Limits of the validity of the mass ratio independence of the Stokes-Einstein relation: molecular dynamics calculations and comparison with the Enskog theory

Author： Willeke Martin

Publisher： Taylor & Francis Ltd

ISSN： 1362-3028

Source： Molecular Physics, Vol.101, Iss.8, 2003-01, pp. : 1123-1130

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Abstract

Detailed molecular dynamics simulations have been carried out to investigate the mass ratio dependence of the tracer self-diffusion coefficient D₂ in binary, atomic Lennard-Jones mixtures as a function of density n and length diameter ratio σ₂₂/σ₁₁. For a compact representation of our results an exponential approach is employed. The results are compared with the Stokes-Einstein relation, which predicts no mass ratio dependence, and the Enskog theory. The validity ranges regarding the mass ratio dependence for Enskog and Stokes-Einstein like behaviour are given as well as the mass ratio dependence in the crossover regions between these two cases. For length parameter ratios σ₂₂/σ₁₁<1, the Stokes-Einstein prediction is not valid for mass ratios 1/16≤m₂/m₁≤50. For length parameter ratios σ₂₂/σ₁₁>2 and for mass ratios m₂&solm₁<1, the Stokes-Einstein regime (regarding the mass ratio dependence) is reached for smaller densities than for the same system but m₂/m₁ > 1.