Comparison of the Titration and Contact Methods for the Water Solubilization Capacity of AOT Reverse Micelles in the Presence of a Cosurfactant

The method used to form a reverse micellar phase determines the amount of water solubilized in the organic phase. The effect of cosurfactant and salt on the water solubilization capacity per mole of surfactant was determined for two methods of solubilization: contact with an excess aqueous phase and the titration method. The two methods are intrinsically different. While in the titration method, there is a transition from a Winsor IV system to either Winsor I or II system, in the contact method, the experiments are always in a Winsor II system. When the titration method was used at low salt concentrations, or even without salt, the water solubilized increased with the concentration of cosurfactant, at low cosurfactant concentrations. When the contact method was used, the water solubilized per mole of surfactant decreased with an increase in the cosurfactant concentration. When titration was used, at a fixed cosurfactant-to-surfactant molar ratio, several solubilization capacities were obtained at low salt concentrations, whereas a single solubilization capacity was found when the titrant had a higher salt content. With the contact method, a single solubilization capacity was observed for all salt concentrations at which reverse micelles were formed. When the same salt solution was used as the titrant in the titration method and the initial excess aqueous phase in the contact method, a larger amount of water was solubilized by the latter method.