Unified treatment of intermolecular and thermodynamic potentials of polar fluids. II. Angle-dependent intermolecular potential

Abstract

A unified treatment is presented of intermolecular and thermodynamic potentials of polar fluids. The theory is specialized to the case in which the rotational and translational correlation times are comparable. It is shown that the latter condition leads naturally to an intermolecular potential which can be decomposed into repulsion, dispersion, electrostatic, and polarization terms, all of which are angle dependent. Expressions are developed for the fluid free energy in a perturbation series and the first few terms are considered in detail. Special attention is focused on the classical limiting form of the free energy for a system of axially-symmetric molecules. It is shown that the free energy expression is consistent with a Boltzmann distribution in which the intermolecular potential is, within the dipolar approximation, represented by a set of infinite order pair potentials, the leading terms of which reduce to the Stockmayer potential plus polarization terms. Comparison is made between our series expansion of the fluid free energy and other perturbation schemes.