STRUCTURE OF THE NE-XE MIXTURE NEAR THE 26-MPA DEMIXING CURVE AT T=275 K

We report neutron diffraction measurements for two Ne-Xe mixtures at T =275 K and P=26 MPa, having a Xe molar fraction of 0.81 and 0.45, resp ectively. These thermodynamic points are chosen to be in the single ph ase region, close to the gas-gas demixing surface. Total structure fac tors and neutron weighted pair distribution functions have been extrac ted. Partial pair distribution functions g(ij)(r) have been calculated by means of molecular dynamics simulation using Lennard-Jones interac tion potentials and the Lorentz-Berthelot mixing rules, obtaining a go od agreement with the experimental data. These partial pair distributi on functions have been analyzed in terms of density expansion and also compared with those derived in a previous experiment on a He-Xe mixtu re. The main difference found between the structural properties of the two mixtures is that, in the Ne containing mixture, a definitely larg er fraction of atoms of the lighter species occupies the Xe-Xe first n eighbor shell. The model already used in interpreting the thermodynami c behavior of the He-Xe mixture in terms of microscopic properties sug gests that this structural difference is related to the different beha vior of the pressure-temperature projection of the critical line in th e two mixtures.