Portugaliae
Electrochimica Acta

DNA and guanine are efficient fluorescence quenchers of the cationic conjugated polymer, poly {9,9-bis[N,N-(trimethylammonium)hexyl] fluorene-co-l,4-phenylene} (CCP). Studies with CCPs, of average chain length ~6, 12 and 100 repeat units, with single strand (ss) DNA, double strand (ds) DNA, and guanine, in 25/75 acetonitrile/water (v/v) mixtures result in Stern-Volmer quenching plots that show upward curvature. Initial Stern-Volmer constants, kSV, are in the range ≈ 3-20 x 107 M-1 which is much higher than possible by diffusional encounter quenching. Aggregation studies in acetonitrile/water mixtures show that aggregation is also an effective quencher of CCP fluorescence, and we note that both aggregation and quenching by DNA or guanine is accompanied by a reduction in solution absorbance at 380 nm. Comparison of the relationship between changes in absorbance and changes in emission intensity suggest that both solvent and chemical induced fluorescence quenching are due to aggregation. We interpret the correlated changes in absorption and emission, high quenching constants, and upward curving Stern-Volmer plots as evidence that the dominant mechanism for fluorescence quenching by DNA or guanine is via induced aggregation of the polymer. The upward curvature of Stern- Volmer plots and high kSV values for DNA and guanine are indicative of “aggregate energy migration quenching” in which CCP aggregates around a DNA or guanine molecule to form an aggregate complex in which excitation energy migrates between and along the polymer chains until it is quenched at an aggregate trap.

A comprehensive photophysical and spectroscopic Raman study guided by Density Functional Theory and Hartree-Fock calculations has been done in a series of fully oxidized oligothiophenes with variable chain length, up to four rings. A comparison with the properties of oligoenes and oligothiophenes is proposed.

The corrosion rates in the presence of nizoral (NZR), an antifungal drug, as mild steel corrosion inhibitor in 0.1 M H2SO4 was measured by the weight loss method in the temperature range from 30 – 50 ºC. Results obtained revealed that NZR acts as an inhibitor for mild steel in sulphuric acid solution. Inhibition efficiency increased with increase in concentration of the inhibitor but decreased with increase in temperature. The addition of KI increased the inhibition of NZR to a considerable extent. The experimental results suggest that the presence of iodide ions in the solution stabilized the adsorption of Nizoral molecules on the mild steel surface thereby improving the inhibition efficiency of Nizoral. The adsorption characteristics of the inhibitor were approximated by Langmuir adsorption isotherm and kinetic/thermodynamic adsorption model of El-Awady et al. Mechanism of physical adsorption is proposed from the activation and thermodynamic parameters calculated.

The use of stainless steel 304(SS) tanks to deposit thiourea in industry has been evaluated by studying the corrosion behaviour in 1% thiourea solution, before and after zinc plating. Weight loss method, 1% copper sulphate test, polarization study and AC impedance spectra have been used. The corrosion resistance of SS decreased when zinc was used for electrodeposition. Hence SS vessels electroplated with zinc should not be used to store thiourea solution, in industry.

The effect of iodide ion on the corrosion inhibition of aluminium in NaOH in the presence of Gum Arabic (GA) was studied using weight loss and hydrogen evolution techniques at 303 and 313 K. The results obtained showed that inhibition efficiency increased with increasing GA concentration. The inhibiting action of GA was considerably enhanced by the addition of KI. The adsorption of KI, GA alone and in combination with iodide ion was found to obey Temkin adsorption isotherm. The inhibiting effect of GA and (GA+KI) increased with increase in temperature of the corrosion medium, indicating chemical adsorption mechanism. Synergism parameter evaluated was found to be greater than unity for the different concentrations of GA, which shows that the enhanced inhibition of GA caused by the addition of iodide ion is due to synergistic effect. It could be deduced from the thermodynamic parameters obtained that adsorption of GA alone and in combination with KI onto the metal surface is spontaneous.

Inhibitive and adsorption properties of ethanol extract of Phyllanthus amarus for the corrosion of mild steel in H2SO4 were investigated using gravimetric, thermometric and gasometric methods. Ethanol extract of Phyllanthus amarus leaves is a good adsorption inhibitor for the corrosion of mild steel in H2SO4. Thermodynamic consideration indicates that the adsorption of the extract is exothermic and spontaneous. Also, the adsorption characteristic of the inhibitor is consistent with the assumptions of Langmuir adsorption isotherm. From the results and findings of the study, a physical adsorption mechanism is proposed for the adsorption of ethanol extract of Phyllanthus amarus on mild steel surface. The inhibition potentials of ethanol extract of Phyllanthus amarus leaves are enhanced by its phytochemical constituents.

The adsorption of [(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-(4-methoxy-phenyl)-methyl]-urea on iron in phosphoric media was studied by using electrochemical technique. The adsorption mechanism was investigated using adsorption isotherms. The experimental data fit localized adsorption models. The values for free energy of adsorption, ΔG0ads, were also calculated at each surface coverage, θ, of the studied compound by applying the mathematical model of Bockris – Swinkels adsorption isotherm. The variation of free energy of adsorption, ΔG0ads with surface coverage, θ, was interpreted in terms of deviation from ideal condition assumed in Langmuir model. A possible adsorption model of HPU2 molecules on to the metal surface was suggested.

1-(3-methyl-4H-1,4-benzothiazin-2-yl)ethanone (1,4-BT) was synthesised via a new mechanism. 1,4-BT was thereafter tested as corrosion inhibitor of mild steel in hydrochloric solution at 293 K by weight loss measurements and electrochemical techniques (potentiodynamic polarisation, polarisation resistance, impedance spectroscopy (EIS)). Inhibiting efficiency (E%) increased with concentration and reached highest values up to 98% even at low concentration with changing the mechanism of the corrosion process. Results obtained showed that the inhibitor studied was an efficient inhibitor, especially in cathodic domain. The temperature effect on the corrosion behaviour of mild steel in 1 M HCl with and without 1,4-BT at 5 10-3 M was studied in the temperature range from 293 to 333 K. E% remains the same even at high temperature. The adsorption free energy and activation parameters for the mild steel dissolution reaction, in the presence of 1,4-BT, were determined. 1,4-BT is adsorbed on the mild steel surface according to a Langmuir isotherm adsorption model.

The effect of some amidopoly ethylamine, with different numbers of ethylamine units, on the corrosion of zinc electrode in ZnCl2, NH4Cl and (ZnCl2 + NH4Cl) electrolytes has been studied using galvanostatic polarization measurements. The inhibition efficiency was found to increase with increasing concentration, number of ethylamine units per molecule and with decreasing the temperature. Inhibition is explained on the basis of adsorption of amidopoly ethylamine molecules on the zinc electrode surface through their ethylamine groups. The inhibitors are adsorbed on the zinc electrode surface according to Langmuir adsorption isotherm. Some thermodynamic parameters are calculated and explained for the tested systems from the data obtained at different temperatures.

The effect of some phenyl phthalimide derivatives on the corrosion of carbon steel in 0.5 M H2SO4 solution was studied using weight loss and galvanostatic polarization techniques. The percentage inhibition efficiency was found to increase with increasing concentration of inhibitor and with decreasing temperature. Inhibition was explained by adsorption of these compounds on the metal surface. The adsorption follows Freundlich adsorption isotherms. Some thermodynamic functions were computed and discussed.

The cyclic voltammetric study on the electro-oxidation of methanol on 30 % Pt and 20 % Pt +10 % Ru catalysts supported on Vulcan XC-72 was carried out in 0.5 M H2SO4. Kinetic parameters such as exchange current density, Tafel slope and heterogeneous rate constants were calculated. The 30 % Pt showed a higher apparent catalytic activity for the electro-oxidation of methanol as compared with 20 % Pt + 10 % Ru catalyst. The presence of Ru did not contribute significantly towards the apparent catalytic activity of a catalyst for the electro-oxidation of methanol but it may be useful to prevent the catalyst from CO poisoning. Thermodynamic parameters, ΔH*, ΔS*, and ΔG298* have been evaluated. The values of ΔH* and ΔG298* were positive showing that the electro-oxidation of methanol is an endothermic and non-spontaneous process.

The corrosion of mild steel in H2SO4 was studied using gravimetric, gasometric and IR methods. The results indicate that the rate of corrosion of mild steel in H2SO4 increases with increase in the concentration of the acid and that ethanol extracts of the seeds and leaves of Azadirachta indica inhibit the corrosion of mild steel in H2SO4. Inhibition efficiencies of the leaves and seeds extract range from 82.62 to 94.24% and from 60.55 to 84.78%, respectively. The inhibition potential of these extracts is attributed to the presence of tannin, saponin, glycoside, anthraquinone, flavonone and other phytochemicals in the extracts. IR study confirms that the inhibitors are adsorption inhibitors and the adsorption of the inhibitors on mild steel surface is exothermic, spontaneous and consistent with the assumptions of Flory-Huggins adsorption isotherm. Based on the values of the activation, free energy of adsorption and the variation of inhibition efficiency with temperature, a physical adsorption mechanism is proposed for the adsorption of ethanol extract of leaves and seeds of Azadirachta indica on the surface of mild steel.

New developments in the utilization of a laccase based polyphenolic biosensor were done in order to optimize its application in wines. Laccase was immobilized on polyether sulphone membranes and applied on an Universal Sensors (US) electrode base system containing a platinum working electrode and silver, silver chloride electrode. Caffeic acid was dissolved either in tartarate standard buffer solution at pH 3.5 or in 12% ethanol tartarate standard buffer solution at pH 3.5. The referred values of pH and ethanol percentage are the average values of table wines. Wines from different regions of Portugal, namely Grão Vasco (Dão), A P_Cabernet (Setúbal), C P_Castelo de Syrah (Setúbal), Ol (Alentejo), A P (Alentejo) and Quinta dos Poços (Douro), were studied. Grão Vasco was taken as a reference. It was observed that Grão Vasco (Dão) wine presented a caffeic acid equivalent concentration superior to the other studied wines, followed by a Douro wine: Quinta dos Poços (values of 17.00 and 12.18 mg/L, respectively, were found). It was also possible to verify that the two latter referred wines did not inhibit the biosensor, which could be reused after having been dipped in those wines. However, the wines from Setúbal and Alentejo, studied in this work, have a caffeic acid equivalent concentration lower than the other two mentioned before and seemed to inhibit the system, since the biosensor response decreased after being used in them. It was, thus, not possible to do several assays with the same membrane, when those wines were used. Reproducibilities were better for caffeic acid standard solutions, Grão Vasco and Quinta dos Poços wines, than for the other wines. In fact RSDs (Relative standard deviations) of the order of 1.1, 1.4 and 5.0% were obtained for caffeic acid standard solution, Grão Vasco and Quinta dos Poços wines, whereas 20% was found in the Setúbal and Alentejo studied wines. Two particular wines, A P_Cabernet and A P, were studied in more detail. Cyclic voltammograms and biosensor responses of these two wines indicate that A P has more caffeic acid equivalents than the A P_Cabernet.

Three polyvinylchloride (PVC) membrane sensors for the determination of moexipril hydrochloride were prepared and characterized. The sensors are based on the use of the ion association complexes of moexipril cation with either ammonium reineckate (sensor 1) or tetraphenyl borate (sensor 2) or phosphotungistic acid (sensor 3) counter anions as ion exchange sites in the PVC matrix. The performance characteristics of these sensors were evaluated according to IUPAC recommendations, which reveal a fast, stable and linear response for moexipril over the concentration range of 10-6 to 10-2 M for the three sensors with cationic slopes of 29.1, 30.1 and 30.2 mV per concentration decade for the three sensors, respectively. The direct potentiometric determination of moexipril hydrochloride using the proposed sensors gave recoveries % of 99.64 ± 0.34, 99.34 ± 0.56 and 99.68 ± 0.42 for the three sensors, respectively. The sensors were used for determination of moexipril hydrochloride in pharmaceutical formulations and in plasma. Validation of the method shows suitability of the proposed sensors for use in quality control assessment of moexipril hydrochloride. The obtained results were in a good agreement with those obtained using the reported spectrophotometric method.

Aluminium metal matrix composites (AMMCs) are now gaining their usage in aerospace and automotive industries. Because of their inherent nature, difficult to machine, they find very little applications in other sectors. Even non traditional processes like Laser Jet Machining and Electro Discharge Machining result in significant sub surface damage to the work. In this paper, an attempt is made to machine the A356/SiCp composite work material using Electro Chemical Machining process. Silicon carbide with an average particle size of 40 microns is tried in three different proportions, namely 5%, 10% and 15% by weight. Taguchi’s L27 orthogonal array is chosen to design the experiments and 54 trials are conducted to study the effect of various parameters like applied voltage, electrolyte concentration, feed rate and percentage reinforcement on maximizing the material removal rate. ANOVA results have shown that all the four selected factors are significant and from the S/N graph the optimum level of each factor is chosen. A mathematical model is also developed using the regression method. Confirmation experiment is conducted and found that the data obtained have close match with the data predicted using the model.

This paper reviews some recent canonical Monte Carlo simulations of the Au(210)/H2O interface using a DFT force field developed by us. New results are reported on the solvent contribution to the potential of mean force (PMF) for the phenol adsorption, from a dilute aqueous solution, onto the Au(210) surface. The Monte Carlo simulations show the common features normally observed in the simulation of water in contact with metallic surfaces, where the water molecules adsorb forming bilayers. The molecules adsorbed over the Top gold sites form hydrogen bonds between the first and second solvent layers. The PMF calculations indicate that the phenol molecule penetrates the solvent layers with the aromatic ring in a perpendicular configuration and the oxygen atom pointing to the surface. The PMF results also suggest the existence of hydrogen bonds between the phenol molecule and the first solvent layer of the water molecules adsorbed onto the Top sites.

The numerical method for the simulation of linear scan voltammetry on the rotating disk electrode is adjusted to the problem of irreversible redox reaction between the adsorbed catalyst and the dissolved reactant under transient conditions. The response consists of the wave and the maximum. The peak current depends on the scan rate in linear scan voltammetry, while the limiting current of the wave depends on the rate of rotation of the working electrode. The rate constant of catalytic reaction is determined from the kinetic current under steady-state conditions.

The corrosion inhibition of aluminium by Ipomoea involcrata (IP) in 1 M NaOH was rapidly assessed using the gasometric technique at 30 and 60 oC . Results obtained showed that Ipomoea involcrata acts as an inhibitor for aluminium corrosion in 1 M NaOH. Inhibition efficiency increased with increase in concentration of IP up to 55.0 % at 50 v/v% and decreased with increase in temperature. The adsorption of IP was in accord with Langmuir adsorption isotherm at the studied temperatures. A mechanism of physical adsorption is proposed from the calculated values of Ea, Qads and ΔGads.