MUTUAL DIFFUSION IN BINARY AR-KR MIXTURES AND EMPIRICAL DIFFUSION-MODELS

Molecular dynamics simulations of four binary Ar-Kr mixtures are used to compute self- and mutual-diffusion coefficients. Results using mean squared displacements and using velocity correlation functions are pr esented. The diffusivity coefficients are also presented in the time a nd frequency domains where a comparatively low frequency structure is evident in some simulations. The computed diffusivities are dependent on the maximum time over which the velocity correlation functions are integrated and the time at which the Einstein relationships are evalua ted. This dependence explains in part the small systematic differences between our results (20-80 ps) and earlier molecular dynamics results (<4 ps) in the system Ar-Kr. We compare the computed mutual diffusion coefficients to two empirical models, Darken's model and the common f orce model. Darken's model is consistent with our results over the ent ire frequency range we resolve. At frequencies lower than about 5 ps(- 1) Darken's model and the common force model converge and we cannot di scriminate between them. At higher frequencies the common force model prediction is significantly different from the computed mutual diffusi on coefficient. Assumptions regarding the contribution of cross correl ations that are implicit in the empirical models are discussed and tes ted against our simulation results. The net contribution of velocity c ross correlations is found to be negligible, as is often assumed in de riving Darken's model, but the individual crosscorrelation terms are s ubstantial and negative-a finding contrary to common assumptions.