Composition Dependence of Gaseous Thermal Diffusion Factors and Mutual Diffusion Coefficients

Abstract

Radioactive tracers are used to measure thermal diffusion factors and mutual diffusion coefficients for several binary gas systems in which one component is present in trace concentration only. The systems are H₂–Ar³⁷, T₂–Ar, T₂–CO₂, and T₂–N₂. These measurements are combined with previous tracer measurements and with earlier measurements on less dilute mixtures to test, over the complete composition range, the theoretical prediction that the reciprocal of the thermal diffusion factor is linear in the mole fraction. The linear relation is found to hold within experimental error. The kinetic theory expressions for the composition dependence are also linear, but do not reliably reproduce the experimental measurements. The implications of these results for the problem of determining intermolecular forces from thermal diffusion measurements are briefly discussed. It is shown that an analogous procedure can be carried out for the composition dependence of the diffusion coefficient, and a new formula for this quantity is derived. This formula can be used to estimate the composition dependence of the diffusion coefficient from measurements of the thermal diffusion factor. This result is checked experimentally.