THERMODYNAMIC PROPERTIES OF METHANE AT HIGH PRESSURES

Abstract

Investigation of thermodynamic properties of methane at high pressures is an actual problem of thermophysics. Interest to the properties of solid methane increased significantly in relation with the search for new energy-intensive molecular systems based on materials suitable for storage and easy retrieval of molecular hydrogen used as an environmentally friendly fuel. In this paper we propose a theoretical equation of state for crystalline methane, developed within the framework of thermodynamic perturbation theory, where a system of spherical molecules serves as a reference system, and octupole  octupole interaction of CH4 molecules is the perturbation. The free energy of the unperturbed crystal is described by the equation of state obtained earlier for the Lennard-Jones FCC- crystal, well proved in calculations for highly anharmonic crystals at high pressures. Thermodynamic functions of solid methane are calculated on the melting line in the temperature range 90-300K, which corresponds to pressures up to 1,4 GPa. We estimated the contribution of the octupole  octupole interaction to the various thermodynamic properties of solid methane. It is shown that on the sublimation line this contribution is larger than on the melting line.