Diffusion interactions in glasses arising from discontinuities in anion concentration

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 (ΣZiCi=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.

Diffusion interactions in glasses arising from discontinuities in anion concentration Journal Articles