The Combinatorial Term for COSMO-Based Activity Coefficient Models

Abstract

COSMO-based activity coefficient models, similarly to the UNIQUAC method, rely on two contributions: combinatorial and residual. The combinatorial term should account for differences in size, shape, and free volume. The residual term should account for electrostatics, dispersion, and hydrogen-bonding effects. The recent literature shows a great effort in improving the residual term, while less attention is given to the combinatorial contribution. This is partly justified by the fact that the residual contribution can be orders of magnitude larger than the combinatorial one. Nevertheless, once the activity coefficient of a substance in mixture is given by the product of these two contributions, both are important. In this work, the combinatorial expressions currently in use in COSMO-based models are reviewed. It is also shown that the model performance can be much improved by assuming a combinatorial contribution similar to that present in the modified UNIFAC model.