Structure of the He-Xe mixture near the 30-MPa demixing curve at T=294 K.

Abstract

We report measurements of the neutron-scattering differential cross section for the He-Xe mixture in the two coexisting states at T=294 K on the 30-MPa isobaric section of the demixing surface corresponding to Xe molar concentration of 0.78 and 0.48, respectively, and for pure orthobaric liquid Xe having the same number density as in the 0.78 mixture. From these data, the total structure factors have been extracted. For the same thermodynamic states we also performed molecular-dynamics simulations using different interatomic potentials. Analyzing the partial pair-correlation functions, we find that, as already found at lower pressure, in this case, where the He concentration is noticeably higher, the partial structure factor of Xe is also essentially unaffected by the presence of He. This supports a simple picture of the He-Xe gas-gas demixing transition in terms of the liquid-gas transition of pure Xe. Analyzing the He-Xe correlations in the mixture, we also discuss the role played by He in the gas-gas immiscibility transition.