Thermodynamics of liquid mixtures of krypton and xenon

The total vapour pressure and the density of liquid krypton + xenon mixtures have been measured at the triple-point of xenon, 161.38 K. Estimates have been made of the excess Gibbs energy of mixing G^E and of the excess volume of mixing V^E. For the equimolar mixture, G^E_½= 114.5 J mol^–1 and V^E_½=–0.695 cm³ mol^–1. These results have been used to test a representative selection of recent theories of solutions. If the geometric mean rule (Berthelot rule) for the energy of interaction of a pair of unlike molecules is assumed to hold, all theories except one predict a negative V^E_½, and a G^E_½ which is too small. The exception is the average potential model, which predicts a positive V^E_½ and a G^E_½ about twice too large. Relatively small departures from the Berthelot rule would bring the calculated G^E into agreement with experiment, but the theories do not necessarily then give an improved estimate of V^E.