Cation Permselectivity at the Phase Boundary of Ionophore-Incorporated Solvent Polymeric Membranes As Studied by Optical Second Harmonic Generation

Abstract

Optical second harmonic generation (SHG) at various plasticized poly(vinyl chloride) (PVC)-based ion-selective membranes was observed. The SHG signal from the ionophore-incorporated membranes in contact with the corresponding aqueous primary cation chloride solutions generally increased with increasing the cation concentration and then leveled off. This result can be explained by the formation of oriented and therefore SHG-active cation-ionophore complexes at the membrane surface. The SHG responses were analyzed on the basis of a Langmuir-type binding isotherm; a closest packed layer of the oriented complexes seems to be formed at the membrane surface at high primary cation concentrations. It was found that the membrane potential and SHG signal changed in parallel: when the lipophilic thiocyanate ion was used as counteranion, decreases in the potentiometric and SHG responses with increasing primary cation thiocyanate concentration were observed in the same concentration range. In this case, the observed membrane potentials were primarily governed by the SHG signal oriented cation complexes at the membrane surface. However, another important property of the SHG response is that the membrane potential still increased steadily at high primary cation chloride concentrations, where saturation of the SHG signals occurred. The latter result suggests that some cation complexes that contribute to the membrane potential are located behind the SHG-active layer. Finally, the observed SHG intensity was assumed to be a measure of the charge density at the very membrane surface. Using the space charge model, the SHG intensities were used to correlated the number of SHG-active surface species with the membrane potentials