Uses and limitations of rigid lattice models for rationalising excess thermodynamic functions of binary liquid mixtures at low pressures

Abstract

The recently introduced model of binary liquid mixtures based on a simplified two-liquid treatment of a rigid lattice (AGAPE) is used to represent the behaviour of their excess thermodynamic functions, in terms first of an unlike-pair intermolecular potential and secondly of this potential taken together with a geometric (steric) parameter. For relatively simple molecules up to ethane or ethene, a rigid lattice model appears capable of giving a logical treatment of the excess Gibbs energies and excess enthalpies. Ab initio predictive procedures using this model however, require a very sensitive appreciation of the ‘whole-molecule’ interaction and geometrical arrangements. Extrapolation of this kind of model to larger polyatomic molecules demonstrates difficulties and falls short of providing a bridge between the numerical molecular group methods based upon rigid lattice principles (UNIFAC) and the models based on modern appreciation of statistical mechanics and intermolecular forces. The need to provide such a bridge is emphasised, to give more physical substance to the vast machinery of numerical engineering predictions currently in use.