Micelle electrode surface

Haapakka and Kankare have studied this phenomenon and used it to determine various analytes that are active at the electrode surface [44-46], Some metal ions have been shown to catalyze ECL at oxide-covered aluminum electrodes during the reduction of hydrogen peroxide in particular. These include mercu-ry(I), mercury(II), copper(II), silver , and thallium , the latter determined to a detection limit of <10 10 M. The emission is enhanced by organic compounds that are themselves fluorescent or that form fluorescent chelates with the aluminum ion. Both salicylic acid and micelle solubilized polyaromatic hydrocarbons have been determined in this way to a limit of detection in the order of 10 8M. 

Schwenz and Moore introduced cyclic voltammetry as a modem approach to electrochemistry experiments. Three new experiments exploit this technique. One uses the technique as a probe or electrode surface area (82). A second uses the method to study adsorption of polyoxometalates on graphite electrodes (83). A third studies the effect of micelles on the diffusion and redox potentials of the well-studied ferrocene system (84). 

The electrochemical behavior of ascorbic acid (1) and uric acid (3) in the presence of micelles and their selective determination were investigated. Aqueous cetylpyridinium bromide (cpb) and sodium dodecylbenzenesulfonate (sdbs) miceUar solutions have been used. The oxidation peak potentials for 1 and 3 are separated by 270 mV in the presence of cpb in aqueous phosphate buffer solution (pH 6.8), thus allowing their selective determination, as well as the selective determination of 3 in the presence of excess of 1. The method is simple, inexpensive and rapid with no need to modify the electrode surface by tedious procedures, and it was applied to 3 determination in samples of human urine and serum. Abnormal levels of uric acid in urine and serum are symptomatic of several diseases (gout, hyperuricaemia and Lesch-Nyhan syndrome) . 

A somewhat different micelle-disruption method was used to prepare thin reactive films on electrode surfaces. For this purpose, micelles containing ferrocenyl surfactant 9 and a dissolved dye, e.g. phthalocyanine or quinone, were electrolyzed. The ferrocenyl micelles broke up into monomers whenever the iron atoms were oxidized electrochemically. The dissolved dyes then precipitated as transparent nanometre films onto the electrode surface. ... 

This technique makes use of the change in polymer solubility upon change in the ionic charge on the polymer. For example, poly(vinylferrocene) films can be deposited onto the surface of an electrode from dichloromethane by oxidizing the polymer to its less soluble ferricinium form which then adsorbs at the electrode surface [134-137]. In some ways this process is similar to metal plating. Recently this approach has been extended to make use of micellar disruption brought about electrochemically. In this approach the coating material is dissolved in a micellar solution and the micelles are disrupted by oxidation. 

The properties of surfactant molecules properties are (i) their ability to form different aggregate structures (micelles) above die critical micellar concentration (CMC), (ii) their ability to solubilize water-insoluble organic molecules (M) by hydrophobic-hydrophobic interactions, and (iii) their adsorption on electrodes changes the solution-metal interface, which alters redox reactions and produces template effects on the electrode surface (79) (Schem 2). SDS can be used to electropolymerize various thiophene derivatives such as EDOT, BT and MOT in aqueous solution. 

A few general features of the surfactant adsorbate structure can be extracted from the information available. At potentials negative of the point of zero charge (PZC) on hydrophilic metal and carbon electrodes, cationic surfactants adsorb head down. Positive of the PZC, anionic surfactants adsorb head down. Nonionic surfactants may adsorb head down on hydrophilic electrodes on either side of the PZC. Adsorption of cationic surfactants may be head down even at potentials positive of the PZC. This may involve adsorbed anions such as chloride on the electrode. Surface aggregate structures above the CMC may include bilayers, surface micelles, or cylinders depending on the nature of the surfactant, the electrode surface, and the applied potential. 

In general, then, surfactant aggregates on electrode surfaces can control electron transfer kinetics. Aggregates such as bilayers, cylinders, or surface micelles adsorb onto electrodes in solutions with surfactant concentrations above the CMC. 

Other catalysts with E° values more positive than —2 V showed smaller rate enhancements because a thick CTAB film apparently does not form on Hg electrodes at these potentials. Negatively charged clays on electrode surfaces have been used to form films of cationic micelles on electrodes for organohalide reductions positive of—2 V. Details of these and other mediated reactions in micellar solutions are found in reviews [4, 30]. 

After the desorption, long-chain alkanethiolates tend to form micelles or aggregates and stay on the electrode surface,... 

A lesser degree of the variation of Ep with n when n > 9 (Fig. 4b), where the desorption accompanies the phase transition of the monolayer, is likely to be attributable to the formation of micelles or aggregates of alkanethiolates formed in the vicinity of the electrode surface after the desorption [12, 25, 71]. When the desorbed species is in the form of such aggregates on the electrode surface, the value of the transfer Gibbs energy of the thiolates from the electrode surface to the aggregates would be much smaller than that into the bulk aqueous phase, giving rise to the smaller shift of the Ep with n. 

Alternatively, thin layers of mesoporous silica can be prepared in a single step by a so-called evaporation-induced self-assembly (EISA) procedure [234] the sol solution is deposited onto the electrode surface, and water evaporation occurs concomitantly with micelle formation and condensation of the material the template surfactant is extracted after that complete dryness is reached. A well defined voltammetric response is obtained at the resulting electrode system, due to the species that can diffuse throughout the coating after surfactant removal. 

The following mechanism was put forward [31] to explain this autocatalysis (1) permeation by cosurfactant (amide) of the water-AOT-toluene interfacial regions as a result of partitioning equilibria with concomitant increase in polarity and dielectric constant in these regions (2) diffusion of swollen micelle to proximity of electrode surface (3) collision of swollen micelle with the electrode surface (de facto hemimicelle formation) or with a hemi-micelle on the electrode surface and diffusion of amide through the AOT interfacial region within the electron transfer distance of the electrode (4) irreversible oxidation of amide. 

The spectra clearly show that the dye molecules do not fluoresce when they are adsorbed at the electrode surface (at Eads)- In contrast, fluorescence was observed at the potential where the surfactant is desorbed from the surface. Therefore, the results of fluorescence experiments support the conclusion that the desorbed molecules reside in the subsurface region in the form of micelles (or other aggregates). The intensity of the fluorescence increased in a nonlinear fashion on going from the aggregates produc-... 

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