EFFECT OF ALCOHOLS ON THE PHASE-BEHAVIOR OF MICROEMULSIONS

In the literature, medium-chain alcohols are frequently considered as cosurfactants that act mainly on the properties of the amphiphilic monolayer at the water/oil interface in microemulsions. In this paper it is suggested that alcohols should rather be considered as cosolvents that distribute between the aqueous and the oil-rich bulk phases, and the interfacial layer, thereby decreasing the effective hydrophilicity of the amphiphile as well as the effective hydrophobicity of the oil. This is supported by the fact that alcohols adsorb rather weakly at the water/oil interface in amphiphile-free H2O-oil mixtures. Experiments on the effect of nonionic amphiphiles on the mutual solubility between water and alcohols, furthermore, give no indication for the formation of (inverse) micelles in the alcohol-rich phase. Finally, it is demonstrated that the effect of alcohols on the phase behavior and, accordingly, on the interfacial tension sigma-ab between the water-rich and the oil-rich phases depends on the position of the experimental temperature T(exp) with respect to the mean temperature TBAR of the three-phase body of the ternary water-oil-amphiphile mixture. With nonionic amphiphiles, sigma-ab increases upon the addition of alcohol if T(exp) > TBAR, but decreases (at first) if T(exp) < TBAR. With ionic amphiphiles, one finds the reverse.