, 5222-5228. (24) Penders, M. H. G. M.; Strey, R. J. Phys. Chem. 1995, 99, 1031310318. (25) Douglas, C. B.; Kaler, E. W. J. Chem. Soc., Faraday Trans. 1994, 90, 471-477. (26) Douglas, C. B.; Kaler, E. W. Langmuir 1991, 7, 1097-1102. (27) Marszall, L. Langmuir 1990, 6, 347-350. (28) Marszall, L. Langmuir 1988, 4, 90-93. (29) Firman, P.; Haase, D.; Jen, J.; Kahlweit, M.; Strey, R. Langmuir 1985, 1, 718-724. (30) Aramaki, K.; Ozawa, K.; Kunieda, H. J. Colloid Interface Sci. 1997, 196, 74-78. (31) Kunieda, H.; Hanno, K.; Yamaguchi, S.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 129-137. (32) Shinoda, K.; Kunieda, H.; Arai, T.; Saijo, H. J. Phys. Chem. 1984, 88 (21), 5126-5129. (33) Schoma¨cker, R.; Strey, R. J. Phys. Chem. 1994, 98, 3908-3912. (34) Roux, D.; Safinya, C. R. J. Phys. France 1988, 49, 307-318.
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the interactions of the surfactant-oil and surfactantwater binary, while greatly increasing the efficiency of hydrocarbon surfactants with respect to silicone oils. This study parallels and supports other work with alkane and ether oils.5,31,35,36 Experimental Section Materials. Water was filtered through a 0.2 µm filter, distilled, and deionized until the specific resistance was 18.3 MΩ cm. N-Dodecyl pentaoxyethylene glycol ether (C12E5) (>99%), n-dodecyl hexaoxyethylene glycol ether (C12E6) (>99%), n-octyl trioxyethylene glycol ether (C8E3) (>99%), n-dodecyl trioxyethylene glycol ether (C12E3) (>99%), and n-decyl tetraoxyethylene glycol ether (C10E4) (>99%) were obtained from Nikko. Decane (>98%), octamethylcyclotetrasiloxane (D4) (>99%), hexamethyldisiloxane (MM) (>99%), decamethyltetrasiloxane (MD2M) (>98%), n-hexyl diethylene glycol ether (C6E2) (>98%), and n-butyl ethlyene glycol ether (C4E1) (>98%) were purchased from Fluka. Didodecyl dimethylammonium bromide (>99%), DDAB, was obtained from TCI America. All materials were used without further purification. Phase Behavior Determination. The procedure used for ternary and quaternary phase diagram determination follows the procedure introduced by Kahlweit and co-workers.19,35 With four-component mixtures, the phase space is defined by temperature, pressure, and three composition variables. To specify the amount of (A) water, (B) oil, (C) nonionic surfactant, and (D) ionic surfactant, we use the following composition variables: the mass fraction of oil neglecting surfactant, R, defined as
R)
B × 100 A+B
the mass fraction of surfactant, γ, defined as
γ)
C+D × 100 A+B+C+D
and the mass fraction of ionic surfactant in the surfactant mixture, δ, defined as
δ)
D × 100 C+D
For a ternary mixture, only the first two mass variables are needed, the third being δ ) 0. If instead of an ionic surfactant the fourth component is a (E) co-oil, R is defined as
R)
B+E × 100 A+B+E
γ is defined as
γ)
C × 100 A+B+C+E
and the mass fraction of co-oil in the oil is
β)
E × 100 B+E
To represent the phase space in two dimensions, only two of the defining variables are allowed to vary. In this work, the pressure will always be the ambient air pressure, and the phase behavior will be read as a function of temperature against one composition variable, all others being held constant. In particular, several sections through the phase prism have proven useful for studying microemulsions. Sections at R ) 0 and 100 correspond to the surfactant-water or surfactant-oil binary, respectively. These give valuable information on the specific interactions between two of the species involved and (35) Kahlweit, M.; Strey, R. J. Phys. Chem. 1988, 92, 1557-1563. (36) Ryan, L. D.; Kaler, E. W. J. Phys. Chem. 1998, 102, 7549-7556.
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Figure 2. Temperature-composition pseudo-binary phase diagram of C12E5-DDAB-D4 mixtures at constant δ.
Figure 1. Temperature-composition pseudo-binary phase diagram of CiEj-DDAB-water mixtures at constant δ. Areas labeled LR contain both single- and multiphase lamellar regions. C12E6-DDAB-water mixtures (top) are shown at δ ) 0, 1, and C8E3-DDAB-water mixtures (bottom) are shown at δ ) 0, 2. have been shown to control the formation of the three-phase body within the phase prism. Sections at R ) 50 are used in determining the least amount of surfactant needed to solubilize equal weights of oil and water. This occurs at the x-point, where the one- and three-phase microemulsions meet at a point at R ) 50, and has the coordinates of γ ) γ˜ and T ) T ˜ . The amount of surfactant at the x-point, γ˜ , is the efficiency of the surfactant being measured and can be compared to efficiencies of other surfactant systems. Varying the oil or surfactant composition allows the tracking of surfactant efficiency as a function of β and δ.
Results CiEj/DDAB/Water. Adding small amounts of DDAB has a profound effect on the phase behavior of solutions of CiEj and water (Figure 1). The changes observed for C12E6 and C8E3 systems are similar to changes induced by anionic cosurfactant in previous studies.25,26,37 At concentrations above δ ) 1, the lower consolute temperature is no longer evident, while a lamellar phase begins to form roughly 15 °C above the former critical point. What was a transition from a single-phase micellar solution to two isotropic phases upon heating becomes a transition from a single-phase micellar solution to a micellar solution plus a liquid crystalline lamellar phase. The phase boundaries do not change upon further addition of DDAB for δ values beyond that needed to form the lamellar phase. C12E5/DDAB/D4. The binary of C12E5 with the silicone oil D4 shows a lower miscibility gap that narrows with an increase in temperature until the critical point TR is reached (Figure 2, with TR ) 21 °C). The addition of DDAB (37) Rajagopalan, V.; Bagger-Jo¨rgensen, H.; Fukuda, K.; Olsson, U.; Jo¨nsson, B. Langmuir 1996, 12, 2939-2946.
Figure 3. Temperature-composition phase diagram of C12E5decane-D4-water at R ) 50 and various β. The shaded area for β ) 0 (decane) contains both single- and multiphase lamellar regions. The three-phase body is not shown for clarity.
raises the temperature required to obtain a one-phase mixture. At δ ) 7, the pseudo-TR ) 27 °C, while at δ ) 18, the pseudo-TR ) 35 °C. C12E3, on the other hand, is completely miscible with D4 for δ from 0 to 10. C12E5/Decane/D4/Water. D4 was blended with decane, with which it is miscible at all concentrations, and the one-phase microemulsion of this oil mixture with C12E5 was followed as a function of oil composition, β, at R ) 50. The results are shown in Figure 3. The shaded area for β ) 0 indicates both single- and multiphase lamellar regions. As β increases, the one-phase region moves to higher temperatures and surfactant concentrations. At β ) 100, the efficiency γ˜ ) 38, so the surfactant is actually the plurality component in the mixture (by weight). The three-phase region of D4 with C12E5 (β ) 100) extends to around γ ) 5 in composition and over a temperature range at least 20 °C wide. The lamellar phase formed with decane retreats to higher γ as β increases, until it is no longer evident in the oil mixture. CiEj/D4/Water. Figure 4 shows the single-phase microemulsion regions at R ) 50 for a selection of CiEj’s. Here, the change in temperature of the three-phase body upon varying i and j can be seen by comparison. Increasing ˜ by 15 °C, while i by 4, from C8E3 to C12E3, lowers T increasing j by 2, from C12E3 to C12E5, increases T ˜ by 40 °C. Notice, however, that none of the CiEj’s have γ˜ < 30, meaning it takes about as much nonionic surfactant as oil to form a one-phase microemulsion. CiEj/DDAB/D4/Water. The addition of cosurfactant DDAB to C8E3 substantially increases the surfactant efficiency, as shown for DDAB and C8E3-D4-water in
Effect of DDAB on Surfactant-Oil Microemulsions
Figure 4. Temperature-composition phase diagram of CiEjD4-water mixtures at R ) 50. Only one-phase regions are shown for clarity. The shaded area for C12E3 indicates a multiphase lamellar region.
Langmuir, Vol. 17, No. 15, 2001 4537
Figure 6. Temperature-composition phase diagram of C8E3MM-water at R ) 50 and various δ. Only one-phase regions are shown for clarity.
Figure 7. Temperature-composition phase diagram of C8E3MD2M-water at R ) 50 and various δ. Only one-phase regions are shown for clarity.
Figure 5. Temperature-composition phase diagram of CiEjDDAB-D4-water at R ) 50 and various δ. Only one-phase regions are shown for clarity. C8E3-DDAB-D4-water mixtures (top) are shown for δ ) 0, 2, 10, and 12. C12E3-DDAB-D4water mixtures (bottom) are shown for δ ) 0, 7, 12, 15, and 18. The shaded area for C12E3 indicates a multiphase lamellar region.
Figure 5a. Only the envelopes of the single-phase microemulsion regions are shown for clarity, but note that by δ ) 10, the three-phase body has disappeared. As δ increases, γ˜ moves to higher temperatures and lower γ. At δ ) 0, γ˜ ) 44 and T ˜ ) 45 °C, while at δ )12, γ˜ ) 14 and T ˜ ) 72 °C. The extent of the one-phase channel for δ ) 10 encompasses the one-phase channels that existed at lower δ. Thus, the addition of 12% ionic surfactant in the surfactant mixture has reduced the amount of surfactant needed to form a one-phase microemulsion by a factor of three from the original nonionic system. Further
addition of ionic surfactant begins to move the one-phase microemulsions above the experimental window in temperature at low surfactant concentrations. Note that as δ increases, the lower phase boundary of the one-phase region does not move. For example, at γ ) 45, the lower phase boundaries overlap at T ) 45 °C for δ ) 0, 2, 10, 12. The effect of addition of DDAB to C12E3 microemulsions is shown in Figure 5b. Again, only the one-phase regions are shown for clarity and the three-phase body disappears as larger amounts of ionic surfactant are added. As for ˜ and lowers γ˜ with an increase in δ. C8E3, DDAB raises T At δ ) 0, γ˜ ) 31, while as δ increases to δ ) 18, γ˜ ) 12. Again, notice how the lower phase boundaries of the onephase region coincide for all δ. C8E3/MM/Water. Adding DDAB to C8E3 with the twounit linear silicone oil hexamethyldisiloxane, MM, also lowers γ˜ (Figure 6), with γ˜ moving from 37 to 15 as δ changes from 0 to 11. The change in temperature with increasing δ is not as large as it is for D4 but still shows the same characteristic of a lower phase boundary that coincides with the original boundary, while the upper boundary increases in temperature. C12E3/MD2M/Water. Similar effects are seen with C12E3, water, and decamethyltetrasiloxane, MD2M (Figure 7). As δ increases, γ˜ decreases while the temperature range of the microemulsions increases. At δ ) 11, γ˜ is lowered to 15, down from 40 when δ ) 0. At the same time, the temperature of the one-phase region increases from 40 to 65 °C. The lower phase boundaries coincide for all δ values, while the upper phase boundary increases with increasing δ.
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Discussion Surfactant-Water Binary. The phase behavior of CiEj surfactants with water is well understood,1 and the changes resulting from the addition of cationic surfactant are similar to those observed in anionic-nonionic systems.26,38-40 The suppression of the upper miscibility gap shown in Figure 1 increases the temperature range of the micellar region of the pseudo-binary mixture, effectively raising Tβ until it is obscured by a liquid crystalline region. It has been suggested that the change in Tβ is due to the adsorption or depletion of salts at the surfactant monolayer, depending on the nature of the salt in question.41 Phenomenologically, increasing Tβ should raise the temperature of the body of heterogeneous phases.29,42 Surfactant-Oil Binary. The typical phase behavior of nonionic surfactant with oil consists of a lower miscibility gap, which, depending upon the differences in hydrophobicity between the compounds, may or may not be visible within the observable temperature range. As surfactants become more hydrophobic, the critical point TR falls below 0 °C. Such is the case for C12E3 with D4. However, TR for C12E5 is around 20 °C with D4. The addition of the ionic surfactant increases the hydrophilicity of the surfactant mixture, raising the pseudo-TR incrementally with each addition of DDAB, as shown in Figure 2. In a ternary mixture, such a change in TR increases the temperature at which the three-phase body forms. Since the threephase body and one-phase microemulsion generally occur in the same temperature range, the temperature of the one-phase microemulsion should also rise. C12E5/Decane/D4/Water. The technique of blending a new “unknown” oil with one whose phase behavior is already known is helpful in locating the three-phase region ˜ for D4 with of the new oil.43 Figure 3 shows that γ˜ and T C12E5 are higher in surfactant concentration and temperature relative to that of decane. The movement up in temperature is expected since TR for C12E5 and decane is below the experimental window, while TR ) 20 °C for C12E5 and D4. In addition, the lamellar phase that forms in the one-phase channel at higher surfactant concentrations with decane diminishes and retracts with the addition of D4. This implies a decrease of long-range order within the microstructure of this system in parallel with the loss of efficiency. Since only the composition of the oil phase is changing in this experiment, the simultaneous loss of microstructural order and surfactant efficiency in the microemulsion clearly reflects changes in oil-surfactant interactions. CiEj/D4/Water. The efficiency of C12E5 with D4 is representative of the results for all the CiEj’s. Indeed, all of the nonionic surfactants shown in Figure 4 require γ above 30 in order to form a single-phase microemulsion. The trends in T ˜ of the CiEj’s shown in Figure 4 are similar to the trends in T ˜ of CiEj’s with alkane oils and n-alkyl methacrylates. In particular, as i increases, T ˜ decreases, and as j increases, T ˜ increases. When the phase behavior of CiEj-D4-water is compared to the results of CiEj-n(38) Nishikido, N. J. Colloid Interface Sci. 1989, 136, 401. (39) Carvell, M.; Leng, C. A.; Leng, F. J.; Tiddy, G. J. T. Chem. Phys. Lett. 1987, 137, 188. (40) De Salvo Souza, L.; Corti, M.; Cantu, L.; Degiorgio, V. Chem. Phys. Lett. 1986, 131, 160. (41) Kabalnov, A.; Olsson, U.; Wennerstrom, H. J. Phys. Chem. 1995, 99, 6220. (42) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 2000, 16, 10201024. (43) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1997, 13, 52495251.
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hexyl methacrylate-water44 or CiEj-octane-water,45 D4 is generally more hydrophobic than either of these two oils. Higher temperatures are required to partition a particular surfactant into D4 than the oil phase of either octane or n-hexyl methacrylate systems. In fact, extrapolating the observed temperature dependence of the alkanes with CiEj’s indicates that D4 behaves as approximately a C20 alkane,46 as one would expect from comparisons of molecular weights or volumes. However, the unique character of the silicone moiety makes linear comparisons with alkanes difficult.47 The body of heterogeneous phases moves in temperature depending on the relative hydrophobicity of the surfactant tested. More hydrophobic surfactants have lower TR’s and Tβ’s and partition into the silicone oil at lower temperatures. This lowers the temperature of the three-phase body, so three liquid phases are formed with C12E3 at lower temperatures than with C8E3 or C12E5. The results of tracking T ˜ and γ˜ from a series of CiEj’s with i decremented by 2 and j by 1 (i.e., C12E5, C10E4, etc.) are similar to the results reported for octane and n-hexyl methacrylate. In those cases, the nonlinear changes in T ˜ and γ˜ as the surfactant becomes less amphiphilic (decreasing i and j) signal the approach to a tricritical point as the threephase bodies shrink, and that is likely what is happening here also. However, C4E1 still forms a three-phase body with D4 and water, so the tricritical point must be “past” C4E1. The change in γ˜ of these solutions correlates to the length of the hydrophobic portion of the surfactant (that is, i). Surfactants with longer hydrocarbon chains are more efficient; they require less surfactant, regardless of temperature, as seen from increasing i from 4 to 12. This is most likely because of the same factors that lead to greater ordering of the surfactant monolayer, such as rigidity and insolubility in the adjacent bulk phases. The ordering ability of the surfactant in solution is seen experimentally by the extent of liquid crystalline phases evident on the surfactant-water binary. For comparison, the phase behavior of C8E3 with water shows just an upper miscibility gap, while C12E5 with water shows a lamellar phase at low concentration and C12E3 with water is dominated by lyotropic phases.48 As γ˜ decreases from C8E3 to C12E5 to C12E3, it follows that the addition of a cosurfactant that can increase the order within the surfactant layer should also affect the efficiency of the surfactant mixture. CiEj/DDAB/D4/Water. The addition of DDAB yields large increases in the efficiency of CiEj-rich surfactant mixtures. Unfortunately, a rise in the temperature of the one-phase region also accompanies the increase in efficiency, and this rise can move the one-phase microemulsion beyond accessible temperatures. Since the homologous series of CiEj surfactants act similarly and predictably, however, the temperature of the original nonionic microemulsion can be controlled by judicious choice of i and j, and thus there is some control of the temperature range over which a microemulsion forms. There are two main effects of adding ionic surfactant to the nonionic microemulsions; T ˜ increases and γ˜ decreases. (44) Lade, O.; Beizai, K.; Sottman, T.; Strey, R. Langmuir 2000, 16, 4122. (45) Burauer, S.; Sachert, T.; Sottman, T.; Strey, R. Phys. Chem. Chem. Phys. 1999, 1, 4299. (46) Kahlweit, M.; Strey, R.; Firman, P. J. Phys. Chem. 1986, 90, 671-677. (47) O’Lenick, A. J.; Parkinson, J. K. Cosmet. Toiletries 1996, 10, 37. (48) van Os, N. M.; Haak, J. R.; Rupert, L. A. M. Physico-Chemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: New York, 1993.
Effect of DDAB on Surfactant-Oil Microemulsions
Understanding the temperature increase of the singlephase microemulsion caused by the addition of DDAB is aided by a closer inspection of the component phase diagrams. In a ternary mixture of surfactant, oil, and water, the Gibbs phase prism encompasses all temperature and composition combinations, allowing one to track the relationship between the binary and ternary phase diagrams. Critical lines directly connect the binary miscibility gaps and the three-phase body, relating TR to Tu, the upper temperature of the three-phase body, and Tβ to Tl, the lower temperature of the three-phase body.19 In a quaternary mixture, however, a phase tetrahedron exists for each temperature, so precise relationships between the pseudo-TR and Tβ and the temperature stability of any interior phase are difficult to ascertain. Inasmuch as the surfactant mixture can be regarded as a pseudo-component, the pseudo-binaries will control the temperature dependence of the pseudo-ternary mixture, but the connection is not as rigorous as it is for a ternary mixture. Keeping this in mind, the addition of DDAB to CiEj’s increases both the pseudo-TR and Tβ (Figures 1 and 2). An increase in both Tβ and TR indicates that a higher temperature is needed to partition the surfactant out of the water phase and a still higher temperature is needed to partition the surfactant into the oil phase. Therefore, the three-phase body (when it exists) and the one-phase microemulsion move to higher temperatures, as seen experimentally in Figures 5-7. Additionally, as ionic surfactant is added to the nonionic microemulsion, the lower phase boundary coincides with that of the original microemulsion, while the upper phase boundary increases in temperature with each DDAB addition. This phenomenon is also evident in previous accounts.36 In terms of the pseudo-binaries, the surfactant-oil binary changes only slightly upon addition of DDAB, as does the lower phase boundary of the one-phase microemulsion. In contrast, the surfactant-water binary changes dramatically, as does the upper phase boundary of the one-phase microemulsion. Since the lower phase boundary corresponds to saturation of the single-phase microemulsion with oil (a Winsor IV to Winsor I transition), it follows that the location of this boundary would be most influenced by the changes in interactions between the surfactant and oil. Similarly, as the upper phase boundary corresponds to saturation of the single-phase microemulsion with water (Winsor IV to Winsor II), the location of this phase boundary is influenced most by the changes in interactions between surfactant and water. Overall, the large movement of the upper phase boundary with DDAB addition indicates that the microemulsion phase is more stable with regard to an excess water phase or that the balanced or HLB temperature has dramatically increased. Recall that the final γ˜ of the CiEj-DDAB surfactant mixture correlates with the formation of liquid crystalline regions with water (Figure 4). The addition of ionic surfactant increases the ability of the surfactant mixture to order in water (as evidenced by the low-concentration lamellar phase), most likely as a consequence of increasing the rigidity of the surfactant monolayers.33 As noted before, the difference in efficiency between alkane oils and silicone
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oils can be traced simplistically to the differences in the surfactant-oil interactions. To mitigate the changes in interactions on the oil side of the surfactant monolayer, the interactions on the water side of the surfactant monolayer can be modified by the addition of an ionic surfactant. Therefore, a relatively “stronger” surfactant mixture is needed to solubilize silicone oils and yield surfactant efficiencies similar to those of CiEj’s with alkane oils. The same arguments and similar formulations have been proposed for alkane systems. However, increasing the amount of lyotropic phases on one side of the phase prism, whether this is done by adding a cosurfactant or by increasing the amphiphilicity, generally obscures isotropic phases throughout the phase prism. For example, the lamellar phase formed with CiEj-DDAB-water extends into the pseudo-phase prism with D4 and obscures the phase behavior of this system for R < 35. However, at R ) 50, observations indicate that the lamellar phase exists only in the multiphase region above the one-phase region plotted in Figures 5-7. Increased surfactant efficiency can be realized only by evading the lyotropic phases in highly amphiphilic surfactant systems. CiEj/DDAB/Linear Oil/Water. Adding ionic surfactant also increases the surfactant efficiency for mixtures with two different linear oils (Figures 6 and 7). As with D4, the temperature of the one-phase region increases with increasing δ, indicating a difference in the partitioning of the surfactant mixture with increasing ionic content. Similarly, the lower phase boundary of the one-phase microemulsions remains relatively constant while the upper phase boundary increases. As the T ˜ and γ˜ increase with the molecular weight of the oil, it becomes more important to choose i and j to lower the temperature of the nonionic microemulsion. Therefore, MM is solubilized with C8E3 while MD2M requires the more hydrophobic C12E3 in order to lower the temperature of the one-phase regions to accessible ranges. This example illustrates the control over the formulation variable, T ˜ , that is obtained by using a homologous series10 and suggests a strategy to study increasing molecular weight linear silicone oils.49 Conclusion Systematic study of water-D4-CiEj-DDAB mixtures shows that adding a cationic cosurfactant greatly changes the phase behavior of water-CiEj and D4-CiEj mixtures. The miscibility gaps in binary CiEj-water mixtures disappear, and a lamellar phase appears at higher temperatures. For CiEj-D4 mixtures, the addition of ionic surfactant increases the temperature at which the miscibility gap appears on the surfactant-oil side of the phase prism. Coincidentally, the addition of DDAB to mixtures containing equal weights of silicone oil and water greatly increases the efficiency of nonionic surfactants but also shifts the single-phase microemulsion to higher temperatures. Acknowledgment. This work was supported by Dow Corning. LA010359G (49) Silas, J. In preparation.
