Thermodynamics of liquid mixtures of xenon and hydrogen chloride

Abstract

The total vapour pressure of the system xenon + hydrogen chloride has been measured as a function of composition at 159.07, 182.26 and 195.42 K. The results have been used to estimate the excess Gibbs function G^E at these temperatures. The volume of mixing V^E has been determined at 195.42 K. The system departs considerably from ideal behaviour, forming a positive azeotrope, and at the lowest temperature is probably not far from an upper critical solution temperature. Estimates have been made of effective Lennard–Jones parameters for the interaction of two hydrogen chloride molecules from the properties of the cubic form of crystalline hydrogen chloride. These have been used to calculate the excess thermodynamic functions of xenon + hydrogen chloride mixtures on the basis of the one-fluid van der Waals theory of Leland, Rowlinson and Sather. To force agreement between the experimental and calculated G^E values requires a value of k₁₂ of ∼0.075, where k₁₂ is defined by the equation for the cross intermolecular energy parameter ε₁₂, namely ε₁₂=(1–k₁₂)(ε₁₁ε₂₂)^½. It is shown on the basis of simple intermolecular force theory that such a value for k₁₂ is reasonable, and that it derives primarily from the permanent dipole–dipole forces between hydrogen chloride molecules.