Salt velocity correlation functions: a microscopic interpretation. Part 1.—Solution of a single binary electrolyte

Abstract

A collective dynamic variable, the mean molecular velocity of a salt, is obtained, by means of which all the isothermal vectorial transport coefficients of a solution of single binary electrolyte can be analysed. It is shown that the dissipative motions of the ion constituents are built up by two contributions: (1) a migrational part, associated to the movement of the ion constituents relative to the salt and (2) a diffusional part, associated to the movement of the salt relative to the solvent. Moreover, both contributions are fully decoupled, i.e. the salt velocity is dynamically orthogonal to the electric current. The new formulation is entirely consistent with the formulation of irreversible thermodynamics and of the velocity correlation coefficients in terms of ion constituents. The expressions of the ionic phenomenological coefficients in terms of empirical transport quantities are reinterpreted in the light of the new formulation. The microscopic salt velocity satisfies the fluctuation–dissipation theorem corresponding to the inter-diffusion process. Our expressions have no resemblance to the dynamic variable proposed by Hertz, and a comparison of our procedure with those of Hertz is given.