Studies on the mechanism of the potential generation at the surface of ion-selective liquid membranes at the molecular level (Accounts)

Abstract

Ion-selective charge separation at the surface of ionophore-incorporated liquid membranes was studied by observing optical second harmonic generation (SHG) and by using lipophilic photoresponsive ionophores as a molecular probe. It was found that SHG signals from the ionophore-incorporated membranes in contact with aqueous primary cation chloride solutions generally increased with increasing cation concentration and then leveled off. This can be explained by the formation of oriented and therefore SHG active cation-ionophore complexes at the membrane surface. It was found that the membrane potential and SHG signal changed in parallel. This result suggests that the observed membrane potentials were primarily governed by SHG active oriented cation complexes at the membrane surface. To further clarify the influence of the surface charge density on the phase boundary potential, photoresponsive ionophores were used as a molecular probe because the ratio of the ionophore conformers with different complexation affinities is controlled quantitatively by light irradiation without any change in the membrane composition. The photoinduced changes in the potentiometric responses were analyzed as a function of the surface charge density by using a surface model based on a double-diffuse layer. As a result, it was found that the photoinduced changes in the potentiometric response behaviors, even when the deviations from a Nernstian response were observed, are in good agreement with the values calculated on the basis of the proposed model. This agreement leads to the important conclusion that the sub-Nernstian response slope is attributed to the low surface charge density due to lower ionophore concentrations.