THEORETICAL FOUNDATIONS OF INTERDIFFUSION

Expressions for the interdiffusion and the tracer‐diffusion coefficients in terms of the time‐integrals of force‐correlation functions are obtained using a microscopic description of the inter‐diffusion and tracer‐diffusion processes in a two‐component liquid. The present approach provides a systematic and direct derivation, as well as an extension, of the results of Bearman et al. in their pioneering work in the late 1950s which have been used recently by Cohen and Foley to discuss the two possible relationships between the interdiffusion and tracer diffusion coefficients, referred to as the “fast‐mode expression” of Hartley and Crank and the “slow‐mode expression” based on the random phase approximation. In our analysis we find an additional three‐particle friction term in the exact expression for the interdiffusion coefficient, which is not present in Bearman's work. It is concluded that not much can be said in favor of the fast‐mode or slow‐mode expressions, unless this additional three‐particle friction is neglected. We also show that the center‐of‐volume reference frame, in which the hydrodynamic fluxes are measured, naturally follows from the microscopic description in the isothermal‐isobaric limit. Copyright © 1990 John Wiley & Sons, Inc.