Surfactants electrochemical

An Undergraduate Physical Chemistry Experiment on Surfactants Electrochemical Study of a Commercial Soap 145... 

PFCA salts (e.g., the sodium and ammonium salts) are an industrially important class of anionic PFCA surfactants. Electrochemical fluorination has been employed to manufacture APFO, an emulsifier used in the production of fluoropolymers and other PFCAs. Hydrolysis of the acid fluorides obtained by electrochemical fluorination results either in the free carboxylic acids or the respective salts. A major problem associated with the hydrolysis of PFCA fluorides is the presence of HF from the electrochemical fluorination and the hydrolysis of the PFCA fluorides with water." The technical process of isolating fluoride-free PFCA derivatives has, therefore, been optimized to reduce the fluoride content of the final product to trace amounts. An example of such a process is shown in Figure 18.6. °... 

Surfactants I-III each undergo a one-electron oxidation to form a dication (N (CH3)3(CH2) Fc ). The halfwave potential for oxidation of a millimolar solution of II (0.1 M Li2S04, pH 2) is 0.17 V (vs. SCE). We have measured the half-wave potential of the series I-in to increase as a function of chain length. Solutions of I-in, when left exposed to air, oxidize over hours and days the rate of oxidation decreases with increasing chain length of the surfactant. Electrochemical characterization of IV demonstrates that the two ferrocene groups oxidize independently at a half-wave potential of 0.15 V (vs. SCE) [4]. 

Alcantara MT, Gomez J, Pazos M, Sanroman MA (2008) Combined treatment of PAHs polluted soils using the sequence extracticm with surfactant-electrochemical degradation. Chemosphere 70 1438-1444... 

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 standard electrode potential for zinc reduction (—0.763 V) is much more cathodic than the potential for hydrogen evolution, and the two reactions proceed simultaneously, thereby reducing the electrochemical yield of zinc. Current efficiencies slightly above 90% are achieved in modem plants by careful purification of the electrolyte to bring the concentration of the most harmful impurities, eg, germanium, arsenic, and antimony, down to ca 0.01 mg/L. Addition of organic surfactants (qv) like glue, improves the quaUty of the deposit and the current efficiency. 

A new generation of mesoporous silica (SG) materials obtained by sol-gel technique where polymers and ionic or non-ionic surfactant act as stmcture - directed templates is widely developed during last year s. Final materials can be synthesized as thin films and used as sensitive elements of optical and electrochemical sensors. 

Sodium dodecyl sulfate has been used to modify polypyrrole film electrodes. Electrodes synthesized in the presence of sodium dodecyl sulfate have improved redox processes which are faster and more reversible than those prepared without this surfactant. The electrochemical behavior of these electrodes was investigated by cyclic voltametry and frequence response analysis. The electrodes used in lithium/organic electrolyte batteries show improved performance [195]. 

Electrochemical analytical techniques are a class of titration methods which in turn can be subdivided into potentiometric titrations using ion-selective electrodes and polarographic methods. Polarographic methods are based on the suppression of the overpotential associated with oxygen or other species in the polarographic cell caused by surfactants or on the effect of surfactants on the capacitance of the electrode. One example of this latter case is the method based on the interference of anionic surfactants with cationic surfactants, or vice versa, on the capacitance of a mercury drop electrode. This interference can be used in the one-phase titration of sulfates without indicator to determine the endpoint... 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

Calculations for Rp as a function of the relevant experimental parameters (eluant ionic species concentration-including surfactant, packing diameter, eluant flow rate) and particle physical and electrochemical properties (Hamaker constant and surface potential) show good agreement with published data (l8,19) Of particiilar interest is the calculation which shows that at very low ionic concentration the separation factor becomes independent of the particle Hamaker constant. This result indicates the feasibility of xmiversal calibration based on well characterized latices such as the monodisperse polystyrenes. In the following section we present some recent results obtained with our HDC system using several, monodisperse standards and various surfactant conditions. 

Abott et al. demonstrated that the flow in a microfluidic channel can be controlled by using the wetting gradient generated by electrochemical reactions ofthe surfactant... 

As alternatives to amphiphilic betaines, a wide range of cationic, anionic, and non-ionic surfactants including environmentally benign sugar soaps have been successfully used as colloidal stabilizers [201]. Electrochemical reduction of the metal salts provides a very clean access to water soluble nanometal colloids [192]. 

ORR catalysis by Fe or Co porphyrins in Nation [Shi and Anson, 1990 Anson et al., 1985 Buttry and Anson, 1984], polyp5rrolidone [Wan et al., 1984], a surfactant [Shi et al., 1995] or lipid films [CoUman and Boulatov, 2002] on electrode surfaces has been studied. The major advantages of diluting a metalloporphyrin in an inert film include the abUity to study the catalytic properties of isolated molecules and the potentially higher surface loading of the catalyst without mass transport Umit-ations. StabUity of catalysts may also improve upon incorporating them into a polymer. However, this setup requires that the catalyst have a reasonable mobUity in the matrix, and/or that a mobile electron carrier be incorporated in the film [Andrieux and Saveant, 1992]. The latter limits the accessible electrochemical potentials to that of the electron carrier. 

Electrochemical redox studies of electroactive species solubilized in the water core of reverse microemulsions of water, toluene, cosurfactant, and AOT [28,29] have illustrated a percolation phenomenon in faradaic electron transfer. This phenomenon was observed when the cosurfactant used was acrylamide or other primary amide [28,30]. The oxidation or reduction chemistry appeared to switch on when cosurfactant chemical potential was raised above a certain threshold value. This switching phenomenon was later confirmed to coincide with percolation in electrical conductivity [31], as suggested by earlier work from the group of Francoise Candau [32]. The explanations for this amide-cosurfactant-induced percolation center around increases in interfacial flexibility [32] and increased disorder in surfactant chain packing [33]. These increases in flexibility and disorder appear to lead to increased interdroplet attraction, coalescence, and cluster formation. 

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. 

This reduction step can be readily observed at a mercury electrode in an aprotic solvent or even in aqueous medium at an electrode covered with a suitable surfactant. However, in the absence of a surface-active substance, nitrobenzene is reduced in aqueous media in a four-electron wave, as the first step (Eq. 5.9.3) is followed by fast electrochemical and chemical reactions yielding phenylhydroxylamine. At even more negative potentials phenylhydroxylamine is further reduced to aniline. The same process occurs at lead and zinc electrodes, where phenylhydroxylamine can even be oxidized to yield nitrobenzene again. At electrodes such as platinum, nickel or iron, where chemisorption bonds can be formed with the products of the... 

FIGURE 13.5 AFM images of Prussian blue-modified monocrystalline graphite (a) conventional Prussian blue deposited without surfactants, (b) Prussian blue electrochemically deposited through liquid crystalline phase of non-ionic surfactant Brij-56. 

The electrochemical response of analytes at the CNT-modified electrodes is influenced by the surfactants which are used as dispersants. CNT-modified electrodes using cationic surfactant CTAB as a dispersant showed an improved catalytic effect for negatively charged small molecular analytes, such as potassium ferricyanide and ascorbic acid, whereas anionic surfactants such as SDS showed a better catalytic activity for a positively charged analyte such as dopamine. This effect, which is ascribed mainly to the electrostatic interactions, is also observed for the electrochemical response of a negatively charged macromolecule such as DNA on the CNT (surfactant)-modified electrodes (see Fig. 15.12). An oxidation peak current near +1.0 V was observed only at the CNT/CTAB-modified electrode in the DNA solution (curve (ii) in Fig. 15.12a). The differential pulse voltammetry of DNA at the CNT/CTAB-modified electrode also showed a sharp peak current, which is due to the oxidation of the adenine residue in DNA (curve (ii) in Fig. 15.12b). The different effects of surfactants for CNTs to promote the electron transfer of DNA are in agreement with the electrostatic interactions... 

Q. Huang, Z. Lu, and J.F. Rusling, Composite films of surfactants, Nafion, and proteins with electrochemical and enzyme activity. Langmuir 12, 5472-5480 (1996). 

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