Diffusion in Binary Systems

Abstract

The Onsager‐Fuoss equation for the concentration dependence of the diffusion coefficient is derived from a kinetic picture, taking into account the asymmetry in the Brownian motion induced by concentration gradients. Mass flow is introduced by considering diffusion in the presence of a pressure gradient. Systems exhibiting volume changes on mixing are discussed, and a modification of the Matano procedure for calculating diffusion coefficients is suggested. It is shown that diffusion coefficients obtained from steady‐state permeability measurements and those obtained from interdiffusion data differ by a factor of V̄₂c₂, the volume fraction of the nonpermeating component.