Electrochemistry of malachite green and crystal violet in aqueous solution and surfactant-based organized media

Abstract

Electrochemical behaviors of malachite green (MG) and crystal violet (CV) in aqueous solutions and different surfactant-based organized media such as, micelles, reverse micelles and microemulsions of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), an anionic surfactant, sodium dodecyl sulfate (SDS) and a non-ionic surfactant, octylphenolpoly(ethyleneglycolether) (Triton X-100, TX-100) at a glassy carbon electrode were studied by using cyclic voltammetry. In aqueous solution, the cyclic voltammograms exhibit a well-defined oxidation peak and a corresponding reduction peak for MG whereas an oxidation peak for CV was apparent without any reduction peak. In case of MG, the oxidation peak corresponds to oxidation of hydrated MG to N, N, N´, N´-tetramethylbenzidine (TMBOx) and the reduction peak is due to the reduction of TMBOx to TMB. The oxidation peak of CV corresponds to oxidation of unhydrated form of CV. The electrochemical oxidation of MG and CV in aqueous solution is a diffusion-controlled process. The electrochemical responses of MG and CV exhibit strong pH dependence. Low pH favors the cationic form; whereas high pH favors the carbinol form of MG and CV. Under highly basic condition, the shape of voltammogram is different. Physicochemical properties of surfactant-based organized media of CTAB, SDS and TX-100 have been studied by measurements of specific conductivity, refractive index, density, viscosity, surface tension etc. In aqueous solution, viscosity increases with increasing concentration of surfactants due to formation of micelles. In micellar solution, the addition of 1-butanol decreased the viscosity of CTAB and TX-100 but increased the viscosity of SDS. Cyclohexane penetrates into the surfactant palisade layer by replacing 1-butanol to increase the viscosity of the microemulsion. The addition of surfactant in water raises the density value whereas the density of the reverse micelles and microemulsions decreases with increasing 1-butanol and cyclohexane content, respectively. At high 1-butanol content, the cores of the reverse micelles are comprised of the hydrophilic ion and the counter ion to lower the specific conductivity of CTAB and SDS. The radius of the droplets of reverse micelles of CTAB is higher than that of micelles in water. In case of SDS, at high 1-butanol content no reverse micelles are formed. Surfactants have profound effect on the cyclic voltammetric behavior of MG and CV in aqueous solution. Oxidation peak currents of MG sharply decreased with increasing SDS concentration, while a slight increase with increasing CTAB and TX-100 was apparent. A sharp decrease in peak current for SDS indicates strong electrostatic interactions of MG and CV with the surfactant. The current and potential of MG and CV fairly depend on the composition of the reverse micelles and microemulsions. As the content of 1-butanol in the reverse micelles of SDS increases, the oxidation peak current decreases first and then increases sharply with increasing 1-butanol. In reverse micelles and microemulsions of TX-100, the increase in cyclohexane content raises the oxidation peak current of MG. In reverse micelles and microemulsions of CTAB, oxidation peaks of MG and CV are merged with the oxidation peak of bromide ion of CTAB. As a result CTAB-based reverse micelles and microemulsions were not suitable to study the electrochemical behavior of MG and CV in the potential range studied. With increasing cyclohexane content the apparent diffusion coefficient, Dapp of CV in microemulsion of TX-100 increased up to 50.0% wt. after that no electrochemical responses found due to formation of thick layer of TX-100 outside the core of reverse micelles which reduces the Dapp of CV towards electrode. The electrochemical behavior of MG and CV can be varied by changing the nature of the medium, aggregation behavior and orientation of the surfactants in the associated states, as well as change in the redox states of the MG and CV. Moreover MG and CV have complex structures. They have both of hydrophobic and hydrophilic characteristics which provide the scope of interaction to varying extent with different type of surfactants. These make the dyes as intriguing probe for electrochemical studies. The analyses of electrochemical behavior have established MG and CV as fascinating electro active substances for electrochemical switching. The dyes, MG and CV can be effectively used for electrochemical switchable devices. Thus MG and CV have the bright prospect in the field of supramolecular chemistry and can serve as a redox active probe for versatile applications.