Electroreduction of dioctadecylviologen in a self-assembled phospholipid monolayer on mercury and its role as an electron mediator

The electroreduction mechanism of dioctadecylviologen (V2+) incorporated in a self-assembled dioleoylphosphatidylcholine (DOPC) monolayer supported on a mercury electrode has been investigated in aqueous 0.1 M KCl by chronocoulometry and other electrochemical techniques. V2+ is reduced in two consecutive charge steps, 2V(2+) + 3e --> V-2(+) and V-2(+) + e --> 2V(0), involving 3/2 and 1/2 of a Faraday per mole of reactant, respectively. Both charge steps are exclusively controlled by the rate of the elementary coupling step Vf + V-0 --> V-2(+). V2+ acts as an electron transfer mediator from Fe(CN)(6)(3-) dissolved in the aqueous solution to the mercury surface across the DOPC monolayer, which is impermeable to Fe(CN)(6)(3-) in the absence of V2+. This mediation takes place only along the first charge step, 2V(2+) + 3e --> V-2(+) and increases with the V2+ concentration in the lipid monolayer less than proportionally; this behavior points to the radical cation V+ as the electron transfer mediator from the mercury surface to the aqueous ferricyanide.