A kinetic study of charge transfer and phase transitions of the methylene blue/leucomethylene blue couple adsorbed at the mercury/aqueous solution interface

Abstract

The interfacial behaviour of a methylene blue (MB)/leucomethylene blue (LMB) redox couple has been studied using fast perturbation potentiostatic techniques at the dropping mercury electrode. The extent of adsorption and the structure of the adsorbed layer above the monolayer coverage are determined by the presence of anions at the interface, anion hydration playing the important role. In 1 M electrolytes (pH=7.9) the tendency of MB dimers towards a stacking interaction in the aqueous phase, as well as to an adsorption at the mercury/aqueous solution interface, increases in the sequence F− ≪NO3−<Cl− ≪ClO4−. Evidence is presented for a coadsorption of LMB and nitrate and well-defined phase transitions in the adsorbed film. There are three distinct potential-dependent phases of LMB at the interface: (1) flat molecules and/or smaller two-dimensional aggregates at low LMB coverages; (2) two-dimensional crystalline layers of flat molecules after a monolayer of LMB is generated by reduction at positive charges; (3) a compact LMB layer at the negatively charged electrode, after desorption of nitrate and a conformational change of LMB molecules. The characteristics of the charge-transfer process are best interpreted in terms of two MB/LMB redox couples: the prewave corresponds to redox reactions of adsorbed species at the mercury surface, while the process at more negative potentials corresponds to the MB/LMB couple at a chemically modified electrode-mercury, covered by a compact and conductive layer of solid LMB.