A combinatorial activity coefficient model for symmetric and asymmetric mixtures

A new combinatorial activity coefficient model, based on the Staverman-Guggenheim (SG) one, is developed in this study. It involves a system-dependent exponent, determined from the size ratio of its components only, and is useful for group-contribution models. The model, referred to as R-UNIFAC, is applied to the prediction of activity coefficients in the following nearly athermal mixtures: (a) alkane / alkane mixtures; (b) alkane / polymer mixtures; (c) polymer / solvent mixtures, where the activity coefficient data were obtained by constant-pressure Monte Carlo simulations. For all these mixtures the residual contribution to the activity coefficient is expected to be close to zero. w model yields satisfactory predictions for all systems considered, independently of system asymmetry, comparable to those obtained with free-volume models such as the Entropic-FV and p-FV ones. It has the advantage, however, that it does not require pure-component liquid molar volumes and, thus, is directly applicable to systems containing compounds where such volumes are not accurately known or supercritical fluids.