Gaseous Diffusion in Porous Media. IV. Thermal Diffusion

Abstract

The pressure dependence of thermal diffusion is derived on the basis of the ``dusty‐gas'' model, in which a porous medium is considered as a collection of giant molecules (dust particles) fixed in space. The results can also be applied to capillaries by suitable substitution for geometric parameters. Thermal separation is zero at zero pressure and constant at high pressures, but the transition can be quite complex and varied, depending upon the temperature, molecular masses, and intermolecular forces of the gas molecules involved. Representative calculations have been made to illustrate the possibility of maxima and sign reversals of the thermal separation as a function of pressure. Comparison with the scanty available experimental results reveals no discrepancies, but further experiments are desirable, particularly for cases in which unusual transition behavior is expected.