Use of ab initio methods to make phase equilibria predictions using activity coefficient models

An important challenge in applied thermodynamics is the prediction of mixtu re phase behavior without the use of any experimental data. Current group c ontribution methods are sometimes, but not always successful in this regard . Activity coefficient models, such as UNIQUAC and Wilson, have explicit en ergy parameters that are usually fitted to experimental data. In this commu nication, we present a unique, new method of obtaining values for these par ameters. We use ab initio quantum mechanics methods to obtain a solution of the Schrodinger equation and determine the molecular interaction energies resulting from hydrogen-bonding and van der Waals interactions. In particul ar, the Hartree-Fock (HF) method is used to determine the interaction energ y between molecular pairs in clusters, and this information is then used in the UNIQUAC model to successfully predict the phase behavior. Examples giv en here are for the methanol + water, ethanol + water, acetic acid + water, and formic acid + water mixtures, the last of which is not accurately desc ribed by current group contribution methods. The analogous procedure using the Wilson model is less successful, suggesting that the energy parameters in the UNIQUAC model are of greater physical meaning. (C) 1999 Elsevier Sci ence B.V. All rights reserved.