Diffusion in equilibrium mixtures of ionized gases

Diffusion in a mixture of two nonreacting homonuclear gases with an arbitrary degree of ionization is considered. It is shown that, provided that an equilibrium composition of the gas mixture can be defined as a function of temperature, pressure, and relative concentrations of the two gases, diffusive mixing of the gases can be fully described by three combined diffusion coefficients. These coefficients describe diffusion due to concentration, temperature, and pressure gradients, respectively, and replace the 1/2(q²+q-2) independent diffusion coefficients required in previous treatments to describe diffusion in a q-species gas mixture. Diffusion due to applied electric and gravitational fields is also considered. Formulas for the combined diffusion coefficients are presented, and combined diffusion coefficients in a mixture of argon and nitrogen at temperatures up to 30 000 K are calculated as an example.