Molecular thermodynamics for fluids at low and high densities. Part I: Pure fluids containing small or large molecules

Abstract

A correlation is presented for the thermodynamic properties of pure fluids containing small or large molecules. The residual Helmholtz energy is given in terms of perturbed-hard-chain (PHC) theory, extended to polar fluids with a multipolar expansion. The novel feature of this correlation is a separation of the Helmholtz energy into low-density and high-density contributions. The low-density contribution follows from a virial expansion and the high-density contribution from a perturbation expansion. For intermediate densities, a continuous function is used to interpolate between the two density limits. This modification of PHC theory improves agreement with experimental second virial coefficients, vapor pressures, and saturated liquid densities. Since all molecular parameters used here have a well-defined physical significance, they can be reliably estimated for high-molecular-weight-fluids where experimental data are scarce. More important, separation into low-density and high-density contributions allows separate mixing rules for each density region; this flexibility in mixing rules significantly improves representation of mixture properties, as discussed in Part II.

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