A phase diagram for non-spherical molecules with spherical square wells

Abstract

The phase diagram for a model fluid of hard ellipsoids with an encircling square well potential has been calculated using a van der Waals like theory. In the present theory for the Helmholtz free energy, the hard repulsive interactions have been treated using ‘exact’ second and third virial coefficients, combined with a resummation of the higher order terms in accord with the Carnahan and Starling hard-sphere limit. Attractive interactions in the Helmholtz free energy have been retained to lowest order by the second virial coefficient alone. The present theory accommodates prolate and oblate bodies with roughly the same level of accuracy. Further, the predicted vapour-liquid coexistence curves are in much closer agreement with the simulations of de Miguel and Allen than that of our earlier mean spherical approximation based theory.