Adsorptive and stripping behavior of methylene blue at gold electrodes in the presence of cationic gemini surfactants

The adsorptive and stripping behavior of methylene blue (i.e. methylene blue chloride, MB) at a gold electrode has been studied with voltammetry, alternating current impedance spectra (ACIS) and quartz crystal microbalance (QCM). MB exhibits a pair of cyclic voltammetry peaks at about −0.3 V (versus SCE) in 0.05 M pH 6.9 phosphate buffer solutions. In the presence of cationic gemini surfactants such as C16H33N(CH3)2-C4H8-N(CH3)2C16H33Br2 (C16-C4-C16), C16H33N(CH3)2-C4H7OH-N(CH3)2C16H33Br2 (C16-C4OH-C16), C16H33N(CH3)2-CH2-C6H4-CH2-N(CH3)2C16H33Br2 (C16-ph-C16) and C16H33N(CH3)2-C12H24-N(CH3)2C16H33Br2 (C16-C12-C16), the anodic peak grows rapidly and moves in positive direction, but the cathodic peak gradually decreases, due to the association adsorption and electrostatic interaction of the geminis with MB and its reduced product (i.e. leuko methylene blue, LMB). With the aid of geminis the adsorption amount of MB increases under open-circuit, but the impedance of the mixed adsorption film to Fe(CN)63−/4− almost keeps unchanged, compared with either bare gold electrodes or MB film, while the adsorption film of geminis exhibits greater impedance. This probably is due to the electron medium action of MB in the film. Gemini surfactants with same alkyl-chain (i.e. -(CH2)15CH3) but different molecular structure, exhibit different influence. The enhancing action of geminis studied follows such order as: C16-ph-C16 > C16-C4-C16 > C16-C4OH-C16 > C16-C12-C16. The change of peak potential was ascribed to the interaction between MB and surfactants, as well as the blocking action of surfactant film. For comparison, the influence of dihexadecyldimethylammonium bromide (DCAB) and cetyltrimethylammonium bromide (CTAB) was studied, and the influence of other factors is discussed as well.