CATION PERMSELECTIVITY AT THE PHASE-BOUNDARY OF IONOPHORE-INCORPORATED SOLVENT POLYMERIC MEMBRANES AS STUDIED BY OPTICAL 2ND-HARMONIC GENERATION

Optical second harmonic generation (SHG) at various plasticized poly(v inyl chloride) (PVC)-based ion-selective membranes was observed, The S HG signal from the ionophore-incorporated membranes in contact with th e corresponding aqueous primary cation chloride solutions generally in creased with increasing the cation-concentration and then leveled off, This result can be explained by the formation of oriented and therefo re SHG-active cation-ionophore complexes at the membrane surface, The SHG responses were analyzed on the basis of a Langmuir-type binding is otherm; a closest packed layer of the oriented complexes seems to be f ormed at the membrane surface at high primary cation concentrations. I t was found that the membrane potential and SHG signal changed in para llel: when the lipophilic thiocyanate ion was used as counteranion, de creases in the potentiometric and SHG responses with increasing primar y cation thiocyanate concentration were observed in the same concentra tion range, In this case, the observed membrane potentials were primar ily governed by the SHG-active oriented cation complexes at the membra ne 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 oc curred, The latter result suggests that some cation complexes that con tribute to the membrane potential are located behind the SHG-active la yer, Finally, the observed SHG intensity was assumed to be a measure o f the charge density at the very membrane surface. Using the space cha rge model, the SHG intensities were used to correlate the number of SH G-adive surface species with the membrane potentials.