VOLTAMMETRIC INVESTIGATION ON ION TRANSFER THROUGH A LIQUID MEMBRANE OR A BILAYER-LIPID MEMBRANE

Voltammograms for ion transfer from one aqueous (W1) to another (W2) through a liquid membrane (LM), VITTM, were recorded by scanning the potential applied between W1 and W2 and measuring the current between W1 and W2. A method to analyze the VITTM by comparing VITTM with voltammograms at the W1/LM and LM/W2 interfaces was proposed. The membrane transport was demonstrated to be controlled mainly by the complementary ion transfers at the W1/LM and LM/W2 interfaces when W1, W2 and LM contained sufficient electrolytes. The influence of coexisting ions in LM or W2 on the membrane transport of an objective ion from W1 to W2 and the change of ion transfers at the W1/LM and LM/W2 interfaces during the electrolysis under an applied membrane potential were discussed. Based on the comparison of the VITTM recorded using a bilayer lipid membrane, BLM, with that recorded using an LM, the ion transport through a BLM was found to be analogous to that through an LM when the BLM contained sufficient ions. When very hydrophobic ion was added to one of the two aqueous phases in the presence of high concentration (e.g., 0.1 M) of hydrophilic salt, large symmetrical current peaks were observed in the VITTM even though the concentration of the added ion was extremely low. These peaks were attributed to transfers of the hydrophilic ion, which had been concentrated spontaneously into BLM by the distribution as the counter ion of the hydrophobic ion, from BLM to W1 and W2. Employing various ions as the additive, 4 types of VITTMs which were determined by hydrophobicities (and hence abilities to distribute into BLM) were observed. Ion transfer reactions determining each type of VITTM were elucidated. The relation between the shapes of VITTMs and properties of added ions, coexisting ions and BLMs was also discussed.