Mutual diffusion in a binary isotopic mixture

Abstract

The mass dependence of the mutual diffusion coefficient, in a binary equimolar mixture ofLennard-Jones fluids, is studied within Mori’s memory function formalism. Aphenomenological form of the memory function is used to study the time evolution of theself- and relative velocity correlation functions. The diffusion coefficients are calculated fromthe relevant velocity correlation functions using the Green–Kubo integral formula. Like theself-diffusion coefficient, the mutual diffusion coefficient is also found to be weaklydependent on the mass ratio. The present study shows that the minimum value that themutual diffusion coefficient in an equimolar mixture of isotopic fluids can have is times the self-diffusion coefficient of any of the species when in isolation. Further, thecontribution of the dynamic/distinct cross correlations to the mutual diffusion coefficient isfound to be small and positive for the whole range of the mass ratio which is consistentwith earlier molecular dynamics results.