Impedance and electrical potential across supported liquid membranes: roleof interfacial potentials on the active transport of a metal cation

The facilitated transport mechanism of a metal cation across a supported li quid membrane was examined in the case of a Cd2+ flux mediated by the natur al ionophore lasalocid; the polypropylene membrane was impregnated with o-n itrophenyl octyl ether. Impedance and membrane potential measurements were carried out along with flux measurements of the Cd2+/H+ counter transport. A proton flux was inferred from the membrane potential measurements perform ed under open circuit, zero electrical current, conditions. This proton flu x was confirmed by impedance measurements performed under closed circuit, n on-zero electrical current, conditions. The main consequence of this proton transfer under open circuit conditions (which are the normal membrane sepa ration conditions) was the building of interfacial potentials. These potent ials modify the ionic concentrations at the membrane interfaces, and thus r ender invalid the mathematical treatment of the cation flux based on the eq uilibrium ion exchange concept which is generally used. When the potential dependence of the effective interfacial reactions is incorporated into the transport model, the theory can account quantitatively for the experimental fluxes measured under open circuit conditions. (C) 1999 Elsevier Science B .V. All rights reserved.