APPLICATION OF THE QUASI-GAUSSIAN ENTROPY THEORY TO THE CALCULATION OF THERMODYNAMIC PROPERTIES OF WATER AND METHANE IN THE LIQUID AND GAS-PHASE

In this article we investigate the applicability of the statistical Ga mma state as following from the quasi-Gaussian entropy theory, where a ll thermodynamic properties at every temperature are obtained from the knowledge of the potential energy distribution at one temperature. We compared for a typically polar system (water) and an apolar one (meth ane) the experimental heat capacity and entropy data with the predicti ons of the theory at various densities, ranging from the almost ideal gas to typical liquids. Interestingly, the behavior of water and metha ne is quite similar. Low-density gases and fluid-liquid systems can be described as weakly perturbed Gamma states. For intermediate densitie s a more complex statistical state arises. In order to describe more a ccurately the fluid-liquid regime, we propose in this paper a confined Gamma state, based on the division of phase-space into two different regions: one of them described by an exact Gamma state and another ver y unstable one. We conclude that typical fluid-liquids can be describe d very well by this new Gamma state approximation. We also try to give a physical interpretation of the two parts of phase space that arise from the model. The high accuracy of the theory over a large temperatu re range makes the approach very suitable for the prediction of thermo dynamical properties at, for example, supercritical conditions. (C) 19 96 American Institute of Physics.