CONSTRUCTION AND APPLICATION OF PHYSICALLY BASED EQUATIONS OF STATE .1. MODIFICATION OF THE BACK EQUATION

In this and a subsequent paper a a class of equations of state with four or five parameters is constructed on the assumption that the Helmholtz energy can be written as F = F(H) + F(A) + F(Pol), where F(H) denotes the hard body contribution, F(A) the contribution due to the attractive dispersion forces and F(Pol) the contribution of either dipolar or quadrupolar interactions. The present paper concentrates on a reformulation of F(H) and F(A). In comparison with the original BACK equation a different temperature dependence of the hard body size is used which closely follows WCA-type perturbation theory. Consequently, F(A) had to be redetermined by a simultaneous fit of Maxwell data, pressures, enthalpies of evaporation and isochoric heat capacities to the experimental data of ethane. This substance was chosen to extend the applicability of the equation to lower temperatures. As a by-product, a closed equation of state for ethane is obtained which, except near the critical point, is in good agreement with the NIST tables.