SALT EFFECTS ON NONIONIC MICROEMULSIONS ARE DRIVEN BY ADSORPTION DEPLETION AT THE SURFACTANT MONOLAYER/

The effects of inorganic salts on the phase equilibrium of the H2O-C(1 2)E(5)-C10H22 system in the Winsor III region have been studied. In ag reement with the previous findings of Kahlweit et al., a regular Hofme ister trend has been observed: adding ''hydrotropic'' stilts (NaI, NaS CN) makes water, in a phenomenological sense, a less polar solvent, an d the bicontinuous microemulsion phase becomes water-rich, whereas wit h ''lyotropic'' salts (NaF, NaCl, NaBr), an opposite trend is observed . This behavior was previously attributed to a modification of water s olution properties, and the brine was modeled as a pseudocomponent. In this paper, we argue instead that the salt effects on the phase equil ibrium have an interfacial origin and are due to the salt adsorption/d epletion at the surfactant monolayer. A simple model relating the incr ement of the monolayer spontaneous curvature to the salt depletion at the monolayer has been proposed. Direct measurements of the salt adsor ption/depletion have been conducted. NaF, NaCl, and NaBr are shown to desorb, whereas NaI and NaSCN are shown to adsorb at the monolayer of the bicontinuous microemulsion phase, the Henry constant increasing in the above series from negative (NaF, NaCl, NaBr) to positive values ( NaI, NaSCN), in a good correlation with the microemulsion phase behavi or. The relationship between the phase behavior and the adsorption/dep letion at the surfactant monolayer has been shown to be of a general n ature and by no way limited to the case of aqueous solutions of inorga nic salts: the same trends have been found with other water-soluble (d extran) and oil-soluble (perfluorohexane) additives.