MASS DEPENDENCE OF SELF-DIFFUSION IN ISOTOPIC FLUIDS

By using Mori memory function formalism we have derived a simple relat ion for the mass dependence of self-diffusion of a particle in an isot opic fluid. It has been shown that the Self-diffusion coefficient of a massive particle for a very large mass ratio (greater than or equal t o 25) has a unique value at a given density and temperature which is 1 /root 2 times the self-diffusion coefficient of the fluid. The predict ion of the formula has been supported by the recent molecular dynamics data on Lennard-Jones fluids. We have also studied the effect of dens ity 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 sta te of the fluid.