Application of the superposition principle to the study of CEC, CE, EC and catalytic mechanisms in cyclic chronopotentiometry. Part III

In this paper we have obtained the general analytical equations corresponding to the response of complicated charge transfer processes with coupled homogeneous chemical reactions in cyclic chronopotentiometry by applying the superposition principle. The analysis of different cycles of this response can be used to obtain accurate and contrasted values of kinetic parameters in each complex reaction scheme analysed. These parameters are of great interest for the chemist and the biochemist. These equations are also applicable to cyclic derivative chronopotentiometry which is a very useful method since the response is obtained with peaks in a similar way to cyclic voltammetry but its mathematical treatment is simpler. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermodynamics of micellization of alkyldimethylbenzylammonium chlorides in aqueous solutions

Specific conductivities of alkyldimethylbenzylammonium chlorides (alkyl=decyl-, dodecyl-, tetradecyl-, and hexadecyl-) in aqueous solutions were measured as a function of molality and temperature. Critical micelle molalities (cmc) and degrees of ionization of the micelles, beta, were estimated from the dependence of the specific conductivity on molality. It was found that temperature dependence of cmc is U-shaped with a minimum shifting toward higher temperatures with a decrease in the chain length of the alkyl group. The temperature dependence of ln xcmc (where xcmc is the cmc in mole fraction units) was fitted to the equation of Muller, which we modified by taking into account the temperature dependencies both of beta and of change in heat capacity upon micellization. From the fitting parameters, Gibbs free energies, enthalpies, and entropies of micellization as a function of temperature were estimated.     

Mechanism of the oxidation of sulfides by dioxiranes: conformational mobility and transannular interaction in the oxidation of thianthrene 5-oxide

The detailed study of the oxidation of thianthrene 5-oxide (1) with methyl(trifluoromethyl)dioxirane (5b) in different solvents and in the presence of (18)O isotopic tracers is reported. Thianthrene 5-oxide (1) is a flexible molecule in solution, and this property allows for transannular interaction of the sulfoxide group with the expected zwitterionic 7 and hypervalent 10-S-4 sulfurane 9 intermediates formed in the oxidation and biases the course of the reaction toward the monooxygenation pathway.     

Effect of glass fiber surface treatments on mechanical strength of epoxy based composite materials

Sizing glass fibers with silane coupling agents enhances the adhesion and the durability of the fiber/polymer matrix interface in composite materials. There are several tests to determine the interfacial strength between a fiber and resin, but all of them present difficulties in interpreting the results and/or sample preparation. In this study, we observed the influence of different aminosilanes fiber coatings on the resistance of epoxy-based composite materials using a very easy fractographic test. In addition, we tried a new fluorescence method to get information on a molecular level precisely at the interface. Strength was taken into account from two standpoints: (i) mechanical strength and (ii) the resistance to hydrolysis of the interface in oriented glass-reinforced epoxy-based composites. Three silanes: gamma-aminopropyltriethoxysilane, gamma-Aminopropylmethyldiethoxysilane, and gamma-Aminopropyldimethylethoxysilane were used to obtain different molecular structures at the interface. It was concluded that: (i) the more accessible amine groups are, the higher the interface rigidity is; (ii) an interpenetrating network mechanism seems to be the most important for adhesion and therefore to the interfacial strength; and (iii) the higher the degree of crosslinking in the silane coupling layer is, the higher the hydrolytic damage rate is.     

Determination of short-chain free fatty acids in lipolyzed milk fat by capillary electrophoresis

The objective of this study was to monitor the release of short-chain free fatty acids (FFA) from milk fat during hydrolysis with lipase using capillary electrophoresis. Sample and run buffer allowed FFA to be maintained in solution by using cyclodextrin and methanol. Indirect UV detection at 270 nm was used, employing p-anisate as a chromophore. Calibration curves constructed for each individual FFA followed linear relationships with highly significant (p < 0.01) correlation coefficients. Electrophoretic FFA profiles of fresh milk fat and lipolyzed milk fat showed marked qualitative and quantitative differences. Butanoic acid (C4) was found in a concentration of 64 ppm, while hexanoic (C6) and octanoic (C8) acids were found in concentrations of 3.8 ppm in fresh milk fat. After a 60-min hydrolysis with commercial lipase, FFA released from milk fat consisted mainly of high concentrations (ppm) of butanoic (C4) (900), followed by hexanoic (C6) (427), octanoic (C8) (282), decanoic (C10) (92), pentanoic (C5) (47), and dodecanoic (C12) (37.5) acids. Ratios of FFA that were associated with flavor balance were calculated. The application of CE for lipolysis monitoring in milk fat offers a simple and fast method for the determination of FFA. Quantitative data can be obtained in 20 min, including sample preparation. The lengthy and laborious steps required in traditional chromatographic techniques, such as lipid extraction, FFA isolation, and derivatization, were not required in this CE method. The implementation of CE for milk fat lipolysis monitoring may be a useful quality control tool for dairy flavor development and production.     

Effect of the Solvent on the Water Properties of Water/Oil Microemulsions

Porous poly(styrene-divinylbenzene) (PS-DVB) particles were modified by adsorption of hydrophobically-modified dextrans, to provide chromatographic matrices for biomolecule chromatography. The dextran distribution and the pore characteristics of various coated PS-DVB beads were examined using nitrogen adsorption–desorption, mercury intrusion, and size exclusion chromatography. It was found that the adsorption of dextran does not result in homogeneous layers but rather in inhomogeneous ones. At high dextran loading and high content of hydrophobic groups in the adsorbed polymer, most of the pores of the macroporous rigid material are filled with a soft and porous dextran network being stabilized by hydrophobic interactions. According to chromatographic experiments, most of the surface was nevertheless expected to be covered at least by a thin and dense protecting layer since proteins—even those that are small enough to penetrate the dextran network—cannot interact nonspecifically with the internal pore surface. At low content of hydrophobic groups, dextran deposits preferentially as a thicker and more diffuse layer. However, the thickness of the coating is expected to be irregular and probably contributes to an increase in the roughness of the polystyrene surface.     

Crystal structure and vibrational spectra of polyamine-copper(II) complexes

The structures of [Cu(en)(H₂O)₂]SO₄ (I), [Cu(en)₂](NO₃)₂ (II) and [Cu(trien)I]I (III) have been determined by single crystal X-ray diffraction. ComplexI is monoclinic, space group C2/c, with unit cell parametera=7.232(1),b=11.725(2),c=9.768(1), =105.50(1)°, andZ=4. ComplexII is also monoclinic, space group P2₁/a, witha=7.978(2),b=9.982(4),c=8.218(3), =111.11(2)°, andZ=2. ComplexIII is orthorhombic, space group P2₁2₁2₁, witha=8.098(1),b=11.902(1),c=13.682(2), andZ=4. The structures were solved by direct methods and refined by full-matrix least-squares to finalR values of 0.031, 0.043 and 0.036 for complexesI, II, andIII, respectively. ComplexesI andII show an octahedral coordination geometry. ComplexIII shows a square pyramidal coordination geometry. ComplexI forms infinite monodimensional chains where the SO ₄ ^2– ions acts as a bridge between two neighboringen molecules. The vibrational spectra of these complexes agree well with their crystal structures. Structure and stability of seven other related Cu(II) complexes of (trien), (dien)₂, (en)₂ and (en) are inferred in this study.     

Magneto-structural characterization of Cu4(prz)2(L)2Cl4 ·CH3CH2OH (LH=1,1-di-2-pyridyl-1-ethoxi methanol,przH=pyrazole)

The title compound, Cu₂C₁₇H₁₆O_2.5Cl₂, is a copper(II) complex, in which each dimeric unit has two copper atoms bridged by a pyrazolato ligand, a di(2-pyridyl)]methane ligand, and an oxygen atom from the modified methylene group that links the two pyridine rings (O-C-O-CH₂CH₃). The complex crystallizes in the monoclinic groupP2₁/n,a=7.7330(1),b=19.589(6),c=13.123(2)Å,=95.020(0)°,V=1980.3(7)ų,Z=4,F _w=514.3,D _Do =1.725 g cm^–3(MoK)=0.71073 Å,F(000)=1032,=2.44 mm^–1, R=0.044 for 2209 observed reflections,Rw=0.047,s=0.086. Each dimer is linked to the next unit by long chlorine bonds (Cu(2)-Cl(1)=2.846(2)Å). These tetrameric units form zigzagged chains through copperchlorine bonds of 3.380(2)Å. The copper(II) complex presents antiferromagnetic behavior withT _m=161 K.     

Packing analysis of racket-like molecules: C6H5MX (M=metal,X=Cl,Br, I). Crystal structure determination of microcrystalline C6H5HgCl by XRD

Relationships between molecular shape and its crystal packing are studied. The XRD crystal structure of macrocrystals and microcrystals of C₆H₅HgX, and their VDW packing analysis, suggest two isoenergetic packing models for racket-like molecules. One of them favoring the crystal growth, and the other more frequent forming microcrystalline aggregates.     

Antibacterial Activity of Biosynthesized Selenium Nanoparticles Using Extracts of Calendula officinalis against Potentially Clinical Bacterial Strains

The use of selenium nanoparticles (SeNPs) in the biomedical area has been increasing as an alternative to the growing bacterial resistance to antibiotics. In this research, SeNPs were synthesized by green synthesis using ascorbic acid (AsAc) as a reducing agent and methanolic extract of Calendula officinalis L. flowers as a stabilizer. Characterization of SeNPs was performed by UV-vis spectrophotometry, infrared spectrophotometry (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) techniques. SeNPs of 40–60 nm and spherical morphologies were obtained. The antibacterial activity of marigold extracts and fractions was evaluated by disk diffusion methodology. The evaluation of SeNPs at different incubation times was performed through the colony-forming unit (CFU) count, in both cases against Serratia marcescens, Enterobacter cloacae, and Alcaligenes faecalis bacteria. Partial antibacterial activity was observed with methanolic extracts of marigold leaves and flowers and total inhibition with SeNPs from 2 h for S. marcescens, 1 h for E. cloacae, and 30 min for A. faecalis. In addition, SeNPs were found to exhibit antioxidant activity. The results indicate that SeNPs present a potentiated effect of both antimicrobial and antioxidant activity compared to the individual use of marigold extracts or sodium selenite (Na₂SeO₃). Their application emerges as an alternative for the control of clinical pathogens.