An expression for activity coefficients of liquid mixtures based on the local surface composition.

Abstract

Based on the energy of mixing with local surface fractions and the Gibbs-Helmholtz equation, a new activity-coefficient equation, referred to as SILS (Simple Local Surface Fractions), is derived. The SILS equation contains only two interaction parameters for a binary system, and it is applicable to both miscible and partially miscible systems. This equation can be generalized to multicomponent systems without any additional parameter. Correlation of binary vapor-liquid equilibria and prediction of ternary vapor-liquid and liquid-liquid equilibria using only binary experimental data are made by means of the SILS equation. The SILS and modified UNIQUAC equations assure almost the same accuracy when applied to calculating vapor-liquid equilibria. Furthermore, the SILS equation is found to give better accuracy than the modified UNIQUAC equation for prediction of liquid-liquid equilibria.