By using a Mori memory function formalism, a simple relationship was derived for the mass dependence of the self-diffusion of a particle in an isotopic fluid. It was shown that the self-diffusion coefficient of a massive particle with a mass ratio of more than 25 had a unique value, at a given density and temperature, which was 0.707 times the self-diffusion coefficient of the fluid. The predictions of the formula were supported by recent molecular dynamics data on Lennard-Jones fluids. A study was also made of the effect of density and temperature upon the mass-dependent self-diffusion coefficient. It was found that the ratio of the self-diffusion coefficient of the heavy particle to that of the fluid was weakly dependent upon the thermodynamic state of the fluid.

K.Tankeshwar: Journal of Physics - Condensed Matter, 1995, 7[50], 9715-22