Diffusion in binary mixtures: An analysis of the dependence on the thermodynamic factor

We study the diffusion process in binary mixtures using transition probabilities that depend on a mean-field potential. This approach reproduces the Darken equation, a relationship between the intrinsic and the tracer diffusion coefficients, D-A and D-A*, through the thermodynamic factor Phi (a function of the derivative of the activity coefficient against the molar fraction). The mean-field approach allows us to go beyond the Darken equation and separately specify the dependence of D-A and D-A* on the thermodynamic factor. We obtain that Phi appears in the expression for D-A*, but the intrinsic diffusivity D-A turns out to be independent of Phi. Experimental results taken from the literature on diffusion in metal alloys are consistent with this theoretical prediction.