The synergistic effect of phosphatidylcholine, PC, and phosphatidic acid, PA, on the ion transfer at a liquid-liquid interface was demonstrated to be caused by interaction with lanthanide ions. Cyclic voltammograms of the potassium ion transfer facilitated by dibenzo-18-crown-6 were recorded at the interface between water containing lanthanide ion and 1,2-dichloroethane containing PC and PA. The mixed adsorbed layer of PC and PA enhanced the blocking effect on the K+ transfer, as compared with the adsorbed layer composed of PC only. When the bulk concentration ratio of PC to that of PA was 1:2, the blocking was most effective and the potential range for the barrier layer stability was extended to the most positive potentials, i.e. the electrochemical stability of the barrier layer increased. On the contrary, an excess of PA resulted in degradation of the mixed barrier layer. The electrochemical stability of the mixed barrier layer was maintained at pH region lower than 4.0. When pH was higher than 4.0, the blocking effect disappeared. The above results are discussed in relation to the complex formation of phospholipids with lanthanide ion and to their protonation. (C) 2008 Elsevier B.V. All rights reserved.