UNIFIED CLASSICAL AND MOLECULAR THERMODYNAMIC THEORY OF SPHERICAL WATER-IN-OIL MICROEMULSIONS

A unified classical and molecular thermodynamic model is developed in order to predict the phase behavior and interfacial properties of spherical water-in-oil microemulsions. A modified Flory-Krigbaum theory is used to describe the interactions between the surfactant tails and solvent, while the ionic head-group interactions are treated with the Poisson-Boltzman equation. The interfacial tension and the bending moment of the interface are calculated explicitly. These values are incorporated into a classical thermodynamic framework that is forced to satisfy the Gibbs adsorption equation on the interface, guaranteeing thermodynamic consistency. Given a surfactant molecular architecture, the model predicts the size of microemulsion droplets as a function of the chain length of the alkane solvent. For bis(2-ethylhexyl) sodium sulfosuccinate (AOT) in the solvents propane through decane, the calculated trends agree with experiment and are explained mechanistically at the molecular level. The microemulsion radius increases for the solvents pentane through propane, an unusual behavior that is explained theoretically.