ISOTHERMAL DIFFUSION IN SOME 2 AND 3-COMPONENT SYSTEMS IN TERMS OF VELOCITY CORRELATION FUNCTIONS

Mass transport coefficients in systems at constant temperature and pressure and composed either of three neutral species, or a simple electrolyte in a solvent, are expressed in terms of molecular velocity correlation functions. These expressions, which are generalizations of the familiar relation of the self-diffusion coefficient to the velocity autocorrelation function in one-component systems, are used to examine the behavior of two Gaussian-Markovian models of diffusion. The first model, based on independent binary collisions, is capable of roughly approximating the concentration dependence of real systems while the second model, which views molecules as moving through a viscous medium, is unsatisfactory for a consistent representation of diffusion in a multicomponent system. The question of the existence of a general functional relation between diffusion coefficients, such as proposed by Darken, is discussed for the special case of binary mixtures.