Study of leuco-methylene blue film growth and its reoxidation on sulphur-modified Au-EQCN electrode

The growth of leuco-methylene blue (LMB) films and their electrodissolution have been studied by voltammetry and piezogravimetry using bare gold (Au) and sulphur modified gold (Au-S) electrodes in methylene blue (MB+) solutions. The growth rate anisotropy has been found for thin LMB films formed on Au-S modified electrodes and on bare Au electrodes. Excellent agreement has been found between the experimental equivalent molmass of thin LMB films deposited from fluoride solutions on Au, with general formula L1−xCxAx·yH2O (M/n=186.9 g/val, x=0.48, n=1.52, y=0) and the theoretical molmass for unhydrated charge-transfer complex salt (M/n=189.0 g/val, x=0.5, n=1.5, y=0) postulated by Svetlicic et al. [V. Svetlicic, V. Zutic, J. Clavilier, J. Chevalet, J. Electroanal. Chem., 195 (1985) 307], where L is the fully reduced LMB0, C is the cation radical LMB*+, A− is the anion, and n is the number of electrons transferred. Thin LMB films contained no water, however, thicker LMB films have been found to contain large number of hydration water molecules. Two different polymorphic LMB phases were found in anodic dissolution of films deposited from fluoride solutions on bare Au-EQCN electrodes: an unhydrated phase with composition L0.5C0.5F0.5 corresponding to the CT-complex salt with all-solid-state electric conductance and a phase with higher hydration number and ionic conductivity. A phase transition facilitated by the release of hydration water is also postulated. A strong effect of anions F−, Cl−, and NO3− on the mass growth rate for LMB films formed under potentiostatic conditions has also been found.