Impedance study of TI+ reduction at gramicidin-modified dioleoylphosphatidylcholine-coated mercury electrodes - Influence of gramicidin concentrationand the nature of the supporting electrolyte

Tli reduction on gramicidin-modified dioleoylphosphatidylcholine-coated mer cury electrodes has been studied by the impedance method in different exper imental conditions. On one hand, experiments at twelve gramicidin concentra tions in 0.1 M KCI solutions have bean performed in order to clarify whethe r radial diffusion plays some role. On the other hand, KCl, KNO3, KClO4, Mg Cl2, and CaCl2 solutions have been used as supporting electrolyte to study the influence of ion nature on both the mass transfer and the electron tran sfer of the process. In all the cases the frequency dependence of the admit tance data fits well to a Randles circuit and the Warburg coefficient, a, a nd the irreversibility coefficient, p ', can be obtained at every de potent ial. However, the potential dependence of a can only be explained if a mech anism is assumed, including two chemical steps, one preceding and one follo wing the electron transfer (CEC mechanism) even at very high gramicidin con centrations. The impedance equations for reactions on partially blocked ele ctrodes have been deduced, but the results do not conform completely to thi s model, so other explanations are proposed for the origin of the two chemi cal steps. Moreover, the standard rate constant, k(s), and the charge trans fer coefficient, a, for the electron transfer were obtained from p '. These parameters are not sensitive to gramicidin concentration or to the nature of the supporting electrolyte. On the contrary, the mass transfer parameter s are strongly dependent on these variables. The behavior can be understood at the light of the mechanisms for ion movement in biomembranes. (C) 2001 The Electrochemical Society. All rights reserved.