Systematic Study on the Influence of Impurities on the Phase Behavior

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Langmuir 1998, 14, 6385-6395

6385

Systematic Study on the Influence of Impurities on the Phase Behavior of Sodium Bis(2-ethylhexyl) Sulfosuccinate Microemulsions W. F. C. Sager* Department of Physical and Macromolecular Chemistry, Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands Received August 26, 1997. In Final Form: August 6, 1998 In this paper we discuss the influence of sodium dodecyl sulfate (SDS) and 1-octanol on the phase behavior of (water/NaCl)-decane-sodium bis(2-ethylhexyl) sulfosuccinate (AOT) microemulsion systems. The two additives were chosen to mimic the effect of surface-active impurities such as the hydrolysis products of AOT, often present in commercial AOT samples or forming in AOT microemulsions. The investigation was carried out to clarify some contrariety of AOT phase diagrams published and to explain the origin of an isolated two-phase island, which so far represented a widely accepted peculiarity of the AOT system.

1. Introduction AOT, sodium bis(2-ethylhexyl) sulfosuccinate, is one of the prevalently investigated anionic surfactants. Due to its double-tailed nature the AOT molecule is almost structurally balanced with respect to its hydrophilic and hydrophobic moieties and is one of the few ionic surfactants that forms microemulsions without the addition of a cosurfactant (e.g., alcohol). Over the last 20 years AOT microemulsions have become a classical example of (ionic) three-component microemulsions and have been used as a model system to test current theories of microemulsions and to study critical behavior (see, e.g., refs 1-3), apart from their many (technical) applications.4 Surprisingly enough, there is no generally accepted or fully understood picture at hand of the phase behavior of AOT microemulsions. Instead there is some contrariety about the coexistence regions and even the sequence of the phases occurring in the phase diagrams published. One of the reasons might be that most of the commercially available AOT samples still contain traces of the starting materials and that AOT, being a diester, can easily undergo hydrolysis, especially at high temperatures and long storage times. In this paper we want to focus on the effect that the hydrolysis products of AOT have on the microemulsion phase behavior. Hydrolysis will be mimicked by the controlled addition of sodium dodecyl sulfate (SDS) and 1-octanol. The paper is organized in such a way that we first summarize common features of the AOT phase behavior and then point out a peculiarity of AOT phase diagrams often observed and reported in the literature. To understand the AOT phase behavior in more general terms, a comparison with nonionic microemulsion systems is made for the different situations encountered. In the results and discussion section we will show the * Current address: Faculty of Chemical Technology, Membrane Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands. (1) Borkovec, M. Adv. Colloid Interface Sci. 1992, 37, 195-217. (2) Chen, S. H.; Ku, C. Y.; Rouch, J.; Tartaglia, P.; Cametti, C.; Samseth, J. J. Phys. IV 1993, 3, 143-161. (3) Chen, S. H.; Kotlarchyk M. In Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M., Eds.; NorthHolland: Amsterdam, 1985; pp 768-792. (4) Johnson, K. A.; Shah, D. O. J. Colloid Interface Sci. 1985, 107269.

respective effects of SDS and octanol on the phase behavior of (water/NaCl)-decane-AOT microemulsion systems, which we have chosen as a reference system. We will demonstrate also for the salt-free AOT microemulsion system how the presence of octanol can be responsible for the formation of the isolated two-phase region, often observed at low temperatures when water-in-oil (w/o) microemulsions form. In conclusion, we discuss the impact of this investigation on studies of the physicochemical properties of AOT w/o microemulsions. 2. The AOT Microemulsion System: Phase Behavior, Hydrolysis, and Reentrant Phase Formation AOT Phase Diagrams: Common Features. The phase behavior of AOT can be summarized as follows: The phase diagram of the binary water-AOT system is dominated by an extended lamellar phase region (LR). A micellar solution (L1) forms at very low AOT concentrations, while cubic and reverse hexagonal liquid crystalline phases appear at higher concentrations of AOT (see, e.g., the water-surfactant side of Figure 1a).5-7 A bicontinuous bilayer phase (L3) is obtained only upon addition of salt, e.g., NaCl.6,7 In hydrocarbons and most common apolar solvents, AOT dissolves completely forming globular reverse micelles (L2).8 A main feature of the ternary water-oil-AOT system is the existence of a w/o microemulsion (L2 phase) over a wide range of concentrations and temperatures, entering the Gibbs triangle from the oil-surfactant side (see, e.g., Figure 1a). Its structure has widely been characterized and attracted a lot of interest as an ideal playground for applying liquid state theories.2 Liquid crystalline phases, especially an LR phase, extend from the water-surfactant side far into the triangle.9,10 Due to the nearly structurally (5) Fontell, K. In Colloidal Dispersions and Micellar Behaviour; ACS Symp. Ser. No. 9; American Chemical Society: Washington, DC, 1975; pp 270-277. (6) Ghosh, O.; Miller, C. A. J. Phys. Chem. 1987, 91, 4528-4535. (7) Strey, R.; Jahn, W.; Skouri, M.; Porte, G.; Marignan, J.; Olson, U. In Structure and Dynamics of Strongly Interacting Colloids and Supermolecular Aggregates in Solution; Chen, S.-H., Ed.; Kluwer: Amsterdam, 1992; pp 351-363. (8) Eicke, H.-F. In Micellization, Solubilization, and Microemulsions; Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 1, pp 429-443.

10.1021/la9709608 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/03/1998

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Figure 1. Schematic representation of a Gibbs triangle (a) for the water-isooctane-AOT system at room temperature (see ref 9). Characteristic for the diagram is the huge isolated twophase region (L2 + W) at lower surfactant concentrations. The arrow on the right side marks the cut for the phase diagram in part b showing the phase behavior as a function of temperature and the amount of water added at constant AOT to oil ratio. The lower phase boundary of the L2 phase is bent. The arrow on the left side indicates the temperature at which the Gibbs triangle is taken. The crosses in (a) and (b) mark the boundaries of the isolated two-phase region at room temperature and an AOT:oil ratio of 3:7.

balanced state of the AOT molecule, the ternary system shows a pronounced temperature dependence.11 At low temperature, the surfactant stays mainly in the oil-rich phase. Depending on the oil used, the Gibbs triangle shows a central miscibility gap (two-phase region) along the water-oil side in which a mixture separates into a water-in-oil (w/o) microemulsion (L2) in equilibrium with an almost pure water phase,10,11 to which we will refer to as 2 h Φ (with the bar indicating in which phase most of the surfactant is dissolved and the microemulsion is basically formed). With an increase of temperature and thus with (9) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1979, 70, 577583. (10) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface Sci. 1970, 33, 215-235. (11) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601606.

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an increase of ionization of the headgroup, water gradually becomes a better solvent for the AOT molecule, and the surfactant moves slowly from the oil-rich into the waterrich phase. At intermediate temperatures, multiphase equilibria occur in which an LR phase is in equilibrium with L2 and/or L1 phases.12 Upon raising the temperature, the declination of the tie-lines in the central miscibility gap reverses sign, and finally an oil-in-water (o/w) microemulsion is in equilibrium with an almost pure oil phase13 (referred to as 2Φ), surrounded now by a singlephase oil-in-water microemulsion (L1 phase). The type of oil used influences the temperature range at which the different phases occur.11 Long chain alkanes (e.g. decane) increase the tendency for AOT to leave the oil-rich phase to such an extent that lamellar multiphase equilibria show up already below room temperature.6,12 The exact positions and sequences of the phases, especially for the multiphase equilibria that form, depend crucially on the AOT used. The ternary system does not exhibit an isotropic threephase region (3Φ) with water and oil excess phases both in equilibrium with a surfactant-rich middle-phase microemulsion. Addition of salt as a fourth component, however, induces the formation of an isotropic three-phase region at intermediate temperatures.11,13,14 The extension of the three-phase region for the quaternary system increases with the amount of salt added, while its mean temperature (T h ) is shifted to higher values. The phase sequence 2 h Φ f 3Φ f 2Φ (corresponding to a Winsor IIIII-I transition) can thus be attained at a given salt concentration by increasing the temperature, whereas the sequence 2Φ f 3Φ f 2 h Φ (corresponding to a Winsor IIII-II transition) is observed at a given temperature as the salt concentration is increased.6,15 For the position of the isotropic three-phase region as a function of salt concentration and of oil chain length, see, e.g., refs 11 and 13. Over the last 20 years, the structure of the L2 phase has been intensively investigated using a variety of techniques. Water droplets covered by a monomolecular AOT layer are stable over a wide range of concentrations and temperatures. The droplets formed were characterized in terms of their size, form fluctuations, and size polydispersity. While the radius of the droplets is basically determined by geometrical factors (e.g., the molecular ratio of the dispersed phase to the surfactant), the droplet stability depends on the elastic properties of the surfactant monolayer surrounding the droplets. With an increase of the temperature, an increasing attractive force between the droplets has been observed leading to droplet aggregation.16 The interaction potential, as well as the aggregation and clustering behavior and the structural transformation of the droplets, has been studied in detail. For a recent overview see, e.g., refs 2, 17, and 18. Influence of impurities: hydrolysis studies. It has been recognized early on that the phase behavior and certain properties of AOT microemulsions depend distinctly on the purity of the AOT used,9,19,20 which actually (12) Assih, T.; Delord, P.; Larche´, F. C. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 3, pp 1821-1828. (13) Kahlweit, M.; Strey, R.; Schoma¨cker, R.; Haase, D. Langmuir 1989, 5, 305-315. (14) Chen, S.-H.; Chang, S.-L.; Strey, R. J. Chem. Phys. 1990, 93, 1907-1918. (15) Shinoda, K.; Kunieda, H. J. Colloid Interface Sci. 1987, 118, 586-589. (16) Huang, J. S.; Safran, S. A.; Kim, M. W.; Grest, G. S. Phys. Rev. Lett. 1984, 592-595. (17) Chevalier, Y.; Zemb, T. Rep. Prog. Phys. 1990, 53, 279-371. (18) Koper, G. J. M.; Sager, W. F. C.; Smeets, J.; Bedeaux, D. J. Phys. Chem. 1995, 99, 13291-13300.

Impurity Influence on the AOT Phase Behavior

itself consists of a mixture of eight optical isomers since the AOT molecule possesses three chiral centers.21 As has been pointed out by Fletcher20 and Luisi22 the impurities in commercially available AOT samples originate mainly from its synthesis. AOT is often prepared by diesterification of maleic or fumaric acids with 2-ethylhexanol. The diester is then sulfonated with sodium bisulfite. Consequently, major impurities are polar AOT components, such as the parent dicarbolic acids and acidic monoesters resulting from incomplete esterification, alcohol (2-ethylhexanol), and salt (sodium bisulfite or sulfate). The amount of impurities present in commercial AOT preparations is known to differ not only between the manufacturers but even from batch to batch of the same commercial source. The various purification procedures that have been applied (see, e.g., refs 9 and 23-27) reduce the amount of impurities to different extents. As a result the phase diagrams appearing in the literature vary with respect to the phase boundaries of the respective phases. For an illustrative example see the differences in the extensions of, e.g., the L2 phase in the phase triangles of the water-isooctane-AOT system as published in refs 9 and 28-30. Hydrolysis studies were performed by Fletcher,20 Luisi23 (w/o microemulsions), and Mukherjee31 (micellar solutions). The hydrolysis rate increases with temperature and can be acid- or base-catalyzed. Delord and Larche32 also observed changes in the phase diagrams (e.g., phase separation) occurring with time, which they associated with the formation of 2-ethylhexanol. In an AOT microemulsion sample left for a month at 25 °C, they found 0.1 mol of 2-ethylhexanol per mole of AOT originally present. After a month at 40 °C already 0.4 mol was formed. Kotlarchyk et al. detected 1 wt % (3.4 mol %) alcohol in microemulsion samples stored for 1 week.33 Formation of the Isolated Two-Phase Region: Reentrant Phase. In one of the classic papers on the phase behavior of AOT microemulsions, Kunieda and Shinoda9 published a phase diagram for the waterisooctane-AOT system at room temperature, which has widely been accepted and repeatedly reproduced in the literature. A schematic presentation of this diagram is shown in Figure 1a. As already noticed a large L2 phase region extends from the AOT-isooctane side far into the Gibbs triangle. Thus w/o microemulsions still form (19) Peri, J. B. J. Colloid Interface Sci. 1969, 29, 6-15. (20) Fletcher, P. D. I.; Perrins, N. M.; Robinson, B. H.; Toprakcioglu, C. In Reverse Micelles; Luisi, P. L., Straub, B. E., Eds.; Plenum: New York, 1984; pp 69-72. (21) Olsson, U.; Wong, T. C.; So¨dermann, O. J. Phys. Chem. 1990, 94, 5356-5361. (22) Luisi, P. L.; Magid, L. J. CRC Crit. Rev. Biochem. 1986, 20, 409-474. (23) Luisi, P. L.; Meier, P.; Imre, V. E.; Pande, A. In Reverse Micelles; Luisi, P. L., Straub, B. E., Eds.; Plenum: New York, 1984; pp 323-337. (24) Maitra, A. N.; Eicke, H.-F. J. Phys. Chem. 1981, 85, 2687-2691. (25) Martin C. A.; Magid, L. J. J. Phys. Chem. 1981, 85, 3938-3944. (26) Smeets, J.; van der Ploeg, J. P. M.; Koper, G. J. M.; Bedeaux, D. Langmuir 1994, 10, 1387-1392. (27) Kotlarchyk, M.; Chen, S.-H.; Huang, J. S.; Kim, M. W. Phys. Rev. A 1984, 29, 2054-2069. (28) Tamamushi, B.; Watanabe, N. Colloid Polym. Sci. 1980, 258, 174-178. (29) Stilbs, P.; Lindman, B. J. Colloid Interface Sci. 1984, 99, 290293. (30) Roux, D.; Bellocq, A. M. In Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M., Eds.; NorthHolland: Amsterdam, 1985; pp 842-856. (31) Mukherjee, K.; Moulik, S. P.; Mukherjee, D. C. Int. J. Chem. Kinet. 1994, 26, 1063-1074. (32) Delord, P.; Larche, F. C. J. Colloid Interface Sci. 1984, 98, 277278. (33) Kotlarchyk, M.; Chen, S.-H.; Huang, J. S.; Kim, M. W. Phys. Rev. A 1984, 29, 2054-2069.

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with even more than 70% of water as internal phase. At lower surfactant concentrations, the w/o microemulsion separates into an upper w/o microemulsion phase in equilibrium with an almost pure water phase at the bottom (L2 + W). The two-phase region forms an isolated island within the Gibbs triangle and is actually completely surrounded by the L2 phase. The L2 phase moulds thus a narrow channel between the L2 + W island and the other two-phase or multiphase regions at very low AOT content. The occurrence of the isolated L2 + W region has not been observed by Fontell,10 Larche,32 and Miller6 for similar systems, who found instead multiphase equilibria with one phase being lamellar. In their paper Kunieda and Shinoda already pointed out that it is not possible to draw tie-lines into the L2 + W island observed. This apparent contradiction with the phase rule foreshadows that they were not dealing with a pure ternary system. In that case, the phase diagram has to be presented in at least a tetrahedron (for a quaternary system). The diagram in Figure 1a would therefore display only a projection of a cut through the “multispaced” two-phase (L2 + W) body onto the (base) triangle. Hence the tie-lines, connecting the compositions of the two phases in equilibrium, are not laying in the plane of the paper. The compositions of the top phases formed correspond to those of the adjacent L2 region (at higher surfactant concentrations); see also ref 34. The bottom phase contains almost pure water within the whole miscibility gap. Figure 1b shows a schematic phase diagram along a line (see arrow in Figure 1a) of constant AOT/oil ratio heading toward the water corner as a function of temperature. Such phase diagram cuts have often been performed to determine the existence region for w/o microemulsions and are obtained, e.g., by adding water to a test tube containing oil with a given concentration of AOT. Below the phase boundary at low temperatures (solubilization-limit curve, open circles in Figure 1b) water is expelled from the microemulsion and an excess water phase forms (2 h Φ). Above the upper phase boundary (cloudpoint curve, filled circles in Figure 1b) the w/o microemulsion splits into two surfactant-rich phases, which can, depending on the amount of water added, be both w/o microemulsions or an L2 phase in equilibrium with an LR phase. Characteristic for this diagram is that the lower phase boundary is bent. The curve first rises but then passes through a maximum. The arrow marks the temperature of the isothermal diagram in Figure 1a. At hΦ this temperature the phase sequence 1Φ (L2 phase) f 2 (L2 + W) f 1Φ (L2 phase) is observed with increasing water content. The two crosses in Figure 1b correspond to the phase boundaries of the isolated two-phase island for the particular AOT/oil ratio in Figure 1a (also marked by crosses). The part of the 1Φ region (L2 phase) that reenters the phase diagram at high water/surfactant ratios (Figure 1b), is located within the narrow onephase channel (Figure 1a). With an increase of temperature the two-phase island shrinks toward the oil corner of the Gibbs triangle to disappear finally at even higher temperatures. Bending of the lower phase boundary and the reentering 1Φ region (reentrant phase) have often been reported in the literature, although the degree to which it occurs differs; see e.g., refs 35 and 36. While in ref 9 the strong influence of impurities on the positions of the phase boundaries with respect to temperature is (34) Sager, W.; Strey, R.; Ku¨hnle, W.; Kahlweit, M. Prog. Colloid Polym. Sci. 1994, 97, 141-145. (35) Eicke, H.-F. J. Colloid Interface Sci. 1979, 68, 440-450. (36) Kon-No, K.; Kitahara, A. J. Colloid Interface Sci. 1971, 37, 469475.

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Figure 2. Phase diagrams at the water corner for the H2Ooctane-C10E4 system (see ref 39). (a) Phase diagram at constant C10E4/H2O ratio as a function of temperature and oil added. The C10E4/H2O ratio corresponds to that of the critical point of the binary water-C10E4 system (for the cut see arrow in part b). The upper phase boundary (filled circles), which passes through a minimum, reflects nearly the trajectory of the critical line. It starts at the lower critical point of the upper miscibility gap of the binary system and ends at T ) Tl when the middle and the water-rich phase merge. (b) The water-corner of the Gibbs triangle shows the closed loop of the reentrant phase (2h ) with two critical points at T ) 17 °C.

Figure 3. Phase diagrams at the oil corner for the H2Ooctane-C10E4 system (see ref 39). (a) Phase diagram at constant C10E4/octane ratio as a function of temperature and water added. The C10E4/octane ratio corresponds to that of the critical point of the binary octane-C10E4 system (for the cut see arrow in part b). The lower phase boundary (filled squares), which passes through a maximum, reflects almost the trajectory of the critical line. It starts at the upper critical point of the lower miscibility gap of the binary system and ends at T ) Tu when the middle and the oil-rich phase merge. (b) The oil corner of the Gibbs triangle shows the closed loop of the reentrant phase (2) with two critical points at T ) 29 °C.

mentioned, the reason for the occurrence of the isolated two-phase region remained unsolved. Comparison with the Loop Formation in Nonionic Microemulsion Systems. Bending of one of the boundaries of the existence region for o/w or w/o microemulsions with the occurrence of a reentrant phase has also been found for nonionic microemulsion systems; see, e.g., refs 37 and 38. In this case the bending observed is of a different origin. To clarify this, Figure 2 and Figure 3 show the phase boundaries of the microemulsion region as a function of the octane (water) content and temperature for o/w (Figure 2a) and w/o (Figure 3a) microemulsions of tetraethylene glycol mono-n-decyl ether (C10E4) at constant

water/surfactant (oil/surfactant) ratio (see arrows in Figures 2b and 3b). The surfactant concentrations chosen correspond to the compositions of the lower and upper critical point of the binary water-surfactant and octanesurfactant systems, respectively (see ref 39). The corresponding Gibbs triangles at temperatures marked by an arrow in Figure 2a and Figure 3a are given schematically in Figure 2b and Figure 3b. Two-phase islands (loops) appear for both situations at hand in the vicinity of the (isotropic) three-phase region; see Kahlweit and Strey.40 Nonionic microemulsion systems exhibit as a function of temperature the phase sequence 2Φ f 3Φ f 2 h Φ. At the temperature at which the three-phase body starts to

(37) Saito, H.; Shinoda, K. J. Colloid Interface Sci. 1967, 24, 10-15. (38) Shinoda, K.; Ogawa, T. J. Colloid Interface Sci. 1967, 24, 5660.

(39) Strey, R. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 182-189. (40) Kahlweit, M.; Strey, R.; Busse, G. Phys. Rev. E 1993, 47, 41974209.

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form (Tl), the surfactant-rich water phase (bottom phase of 2Φ) separates into the surfactant-rich middle phase and a surfactant-containing water phase.41 At Tl the threephase triangle forms out of a tie-line (critical tie-line) of the central miscibility gap (2Φ) in the Gibbs triangle. The water-rich end of the critical tie-line at Tl marks the critical end point of the critical line that enters the Gibbs triangle at the critical point of the binary water-surfactant (w-s) system at (much) higher temperatures. The trajectory of the critical line is nearly represented by the upper phase boundary (cloud point curve) of Figure 2a (filled circles), which marks the existence region of the o/w microemulsion at H2O/C10E4 ) 96/4. The H2O/C10E4 ratio of the compositions of the critical points does not change significantly upon addition of octane. Entering from the watersurfactant side the critical line first decreases steeply, passes through a minimum at Tm (Tm < Tl), and approaches from below its critical end point at T ) Tl. At T ) Tm the isolated two-phase region shows up for the first time as a point in the Gibbs triangle. With increasing temperature the loop fully develops (see Figure 2b) and tacks on the three-phase triangle at T ) Tl. The loop shows two critical points, one of them faces the water-surfactant side of the Gibbs triangle and the other the central miscibility gap (2Φ). Within the loop the tie-lines lie in the plane of the paper and connect a critical micelle concentration (cmc) phase (L1 phase) with a surfactant-rich phase. The lower phase boundary in Figure 2a is the solubilization-limit boundary, below which oil is expelled from the o/w microemulsion into an excess oil phase (2Φ). Figure 3 shows the situation for the oil-rich side. Above Tl the middle-phase microemulsion takes up gradually more oil and finally merges with the upper oil-rich phase at Tu. At T ) Tu the oil side of the critical tie-line of the central miscibility gap (2 h Φ) marks the critical end point of the critical line coming into the Gibbs triangle at the critical point of the binary oil-surfactant (o-s) system (Tcp(o-s) < Tl < Tu < Tcp(w-s)). The diagrams in Figures 2a and 3a are quite symmetrical but reversed with respect to temperature. In this case, the critical line is represented by the lower phase boundary of the w/o microemulsion region (haze-point curve, filled squares). It first increases steeply and passes at TM through a maximum. The upper phase boundary in Figure 3a is the solubilization-limit curve, above which water is expelled from the w/o microemulsion into an excess water phase (2 h Φ). The loop disconnects at T ) Tu from the three-phase triangle and disappears at T > TM. The tie-lines within the loop connect an oil phase with a surfactant concentration just above its cmc in the oil (L2 phase) and a surfactant-rich phase. For a more thorough description of the temperature evolution of the isothermal loops within the Gibbs triangle and the trajectories of the critical lines, including schematic diagrams, the reader is referred to ref 40. Loop formation has only been observed for middle- or long-chain nonionic surfactants. It has been attributed to structural changes (cylinder-sphere shape transformations) of the microemulsion domains, which allow for the additional degree of freedom needed to model the reentrant phase.42,43 For small-chain surfactants the critical lines descend or ascend monotonically with temperature. The bending of the phase boundaries observed for ternary nonionic microemulsion systems appears at the (41) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 38813894. (42) Menes, R.; Safran, S. A.; Strey, R. Phys. Rev. Lett. 1995, 74, 3399-3402. (43) Tlusty, T.; Safran, S. A.; Menes, R.; Strey, R. Phys. Rev. Lett. 1997, 78, 2616-2619.

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cloud point (Figure 2a) or haze point (Figure 3a) boundaries, above (below) which two microemulsion phases coexist. It does not appear at the solubilization limit boundary, at which the respective microemulsion is in equilibrium with an excess oil (Figure 2a) or water (Figure 3a) phase. Due to the opposite phase behavior of nonionic and ionic surfactant systems with respect to temperature, the situation of a nonionic surfactant on the water side (Figure 2) would correspond to the one for AOT on the oil side; see, e.g., ref 41. If the 2 h Φ (L2 + W) island would be of the same origin, it should be the upper and not the lower phase boundary that bends. The isolated two-phase region revealed in Figure 1 remains a peculiarity of the AOT system. 3. Experimental Section Choice of the AOT Microemulsion System. In this work we want to study the influence of sodium dodecyl sulfate (SDS) and octanol on the phase behavior of AOT microemulsions. To follow systematically changes in the phase behavior of systems containing more than three components, a reference point is required. Changes in the position of the phase boundaries itself might not be physically meaningful, since one could just compare different projections of the multispaced phase bodies they belong to onto the plane of the paper. In this investigation we use the isotropic three-phase body as reference. In a ternary system the three-phase region forms an isosceles triangle within the Gibbs triangle at T ) T h . In a quaternary or quinary system the 3Φ region can be obtained not only by varying the temperature but also by changing the concentration of the fourth and/or fifth component. Thus T h becomes meaningless, but the composition of the surfactant-rich middle phase at, e.g., equal amounts of water and oil is still well-defined and corresponds to a point, line, or plane in the ternary, quaternary, or quinary system, respectively.41,44 We will in the following refer to this point as X ˜ (γ˜ ,T ˜ ), where γ is the total surfactant concentration (see also the inset of Figure 5). As mentioned earlier, the AOT system shows an isotropic threephase region only upon addition of salt; see, e.g., refs 11 and 13. Gosh and Miller6 studied in detail the brine-AOT-hydrocarbon (decane-tetradecane) system. They found that if the aqueous phase (brine) contains more than 1 wt % of NaCl, the NaCl/ water ratio is not the same throughout the equilibrating phases formed. For lower NaCl concentrations and low surfactant concentration, the brine (water + NaCl) can be treated well as one component (pseudocomponent). We have therefore chosen the (water/NaCl)-AOT-decane system for our investigation with NaCl concentrations in the aqueous phase of 0.4 and 0.6 wt %. To demonstrate that the observations we made for the quaternary system hold also for the salt-free microemulsions, we have performed phase diagram studies for the system water-AOThexane with and without octanol added. Materials. Throughout the whole investigation we used AOT Microselect purchased from Fluka. The purity was checked using thin-layer chromatography. AOT (Fluka) and SDS (Henkel) were used as obtained without further purification. Decane (Fluka), hexane, 1-octanol (Fluka), and NaCl were of a high commercially available purity. The water used was doubly distilled. Phase Diagrams. Samples were prepared by mixing appropriate masses of aqueous sodium chloride solutions (w), decane (o), and surfactant (s). The concentration of salt in the aqueous phase (brine) was given by  ) NaCl/(NaCl + water). Phase diagrams were taken at equal amounts of brine and oil (R ) o/(o + w) ) 50 wt %) as a function of the surfactant concentration γ (γ ) s/(s + o + w)) and temperature. The temperatures of the phase boundaries were measured by equilibrating the samples in a thermostated water bath using a magnetic stirrer for homogenizing. The concentration of the additive (SDS or octanol) in the surfactant mixture was given by δ ) additive/(additive + AOT). To obtain the phase diagram at a given δ, samples at high surfactant concentration were consecutively diluted with equal amounts of NaCl solution and decane. When octanol was used (44) Lang, J. C., Jr.; Widom, B. Physica A 1975, 81, 190-213.

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Figure 4. Phase diagrams of the (H2O/NaCl)-decane-(AOT/ SDS) system for different SDS to AOT ratios (δ ) 0-7 wt %) as a function of the total surfactant concentration (AOT + SDS) at R ) 50 wt % and  ) 0.6 wt %. The quaternary system (δ ) 0) is presented by filled squares. The inset shows along which line the phase diagram has been taken. as additive, samples were also made with a constant octanol/ decane ratio. In this case, a solution of fixed β (β ) octanol/ (octanol + decane)) was prepared. This solution was used for the preparation of concentrated microemulsions and their successive dilutions. The phase diagrams for the salt-free system were obtained by subsequently adding water to samples with a given concentration of AOT or AOT and octanol, respectively, in hexane. Phase separation took place within a few minutes up to several hours. The positions of the phase boundaries were reproducible on both raising and lowering the temperature. Liquid crystalline phases (LR) were detected using cross polarizers.

4. Results and Discussion In this section we present results of a phase diagram study on the controlled addition of SDS and octanol to the quaternary microemulsion system (water/NaCl)-decaneAOT at equal amounts of brine and oil. This study has been performed to analyze the contrarieties in the AOT phase diagrams published and to clarify the origin of the isolated two-phase region observed. To understand the influence of octanol in more detail, we also studied the salt-free system water-hexane-AOT and compared our findings with those of the nonionic system water-octaneC10E5-octanol. In the last part we discuss the impact of our observations on the physicochemical properties of AOT microemulsions. Addition of SDS. To study the influence of SDS on the phase behavior of the quaternary system (water/ NaCl)-decane-AOT, two different brine concentrations () were used. Figure 4 shows the phase diagrams at equal amounts of brine ( ) 0.6 wt %) and oil (R ) 50 wt %) as a function of the total surfactant concentration (γ) and temperature for different amounts of SDS added (δ ) SDS/ (SDS + AOT) ) 0-7 wt %). The inset indicates along which line in the Gibbs triangle the diagram has been taken. The upper phase diagram with the filled squares corresponds to the “pure” quaternary system (δ ) 0). At

Sager

high surfactant concentrations (e.g., γ ) 12 wt %) the phase sequence 2h Φ f 1Φ f 2Φ is observed with increasing temperature. No liquid crystalline phases (LR) form within the 1Φ region. Upon dilution (toward smaller γ) the temperature interval for the 1Φ region narrows. At surfactant concentrations γ < 6 wt % three coexisting phases form. The crossing X ˜ (γ˜ ,T ˜ ), see inset of Figure 5, gives the minimum amount of surfactant at which microemulsions of equal amounts of brine and oil (R ) 50 wt %) form and thus the position of the surfactant-rich middle phase at R ) 50 wt % in the T,γ space. We will in the following refer to this point as the “fish tail end point”. Upon addition of SDS the phase boundaries and therefore T ˜ are shifted to lower temperatures. While the fish tail end point lies for the quaternary system (δ ) 0) at 40 °C, it is shifted down to 14 °C at δ ) 7 wt %. The shape of the boundaries remains unchanged. In Figure 5T ˜ is given as a function of δ for  ) 0.6 wt % and  ) 0.4 wt %. T ˜ depends almost linearly on δ, independent of the salt concentration () necessary to obtain the three-phase body. Addition of the hydrophile SDS makes the surfactant mixture in the interface more hydrophilic. Thus phase inversion and the formation of the three-phase body occur at lower temperatures. Addition of Octanol. Addition at Constant AOT/ Octanol Ratio (δ ) 0-5 wt %). Figure 6 shows the phase diagrams upon addition of octanol with δ ) octanol/ (octanol + AOT) ) 0-5 wt % for a NaCl concentration of  ) 0.4 wt %. At this salt concentration a lamellar phase (LR) forms within the 1Φ region, which is not shown for simplicity. At the dashed lines the upper phase boundary (1Φ f 2Φ) coincides with the upper boundary of the LR phase. For δ ) 0-1 wt %, one of the phases that coexist in the three-phase region is lamellar (LR). The lamellar phase does not coincide with the lower boundary, to which we want to draw the reader’s attention. With an increasing amount of octanol added, the phase boundaries and the fish tail end point are shifted to higher temperatures. T ˜ moves from 24.5 °C for δ ) 0 (quaternary system at  ) 0.4 wt %, filled squares) to 30 °C for δ ) 5 wt %. Addition of octanol changes the shape of the lower phase boundary of the 1Φ region significantly. The tendency to bend increases with the amount of octanol added. The diagram at δ ) 3 wt % reveals clearly a reentrant phase, showing the phase sequence 1Φ f 2 hΦ f 1Φ f 2Φ with decreasing total surfactant concentration (γ) at 30 °C. In Figure 5 the dependence of T ˜ on the amount of octanol added (δ ) 0-5 wt %) is given for both salt concentrations. As for the case when SDS was used as additive, T ˜ depends almost linearly on δ. For octanol dT ˜ /dδ depends slightly on the salt concentration used. Upon addition of octanol the “surfactant mixture” becomes more hydrophobic. The tendency of the surfactant(s) to leave the oil-rich phase is thus reduced. Phase inversion and three-phase body formation take place at higher temperatures. The changes in the shape of the 1Φ region (adjacent to the three-phase body) occurring with increasing octanol concentration indicate that the concentration of octanol in the interfacial surfactant layer changes upon dilution. Contrarily to SDS, octanol dissolves to a certain extent in the bulk-oil phase. If, for the cut shown in the inset of Figure 6, samples were prepared at constant AOT/octanol ratio, then by dilution of concentrated microemulsion samples (decreasing γ) with pure decane (and brine), the octanol/decane ratio changes in the samples. To retain its equilibrium value in the bulk-oil phase of the microemulsion (continuous phase), the octanol has to leave the interfacial surfactant layer.

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Figure 5. Dependence of T ˜ on δ for SDS and octanol at  ) 0.4 wt % and  ) 0.6 wt %. The schematical diagram shows how the fish tail end point X ˜ (γ˜ ,T ˜ ) is defined.

Figure 6. Phase diagrams of the (H2O/NaCl)-decane-(AOT/ octanol) system for different octanol to AOT ratios (δ ) 0-5 wt %) as a function of the total surfactant concentration (AOT + octanol) at R ) 50 wt % and  ) 0.4 wt %. The inset shows along which line the phase diagram has been taken.

At a given temperature the interfacial layer changes, and thus its hydrophile-lipophile state, if γ decreases. Since it becomes less hydrophobic, the 1Φ region is reentered at low γ. It is therefore the partitioning of the octanol between the interface and the bulk-oil phase that causes the formation of the isolated two-phase region (L2 + W) observed in the literature. The size of the island depends on the amount of octanol present. Addition at Constant Octanol/Decane Ratio (β ) 1 wt %). If octanol partitions between the interface and the bulk-oil phase, the shape of the phase boundary should depend on the way the dilution is performed. Figure 7 reveals the phase diagram at fixed octanol/decane ratio. To obtain this phase diagram, a microemulsion was diluted with brine and a solution of a given amount of octanol in decane (β ) octanol/(octanol + decane) ) 1 wt %); see inset of Figure 7. The lower phase boundary (2 h Φ f 1Φ) rises monotonically with decreasing γ. While no bending is observed for the lower boundary, the upper boundary (1Φ f 2Φ) bends slightly just adjacent to the fish tail end point. The three-phase region is not tilted downward, as in Figure 6, but bends slightly upward. Upon dilution, octanol is in this case added with the decane. If the concentration of octanol in the octanol/decane solution added is higher than the equilibrium concentration of

Figure 7. Phase diagrams of the (H2O/NaCl)-(decane/octanol)-AOT system for a constant octanol-to-decane ratio (β ) 1 wt %) as a function of the total surfactant concentration (AOT) at R ) 50 wt % and  ) 0.4 wt %. To obtain the phase diagram samples were diluted with equal amounts of water and a solution of octanol in decane (β ) 1 wt %). The inset shows along which line the phase diagram has been taken.

octanol in the bulk-oil phase of the microemulsion, the additional octanol accumulates in the interface. The interface thus becomes more hydrophobic as γ is decreased. At γ > 8 wt % an LR phase forms within the 1Φ region at higher temperatures. At γ ) 8.7 wt % the composition of the microemulsion corresponds to the one at γ ) 9.2 wt % at δ ) 5 wt % in Figure 6. At low γ the microemulsion phase is colored and appears deeply red in and close to the three-phase region indicating the presence of very large structures. Addition of Octanol to the Salt-Free System Water-AOT-Hexane. To show that the observations concerning the appearance of the isolated two-phase region (L2 + W) and the reentering phase formation do not only hold for AOT microemulsions prepared with brine (quaternary system), Figure 8 displays phase diagrams for the system water-AOT-hexane with and without octanol added. This also allows a direct comparison with the phase diagram by Kunieda and Shinoda9 and phase diagram studies by Delord and Larche.32 The phase diagrams were performed by adding water to samples of a fixed concentration of AOT in hexane (AOT/hexane ) 0.074) for the octanol-free system (filled circles) and AOT and octanol in hexane ((AOT + octanol)/hexane ) 0.075) for an octanol-

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Figure 8. Phase diagrams of the H2O-hexane-(AOT/octanol) system as a function of the water content at AOT/hexane ) 0.074 (filled circles) for the ternary system and at constant (AOT + octanol)/hexane ) 0.075 (open circles) for an octanolcontaining system with δ ) 6.42 wt %. To obtain the phase diagram water was added to a solution of AOT or AOT + octanol, respectively, in hexane. The inset shows along which line the phase diagram has been taken.

containing system with δ ) 6.42 wt % (open circles). The inset indicates along which line in the Gibbs triangle the diagram has been taken. For this cut the surfactant/oil ratio and therefore the ratio between the total amount of interface and the oil-continuous phase stays constant upon water addition. Hexane was used as oil instead of decane, since without salt added the lower phase boundary lies for microemulsions prepared with decane below 2 °C. Figure 8 displays the existence regions of the w/o microemulsions (L2 phase) for the ternary and the octanolcontaining system. Below the lower phase boundary water is expelled (2 h Φ) and the microemulsion samples separate into an L2 phase in equilibrium with an almost pure water phase (L2 + W). Above the upper phase boundary two oil-continuous microemulsions form at water contents smaller than 12 wt %, while an LR phase in equilibrium with an L2 phase is stable at higher water contents. For both cases the lower boundary shifts to lower temperatures at water content larger than 12 wt %. For the octanolcontaining system (open circles) the lower phase boundary shows a pronounced maximum below a water content of 15 wt %. In the case of a salt-free microemulsion the ionic strength inside the waterpool of the microemulsion droplets changes with the amount of water added within the L2 phase region. At low water content and high ionic strength the effective charge of the ionic surfactant headgroups is screened, while for large droplets electrostatic headgroup repulsion becomes important. This changes the spontaneous curvature of the interfacial surfactant film or, in other words, its hydrophile/lipophile status. For a general description of the influence of electrostatics on the elastic properties of the interfacial surfactant film, such as the radius of spontaneous curvature and the rigidity constant, the reader is referred to refs 45-48. As a result the interfacial layer becomes more hydrophilic for larger water content and the L2 phase

Sager

Figure 9. Phase diagram for the nonionic system (H2O)octane-C10E5-octanol system (see ref 50). The ternary system is represented by empty circles and a dotted line. The quaternary systems (fully drown lines) are shown for (A) constant octanol to C1OE5 ratio of δ ) 10 wt % (filled circles) and (B) constant octanol-to-octane ratio of β ) 2 wt % (open squares). The schematic diagrams show along which line the diagrams have been taken and the definition of X ˜ (T ˜ ,γ˜ ) displaying the different temperature behavior of nonionic surfactants. For case (A) s stands for C10E5 + octanol while for case (B) o stands for octane + octanol.

forms at lower temperatures, which can be seen for the ternary system (filled circles) in Figure 8 at water contents larger than 12 wt %. The lower phase boundary of the octanol-containing system shows a pronounced maximum. Since the cut at (AOT + octanol)/hexane ) 0.075 is performed close to the oil corner of the Gibbs triangle, it cuts at the small end through the isolated two-phase region (L2 + W); see Figure 1a. Contrarily to the phase diagram displayed in Figure 6, the octanol/hexane ratio does not change in this cut. In this case, the changes in the partitioning of octanol between the interface and the bulk-oil phase, which lead to the bending of the lower phase boundary, can only be explained by the differences of the hydophile/lipophile status of the interfacial surfactant layer, caused by the differences in the electrostatics upon addition of water. Phase diagram studies performed at equal amounts of water and hexane49 (R ) 50 wt %) reveal the bending of the lower phase boundary to the same extent as the saltcontaining microemulsion systems shown in Figure 6. Comparison with Quaternary Nonionic Microemulsion Systems. To show the partitioning effect of (a medium-chain) alcohol on the phase boundaries of the adjacent three- and one-phase regions more generally, we will examine a comparable situation for a nonionic system. Figure 9 shows phase diagrams for the nonionic system water-octane-pentaethylene glycol mono-n-decyl ether (C10E5)-octanol at equal amounts of water and oil (R ) 50 wt %); see ref 50. The inset demonstrates along which cuts the diagrams have been performed. The upper diagram (dashed line) reveals the phase diagram for the ternary system. With increasing temperature the phase sequence 2Φ f 1Φ f 2 h Φ (γ ) 20 wt %) is observed. The (45) Winterhalter, M.; Helfrich, W. J. Phys. Chem. 1988, 92, 68656867. (46) Lekkerkerker, H. N. W. Physica A 1989, 159, 319-328. (47) Daicic, J.; Fogden, A.; Carlsson, I.; Wennerstro¨m, H.; Jo¨nsson, B. Phys. Rev. E 1996, 54, 3984-3998. (48) Fogden, A.; Daicic, J. Colloids Surf. A 1997, 130, 157-165. (49) Sager, W. F. C. Unpublished phase diagrams. (50) Kahlweit, M. Tenside, Surfactants, Deterg. 1993, 30, 1-7.

Impurity Influence on the AOT Phase Behavior

three-phase body is not tilted and the adjacent 1Φ region is not bent, exhibiting an almost symmetrical shape with respect to temperature. Case A (filled circles) reflects the situation for the quaternary system when the microemulsion is only diluted with pure oil, i.e., for constant alcohol/ surfactant ratio (δ ) 10 wt %). At γ ) 20 wt % the 1Φ region appears at lower temperatures with respect to the ternary system, since the surfactant mixture in the interface contains the more hydrophobic alcohol. Due to the opposite temperature dependence of ionic and nonionic microemulsion systems (see, e.g., ref 41), the tendency of the surfactant(s) to leave the water-rich phase is increased. Upon dilution (decreasing γ) partitioning of the alcohol lessens the alcohol concentration in the interface. The interfacial layer becomes less hydrophobic and the phase boundaries are shifted to higher temperatures. The threephase region is significantly tilted upward, and the upper phase boundary of the 1Φ region (1Φ f 2 h Φ) bends, revealing a reentrant phase. At high dilution (γ < 3 wt %) the interface is so lean of alcohol that the phase boundaries of the three-phase body approach those of the ternary system. In case B (empty squares) the quaternary microemulsion is diluted with a solution of octanol in octane. The phase diagram reflects the situation for a constant alcohol/octane ratio (β ) 2 wt %). Dilution with the octanol-containing oil solution leads to an enrichment of alcohol in the interface at low γ. In case B it is thus the low γ region that differs mostly from the ternary system. Since the interface contains the more hydrophobic alcohol, the phase boundaries of the 3Φ phase region are shifted to lower temperatures. The three-phase body is thus tilted downward and the lower phase boundary of the 1Φ region (2Φ f 1Φ) bends. The comparison with the nonionic microemulsion system manifests the origin of the isolated two-phase region observed for AOT microemulsions. Despite the opposite temperature behavior the boundary that bends is for both the ionic and the nonionic system, the one at which water is expelled (1Φ f 2 h Φ). A reentrant phase can thus be observed at the solubilization boundary when the surfactant (mixture) contains a medium- or long-chain alcohol. Bending of the solubilization boundary shows that for both types of surfactant the surfactant/alcohol mixture cannot be treated as a pseudocomponent and that the phase behavior should instead be presented in a phase tetrahedron (for the representation of four-component systems see, e.g., refs 51 and 52). Impact on Microemulsion Studies. At the beginning of this section we have seen that small amounts of surfaceactive impurities such as the hydrolysis products of AOT have a significant influence on the AOT phase behavior. Changes in the positions of phase boundaries can naturally be found back in the properties of the microemulsion systems studied. In this part we want to discuss the impact our investigation has on studies of the physicochemical properties of AOT microemulsions. Over the last 2 decades AOT microemulsions have been investigated with all the diverse techniques available to study these systems, including static and dynamic light scattering, small-angle X-ray (SAXS) and neutron (SANS) scattering, NMR self-diffusion, fluorescence quenching, tensiometry, viscometry, dielectric permittivity, conductometry, ellipsometry, electrooptic birefringence, and (51) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1991, 95, 53445352. (52) Bourell, M.; Schechter, R. S. In Microemulsions and Related Systems: Formulation, Solvency, and Physical Properties; Marcel Dekker: New York, 1988; pp 247-256.

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neutron spin-echo measurements (for a recent overview see refs 17, 18, and 53). Due to the model character of the ternary AOT system, researchers felt engendered to study AOT microemulsions extensively in order to gain a more fundamental insight and to draw generally valid conclusions. AOT w/o microemulsion droplet phases (L2 phase) have thus been investigated, mainly by scattering techniques, to test liquid state theories and to study critical behavior (see, e.g., refs 2 and 54). When the Helfrich energy,55,56 originally proposed to describe the nonspherical shapes of membranes and vesicles, was applied to microemulsions, the AOT system was one of the first studied to determine the elastic moduli or rigidity constants, namely, the bending modulus κ and the saddlesplay modulus κj, using (shape) fluctuation measurements of the droplets57,58 or of the (macroscopic) interface.59,60 In particular, AOT droplet phases and bicontinuous monoand bilayer as well as liquid crystalline phases have been investigated to obtain a better understanding of the stability of structurally differing phases and to expound the phase diagrams of microemulsion systems.61,62 But one should keep in mind that the presence of surfaceactive impurities, e.g., AOT hydrolysis products, can affect studies on AOT microemulsions in different ways. We first want to concentrate on the influence of upward or downward shifts of the phase boundaries caused by the impurities. Within the one-phase region of a microemulsion system (see L2 phase in Figure 1b) structural changes occur between the lower (here solubilization limit) and the upper (cloud point) phase boundary. As mentioned earlier, single spherical microemulsion droplets are stable close to the solubilization limit boundary. With increasing temperature, droplet aggregation occurs in the L2 phase of AOT microemulsions leading to the formation of large(r) clusters of droplets or even to channel formation (cylinder forming) close to the cloud point boundary.14 Experiments to determine the size and shape of the microemulsion droplets and to study droplet-droplet interactions have often been performed at constant temperature. If due to surface-active impurities the phase boundaries are shifted to lower (monoester, SDS) or higher (octanol) temperatures, the state of aggregation or the general structure at constant temperature is likely to be different. At a given temperature the addition of SDS favors aggregation.63 Upon addition of octanol, on the other hand, the phase boundaries are shifted upward, bringing the microemulsion studied closer to its lower (solubilization) boundary, at which isolated droplets form. Disfavoring of aggregation upon addition of decanol to water-isooctane-AOT microemulsion has been found by Nazario et al.64 using dynamic light scattering and conductometry. Aggregation phenomena, such as dimer (53) Langevin, D. In Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989; pp 13-41. (54) Chen, S. H.; Rouch, J.; Tartaglia, P. Physica A 1994, 204, 134151. (55) Helfrich, W. Z. Naturforsch. C 1973, 28, 693-703. (56) Safran, S. A. In Statistical Thermodynamics of Surfaces, interfaces, and Membranes; Addison-Wesley: Reading, MA, 1994; pp 248-263. (57) Borkovec, M.; Eicke, H.-F. Chem. Phys. Lett. 1989, 157, 457461. (58) Huang, J. S.; Milner, S. T.; Farago, B.; Richter, D. Phys. Rev. Lett. 1987, 59, 2600-2603. (59) Binks, B. P.; Meunier, J.; Abillon, O.; Langevin, D. Langmuir 1989, 5, 415-421. (60) Kellay, H.; Binks, B. P.; Meunier Phys. Rev. Lett. 1993, 70, 14851488. (61) Kellay, H.; Meunier, J. J. Phys.: Condens. Matter 1996, 8, A49A64. (62) Kellay, H.; Binks, B. P.; Hendrikx, Y.; Lee, LT.; Meunier, J. Adv. Colloid Interface Sci. 1994, 49, 85-112. (63) Smeets, J. Unpublished SAXS and viscosity data.

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and higher agglomerate formation, contribute significantly to the virial coefficients of osmotic pressure and the (collective) diffusion coefficient.18 Even if the formation of (small) droplet clusters, e.g., dimers, does not show up in the experiments performed, changes in the virial coefficients and thus the droplet interaction will certainly manifest themselves. In particular conductivity measurements to study percolation phenomena can be misleading. With increasing temperature the conductivity of L2 phase AOT microemulsions can exhibit a steep increase over a few orders of magnitude within a very small temperature interval; see, e.g., ref 65. Due to temperature shifts of the boundaries one might, at a given temperature, deal with a system just below (low conductivity) or just above (high conductivity) the percolation threshold depending on the kind and amount of impurities. Nazario et al.64 found a rise in the percolation temperature with increasing concentration of decanol added. Another important impact on AOT microemulsion studies is the partitioning of alcohol between the interfacial layer and the bulk-oil phase. A number of single droplet properties such as the radius, shape fluctuations, and polydispersity, are obtained by extrapolation of the value of the quantity measured to infinite dilution. Upon dilution of an octanol-contaminated microemulsion with pure oil, the properties of the interfacial layer and thus of the microemulsion droplets themselves change. Accordingly the microemulsion at infinite dilution can differ in, e.g., the interfacial composition, hydrophile-lipophile balance, and the elastic properties, from that at higher concentrations. Experimentally accessible quantities, which are related to the bending elasticity, e.g., interfacial tension and fluctuations, are especially sensitive to changes in the alcohol concentration. The influence of octanol on the interfacial tension between a w/o AOT microemulsion and an excess water phase has been studied in detail by Aveyard et al.66 In measuring the shape fluctuations of AOT microemulsion droplets (L2 phase) by SANS and neutron spin-echo techniques Farago et al.67 found that both elastic moduli κ and κj decrease significantly if octanol is added. 5. Conclusions In this investigation we studied systematically the influence of SDS and 1-octanol on the phase behavior of (NaCl/water)-decane-AOT microemulsions to mimic the effect of AOT hydrolysis. We demonstrated that even small amounts of SDS and octanol lead to drastic shifts of the phase boundaries. This indicates that minor amounts of surface-active impurities originating from the synthesis or produced by hydrolysis of AOT can cause the differences and inconsistencies in the phase diagrams published. Since both additives act in different directions, T ˜ or the HLB temperature cannot be used as a purity criterion, as suggested earlier.9 Addition of SDS shifts the three-phase body and thus T ˜ , which we used as reference, to lower temperatures. For both brine concentrations investigated ( ) 0.4 and  ) 0.6 wt %) an almost linear dependence of T ˜ on δ, the weight fraction of SDS in the “surfactant mixture”, is found yielding dT ˜ /dδ ) -3.7°. Expressed in molar ratios, only (64) Nazario, L. M. M.; Hatton, T. A.; Crespo, J. P. S. G. Langmuir 1996, 12, 6326-6335. (65) Sager, W.; Sun, W.; Eicke, H.-F. Prog. Colloid Polym. Sci. 1992, 89, 284-287. (66) Aveyard, R.; Binks, B. P.; Mead, J. J. Chem. Soc., Faraday Trans. 1 1986, 1755-1770. (67) Farago, B.; Richter, D.; Huang, J. S.; Safran, S. A.; Milner, S. T. Phys. Rev. Lett. 1990, 65, 3348-3350.

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one SDS molecule per 100 AOT molecules shifts T ˜ by 2.4°. To point out the significance of the influence of SDS on AOT phase diagrams, we would like to refer again to Figure 5. At 4.4 wt % of SDS in the surfactant mixture the downward shift of T ˜ observed is of the same order as the one resulting from changing the salt concentration in the aqueous phase from  ) 0.6 wt % to  ) 0.4 wt %. A comparable dependence of the fish tail end point on the amount of SDS added has been reported by Kahlweit for the (NaCl/water)-hexane-AOT system.68 This suggests that for SDS dT ˜ /dδ does not depend on the oil used. Kunieda and Shinoda9 replaced 10 wt % of AOT with SDS in a microemulsion containing 10 wt % of surfactant and found a congruent downward shift of the lower phase boundary of the L2 phase of 40° using cyclohexane as oil. The shape of the phase boundaries does not seem to change with the amount of SDS added. This indicates that the AOT/SDS ratio in the interfacial film does not change upon dilution and that a mixture of the two surfactants can be treated as a pseudocomponent in the δ range investigated. In fact, phase diagrams of the salt-free system water-isooctane-(AOT/SDS) with δ ) 5 wt % match exactly on the oil-rich side with those of the pure water-decane-AOT system.69 Addition of 5 wt % SDS would thus correspond to a change from isooctane to decane. At constant temperature the tendency of the surfactant(s) to leave the oil-rich phase increases with the amount of SDS added. The more hydrophilic SDS thus shifts the phase boundaries of the three- and onephase regions to lower temperatures. If, on the other hand, the more hydrophobic octanol is added, T ˜ moves upward. The influence of δ on the position of T ˜ is less strong and depends slightly on the brine concentration used (see Figure 5). For  ) 0.4 wt % the upper phase boundary (1Φ f 2Φ) coincides at low γ for δ ) 0-1 wt % with the upper boundary of the LR phase. T ˜ could therefore only be obtained by extrapolation of the upper phase boundary to low surfactant concentrations (γ). For  ) 0.4 wt % an upward shift of 1.1° per δ has been observed. This corresponds to a temperature shift of 0.32° if one octanol molecule is added to 100 AOT molecules. After leaving a w/o microemulsion for 1 month at 25 °C, Delord and Larche32 have found 0.1 mol of octanol per 1 mol of AOT originally present, corresponding to δ ) 3 wt %. By replacing 10 wt % of AOT with decanol, Kunieda and Shinoda9 found an upward shift of 25° for a microemulsion containing 10 wt % of surfactant and cyclohexane as oil, which is in agreement with what we found. Upon addition of octanol the shape of the lower phase boundary (2h Φ f 1Φ) changes significantly. For δ ) 3 wt % at 30 °C the phase sequence 1Φ f 2 h Φ f 1Φ f 2Φ can be observed with decreasing γ, exhibiting clearly a reentrant phase. The two points at which a line at 30 °C crosses the lower phase (2Φ f 1Φ) boundary (see Figure 6 and also Figure 1b) correspond to the boundaries of the isolated one-phase region at low and high surfactant concentration for R ) 50 wt % and T ) 30 °C. Since at δ ) 3 wt % the isolated two-phase island is already fully developed, Delord and Larche32 found significant changes in the phase behavior after leaving the samples for 1 week at 25 °C. They observed a separation of an originally homogeneous L2 phase into an upper w/o microemulsion and a bottom water phase with time. The progressing change in the shape of the lower phase boundary with increasing δ is a clear sign that AOT and octanol cannot be treated as a pseudocomponent. Octanol partitions (68) Kahlweit, M. J. Phys. Chem. 1995, 99, 1281-1284. (69) Sager, W. F. C. Unpublished phase diagrams.

Impurity Influence on the AOT Phase Behavior

between the interface and the bulk-oil phase. As a consequence, the AOT/octanol ratio is not the same in the top and bottom phases that form. Due to the partitioning of octanol between the interface and the bulk-oil phase, the surfactant mixture in the interfacial film becomes more hydrophobic upon alcohol addition and the bulk-oil phase becomes less hydrophobic. Both changes let T ˜ rise. Controlled addition of octanol at constant AOT/octanol and decane/octanol ratios clarified the origin of the isolated two-phase region (L2 + W) in the Gibbs triangle first reported by Kunieda and Shinoda9 and presented in Figure 1a. In particular the comparison with the quaternary nonionic system demonstrates that partitioning of octanol is responsible for the loop formation and the reentrant phase in AOT phase diagrams observed in the literature. The characteristic findings on the bending of the lower phase boundary do not depend on the salt concentration investigated and are observed also for the salt-free system. Significant bending of the lower phase boundary of the L2 phase (solubilization limit boundary) can thus be used for AOT microemulsions as a reliable criterion for the presence of alcohol, e.g., 2-ethylhexanol from AOT hydrolysis. Explaining the origin of the two-phase island in Shinoda’s phase diagram is a first step in understanding AOT phase diagrams. Knowing the effect of alcohol and changes in the phase behavior possibly occurring with time, it should now be possible to investigate the effect of ionic strength (and thus electrostatics) on the stability of the different phases formed. Of special interest are the stability regions of L3 and LR phases. It is not at all clear yet how the LR phase intersects precisely with the central two-phase region and how the different phase equilibria thereby obtained develop with temperature and brine

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concentration. First attempts have been performed by Gosh and Miller6 in 1987 and Assih et al.12 in 1984, but a much more complicated phase behavior is to be expected. AOT could serve as an ideal system to verify and understand trends that have been observed for ionic surfactants in multicomponent systems and systems in which partitioning of one or more components between the interface and the bulk phase causes unnecessary complications (see, e.g., refs 70 and 71). A still open question is whether a reentrant phase at the haze-point curve and a real two-phase loop showing two critical points at a given temperature can also form with ionic surfactants, as has been observed for nonionic surfactants. Acknowledgment. A substantial part of the experimental work has been performed in the Department of Professor M. Kahlweit at the Max-Planck Institute for Biophysical Chemistry, Go¨ttingen. The author thanks him for his support. She would further like to thank M. Borkovec, R. Strey, and G. J. M. Koper for continuous discussions on the AOT phase behavior, T. Lieu for his (experimental) assistance with some of the phase diagrams, J. Da Corte and D. Luckmann for drawing part of the figures, and H. Verweij for his comments on the manuscript. This work has been supported by The Netherlands Foundation for Chemical Research (SON) in collaboration with The Netherlands Technology Foundation (STW). LA9709608 (70) Kunieda, H.; Nakamura, K. J. Phys. Chem. 1991, 95, 88618866. (71) Guerin, G.; Bellocq, A. M. J. Phys. Chem. 1988, 92, 2550-2557.