Dynamic cross correlation in isotopic two-component liquids: Molecular-dynamics calculation results compared with predictions of kinetic theory.

Abstract

For thermodynamically ideal two-component systems, as for example isotopic systems, the mass effect on the mutual diffusion coefficient is negligibly small for mass ratios of order 2. For higher mass ratios a significant influence on the diffusion coefficient may be expected. We present here accurate molecular-dynamics calculation results for the hard soft sphere and the Lennard-Jones liquids, which indicate quantitatively the mass-ratio effect on the mutual diffusion coefficient. The predictions of the Enskog kinetic theory are found to give the correct trend and order of magnitude of these effects but overestimate them at all the mass ratios studied. Both systems, the hard soft sphere liquid (the softness of the interaction is so small that it can be regarded as a hard one) and the Lennard-Jones liquid, show appreciable mass-ratio dependence of the diffusion coefficient only for ratios larger than 10, in agreement with our previous numerical work [Schoen and Hoheisel, Mol. Phys. 52, 1029 (1984); 53, 1367 (1984); 57, 65 (1986)].