The statistical mechanical local composition theory II. Effects of molecular size and energy differences in Lennard-Jones and Kihara mixtures

Abstract

Abstract Integral equation theory is used to determine the variations of the local mole fractions in binary mixtures of molecules differing considerably in size and energy of interaction. We model two types of mixtures, i.e., molecules interacting with the Lennard—Jones potential and molecules with the Kihara potential. The size ratio ranges from σ BB /σ AA = 1.0 to 1.75 and energy ratio for ϵ BB /ϵ AA = 1.21 to 9.14. Results are compared with simulation data of Hoheisel and Kohler and Nakanishi et al. Findings indicate that the coordination numbers and nearest neighbor numbers are strong functions of size ratio, but depend only weakly on energy disparity. In a mixture, large differences in size promote ‘mixing’ while large differences in energy induce ‘demixing’ of the components. It is found that the packing of nearest neighbors depends, to a large extent, on available ‘local surfaces’. the Seaton—Glandt ‘solid angle’ fractions turn out to be good correlators in nearest neighbor statistics. All local quantities are variable with changes in temperature, density and composition. The assumption of constant Wilson parameters is not valid for the systems studied.