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(12)E(5)-C10H22 system in the Winsor III region have been studied. In agreement with the previous findings of Kahlweit et al., a regular Hofmeister trend has been observed: adding ''hydrotropic'' stilts (NaI, NaSCN) makes water, in a phenomenological sense, a less polar solvent, and the bicontinuous microemulsion phase becomes water-rich, whereas with ''lyotropic'' salts (NaF, NaCl, NaBr), an opposite trend is observed. This behavior was previously attributed to a modification of water solution properties, and the brine was modeled as a pseudocomponent. In this paper, we argue instead that the salt effects on the phase equilibrium have an interfacial origin and are due to the salt adsorption/depletion at the surfactant monolayer. A simple model relating the increment of the monolayer spontaneous curvature to the salt depletion at the monolayer has been proposed. Direct measurements of the salt adsorption/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 behavior. The relationship between the phase behavior and the adsorption/depletion at the surfactant monolayer has been shown to be of a general nature and by no way limited to the case of aqueous solutions of inorganic salts: the same trends have been found with other water-soluble (dextran) and oil-soluble (perfluorohexane) additives.