Estela M. Andrade

Electropolymerization of 2-methoxy aniline. Electrochemical and spectroscopical product characterization

Poly(2-methoxyaniline) (PMOA) was electrosynthesized with varying monomer concentrations in 1 M HClO4. The polymers obtained were characterized using cyclic voltammetry, Fourier transform IR diffuse reflectance spectroscopy (FTIR-DRS), UV–vis spectroscopy and electrochemical impedance. At relatively high monomer concentrations (>20 mM), the polymer obtained is similar to aryl amine conducting polymers, but shows in its voltammogram an intermediate peak, which becomes larger as the monomer concentration decreases. For monomer concentrations below ∼20 mM, the polymer becomes more resistive, behaving similarly to aryl amine redox polymers. The different techniques employed show results consistent with the voltammetric experiments. This change of behaviour indicates that different polymers are obtained as the monomer concentration is modified.

IR response of poly-(o-toluidine) : spectral modifications upon redox state change

The IR spectra of poly(o-toluidine) in the reduced, half and fully oxidized states have been studied in both salt and base forms. The bands in the range 1500 to 1600cm−1 undergo important intensity changes as the degree of oxidation increases. The IR spectra show that during the electrochemical polymerization there is an influence of the nature of the anion present in the electrolyte. This shows up as conformational differences in the polymer backbone, but it does not affect the chemical structure. Also, spectral changes are observed when the polymer is oxidized in supporting electrolytes containing different anions. This has been interpreted by considering that the less hydrated anions can approach closer to the positive centres of the protonated amine groups.

Pb(II) binding to humic substances: an equilibrium and spectroscopic study

The binding of Pb(II) to humic acids is studied through an approach combining equilibrium and spectroscopic measurements. The methods employed are potentiometric and fluorometric titrations, fluorescence excitation-emission matrices (EEM) and IR spectroscopy. Potentiometric titration curves are analyzed using the NICA equations and an electrostatic model treating the humic particles as an elastic polyelectrolyte network. EEMs are analyzed using parallel factor analysis, decomposing the signal in its independent components and finding their dependence on Pb(II) activity. Potentiometric results are consistent with bimodal affinity distributions for Pb(II) binding, whereas fluorometric titrations are explained by monomodal distributions. EEM analysis is consistent with three independent components in the humic fluorescence response, which are assigned to moieties with different degree of aromaticity. All three components show a similar quenching behavior upon Pb(II) binding, saturating at relatively low Pb(II) concentrations. This is attributed to metal ion induced aggregation of humic molecules, resulting in the interaction between the aromatic groups responsible for fluorescence; this is also consistent with IR spectroscopy results. The observed behavior is interpreted considering that initial metal binding (observed as strongly binding sites), correspond to bi- or multidentate complexation to carboxylate groups, including binding between groups of different humic molecules, promoting aggregation; further metal ions (observed as weakly binding sites) bind to single ligand groups.

Electropolymerization of 2-methoxyaniline. Polymerization kinetics and phenazine insertion at low monomer concentrationElectronic supplementary information (ESI) available: Cyclic voltammograms. See http://www.rsc.org/suppdata/cp/b1/b110705d/

The electropolymerization of 2-methoxyaniline (MOA) has been found to yield products having structure and properties dependent on the monomer concentration, with the insertion of phenazinic units giving a redox-type polymer at low concentrations. The kinetics of this process is studied in 1 M HClO4 solutions containing different monomer concentrations. The voltammetric results are compared with the electropolymerization, in similar experimental conditions, of other aryl amines such as aniline and 2-methylaniline. The effect of dimeric additives on the polymerization rate of MOA is also studied. A change in the kinetic behavior is found at MOA concentrations of about 30–60 mM. An electropolymerization mechanism for 2-methoxyaniline is proposed through a reaction scheme similar to that of aniline, with the inclusion of a step consisting in a head-to-head coupling of oligomers and/or polymer chains yielding phenazine-type units, that effectively works as a termination step. The resistivity increase, in the conductive state, due to the insertion of redox units is also considered. The experimental results are interpreted on the basis of this mechanism.

Modeling ion binding to humic substances: elastic polyelectrolyte network model.

A new model for the electrostatic contribution to ion binding to humic substances is proposed and applied to published data for proton binding to fulvic and humic acids. The elastic polyelectrolyte network model treats humic substance particles as composed by two parts, an external one directly in contact with the solution, and an internal part or gel fraction which is considered, from a statistical point of view, as a charged polymer network swelled by the electrolyte solution, in the framework of the Flory polymer network theory. The electrostatic effect is given by a Donnan-like potential, which can be regarded as an average value over the gel fraction of the humic particle. The gel fraction expands as the pH and humic charge are increased, determining the Donnan potential and consequently the ion activity inside the gel. The model was fitted to published experimental data with good agreement. The model predictions are discussed, and the behavior suggests, for some cases, the presence of a transition between closed and open structures attributed to the presence, at low pH, of intramolecular hydrogen bonds which are removed as the carboxylic sites become deprotonated.

Application of a constrained regularization method to extraction of affinity distributions: Proton and metal binding to humic substances

The binding of proton and metal cations to humic substances has been analyzed with a regularized fitting procedure (using the CONTIN software package) to extract conditional affinity distributions, valid at a given ionic strength, from binding (titration) curves. The procedure was previously tested with simulated titration curves using a simple bi-Gaussian model, the NICA-Donnan model, and the Stockholm humic model. Application to literature data for proton binding shows that in several cases the affinity distribution found is bimodal (carboxylic and phenolic sites) as usually assumed; however in other cases, specially for fulvic acids, a trimodal distribution is clearly discerned, with a smaller peak between the two noted above attributed to the presence of vicinal carboxylic groups. The analysis of metal binding curves has been performed in a few cases where the available data could be reliably processed, separating the proton affinity distribution and obtaining the conditional affinity spectra. For Cd(II) and Pb(II) a bimodal distribution is found, attributed in principle to mono- and bidentate binding, based on spectroscopic data. In the case of Cu(II), a more complex affinity distribution is found showing 3-4 peaks; this is consistent with spectroscopic studies, where different binding modes, up to tetradentate, have been observed.

Electrochemical impedance of mercury electrodes with hematite particles adhered

The effect of hematite particles adhesion on the electrochemical impedance of mercury electrodes was studied at different electrode potentials. The impedance decreases as the number of attached particles increases; this impedance decrease is related to strong adhesion of particles. The impedance diagrams show, in the low frequency range, the presence of a constant phase element (CPE), with an exponent of ca. 0.5. The experimental results are analyzed in terms of an equivalent circuit including the CPE. The magnitude of this CPE is directly related to the coverage of the electrode. A qualitative interpretation for this behavior, when an AC signal is applied, is proposed in terms of a pore model for the metal/hematite particles interphase.