Molecular dynamics simulation on devitrification: Isothermal devitrification and thermodynamics of PbF2 glasses

The vitrification and devitrification features of lead fluoride are investigated by means of molecular dynamic simulations. The influence of heating rate on the devitrification temperature as well as the dependence of the glass properties on its thermal history, i.e., the cooling rate employed, is identified. As expected, different glasses are obtained when the cooling rates differ. Diffusion coefficient analysis during heating of glass and crystal, indicates that the presence of defects on the glassy matrix favors the transition processes from the ionic to a superionic state, with high mobility of fluorine atoms, responsible for the high anionic conduction of lead fluoride. Nonisothermal and isothermal devitrification processes are simulated in glasses obtained at different cooling rates and structural organizations occurring during the heat treatments are clearly observed. When a fast cooling rate is employed during the glass formation, the devitrification of a single crystal (limited by the cell dimensions) is observed, while the glass obtained with slower cooling rate, allowing relaxations and organization of various regions on the glass bulk during the cooling process, devitrifies in more than one crystalline plane. (C) 2004 American Institute of Physics.

Structural organization and thermal properties of the Sb2O3-SbPO4 glass system

The structural organization of Sb2O3-SbPO4 glasses has been studied by FTIR, Raman, P-31 MAS and spin echo NMR, Mossbauer and X-ray absorption spectroscopy (EXAFS and XANES at K and L-3,L-1-Sb edges). The combined results can be explained in terms of two potential mechanisms describing the change of the Sb(m) local environment upon incorporation of Q((4))-type phosphate. The formation of the latter species requires anionic compensation that may be adjusted by (a) formation of non bridging oxygen or (b) formation of SbO4E- groups (E = non-bonding electron pair). The second model is favored.

Structural studies of NaPO3-WO3 glasses by solid state NMR and Raman spectroscopy

Vitreous samples were prepared in the (100 2 x) NaPO3-x WO3 (0 <= x <= 70) glass forming system using conventional melting-quenching methods. The structural evolution of the vitreous network was monitored as a function of composition by thermal analysis, Raman spectroscopy and high resolution one- and two-dimensional P-31 solid state NMR. Addition of WO3 to the NaPO3 glass melt leads to a pronounced increase in the glass transition temperatures, suggesting a significant increase in network connectivity. At the same time Raman spectra indicate that up to about 30 mol% WO3 the tungsten atoms are linked to some non-bridging oxygen atoms (W-O- or W=O bonded species), suggesting that the network modifier sodium oxide is shared to some extent between both network formers. W-O- W bond formation occurs only at WO3 contents exceeding 30 mol%. P-31 magic angle spinning (MAS)-NMR spectra, supported by two-dimensional J-resolved spectroscopy, allow a clear distinction between species having two, one, and zero P-O-P linkages. The possible formation of some anionic tungsten sites suggested from the Raman data implies an average increase in the degree of polymerization for the phosphorus species, which would result in diminished P-31/Na-23 interactions. This prediction is indeed confirmed by P-31{Na-23} and Na-23{P-31} rotational echo double resonance (REDOR) NMR results, which indicate that successive addition of WO3 to NaPO3 glass significantly diminishes the strength of phosphorus-sodium dipole-dipole couplings.

Collagen-hydroxyapatite films: piezoelectric properties

In this paper we report a study of the physicochemical, dielectric and piezoelectric properties of anionic collagen and collagen-hydroxyapatite (HA) composites, considering the development of new biomaterials which have potential applications in support for cellular growth and in systems for bone regeneration. The piezoelectric strain tensor element d(14), the elastic constant s(55) and the dielectric permittivity 8(11), were measured for the anionic collagen and collagen-HA films. The thermal analysis shows that the denaturation endotherm is at 59.47 degreesC for the collagen sample. The collagen-HA composite film shows two transitions, at 48.9 and 80.65 degreesC. The X-ray diffraction pattern of the collagen film shows a broad band characteristic of an amorphous material. The main peaks associated to the crystalline HA is present in the sample of collagen-HA. In the collagen-HA composite, one can also notice the presence of other peaks with low intensities which is an indication of the formation of other crystalline phases of apatite. The scanning electron photomicrograph of anionic collagen membranes shows very thin bundles of collagen. The scanning electron photomicrography of collagen-HA film also show deposits of hydroxyapatite on the collagen fibers forming larger bundles and suggesting that a collagenous structure of reconstituted collagen fibers could act as nucleators for the formation of apatite crystal similar to those of bone. The piezoelectric strain tensor element d(14) was measured for the anionic collagen, with a value of 0.062 pC N-1, which is in good agreement compared with values reported in the literature obtained with other techniques. For the collagen-HA composite membranes, a slight decrease of the value of the piezoelectricity (0.041 pC N-1) was observed. The anionic collagen membranes present the highest density, dielectric permittivity and lowest frequency constant f.L. (C) 2001 Elsevier B.V. B.V. All rights reserved.

Regeneration of poly-L-lysine modified carbon electrodes in the accumulation and cathodic stripping voltammetric determination of the cromoglycate anion

Cromoglycate is accumulated on a poly-L-lysine (PLL) modified carbon electrode best from pH 4 solution, where it is anionic and the PLL is cationic, and at which pH the cromoglycate gives a good reduction peak at -0.82 V. The PLL film can be regenerated readily by washing the electrode with 3 M sodium hydroxide solution, in which the PLL is deprotonated. Regeneration of the film is not required as frequently when larger amounts of PLL are incorporated into it. This allows standard addition procedures to be carried out without regenerating the electrode. Linear calibration graphs have been obtained typically in the range 0.1 - 1.5 mug ml(-1). Detection limits have been calculated to be 10 ng ml(-1). The standard addition method has been applied satisfactorily to diluted urine solutions. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Modification of glassy carbon electrodes with a poly-L-lysine/glutaraldehyde film

A film of poly-L-lysine (PLL) adheres better to the surface of a glassy carbon electrode when the PLL is partially cross-linked by means of glutaraldehyde. A film composition of 97.5% PLL/2.5% glutaraldehyde gives good adhesion and retains the anionic exchange capability of the PLL. The performance of the film was tested with hexacyanoferrate(III) using electrochemical and nonelectrochemical accumulation.

Fractionation and identification of the nucleic acid-related substances secreted by Streptomyces aureofaciens

S. aureofaciens growth in a chemically defined medium was associated with the active secretion of nucleic acid-related substances in the medium. High secretion depended on low availability of phosphate, and fractionation showed 7 anionic substances were secreted as major components. When compared to 76 known purine and pyrimidine derivatives only erotic acid was identified. Cationic components are among the minor concentration components secreted which have been identified as cytosine, inosine, cytidine, adenine, guanine and, probably, 1-methyl-adenine.

Formation and characterization of soy bean oil/surfactant/water microemulsions

Pseudoternary phase diagrams, at 25 degrees C, were constructed for the systems soy bean oil (SBO)/surfactant/water, with single anionic sodium bis(2-ethylhexyl)sulfosuccinate (AOT), nonionic monoolein (MO) and mixtures of these surfactants, showing the isotropic phase of W/O microemulsions (MEs). The area of ME formation in the phase diagrams was shown to be dependent of the relative amount of surfactants, being larger for MO:AOT equals to 2:1. Rheological and dynamic light scattering (DLS) studies indicated that the viscosity of the isotropic ME phase exhibited two different behaviors depending on composition. The viscosity of dry MEs initially decreased with increasing amount of water following a dilution line in the phase diagram, i.e., a constant surfactant:SBO percentage ratio. As the water content increased the relative viscosity attained a minimum and then increased. This minimum could be related to the transition between two ME regions, L-2 and L'(2), having different characteristics. DLS measurements confirm the existence of ordinary W/O ME droplets in the L-2 region and suggest the existence of another structure in the L'(2) region. The size of the MEs droplets in L-2 phase ranges from 3.6 to 16.5 nm, depending on composition of SBO, surfactant and water. Small angle X-ray scattering (SAXS) also indicates the existence of structures with different characteristics, for the SAXS curves exhibit a typical micelle asymmetrical peak at low scattering vector q for MEs in L-2 but a symmetrical correlation peak at higher q vector in L'(2). (c) 2006 Elsevier B.V. All rights reserved.

beta-ZnMoO4 microcrystals synthesized by the surfactant-assisted hydrothermal method: Growth process and photoluminescence properties

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

Highly swollen liquid crystals as new reactors for the synthesis of nanomaterials

Synthesis and self-assembly of nanomaterials can be controlled by the properties of soft matter. on one hand, dedicated nanoreactors such as reverse microemulsions or miniemulsions can be designed. on the other hand, direct shape control can be provided by the topology of liquid crystals that confine the reacting medium within a specific geometry. In the first case, the preparation of micro- or miniemulsions generally requires energetic mechanical stirring. The second approach uses thermodynamically stable systems, but it remains usually limited to binary (water + surfactant) systems. We report the preparation of different families of materials in highly ordered quaternary mediums that exhibit a liquid crystal structure with a high cell parameter. They were prepared with the proper ratios of salted water, nonpolar solvent, surfactant. and cosurfactants that form spontaneously swollen hexagonal phases. These swollen liquid crystals can be prepared from all classes of surfactants (cationic, anionic, and nonionic). They contain a regular network of parallel cylinders, whose diameters can be swollen with a nonpolar solvent, that are regularly spaced in a continuous aqueous salt solution. We demonstrate in the present report that both aqueous and organic phases can be used as nanoreactors for the preparation of materials. This property is illustrated by various examples such as the synthesis of platinum nanorods prepared in the aqueous phase or zirconia needles or the photo- or gamma-ray-induced polymerization of polydiacetylene in the organic phase. In all cases, materials can be easily extracted and their final shapes are directed by the structure-directing effect imposed by the liquid crystal.

Implants of polyanionic collagen matrix in bone defects of ovariectomized rats

In recent years, there has been a great interest in the development of biomaterials that could be used in the repair of bone defects. Collagen matrix (CM) has the advantage that it can be modified chemically to improve its mechanical properties. The aim of the present study was to evaluate the effect of three-dimensional membranes of native or anionic (submitted to alkaline treatment for 48 or 96 h) collagen matrix on the consolidation of osteoporosis bone fractures resulting from the gonadal hormone alterations caused by ovariectomy in rats subjected to hormone replacement therapy. The animals received the implants 4 months after ovariectomy and were sacrificed 8 weeks after implantation of the membranes into 4-mm wide bone defects created in the distal third of the femur with a surgical bur. Macroscopic analysis revealed the absence of pathological alterations in the implanted areas, suggesting that the material was biocompatible. Microscopic analysis showed a lower amount of bone ingrowth in the areas receiving the native membrane compared to the bone defects filled with the anionic membranes. In ovariectomized animals receiving anionic membranes, a delay in bone regeneration was observed mainly in animals not subjected to hormone replacement therapy. We conclude that anionic membranes treated with alkaline solution for 48 and 96 h presented better results in terms of bone ingrowth.

Pt(II) and Ag(I) complexes with acesulfame: Crystal structure and a study of their antitumoral, antimicrobial and antiviral activities

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

Direct analysis of biodiesel microemulsions using an inductively coupled plasma mass spectrometry

An analytical procedure for direct introduction of biodiesel samples into an inductively coupled plasma mass spectrometer (ICP-MS) by using microemulsion for sample preparation was developed here. Cadmium, Co, Cu, Mn, Ni, Pb, Ti, and Zn were determined in biodiesel microemulsified samples prepared from different oleaginous sources (African palm, castor beans, palm, soybeans and an unknown oleaginous). Microemulsions were prepared using 0.25 mL Triton X-100, 0.25 mL 20% v v(-1) HNO(3), 0.50 mL biodiesel sample and 4.0 mL n-propanol. Argon-oxygen mixture was added to the plasma as auxiliary gas for correcting matrix effects caused by the high carbon load due to biodiesel microemulsions. The oxygen gas flow rate was set in 37.5 mL min(-1). The accuracy of the developed procedure was evaluated by applying addition-recovery experiments for biodiesel samples from different sources. Recoveries varied from 76.5 to 116.2% for all analytes but Zn in castor beans biodiesel sample (65.0 to 76.2%). Recoveries lower than 86.6% were obtained for palm biodiesel sample, probably due to matrix effects. Detection limits calculated by using oxygen in the composition of the auxiliary gas added to the plasma were higher than those calculated without using it, probably due to the highest formation of oxides. Despite oxides formation, best analytical performance was reached by using oxygen as auxiliary gas and by proper correction of transport interferences. The developed procedure based on microemulsion formation was suitable for direct introduction of biodiesel samples in ICP-MS. (C) 2010 Elsevier B.V. All rights reserved.

Effect of different surfactants on the shape, growth and photoluminescence behavior of MnWO4 crystals synthesized by the microwave-hydrothermal method

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

Determination of Ca, Mg, and Zn in biodiesel microemulsions by FAAS using discrete nebulization

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