Ion transfer across ion-exchange membranes with homogeneous and heterogeneous surfaces.

A homogeneous (AMX) and two heterogeneous (MA-40, MA-41) anion-exchange membranes, as well as a heterogeneous cation-exchange membrane (MK-40), are studied by electronic scanning microscopy, voltammetry, and chronopotentiometry. The presence of conducting and nonconducting regions on the surfaces of heterogeneous membranes is established by means of element analysis. The fraction of conducting regions is found by an image treatment. The surface of the AMX membrane was partially coated with microspots of a paint to make it heterogeneous (AMXheter). Voltammetric and chronopotentiometric measurements for AMX, AMXheter, and MA-41 membranes in NaCl solutions are carried out and the pH changes in the solution layers adjoining to these membranes are recorded. Analysis of obtained results shows that the concentration polarization of studied membranes characterized by the potential drop and the rate of water dissociation at the interface is mainly governed by the properties of their surfaces. It is found that the local limiting current density through conducting regions of a heterogeneous membrane is several times higher than the average limiting current through a homogeneous membrane.