Classification of solubilization isotherms of water by oil—soluble surfactants in nonaqueous solutions

Abstract

The solubilization isotherms of water by oil-soluble surfactants in nonaqueous solutions could be classified into four types when the relative vapor pressure of water solubilized by the surfactants was plotted against the molar ratio of water to the surfactant. Type I showed an S-shaped solubilization isotherm. Water in the initial stage of solubilization was more firmly bound than in the later stage, the amount of strongly bound water being about one molecule per surfactant molecule. Type II was the most frequently encountered solubilization isotherm and had a concave shape to the vapor pressure axis in the initial stage of solubilization, whereas the later stage was similar to Type I. Type III was similar to Type II except for the range neighboring the saturated water vapor pressure, i.e., the isotherm did not reach the saturated vapor pressure. The isotherm classified as Type IV reached the saturated vapor pressure at small ratio of water to the surfactants and was encountered for the system of more oleophilic surfactants. The classification of solubilization of polar substances by dinonylnaphthalenesulfonates and phenylstearates in nonaqueous solutions reported by Kaufman and Bascom et al. was also tried in connection with that of solubilization of water.