A COMBINATORIAL ACTIVITY-COEFFICIENT MODEL FOR SYMMETRICAL 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 Ca rlo simulations. For all these mixtures the residual contribution to t he activity coefficient is expected to be close to zero. The new model yields satisfactory predictions for all systems considered, independe ntly of system asymmetry, comparable to those obtained with free-volum e 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 wher e such volumes are not accurately known or supercritical fluids.