Thermal Diffusion in Isotopic Gas Mixtures and Intermolecular Forces

Available thermal diffusion data on He, Ne, Ar, Kr, Xe, ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, CO, C${\mathrm{H}}_4$, N${\mathrm{H}}_3$, HCl of different workers have been discussed. This study has revealed the need and nature of the additional measurements which will be useful and is thus of help in planning new experiments. The problems of temperature assignment and composition dependence of the thermal diffusion factor ${\alpha }_T$ are examined. The adequacy of the theoretical ${\alpha }_T$ expression used while interpreting the experimental results is also given proper consideration. Comparison of the experimental data are made with the prediction of the regorous Chapman-Enskog kinetic theory in conjunction with the three spherically symmetric potentials (Lennard-Jones 12-6, modified Buckingham exp-6, and Morse) for nonpolar gases, while a Stockmayer type 12-6-3 potential for polar gases. Reasonably satisfactory agreement is obtained even for polyatomic gases. The extent to which Chapman-Enskog theory in conjucntion with central force-field models may be expected to hold for polyatomic gases is investigated The up-to-date knowledge of the kinetic theory which takes into consideration inelastic molecular collisions is reviewed