Analytic equation of state for Mie(α, β) fluids based on an improved Ross variation perturbation theory

Abstract

The analytic expressions of thermodynamic properties obtained by Sun for multi-Yukawa fluids from the Ross variation perturbation theory (Mol. Phys. 105 (2007) 3139) can be used to study other practical fluids over wide ranges of temperature and density by mimicking their intermolecular potentials with two-Yukawa (TY) functions. Using the Lennard-Jones LJ (12, 6) as an example, Sun noticed that the accuracy obtained is sensitive to the choice of the fitted TY potential. To remedy this drawback we propose in this work to introduce into Sun Ross analytical (RA) theory correction terms that take into account the discrepancies between the more general Mie(α, β) potential (the LJ is the Mie(12, 6)) and its two-Yukawa representations leading to an improved Ross analytical corrected (RAC) theory. By using three different TY potentials fitted on the LJ from the literature the corresponding excess Helmholtz free energy, excess internal energy and compressibility factor Z obtained from the Ross analytical corrected (RAC) theory are seen to be all in good agreement with the values obtained from the original Ross numerical (RN) theory applied directly to the LJ potential for a wide range of temperatures and densities. These results show that the introduction of these analytical correcting terms increases the accuracy of Sun Ross analytical (RA) theory, which also becomes less dependent on the choice of the TY parameters.