Phase diagrams of interaction site fluids

Abstract

We present a theoretical investigation of the thermodynamics and phase equilibria of fluids of homonuclear diatomic molecules modelled with site-site Lennard-Jones 12-6 potentials. The Helmholtz free energy and distribution functions are obtained using a recently developed cluster perturbation theory (CPT). This is a nonspherical reference perturbation theory with the influence of the attractive forces upon the Helmholtz energy and distribution functions summed approximately to infinite order using cluster expansion techniques. The theoretical predictions of CPT for the Helmholtz free energy and the pressure agree well with the results of computer simulation studies. In the course of the work, the phase diagrams for diatomic molecules with a variety of bond lengths have been determined. The theory predicts the departures from the principle of corresponding states arising from the molecular anisotropy. Comparisons of the theoretically determined critical properties and coexistence curves against those of real fluids show satisfactory agreement given the simplicity of the potential model employed.