Diffusion interactions in glasses arising from discontinuities in anion concentration

Abstract

A multicomponent diffusion formalism based on the Nernst‐Planck equation is presented which is capable of generating from tracer diffusion data the discontinuous cation diffusion profiles which arise in diffusion couples when a stationary anion concentration profile contains a step discontinuity. Such couples can be constructed with ion exchange resins, zeolites, and glasses. This formalism introduces the electroneutrality condition through the zero flux (ΣZ₁J₁=0) condition alone, thus avoiding the mathematical difficulties associated with the application of the discontinuous neutrality condition (ΣZ_iC_i=0) to a set of differential equations. The agreement between the predictions and the experiments for a series of silica glass couples is found to be good. It is argued that the well‐known contradiction between the electroneutrality constraint on the real charge and the nonuniform field predicted within the Nernst‐Planck formalism can be removed if the proposition is accepted that a distribution of dipoles, and therefore of dipole charge, is associated with each diffusion profile of such magnitude that solutions of the Poisson equation become exactly consistent with the diffusion‐generated electric field.