Local composition in liquid mixtures

Abstract

Nonrandomness and local compositions in model liquid mixtures have been investigated by computer simulation (molecular dynamics) and perturbation theory. The computer simulation represents nonrandomness due to both attractive and repulsive forces, whereas perturbation theory is restricted to the effect of repulsive forces (size differences). In the case of mixtures of equal-sized molecules, the quasichemical approximation of Rushbrooke and Guggenheim gives good results concerning both the magnitude of the effect and its concentration dependence. For a quantitative description, some corrections are necessary for the difference in coordination number around strongly and weakly interacting molecules. Perturbation theory neglects the effect of nonrandomness. However, as emphasized by Rushbrooke and Guggenheim, this effect is not of great thermodynamic significance. For mixtures of molecules of different sizes, packing effects (included in perturbation theory) are dominant, and may be of opposite sign to the effect caused by attractive forces.