SPECTROELECTROCHEMISTRY OF METHYL VIOLOGEN IODIDE SOLUTIONS AT MERCURY FILM ELECTRODES

Methyl viologen (MV(2+)) has been studied at a thin mercury film elect rode in the presence of iodide ions. Conventional electrochemistry and in-situ optical probes have been applied to investigate processes not previously observed in fluoride solutions. As the potential is made n egative, iodide desorption is seen attended by an intense increase in reflectivity. The MV(2+) first reduction coincides with the formation of the (MV(.+)-I-) species on the surface, at the monolayer level, sho wing strong adsorbate-adsorbate interactions and a flat orientation of the pyridyl groups. The ac current response accompanying the appearan ce of the (MV(.+)-I-) film was found to be similar to that of a phase transition (condensation) in the adsorbed state. The dissociation of(M V(.+)-I-) clusters, to yield MV((solid))(0) + I-(aq)(-), at the onset of the second reduction wave of MV(2+), leads to a reflectivity enhanc ement in the in-phase optical signal paralleled by a sharp doublet in the imaginary current response. The splitting of the reduction peak of (MV(.+)-I-) may arise from different orientations of similar clusters or from distinct oligomer sizes.