Thermodynamic properties of He-H2 fluid mixtures over a wide range of temperatures and pressures.

The effect of pressure P and temperature T on the properties of mixing of helium-hydrogen (He-H2 ) fluid mixture is studied based on statistical mechanical perturbation theory. The constituent species are considered to be interacting via a pair potential consisting of short range repulsion and a long range attraction which has been included through a double Yukawa (DY) potential. He and H2 are the lightest elements; therefore, the quantum effect is included via first-order quantum correction in the framework of Wigner-Kirkwood expansion. The dimerization of the H2 molecule is treated as a hard convex body fluid for which the equation of state (EOS) can be derived from hard sphere system based on scaled particle theory. An advanced and most recent EOS has been used for our investigation. The use of the DY potential, which can readily be simulated to empirical potentials, has enabled us to obtain analytical expressions for attractive and quantum energy contributions in terms of Laplace transforms. With a view to ensure internal consistency of the various thermodynamic functions to extract information on segregation and order in the mixture, different functions, such as compressibility factor, Gibbs free energy of mixing, entropy of mixing, and concentration fluctuations in the long wavelength limit, have been calculated as functions of composition of the mixture over an extended region of P and T. The results suggest that segregation, heterocoordination, or both may occur in the He-H2 mixture depending upon its composition, pressure, and temperature.