Preaveraged potential model and equilibrium properties of polar gases

Abstract

The second and third virial coefficients of polar gases have been studied using a preaveraged pair potential model which includes the dipole repulsion term. The dipole, dispersion and repulsion three-body nonadditive interactions have been taken into account in evaluating the third virial coefficient. A large number of sets of force parameters have been found and all reproduce the second virial data with equal consistency and accuracy. Using these parameters, the third virial coefficient has been calculated. It has been found that the values of the third virial coefficient are different for a different set of force parameters. A comparison of experimental and calculated values of third virial coefficient, therefore, leads to a unique set of force parameters for an assumed potential model. The theoretical results have been compared with the experimental data of CH3F and C6H5F. For CH3F it is found that a set of force parameters given by y=0.3, sigma 0=4.598 ( degrees A) and epsilon 0/k=182.0 (K), gives good agreement between theory and experiment. No set of force parameters obtained from the second virial data, however, explains the third virial coefficient data of C6H5F. This may be due to large uncertainties in the experimental data.