BACK equation of state for simple compounds

The BACK equation of state represents a powerful tool in the description of the P-V-T behavior of fluid systems in a broad range of independent variables - temperature, pressure and composition; this equation comprises four compound-specific parameters which are adjusted to the equilibrium data. In the previous contribution, the possibility was studied of determining one of the parameters, the non-sphericity one, from a theoretical approach; here we extend our study to theoretical determination of two parameters, one connected with the size and the other with shape of molecular models. On the basis of the structural data - bond distances and bond angles - three characteristics of the hard body models of the studied molecules - the reduced volume, surface area and the mean curvature integral, divided by 4π - are determined and used to evaluate the non-sphericity parameter, α, and close-packed volume, V00. The method is applied to argon, krypton and methane (with a spherical shape of molecular model) and nitrogen, oxygen and ethane (modelled as diatomics) plus to air (a mixture of nitrogen and oxygen); its prediction is compared with results obtained from an approach where all four parameters were adjusted on the same sets of data. It is concluded that the accuracy of the P-V-T description obtained within the proposed method is reduced only marginally in comparison with the latter case.