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Electrochimica Acta

Multi-layered films of poly(3,4-ethylenedioxythiophene), and poly(N-methylpyrrole), have been prepared using a layer-by-layer electrodeposition technique. The electrochemical properties and the conductivity of the films formed by 3, 5 and 7 layers have been compared with their homopolymers and with the copolymers prepared from 3,4-ethylenedioxythiophene and N-methylpyrrole with different concentration ratios. The electroactivity and stability of the multi-layered systems were higher than those of PEDOT homopolymer and copolymers. Furthermore, the electrochemical properties improved when the number of layers increased. In addition, the electrical conductivity of the multi-layered films is higher than those of poly(N-methylpyrrole) and copolymers, and comparable to the PEDOT homopolymers. These results indicate that multi-layered systems show better performance than their homopolymers and/or copolymers derivatives for some applications like anticorrosive coatings or sensor materials.

It was prepared a new polyvinyl chloride (PVC)/tetrathiafulvalene–tetracyanoquinodimethane (TTF-TCNQ) composite electrode which showed good physical and electrochemical characteristics. Different PVC/TTF-TCNQ–graphite proportions were tested and the relation 1/10 (p/p) PVC/TTF-TCNQ provided the best results in terms of sensitivity, coefficients of variation and mechanical resistance. This electrode has interesting applications as a working electrode. So, employment is remarkable in a flow injection system and had been satisfactory checked to detect ascorbic acid. Also, the electrode exhibits selective voltammetric behaviour in aqueous solution that affords its use as an ion detector. The proposed electrode showed good kinetics property with a low background current and a relatively reproducible signal. Moreover, the electrode can be prepared in a simple way and its surface easily recovered.

Electrochemical characterization of a symmetric track-etched polyethylenthereftalate (PET) membrane in contact with KCl solutions at different concentrations was carried out by measuring impedance spectroscopy (IS) and membrane potential (MP), which allow the estimation of membrane electrical resistance and ion transport numbers, respectively. IS measurements permit us the characterization of membranes in “working conditions” (in contact with electrolyte solutions) and the determination of electrical parameters for the membrane and the membrane/solution interface (resistance, capacitance and Warburg impedance) by analyzing the impedance plots and using equivalent circuits as models. The non-reproducibility of membrane potential values for two series of measurements and the asymmetry of the impedance curves show the modification of the membrane/aqueous solution interface. This point was confirmed by the time evolution of the membrane system electrical resistance and the increase of nitrogen content obtained from X-ray photoelectron spectroscopy (XPS) analysis for dry and PET samples maintained in water for different periods of time, and the results indicate an increase in nitrogen content, which is attributed to bacterial presence on the membrane surfaces. These results seem to indicate modification of the PET-membrane/solution interface due to fouling and the possibility of its determination by electrical measurements.

Simple photochromic dithienylethylenes with either perfluoro or perhydro cyclopentene ring, and a variety of substituents have been prepared and their electrochemical behavior explored by cyclic voltammetry. All present two electron irreversible oxidation waves in their open form, but the radical cation of the open isomers can follow three different reaction pathways: dimerisation, ring closure, or ring reopening. Whereas the chloro derivative follows a dimerisation mechanism (EC2E mechanism), the phenylthio substituted compound displays an efficient oxidative ring closure (ECE or DISP1 mechanism). Interesting electrochromic behavior is associated with this compound, a redox process occurring in the range 0.5-1.5V is observed by monitoring the absorption species changes (colored species) in function of the applied potential. Furthermore, electrochromic properties are also found in the corresponding ring closed isomers. Depending on the substituents on the thiophene ring and the perfluro or perhydro cyclopentene ring, open isomers can be obtained from oxidation (chemical or electrochemical) of the corresponding ring closed isomers via EC mechanism. These observations should be taken into account for the potential design of three-state conjugated systems and photoelectrical molecular switching.

The study of the reversible reduction/oxidation molecules containing multiple interacting or noninteracting redox centers has been carried out using the multipotential step technique Differential Staircase Voltammetry (DSCV). Methods to obtain the formal potentials and other characteristic parameters of the process are proposed for any interaction degree between the centers, when using spherical electrodes of any size (from planar to ultramicrospherical electrodes) and, in particular, the advantages of microelectrodes are discussed. The theoretical predictions have been tested with two experimental systems: ethyl viologen in acetonitrile and pyrazine in aqueous acid media, finding an excellent agreement between the theory and experiments. Therefore, the formal potentials for these systems have been evaluated using DSCV.

The electroreduction behaviour of cysteine was investigated using cyclic, square wave and differencial pulse voltammetric techniques at a platinum working electrode. The reduction of cysteine occurs at a potential of -0.36 V independent of pH. It is a reversible process, controlled mainly by diffusion and in the mechanism of reduction 1 electron per molecule is involved. Using the voltammetric techniques: Cyclic Voltammetry, Square Wave Voltammetry and Differencial Pulse Voltammetry, different parameters (pH, frequency, step potential, pulse amplitude, scan rate) were optimized in order to develop an electrochemical procedure for determination of cysteine in pharmaceutical products. The repeatability, reproducibility, precision and accuracy of the methods were studied. No electroactive interferences from the excipient were found in the pharmaceutical compounds.

The objective of this investigation has been the study of corrosion resistance of Fe2.5Co64.5Cr3Si15B15, Fe3Co67Cr3Si15B12 and Fe5Co70Si15B10 amorphous metallic alloys obtained by the melt spinning technique, used as based materials to create a new type of giant magnetoimpedance (GMI) biosensor. The corrosion behaviour has been studied in phosphate buffered saline (PBS) solutions at pH 7.3 and 37.5 ºC. The electrochemical characterization of alloys has been made by means of DC techniques, obtaining the corrosion potential, pitting and protection potentials, as well as the perfect and imperfect passive regions of alloys. In this work, the experimental results obtained are discussed in order to study their corrosion behaviour in artificial biological solutions and thus determine their possible use as GMI-biosensor prototype materials.

A Langmuir-Schaefer (LS) film of a thiomacrocyclic (ThM) compound was deposited on the surface of a glassy carbon electrode (GCE) rod, from a subphase containing Cu(II) ions. The voltammetric response of this modified GCE when the ThM was bonded to Cu2+, showed that the LS film moved the oxidation peaks of copper to more positive values. On the other hand, a LS film of the ThM compound was deposited on the surface of a GCE rod from a subphase of pure water. When the voltammetric response of the GCE-ThM electrode was studied in a Cu2+-SO42- solution, it was found that the modified electrode increases its sensitivity respect to Cu2+ at low bulk Cu2+ concentrations in solution, and a surface-complexation reaction is proposed to explain the effect of the LS film on the GCE surface.

A nano-porous anodized aluminium oxide layer was synthesized using two step anodization process, in which the dents of aluminium formed in the first anodization step (one day) worked as the initial sites of the pore growth in the second anodization step (one hour). The influence of the voltage on the structure of aluminium oxide has been studied. For different anodization temperatures, it has been found that higher temperatures decrease the pore size, although lower temperatures yielded higher symmetry. The electrolyte used has high influence on the ordered pore domain size. Sulfuric acid generated very small pore sizes, phosphoric acid produced a very big pore size and the largest domain size was observed for samples anodized in oxalic acid.

In this paper we present experimental measurements of the temperature dependence of the electrical conductivity, s, in four ionic liquid compounds (ILs) in both the liquid and solid states and at atmospheric pressure. The chemicals measured are composed by the 1-ethyl-3-methyl-imidazolium (EMIM+) cation, which has been combined with four different anions: Cl-, Br-, BF4- and ethyl sulfate (ES-). In the liquid state, the temperature dependence of s, for the four ILs follows the Vogel-Tamman-Fulcher (VTF) equation with high precision. Around the transition between the solid and liquid states, the electrical conductivity of the three ILs with lighter anions presents an hysteresis loop, that can be explained as the apparition of supercooled liquid. In contrast, the EMIM-ES presents a smooth transition, without any jump in the s, value or any hysteresis loop (probably because its melting point is reported to be below the minimum temperature measured by us). Finally, the jump in s, is not related with the glass transition because its temperature value is well below the minimum temperature measured, and at that glass transition temperature the s, value is below the resolution of our conductivity meter (2 nS/cm).

The voltammetric behaviour of glutamate at a gold electrode was studied by means of cyclic voltammetry. From 2.6 to 11.5 pH values, glutamate originated a single cathodic peak. Potential of the peak, Ep, was -0.8 V, and it was independent from pH. The reduction of glutamate was found irreversible and mainly controlled by diffusion. An analytical approach for analysis of glutamate by means of square wave voltammetry was developed. Plots of current versus concentration presented a linear behaviour from 2.5×10-4 to 2.7×10-3 M. The detection limit was 6.3×10-5 M. Interference from compounds co-existing with glutamate in food was negligible, and in favour of an application of the proposed method to the analysis of real samples. Determination of glutamate in pure solutions resulted in acceptable deviation from the stated concentration. Relative errors ranged -2.5 to +1.6 %.

A new alkaline electrolytic solution based on ammonia for copper electroplating has been investigated. Upon addition of alkaline metal hydroxide solution and ammonia followed by the immediate addition of a strong inorganic acid, a highly exothermic reaction takes place. At optimum solution condition ammonia remains in solution as NH2 and forms a complexing agent.

The corrosion behaviour of 1018, 410 and 800 steels exposed to synthetic wastewater has been studied using linear polarization resistance (LPR), cyclic potentiodynamic curves (CPC), electrochemical noise (EN), and electrochemical impedance spectroscopy (EIS) tests. The conditions were: biochemical oxygen demand (BOD) of 776 ppm, a chemical oxygen demand (COD) of 1293 ppm, pH = 8 and the cell temperature was 24 °C. From the CPC and EN results, no localized corrosion was found for the stainless steels. However, the reverse was true for the 1018 steel. The EIS results showed that different corrosion mechanism occurred for the carbon steel compared with the stainless steels. This shows that the corrosion mechanism strongly depends on the type of steel. Overall, the 1018 steel exhibited the highest corrosion rate, followed by the 410 alloy. The highest corrosion resistance was achieved by the 800 alloy. In addition, SEM analyses were carried out to explain the experimental findings.

Electroless nickel (EN) process is the controlled auto catalytic reduction of nickel ions using suitable reducing agent such as sodium hypophosphite on certain catalytic surfaces, which results in the production of sound, coherent coatings for a number of applications. Frequently the rate of deposition of EN with hypophosphite as reducing agent is below 20 microns per hour. Hence the deposition of the electroless nickel with hypophosphite solution in the presence of small concentration of thiourea and its derivatives as accelerators is investigated. The mechanism of accelerated deposition is studied by impedance measurements. The performance of various additives during the plating process is being evaluated by increased Constant Phase Element (CPE) and decreased charge transfer resistance values. A model is proposed for the adsorptive behavior of additives.

The effect of different concentrations of 1,3-dihydroxypropane (DHP) on the electrodeposition of copper powder from acidified copper sulphate solution has been studied at different temperatures and different speeds of rotation. Copper powder was electrodeposited onto rotating cylinder electrode (RCE) that made of pure copper. The inhibition percentage, P, in the electrodeposited copper powder was 0.00 – 92.91%, depending on the experimental variables. P was affected by temperature and mole fraction of DHP, while rotation did not show any influence whatsoever. Values of the activation energy of electrodeposition process, Ea, were found to be less than 28 k J mol-1 indicating diffusion controlled process. The overall mass transfer correlations under the present conditions have been obtained using the dimensional analysis method. The data were valid for 80 < Sh (Sh = Sherwood number) < 3970, 290 < Sc (Sc = Schmidt number) < 59284 and 271 < Re (Re = Reynolds number) < 52705 and the results agreed with the previous studies of mass transfer to rotating cylinders in turbulent flow regimes. Experimental determination of the solution critical velocity was obtained for blank and 20% (v/v) DHP solutions at 298 K. The effect of time, DHP content, temperature and the speed of rotation on the morphological changes of the electrodeposited copper powder as well as deposits composition and particle size have been studied. Various particle sizes ranged 60.5 – 203.4 nm were obtained, characterized by EDS and XRD and found to be pure copper with small amount of oxygen. Different topographs proved that the rate of copper electrodeposition increased by increasing time, temperature and the speed of rotation. In addition, they proved that the deposition rate decreased by adding DHP to the solution. Therefore, the results obtained by SEM supported those results obtained by electrochemical measurements. The morphological structure of deposited copper powder from 20% (v/v) DHP at 1000 rpm and 298 K was unique, rounded-crystalline aggregates with voids.

This work presents the results of corrosion inhibition of muntz alloy (63% Cu,  37% Zn) in 1.0 M HCl by water extracts of some naturally occurring plants. These are: outer brown skin of onion (A), onion bulb (B), the cloves of garlic bulb (C), orange peels (D), and henna leaves (E). The techniques of measurements for the determination of the amount of each zinc and copper dissolved from the alloy in the aggressive solution were: weight-loss, galvanostatic polarization, linear polarization and atomic absorption spectroscopy. From these measurements the values of surface coverage, , and inhibition efficiency were calculated. It was found that the investigated extracts have high inhibition efficiency on the corrosion of muntz alloy in 1.0 M HCl. Their inhibition efficiency decreases according to the order: C > D > E > B > A. These extracts behave as mixed inhibitors, i.e., they affect both the cathodic and anodic processes. The activation energy of corrosion was calculated in absence and in presence of extracts. It was found that the presence of extracts in 1.0 M HCl solutions increases the values of activation energy of corrosion in that order of their inhibition efficiency. The inhibiting effect of these extracts results from their adsorption on the electrode surface via the adsorption centers of the compounds present in the extracts. The adsorption of these extracts onto the surface of muntz follows Frumkin,s isotherm. The atomic absorption spectroscopic measurements showed that the presence of these extracts greatly inhibits the preferential dissolution of zinc from the alloy and the occurrence of simultaneous dissolution of both zinc and copper.

The effect of natural occurring extract of artemisia on the corrosion of steel in 0.5 M H2SO4 in the temperature range 298 – 353 K is studied by weight loss method, electrochemical polarisation and linear polarisation Rp measurements. Results obtained reveal that extract reduces the corrosion rate. The inhibition efficiency increases with the increase of artemisia content at 10 g/L to reach 95% and 99% at 298 and 353 K, respectively. Results obtained by gravimetric and electrochemical polarisation are in good agreement. Polarisation studies clearly reveal that the presence of the natural artemisia does not change the mechanism of the hydrogen evolution reaction and acts as a mixed type inhibitor. The inhibition efficiency increases with temperature. The adsorption of artemisia on the steel follows Langmuir adsorption isotherm.

In this work the characteristics of corrosion and inhibition of 1008 stainless steel in 0.5 mol.L-1 H2SO4 were investigated by physical and electrochemical methods. Organic compounds containing the heteroatom of N as benzotriazole (BTAH), tolytriazole (TTAH) and mixtures of BTAH + TTAH were used as corrosion inhibitors. The techniques used were: open circuit potential, anodic potentiodynamic polarization measurements, chronoamperometry, electrochemical impedance spectroscopy, weight loss and optical microscopy. The anodic polarization showed that the increase of both concentrations BTAH or TTAH diminishes the current density in all the potential anodic range studied; these results together with the weight loss measurements suggest that the inhibitors act by blocking the surface and that the interaction among them is only of destructive nature when inhibiting efficiency is approximated 100%, revealing that at low concentration both inhibitors act without affecting the mechanisms of the cathodic processes, but the anodic polarization curves in presence of the TTAH showed a strong inhibitive effect in all the potential range studied. However, both gravimetric and electrochemical results suggest a synergic effect for the inhibitive efficiency of the mixture 1x10-3 mol.L-1 BTAH + 1x10-3 mol.L-1 TTAH.