Surfactant monomer -electrode

Surfactant Activity in Micellar Systems. The activities or concentrations of individual surfactant monomers in equilibrium with mixed micelles are the most important quantities predicted by micellar thermodynamic models. These variables often dictate practical performance of surfactant solutions. The monomer concentrations in mixed micellar systems have been measured by ultraf i Itration (I.), dialysis (2), a combination of conductivity and specific ion electrode measurements (3), a method using surface tension of mixtures at and above the CMC <4), gel filtration (5), conductivity (6), specific ion electrode measurements (7), NMR <8), chromatograph c separation of surfactants with a hydrophilic substrate (9> and by application of the Bibbs-Duhem equation to CMC data (iO). Surfactant specific electrodes have been used to measure anionic surfactant activities in single surfactant systems (11.12) and might be useful in mixed systems. ... 

Micelles of cationic surfactants have been found to form both in glycerol [44] and in ethylene glycol [18], The micelle formation of Ci6PyBr in ethylene glycol and glycerol was studied with surfactant-selective electrodes [45,46], The monomer concentration could in this way be measured at different total surfactant concentrations, and it was concluded that there is some premicellar aggregation... 

If one considers solely the consecutive equilibria, the concentration of monomer can only increase with increasing total amphiphile concentration even above the CMC. (Apart from the trivial decrease in the monomer concentration calculated on the total volume which may arise when the micelles occupy a substantial volume fraction). However, if one realizes that micelles are not only composed of amphiphile, the result may be different. Thus counterion binding helps to stabilize the micelles and for ionic surfactants it can be predicted that the monomer activity may decrease with increasing surfactant concentration above the CMC. Good evidence for a decreasing monomer concentration above the CMC has been provided in the kinetic investigations of Aniansson et al.104), and recently Cutler et al.46) demonstrated, from amphiphile specific electrode studies, that the activity of dodecylsulfate ions decreases quite appreciably above the CMC for sodium dodecylsulfate solutions (Fig. 2.14). 

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. ... 

Ultrasound was also used for the dispersion of a surfactant pyrrole, prior to electrooxidation to the conducting polymer [233]. An amphiphilic (pyrrolylalkyl) ammonium monomer dispersion was used to coat the electrode surface with monomer, subsequently electropolymerized to thin films using an aqueous electrolyte for this step. Ultrasound has also been used to assist impregnation of pyrrole monomer into, for example, a conventional polymer matrix prior to polymerization to yield a composite of the conducting and conventional polymers, but is also a pretreatment effect of ultrasound rather than a sonoelectrochemical one [234],... 

Special examples of mixture adsorption are competitive adsorption of the different forms of the same substance, such as pH-dependent ionic and undissociated molecular forms, monomers, and associates of the same substance, as well as potential-dependent adsorption of the same compound in two different orientations in the adsorbed layer. Different orientations on the electrode surface—for example, flat and vertical—are characterized with different adsorption constants, lateral interactions, and surface concentrations at saturation. If there are strong attractive interactions between the adsorbed molecules, associates and micellar forms can be formed in the adsorbed layer even when bulk concentrations are below the critical micellar concentration (CMC). These phenomena were observed also at mineral oxide surfaces for isomerically pure anionic surfactants and their mixtures and for mixtures of nonionic and anionic surfactants (Scamehorn et al., 1982a-c). 

The electropolymerization of these monomers at constant current under the same micellar conditions led to the formation of thin, electroactive polymer films. The electropolymerization of 0.05 M EDOT in 0.1 M SDS containing O.IM LiC104 in water at a Pt electrode began at very low current (j = 0.1 mA/cm ), compared to that found in acetonitrile without SDS (j = 0.5 mA/cm ). This phenomenon nuy be attributed to a specific effect of the SDS surfactant, which alters the oxidation potentials of EDOT. Thin, electroactive and conductive poly(EDOT) films can be synthesized in the above aqueous micellar solution at constant currents ranging firom j = 0.1 mA/cm to j = 5 mA/cm. For j > 5 mA/cm the resulting poly(EDOT) films were non-electroactive and extremely degraded owing to the reaction between water molecules and the thienyl radical-cations formed (8). 

Finally, nanocomposites incorporating electrogenerated PPy have known a growing interest in the last decade, especially those based on carbon nanotubes (CNT). Indeed CNT-PPy hybrids exhibit very high capacitance that makes them promising materials for supercapacitors. Several methods can be envisaged for the obtention of CNT-PPy hybrids electropolymerization on a CNT-array-modified electrode, coelectrodeposition from mixtures of surfactant stabilized aqueous CNT dispersion with the monomer or from mixtures in an ionic liquid layer on the electrode, and alternatively direct electrodeposition of PPy on CNT in a cavity microelectrode. ... 

Cosnier and Innocent [135] recognized tliat one of the problems with incorporation of enzymes from solutions into CP films during electropolymerization was that the concentration of the enzyme in the CP film could only be approximately controlled. They took a derivatized pyrrole monomer containing a surfactant group (Eq. (17.211 which has a strong affinity for the enzyme tyrosinase within specific pH ranges. A film of the monomer-tyrosinase complex, with predetermined monomer-tyrosinase concentrations, was cast from solution onto a Pt electrode. The electrode was then removed from this solution and immersed in a second electrolyte for polymerization of the adsorbed monomer-tyrosinase complex, yielding a P(Py)-derivative/tyrosinase film. Tyrosinase catalyzes the oxidation of monophenols and o-diphenols to o-quinones in the presence of O2. The sensor could thus be used for detection of such analytes as catechol, as shown in Fig. 17-12. Response times were of the order of seconds. 

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