Effect of Added Salt and Poly(ethylene glycol) on the Phase Behavior

The phase behavior of AOT−water−NaCl−poly(ethylene glycol) (PEG; Mw = 20 000)−isooctane mixtures at equal volumes of water and oil has been ...
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Effect of Added Salt and Poly(ethylene glycol) on the Phase Behavior of a Balanced AOT-Water-Oil System Maryse Maugey and Anne-Marie Bellocq* Centre de Recherche Paul Pascal, CNRS, Avenue Albert Schweitzer, 33600 Pessac, France Received April 12, 1999. In Final Form: August 2, 1999 The phase behavior of AOT-water-NaCl-poly(ethylene glycol) (PEG; Mw ) 20 000)-isooctane mixtures at equal volumes of water and oil has been systematically explored as a function of AOT, NaCl, and PEG concentrations. The effect of adding PEG is in many respects similar to that of adding salt although the changes are less pronounced. Each additive, taken separately, shifts the phase behavior to higher temperature, promotes the formation of bicontinuous microemulsions, and induces a lamellar-to-bicontinuous microemulsion phase transition. The simultaneous presence of both additives enhances the effect of salt. These phase behavior results suggest that the two variables salt and PEG act in the same direction on the spontaneous curvature C0, the bending rigidity κ, and the Gaussian bending modulus κj of the AOT monolayer.

I. Introduction Mixtures of water-oil-surfactant lead to a wide variety of structures corresponding to different topologies of the surfactant interfaces.1 Experimentally, globular and bicontinuous microemulsions as well as lamellar phases where water and oil build periodic arrays of alternating water and oil layers separated by a surfactant layer are commonly observed. The type of microstructure is closely related to the sign of the spontaneous curvature C0, which describes the tendency of the surfactant film to bend toward either the water (C0 > 0) or the oil (C0 < 0). Roughly when C0 is sufficiently large, a droplet structure is formed, and when C0 vanishes, bicontinuous and lamellar structures are obtained. These structures may occur either as single phases or in large parts of the phase diagrams in two/three-phase coexistence. The characteristic structures of the microemulsions associated with the spontaneous curvature of the interface reflect themselves in the equilibrium phase diagram. Since the pioneering work of Winsor,2 one knows that a mixture containing a sufficient amount of surfactant and equal volumes of water and oil separates into a water-rich microemulsion in equilibrium with an excess oil phase (Winsor I, 2), an oil-rich microemulsion in equilibrium with an excess aqueous phase (Winsor II, 2 h ), or three phases with a middle-phase microemulsion in simultaneous equilibrium with excess oil and water phases (Winsor III, 3). Typically, globular structures are found in Winsor I (oil-in-water (O/W) droplets) and Winsor II (water-in-oil (W/O) droplets) equilibria and bicontinuous structures in Winsor III states. Systematic phase progression 2-3-2 h and curvature inversion are obtained when a suitable tuning parameter is varied. For the balanced systems with C0 ) 0, the Winsor III equilibria exist at low values of the surfactant concentration γ; as γ is increased, a bicontinuous microemulsion occurs at γ˜ m, and at larger values of γ, the system undergoes a first-order phase transition to a lamellar phase. One very successful approach to understanding the phase behavior of microemulsions has been to describe these systems as ensembles of interfacial surfaces with (1) For a review see for instance: Micelles, membranes, microemulsions, and monolayers; Gelbart, W. M., Ben-Shaul, A., Roux, D., Eds.; Springer-Verlag: Berlin, 1994. (2) Winsor, P. A. Solvent properties of Amphiphilic compounds; Butterworths: London, 1954.

conformations controlled by a bending energy of the form proposed by Helfrich.3 In this approach, the behavior of the system is determined by a competition between the entropy of mixing of the oil and water domains and the curvature bending energy, which is characterized by three elastic parameters: the spontaneous curvature C0 and two bending moduli κ and κj associated with the mean curvature C and the Gaussian curvature G, respectively.4,5 The modulus κj is coupled to the topology of the surface and plays an important role in the phase transitions where the topology of the surface is changed: positive κj favors connected structures such as bicontinuous microemulsions, while negative κj favors lamellar phases or spherical droplets. The phenomenological interfacial model for microemulsion phase behavior of Andelman et al.,4 developed in the case κj ) 0, predicts that the efficiency of the microemulsion and the competition between the lamellar and microemulsion phases are controlled by the bending modulus κ. For small κ (very flexible layers), the lamellar phase is stable only for very high γ and the microemulsion is stable for a large range of concentration, though the minimum amount of surfactant needed to form a balanced microemulsion, γ˜ m, is large. When κ is increased, γ˜ m decreases, and in parallel, the lamellar phase is more stable and occurs at smaller γ. Finally, for larger values of κ, the bicontinuous microemulsion is predicted to disappear and complex polyphasic equilibria should be observed. Experimental results show that the competition between LR and the microemulsion is qualitatively correctly described by theory. However, quantitatively there are differences which reveal the necessity to take into account the Gaussian bending elastic modulus κj.6 In practice, the bending properties of interfacial surfactant layers can be influenced by various additives. In the case of ionic surfactants, alcohol, salt, and temperature are parameters which are usually used to monitor the curvature. The electrostatic contribution to the bending moduli κ and κj has been studied theoretically quite extensively.7,8 It depends on the surfactant charge, on the (3) Helfrich, W. Z. Naturforsch. 1973, 28c, 693. (4) Andelman, D.; Cates, M. E.; Roux, D.; Safran, S. J. Chem. Phys. 1987, 87, 7229. (5) Daicic, J.; Olsson, U.; Wennestrom, H. Langmuir 1995, 11, 2451. (6) Kegel, W. K.; Lekkerkerker, N. W. J. Phys. Chem. 1993, 97, 11124; Colloids and Surf., A 1993, 76, 241.

10.1021/la990438g CCC: $18.00 © 1999 American Chemical Society Published on Web 11/10/1999

Phase Behavior of a AOT-Water-Oil System

ionic strength, and on the distance between the surfactant layers. Electrostatics makes the surfactant layer stiffer and favors the formation of disconnected aggregates. Also, when salt is added to charged membrane systems, both moduli κ and κj are changed. Calculations indicate that the addition of salt leads to a decrease of κ and an increase of κj. An alternative way to change the curvature moduli is to adsorb a polymer on the surfactant layer. This problem has recently been the focus of several theoretical papers.9-12 Polymer adsorption was first discussed by De Gennes.9 In all cases, reversible and irreversible adsorption of the polymer, the calculated polymer contribution to the bending modulus κ is negative and the contribution to κj is positive.10-12 Thus, the adsorbed polymer should make the surfactant layer less stiff and favor connected structures. These effects begin to be investigated experimentally in lamellar phases and also in microemulsions.13-28 In the particular case of AOT-water-NaCl-oil systems, Kunieda and Shinoda29 and Kalhweit et al.30 have extensively studied the combined effects of salinity and temperature on the phase behavior. Moreover, the elastic bending moduli κ and κj of the AOT monolayer at the water-oil interface have been measured as a function of the oil chain length and salinity.31 In a recent study, Bellocq reported the influence of the adsorption of the water-soluble polymer poly(ethylene glycol), PEG, on the phase behavior of the AOT-water-isooctane system.32 It has been found that the polymer solubility in W/O droplets is strongly dependent on the size of the polymer coil, RG, relative to the water core radius, RW, of the droplet. Moreover, the polymer enhances the solubilization of water in oil and induces, like salt, a decrease of the spontaneous (7) For a review see: Andelman, D. In Structure and dynamics of membranes; Lipowski, R., Sackmann, E., Eds.; Elsevier Science: New York, 1995; p 603. (8) Daicic, J.; Fogden, A.; Carlsson, I.; Wennerstro¨m, H.; Jonsson, B. Phys. Rev. E 1996, 54, 3984. (9) De Gennes, P. G. J. Phys. Chem. 1990, 94, 8407. (10) Podgornik, R. Europhys. Lett. 1993, 21, 245. (11) Brooks, J. T.; Marques, C. M.; Cates, M. E. J. Phys. II 1991, 1, 673. Brooks, J. T.; Cates, M. E. J. Chem. Phys. 1993, 99, 5467. (12) Clement, F.; Joanny, J. F. J. Phys. II 1997, 7, 973. (13) Ligoure, C.; Bouglet, G.; Porte, G. Phys. Rev. Lett. 1993, 71, 3600. Ligoure, C.; Bouglet, G.; Porte, G.; Diat, O. J. Phys II 1997, 7, 473. (14) Porcar, L.; Ligoure, C.; Marignan, J. J. Phys. II 1997, 7, 493. (15) Singh, M.; Ober, R.; Kleman, M. J. Phys. Chem. 1993, 97, 11108. (16) Ficheux, M. F.; Bellocq, A. M.; Nallet, F. J. Phys. II 1995, 5, 823. (17) Radlinska, E. Z.; Gulik-Krzywick, T.; Lafuma, F.; Langevin, D.; Urbach, W.; Williams, C. E.; Ober, R. Phys. Rev. Lett. 1995, 74, 4237. (18) Zhang, K.; Linse, P. J. Phys. Chem. 1995, 99, 9130. (19) Bouglet, G., Ligoure, C.; Bellocq, A. M.; Dufourc, E.; Mosser, G. Phys. Rev. E 1998, 57, 834. (20) Ficheux, M. F.; Bellocq, A. M.; Nallet, F. Colloids Surf. 1997, 123, 253. (21) Radiman, S.; Fountain, L. E.; Troprakcioglu, C.; de Vallera, A.; Chieux, P. Prog. Colloid Polym. Sci. 1990, 81, 54. (22) Lianos, P.; Modes, S.; Staikos, G.; Brown, W. Langmuir 1992, 8, 1054. (23) Papoutsi, D.; Lianos, P.; Brown, W. Langmuir 1993, 9, 663; 1994, 10, 3402. (24) Suarez, M. J.; Levy, H.; Lang, J. J. Phys. Chem. 1993, 97, 9808; 1995, 99, 4626. (25) Lal, J.; Auvray, L. J. Phys. II 1994, 4, 2119. (26) Kabalnov, A.; Olsson, U.; Wennerstro¨m, H. Langmuir 1994, 10, 4509. (27) Kabalnov, A.; Olsson, U.; Wennerstro¨m, H. Langmuir 1994, 10, 2159. (28) Rajagopalan, V.; Olsson, U.; Iliopoulos, I. Langmuir 1996, 12, 4378. (29) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601; 1987, 118, 586. (30) Kalhweit, M.; Strey, R. J. Phys. Chem. 1988, 92, 1557. Kalhweit, M.; Strey, R.; Schomacker, R.; Haase, D. Langmuir 1985, 5, 305. Kalhweit, M. J. Phys. Chem. 1995, 99, 1281. (31) Kellay, H.; Binks, B. P.; Hendricks, Y.; Lee, L.; Meunier, J. Adv. Colloid Interface Sci. 1994, 49, 85. (32) Bellocq, A. M. Langmuir 1998, 14, 3730.

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curvature of the AOT layer. Also, from Kerr effect measurements and a conductivity study of W/O droplets, it has been concluded that the adsorption of PEG leads to an increase of the bending modulus κ of the AOT layer.33,34 As discussed above, theoretical studies predict that upon addition of salt and adsorption of a polymer the curvature moduli of a surfactant layer change in the same direction. Thereby, these two additives should have similar effects on the phase behavior of a surfactant system. In the present work, we report the effects of PEG and NaCl concentration on the stability of bicontinuous microemulsions and lamellar phases formed in AOT-water-isooctane mixtures. In this study, the volumes of oil and water were kept equal and the phase behavior was investigated as a function of temperature and surfactant concentration for different salt and polymer concentrations. II. Experimental Section The surfactant AOT (Fluka) was purified as previously described.32 Poly(ethylene glycol) and isooctane were supplied by Aldrich and Fluka, respectively. We measured by gel permeation chromatography the polymer molecular weight and its polydispersity: Mw ) 22 600; Mw/Mn ) 1.1. The mixtures studied here consist of four or five components. At fixed pressure, a mixture of n components is defined by n independent thermodynamic variables, namely, temperature, and n - 1 composition variables. The most convenient composition variables for our purpose are the fraction of the oil in the mixture of oil and brine

R)

oil oil + water + NaCl + PEG

that of the AOT in the mixture

γ)

AOT oil + water + NaCl + PEG + AOT

and those of the polymer and NaCl in the aqueous medium.

Cp ) )

PEG water + NaCl + PEG

NaCl water + NaCl + PEG

all expressed in weight percent. The phase diagrams of these mixtures were obtained by studying sections where two (for the quaternary systems) or three (for the quinary systems) composition variables are kept constant. In all the sections investigated R ) 0.4. The lamellar phase was detected by using crossed polarizers and was identified by polarizing microscopy.

III Results 1. Effect of Salt. We start with the quaternary system AOT-water-NaCl-isooactane, that is, keeping Cp ) 0 and R ) 40%. In this study we investigate for various  the evolution with temperature of the phase behavior. For such sections with fixed R and , the variables are T and γ. Sections with  > 0.5% have been investigated previously.29,30 Figure 1 shows the composition-temperature phase diagrams for  ) 0, 0.2, 0.3, and 0.4%. In the four sections, two isotropic phases, L1 and L2, and a lamellar phase, LR, are occurring. In the surfactant concentration range between L1 and L2 the mixtures with  ) 0 do not separate at low temperature even after several months; those with  ) 0.2% give rise to three-phase equilibria L1L2LR. When  ) 0.3 and 0.4%, the isotropic (33) Meier, W. Langmuir 1996, 12, 1188. (34) Meier, W. J. Phys. Chem. B 1997, 101, 919.

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Figure 2. Temperature dependence of electrical conductivity for quaternary mixtures of AOT-water-NaCl-isooctane at R ) 40% and  ) 0.3% for γ ) 3, 7, 10, and 15%.

Figure 1. Sections of the phase diagram of the quaternary system AOT-water-NaCl-isooctane at R ) 40% for  ) 0, 0.2, 0.3, and 0.4%. L1 and L2 are isotropic microemulsions, and LR is a lamellar phase. In region 2R, a microemulsion coexists with a lamellar phase. In region 2 h at low temperature, the microemulsion is in equilibrium with excess water. In region 2 at high temperature, the microemulsion is in equilibrium with excess oil and in region 3 (4) there are Winsor III equilibria. The symbol 2 represents three-phase equilibria where a lamellar phase coexists with two isotropic phases.

phases L1 and L2 merge at low temperature and low γ and form a bicontinuous microemulsion which touches the three-phase volume Winsor III in a point of coordinates ˜ m. For all the salinities, the Winsor II equilibria γ˜ m and T (2 h ) are found at low temperature and the Winsor I equilibria (2) at high temperature. These results and those previously published for  > 0.5% show that both γ˜ m and T ˜ m increase with increasing . The lamellar phase appears at T ˜ LR and surfactant concentration γ˜ LR. As  increases T ˜ LR continuously rises whereas γ˜ LR exhibits a minimum for  ) 0.3%. Thus, at low concentration salt increases the efficiency of the amphiphile, and at higher concentration it reduces the lamellar swelling. This effect of salt is similar to that previously observed in AOT-brine mixtures.35,37 In abscence of oil, a maximum in the lamellar swelling is obtained for  ) 0.1%. Although the diagrams in Figure 1 seem similar to those previously published, they exhibit some differences. It has been early recognized that the phase behavior of AOT microemulsions is very sensitive to the purity of the AOT used.29 Especially it has been found that the shape of phase boundaries can be taken as a reliable criterion for the presence of the hydrolysis products of AOT.29,38 In a pure AOT mixture with R ) 0.5 and  ) 0.5%, the lower phase (35) Fontell, K. J. Colloid Interface Sci. 1973, 44, 318. (36) Ghosh, O.; Miller, C. A. J. Phys. Chem. 1987, 91, 4528. (37) Skouri, M.; Marignan, J.; Appel, J.; Porte, G. J. Phys. II 1991, 1, 1121. (38) Sager, W. F. C. Langmuir 1998, 14, 6385.

boundary of L2 is lying at roughly constant temperature, while it exhibits a significant bending in the presence of alcohol. In Figure 1, this boundary is tilted toward lower temperatures with decreasing AOT concentration. This downward tilt could be due either to a different value of R or to traces of inorganic impurities which have not been totally removed by the purification procedure used. The microstructure of AOT-brine ( ) 0.6%)-decane mixtures has been explored by use of small-angle neutron scattering,39 conductivity,39 and NMR self-diffusion.40,41 A continuous inversion from a W/O microemulsion at low temperatures to an O/W microemulsion at high temperatures through a disordered bicontinuous microemulsion has been clearly established. Conductivity measurements show that the same structural evolution is found in brineisooctane-AOT mixtures as temperature is varied (Figure 2). In the L1 phase, the conductivity σ is of the same order of magnitude as brine conductivity. In the L2 phase, a percolative behavior is evidenced, and as the temperature is raised toward the boundary where LR becomes stable, a bicontinuous microstructure is rapidly approached. Conductivity results show that such a structure is also present at low γ, in the region where the L1 and L2 channels have merged. 2. Effect of PEG. Figure 3 shows the temperature dependence of the phase behavior of quaternary mixtures AOT-water-PEG-isooctane with R ) 40% and Cp ) 5, 10, 15, and 20%. For Cp ) 5, 10, and 15%, two isotropic single-phase regions, L1 and L2, exist above a certain temperature; they merge at high T and γ. For Cp ) 5 and 10%, a lamellar phase is seen between L1 and L2. As Cp increases a gradual lowering of the L1/L2 merging phase transition on the temperature scale is found. The merging temperature T ˜ decreases from 75 to 25 °C as Cp increases from 5 to 17%. The comparison of the diagrams reveals that one of the main effects of PEG is to destabilize the concentrated lamellar phase at the benefit of an isotropic microemulsion and also to increase the swelling of the dilute lamellar phase (Figure 3); γ˜ LR, shifts from approximately 12 to 8% with the increase of Cp from 0 to 10%. LR exists within a well-defined interval of AOT concentration and temperature. Upon increasing Cp, both intervals shrink to vanish at T ≈ 28 °C and Cp ≈ 13%. As (39) Chen, S. H.; Chang, S. L.; Strey, R. J. Chem. Phys. 1990, 93, 1907. Chen, S. H.; Strey, R.; Samseth, J.; Mortensen, K. J. Phys. Chem. 1991, 95, 7427. (40) Carnali, J. O.; Ceglie, A., Lindman, B.; Shinoda, H. Langmuir 1986, 2, 417. (41) Xie, M.; Zhu, X.; Miller, W. G.; Bohlen, D. S.; Vinson, P. K.; Davis, H. T.; Scriven, L. E. Surfactant Science Series; Marcel Dekker: New York, 1992; Vol. 44, p 145.

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Figure 3. Sections of the diagram of the quaternary system AOT-water-PEG-isooctane at R ) 40% for Cp ) 5, 10, 15, and 20%. The symbol 9 represents a four-phase equilibrium where a lamellar phase coexists with three isotropic phases.

Figure 4. Temperature dependence of electrical conductivity for quaternary mixtures of AOT-water-PEG-isooctane at R ) 40%, and γ ) 15% for Cp ) 5, 10, 15, and 20%.

Cp is further increased, the L1 and L2 channels come closer and are completely merged at Cp ) 20%. Whatever Cp, the L1 mixtures separate at high temperature with an excess oil phase (2) and the L2 mixtures at low temperature with an aqueous excess phase (2h ). In the surfactant concentration range between L1 and L2, the mixtures with Cp ) 0 and 5% do not separate at low temperatures; for those with Cp ) 10 and 15% separations between two, three, or four phases, either isotropic or lamellar, occur. For Cp ) 15%, the phase progression obtained by varying γ at 20 °C is 2-3-4R-3R-2 h . In the three- and four-phase equilibria denoted 3R and 4R, respectively, a lamellar phase coexists with isotropic phases. On increasing Cp, the equilibria 4R and 3R disappear, and for Cp ) 20%, the Winsor III region touches the single-phase microemulsion region at γ˜ m ≈ 11% and T ˜ m ≈ 35 °C. The sequences of phases observed by varying temperature are dependent on γ and Cp, but in all cases increasing T leads to a curvature inversion which occurs via either a lamellar phase or a bicontinuous phase. Phase behavior suggests that L1 and L2 mixtures are water-continuous and oil-continuous, respectively. This is supported by electrical conductivity measurements. Figure 4 presents the temperature dependence of electrical conductivity for samples with γ ) 15% and various Cp values. For Cp ) 5%, the extremely low values of σ(∼10-7 S cm-1) at low

Figure 5. Sections of the phase diagram of the quinary system AOT-water-NaCl-PEG-isooctane at R ) 40% and  ) 0.2% for Cp ) 2.5, 5, and 10% and R ) 40% and  ) 0.3% for Cp ) 1, 2.5, and 5%.

temperature and conversely the high values (∼10-2 S cm-1) obtained at high temperature are consistent with a waterin-oil structure for L2 and an oil-in-water structure for L1. For Cp ) 10 and 15%, the curves exhibit an electrical percolation phenomenom in the L2 phase. Finally, the continuous increase of σ, from 4 × 10-7 to 8 × 10-3 S cm-1, found for Cp ) 15% as T increases suggests a continuous inversion of the microstructure. Thus, for γ ) 15%, increasing temperature drives the preferred mean curvature of the surfactant film toward zero, a value favoring the lamellar phase at low Cp and a bicontinuous microemulsion at high Cp. 3. Combined Effects of NaCl and PEG. Let us now consider quinary mixtures containing AOT-waterNaCl-PEG-isooctane. For studying the effect of Cp on mixtures with salt, we have investigated sections at constant R, , and Cp (Figure 5). In salt-free mixtures, three-phase Winsor III equilibria are occurring at Cp ) 20%, for those with  ) 0.1% and  ) 0.2%, they start to exist, respectively, at Cp ) 15 and 1.25%, and in the cases with  ) 0.3 and 0.4%, they are present even at Cp ) 0. The coordinatesstemperature, T ˜ m, and surfactant concentration, γ˜ msof the points where the one-phase and three-phase regions meet are plotted in Figure 6. At fixed , increasing Cp shifts the phase behavior up in temperature (T ˜ m rises) and produces a decrease in efficiency. For instance at  ) 0.2%, γ˜ m is 2% for Cp ) 1.25% and 4% for Cp ) 10%. Figure 6 also reports the

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Figure 7. Temperature dependence of electrical conductivity for quinary mixtures of AOT-water-NaCl-PEG-isooctane at R ) 40%,  ) 0.3%, and Cp ) 2.5% for γ ) 4, 6, 12, 15, 20, and 25%.

Figure 6. Dependence of T ˜ m, T ˜ LR, γ˜ m, and γ˜ LR on polymer concentration for the quinary mixtures AOT-water-NaClPEG-isooctane for R ) 40% and with  as a parameter.

positionsstemperature, T ˜ LR, and surfactant concentration γ˜ LRsof the lamellar phase (minimum concentration of AOT ˜ m continuously to form LR) versus Cp for several  values. T rises with increasing Cp whatever , whereas the Cp dependence of γ˜ LR depends on salinity. With increasing Cp, γ˜ LR decreases at low  (0, 0.1%), increases at high  (0.3, 0.4%), and exhibits a minimum for  ) 0.2%. At  ) 0.3 and 0.4% these trends are similar to those found for the bicontinuous microemulsion, although the variations in efficiency are more pronounced for the lamellar phase than the microemulsion. The addition of PEG also alters the phase behavior of concentrated surfactant mixtures. For a given , increasing Cp strongly reduces the extent of the lamellar phase and produces the merging of the L1 and L2 phases at high γ. Thereby, for certain couples of values of  and Cp, the lamellar phase LR and the twophase region 2R, where LR and a microemulsion coexist, are totally surrounded by an isotropic single-phase region (Figure 5). This is also observed in the absence of polymer but at a much higher temperature.29 Conductivity data show that mixtures with low and high γ have a bicontinuous structure, while those with intermediate γ keep a W/O structure at low temperature (Figure 7). Further increasing Cp leads to the disappearance of both LR and 2R and to the formation of an isotropic region which covers a large range of surfactant concentration (Figure 5). These isotropic mixtures which result from the complete merging of the L1 and L2 channels exhibit a high conductivitiy whatever γ and T, suggesting a bicontinuous structure. The solubility of PEG in LR depends on ; it decreases upon increasing . The lamellar phase disappears around Cp ) 1% for  ) 0.4%, while it is still stable at Cp ) 10% for  ) 0. In parallel with that, the large isotropic bicontinuous phase formed after the disappearance of LR is obtained for smaller values of Cp when  is increased. (Cp ) 20% for  ) 0, Cp)10% for  ) 0.2%, Cp ) 5% for  ) 0.3%, and Cp ) 2.5% for  ) 0.4%).

Figure 8. Extent of the lamellar region in (a) the AOT-waterNaCl-isooctane system as a function of  and (b) the AOTwater-PEG-isooctane system as a function of Cp.

IV. Discussion Salt and PEG have a profound influence on the phase behavior of AOT-water-isooctane mixtures. Adding salt shifts the phase behavior up in temperature, promotes the formation of a bicontinuous phase, produces a maximum in the swelling of the lamellar phase, and destabilizes, at high temperature, concentrated LR phases (Figure 8). The first effect reflects a decrease of the spontaneous curvature C0 of the AOT layer, due to a change in the HLB of the surfactant, which becomes more lipophilic in the presence of salt. The other effects can be explained in terms of interaction between the surfactant layers and flexibility of the layers.7,8 In the mixtures with equal volumes of oil and water, the ionic AOT layers interact via electrostatic interactions and entropic undulation interactions.1 In recent years, theoretical work on lamellar phases of charged bilayer membranes has concentrated on the interplay between these two types of interactions.7 For highly charged membranes, undulations are unimportant as long as the distance d between the membranes is smaller than or comparable to the Debye length. In such a case, the free energy is dominated by the

Phase Behavior of a AOT-Water-Oil System

electrostatic energy. Upon adding salt or increasing d, undulations become stronger and stronger. In the limit of very high salt concentration or large distances, the free energy is dominated by the undulation interactions.42-43 A number of theories have described the transition between these two regimes.44-46 It has been shown that if the amplitude of the undulations is appreciably larger than the Debye length, there is a substantial enhancement of the electrostatic repulsion. Experimental evidence for undulation-enhanced electrostatic forces has been found in lamellar phases47,48 and a hexagonal polyelectrolyte gel.49 This mechanism may account for the increased swelling of LR found at low salinity for  < 0.3%. In agreement with theoretical predictions, the experimental values of κj obtained for various AOT-brine-oil systems increase with salt concentration.31,50 Therefore, the simultaneous increase of κj along with the high flexibility of the AOT layer accounts for the occurrence of dilute bicontinuous microemulsions as the salt concentration is above 0.05 M. The effect of adding PEG on the phase behavior of the ternary mixtures AOT-water-isooctane is in many respects similar to that of adding salt, although the changes observed are much less pronounced. Experimentally one finds that PEG causes a slight shift of the phase behavior to higher temperatures and the formation of concentrated bicontinuous microemulsions at high Cp (γ˜ m ≈ 11%, Cp ) 20%). Also, like salt at low content ( ) 0.1 and 0.2%), PEG increases the swelling of LR and destabilizes concentrated LR phases (Figure 8). The similarity of the effects produced by both additives appears also as one compares the phase behavior of one sample with fixed composition (γ ) 10%, R ) 40%) as a function of  or Cp (Figure 9). First, at constant temperature increasing  or Cp leads to the phase progression L1 f LR f L2. So, salt and PEG taken separately induce a decrease of the spontaneous curvature of the interface. The simultaneous presence of both additives enhances the effect of salt on ˜ LR the curvature as evidenced by the increase of T ˜ m and T with Cp regarless of  (Figure 6). Moreover, each additive destabilizes the lamellar phase and induces a transition toward a bicontinuous microemulsion. However, a major difference between NaCl and PEG concerns the thermal stability of the LR phase relative to that of the bicontinuous microemulsion. In the salted mixtures, the bicontinuous microemulsion starts to exist for  ) 0.3% at low γ and low T and the lamellar phase is stable over a large range of temperature. In the polymer-containing system, it appears that increasing Cp induces the formation of a concentrated bicontinuous microemulsion at moderate temperature and high surfactant concentration. The loss of stability of LR phases upon addition of PEG is also observed in the salted mixtures. In the quinary mixtures, the complete disappearance of LR to the benefit of a large bicontinuous microemulsion occurs at lower values of Cp as  increases. (42) Roux, D.; Safinya, C. R. J. Phys. (Paris) 1988, 49, 307. (43) Bassereau, P.; Marignan, J.; Porte, G. J. Phys. (Paris) 1987, 48, 673. (44) Odijk, T. Langmuir 1992, 8, 1690; Europhys. Lett. 1993, 24, 177. (45) de Vries, R. J. Phys. II 1994, 4, 1541. (46) Podgornik, R.; Parsegian, V. A. Langmuir 1992, 8, 557. (47) Evans, E. A.; Parsegian, V. A. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 7132. (48) (a) Dubois, M.; Zemb, T. Langmuir 1991, 7, 1352. (b) Zemb, T.; Dubois, M.; Belloni, L.; Marcelja, S. Prog. Colloid Polym. Sci. 1992, 89, 33. (49) Strey, H. H.; Parsegian, V. A.; Podgornik, R. Phys. Rev. Lett. 1997, 78, 895. (50) Skouri, M.; Marignan, J.; May, R. Colloid Polym. Sci. 1991, 269, 929.

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Figure 9. (a) Section of the phase diagram of the quaternary system AOT-water-NaCl-isooctane at R ) 40% and γ ) 10%. (b) Section of the phase diagram of the quaternary system AOTwater-PEG-isooctane at R ) 40% and γ ) 10%.

PEG is insoluble in inverse microemulsions when its size RG is larger than the radius RW of the water-core droplet.32,33 Such a confinement effect is not found in lamellar phases and bicontinuous microemulsions. Indeed, these phases dissolve large amounts of PEG, even when the water layer size is less than RG. However, the solubility is larger in the microemulsion than in the LR phase. A similar dependence of the solubility of polymer on the surfactant film topology was observed previously for other polymer-surfactant systems.28 The disappearance of concentrated lamellar phases upon addition of PEG is related to the polymer concentration Cp rather than to the size of the coil compared to the thickness of the water layer dW. In a lamellar phase, the thickness dW is given by

dW ) δΦW/ΦS where δ is the thickness of the AOT bilayer [δ ) 20 Å51] and φW and φS are the water and surfactant volume fractions, respectively. dW is estimated to be 30 Å for γ ) 25% and 200 Å for the most dilute samples at γ ≈ 5%. Figure 8 shows that the concentrated lamellar phase with γ ) 25% may incorporate PEG up to a concentration of 2.5% in water, although the size of the polymer (2RG ≈ 60 Å16] is larger than dW. The phase transformation LR-bicontinuous microemulsion observed with increasing Cp at constant γ and  may be due to a decrease of the bending constant κ or/ and to an increase of the Gaussian elastic constant κj.4 The analoguous effects of PEG and salt on the phase behavior of the AOT-water-isooctane mixtures suggest that PEG influences the elastic parameters in the same direction as salt. Thus, our results suggest that PEG (51) Khan, A.; Fontell, K. Colloids Surf. 1984, 11, 401.

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slightly decreases κ and increases κj. These trends are consistent with the polymer concentration dependence of γ˜ m and γ˜ LR shown in Figure 6. For the  values where both phases are stable, we found that, in a large range of Cp, the polymer slightly affects the swelling of the microemulsion, while it reduces that of the lamellar phase. At higher polymer concentration the bicontinuous microemulsion phase becomes more concentrated in AOT. For instance in the case of  ) 0.3%, the microemulsion swells to approximately 2% of the surfactant as Cp is in the range 0-5%, while it swells to approximately 10% of AOT when Cp is 10%. All the changes produced by PEG on the phase behavior of the salted mixtures are in agreement with the theoretical predictions of Brooks et al.10 and Clement and Joanny.11

Maugey and Bellocq

V. Conclusions The results presented in this study show that the phase behavior of AOT-water-salt-PEG-isooctane mixtures is very sensitive to changes in temperature and salt, PEG, and surfactant concentrations. Changes in monolayer topology may be induced through changes in one of these system parameters. In particular the addition of either polymer or salt leads to a lamellar-to-bicontinuous microemulsion phase transition. The phase behavior results obtained suggest that the two variables, PEG and salt, act in the same direction on the spontaneous curvature and the bending moduli κ and κj. Both additives decrease C0 and κ and increase κj. LA990438G