Fluid phase equilibrium of dilute mixtures of helium in near-critical mercury

ERNESTO MARCECA; GUDUN SCHÄAFER; FRIEDRICH HENSEL

JOURNAL OF CHEMICAL THERMODYNAMICS

Elsevier

Año: 1996 vol. 28 p. 647 - 666

0021-9614

The phase behaviour of dilute mixtures of the He in near-critical fluid Hg has been studied within the ranges 69 MPa to 332.5 MPa, 1433 K to 1882 K and amount-of-substance fractionsx(He) within the values 0.0018 and 0.1020. A high pressure and high-temperature vessel was filled with known amounts of He and Hg (synthetic method) and heated; an isochoric record ofpandTwas made while the temperature was gently decreased and the transition from homogeneous to two-phase systems was indicated by abrupt slope changes in thep-against-Tisochores. Two distinct fluid phases are shown to exist in (helium + mercury) at temperatures and pressures higher than those corresponding to the critical point of pure mercury (“fluid–fluid equilibrium of the first type”) and the system´s critical line, beginning at the critical point of pure Hg, runs steeply to higher pressures and temperatures. Solubility values as high as 0.28 mol·dm−3were measured for He in near-critical fluid Hg, in contrast to the much lower values estimated for He dissolved in metallic liquid Hg at sub-critical temperatures. The isobaric solubility of He in Hg increases with temperature for super-critical pressures so that (helium + mercury) shows a positive enthalpy and entropy of dissolution under these conditions. At sufficiently high density, non-metallic near-critical Hg experiences a transition to a metallic state, resulting in a strong repulsive interaction between dissolved He and the surrounding conduction electrons, and hence the isothermal solubility of He decreases.