While calculating the diffusion coefficient [tex]D[/tex] of a binary gaseous mixture as Hirschfelder do so based in Lennard Jones potential in his book about "Molecular Theory of Gases and Liquids", I have realised that [tex]D[/tex] does not depend on the local composition of the mixture (i.e. [tex]D[/tex] does not depend on the mass fraction of each substance). On the contrary, the coefficient of viscosity depends on such local composition. I am trying to figure out the reason of this disparity, and I think that it has to do with the Brownian motion involved in diffusion mechanism. Have you got any explanation of why [tex]D[/tex] does not depend on local composition?.