Redox processes in microdroplets studied by voltammetry, microscopy, and ESR spectroscopy: Oxidation of N,N,N′,N′-tetrahexylphenylene diamine deposited on solid electrode surfaces and immersed in aqueous electrolyte solution

Electrochemical processes in nonhomogeneous media such as emulsions allow the electrolysis of materials of low solubility, which gives rise to new ways of controlling reaction environments, and/or simpler separation of products. However, the mechanistic details of these redox processes are not well understood. Two types of processes may be considered: (i) reactions in the two-phase solution environment coupled by simple mass transport in one phase to the interfacial electron transfer, and (ii) reactions involving both liquids in direct contact to the electrode surface (wetting). The latter type of process can be studied separately and is analyzed in this work for the one electron oxidation of the oil N,N,N′,N′-tetrahexylphenylene diamine deposited in the form of microdroplets on basal plane pyrolytic graphite, gold, or siliconized indium tin oxide (ITO) electrodes and immersed in aqueous electrolyte. The bulk conversion of the nonconducting oily material proceeds via the uptake of anions from the aqueous phase and the formation of a dark blue product monitored by voltammetry, optical microscopy, and ESR spectroscopy. The process is extremely sensitive to the type of anion which is electroinserted into the organic phase as is shown for ClO^-₄, PF^-₆, Cl^-, I^-, and OH^-. In the case of electroinsertion of hydroxide, a coupled chemical process occurs that can be monitored voltammetrically.