Evolution of the Clausius–Mossotti function of noble gases and nitrogen, at moderate and high density, near room temperature

Abstract

The static dielectric constant of noble gases (excepting xenon) and nitrogen has been measured near room temperature in a large range of pressures (up to 12 kbar for He, Ne, Ar, and N₂). The Clausius–Mossotti function (C.M.) has been deduced, when density data are known, with a precision of 10⁻³ in the moderate pressure range and 3×10⁻³ above 1.0 kbar. Values of the first, second, and in some cases third dielectric virial coefficient have been determined. We have found, in agreement with previous results obtained at low pressure, that the light gases and the heaviest ones show a different behavior of their C.M. function in terms of the density. For He and Ne, the second dielectric virial coefficient is negative; for Ar, Kr, and N₂ it is positive, but the C.M. function goes through a maximum and then decreases. The effect of density on the C.M. function of Ar and N₂ is approximately a drop of 3.5% at 12 kbar. These results have been compared with the predictions of molecular theories of dielectric properties of nonpolar fluids in the case of a hard spheres model. We have found that the Kirkwood–Yvon theory based upon the DID approximation does not reproduce completely the results of experiments. By contrast, we have been able to show that the semiquantal theory of Heller and Gelbart which takes into account the influence of the electronic distortion is more in agreement for a wide range of densities. This is the case for Ne and Ar when the excess of the pair polarizability tensor of a diatom is computed within the electron–gas approximation, and for He only if it is deduced from the Hartree–Fock scheme.