Impedance study of thallous ion movement through gramicidin–dioleoylphosphatidylcholine self-assembled monolayers supported on mercury electrodes: the C–(C)–CE mechanism

A new mechanism (the C–(C)–CE mechanism) is proposed for Tl+ reduction and Tl–amalgam oxidation on gramicidin-modified dioleoylphosphatidylcholine (DOPC) mercury and Tl–amalgam electrodes. The faradaic impedance equations are derived, and applied to the experimental results obtained in 0.1 M KCl solutions at a high gramicidin concentration. The mechanism includes two consecutive heterogeneous chemical steps with an intermediate, which decomposes in a parallel homogeneous chemical step. The two heterogeneous chemical steps are assumed to mimic the interaction of the ion with the channel mouth and the further translocation across the channel, respectively. The results fit the mechanism well, and the analysis provides the equilibrium constant for the first chemical step and the forward rate constant for the translocation step. The latter was found to depend on the drop in potential across the interface, with a transfer coefficient α=0.2, in both the reduction and the oxidation experiments. However, the translocation step standard rate constant provided by the oxidation experiment is lower than that by the reduction experiment, indicating some asymmetry in the half-channel behaviour.