Excess Thermodynamic Functions for Binary Mixtures as Calculated from the Percus—Yevick Equation for the Lennard-Jones Potential

Abstract

Using the Percus—Yevick equation and the Lennard-Jones 6–12 potential for the noble gases, we have calculated the thermodynamic functions for a number of binary mixtures for supercritical isotherms and at densities up to twice the critical density. From these thermodynamic functions, we have calculated the excess functions along the isotherms as a function of density for two mixing processes, one at constant pressure and one at constant volume. The results are presented graphically but the numerical data are available from the thesis of Throop. For the constant pressure process, we have found that the excess energy, volume, and Gibbs free energy are positive at low densities, increase with increasing density to a maximum, and then decrease to small positive or negative values at higher densities. The excess functions for the constant-volume process, in the density and temperature ranges of our calculations, are much smaller than the constant-pressure functions for the same density and temperature. The excess energy of the constant volume process is always positive at low densities, passes through a maximum, and becomes negative at high densities. The variation with density of the excess Gibbs free energy is more complicated, and it is usually negative and decreasing with density, although at temperatures near a critical temperature, this function is positive at low densities. For both of these mixing processes, the excess functions are not quite symmetrical functions of mole fraction, with the constant-volume excess functions being more symmetric.