Development of a Molecular Based Computational Approach for Compressed and Supercritical Fluids

When applied to nonpolar fluids, the virial equation of state has been shown to be describe accurately the PVT behavior of compressed fluids at conditions near the critical point, as well has high-density supercritical fluids. In a polar molecular system, hydrogen bonding can cause a breakdown of the virial series at subcritical densities, in part because the series does not explicitly consider fluid association via hydrogen bonding. Moreover, calculation of the higher order virial coefficients such as B6 for water molecules and other polar systems requires considerable CPU time. Therefore, it would be helpful to find an alternative thermodynamic method which maintains molecular-level detail, but incorporates fluid association. We have used the 2-density series expansion prescribed by Wertheim's association fluid theory for this purpose. We have tested the approach with a single attraction site model comprising a Lennard-Jones interaction and a short-ranged square-well site-site interaction. The cluster diagrams from the theory are evaluated using the Mayer Sampling Monte Carlo algorithm. The cluster diagram expansion of the association theory describes the P-V-T behavior of the model better than the conventional virial expansion. Extensions to other site interaction models and more realistic molecular models are discussed.