Generalized van der Waals theory. IX. Excess properties of simple fluid mixtures

The generalized van der Waals theory, recently developed and applied to a wide range of problems involving simple single component fluids, is here extended to uniform simple fluid mixtures. Binary mixtures of hard spheres or particles interacting by Lennard-Jones pair potentials are considered, the Lorentz-Berthelot rules being used to generate the between-species pair-potential. The basic theory is very simple, combining features of cell and mean field theories. Yet it differs considerably from the presently favoured theories of mixtures. The new GvdW theory is implemented at four levels of sophistication but the calculations remain very simple by present standards even for the most accurate form of the theory wherein the effective hard-sphere diameters are determined variationally. The results obtained for the equation of state and thermodynamic excess properties indicate that the very favourable accuracy relative to the simplicity of the theory observed for single component fluids is retained for mixtures. In the case of excess properties calculations for a wide range of equimolar mixtures suggest that the GvdW accuracy is substantially better than that obtained by random mixture or average potential theories but not as good as that obtained by the best theories based on the estimation of the radial distribution functions of the fluid.