The driving factors of electro-convective instability in concentration polarization

Abstract

Ionic current through a charge-selective interface in a binary electrolyte is a basic element of many electrochemical engineering and microfluidic processes. Such current passage is diffusion-limited: it induces a decrease of electrolyte concentration towards the interface (concentration polarization, CP), expressed in the saturation of current upon increasing voltage at some value (limiting current, LC). With further increase of voltage, this saturation breaks down (overlimiting conductance, OLC). In open systems OLC is mediated by a microscale vortical flow which develops as a result of electroconvective instability (ECI) of quiescent CP near LC. Electroconvection (EC) is a flow driven by the electric force acting either upon the space charge of the interfacial EDL (electroosmosis, EO) or the residual space charge of the quasielectroneutral bulk (bulk EC). There are two types of EO, the equilibrium and the nonequilibrium one. The former relates to the action of the tangential electric field upon the space charge of the EDL, and the latter pertains to the similar action upon the extended space charge, forming next to the EDL near LC. For a perfectly charge-selective interface, CP is stable under the equilibrium EO or bulk EC, but nonequilibrium EO may cause ECI. For this reason until recently, ECI in CP was attributed to this latter mechanism. Lately, it was shown that imperfect charge-selectivity of the interface makes equilibrium ECI possible, driven by either equilibrium EO or bulk EC, or both. Here we identify and analyze the major surface and bulk factors affecting the ECI. These factors (diffusioosmosis, EO, bulk EC, and some newly identified ones) are manifestations of the electric force and pressure gradient, balanced by the viscous force acting in various locations in solution. The contribution of these factors to ECI in CP is analyzed for a varying interface permselectivity.