Portugaliae
Electrochimica Acta

In the present study, vicine was extracted from sunflower seeds, and has been evaluated as a corrosion inhibitor for carbon steel alloy, at different concentrations and temperatures. Vicine showed maximum inhibiting efficiency of 97%, at 25 ºC. The inhibitor’s efficiency was reduced when the temperature increased. Kinetic parameters (Ea, ΔH*, ΔG* and ΔS*) were calculated. The corrosion reaction was suppressed by vicine, because the energy barrier of the corrosion reaction was increased, and it became non-spontaneous, by an endothermic process. Furthermore, ΔHads, ΔGads and ΔSads were also calculated, and showed that the inhibitor was physically adsorbed by a spontaneous and exothermic process. The corrosion was inhibited by simple blocking the reaction sites. The adsorption process obeyed the Langmuir adsorption isotherm. The theoretical and experimental studies depicted that the inhibitor worked by a mixture of physical-chemical adsorption modes.

Three substituted imidazoles – 2-(3-methoxyphenyl)-4,5-diphenyl-1H-imidazole (IM-1), 2,4,5-triphenyl-1H-imidazole (IM-2), and 2-(3-nitrophenyl)-4,5-diphenyl-1H-imidazole (IM-3) – were synthesized, and their inhibiting action was tested using mass loss, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) methods. The results show that methoxy substituted imidazole performed better as a corrosion inhibitor than NO2 substituted imidazole. These findings were corroborated by density functional theory (DFT) and molecular dynamics (MD) simulations methods. IM-1 was found to exhibit maximum IE of 97.5%, at 100 mgL-1, among the studied IMs. PDP study revealed that all the three IMs inhibitors predominantly acted as cathodic inhibitors, and the adsorption study showed that they followed Langmuir adsorption isotherm. The formation of an inhibitor film on the MS surface was confirmed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). MD study revealed that binding energy and interaction energy of the inhibitors molecules on the MS surface followed the order IM-1> IM-2> IM-3. All the three IMs molecules adsorbed onto the mild steel surface by flat orientation. DFT and MD study results corroborated the experimental results.

The inhibiting effect of benzaldehydethiosemicarbazone (BTSC) on the mild steel alloy corrosion in a 1 M sulfuric acidic solution was potentiostatically investigated at four temperatures, in the range of 298.15 to 328.15 K. Three BTSC concentrations, ranging from 100 to 300 mg/L, were tested. Mild steel corrosion feasibility decreases with increasing inhibitor concentrations, and also with the rise in temperature. A protection efficiency of 96% was obtained at 300 mg/L, and 328.15 K. Potentiostatic polarization studies showed that BTSC acted as a mixed type inhibitor. The main kinetic effect of the BTSC inhibitor added to the sulfuric acid solution was to considerably enhance activation energy values, pre-exponential factor and activation entropy of the alloy corrosion. This was because BTSC shifted the corrosion reaction on the mild steel surface to reaction sites where energy was relatively higher than that on which the corrosion occurred in the inhibitor absence. The inhibitor adsorption followed the Langmuir adsorption isotherm. The activation thermodynamic functions (Ea, Kads, ΔGads., ∆Hads. and ∆Sads) were evaluated. The obtained activated parameters revealed that BTSC adsorption took place through chemisorption. Scanning electron microscopy (SEM) technique was used to provide insight into the formation of a protective film on the alloy surface. To provide a relationship between the BTSC’s molecular structure and its corrosion inhibition capability, quantum chemical studies were achieved using density functional theory (DFT) at the B3LYP/6-311G level.

In the present work, we report the development of a polyphenol oxidase (PPO) sensor for the selective and sensitive detection of catechol, using biosynthesized silver nanoparticles (AgNPs). For the sensor development, AgNPs biosynthesized using the leaf extract of Convolvulus pluricaulis, were successfully deposited onto a polypyrrole modified graphite electrode (Gr/PPy). The resulting Gr/PPy/AgNPs electrode was further used as a matrix for the immobilization of the PPO enzyme extracted from Manilkara Zapota (sapota). The morphological characteristics of the developed Gr/PPy/AgNPs/PPO sensor were studied using a scanning electron microscope (SEM). The sensor performance was evaluated and optimized using cyclic voltammetry (CV), differential pulse voltammetry (DPV), chrono amperometry (CA) and electrochemical impedance spectroscopy (EIS) methods. Under neutral pH conditions, the developed sensor showed excellent electro catalytic activity towards catechol detection. The sensor performed well in the concentration range of 0.001 to 0.015 mM, with the detection limit of 0.47 µM, and sensitivity of 13.66 µM-1cm-2. The biosensor response was found to be uninfluenced by some common interferents, and it also showed good storage stability and repeatability. The practical applicability of the PPO biosensor for catechol detection in real samples was assessed by examining the catechol content in a green tea sample. The sensor could detect catechol content in a green tea sample, to an accuracy of about 98%, thereby establishing its efficiency in real sample analysis.

The electrochemical behaviour of 4-tertbutylcyclohexanone semicarbazone (TBCHSC) and its Co(II) complex was investigated by a glassy carbon electrode, cyclic voltammetry technique. Semicarbazone synthesized from the condensation of 4-tertbutylcyclohexanone and semicarbazide hydrochloride (1:1 molar ratio) was further treated with cobalt(II) acetate (2:1 molar ratio), to form the Co(II)-semicarbazone complex. The semicarbazone was studied in buffer solutions (Britton-Robinson universal buffer and phosphate buffer), with a pH in the range of 3 to 11. The reduction process was found to be irreversible and diffusion controlled, for both semicarbazone and its Co(II) complex. The effects of change on the sweep rate, concentration, pH and solvent were evaluated. The semicarbazone reduction mechanism was suggested on the basis of the obtained results. Kinetic parameters, such as charge transfer coefficient (αn), diffusion coefficient (D01/2), and rate constant (k°f,h), were calculated from cyclic voltammetric measurements. Semicarbazone and its Co(II) complex were tested against bacterial and fungal species. The metal complex had higher activity than the free ligand.