Chemical potentials of model and real dense fluid mixtures from perturbation theory and simulations

Abstract

The calculation of chemical potentials from a WCA-type perturbation theory (PT) is described and two consistency tests are given. To check the results at infinite dilution of component A, Widom's test particle method is applied in combination with molecular dynamics (MD) simulations using vectorized codes on a CYBER 205. Technically, with increasing size of the test particles firstly the test particle cut-off radius and secondly the number of time steps has to be increased. Comparison of PT with MD results shows good agreement especially for σ _AB /σ _BB < 1 and ε _AB /ε _BB < 1 which is the parameter region for the solubility of gases (A) in liquids (B). The agreement is least satisfactory for decreasing σ _AB /σ _BB while simultaneously increasing ε _AB /ε _BB which is a typical ‘local composition’ effect for which PT does not account. Comparison with the results of Shing and Gubbins 1982, [Molec. Phys., 46, 1109] shows, however, discrepancies in some cases. Finally, predictions of Henry's constants for the mixtures He/CH₄, H₂/N₂, H₂/Ar, H₂/CH₄, Ne/Ar, and Ne/Kr are reported using Lennard-Jones potentials and three different combining rules. For the hydrogen-systems the best results were obtained with Kohler's original combining rule. For the other systems the experimental data are bracketed by the results of Kohler's new combining rule and from the Kohler-Smith-Kong combining rule.