2010

Resumo: The kinetics of zinc electrodeposition from acid sulphate solution on a platinum electrode was investigated by means of stationary polarization curves, interfacial pH measurement and electrochemical impedance spectroscopy. The effect of pH, namely pH 2, 3 and 4, was analyzed. A significant dependence of Zn electrodeposition with solution pH was verified. The results obtained cannot be predicted by the available models for Zn electrodeposition. A reaction model is then proposed based on the predominant steps as a function of the potential and the electrode surface nature.

Resumo: In this work are described the crystal structures of two alkaline earth salts as well the vibrational spectra of three alkaline earth salts of 3,5-bis-(dicyanomethylene)cyclopentane-1,2,4-trionate (known as croconate violet – CV2−). The compounds obtained are a mixed salt of potassium and magnesium [MgK2(CV)2], beyond of calcium and barium salts (CaCV and BaCV, respectively). The structures of MgK2(CV)2 and BaCV have been confirmed by single crystal X-ray diffraction and spectroscopic analysis. These compounds crystallize in monoclinic P21/n space group, showing four and five water molecules for MgK2(CV)2 and BaCV, respectively. The dianionic units [MgK2(CV)2(H2O)4] adopt a slight distorted octahedral geometry in which the metallic center is coordinated by six oxygen atoms (four from CV2− and two from water molecules). These units are connected through intermolecular hydrogen bonding giving rise to a supramolecular array in a two-dimensional arrangement (2D). The potassium ion shows monodentate coordination, whereas the magnesium ion presents a chelate coordination in the MgK2(CV)2 salt and for the BaCV, the coordination geometry of metal site is monodentate and chelate coordination; it is straightforward to mention that the barium compound is the only pure divalent salt obtained until now in the literature. In the vibrational spectra, the most important vibrational markers for croconate violet ion are the ν(Ctriple bondN) and ν(Cdouble bondO) modes, which present different behavior due to the modification of chemical. In general way, the bands assigned to ν(Ctriple bondN) and ν(Cdouble bondO) in MgK2(CV)2, CaCV and BaCV present small wavenumber shifts, which can be associated to the different interionic interactions between anion and cations.

Resumo: The anticorrosive performance of medium-long (54–59%) alkyd paints modified with linseed and soy oils was compared by accelerated tests (Prohesion Cycle) and natural exposition in marine and industrial atmospheres. Differences on the protection mechanism of anticorrosive pigments due to substitution of linseed oil by soy oil were investigated by polarization curves and electrochemical impedance spectroscopy (EIS). Complementary tests such as water vapor and ions permeability in freestanding films were also performed. Results suggested that the type of oil influenced the barrier properties of the paint pigmented with zinc phosphate. The same tendency was verified by resistance values obtained from impedance diagrams. Polarization curves suggest that the action of the pigments in the alkyd paintings is practically the same for both oils. The substitution of linseed oil by soy oil did not impair the anticorrosive performance of alkyd paints and from the economic point of view this substitution could be very interesting.

Resumo: The osmotic compressibility factor of several polymer solutions was correlated with the polymer concentration and temperature through different cubic equations of state, with particular focus on the McMillan–Mayer solution theory. The equations of state employed were those of van der Waals, Redlich–Kwong, Peng–Robinson, and Soave—Redlich–Kwong. All of these equations present two parameters that take into account the attractive and repulsive interactions between the solute molecules in a given solvent, with these parameters being dependent on the nature of the system (solute and solvent) and on the temperature. As the attractive and repulsive interactions are well defined in the parameters of the cubic equations of state, the theta temperature for each polymer system studied may be calculated by a simple and efficient procedure. For the purposes of comparison, the virial equation truncated at the third term was also included in this study. It was confirmed that the attractive parameter has a linear dependence on the temperature, while the repulsive parameter varies according to a quadratic profile. Accordingly, the model to be minimized presents five adjustable parameters that depend only on the nature of the polymer system. The agreement between the experimental and calculated values is within the experimental error.