Membrane polarisation at high current densities

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Abstract
An extension of the Nernst model of the current against voltage relation for ion-selective membranes is proposed. It assumes that the salt concentration at the surface of the membrane on the side of the depleted layer reaches a lower limit when the current approaches the Nernst limit of high current density. Beyond that limit the depleted layer is assumed to separate into two sub-layers, one containing that lower limit concentration and the other a concentration gradient towards the bulk concentration. The physical basis for this modification of the Nernst model is indicated to be the existence of a laminar convection at high current densities, which injects salt into the depleted layer. Beyond the Nernst limiting current density the convection turns turbulent, eventually, as deduced from the analysis of the current noise. Turbulence enhances the injection of salt into the depleted layer, thus increasing the conductance with increasing current density.

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