Thermal Diffusion

the transport of the components of gaseous mixtures or solutions when subjected to a temperature gradient. If the temperature difference is held constant, thermal diffusion in a mixture will produce a concentration gradient. The production of such a gradient causes ordinary diffusion. Under steady-state conditions, when there is no mass flux, ordinary diffusion counterbalances thermal diffusion, and a concentration difference, which may be utilized for isotope separation, is established.

Thermal diffusion in solutions—the Soret effect—was discovered by the German scientist C. Ludwig in 1856 and studied by the Swiss scientist C. Soret between 1879 and 1881. Thermal diffusion in gases was predicted on the basis of the kinetic theory of gases by the English scientist S. Chapman and the Swedish scientist D. Enskog between 1911 and 1917 and was observed experimentally by Chapman and the British scientist F. Dootsen in 1917.

In the absence of external forces, the total diffusion mass flux in a binary mixture at constant pressure is equal to j_i = – nD₁₂ grad c_i - D_T/T) grad T, where D₁₂ is the coefficient of ordinary diffusion, D_T is the coefficient of thermal diffusion, n is the number of particles of the mixture per unit volume, and c_i = n_i/n is the particle concentration of the ith component (i = 1,2). The steady-state distribution of concentrations can be found from the condition j_i = 0; in this case grad c_i = – (k_T/T) grad T, where k_T = D_T/D₁₂ is the thermal diffusion ratio, which is proportional to the product of the component concentrations. Since the coefficient of thermal diffusion is highly dependent on molecular interactions, knowledge of this coefficient makes it possible to study intermolecular forces in gases.