Thermal diffusion in mixtures of the inert gases

Abstract

Measurements have been made of the separation produced by thermal diffusion in all the binary mixtures of the inert gases (except Kr-X and Rn mixtures) at temperatures ranging from -180 to 400° C. The temperature dependence of the thermal diffusion ratio, k T , and of R T ( R T = k T /[ k T (∞)] 1 , where [ k T (∞)] 1 is the theoretical value for molecules behaving like rigid elastic spheres) has been deduced. The results indicate that, in general, R T increases with the temperature in the lower range and becomes constant at high temperatures. The constant value is first attained at a temperature which varies from mixture to mixture, but its magnitude is nearly the same for all mixtures. It corresponds with a repulsive force index of 11, if the molecules are treated as centres of repulsive force only. The results have been compared with those calculated by R. Clark Jones (1940, 1941) for a gas mixture in which the two molecular species have identical force fields of the Lennard-Jones type, with repulsive force index v = 9 and attractive force index v' = 5. The theoretical results give R T as a function of T / T c , where T c is the critical temperature of the gas. This relation with the critical temperature is supported by the experimental results, though there are difficulties in the comparison due to the non-identical nature of the force fields in the experimental mixtures. It appears, however, that the observed behaviour could be explained by a Lennard-Jones model if the force indices were suitably chosen, and that a study of the temperature dependence of R T in isotopic mixtures would enable the indices to be determined with some precision.