Correlation between surface and bulk structures of alcohol-water mixtures

Adsorption isotherms of binary aqueous solutions of methanol, ethanol, 1-propanol, 2-propanol, tert-butanol, and 1-butanol are demonstrated, being calculated by using the Gibbs adsorption equation with experimental data of surface tension and vapor pressure found in the literature. For all of the alcohol-water mixtures, the maximum value in the adsorption isotherm, namely, the maximum surface excess is about that expected for the formation of a monolayer. Furthermore, the composition of the mixture for the maximum surface excess coincides with that corresponding to the minimum in the excess partial molar volume of the solutes. These results indicate that the hydrophobic hydration in bulk induces the surface excess of the alcohols and after a monolayer is formed, the hydrophobic hydration itself is no longer retained.