Characteristics of faradaic phase transition of an adsorption layer of heptyl viologen at a basal plane HOPG electrode

At a HOPG electrode surface horizontally touched to an aqueous solution of heptyl viologen (HV), the adsorption layer of HV exhibits a faradaic phase transition of the first order showing a nucleation-growth type current transient. The characteristics of this transition were studied herein in terms of (i) HV concentration dependence, (ii) temperature dependence, (iii) adsorption isotherm model, (iv) Avrami plot, and (v) effect of the coexistence of a water-soluble cationic surfactant. It was confirmed that the transition takes place between a gaseous expanded adsorption layer of oxidized form and a 2D insoluble condensed monolayer of one-electron reduced form. The temperature dependence suggested that the desolvation of HV dication upon its reduction may play an important role in the transition as the nucleation barrier. A Frumkin isotherm with a strong attractive interaction between reduced molecules (HV radical cations) was applicable as an approximated model to explain the transition potentials. The coexistence of an additive cationic water-soluble surfactant, cetyltrimethylammonium, inhibited the sharp voltammetric spike at surfactant concentrations far lower than the cmc. The surfactant may act even at low concentrations as an impurity, weakening the attractive interaction between reduced viologen moieties.