Potential reversals across site-free passive membranes A simulation analysis

Steady-state membrane potentials can be reversed by changing membrane thickness, according to calculations for reversible ion exchanging membranes with ion transport obeying Nernst-Planck equations. Requirements are illustrated for single uni-univalent bathing electrolytes with (1) one mobile ion relatively favored in the membrane, and (2) the opposite charge ion has the greater mobility in the membrane. The technique of digital simulation isemployed in the solution of the Nernst-Planck and Poisson equations for the extraction of a salt M+X− into a membrane phase. Single ion extraction coefficients k+ and k− are assumed such that ksalt = √K+K−=1. Ion M+ is favored in the membrane phase by choosing k+>1. Diffusion coefficients are D−≫D+. When membrane thickness is large compared to the Debye length, electroneutrality is forced throughout the bulk, and the system behaves as a classical constrained liquid-junction with preferential anion response. At smaller membrane thicknesses, the interior of the cell deviates from electroneutrality, and the system responds preferentially to M+. approaching the Nernstian behavior of the single cation case.