Highlighting coupling effects in ionic diffusion

Abstract

The proper description of ion diffusion fluxes is important in the design and development of separation processes such as ion exchange, electrodialysis, metals extraction and fuel cells. The primary objective of this article is to highlight the several distinguishing characteristics of ionic diffusion. Due to the requirements of electro-neutrality and the no-current prescription, an electrostatic potential gradient is induced that tends to accelerate or decelerate ions depending on the sign of their charges. Furthermore, for mixed ion systems the diffusion fluxes are strongly coupled to one another. Diffusional coupling effects result in overshoots and uphill ion diffusion in bulk electrolytes and within charged ion-exchange particles. The induced electrostatic potential also cause forward and reverse ion exchange processes to proceed at significantly different rates, i.e. these are asymmetric. For exchange of three counter-ions, the transient adsorption/desorption trajectories follow completely different equilibration trajectories in composition space. The Nernst–Planck equation is found to be of sufficient accuracy to capture all of the essential features of mixed ion diffusion in the variety of systems examined.