Investigação de Er3+ nos dois sítios cristalográficos de Gd2SiO5 através da fotoluminescência resolvida no tempo

Gadolinium oxyortho-silicate, Gd2SiO5, presents a monoclinic structure with two crystallographic sites in which Gd3+ ions are equally distributed with coordination numbers CN, 7 and 9, respectively. By doping this host with Er3+ it is possible to distinguish and attribute the two sites by means of lifetime determination of the 4S3/2 state, (in this case, Er3+ substitutes Gd3+ ions). Samples doped with 0.1 and 5.0% molar Er3+ were prepared by solid state reaction and characterized by X-ray Diffractometry, Vibrational and Electronic Absorption Spectroscopies, and Time Resolved Photoluminescence. Based on the experimental results, it is possible to verify that, for the 5,0% doped sample, the lifetime value of the 4S3/2 state of the erbium ion inserted in site 1, (CN = 9), is 2.7 ± 0.1 mus, and for the one inserted in site 2, (CN = 7), it is 1.5 ± 0.1 mus.

A síntese do aduto binuclear [Fe(CNBu t)(CO)4(HgSO4 )]

The binuclear [Fe(CNBu t)(CO)4(HgSO4 )] adduct was obtained in the reaction of HgSO4 with [Fe(CNBu t)(CO)4] in methanol. This adduct, without a similar in the homoleptic pentacarbonyliron, was characterized by analytical and spectroscopic data. Further Mössbauer and molar conductivity studies have confirmed it's adduct nature.

Preparação e caracterização de pós cerâmicos de aluminatos de lítio

The preparation of gamma-LiAlO2 by coprecipitation and sol-gel synthesis was investigated. Ceramic powders obtained by coprecipitation synthesis were prepared from aqueous solutions of aluminum and lithium nitrates using sodium hydroxide as precipitant agent. By sol-gel synthesis, the ceramic powders were prepared from hydrolysis of aluminum isopropoxide. The materials obtained by two routes of synthesis were dried at 80ºC and calcined at 550, 750, 950 and 1150ºC. The characterization was done by X-ray diffraction, infrared spectroscopy, emission and absorption atomic spectrometry, helium picnometry, specific surface area (BET method) and scanning electronic microscopy. Mixtures of crystalline phases were obtained by coprecipitation synthesis: 80ºC- LiAl2(OH)7.2H2O + Al(OH)3; 550 and 750ºC- alpha-LiAlO2 + eta-Al2O3; 950 and 1150ºC- gamma-LiAlO2 + LiAl5O8. Chemical analysis showed molar ration Al/Li @ 3. Crystalline single-phases were obtained by sol-gel synthesis above 550ºC: 550ºC-alpha-LiAlO2; 750, 950 and 1150ºC-gamma-LiAlO2. These powders presented molar ration Al/Li @ 1. Thus, gamma-LiAlO2 crystalline phase was obtained at 750ºC by sol-gel synthesis while by coprecipitation synthesis, a mixture of crystalline phases was obtained. These results showed the superiority of the sol-gel synthesis for the preparation of pure gamma-LiAlO2.

Síntese de membranas cerâmicas via método sol-gel utilizando TEOS e N,N-dimetilformamida

In this work we obtained microporous and mesoporous silica membranes by sol-gel processing. Tetraethylortosilicate (TEOS) was used as precursor. Nitric acid was used as catalyst. In order to study the affect of N,N-dimethylformamide (NDF) as drying additive, we used a molar ratio TEOS/NDF of 1/3. The performance of N,N-dimethylformamide was evaluated through monolithicity measurements. The structural evolutions occurring during the sol-gel transition and in the interconnected network of the membranes during thermal treatment were monitored by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analyses and nitrogen sorption. We noted that in the presence of N,N-dimethylformamide, polymerization goes through a temporary stabilization of oligomers. The Si-O(H) bonds are stronger and belong to a more cross-linked structure for the N,N-dimethylformamide containing sol. The membranes obtained in the presence of N,N-dimethylformamide have larger surface area and its pore structure is in the range of mesoporous. The membranes obtained without additive have pore structure in the range of microporous.

SSPBE: um programa para solução numérica da equação de Poisson-Boltzmann em simetria esférica com modelo de adsorção

A Fortran77 program, SSPBE, designed to solve the spherically symmetric Poisson-Boltzmann equation using cell model for ionic macromolecular aggregates or macroions is presented. The program includes an adsorption model for ions at the aggregate surface. The working algorithm solves the Poisson-Boltzmann equation in the integral representation using the Picard iteration method. Input parameters are introduced via an ASCII file, sspbe.txt. Output files yield the radial distances versus mean field potentials and average molar ion concentrations, the molar concentration of ions at the cell boundary, the self-consistent degree of ion adsorption from the surface and other related data. Ion binding to ionic, zwitterionic and reverse micelles are presented as representative examples of the applications of the SSPBE program.

Distribuição de metais e caracterização das constantes de troca entre espécies metálicas e frações húmicas aquáticas de diferentes tamanhos moleculares

In this work the metal distribution and exchange constants between metal species and aquatic humic fractions with different molecular sizes were studied. The aquatic humic substances (AHS) were extracted by XAD-8 resin from water sample collected from Itapitanguí river, São Paulo State, Brazil. The AHS were fractionated in six fractions with different molecular sizes (>100 - <5 kDa) and characterized by several techniques. Molar ratios H/C suggested higher aromaticity for fractions F1 and F6 whereas molar ratios C/N didn´t show any differences regarding the humification degree between the fractions. The UV-Vis absorbance a254/a436 ratio showed higher results for F4 and F5, probably by less condensed features. FTIR studies showed high similarity in the functional groups in the fractions. The highest percentage of traces of Co, Al, Fe, Mn, Cu, Zn and Ni (determined by ICP-AES) was preferably complexed by fractions F3 and F4 with a greater amount of dissolved organic carbon (DOC). In addition, the exchange constants, determined by ultrafiltration method, showed complexes AHS-Fe and AHS-Al with higher stability than complexes AHS-Co in all fractions.

Review of the synthesis of layered double hydroxides: a thermodynamic approach

The synthesis of layered double hydroxides (LDHs) by hydrothermal-LDH reconstruction and coprecipitation methods is reviewed using a thermodynamic approach. A mixture model was used for the estimation of the thermodynamics of formation of LDHs. The synthesis and solubility of LDHs are discussed in terms of standard molar Gibbs free energy change of reaction. Data for numerous divalent and trivalent metals as well as for some monovalent and tetravalent metals that may be part of the LDH structure have been compiled. Good agreement is found between theoretical and experimental data. Diagrams and tables for the prediction of possible new LDH materials are provided.

Fotoluminescência e adsorção de CO2 em nanopartículas de CaTiO3 dopadas com lantânio

Ca1-xLa xTiO3 powders were prepared by the polymeric precursor method. X-ray diffraction (XRD), FT-Raman spectroscopy, transmission electron microscopy (TEM), and N2 and CO2 adsorption were used for the microstructural and surface characterization of the powders. Room temperature photoluminescence (PL) was observed in Ca1-xLa xTiO3 amorphous particles. The PL intensity of these powders was found to be dependent on the lanthanum molar concentration.

Estudo da influência do solvente, carboidrato e ácido graxo na síntese enzimática de ésteres de açúcares

The aim of this work was to gain knowledge of enzymatic processes for the synthesis fatty acid esters of sugar, with the objective to develop an enzymatic process for the preparation of non-toxic biodegradable surface-active agents derived entirely from renewable resources. A wide range of data were collected for reaction conditions involving different sugars (glucose, fructose and sucrose), fatty acids (oleic, palmitic, lauric), solvents (hexane, heptane and t-butanol) and different sources of lipases in both free and immobilized forms. As a solvent t-butanol provided the best conditions to create a catalytic liquid phase in which the reaction occurs. Sugars were preferentially esterified in the following order: fructose > glucose > sucrose, depending on the enzyme preparation. For fructose no influence was found concerning de acyl donor and similar rates were achieved for all tested fatty acids. Ester synthesis was maximized for substrates containing fructose, lauric or oleic acids, t-butanol and lipase from porcine pancreas immobilized on polysiloxane-polyvinyl alcohol particles. Under such conditions molar conversions were higher than 50%.

Uso de membranas de Nafion para a construção de sensores ópticos para medidas de pH

The behaviour of Nafion® polymeric membranes containing acid-base dyes, bromothymol blue (BB) and methyl violet (MV), were studied aiming at constructing an optical sensor for pH measurement. BB revealed to be inadequate for developing sensing phases due to the electrostatic repulsion between negative groups of their molecules and the negative charge of the sulfonate group of the Nafion®, which causes leaching of the dye from the membrane. On the other hand, MV showed to be suitable due to the presence of positive groups in its structure. The membrane prepared from a methanolic solution whose Nafion®/dye molar ratio was 20 presented the best analytical properties, changing its color from green to violet in the pH range from 0.6 to 3.0. The membrane can be prepared with good reproducibility, presenting durability of ca. 6 months and response time of 22 s, making possible its use for pH determination in flow analysis systems.

Desenvolvimento de colunas cromatográficas de meios de acesso restrito proteína-imobilizada e suas avaliações para análise de fármacos com injeção direta de plasma humano

A series of bovine serum albumin-immobilized supports have been prepared and used as restricted access media (RAM) columns. Restricted-access supports combine size-exclusion of proteins and other high-molar-mass matrix components with the simultaneous enrichment of low-molar mass analytes. These characteristics were chromatographically evaluated for the columns. The RAM-BSA (Bovine Serum Albumin) columns showed excellent performance for exclusion of human plasma protein with good retention capacity for a series of acidic, basic, and neutral drugs.

Integração de técnicas analíticas e proposta de experimentos para cursos de graduação em análise instrumental: uso de espectrômetro de absorção atômica para medidas de absorção molecular

Simple experiments are proposed for measuring molecular absorption of chromate and dichromate ions using an atomic absorption spectrometer. The experiments can help undergraduate students in instrumental analysis courses understand important aspects involving conceptual and instrumental similarities and differences between frequently used analytical techniques. Hollow cathode lamps were selected with wavelengths in the region of molecular absorption of chromate and dichromate. Calibration curves were obtained and the linear dynamic range was evaluated. Results were compared with those obtained in a molecular absorption spectrometer. The molar absorptivities obtained were also compared.

Polimerização de estireno utilizando compostos organolantanídeos ativados por MAO

In an attempt to improve the performance of organolanthanide catalysts we investigated the use of the industrially important cocatalyst methylaluminoxane (MAO) to activate organolanthanide compounds in olefin polymerization. The catalytic systems LnBrCp2(THF)2/MAO (Cp=cyclopentadienyl) and LnBrCp*2THF/MAO (Cp*= pentamethylcyclopentadienyl), Ln=Pr and Yb, were active in styrene polymerization but inactive in ethylene and propylene polymerization. These systems produced atactic polystyrene with conversions of up to 8.2% (PrBrCp*2THF, Al/Ln=200, T=80ºC, t=4 h) in toluene. In the absence of solvent, the conversion is 26.0% (1.5 h) and the molar mass of the atactic polystyrene is almost ten times higher (43 kg/mol).

Produção de hidrogênio a partir da reforma a vapor de etanol utilizando catalisadores Cu/Ni/gama-Al2o3

Cu/Ni/gamma-Al2O3 catalysts were prepared by an impregnation method with 2.5 or 5% wt of copper and 5 or 15% wt of nickel and applied in ethanol steam reforming. The catalysts were characterized by atomic absorption spectrophotometry, X-ray diffraction, temperature programmed reduction with hydrogen and nitrogen adsorption. The samples showed low crystallinity, with the presence of CuO and NiO, both as crystallites and in dispersed phase, as well as of NiO-Al2O3. The catalytic tests carried out at 400 ºC, with a 3:1 water/ethanol molar ratio, indicated the 5Cu/5Ni/Al2O3 catalyst as the most active for hydrogen production, with a hydrogen yield of 77% and ethanol conversion of 98%.

Emprego do sal di-sódico de edta como padrão no preparo de soluções

The hydrated sodium salt of EDTA, Na2H2Y·2H2O, cannot be used as a primary standard for titrations due to uncertainties in the water content. An alkalimetric titration of the homogenized solid in the presence of a small excess of BaCl2·2H2O allows one to titrate quantitatively the released two hydrogen cations with end-point indication by phenolphthalein or potentiometry. This leads one to calculate the average molar mass of the reagent and its water content, allowing to use it to prepare EDTA standard solutions. One titrated sample led to the formula Na2H2Y·1.876 H2O, and 370.01 g.mol-1 for the average molar mass.