Hybrid Sol-Gel Coatings Produced from TEOS and γ-MPS

Hybrids sols from tetraetoxysilane (TEOS) and 3-(methacryloxypropyl) trimethoxysilane (-MPS) were prepared in acid medium for different TEOS/-MPS ratios and were modified by the addition of a colloidal silica suspension. The stability of the different sols was evaluated by viscosity measurements; the sols showed a Newtonian behaviour and the ageing effect was negligible even after two months from their preparation. Coatings were obtained by dipping at different withdrawal rates and heat-treated between 150 and 250°C. Transparent coatings with thickness higher than 4 m were reached for most of the studied compositions. The surface microhardness of coatings for each composition and thermal treatment was evaluated by the pencil hardness test. The thermal stability was followed by simultaneous thermogravimetric and differential thermal analysis (TGA-DTA) determining the limit temperatures at which the coatings can be treated without losingits hybrid character. A structural analysis was made by deconvolution of Fourier transformed infrared spectra (FTIR) of self-supported films observing the influence of the organic groups on the silica network.     

Direct multicomponent analysis of beer samples constituents using micellar electrokinetic capillary chromatography

A capillary electrophoretic method was developed using micellar electrokinetic capillary chromatography (MEKC) with diode-array detection to analyze simultaneously 26 beer constituents in a single procedure, including alcohols, iso-alpha-acids, amino acids, flavonoids, isoflavonoids, a vitamin, purine and pyrimidine bases. After filtration, sample components were separated with an uncoated capillary and a 25 mM sodium borate and 110 mM SDS buffer at pH 10.5. Analyses were run at 14 kV and 8 s of hydrodynamic injection with UV detection at 210 nm and 270 nm. The proposed method was successfully applied to the direct determination of beer constituents without any sample cleanup procedures.     

Observation of wall wettability under imposed flow by fluorescence depolarization: dependence on surface oxygen content and degree of polymer branching

This work was concerned with the dependence of the interfacial tension (Gamma(SL)) on surface degree of oxygen content and on polymer branching degree. The static Gamma(SL) was evaluated by contact angle (theta;(c)) and the dynamic Gamma(SL) by fluorescence depolarization of molecular probes seeded in induced flows of monoethylene glycol. The latter results were interpreted using statistical covariant analysis. Two different systems of flowing films were studied: free films flowing on the surfaces on which they impinge and films flowing inside 1-mm-thick microflow cells. The solid surfaces were polyethylene of low density, medium density, high density, and linear with low density, polypropylene, vinyl acetate co-polymer with oxygen content of 15% and 28%, borosilicate, and tin dioxide. Increase in oxygen content of the surface decreased both the static and the dynamic Gamma(SL), which demonstrated that the presence of oxygen atoms hindered wetting. Only the dynamical Gamma(SL) was sensitive to polymer branching, and it increased as branching degree decreased. This was attributed to the higher hydrogen-atom density at the surface, which favored temporary intermolecular bonds between the surface and the flowing liquid.     

Concerted mechanisms of the reactions of 2,4,6-trinitrophenyl methyl carbonate and 2,4,6-trinitrophenyl acetate with secondary alicyclic amines

The reactions of secondary alicyclic amines with 2,4,6-trinitrophenyl methyl carbonate (TNPMC) and 2,4,6-trinitrophenyl acetate (TNPA) are subjected to a kinetic study in aqueous solution, 25.0 degrees C, ionic strength 0.2 (KCl). The reactions are studied by following spectrophotometrically (360 nm) the release of the 2,4,6-trinitrophenoxide anion. Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) vs [amine] are linear, with the slope (kN) independent of pH. The Br?nsted-type plots (log k(N) vs pK(a) of the conjugate acid of the amines) are linear, with slopes beta = 0.41 and beta = 0.36 for the reactions of TNPA and TNPMC, respectively. The predicted breaks of the Br?nsted plots for stepwise mechanisms are pK(a)0 = 6.8 and 7.3, respectively. The lack of Br?nsted breaks for these reactions and the values of the Br?nsted slopes are consistent with concerted mechanisms. By comparison of the reactions under investigation among them and with similar aminolysis and pyridinolysis, the following conclusions can be drawn: (i) Secondary alicyclic amines react with TNPA and TNPMC by concerted mechanisms. (ii) TNPA is more reactive toward these amines than TNPMC due to the greater electron release of MeO from the latter substrate. (iii) The change of 2,4-dinitrophenoxy to 2,4,6-trinitrophenoxy in the zwitterionic tetrahedral intermediate (T+/-) formed in the reactions of the title amines with 2,4-dinitrophenyl acetate greatly destabilizes T+/-. (iv) Secondary alicyclic amines destabilize T+/- relative to pyridines. (v) The intermediate T+/- formed in the reactions of the title amines with S-(2,4,6-trinitrophenyl) acetate is greatly destabilized by substitution of S-(2,4,6-trinitrophenyl) by O-(2,4,6-trinitrophenyl) as the leaving group.     

Electrochemical synthesis of metal complexes of schiff bases: The crystal structure of bis{2-[(4-methylphenyl)iminomethyl]pyrrolato}copper(II)

The electrochemical oxidation of anodic nickel, copper, zinc or cadmium in acetonitrile solutions of Schiff bases (HL) derived from H-pyrrole-2-carbaldehyde and substituted anilines gives compounds of general formula ML₂. The crystal structure of bis{2-[(4-methylphenyl)iminomethyl]pyrrolato}copper(II) has been determined by X-ray diffraction. The compound crystallizes in the monoclinic space group P2₁/n with a = 9.356(2), b = 16.697(2), c = 14.145(2) Å and β = 108.47(2)°. The crystal structure consists of monomeric molecules in which the central CuN₄ unit has distorted square-planar geometry with a dihedral angle of 25.8(3)° between the coordination planes. The IR, ¹H NMR and UV-visible spectra of the complexes are discussed and related to the structure.     

Determination of glucose in alcoholic beverages by flow injection with two internally coupled injection valves and an enzyme reactor

Two new flow configurations are described for the determination of a component in samples for which the matrix provides varying blank values. They are applied to the determination of glucose in alcoholic beverages, by using immobilized hexokinase and glucose-6-phosphate dehydrogenase, and spectrophotometric determination of the NADPH formed.     

Nouveaux complexes π-allyliques cationiques du nickel(II)

The air stable complexes

are readily obtained in high yields by reaction of tetracarbonylnickel and allyloxytris(dimethylamino)phosphonium salts, R²CHCR¹CH₂OP(NMe₂)₃A^?, in acetonitrile, followed by addition of triphenylphosphine.     

The thermal behaviour of solid phases in the TeSeO2 system

A comparative study is made of the thermoanalytical stabilities of various tellurium and selenium mixed oxides (TeSeO₄, Te₂SeO₆, Te₃SeO₈, Te₄SeO₁₀ and Te₈SeO₁₈). The observed stabilities are then approximately correlated with data given by a kinetic analysis of the thermal decomposition processes in the non-isothermal regime obtained from TG and DSC by using two calculation methods (Freeman and Carroll and modified Freeman and Carroll).     

Kinetics and mechanisms of the pyridinolysis of phenyl and 4-nitrophenyl chlorothionoformates. Formation and hydrolysis of 1-(aryloxythiocarbonyl)pyridinium cations

The title reactions are subjected to a kinetic study in water, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). By following the reactions spectrophotometrically two consecutive reactions are observed: the first is formation of the corresponding thionocarbamates (1-(aryloxythiocarbonyl)pyridinium cations) and the second is their decomposition to the corresponding phenol and pyridine, and COS. Pseudo-first-order rate coefficients (k(obsd1) and k(obsd2), respectively) are found under excess amine. Plots of k(obsd1) vs free pyridine concentration at constant pH are linear, with the slope (k(N)) independent of pH. The Br?nsted-type plots (log k(N) vs pK(a) of the conjugate acids of the pyridines) are linear with slopes beta = 0.07 and 0.11 for the reactions of phenyl and 4-nitrophenyl chlorothionoformates, respectively. These Br?nsted slopes are in agreement with those found in other stepwise reactions of the same pyridines in water, where the formation of a tetrahedral intermediate is the rate-determining step. In contrast to the stepwise mechanism of the title reactions that for the reactions of the same substrates with phenols is concerted, which means that substitution of a pyridino moiety in a tetrahedral intermediate by a phenoxy group destabilizes the intermediate. The second reaction corresponds to the pyridine-catalyzed hydrolysis of the corresponding 1-(aryloxythiocarbonyl)pyridinium cation. Plots of k(obsd2) vs free pyridine concentration at constant pH are linear, with the slope (k(H)) independent of pH. The Br?nsted plots for k(H) are linear with slopes beta = 0.19 and 0.26 for the reactions of the phenyl and 4-nitrophenyl derivatives, respectively. These low values are explained by the fact that as pK(a) increases the effect of a better pyridine catalyst is compensated by a worse leaving pyridine from the corresponding thionocarbamate