Thermal diffusion factors and intermolecular potentials for noble gas–SF6 systems

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Abstract
Experimental thermal diffusion factors for equimolar mixtures of He–, Ne–, Ar–, Kr–, and Xe–SF6 have been measured in the temperature range from 225 to 500 K. The data were obtained in a 20‐tube trennschaukel, or ‘‘swing separator.’’ The systems containing the four lighter noble gases all exhibited a ‘‘normal’’ thermal diffusion factor, αT, i.e., concentration of the heavy species, SF6, in the cold region of the apparatus and increase of αT with temperature. Xe–SF6, the system with the smallest mass difference, exhibited ‘‘abnormal’’ behavior. The spherically symmetric Pack potentials were used to calculate the thermal diffusion factor with reasonable success. Recently published dipole–dipole dispersion coefficients were used to construct intermolecular potentials of the Hartree–Fock‐dispersion functional form with individually damped attractive terms. The potentials, when tested against the available transport and thermodynamic data, improved the fit to experiment in almost all cases.