SODIUM ALKYL SULFATE INDUCED DEPOSITION OF METHYL VIOLOGEN CATION RADICAL ON GLASSY-CARBON SURFACES

The effect of sodium n-alkyl sulfates (Cm, where m is the carbon number, m = 5-14) on the reduction of methyl viologen from aqueous solutions was investigated. C5 through C9 had no effect, but for C10 and longer alkyl groups, an induced deposition of the cation radical was observed by cyclic voltammetry, rotating ring-disk electrode (RRDE) voltammetry, and in situ optical microscopy. The amount deposited was shown to exceed monolayer quantities. The electrochemical data are consistent with the formation of an adherent porous deposit from which particles can be dislodged during anodic stripping with the RRDE. Discontinuities in the disk current-potential curve and an associated maximum ring current have been interpreted in terms of surpassing a critical surface concentration of cation radical followed by nucleation and growth of the deposit. The product of the second step of reduction, neutral methyl viologen (MV), deposits on the electrode even in the absence of surfactant. The voltammetric data suggest that two forms exist: an initially produced deposit that is reversibly oxidizable to the soluble cation radical and a form that appears at longer times and is more difficult to oxidize. Optical microscopy during prolonged electrolysis (>20 min) reveals a crystalline layer of MV on the surface of the electrode. The number density of MV crystals is reduced in the presence of C11 and C12 surfactants.