High-temperature behaviour of the cohesion parameter of cubic equations of state
Coray M Colina, Jorge W Santos, Claudio Olivera-Fuentes
The high-temperature behaviour of the cohesion function of cubic equations of state is studied by computing cohesion parameters from four thermodynamic properties of pure fluids: saturation pressures; pressures on the critical isochore; second virial coefficients; Joule – Thomson inversion curves. Results for three typical cubic equations of state (EOS) (Van der Waals, Redlich – Kwong, and Peng – Robinson) consistently show that the limiting value of the cohesion parameter is negative, contravening its usual interpretation as a measure of the intermolecular attraction forces in the fluid. A theoretical explanation is presented for these results, based on the requirement that the EOS reduce to a hard-body model in the zero-coldness limit. For cubic EOS that use the incorrect Van der Waals repulsion term, the cohesion function is shown to contain a hard repulsion contribution that in general does not vanish at infinite temperature, but should become homogeneously linear in temperature, T, with constant first derivative, and second derivative converging to zero as 1/(Trn) with n > 1.