Microemulsions electrochemical measurements

Preliminary conductivity measurements indicate that the polymers based on the anionic system are ionically conductive, whereas the nonionic based polymers are non-conductive. AC impedance tests were done on a thick film ( limn thick) using sodium sulfate as the electrolyte in a specially designed closed cell. The resistivity of polystyrene obtained from middle phase microemulsions was found to be in the rjange of lOMO ohm-cm, compared to lO o -10 2 ohm-cm for bulk polystyrene. A thin film of the polymer was also obtained on graphite electrodes by UV irradiation. Electrochemicd measurements using such polymer coated electrodes also suggest that the film is conductive. SEM micrographs before and after the electrochemical measurements indicate that the polymeric film is stable and porous. 

A recent SECM study of electrochemical catalysis at the ITIES was based on a similar concept (23). The ITIES was used as a model system to study catalytic electrochemical reactions in microemulsions. Microemulsions, i.e., microheterogeneous mixtures of oil, water, and surfactant, appear attractive for electrochemical synthesis and other applications (63). The ITIES with a monolayer of adsorbed surfactant is of the same nature as the boundary between microphases in a microemulsion. The latter interface is not, however, directly accessible to electrochemical measurements. While interfacial area in a microemulsion can be uncertain, the ITIES is well defined. A better control of the ITIES was achieved by using the SECM to study kinetics of electrochemical catalytic reduction of //zms-l, 2-dibromo-cyclohexane (DBCH) by Co(I)L (the Co(I) form of vitamin B12) ... 

Table 2 lists some measurements of diffusion coefficients in microemulsions of different surfactants [51]. Some data on SDS micellar solutions are included for comparison. The electrochemical measurements in all SDS systems were performed using ferrocene as probe to obtain the apparent diffusion coefficients. These values are compared to QELS... 

When small electroactive ions or molecules are bound to larger aggregates like micelles or microemulsion droplets, the reactant (probe) is transported to the electrode along with the larger, slower diffusing aggregate. Equation (12) describes the influence of concentration of surfactant or reactant on electrochemically measured diffusion coefficients. At [X] >2 mM, the measured apparent diffusion coefficient D approaches the diffusion coefficient of the micelle D. This implies electrolysis of one reactant X per micelle. This electrolyzed X could reside within MX , or be released by dissociation, as illustrated in Eqs. (17 and 18) for an oxidation ... 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

J. Georges and J. W. Chen. Microemulsion studies correlation between viscosity, electrical conductivity and electrochemical and fluorescent probe measurements. Colloid Potym, Sci 264 896-902, 1986. 

The water-in-C02 microemulsion mentioned previously in this section may provide an effective medium for generating electrical conductivity in supercritical CO2. In 2000, Ohde et al. first reported the results of voltammetric measurements for the redox reactions of ferrocene (FC) and A,fV,iV fV tetramethyl-jc-phenylenediamine (TMPD) in supercritical CO2 in the presence of a water-in-CO2 microemulsion (14). The design of their high-pressure electrochemical cell is shown in Figure 16. The same AOT/PFPE-PO4 water-in-C02 microemulsion described in Section IV.A was used in their voltammetric experiments. Well-defined voltammetric waves were obtained for FC and for TMPD in the microemulsion system as shown in Figure 17. An obvious diffusion current for the redox reaction of FC or TMPD was observed. An electrolysis experiment was also performed with TMPD. After the electrolysis at +0.3 V, the UV-Vis absorption spectrum of the sample collected in hexane was measured. The absorption peak wavelength and the shape of the peak were identical to that for TMPD + in water. The result suggests that TMPD " produced at the electrode surface was in the water core of the water-in-C02 microemulsion, as shown in Fq. (12) ... 

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