Thermodynamics of a square well octopolar fluid

Abstract

A simple octopolar fluid is considered whose particles interact via a square well (SW) potential plus point octopoles. The SW potential models the effects of quasi-spherical overlap and dispersion forces. An analytical equation for the free energy, which contains the explicit dependence on the potential parameters is derived by perturbation theory. The theory is used to analyse the influence of the octopole–octopole interaction on the thermodynamics. The octopolar effects on the vapour–liquid equilibrium are found to be much stronger than those of equivalent dipoles or quadrupoles. The theory is applicable to octopolar quasi spherical molecules and is used as a simple model for the vapour–liquid equilibrium of methane.