Excess Volumes in Binary Liquid Mixtures of Simple Molecules

Abstract

RECENT statistical theories of liquid mixtures have made it possible to calculate the excess thermodynamic functions of binary mixtures from the parameters characterizing the intermolecular potential-energy curves for the two pairs of like molecules. For small molecules, between which only dispersion forces are supposed to operate, these parameters may conveniently be taken to be ε₁₁, the minimum value of the potential energy, and r₁₁ the molecular separation corresponding to this minimum (for a pair of like molecules of species 1). The theories are of two types. Those of the first type use a definite model for the liquid, and enable the excess functions to be calculated from the appropriate ε and r values alone. This applies to the treatment of Prigogine and Mathot¹ (and its extension by Prigogine and Bellemans²) and to that of Salsburg and Kirkwood³. In treatments of the second type^4,5, based on the theory of corresponding states, the excess functions are expressed in terms of (a) the ratios ε₁₁/ε₂₂ and r₁₁/r₂₂ and (b) the macroscopic properties of one of the two pure components, chosen as reference substance. Thus, the excess volume, V^E, depends on the molar volume of the reference liquid and the temperature variation of this volume.