Electrochemical impedance study of Tl+ reduction through gramicidin channels in self-assembled gramicidin-modified dioleoylphosphatidylcholine monolayers on mercury electrodes

Impedance measurements for the reduction of Tl+ on gramicidin-modified diol eoylphosphatidylcholine-coated mercury electrodes have been performed. The frequency dependence of the admittance data fits well to a Randles circuit, and the Warburg coefficient, sigma, and the irreversibility coefficient, p ', can be obtained at every de potential from the frequency analysis confor ming to this circuit. However, the potential dependence of the Warburg coef ficient is different from the one expected for a simple electron transfer. Instead, the sigma-E data can be analyzed conforming to a mechanism includi ng preceding and following homogeneous chemical steps to the electron trans fer (CEC mechanism). In addition, from the irreversibility coefficient, p', a value of the standard rate constant for the electron transfer of 0.035 c m s(-1) and a potential independent value for the transfer coefficient, alp ha, of close to 0.5 are obtained. The possibility that the CEC mechanism or iginates partly from nonlinear diffusion is considered, and the results are discussed in comparison with those given in the literature for Tl+ reducti on on pure mercury.