MICROCALORIMETRIC INVESTIGATION OF THE SOLUBILIZATION OF WATER IN REVERSED MICELLES AND WATER-IN-OIL MICROEMULSIONS

Microcalorimetry has been used to study the solubilization of water in reversed micelles and water-in-oil (w/o) microemulsions. The systems were based on Aerosol OT (AOT) and two isomerically pure sodium alkylarenesulfonates as surfactants. The solubilization of water can be described in terms of hydration of the head group (ca. three H2O molecules per AOT), swelling of the water droplet, and phase separation. All steps are endothermic, indicating that a gain in entropy is the driving force for solubilization. Above 60-degrees-C the hydration energy for AOT is strongly affected by temperature. The swelling of alkylarenesulfonate w/o microemulsions droplets in n-heptane is more endothermic than that of AOT w/o microemulsion droplets in n-heptane, indicating a significant influence of surfactant structure.