New model for classical fluids. II

Abstract

A model for classical fluids given previously has been extended in two ways. An approximate Helmholtz free energy is obtained by using the equation of state of hard spheres to continue a virial series truncated after N+1 terms into the whole density-temperature plane. The model given earlier corresponded to N=1. A modified version of the model is given, which contains an additional function of the excess entropy. It is shown that the two versions of the model constitute truncation after one and two terms of an expansion of the thermodynamic function which transforms from the model free energy to the exact free energy. The expansion is made around the line of zero excess pressure, suggesting that the modified model is appropriate for liquids. It is formulated entirely in terms of experimentally accessible thermodynamic quantities. Comparison with experimental data for argon favors the modified version.