  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 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.
