Light Scattering and Electrical Conductivity Studies of the Aerosol OT Toluene Water-In-Oil Microemulsions M. Mercedes Vela ´ zquez,* ,² Margarita Valero, ‡ and Francisco Ortega § Departamento de Quı ´mica Fı ´sica, Facultad de Quı ´mica, UniVersidad de Salamanca, Plaza de la Merced s/n, 37008 Salamanca, Spain, Departamento de Quı ´mica Fı ´sica, Facultad de Farmacia, UniVersidad de Salamanca, Apdo. 449, 37080 Salamanca, Spain, and Departamento de Quı ´mica Fı ´sica I, Facultad de Ciencias Quı ´micas, UniVersidad Complutense, 28040 Madrid, Spain ReceiVed: August 2, 2000; In Final Form: July 23, 2001 By using electrical conductivity and static and dynamic light scattering measurements, the behavior of water - oil (w/o) microemulsions formed by Aerosol OT dissolved in toluene has been examined. To study the effect of the droplet volume fraction on the microemulsion behavior, the water and the surfactant concentrations were systematically modified and the temperature dependence was also studied. The scattering intensity and the apparent diffusion coefficients are analyzed with a model involving the Carnahan-Starling hard-sphere model including an attractive term. It was found that attractive interactions become important in w/o microemulsions containing w 0 > 37. No percolation threshold is detected at the droplet volume fraction and temperature values used in this work. Introduction Microemulsions are thermodynamically stable, isotropic transparent dispersions of two immiscible liquids, i.e., water and hydrocarbon, and one or more surfactants. A water-oil (w/o) microemulsion contains isolated water pools surrounded by a monolayer of surfactant molecules dispersed in a continu- ous oil phase. AOT as surfactant is well-known to form w/o microemulsions in several alkane oils. The aggregation process is fairly well characterized with respect to size and shape at various water contents. 1-5 All of these works concluded that the w/o microemulsion structure primarily depends on the water/ surfactant molar concentration ratio, w 0 ) [water]/[surfactant], and on the bulk solvent. 6-9 In previous papers, we reported structural properties of AOT w/o microemulsions dissolved in cyclohexane, isooctane, and toluene and we also studied the effect of the addition of water- soluble polymers on the structure and properties of these microemulsions. 9-12 Infrared and fluorescence spectroscopy, dynamic light scattering, and conductivity measurements were used. Results showed significant differences between the water structure of AOT microemulsions dissolved in toluene and those dissolved in cyclohexane and isooctane. 9 On the other hand, the electrical conductivity results shown that in microemulsions composed of AOT dissolved in toluene conductivity increases until a certain critical w 0 value, around 25-35, and above this water concentration, a sharp increase of the conductivity was observed. 10 However, this increase of conductivity is not sharp enough to attain the percolation threshold. The addition of poly- (vinylpyrrolidone) and poly(sodium-4-styrenesulfonate) poly- mers prevents this behavior. 10 The breakpoint of conductivity/ water concentration curves was also observed in the photophysical properties of Acridone solubilized in these microemulsions. 13 It is well established that AOT forms in toluene w/o microemulsions. 14 The size and the aggregation number of these microemulsions have been determined by dynamic light scat- tering and viscosity measurements. 8 Results indicate that at relatively low water concentration, w 0 e 10, the droplet radius linearly increases with w 0 . Significant differences between results obtained by dynamic light scattering and viscosity were observed. In all cases, the radii obtained by DLS were lower than those measured by viscosity method, differences were interpreted as being due to polydispersity. 8 The linear relation between the droplet radius and w 0 was observed in AOT microemulsions dissolved in some organic solvents. 8 However, by using synchrotron radiation small-angle X-ray scattering, some deviation from the linear relation were detected in apolar solvents such as n-hexane, n-heptane, n-octane, and iso- octane. 15,16 Recently, 17 using small angle neutron scattering (SANS), the radius of microemulsions of D 2 O/AOT/C 6 D 5 CD 3 without and with additives has been reported for w 0 e 20. Neglecting interdroplet interactions, these results show that at low water content (w 0 e 7) the radius linearly increases with w 0 , whereas at w 0 ≈ 7.5, a sharp increase of the droplet radius is reported; however, above this water content, the radius increases. When additives are present, the droplet radius is smaller and a slight change with w 0 is observed. There are no results of the size of AOT toluene w/o microemulsions containing a water concentration greater than 20. Geiger reported that the maximum water concentration solubilized in these microemulsions was w 0 ) 12 at 298 K and that increasing the water content leads to a phase separation. 18 We will show in this work that stable microemulsions containing until w 0 ) 50 can be prepared. In this work, we are interested in the origin of the sharp increase of conductivity of AOT toluene w/o microemulsions containing high water concentrations. Therefore, by using dynamic light scattering, we determine the effect of water and surfactant concentrations on the droplet size. In addition, the effect of the temperature on the behavior of these w/o micro- * To whom correspondence should be addressed. Fax: 00-34-923- 294574. E-mail: mvsal@usal.es. ² Facultad de Quı ´mica, Universidad de Salamanca. ‡ Facultad de Farmacia, Universidad de Salamanca. § Universidad Complutense. 10163 J. Phys. Chem. B 2001, 105, 10163-10168 10.1021/jp002812n CCC: $20.00 © 2001 American Chemical Society Published on Web 10/03/2001