Study of the correlation of the cyclic voltammetric responses of a nonionic surfactant containing an anthraquinone group with the dissolved states

Cyclic voltammetric behavior of a nonionic surfactant, alpha-(anthraquinony loxyhexyl)-omega-hydroxy-oligo- (ethylene oxide), ACPEG, has been studied i n detail in aqueous media with NaCl as the supporting electrolyte. Attempts have been made to correlate the electrochemical responses with the dissolv ed states of the surfactant. The shapes of the cyclic voltammograms at a gl assy carbon electrode have proved to be fairly dependent on the concentrati on of ACPEG and on the redox state of the anthraquinone group. Below the cr itical micelle concentration (cmc), the cyclic voltammogram (v = 10 mV/s) c orresponds to the surfactant molecules adsorbed onto the electrode surface. The adsorption of ACPEG molecules at the electrode surface is very weak in nature owing to its low surface activity. A high cmc value and weak adsorp tion allow diffusion of the monomeric species prior to the reduction to con tribute to the total current even below the cmc. At concentrations higher t han the cmc, the diffusion-controlled waves are superimposed on the surface waves, and far above the cmc, the voltammetric shape is of a typical diffu sion-controlled species. Above the cmc, self-association of ACPEG results i n the formation of micellar aggregates, which diffuse to the electrode surf ace to dominate the adsorption wave. The redox process is influenced by the formation of a quinhydrone-like charge transfer complex. The cyclic voltam metrically determined apparent diffusion coefficient of the micellar soluti ons changes continuously with concentration; with decreasing concentration the apparent diffusion coefficient approaches the monomeric diffusion coeff icient while with increasing concentration it approaches the micellar value . This has been interpreted in terms of change in the diffusion species by dissociation of the micelles formed above the cmc to monomers in the concen tration gradient in the diffusion layers. The formation and dissociation re action can be reversibly controlled by a change in the redox state. A CEC m echanism with the electrochemical reaction coupled with the preceding disru ption reaction of the micelles (CE) and the following protonation reaction of the reduced state (EC) has been inferred.