Ionic Strength Effects of Electrolytes on Solubilized States of Water in AOT Reversed Micelles

Abstract

The limiting amounts of solubilized aqueous NaCl, NaNO3, MgCl2, and AlCl3 in sodium 1,2-bis-(2-ethylhexyloxycarbonyl)-1-ethanesulfonate (aerosol OT)/isooctane solutions have been measured as a function of the ionic strength of the electrolytes. In any system, the limiting amounts increased up to the optimal ionic strength (Ioopt), and afterward, as ionic strength increased, they decreased and were followed by a constant. These increased and decreased curves of water solubilization below and above Ioopt, called, respectively, salting-in and salting-out curves, could be interpreted, respectively, from the counteracting effects of attractive intermicellar interaction and interfacial bending stress. The effects of electrolytes on the solubilized states of water by micelles were examined using NMR and near-infrared spectroscopic techniques. Consequently, two types of waters, i.e., water bound directly to the ionic head groups of the surfactant in reversed micelles and water interacting with the hydrated head groups in swollen micelles through hydrogen bonds, were almost unaffected by any electrolytes, whereas much greater effects were observed for bulklike water in W/O microemulsions. The order of the degree of lowering of salting-out curves in W/O microemulsion regions in each system was found to agree with the Hofmeister series.