Mass dependence of self-diffusion in isotopic fluids

By using Mori memory function formalism we have derived a simple relation for the mass dependence of self-diffusion of a particle in an isotopic fluid. It has been shown that the self-diffusion coefficient of a massive particle for a very large mass ratio (>or=25) has a unique value at a given density and temperature which is 1/ square root 2 times the self-diffusion coefficient of the fluid. The prediction of the formula has been supported by the recent molecular dynamics data on Lennard-Jones fluids. We have also studied the effect of density and temperature on the mass-dependent self-diffusion coefficient. It is found that the ratio of self-diffusion coefficient of the heavy particle to that of the fluid is weakly dependent on thermodynamic state of the fluid.