The Dependence of the Thermal Diffusion Factor on Temperature

The thermal diffusion factor for isotopic mixtures has been calculated from values of the collision integrals given by Hirschfelder, Bird and Spotz which refer to molecules exerting an inverse 13th power repulsion with an inverse 7th power attraction - the (13, 7) model. The theoretical results are compared with experimental ones for the inert gas mixtures previously considered by Grew in relation to the (9, 5) model. The agreement is, as expected, better than for the (9, 5) model. Experimental results for hydrogen-helium mixtures are also given; the behaviour of these mixtures is exceptional in that the thermal diffusion factor appears to be independent of temperature over the range 12 to 600° k.