Juana J. Silber

Formation of a novel electroactive film by electropolymerization of ortho-aminophenol: Study of its chemical structure and formation mechanism. Electropolymerization of analogous compounds

Abstract The anodic oxidation of ortho -aminophenol ( o AP) in acid aqueous media on different electrode materials, leads to the formation of an electroactive polymer, poly( o AP). Cyclic voltammetric studies allow the inference of a quite complex mechanism for the formation of this film. These studies and the spectroscopie data suggest that the structure of poly( o AP) is mainly a polymer of phenoxazine rings. To confirm these studies the electroactive polymer was also prepared by chemical oxidation of o AP and its properties were compared with those of the electrochemically produced poly( o AP). Moreover, a film was obtained by electrooxidation of the cyclic dimer of o AP, the 3-aminophenoxazone (3APZ). The differences between these films are discussed, leading to the conclusion that the actual monomer in the formation of poly( o AP) is 3APZ. However, the fact that the film obtained from pure 3APZ has an ideal nernstian behavior while the one obtained from o AP does not, gives evidence that side reactions and consequently a “side” polymer make the real structure of poly( o AP) more complex. Then the conditions for obtaining reproducible poly( o AP) modified electrodes are proposed. In addition, the electrochemical oxidation of compounds structurally related to o AP was carried out. The studies with m - and p -aminophenol show that o -hydroxy-substitution of anilines is required for the formation of ring chain films like poly( o AP). The o -anisidine gives a linear polymer of the polyaniline type, since the formation of a cyclic dimer is hindered. The mechanism of electropolymerization of o -phenylenediamine and its cyclic dimer (2,3-aminophenazine) seems to have similar features to that of o AP.

Electrochemical properties of poly-ortho-aminophenol modified electrodes in aqueous acid solutions

Abstract Poly-ortho-aminophenol (PolyoAP) modified electrodes were obtained from the electro-oxidation of the corresponding monomer on platinum and glassy carbon electrodes in aqueous solution. The electrochemical behaviour of the film was studied by cyclic voltammetry under various experimental conditions. At low scan rates (v 7, there is practically no response. However, the film is not destroyed and its response is recovered at pH 1. The effect of temperature on the peak current was also studied. The anomalies observed may be ascribed to the effect of temperature on the interaction parameters. On the other hand, kinetic effects were observed on the cathodic response of the film. Thus, Epc shifts with the sweep rate. This behaviour can be explained by assuming a chemical reaction coupled with the charge transfer. This kinetic effect depends on the temperature, and Epc shifts with T to more positive values, as expected. A redox mechanism of the polyoAP-modified film is proposed which involves addition-elimination of protons coupled to reversible one-electron transfer. The electrocatalytic properties of the polyoAP film electrode were tested for several typical substrates.

Anodic oxidation of 1-naphthylamine in acetonitrile

Abstract The anodic oxidation of 1-naphthylamine (ArNH 2 ) has been studied at the platinum electrode in acetonitrile by controlled potential electrolysis, chronopotentiometry, cyclic voltammetry, cyclic voltammetry and spectrometric methods. From the electrochemical data a complex ECE type of mechanism is inferred with an overall efficiency of one electron per molecule of ArNH 2 . The electrochemical steps are reversible and the chemical process seems to follow second order kinetics and is extremely fast. The cation radical produced in the first charge transfer step suffers fast decomposition to give dimers which are easier to oxidize than ArNH 2 . The dimeric products were identified by comparison with authentic samples. Further complications arise due to slow reactions of the oxidized dimers. Electrochemical studies in basic media (pyridine and 2,6-lutidine) and acid media (anhydrous trifluoroacetic acid) were also performed.

Studies of surface-modified glassy carbon electrodes obtained by electrochemical treatment: Its effect on Ru(bpy)2+3 adsorption and the electron transfer rates of the Fe2+/Fe3+ couple

Glassy carbon electrodes were treated electrochemically by cycling the electrode between - 500 and 1860 mV (vs. SCE) in 1 M H2SO4 at a sweep rate of 150 mV/s. n nThe modified glassy carbon (MGC) electrode surfaces obtained have been characterized by cyclic and step potentiostatic techniques. The redox reactions of the surface functionalities attributed to the quinone-like structures of these MGC electrodes are slow and exhibit apparent diffusion control. Thus, when a given catalytic effect depends on the redox state of the surface, it will also depend on the time scale of the experiment. The conditions for obtaining reproducible MGC electrodes are given. Cyclic voltammetric studies show that Ru(bpy)2+3 adsorbs strongly on MGC electrodes while the adsorption is negligible on freshly polished electrodes. The adsorption fits the Temkin isotherm, giving the value of the repulsive interaction between the molecules. The value of †G = −39.6 kJmol was calculated for the process. A specific interaction between the bipyridyl moieties and the surface functionalities is proposed. Appreciable photocurrents are observed for Ru(bpy)2+3 adsorbed on a MGC electrode which may lead to future applications. On the other hand, by measuring the formal heterogeneous rate constants (ks) for Fe2+/Fe3+ it is possible to show how the catalytic efficiency of MGC depends on the degree of modification of its surface.

Mechanism of electropolymerization of 1-naphthylamine in aqueous acid media

Abstract The electrooxidation of 1-naphthylamine (ArNH 2 ) in aqueous acid media was studied. The formation of a film adhered to the electrode surface is observed as principal product and for which a polymer structure through Cue5f8N coupling is proposed. Another soluble product is detected in smaller proportion which has been characterized as the dimer 1,1′-naphthidine. On the other hand, from the analysis of the voltammetric and voltacoulometric experiments some aspects of the mechanism of electrooxidation of ArNH 2 for the formation of both products is proposed. This mechanism allowed us to establish quantitative ratios between the charge and number of moles of ArNH 2 consumed, with the redox process of the film. The electrochemical behaviour of the film is reported as a function of ArNH 2 concentration and pH. The film behaves as a thin layer cell when it is obtained at low pH ( ie pH = 1) and dilute solutions of ArNH 2 . Moreover the voltammetric response of the film seems to fit the “unusual quasi-reversibility” behaviour.

Spectroelectrochemical study of poly-o-aminophenol

We report a Spectroelectrochemical study in the wavelength range 300 nm < λ < 800 nm, of poly (o-aminophenol) (POAP) in aqueous acid solutions of different pHs and with different supporting electrolytes. Four features were observed in the absorption spectra: the uv tail of the band of the aromatic ring structures, a weak absorption band at 340 nm corresponding to the polymer in the reduced state, a strong broad band at 440 nm corresponding to the oxidized state of the polymer and a maximum in absorbance at about 750 nm when going from the reduced to the oxidized states. Cycling the potential while holding λ at 750 nm yields a transient response in the absorbance peaking at the peak potential of the voltammetric response, which also is the potential of maximum conductivity of POAP. From the absorbance data the number of exchanged electrons during the oxidation cycle was 0.4 which agrees well with that estimated from the oxidation charge and the polymer thickness. This indicates that one in every four or five monomer units is being oxidized. The dependence of the absorbance on pH shows that as the pH increases the protonation of the amino groups decreases. The experimental results suggest that the imino groups in the oxidized state are not protonated. This is in agreement with the fact that POAP is easily oxidable and a very poor conductor. This is also in agreement with the increasing difficulty in reducing the polymer as the pH increases. Hysteresis effects were observed both in the voltammogram and in the absorbance during potential cycling. In the presence of anions bigger than perchlorate, such as benzenesulphonate, both the oxidation charge and the absorbance change decrease.

Impedance characteristics of poly-o-aminophenol electrodes

The impedance characteristics of poly-o-aminophenol film electrodes in NaClO4 0.4 M + HC1O4 0.1 M electrolyte have been measured in the frequency range 0.01 <f< 103Hz and at two different thicknesses. The experimental results can be interpreted through a simple charge transfer resistance in parallel with a constant, capacity at high frequencies and the redox capacity in series with the film resistance at low frequencies. The analysis of the impedance response allows to the electrochemical rate constant, the concentration of redox centers in the film, the film conductivity and the diffusion coefficient of the elections to be determined.

Solvent effects in electrochemical oxidation of 1-naphthylamine: Dimethylsulfoxide vs. acetonitrile

The anodic oxidation of 1-naphthylamine (ArNH2) has been studied at the platinum electrode in dimethylsulfoxide (DMSO) by cyclic voltammetry and controlled-potential electrolysis. From the electrochemical data a complex ECE type of mechanism is inferred. Dimeric products are obtained by Cp-N and N−N coupling. These results compared with similar studies in acetonitrile show an important solvent effect on product distribution and faradaic efficiency. These can be explained considering the basicity of DMSO and its influence in the deprotonation reactions following the charge-transfer steps.

Preparation and electrochemical behavior of conducting films obtained by electropolymerization of benzidine in aqueous media

Abstract The electrochemical oxidation of benzidine to obtain an electroactive film of polybenzidine (poly-Bz) is reported. The yield of the electropolymerization reaction depends on temperature and pH in a way that indicates that there is a competitive reaction to film formation. A mechanism that explains these effects and that is used to calculate the rate coefficients of early stages of the process is proposed. The voltammograms of poly-Bz are highly asymmetric, suggesting a complex redox behavior. This film shows thin-layer behavior at sweep rates lower than 0.100 V s −1 , while at higher rates semi-infinite diffusion behavior is observed. Poly-Bz exhibits electrochromic behavior. It is colorless in the reduced state and blue in the oxidized state. Spectroelectro-chemical studies on an indium tin oxide electrode show that, upon oxidation, an absorption band with a maximum at 560 nm develops and a well-defined isosbestic point is observed at 415 nm. The variation of the absorbance at 540 nm on the potential shows that the poly-Bz film has a Nernstian behavior with interaction between active sites.

Binding of nitroanilines to reverse micelles of AOT n-hexane

Abstract The electronic absorption spectra of a series of nitroanilines: p-nitroaniline (pNA); o-nitroaniline (oNA); 2,4-dinitroaniline (dNA) and N,N-dimethyl-p-nitroaniline (N,NpNA) have been measured in solutions of sodium 1,4-bis (2-ethylhexyl) sulfosuccinate (AOT) in n-hexane at different W=[H2O]/[AOT]. On increasing AOT concentration, it is observed that the characteristic absorption band of pNA, oNA and dNA in n-hexane decreases in intensity, and a new band develops at higher wavelength. A net isosbestic point is detected in every case. The spectral changes are interpreted as a consequence of an equilibrium between nitroaniline free and bound to AOT in the micellar system. These changes allowed us to determine the binding constant (Kb) between the nitroanilines and AOT. The values of Kb vary from 73 M−1 for oNA to 3370M−1 for dNA at W = 0. For N,NpNA only a shift of the absorption band in n-hexane is observed. The magnitude of the possible solute-solvent interactions of these compounds was analyzed by means of the Taft and Kamlets solvatochromic comparison method. Thus, taking into account our studies in pure solvents, the strength of binding is interpreted considering their polarity and hydrogen-bond donor ability as well as their solubility in n-hexane. The binding of oNA to AOT shows a decrease in Kb followed by a plateau value as W is increased. For pNA and dNA, which have large values of Kb, no variation is observed as W is increased. This effect is discussed in terms of a competition between the nitroaniline and water for interfacial binding sites. In addition the absorption maxima of the free nitroaniline tends toward that on hexane (hypsochromic shift) when increasing the water concentration and mainly for oNA, indicating that water competition for the hydrogen bond acceptor sites of AOT is greater for the weaker bounded nitroaniline. The absorption maxima of the bound nitroanilines shifts bathochromically on water addition as expected for the increasing polarity of the micellar interface.