Dielectric analysis of concentration polarization phenomena at cation-exchange membrane/solution interfaces by frequency variation and d.c. bias application

Abstract

Dielectric measurements were carried out of Selemion CMV cation-exchange membranes immersed in distilled water under application of d.c. bias voltage. A distinct double dielectric relaxation was observed for the membrane—aqueous phase system in the frequency range from 100 Hz to 13 MHz. The dielectric relaxation at the lower frequencies around 3kHz is caused by a concentration polarization boundary layer developing alongside the membrane. The relaxation at the higher frequencies around 200 kHz is caused by interfacial polarization due to a series combination of the two aqueous phases adjacent to the membrane. On the basis of a model of the series combination of a left aqueous phase, a concentration polarization layer, and a right aqueous phase, the complex capacitance of the whole system is formulated theoretically so that the frequency profile may be computed. Next, curve fittings to the observed data based upon the formula derived were performed by a nonlinear least-squares minimization algorithm to yield the best-fit parameters, yielding the following values: the thichness and the reduced conductivity of the concentration polarization, the capacitances and the conductances of both aqueous phases. The thickness estimated for the concentration polarization is about 1 mm which is conform to the literature values. The reduced conductivity is about 0.04 μS-cm −1, which is comparable with the value for pure water.