Interfacial tension and interfacial profiles of fluids and their mixtures

A quasi-thermodynamic treatment of inhomogeneous systems is presented and applied for the estimation of the interfacial tension of pure fluids and their mixtures. The density gradient theory is combined with a recently developed equation-of-state model of fluids and their mixtures. The combined model can be applied to nonpolar fluids as well as to systems of fluids interacting with strong specific forces, such as strong hydrogen bonds. The predictions of the model are rather satisfactory in the case of pure fluids and for mixtures of nonpolar or weakly polar fluids. In the case of associated mixtures, the predictions are in qualitative agreement with the experimental data. The calculated composition profiles across the interface in the interesting case of azeotropic systems exhibit a peculiar behavior. An attempt has been made for a plausible justification of this behavior.