The charge separation process in non-homogeneous electrolyte solutions

Abstract

The charge separation process occurring in two different solutions of the same electrolyte brought into contact is studied using Poisson's equation and the (simplified) equations of transport. The process is characterized on the basis of the change in observable physical magnitudes. The relevance of the “diffusional” and “electric” relaxations is analysed. The results obtained can be applied to problems of ionic transport across membranes and liquid junctions, and contribute to the study of the transport of charged matter during the time interval in which the “charge” plays a significant role. Although, unfortunately the time domain involved in the electric relaxation seems to be inaccessible to precise experimental measurement, the physical model provides a detailed description of the way charge separation takes place. The latter is consistent with experimental observations at “large times”. An equation for the current density has been obtained from the ionic transport equations and Poisson's equation. By using the former, it has been shown that the classical treatment by Planck (commonly used for describing the diffusion potential in ionic transport through membranes) implies neglecting the whole charge separation process (assuming τ e = 0, where τ e is the electric relaxation time). The inconsistencies involved in this have been shown. Finally, the significance of the two terms “conduction” and “displacement” current) in the equation for the total current density is discussed. Both terms play an important role in ionic transport processes through totally or partially blocked interfaces.