Ionic strength effects of electrolytes on solubilized states of water in AOT reversed micelles

The limiting amounts of solubilized aqueous NaCl, NaNO3, MgC1(2), 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 t he optimal ionic strength (Io(opt)), and afterward, as ionic strength incre ased, they decreased and were followed by a constant. These increased and d ecreased curves of water solubilization below and above Io(opt) called, res pectively, salting-in and salting-out curves, could be interpreted, respect ively, from the counteracting effects of attractive intermicellar interacti on and interfacial bending stress. The effects of electrolytes on the solub ilized states of water by micelles were examined using NMR and near-infrare d spectroscopic techniques. Consequently, two types of waters, i.e., water bound directly to the ionic head groups of the surfactant in reversed micel les and water interacting with the hydrated head groups in swollen micelles through hydrogen bonds, were almost unaffected by any electrolytes, wherea s much greater effects were observed for bulklike water in W/O microemulsio ns. The order of the degree of lowering of salting-out curves in W/O microe mulsion regions in each system was found to agree with the Hofmeister serie s. (C) 2001 Academic Press.