ION ADSORPTION IN WEAKLY CHARGED MEMBRANES - EFFECTS ON SALT FLUX AND MEMBRANE-POTENTIAL

Ion transport across weakly charged membranes with surface charge due to ion adsorption is analyzed and particular attention is paid to the case of Langmuir-type adsorption. Coupling between electrodiffusion of ions and their adsorption to the membrane matrix results in a number of interesting nonlinear phenomena. The steady-state transport equations lead to a first-order differential equation that can be integrated either analytically or numerically to give the ion fluxes, the membrane potential and the concentration profiles. Comparison is made to the approximate results obtained by using the Henderson profile assumption and it is shown that this assumption, while valid for homogeneous membranes, leads to wrong conclusions when applied to nonhomogeneous membranes. Results make clear also that Henderson's approximation is good for the limiting cases: adsorption constant times bulk concentration very small or very high.