4528 J . Phys. Chem. 1987, 91, 4528-4535

Liquid-Crystalline and Microemulsion Phase Behavior in Alcohol-Free Aerosol-OT/Oil/Brine Systems

Olina Ghosh and Clarence A. Miller*

Department of Chemical Engineering, Rice University, Houston, Texas 77251 (Received: September 19, 1986)

The phase behavior of systems containing the pure anionic surfactant Aerosol OT or sodium bis(2-ethylhexyl) sulfosuccinate was studied as a function of salt concentration, surfactant concentration, alkane carbon number, and water-to-oil ratio. Since the hydrophilic and lipophilic properties of Aerosol OT are nearly balanced, the surfactant forms microemulsions with water and oil in the absence of cosurfactant, allowing for simplified representation of phase behavior. In particular, this property aided in the understanding of transitions between the aqueous surfactant phase behavior and the well-studied oil-rich microemulsion regime. With the addition of salt to dilute alcohol-free surfactant-water mixtures, transitions in the liq- uid-crystalline phases similar to those seen previously for systems containing petroleum sulfonates and other anionic surfactants with alcohol cosurfactants were found. When hydrocarbons of various chain lengths were equilibrated with the aqueous surfactant solutions, again behavior similar to that of anionic surfactants with alcohol cosurfactants was observed. Pseudoternary diagrams of surfactant-brine-oil were constructed at various brine salinities with n-dodecane as the oil. The assumption that brine acts as a pseudocomponent was found to work best at salinities well below and well above the optimum and at low surfactant concentrations. In any case, the results provide extensive information on phase behavior of a four-component system containing a pure anionic surfactant, a pure hydrocarbon, and sodium chloride brine over a region of considerable interest for enhanced oil recovery and other applications.

Introduction Aerosol OT, also known as AOT or sodium bis(2-ethylhexyl)

sulfosuccinate, is a widely used anionic surfactant. Some of the many commericial applications of this surfactant include use in emulsion and suspension polymerization, in paint formulations, in dry cleaning and spotting, in dispersion of colors and dyes in plastics, and in lubricants, coolants, and rust inhibitors.'S2 It is not a t all surprising therefore to find that the physical properties of this surfactant and its solubilization characteristics with oil and water have been extensively ~ t u d i e d . ~ - ~

Aerosol OT has also been used as a model surfactant by in- vestigators to obtain a fundamental understanding of surfactant systems. One of the reasons that it is an attractive system for such studies is that its hydrophilic and lipophilic properties are nearly balanced.I0 Further, since microemulsions form without requiring the presence of any cosurfactant such as alcohol, analysis of the phase behavior is considerably simplified. This surfactant system was therefore chosen by Huang et al.l*-15 for studies on the critical behavior of microemulsions. Such alcohol-free mi- croemulsions have important applications in pharmaceutical preparations and photochemical studies where the presence of alcohol is undesirable.16 Aerosol OT has also been used for extensive studies on the effects of nonpolar solvent^,^-'^ electro-

(1) Candau, F.; Leong, Y. S.; Pouyet, G.; Candau, S. J. Colloid Interface

(2) Surfactants by Cyanamid, American Cyanamid Co., Wayne, NJ, Jan Sci. 1984, 101, 167.

1983. (3) Peri, J. B. J. Colloid Interface Sci. 1969, 29, 6. (4) Kitahara, A,; Watanabe, K.; Konno, K.; Ishikawa, T. J. Colloid In-

(5) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979, 83, 48. (6) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface Sci. 1970, 33,

(7) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1979, 70, 577. (8) Tamamushi, B.; Watanabe, N. Colloid Polym. Sci. 1980, 258, 174. (9) Frank, S. G.; Zografi, G. J. Colloid Interface Sci. 1969, 29, 27. (IO) Shinoda, K.; Kunieda, H.; Arai, T.; Saito, H. J. Phys. Chem. 1984,

(11) Kotlarchyk, M.; Chen, S. H.; Huang, J. S. Phys. Reu. A 1983, 28,

(12) Kotlarchyk, M.; Chen, S. H.; Huang, J. S. Phys. Reu. 1984, 29, 2054. (13) Huang, J. S.; Kim, M. W. Soc. Petrol. Eng. J. 1984, 24, 197. (14) Kotlarchyk, M.; Chen, S. H.; Huang, J. S. J. Phys. Chem. 1982.86,

terface Sci. 1969, 29, 48.

215.

88, 5126.

50s.

3273. (15) Huang, J. S.; Safran, S. A.; Kim, M. W.; Grest, G. S.; Kotlarchyk,

(16) Johnson, K. A,: Shah, D. 0. J. Colloid Interface Sci. 1985, 107, 269. M.; Quirk, N. Phys. Reu. Lett. 1984, 53, 592.

0022-36541871209 1 -4528s01 . 5 0 p

l y t e ~ , ~ ? ' ~ ~ ' ~ and cosurfactantZo on the interfacial activity of the surfactant.

Various techniques have been utilized for the investigation of the micelles and microemulsions formed by AOT with water and oil. Information on processes occurring at the molecular level has been obtained from conductivity, vapor pressure, and 'H and I3C N M R measurements, while the size of the aggregates has been investigated by light scattering, ultracentrifugation, fluorescence depolarization, and photon correlation s p e c t r o s c ~ p y . ~ ~ ~ ~ ~ ~ - ~ ~ Further, the distribution of surfactant between aqueous solution and a normal alkane, heptane, has been studied with respect to both salt concentration and surfactant concentration, and the behavior related to the Occurrence of interfacial tension minima (26).

The liquid-crystalline phases formed by AOT with polar and nonpolar solvents have also been studied in great detail. Here, microscopy, IH and I3C NMR,z7 X-ray d i f f r a c t i ~ n , ~ ~ . ~ ~ and conductivitys measurements have been utilized to provide infor- mation on the structure and properties of these anisotropic phases.

Several aspects of the phase behavior of the AOT system make its study particularly interesting. First, as mentioned before, the hydrophilic-lipophilic balance in this surfactant enables it to form microemulsions with water and oil in the absence of any cosur- factant. The transitions through the lower, middle, and upper phase microemulsions can be induced by increasing the hydro- phobic character of the system, in particular by increasing brine salinity or decreasing hydrocarbon chain length. Such changes

(17) Eicke, H. F. "Micelles in Polar Media", In Micellization, Solubili- zation, and Microemulsiom, Vol. 1, Mittal, K. L., Ed.; Plenum: New York, 1917; p 429.

(18) Konno, K.; Kitahara, A. J. Colloid Interface Sci. 1972, 41, 47. (19) Bedwell, B.; Gulari, E. J. Colloid Interface Sci. 1984, 102, 88. (20) Eicke, H. F. J. Colloid Interface Sci. 1979, 68, 440. (21) Eicke, H. F.; Arnold, V. J. Colloid Interface Sci. 1974, 46, 101. (22) Eicke, H. F.; Shepherd, J. C. W.; Steinemann, A. J. Colloid Interface

(23) Eicke, H. F.; Zinsli, P. E. J. Colloid Interface Sci. 1978, 65, 131. (24) Ueno, M.; Kishimoto, H.; Kyogoku, Y. J. Colloid Interface Sci. 1978,

(25) Peyrelasse, J.; Boned, C.; Marin, G. Colloid Polym. Sci. 1986, 264,

(26) Aveyard, R.; Binks, B. P.; Clark, S.; Mead, J. J. Chem. Soc., Faraday

(27) Frames, E. I.; Hart, T. J. J. Colloid Interface Sci. 1983, 94, 1. (28) Fontell, K. J. Colloid Interface Sci. 1973, 43, 156 (29) Fontell, K. J. Colloid Interface Sci. 1973, 44, 318.

Sci. 1976, 56, 168.

63, 113.

143.

Trans. I 1986,82, 125.

0 1987 American Chemical Society

Alcohol-Free Aerosol-OT/Oil/Brine Systems The Journal of Physical Chemistry, Vol. 91, No. 17, 1987 4529

reverse the direction of the natural curvature of the surfactant film from oil-in-water to water-in-oil and hence produce an in- version in phase continuity of the microemulsion. Second, the ability of the surfactant to form microemulsions and liquid crystals a t different compositions of oil and water can be used to provide an improved understanding of how and when these phases form. This factor is especially of interest here as it enables a comparison to be made with similar studies performed with alcohol-containing surfactant systems.30 Aerosol OT has been chosen previously for a similar study by Tamamushi and Watanabe,8 who discussed one mechanism by which a transition between an oil-continuous microemulsion and liquid crystal could occur.

AOT is very oil-soluble and, indeed, dissolves completely in common nonpolar solvents forming globular micelles. When sufficient water is added, spherical droplets of a water-in-oil microemulsion are ~ e e n . ~ . ~ Using proton and sodium-23 NMR, Wong et aL31 investigated water-in-oil microemulsions of AOT in heptane. Their results show that in small micelles the water is immobilized, but that it becomes more mobile as the amount of water in the droplets increases. From density measurements, Ekwall et aL6 have found that the water is energetically bound to the AOT micelles for up to 5 to 6 mol of water/mol of AOT, i.e. until the sodium ions of the surfactant are surrounded by a complete hydration layer. Using small-angle neutron scattering, Kotlarchyk et a1.I2 have studied single-phase water-in-oil micro- emulsions up to droplet volume fractions of 15 vol %. They reported the presence of spherical droplets having a water core of radius 45-50 A coated with an 8-A monolayer of surfactant tails. The phase behavior a t higher water contents has also been studied. Assih et al.32 have provided a salt-free ternary diagram of the Aerosol OT-water-decane system showing the multiphase regions that occur as the water-to-oil ratio is varied.

The effect of the addition of inorganic salts to these water-in-oil microemulsions has also been investigated. Kunieda and Shinoda' have shown that the domain of existence of the M E phase is considerably reduced when inorganic salts are present. The latter decrease the solubility of water and shrink the ME region of the ternary phase diagram.I8 Further, using viscometry and dynamic light scattering, Bedwell and Gulari19 show that NaCl moderates the attractive interactions between the microemulsion droplets by making the drops smaller.

Using dynamic light scattering, Bedwell and GulariI9 also studied the effect of different amounts of water on microemulsions at a constant surfactant-to-salt molar ratio. The results show that the droplets behave more like hard spheres a t low water content than at high water content. These investigators suggest that this behavior can be interpreted as the NaCl being dispersed uniformly in the droplet core so that it does not have a preferential interaction with the interior surface occupied by the surfactant.

As these investigations have been made primarily on water-in-oil microemulsions, further studies are needed to understand the complete picture of how electrolyte alters the characteristics of AOT microemulsions. In the present study, the effect of a com- mon electrolyte, sodium chloride, on aqueous solutions of AOT at high water contents has been investigated. Also examined is the influence of hydrocarbons of varying chain lengths on phase behavior a t equal water and oil contents. From these results, the straight-chain hydrocarbon n-dodecane was chosen to study the transitions from the liquid-crystalline to the microemulsion phases as the system composition was varied. Pseudoternary diagrams of AOT-brine-dodecane have been constructed at several brine salinities to provide an insight into various aspects of the phase behavior.

Materials The Aerosol OT molecule consists of two branched hydrocarbon

chains held together by a rather large polar sulfosuccinate group.

(30) Ghosh, 0. Ph.D. Thesis, Rice University, Oct, 1985. (31) Wong, M.; Thomas, J. K.; Novak, T. J . Am. Chem. Soc. 1977, 99,

(32) Assih, T.; Delord, P.; Larche, F. In Surjacfants in Solution, Vol. 3, 4730.

Mittal, K. L.. Lindman, B., Ed.; Plenum: New York, 1984; p 1821.

The surfactant molecule has the following structure:

C2H5 I

I

CH,COOCH2CH(CHp),CH3 I

Na03S-CHCOOC H ,CH (C H p )3CH3

C2H5

AOT has a molecular weight of 444.56 and, at 30 "C, a density of 1.130 g/cm3.

The surfactant, of purity greater than 99%, was supplied by Fluka and American Cyanamid Company (U.S.P. Grade). The hydrocarbons were obtained from Humphrey Chemical Co. and sodium chloride from EM Science, all the chemicals used being of reagent grade. ASTM Grade Type I water was obtained from a Bamstead water purification system. In a majority of the studies, the surfactant was used as received after being dried in a vacuum oven at 30 OC. Since the presence of impurities in the surfactant can affect its phase behavior significantly,' several data points were repeated by using a purified batch of surfactant. The surfactant was purified following the procedure of Kotlarchyk et al.I2 A comparison of phase behavior of the original surfactant with that of the purified batch showed no apparent differences even at compositions near phase boundaries where the presence of impurities would be expected to have an effect.

Methods Solution Formulation. All phase behavior studies were per-

formed at 30 OC. The solutions were formulated as follows. The amount of AOT required to make up the stock solution was first weighed, after which measured volumes of either brine or hy- drocarbon were added. This solution was mixed thoroughly with a magnetic stirrer until the surfactant was completely dissolved, and then appropriate volumes of the stock solution and brine or hydrocarbon were mixed for 15 s on a vortex mixer to obtain the desired composition. To minimize the rate of hydrolysis, the solutions were formulated at room temperature, then gently mixed on a rotator for 24 h and equilibrated in a constant temperature room at 30 "C. Because of the relatively short equilibration times required, the system phase behavior could be observed within 1 week after the solutions were made so that changes due to the effects of hydrolysis were minimized.

The polarized light screening technique was used for observing macroscopic phase behavior of surfactant solutions. Here, diffused light was transmitted through a polarizer and an analyzer with its optic axis rotated 90" with respect to the polarizer. The test tube containing the surfactant solution was placed between the polarizer and analyzer and its behavior observed. This technique was used extensively to provide information about the isotropy, anisotropy, and scattering of solutions. Liquid-crystalline phases are easily distinguished by the birefringence displayed with polarized light.

Phase Volume Measurements. The 13-mm-i.d. flat-bottomed test tubes used for storing the solutions were calibrated and the phase volumes were measured with an accuracy of 1%. Equi- librium was considered to be reached when no further change in the phase volumes was detected.

Density Measurements. A Mettler/Paar DMA 45 model di- gital density meter was used to measure the densities of the various phases up to four decimal places, the accuracy of measurement being about 0.1%. The temperature of the sample was regulated by water from a bath controlled to f0.5 "C.

Optical M i ~ r o s c o p y . ~ ~ Samples of the solutions to be studied were introduced into rectangular optical glass capillaries by ca- pillary action. These capillaries were 50 mm long, 2 mm wide, and had an optical pathlength of 100 Wm. The capillaries were filled and then sealed and fastened to standard microscope slides with an ultraviolet light sensitive polymer adhesive which was cured with an ultraviolet fiber optic gun.

Polarized Light Screening System.33

(33) Benton, W. J.; Miller, C. A. J . Phys. Chem. 1983, 87, 4981. (34) Benton, W. J.; Miller, C. A. Progr. Colloid Polym. Sci. 1983, 68, 71.

4530 The Journal of Physical Chemistry, Vol. 91 I No. 17 , 1987 Ghosh and Miller

I /

Figure 1. Phase behavior of the Aerosol OT-NaCI-H20 system near the water corner of the diagram at 30 OC.

The observations were made with a Nikon PoH polarizing microscope. Temperature control for the samples at 30 OC was obtained with a Mettler FP-52 controller and FP-5 microscope hot stage. The samples were studied by conventional polarizing microscopy to identify the birefringent textures of the liquid- crystalline solutions.

Interfacial Tension Measurements. Interfacial tensions between equilibrium phases were measured with a spinning-drop ten- siometer developed at the University of Texas.35 All measure- ments were performed at 30 OC inside a constant temperature room.

Refractive Index Measurements. A Bausch and Lomb Abbe-3L refractometer was used to measure refractive indices of solutions with an accuracy of f0.1%.

Effect of Sodium Chloride Brine Concentration The binary system Aerosol OT-water has been extensively

s t ~ d i e d . ~ ~ ~ ~ ~ ~ ~ ~ , ~ ~ The solubility of AOT in water is quite low; at 30 O C it was found to be about 1.8 wt %. This result compares well with published data.* Beyond this solubility limit, an unstable dispersion of liquid-crystalline particles exists having as the continuous phase an isotropic aqueous solution. Various types of liquid-crystalline phases occur a t higher surfactant concen- trations.*' The final anhydrous compound is also found to be birefringent and exhibits the two-dimensional structure of the reverse hexagonal phase.6

As mentioned previously, the effect of electrolytes on the phase behavior of this surfactant system has been studied to some ex- tent.7J8*19 A detailed study by F0nte11~~ on the effect of the electrolyte NaCl on the aqueous AOT system has provided much information on the phase behavior of this system. These studies indicate that, at low surfactant and salt concentrations, the transitions observed upon the addition of electrolyte are similar to those exhibited by petroleum sulfonate^^^*^^ and are a further confirmation of the generality of the phase behavior described there. Since the phase behavior provided by Fontell was at a temperature of 20 OC, a similar study was performed at 30 O C

to maintain consistency with the other data reported here. When varying amounts of the electrolyte sodium chloride were

added to the AOT-water mixtures a t high water contents, the phase behavior illustrated in Figure 1 was observed (nomenclature after ref 40). This diagram was drawn by using observations from over 80 data points at different compositions on the diagram. Although the transitions are similar to those at 20 O C , the range of appearance of the individual phases has changed to some extent.

(35) Gardner, J. E.; Hayes, M. E. University of Texas Model 300 Spinning Drop Interfacial Tensiometer Instruction Manual: University of Texas: Austin, TX.

(36) Philippoff, W.; McBain, J. W. Nature (London) 1949, 164, 835. (37) Park, D.; Rogers, J.; Toft, R. W.; Winsor, P. A. J . Colloid Interface

Sci. 1970, 32, 8 1. (38) Fontell, K. In Colloidal Dispersions and Micellar Behavior; Amer-

ican Chemical Society: Washington, DC, 1975; ACS Symp. Ser. No. 9. (39) Ghosh, 0.; Miller, C. A. J . Colloid Interface Sci. 1984, 100, 444. (40) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald,

M. P.; Wood, R. A. J . Chem. SOC., Faraday Trans. 1 1983, 79, 975.

16 14 12 IO ACN -+

Figure 2. Phase diagram of wt % NaCl vs. alkane carbon number at WOR = 1 for 2.5 wt % Aerosol O T in the system.

At surfactant concentrations above 5 wt %, the condition for transition from the liquid-crystalline (L, and L1 + L,) to the isotropic regions (L3 and L3 + L,) is almost identical with that at 20 O C . The isotropic surfactant-rich phase found at the higher salinities and termed L2 by Fontel13* exhibits scattering and streaming birefringence and has therefore been identified as the L3 phase commonly encountered in other surfactant systems.

The existence of the pure lamellar phase at concentrations above 5 wt % AOT and between 1.3 and 1.7 wt % NaCl has been confirmed by optical microscopy. At lower salinities, a dispersion of liquid-crystalline particles termed spherulites is found in equilibrium with an isotropic solution, L,. While this dispersion is unstable at very low salinities, a stable dispersion is found with increased concentrations of electrolyte, that is, no macroscopic phase separation is observed even after 1 month of equilibration. In Figure 1, the dashed line demarcates the regions corresponding to the stable and unstable dispersions.

Microscopy of the stable liquid-crystalline dispersions and phases showed details of the transitions that occur as electrolyte concentration is increased in a system containing a constant surfactant concentration of 5 wt %. With 0.5 wt % NaCl in the system, clusters of monodisperse spherulites were found immersed in an isotropic aqueous phase. As the salt content was increased up to about 1.2 wt %, the liquid-crystalline spherulitic particles increased in number and became more uniformly distributed in the solution. Also, a wider size distribution of particles occurred with increase in salinity. Not all the particles observed were spherulites and some were even stretched out like myelins. Finally, at 1.5 wt % NaCI, a pure lamellar phase was seen. These results therefore confirm the expected increase in liquid crystal content as brine salinity is increased in the system.

The three-phase region L, + L3 + L, of Figure 1, also found previously,38 appears at surfactant concentrations between 0.5 and 5.2 wt %. The compositions of the vertices of the triangle, cal- culated by using information on phase volumes and densities, were found to match the experimentally determined data points. It is especially interesting to note that, for the L, and L, vertices, the molar ratios of salt-to-surfactant are found to be 2.89 and 2.67 and salt-to-water molar ratios are 3.73 X lo-, (1.21 wt 7% NaCl in brine) and 6.09 X (1.98 wt % NaCl in brine), respectively. Therefore, although the salt-to-surfactant ratio is nearly the same for both phases, the brine salinity corresponding to the L3 is approximately 70% higher than that for the lamellar phase, L,.

The brine salinities at the three vertices of the three-phase region are in the sequence L, > L, > L,. On comparing L, with L,, the lower salinity in the L, can be attributed to the presence of the electrical double layer associated with the bilayers of the L,. Here there is a decrease in salinity to counteract the effect of the negatively charged surfactant head groups at the bilayer surface. The higher brine salinity in the L, as compared to the L, can perhaps be rationalized as follows. The electrolyte acts to reduce the interfacial tension of the platelike particles, allowing them to be of smaller diameter and form the isotropic L3 phase which has a higher entropy of dispersion than the L, phase.41

(41) Miller, C. A.; Ghosh, 0. Langmuir 1986, 2, 321

Alcohol-Free Aerosol-OT/Oil/Brine Systems The Journal of Physical Chemistry, Vol. 91, No. 17, 1987 4531

Figure 3. Phase behavior of Aerosol OT-brine-n-dodecane at WOR = 1 and 2.5 wt %J Aerosol OT as the brine salinity is varied.

Effect of Alkane Carbon Number While several studies on the effect of electrolytes on AOT-

water-oil systems have been made, a systematic investigation of the combined effect of hydrocarbon chain length and brine salinity on phase behavior has not been made previously. To obtain such information, straight-chain hydrocarbons of various chain lengths were equilibrated with equal volumes of the AOT-brine mixtures. Figure 2 illustrates the sequence of phases observed as system composition is varied. In this diagram, the brine salinity in the aqueous surfactant solution is plotted as a function of the alkane carbon number (ACN). The aqueous solutions consist of 5 wt % AOT and 95% brine.

One of the interesting features of this diagram is the appearance of liquid-crystalline phases in equilibrium with excess oil. The L, + 0 and L, + ME + 0 regions, which appear at low salinities and high alkane chain lengths, can be compared with similar phase behavior found in a petroleum sulfonate system!* The behavior at low ACN is somewhat different in the two systems. For the commercial sulfonates, the absence of liquid crystals in these regions could be due to the presence of alcohol in the system which typically acts to increase surfactant film flexibility and also appears to dissolve liquid crystals as has been found in recent e~periments.4~ For example, studies with an orthoxylenesulfonate surfactant have indicated that decreasing the alcohol content in the system converts an oil-in-water microemulsion to a lamellar liquid crystal, both phases being in equilibrium with excess oil.

As reported previously,4 the L, + 0 region appears to be associated with low interfacial tensions. For example, L,-O interfacial tensions for the AOT-brine-decane system have been found to be of the order of 0.01 dyn/cm. While further inves- tigation of the phase behavior and properties of these phases is in progress, Table I provides some information on the interfacial

(42) Miller, C. A.; Ghosh, 0.; Benton, W. J. Colloids Surf. 1986, f9,197. (43) Ghosh, 0.; Miller, C. A., unpublished research. (44) Ghosh, 0.; Miller, C. A. J. Colloid Inferface Sci. 1987, 116, 593.

TABLE I: Interfacial Tensions in tbe AOT-Brine-Dodecane System 5 n t % AOT in Aqwous Solution, Water-to-Oil Ratio of Unity, and T =30°co

wt % NaCl phases at interfacial in brine equilibrium interface tension, dyn/cm ! .O La + 0 L U - 0 0.1 24 1.1 M E + O ME-O 0.101 1.2 M E + B + O ME-O 0.084 1.4 B + M E + O B - 0 0.044 1.6 B + M E B-ME 0.080

tensions of these and microemulsion phases for the AOT-brine- dodecane system. Here, the L,-O interfacial tension is found to be an order of magnitude higher than that in the AOT-decane system. As expected, when microemulsions appear at higher brine salinities, lower interfacial tensions are found with a minimum occurring in the region near optimum salinity where the mid- dlephase microemulsion solubilizes equal amounts of oil and brine. It should be noted that the interfacial tension values reported in Table I were found to be independent of the speed of rotation of the spinning drop tensiometer.

With increasing brine salinity, the expected microemulsion transition from the lower to middle to upper phase is observed. This transition is shown in Figure 3 where the brine salinity is varied between 1.0 and 2.0 wt %. One of the interesting features of this diagram is that a t the onset of the three-phase brine- microemulsion-oil region, the microemulsion phase appears as the most dense phase instead of appearing between the brine and the oil phases as the commonly found "middle-phase" micro- emulsion. This difference in behavior is due to the slightly higher density of Aerosol OT as compared to other commonly used surfactants which results in the microemulsion being heavier than the excess brine until it solubilizes sufficient amounts of oil and appears as a "middle-phase" microemulsion. This transition also occurs upon decreasing alkane carbon number. Also as seen in other surfactant systems, the three-phase region containing the

d

4532 The Journal of Physical Chemistry, Vol. 91. No. 17, 1987 Ghosh and Miller

m i i 01 I I

B

d 49 0 50 0 51 0 5 2 0 . 5 3 WT % N A C L

Figure 4. Volume fraction as a function of wt % NaCl at WOR = 1 for the Aerosol OT-brine-n-decane system.

0 m l

0 = I 0 I i I

W T . % N A C L Figure 5. Volume fraction as a function of wt % NaCl at WOR = 1 for the Aerosol OT-brine-n-dodecane system.

middle phase microemulsion widens with increasing hydrocarbon chain length as shown in Figures 4 and 5. For example, the middle phase spans a salinity range of 0.4 wt % NaCl with n- dodecane as the hydrocarbon as compared to the significantly smaller range of 0.06 wt % NaCl with n-decane as the oil.

Solubilization parameters, the ratios of water to surfactant and oil to surfactant in the microemulsion phase, were calculated to determine the optimum salinity a t which these ratios are equal. These parameters decrease with increasing hydrocarbon chain length as shown in Figure 6. Also plotted in the figure is the variation in optimum salinity which is indicated by the dashed line. The latter compares well with Salager's c o r r e l a t i ~ n ~ ~ when a value of 0.56 is used for the constant k .

Although the studies of Figure 2 were performed at equal water and oil contents, the effects of adding smaller amounts of oil to the surfactant-brine mixtures are also of interest. Table I1 il- lustrates the phase behavior observed when small amounts of n-decane were added to solutions of 5 wt % AOT in brine. The aqueous solutions, in which the brine salinity is varied between 0 and 0.5 wt % NaCI, consist of dispersions S and S' of liquid- crystalline particles in an isotropic aqueous solution. Although such dispersions are unstable in the absence of salt as shown in Table 11, they are stabilized by the addition of salt. Above 0.2 wt % NaCl, a second dispersion S' is found to coexist with the S. As Table I1 indicates, less than 2 vol % n-decane is required to form a single homogeneous phase or stable dispersion for all

a a

a N H m

/

t"

t 2

9 10 11 12 13 ALKANE CARBON NUMBER

Figure 6. Optimum salinity and solubilization parameters at optimum salinity vs. alkane carbon number for the Aerosol OT system.

TABLE II: Effect of n-Decane on Aqueous Solutions of 5 wt % AOT in Brine" wt '% aqueous NaCl solution 1% oil 2% oil 3% oil 4% oil 5% oil 0.0 s + L, s + L, L, L, + 0 L, + 0 L, + 0 0.1 s s + S' s L, L , + O L , + O 0.2 s + S' s s s S + L , S + L , 0.3 S + S' S s (S + L,) (S + La) s + S' 0.4 S + S' S s (S + La) (S + Le) (S + Le) 0.5 s + S' (S + L,) La L, L a Le + L, "L1 = isotropic surfactant solution; S , S' = dispersions of liquid

crystals; L, = lamellar phase; L3 = isotropic surfactant phase which exhibits streaming birefringence; 0 = oil phase.

salinities up to 0.5 wt % NaCl. A similar result has been reported previously for studies with a commerical petroleum sulfonate.30 Also as noted previously, the hydrocarbon acts to dissolve the liquid crystal at lower salinities forming the isotropic solution, L,. Above optimum salinity, which is about 0.5 wt % NaCl for the AOT- decane system, the liquid crystal content is increased until a pure lamellar phase appears. With further addition of oil, it is transformed into the isotropic L3 phase.

Figure 2 indicated n-dodecane as being the hydrocarbon of the smallest chain length for which all the multiphase regions appear over sufficiently wide brine salinities for performing practical studies. The aqueous solutions corresponding to this range of compositions are seen from Figure 1 to consist of homogeneous liquid-crystalline phases which are comparable to those found in the aqueous petroleum sulfonate solutions. To provide an improved understanding of the transitions between the aqueous liquid- crystalline phases and the microemulsions formed with added oil, the phase behavior of the AOT-brine-dodecane system was in- vestigated.

The AOT-Brine-Dodecane System Pseudoternary diagrams of AOT-brine-dodecane were con-

structed at various salinities around the optimum of 1.5 wt % NaC1. The optimum refers to the salinity where, at equal water-to-oil ratios, a three-phase region consisting of brine, hy- drocarbon, and a middle-phase microemulsion which solubilizes equal amounts of oil and water is found. Since no cosurfactant is present in the system, the phase behavior can possibly be rep- resented on a ternary diagram using brine as a pseudocomponent. That is, the ratios of sodium chloride to water in all the phases in equilibrium are assumed to be equal. Since brine was found to be a suitable pseudocomponent under some conditions in multicomponent commerical surfactant systems,46 it is expected to be a suitable pseudocomponent in this analysis as well. The following studies will indicate how well this scheme works here

(45) Salager, J. L.; Bourrel, M.; Schecter, R. S.; Wade, W. H. Soc. Petrol. Eng. J . 1979, 19, 271. (46) Fleming, P. D.; Viqatieri, J. E. J . Chem. Phys. 1977, 66, 3147.

Alcohol-Free Aerosol-OT/Oil/Brine Systems

0

\

0.0 0 : 2

0

Figure 7. Pseudoternary diagram of the Aerosol OT-brine-dodecane system at 1.0 wt % NaCl brine.

since it provides major simplification in representing phase be- havior.

Surfactant compositions up to 25 wt % were studied as the phase behavior at higher surfactant contents is not of practical interest.

Low Salinity 1 .O% NaCl. Figure 7 shows the phase behavior found at various

compositions of surfactant, brine, and n-dodecane, when brine salinity is fixed at 1 .O wt % NaCl. As indicated by Figure 2, the L, + 0 region occurs a t this salinity for a system containing 5 wt % AOT in the aqueous solution. Figure 7 was constructed with observations from over 60 data points a t different compositions. This diagram indicates that several multiphase regions occur as surfactant, oil, and brine contents are varied.

The two-phase region, L, + 0 in which a liquid crystal coexists in equilibrium with excess oil, is present over a wide range of water and oil contents. Optical microscopy on the liquid-crystalline phases has shown that these are pure lamellar phases, and they are analogous to those found when aqueous solutions of petroleum sulfonates are equilibrated with hydrocarbons of long chain lengths.42 Also found in both systems is the three-phase region, L, + M E + 0, which occurs over a narrower range of compo- sitions as compared to the L, + 0 region. Figure 7 indicates that the L, + M E + 0 region appears to follbw a iine of constant brine-to-surfactant ratio equal to 4.5.

At slightly higher surfactant concentrations, another two-phase region, M E + 0 occurs, which is followed by the three-phase region, B + M E + 0 containing the middle-phase microemulsion. That this region is not bounded by three sides of a triangle is indicative of the fact that brine does not act as a good pseudo- component in this region. Here, electrical double layer effects, as in the L, and L, equilibrium discussed earlier, likely play a major role in inducing such behavior. However, at lower surfactant concentrations, as indicated in Figure 7, the pseudocomponent scheme seems to work very well.

With further increase in surfactant content the excess oil phase disappears, and excess brine and a microemulsion phase are found to coexist in equilibrium. As the water-to-oil ratio is varied in this region, a continuous transition in the structure of this up- per-phase microemulsion occurs. Calculations of solubilization parameters from phase volumes indicate that, while the surfac- tant-containing phase is water continuous at high water contents, it becomes a water-in-oil microemulsion at compositions with large amounts of oil. Therefore, a bicontinuous phase is expected in the region where the transition takes place. A similar transition has been noted in the petroleum sulfonate surfactants when the water-to-oil ratio is varied at salinities above the optimum.39

On the brine-surfactant side of Figure 7, liquid-crystalline phases occur. Here, the sequence of phases is found to correspond roughly to the sequence of aqueous phases seen when surfactant concentration is varied at 1.0 wt % NaCl (Figure 1). When oil was added to these aqueous solutions, less than 5 vol % hydro- carbon was required to convert the liquid-crystalline phases into multiple phases even for surfactant-brine mixtures containing up to 20 wt % surfactant.

At surfactant concentrations below 5 wt %, the aqueous solu- tions consisted of dispersions of liquid-crystalline material. Less than 2 vol % n-dodecane was required to transform these into

The Journal of Physical Chemistry, Vol. 91, No. 17, 1987 4533

0

d

m 0

z 0 H I-m

U L L

W

30 J 0 >

zo

x:

N 0

0

0

1 . 0 [ v b / (vb v,) 1

Figure 8. Volume fraction as a function of brine content for the Aerosol OT-brine-dodecane system at 1.0 wt % NaCI.

TABLE III: Total Concentration of Counterions in 10 wt % AOT-1 wt % NaCl Brine-Dodecane System

counterions counterions total Na+ water-to-oil from from concn in

ratio phases brine" surfactant" brine, M 3.50 La + 0 0.01 1 97 0.022 49 0.490 1.25 L, + M E + 0 0.008 55 0.02249 0.620 0.80 M E + 0 0.00684 0.02249 0.731 0.38 B + M E + 0 0.00427 0.02249 1.071 0.20 B + M E 0.002 56 0.02249 1.670 0.06 M E ( 0 ) 0.000 85 0.022 49 4.669

" Moles of Na in 100 mL of the overall system.

isotropic surfactant-rich phases termed L1 which coexist with excess oil a t higher hydrocarbon contents. The dotted region in Figure 7 shows where detailed phase behavior was not determined as changes in phase behavior occurred within an extremely narrow range of oil contents which unfortunately was less than the ex- perimental accuracy. A thermodynamically consistent phase diagram which agrees with available data in this region is shown in the small insert.

Figure 7 indicates that the system becomes more hydrophilic when brine is added to the surfactant-oil mixture, or, more generally, when water-to-oil ratio (WOR) is increased, or sur- factant concentration is decreased. The progression of phase behavior that occurs when brine is added to a mixture of 20 wt % surfactant in oil is illustrated in some detail in Figure 8. Here the phase volume fraction is plotted as a function of brine content in the system.

The following argument can be made to explain these transi- tions. Table I11 shows calculations of the total concentration of counterions in a system containing 10 wt % surfactant. The table illustrates that the counterion concentration increases with de- creasing WOR. If it is assumed that the total concentration of counterions determines phase behavior, that is, increasing coun- terion concentration makes the system more hydrophobic, then the observed transitions can be easily explained by the calculations of Table 111.

When the surfactant concentration is increased at a constant water-to-oil ratio, the phase transitions indicate that the system becomes less hydrophilic as illustrated by line AB' in Figure 7 along which the water-to-oil ratio equals unity. The sequence of phases encountered on increasing surfactant content is found to be similar to that seen when the brine salinity is increased. The explanation for the behavior is basically the same as in the pre- ceding paragraph. As surfactant concentration increases, the overall concentration of counterions in water increases, as Table IV demonstrates. Thus, even though the brine content in the system decreases, the increasing counterion concentration in the system makes the surfactant less hydrophilic.

4534 The Journal of Physical Chemistry, Vol. 91, No, 17, 1987 Ghosh and Miller

TABLE I V Total Concentration of Counterions in Brine for AOT-1 wt '?& NaCl Brine-Mecane Svstem at a Water-to-Oil Ratio of Unity

~~

wt % counterions counterions total Na' AOT in from from concn in svstem ohases brinea surfactant" brine, M

~~~ ~

2.0 L, + L, + 0 0.0084 0.0045 0.260 5.0 L, + 0 0.008 I 0.0112 0.407

10.0 L, + ME + 0 0.0077 0.0225 0.671 12.0 ME + 0 0.0075 0.0270 0.784 15.0 B + ME + 0 0.0073 0.0337 0.965 20.0 B + ME 0.0068 0.0450 1295

"Moles of Nat in 100 mL of the overall system.

Figure 9. Pseudoternary diagram of the Aerosol OT-brine-dodecane system at I . 1 wt '3% NaCl brine.

I . I % NaCl. The phase behavior was next studied at a slightly higher brine salinity of 1.1 wt % NaCl and the resulting pseu- doternary diagram is illustrated in Figure 9. Here the sequence of phases is quite similar to that in Figure 7 . One difference is the absence of the L, + 0 and the L, + L, + 0 regions at this salinity. The domain over which the liquid-crystalline phases appear therefore seems to decrease with increasing salinity. Further, because of the absence of the L, + 0 region in this diagram, pesudoternary behavior is not found over much of the phase diagram, e.g. the L, + ME + 0 region would be triangular and surrounded by three two-phase regions, one of them being L, + 0, if the behavior were pseudoternary.

The three-phase region L, + ME + 0 now appears over a somewhat wider region than in Figure 7. At lower surfactant concentrations, in place of the L, + 0 region observed previously, a two-phase region consisting of excess oil and an isotropic sur- factanbrich phase appears. Near the surfactant-brine axis, the latter phase exhibits streaming birefringence as seen in the L, phase,41 which is indicative of the possible presence of anisotropic particles in the system. With the addition of hydrocarbon, the surfactant phase gradually transforms into a microemulsion as is indicated by the decrease and eventual disappearance of streaming birefringence. For example, when small amounts of oil are added to aqueous solutions containing 5 wt % surfactant, solutions with up to 2 vol % oil exhibit birefringence. As in Figure 7 , the transitions occurring at very low oil contents near the surfactant-brine axis were not determined here. However, a phase diagram that is thermodynamically consistent with the data is indicated in the small inset diagram in Figure 9.

At smaller concentrations of surfactant, an excess brine phase separates so that another three-phase region is found in which a surfactant-containing phase coexists in equilibrium with excess oil and brine. As shown in Figure 9, this three-phase region extends over a wide range of brine-to-oil ratios as was also seen at the lower salinity in Figure 7 . Brine appears to be a reasonable pseudocomponent a t these low surfactant concentrations where there are fewer aggregates having electrical double layers. At higher surfactant contents where the middle- and upper-phase microemulsions are found, the approximation becomes invalid. However, it is important to note that in Figures 7 and 9 particular multiphase regions seem to be characterized by definite ranges of surfactant-to-brine ratios.

1.2% NaCI. When brine salinity is increased to 1.2 wt % NaCI, a marked change in phase behavior occurs as shown in Figure 10. The liquid-crystalline phases that appeared over a wide range of water-to-oil ratios in the previous diagrams corresponding to lower salinities now are found only close to the surfactant-brine axis.

\ 1.+1

f

-.. \

0

Figure 10. Pseudoternary diagram of the Aerosol OT-brine-dodecane system at 1.2 wt % NaCl brine.

'i \

0 0 1 2 3 4 5 6

W A T E R - T O - O I L R A T I O Figure 11. Solubilization parameters in middle phase microemulsion vs. overall water-to-oil ratio in the 10 wt % Aerosol OT-1.2 wt 76 NaCl brine-dodecane system.

With more than 5% hydrocarbon in the system, the liquid crystals disappear forming the isotropic microemulsions.

The three-phase region containing the middle-phase micro- emulsion appears over a wide range of compositions as shown in Figure 10. While a detailed analysis of the phases was not performed, phase volume and density measurements indicated that the composition of the phases in this three-phase region changed with the overall composition of the system, Le., behavior was inconsistent with that characteristic of a true ternary system. It occurred because the sodium chloride in the brine partitioned differently than the water among the phases. Such difference in partitioning was seen previously in the oil-free diagram (Figure l), where a three-phase region occurred. Thus, some of the same reasoning may apply here as well. For example, the salinity difference between brine and the microemulsion phases in the two-phase and three-phase regions with oil may be due to electrical double-layer effects as discussed earlier. Indeed, measurements of salinities of the excess brine and microemulsion phases in other surfactant systems have indicated differences in partitioning be- tween these phases.47

Solubilization parameters, which are volume ratios of water- to-surfactant and oil-to-surfactant in the surfactant-rich phase, were calculated at several compositions to provide an analogy with the petroleum sulfonate systems. These parameters are plotted in Figure 11 as a function of WOR. The surfactant concentration was kept constant at 10 wt %. As shown in this figure, a con- tinuous change in microemulsion composition occurs with in- creasing WOR, including inversion from an oil-continuous to a water-continuous microemulsion at a WOR of about 0.56. Figure

(47) Robertson, S. D. "An Empirical Model for Microemulsion Phase Behavior", presented at the SPE/DOE Fifth Symposium on Enhanced Oil Recovery of the Society of Petroleum Engineers and the Department of Energy, Tulsa, OK, April, 1986.

Alcohol-Free Aerosol-OT/Oil/Brine Systems The Journal of Physical Chemistry, Vol. 91, No. 17, 1987 4535

0 I

0 2 4 6 8 10 12 W A T E R - T O - O I L R A T I O

Figure 12. Solubilization parameters in upper phase microemulsion vs. overall water-to-oil ratio in the 20 wt % Aerosol OT-1.2 wt % NaCl brine-dodecane system.

/Y I

Figure 13. Pseudoternary diagram of the Aerosol OT-brine-dodecane system at 1.5 wt % NaCl brine.

12 indicates the solubilization parameters calculated for the two-phase region B + ME in which the overall surfactant content was fixed at 20 wt 5% while the WOR was varied. As found previously in the three-phase region, the microemulsion phase gradually inverts from oil-in-water to water-in-oil as the hydro- carbon content in the system is increased.

Optimum Salinity The phase behavior of the system was also studied at salinities

near the optimum as shown in Figure 13 which corresponds to a brine salinity of 1.5 wt %. This diagram is considerably different from idealized phase diagrams sometimes used to describe anionic surfactant systems at compositions near optimum salinity.

Because the electrolyte in the brine acts as a fourth component, the three-phase region B + M E + 0 does not appear as a tri- angular region in this diagram. A comparison of Figures 10 and 13 indicates that the latter exhibits a smaller three-phase region closer to the brine-oil axis and, consequently, a higher solubili- zation of both brine and oil. As before, the composition of the middle-phase microemulsion varies with the overall composition of the system. Between WOR of 0.5 and 2.0, as the two-phase (B + ME) region is approached through the three-phase (B + M E + 0) region, the interface between the ME and 0 appears diffuse with increasing amounts of the surfactant being present in the oil. This observation is indicative of nearness to a critical end point.

At surfactant concentrations above the domain of the three- phase region, the two-phase region B + ME is found. Here again, the composition of the M E phase gradually inverts from water- continuous to oil-continuous as WOR is decreased. The per- formance of brine as a pseudocomponent is poor a t these com- positions, as is further indicated by the appearance of the

Figure 14. Pseudoternary diagram of the Aerosol OT-brine-dodecane system at 2.0 wt % NaCl brine.

three-phase region B + La + L3 on the surfactant-brine side of the diagram.

High Salinity At salinities above the optimum, the surfactant becomes even

less hydrophilic so that it exists as an upper phase microemulsion in equilibrium with excess brine. Figure 14 shows the resulting phase behavior when the compositions are varied at a constant brine salinity of 2.0 wt % NaC1.

The two-phase region B + M E appears over a wide composition range in the diagram. At these salinities, the brine appears to act as a single component so that the choice of brine as a pseu- docomponent is excellent here. The binodal line could therefore be calculated from phase volume and density measurements of the phases a t different compositions in the region.

A continuous transition from the water-continuous surfactant phase L3 to an oil-continuous microemulsion M E occurs as oil is added. As illustrated in Figure 14, this inversion of the sur- factant phase occurs while it coexists with excess brine in the two-phase region and as well as when it is present as a single phase at higher surfactant concentrations.

Summary and Conclusions The phase behavior of the model surfactant Aerosol OT has

been studied by investigating the effect of electrolyte, alkane carbon number, and water-to-oil ratio a t various surfactant concentrations. These investigations provide extensive knowledge of the phase behavior of a model four-component alcohol-free system.

With the addition of an electrolyte to AOT-water mixtures dilute in surfactant, the transitions in the liquid-crystalline phases are found to be similar to those of petroleum sulfonates and other anionic surfactant systems. When hydrocarbons of various chain lengths were equilibrated with the aqueous surfactant solutions, again behavior similar to that of the petroleum sulfonates is observed.

Since the hydrophilic and lipophilic properties of Aerosol OT are nearly balanced, the surfactant is able to form liquid crystals and microemulsions at different compositions of water and oil in the absence of any cosurfactant. This simplifies the analysis of phase behavior as pseudoternary diagrams using brine as a pseudocomponent can be constructed to provide an improved understanding of how the various phases form. This assumption of brine as a pseudocomponent was found to work best at salinities well below and well above the optimum and at low surfactant concentrations, Le., the phase diagrams for these conditions were consistent with true ternary behavior. Finally, the results indicate that the surfactant becomes less hydrophilk as the amount of surfactant in the system increases or as that of brine decreases. Since the surfactant is dissociated, both these effects produce an increase in the ratio of total counterions (Na') present to water.

Acknowledgment. This research was supported by grants from Amoco Production Co., Arm Oil and Gas Co., Gulf Research and Development Co., Exxon Production Research Co., the Mobil Foundation, and Shell Development Co. Assistance with optical microscopy and surfactant purification provided by W. J. Benton is greatly appreciated.

Registry No. NaCI, 7647-14-5; AOT, 577-1 1-7; decane, 124-18-5; dodecane, 112-40-3.