Thermodynamic properties of molecular fluids from the site-site Ornstein-Zernike equation

Abstract

Thermodynamic properties of molecular fluids interacting with site-site Lennard-Jones potentials have been computed from the site-site Ornstein-Zernike equation using the compressibility equation of state. The critical densities and temperatures have been estimated from isotherms of the bulk modulus. The variation of the critical parameters with the shape of the molecules is discussed in the light of these results. For slightly nonspherical systems the results are compared with those from a Mayer function perturbation theory based on a spherical reference potential. It is shown that the compressibility pressure can be obtained in closed form if the ‘Percus-Yevick like’ closure is used to solve the SSOZ equation. The results from this expression are compared with computer simulation results for diatomic liquids. In all cases considered the compressibility pressure is substantially lower than the simulation results. The compressibility equation of state thus obtained is found to obey quite accurately the simple principle of corresponding states for a range of diatomic and tetrahedral fluids if the compressibility factor is tabulated as a function of reduced variables defined in terms of the critical temperature and density.