Structural and phenomenological characterization of the thermoreversible sol-gel transition of a zirconyl aqueous precursor modified by sulfuric acid

The thermoreversible sol-gel transition is well-known in biological and organic polymeric systems but has not been reported for inorganic systems. In this paper we put in evidence a thermoreversible sol-gel transition for zirconyl chloride aqueous solutions modified by sulfuric acid in the ratio 3:1 Zr:SO4. The synthesis conditions are detailed and a variety of experimental techniques (turbidimetry, dynamic rheology, and EXAFS) have been employed for investigating the thermal reversibility and the chemical structure of this new material. Turbidimetric measurements performed for solutions containing different concentrations of precursor have evidenced that the sol-gel transformation temperature increases from 50 to 80 degrees C as the concentration of zirconyl chloride decreases from 0.22 to 0.018 mol L-1. A more detailed study has been done for the sample with [Zr] = 0.156 mol L-1, in which the sol-gel-sol transformation has been repeated several times by a cyclic variation of the temperature. The mechanical properties of this sample, evaluated by measuring the storage and the loss moduli, show a change from liquid like to viscoelastic to elastic behavior during the sol-gel transition and vice versa during the gel-sol one. In situ EXAFS measurements performed at the Zr K-edge show that no change of the local order around Zr occurs during the sol-gel-sol transition, in agreement with the concept of physical gel formation. We have proposed for the structure of the precursor an inner core made of hydroxyl and oxo groups bridging together zirconium atoms surrounded in surface by complexing sulfate ligands, the sulfate groups act as a protective layer, playing a key role in the linking propagation among primary particles during sol-gel-sol transition.

TG studies of a composite solid rocket propellant based on HTPB-binder

Thermal decomposition kinetics of solid rocket propellants based on hydroxyl-terminated polybutadiene-HTPB binder was studied by applying the Arrhenius and Flynn-Wall-Ozawa's methods. The thermal decomposition data of the propellant samples were analyzed by thermogravimetric analysis (TG/DTG) at different heating rates in the temperature range of 300-1200 K. TG curves showed that the thermal degradation occurred in three main stages regardless of the plasticizer (DOA) raw material, the partial HTPB/IPDI binder and the total ammonium perchlorate decompositions. The kinetic parameters E-a (activation energy) and A (pre-exponential factor) and the compensation parameter (S-p) were determined. The apparent activation energies obtained from different methods showed a very good agreement.

Role of the surface state and structural feature in the thermoreversible sol-gel transition of a zirconyl aqueous precursor modified by sulfuric acid

The sols produced by admixture of ZrOCl2 acidified solutions to hot H2SO4 aqueous solutions were studied to clarify the effects of Cl- and SO42- ions on the kinetic stability of nanoparticles and to obtain some new evidence concerning the mechanism of a thermoreversible sol-gel transition observed in this system. The study of suspensions prepared with different molar ratios R-S = [Zr]/[SO42-] and R-Cl = [Zr]/[Cl-] revealed domains of composition of formation of thermoreversible gels, thermostable sols, and powder precipitation. The effects of R-S and R-Cl on the structural features of nanoparticles and on the particle solution interface were systematically analyzed for samples of thermoreversible and thermostable sol domains. Small-angle X-ray scattering measurements revealed the presence of small fractal aggregates in all samples of thermoreversible domains, while compact packing aggregates of primary particles are present in the thermostable sol. Extended X-ray absorption fine structure and elemental chemical analysis revealed that irrespective of the nominal value of R-S and R-Cl all studied samples of the thermoreversible domain are constituted by a well-defined compound possessing an inner core made of hydroxyl and oxo groups bridging together zirconium atoms surrounded on the surface by complexing sulfate ligands. zeta potentials of powders extracted by freeze-drying from the thermoreversible gel revealed a point of surface charge inversion attributed to the specific adsorption of SO42- ion. Thermoreversible gel formation is rationalized by considering the effect of the specific adsorption on the electrical double-layer repulsion together with the temperature dependency of the physical chemical properties of ions in solution.

Synthesis and characterization of solid molybdenum(VI) complexes with glycolic, mandelic and tartaric acids. Photochemistry behaviour of the glycolate molybdenum complex

The complexes (NH4)(2)[ MoO2( C2H2O3)(2)]center dot H2O, (NH4)(2)[MoO2(C8H6O3)(2)] and (NH4)(2) [MoO3(C4H4O6)]center dot H2O were prepared by reaction of MoO3 with glycolic, mandelic and tartaric acids, respectively. The complexes were characterized by elemental and thermal analysis, IR spectroscopy and X- ray diffraction. Crystals of the glycolate and tartarate complexes are orthorhombic and the mandelate complex is monoclinic. Elemental and thermal analysis data showed that the glycolate and tartarate complexes are monohydrated. Hydration water is not present in the structure of the mandelate complex. IR spectra showed COO- is involved in coordination as well as the oxygen atom of the deprotonated hydroxyl group of the alpha-carbon. The glycolate molybdenum complexes with general formula M-2[MoO2(C2H2O3)(2)]center dot nH(2)O, where M is an alkali metal and n=1 or 1/2, were also prepared and characterized. Aqueous solutions of the glycolate complex become blue and mandelate and tartarate complexes change to yellow or brown when exposed to UV- radiation.

Effects of the magnesium and chloride ions and shikimate on the structure of shikimate kinase from Mycobacterium tuberculosis

Bacteria, fungi and plants can convert carbohydrate and phosphoenolpyruvate into chorismate, which is the precursor of various aromatic compounds. The seven enzymes of the shikimate pathway are responsible for this conversion. Shikimate kinase (SK) is the fifth enzyme in this pathway and converts shikimate to shikimate-3-phosphate. In this work, the conformational changes that occur on binding of shikimate, magnesium and chloride ions to SK from Mycobacterium tuberculosis (MtSK) are described. It was observed that both ions and shikimate influence the conformation of residues of the active site of MtSK. Magnesium influences the conformation of the shikimate hydroxyl groups and the position of the side chains of some of the residues of the active site. Chloride seems to influence the affinity of ADP and its position in the active site and the opening length of the LID domain. Shikimate binding causes a closing of the LID domain and also seems to influence the crystallographic packing of SK. The results shown here could be useful for understanding the catalytic mechanism of SK and the role of ions in the activity of this protein.

Preparation and characterization of erbium and ytterbium co-doped sol-gel SiO2 : HfO2 films for planar waveguides

This work reports on the preparation of erbium and ytterbium co-doped SiO2:HfO2 single mode planar waveguides using the sol-gel method. Silica nanoparticles were prepared from tetraethylorthosilicate in basic media and the films were characterized by transmission electron microscopy, scanning electron microscopy, mechanical profilometry, M-lines spectroscopy based on prism coupling technique, X-ray diffractometry, infrared spectroscopy and photoluminescence spectroscopy. The film thicknesses and the refractive indexes were adjusted in order to satisfy a future efficient coupling to single mode optical fiber. Films suitable for both weak and strong light confinement were prepared varying hafnia concentration into the silica matrix. The lifetime values of erbium I-4(13/2) state were measured in order to investigate the influence of clustering and hydroxyl groups on the fluorescence quantum efficiency of the I-4(13/2) level, responsible for the emission at 1.55 mu m attributed to the I-4(13/2) -> I-4(15/2) transition. The high lifetime values suggest the absence of erbium clusters and the elimination of hydroxyl groups by rapid thermal process. (c) 2007 Elsevier B.V. All rights reserved.

The electrochemical cleavage of the nitrobenzoyl group from butyl nitrobenzoates in N,N-dimethylformamide

The applicability of the nitrobenzoyl group [NO2C6H4CO-] to protecting the functional hydroxyl group was investigated through study of the electrochemical behaviour of the butyl 4-, 3- and 2-nitrobenzoate compounds. These isomers are reduced in two cathodic steps. The first, at potentials of ca. -0.9 V vs. SCE, is attributed to the formation of rather stable anion radicals, involving one-electron transfer. The second, at potentials of ca. -1.7 V vs. SCE, occurs with a two-electron transfer in an ECE process, in which the dianion produced undergoes scission of the C-O bond giving n-butanoate ions with high yields (similar to 80%)

Density improvement of the sol-gel dip-coated SnO2 films by chemical surface modification

We compare the effect of organic (Tiron (R)) and inorganic (Mn(11)) additives on the low temperature (< 600 degrees C) densification of the sol-gel dip-coated SnO2 films. The structural and compositional properties of the samples were investigated by X-ray reflectometry (XRR), X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The results suggest that the replacement of hydroxyl groups at the particle surface by Tiron (R) reduces the level of agglomeration of the sol, increasing the particles packing and the apparent density of the coatings. Undoped and Mn-doped films drawn from a Tiron (R) containing suspension show after firing at 500 degrees C a porosity reduction of 12 and 8.6%, respectively. The porosity decrease is less pronounced (4.3%) for the film without additives. Both XAS and XPS data show the presence of trivalent manganese. The formation of a non-homogeneous solid solution characterised by the presence of Mn(111) replacing tin atom near to the crystallite surface was evidenced by XAS. Additionally, XPS results reveal the presence of metallic Sn at the surface of films containing Tirono. (c) 2005 Elsevier Ltd. All rights reserved.

Thermogravimetric and UV-vis spectroscopic studies of chromium redox reactions in rutile pigments

In this study undoped and Cr, Sb or Mo doped TiO(2) were synthesized by polymeric precursor method and characterized by X-ray diffraction, UV-VIS spectroscopy, infrared spectroscopy and thermogravimetry (TG). The TG curves showed a continuous mass loss assigned to the hydroxyl elimination and Cr(6+) reduction. Doped TiO(2) samples showed a higher mass loss assigned to water and gas elimination at lower temperatures. In these doped materials a decrease in the anatase-rutile phase transition temperature was observed. After calcination at 1,000 A degrees C, rutile was obtained as a single phase material without the presence of Cr(6+).

A joint computational and experimental study of a novel dioxomolybdenum(VI) complex bearing chiral N,N-dimethyllactamide ligand

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Theoretical study on the reaction mechanism of VO2+ with propyne in gas phase

Possible molecular mechanisms of the gas-phase ion/molecule reaction of VO2+ in its lowest singlet and triplet states ((1)A(1)/(3)A '') with propyne have been investigated theoretically by density functional theory (DFT) methods. The geometries, energetic values, and bonding features of all stationary and intersystem crossing points involved in the five different reaction pathways (paths 1-5), in both high-spin (triplet) and low-spin (singlet) surfaces, are reported and analyzed. The oxidation reaction starts by a hydrogen transfer from propyne molecule to the vanadyl complex, followed by oxygen migration to the hydrocarbon moiety. A hydride transfer process to the vanadium atom opens four different reaction courses, paths 1-4, while path 5 arises from a hydrogen transfer process to the hydroxyl group. Five crossing points between high- and low-spin states are found: one of them takes place before the first branching point, while the others occur along path 1. Four different exit channels are found: elimination of hydrogen molecule to yield propynaldehyde and VO+ ((1)Sigma/(3)Sigma); formation of propynaldehyde and the moiety V-(OH2)(+); and two elimination processes of water molecule to yield cationic products, Prod-fc(+) and Prod-dc(+) where the vanadium atom adopts a four- and di-coordinate structure, respectively.

Luminescent nano-composites generated from a spray

The spray-pyrolysis (SP) synthesis technique has been employed to obtain SiO2:Eu3+ and gamma-AlOOH:Eu3+, It was possible to obtain sub-micrometric spherical particles of SiO2 with luminescent Eu3+ ions bonded to the silica surface or embedded in amorphous silica beads, by controlling the synthesis and annealing process. Boehmite y-AlOOH doped with Eu3+ nanoparticles were synthesized by SP at moderate temperature (200 degrees C) with Eu3+ ions bonded to the surface hydroxyls of the boehmite nanocrystals. Luminescent nanocomposites were obtained by controlled reaction of gamma-AlOOH:Eu3+ nanocrystals with ASN (asparagine). In these nano-composites, the Eu3+ are held at the surface of the boehmite nanocrystals and partially shielded from interactions with additional luminescence quenchers (hydroxyl groups, water molecules). (C) 2008 Elsevier B.V. All rights reserved.

Synthesis and electrochemical behavior of single-crystal magnetite nanoparticles

Water-dispersed magnetite nanoparticle synthesis from iron(II) chloride in dimethyl sulfoxide (DMSO)-water solution at different DMSO-water ratios in alkaline medium was reported. TEM and XRD results suggest a single-crystal formation with mean particle size in the range 4-27 nm. Magnetic nanoparticles are formed by the oxidative hydrolysis reaction from green rust species that leads to FeOOH formation, followed by autocatalysis of the adsorbed available Fe(II) on the FeOOH surfaces. The available hydroxyl groups seem to be dependent on the DMSO-water ratio due to strong molecular interactions presented by the solvent mixture. Goethite phase on the magnetite surface was observed by XRD data only for sample synthesized in the absence of DMSO. In addition, cyclic voltammetry with carbon paste electroactive electrode (CV-CPEE) results reveal two reduction peaks near 0 and +400 mV associated with the presence of iron(III) in different chemical environments related to the surface composition of magnetite nanoparticles. The peak near +400 mV is related to a passivate thin layer surface such as goethite on the magnetite nanoparticle, assigned to the intensive hydrolysis reaction due to strong interactions between DMSO-water molecules in the initial solvent mixture that result in a hydroxyl group excess in the medium. Pure magnetite phase was only observed in the samples prepared at 30% (30W) and 80% (80W) water in DMSO in agreement with the structured molecular solvent cluster formation. The goethite phase present on the, magnetite nanoparticle surface like a thin passivate layer only was detectable using CV-CPEE, which is a very efficient, cheap, and powerful tool for surface characterization, and it is able to determine the passivate oxyhydroxide or oxide thin layer presence on the nanoparticle surface.

Role of polyethylene-graft-glycidyl methacrylate compatibilizer on the biodegradation of poly(epsilon-calprolactone)/cellulose acetate blends

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

pH changes at the surface of root dentin when using root canal sealers containing calcium hydroxide

The purpose of this study was to investigate long-term pH changes in cavities prepared in root surface dentin of extracted teeth after obturation of the root canal with gutta-percha and a variety of sealers containing calcium hydroxide. After cleaning and shaping, root canals in 50 recently extracted, human single-rooted teeth were divided into five groups. Each of four groups was obturated with gutta-percha and either Sealapex, Sealer 26, Apexit, or CRCS, all of which contain calcium hydroxide. The remaining group served as the control and was not obturated with gutta-percha or sealer. Cavities were prepared in the facial surface of the roots in the cervical and middle regions. The pH was measured in these dentinal cavities at the initiation of the experiment, and 3, 7, 14, 21, 28, 45, 60, 90, and 120 days after obturation. Results indicate that the pH at the surface of the root does not become alkaline when calcium hydroxide cements are used as root canal sealers. Regardless of the sealer used, the observed pattern of pH change was not different from that seen in the control group of roots that were not treated with sealer. It is concluded that calcium hydroxide-containing cements, although suitable for use as root canal sealants, do not produce an alkaline pH at the root surface. If such a pH change is related to treatment of root resorption, these sealants do not contribute to this treatment. Copyright © 1996 by The American Association of Endodontists.