STUDIES ON THE MECHANISM OF THE POTENTIAL GENERATION AT THE SURFACE OF ION-SELECTIVE LIQUID MEMBRANES AT THE MOLECULAR-LEVEL

Ion-selective charge separation at the surface of ionophore-incorporat ed liquid membranes was studied by observing optical second harmonic g eneration (SHG) and by using lipophilic photoresponsive ionophores as a molecular probe. It was found that SHG signals from the ionophore-in corporated membranes in contact with aqueous primary cation chloride s olutions generally increased with increasing cation concentration and then leveled off. This can be explained by the formation of oriented a nd therefore SHG active cation-ionophore complexes at the membrane sur face. It was found that the membrane potential and SHG signal changed in parallel. This result suggests that the observed membrane potential s were primarily governed by SHG active oriented cation complexes at t he membrane surface. To further clarify the influence of the surface c harge density on the phase boundary potential, photoresponsive ionopho res were used as a molecular probe because the ratio of the ionophore conformers with different complexation affinities is controlled quanti tatively by light irradiation without any change in the membrane compo sition. The photoinduced changes in the potentiometric responses were analyzed as a function of the surface charge density by using a surfac e model based on a double-diffuse layer. As a result, it was found tha t the photoinduced changes in the potentiometric response behaviors, e ven when the deviations from a Nernstian response were observed, are i n good agreement with the values calculated on the basis of the propos ed model. This agreement leads to the important conclusion that the su b-Nernstian response slope is attributed to the low surface charge den sity due to lower ionophore concentrations.