Kinetics of the interfacial ion exchange in winsor II microemulsion systems

Abstract

The kinetics of the ion exchange between an aqueous and a reverse micellar phase (Winsor II system) was studied in detail using a two-phase stirred cell. The observation that the solubilization rate was independent of convection in all experiments leads to the conclusion that the formation of reverse micelles with new counterions (i.e., the ion solubilization) is controlled by an interfacial process. Furthermore, it was observed that main group cations differ strikingly from cations of the transition groups of the periodic system of elements in solubilization rate, in interfacial tension, and in the influence of electrolyte concentration. This different behavior was explained by specific interactions, probably by some kind of chelate structures being possible between transition metals and the sulfonate group of the surfactant molecule (AOT) at the interface. A further series of experiments showed that the solubilization rate pronouncedly increases with the number of micelles. From this the conclusion can be drawn that the AOT coverage at the macroscopic interface increases as well. These facts point to a specific mechanism of adsorption being controlled by coalescence and new formation of micelles. The observed influence of solvents on the formation of reverse micelles is interpreted by the different ability of solvent molecules to penetrate into the palisade layer of AOT molecules at the interface. This penetration ability is higher for short-chain or cycloalkanes which therefore can stabilize the interfacial layer more effectively. Thus, the rate of micelle formation is significantly reduced.