Low pressure thermodynamic excess functions for mixtures of low molecular mass liquids at 20–30 K Attempted interpretations using van der Waals equation of state

Abstract

Accepted interpretations of the highly non-ideal excess functions for the liquid mixtures of similar sized particles of neon with deuterium and with hydrogen, purely in terms of mass (quantum) differences are reviewed. Application of van der Waals equation to all three combinations of these liquids shows that when using like pair fluid parameters corrected to fit liquid vapour pressures, the excess functions of the H₂–D₂ mixture can be explained using the parameters of the pure fluids together with the Lorentz-Berthelot combining rules for the unlike pair. In contrast, substantial unlike pair departures from a geometric mean combining rule are required to fit the thermodynamic excess functions of the neon mixtures similarly. Even after correction for differences in ionisation potential and in size between neon and the hydrogen isotopes, as it affects the unlike molecular pair forces relative to the like pair forces in these mixtures, there is still a considerable residue unexplained. It seems necessary to presume either that the b parameters are strongly temperature dependent as well as the a parameters in the van der Waals equation, or more likely, that the anisotropic intermolecular interactions of the hydrogen isotopes are involved in these low temperature liquids giving rise to weak unlike pair interactions with neon