ELSEVIER Journal of Electroanalytical Chemistry 389 (1995) 61-70 JOURNAL OF Ion transfer through a liquid membrane or a bilayer lipid membrane in the presence of sufficient electrolytes Osamu Shirai, Sorin Kihara *, Yumi Yoshida, Masakazu Matsui Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan Received 12 July 1994; in revised form 15 November 1994 Abstract Voltammograms for the ion transfer from one aqueous solution (Wl) to another (W2) through a liquid membrane (LM) (vITrM) were recorded under various conditions by scanning the potential applied between two aqueous phases and measuring the current between two aqueous phases. A method of analysing the VITTM was proposed based on the comparison of the VITI'M with voltammograms at the W1 ] LM and LM I W2 interfaces observed simultaneously with the VITTM. The membrane transport was demonstrated to be controlled mainly by the complementary ion transfers at the WllLM and LM IW2 interfaces, when W1, LM and W2 contain sufficient electrolytes. The influence of different ions in LM or W2 on the membrane transport of an objective ion and the change in ion transfer reactions at the WI b LM and LM iW2 interfaces during the electrolysis under an applied membrane potential were elucidated taking into account the relation among the VITTM and voltammograms at two interfaces. The ion transport through a bilayer lipid membrane (BLM) was found to be analogous to that through an LM when the BLM contains sufficient ions, although the BLM is very much thinner than the LM. Keywords: Ion transfer; Liquid membranes; Bilayer lipid membranes I. Introduction The membrane potential which determines the membrane transport of an ion from one aqueous solu- tion, W1, to another, W2, through a membrane, M, is composed of potentials due to transfers of the ion at the WliM and Mew2 interfaces and those due to mass transfers within W1, M and W2. Here, the mem- brane potential is defined as the potential difference between W1 and W2. It has been widely supposed that the potential for the mass transfer in M contributes significantly to the membrane potential, since most of investigations on the membrane transport so far car- ried out employed membranes of high electric resis- tance containing negligible concentrations of ions. However, the membrane transport in the presence of a high concentration of ions in the membrane is also important especially in connection with the ion transfer at biomembranes or bilayer lipid membranes (BLMs). These membranes concentrate hydrophobic ions sport- * Corresponding author. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0022-0728(94)03852-X taneously into their lipid phases [1-3] and even hy- drophilic ions when the membranes contain hydropho- bic complexing agents [2, 3]. In this case, the share of the potential for the interfacial ion transfer in the membrane potential is considered to be more signifi- cant than that for the mass transfer in the membrane. One of the powerful methods for elucidating the ion transfer at the aqueous ]organic solutions interface is voltammetry for ion transfer at the interface of two immiscible electrolyte solutions [4-6] by which transfer energies of ions and amounts of ions transferred can be evaluated precisely. The kinetics of the ion transfer and the inteffacial adsorption can also be investigated by the method [7-9]. Hence, voltammetry for ion trans- fer at the interface of two immiscible electrolyte solu- tions is expected to offer much information for the analysis of ion transfer at the aqueous Imembrane in- terface [10-12], if the organic solution is regarded as the membrane. The voltammogram for the ion transfer through a membrane (VITTM) has also been studied by several researchers by scanning the membrane potential and measuring the membrane current which will be defined