INTERDROPLET ATTRACTIVE FORCES IN AOT WATER-IN-OIL MICROEMULSIONS FORMED IN SUBCRITICAL AND SUPERCRITICAL SOLVENTS

The van der Waals attractive interactions between aqueous droplets in water-in-oil type microemulsions have been investigated for a range of continuous-phase solvents including the alkanes from methane to isooctane and the noble gases, krypton and xenon. Hamaker constants for water droplets with surfactant shells of the sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in subcritical and supercritical solvents were calculated by using Lifshitz theory and the resulting interaction potential calculations qualitatively account for many features of the phase behavior of these systems. A correlation was experimentally established between the continuous-phase dielectric constant and the maximum water-to-surfactant ratios, W0, which vary from W0 = 1 for methane at 25°C (and 1650 bar) to greater than 100 for the near-critical and supercritical fluids. Both the aqueous core and the hydrocarbon shell of the droplet are important components of the total attractive potential for the lower dielectric continuous-phase solvents, including methane, ethane, propane, krypton, and xenon. Phase behavior studies suggest that the van der Waals attractive interactions are a major component of the total interdroplet potential for these lower dielectric solvents; however, the relative contribution to the total attractive potential of a second type of interaction, the overlap and intersolvation of the hydrocarbon tails of two droplets, is unresolved. The important role of the second phase which can be either water or oil continuous, upon phase behavior, is indicated. © 1990 American Chemical Society.