Thermal decomposition of sodium dithionite

The thermal decomposition of sodium dithionite (Na₂S₂O₄) in the solid state was investigated by differential scanning calorimetry and the exothermic heat of decomposition was determined. The effect of isothermal pretreatment at various selected temperatures on the subsequently determined decomposition temperature was studied, together with the effect of sample weight on the decomposition temperature and degree of exothermicity.     

The kinetics of dissolved oxygen reaction in aqueous sodium dithionite solutions

The kinetics of the reaction between dissolved oxygen and sodium dithionite in alkaline aqueous solution was investigated in a stirred cell. For dithionite concentrations below 0.08 kmol/m³ and sodium hydroxide concentrations from 0.044 to 0.27 kmol/m³, the results show the reaction to be first order in dithionite and zero order in oxygen. The Arrhenius activation energy was determined to be 76.2 MJ/kmol in the temperature range 15 to 34°C. Addition of other electrolytes (sodium sulphate or potassium chloride) had no effect on the kinetics.     

Electrosynthesis of sodium dithionite in a trickle-bed reactor

Sodium dithionite was generated by electroreduction of sulphur dioxide on a graphite fibre mat cathode in a continuous ‘flow-by’ trickle-bed electrochemical reactor. Reactor performance was measured with respect to variation of the feed catholyte NaOH concentration [0.5-1.0 mol/L], feed gas SO₂ concentration [4-40 vol%], product catholyte pH [2.4-12.0] and applied current [10-50 A (0.4-2.0 kA/m²)]. Product catholyte temperature was in the range 18 to 30°C. With product catholyte pH between 2.4 and 6.9, dithionite was generated at current efficiency from 42 to 100%, concentration from 0.05 to 0.78 mol/L, yield from SO₂ up to 69% and specific energy from about 2 to 5 kWh/kg Na₂S₂O₄. Current efficiency fell with increasing current and rose with increasing gas load. High levels of thiosulphate [0.07-0.3 mol/L] and sulphide [0.01-0.13 mol/L] measured in the product catholyte would compromise the use of this process in commercial applications such as brightening wood pulp.     

The crystal structure of {alpha}-bungarotoxin at 2.5 ? resolution: relation to solution structure and binding to acetylcholine receptor

We report collection of 2.5 ? resolution X-ray diffraction datafrom newly grown crystals of the rare ‘small unit cell’form of the long snake neurotoxin, -bungarotoxin. The previousmodel of the molecule has been rebuilt, and refined using least-squaremethods to a crystallographic residual of 0.24 at 2.5 ? resolution.-Bungarotoxin's crystal structure is compared with the crystalstructures of two other snake neurotoxins (cobratoxin and erabutoxin),and with its solution structure inferred from spectroscopicstudies. Significant differences include less ß-sheetin bungarotoxm's crystal structure than in solution, or in thecrystal structures of other neurotoxins, and an unusual orientationin the crystal of the invariant tryptophan. The functional,binding surface of bungarotoxin is described; it consists primarilyof hydrophobic and hydrogen-bonding groups and only a few chargedside chains. The structure is compared with experimental bindingparameters for neurotoxins.     

Some problems of the liquid crystal state of rigid-chain polymers

Conclusions A review has been given of recent theoretical work on the liquid crystal state of rigid-chain polymers.The effect of polydispersity in molecular weight and the role of partial flexibility of macromolecules on the concentration and temperature ranges for transition of solutions of these polymers into the anisotropic state have been noted.The problem of transition of solid polymers containing no solvent into the liquid crystal state has been discussed.Translated from Khimicheskie Volokna, No. 6, pp. 39–43, November–December, 1983.     

Kinetics of oxygen absorption in aqueous sodium dithionite solutions

The absorption and reaction of oxygen in aqueous alkaline solutions of sodium dithionite has been experimentally investigated in a novel gas-liquid contactor. The novel gas-lift bubble column contactor was used to study the kinetics over wide ranges of reactant concentrations, temperature, and pH. The oxygen-sodium dithionite reaction was found to be first-order with respect to dithionite in the range of dithionite concentration < 0.1 M, and second-order in the range of dithionite concentration > 0.1 M. The reaction with respect to oxygen was found to be zero-order for all dithionite concentrations. These results and experimental investigations of the effect of solution alkalinity and temperature on the reaction rate are consistent with previous findings obtained in different gas-liquid contactors. The results thus confirm the feasibility of using the gas-lift bubble column for the kinetics of gas-liquid reactions.     

The binding of micellar lipids to chitosan

The lipid binding capacity of chitosan (partially deacetylated chitin) was determinined with respect to micellar solutions of bile salts, dodecyl sulfate, natural ox bile and an artificial mixed microemulsion. The stoichiometry was determined following the separation of the solid phase by filtration or centrifugation. The major variables in the extent of binding were the pH and ionic strength, suggesting that the interactions are mainly of ionic nature. It is noteworthy that under optimal conditions chitosan could bind, i.e., coprecipitate, with 4–5 times of its weight with all the lipid aggregates tested. These results have a bearing on the nutritional and pharmacological applications of chitosan. The analyses of the components from the precipitates with microemulsion and ox bile show a significant selectivily of binding caused by hydrophobic interactions.     

Characterization of kraft pulp delignification using sodium dithionite as bleaching agent

Abstract

The delignification of kraft pulp with sodium dithionite was studied to remove lignin content in the pulp. The sodium dithionite dissolves the chromophoric groups and residual lignin present in the pulp. The increase in dosage of sodium dithionite and reaction temperature of delignification has positive effect on kappa reduction of pulp. X ray diffraction was used to determine the crystallinity index of bleached pulp. The crystallinity increased from 83.3% for unbleached pulp to 86.7% after delignification. Fourier transform infrared spectroscopy shows the reduction in hydrogen bonding in bleached pulp and also the conversion of cellulose I to cellulose II. FT-Raman spectra shows that the fluorescence observed in the spectra of unbleached pulp reduced significantly in comparison to the spectra of bleached pulp resulting in removal of residual lignin and chromophoric groups present in the pulp. Scanning electron imaging shows the smoothening of fiber surface after bleaching. The delignification reaction followed first-order kinetics and activation energy is 33.57kJ/mol.     

Master curve of crystal growth rate and its corresponding state in polymeric materials

The temperature dependence of linear crystal growth rate (G) for various polymers shows a bell shape with the maximum growth rate (G_max). The G_max shows remarkable molecular weight dependence. The G_max was formulated on the basis of a crystallization theory, both for Arrhenius and WLF expressions in the molecular transport term. The plots of the reduced growth rate (G/G_max) against the reduced temperature (T/T_cmax) for a given polymer showed a single master curve without molecular weight dependence. The ratio of G₀/G_max gave a constant value for each polymer, indicating a material constant. On the basis of G_max for many polymers, a universal master curve was observed when the ratio of ln(G/G_max)/ln(G₀/G_max) was plotted against the reduced temperature (T/T_cmax).     

Single crystal growth of CoTi2O5 by solid state reaction synthesis

Materials in single crystal form are often sought after because the absence of grain boundaries can result in unique properties relative to the polycrystal, but producing these materials is typically a slow and complex process. In this work, pseudo single crystals of the pseudobrookite compound CoTi₂O₅ were synthesized by solid-state reaction from a duplex grain mixture of CoTiO₃ and TiO₂. The size of the crystallites was >250 µm. The transformation and subsequent microstructural evolution of the CoTi₂O₅ was studied by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD). A novel growth mechanism was identified whereby a single crystal CoTi₂O₅ front advances simultaneously along multiple CoTiO₃/TiO₂ diphasic boundaries. The single crystal domains were composed of subgrains approximately 5 µm in diameter; differences in the subgrain size and misorientation were related to the growth mechanism and the initial grain size of the duplex CoTiO₃–TiO₂ mixture. CoTi₂O₅ is a little characterized compound, and this study represents the most significant microstructural study of CoTi₂O₅ to date. The findings may be applied to similar pseudobrookite compounds such as MgTi₂O₅ and Al₂TiO₅.     

Ultrahigh-performance [001]-oriented porous PZT-5H single crystal grown by the solid state crystal growth method

Porous PZT-5H single crystals are grown by the solid state crystal growth (SSCG) method. The microstructure, phase structure and dielectric/piezoelectric properties are investigated for [001]-oriented porous PZT-5H single crystal. Evolution of phase structure with temperature is researched using in-situ temperature-dependent X-ray diffraction. The effect of pores on performance parameters is simulated using COMSOL Multiphysics® software. Ultrahigh piezoelectric coefficient d₃₃ of up to about 1700 pC/N and effective piezoelectric coefficient d₃₃* of up to about 3700 pm/V at 5 kV/cm are obtained. Moreover, the effective piezoelectric coefficient d₃₃* is stable around 1900 pm/V under 3 kV/cm and 5 kV/cm in the temperature range of 70–160 °C. Importantly, the sample possess an extremely large figure of merit g₃₃*d₃₃ (111 × 10⁻¹² m²/N), which is related to the presence of pores in the single crystal. This work expands the scope of PZT based single crystal and highlights their significant application possibilities in piezoelectric energy harvester, and actuator at high temperature.     

Solid state 19F NMR study of crystal transformation in PVDF and its nanocomposites

The polymorphism of poly(vinylidene fluoride) (PVDF) and its nanocomposites was studied by means of solid state nuclear magnetic resonance spectroscopy. ¹³C cross polarization magic angle spinning (¹³C CP MAS) NMR spectra were recorded using simultaneous high‐power decoupling on both the proton and fluorine channels. Both ¹H → ¹³C and ¹⁹F → ¹³C CP experiments were conducted, giving identical results apart from intensity variations due to the CP efficiency. Two main resonances for the CF₂ and the CH₂ groups were observed for both neat PVDF (PVDF‐C0) and the nanocomposite containing 2 wt% clay (PVDF‐C2) samples. ¹⁹F CP MAS spectra were obtained from long proton spin‐lock experiments with a shorter contact time. The results showed two strong resonances at ?84 and ?98 ppm with equal intensities, representing the α‐form crystalline structure of PVDF. It was shown that the clay induces the crystallization of PVDF in β‐form. Our earlier investigations using thermal analysis and X‐ray scattering methods also showed crystal transformation of PVDF in its clay nanocomposites. POLYM. ENG. SCI. 46:1684–1690, 2006. © 2006 Society of Plastics Engineers     

Preparation of nanosize polyaniline by solid‐state polymerization and determination of crystal structure

BACKGROUND: Nanosize polyaniline has several advantages in both the fabrication of nanodevices and for preparing nanoscale electrical connections in highly conducting polymer composites. RESULTS: Nanosize polyaniline with a diameter of 30–60 nm was prepared using a solid‐state polymerization process (PANI‐S) by mixing an equimolar quantity of ammonium persulfate and anilinium chloride crystals with a mortar and pestle. Polyaniline was also synthesized using a conventional oxidative polymerization method (PANI‐C) in an aqueous medium for comparison. Conductivity and contact angle measurements, infrared spectroscopy, ultraviolet spectroscopy, transmission electron microscopy and thermogravimetric analysis were carried out. An in‐depth investigation of the crystal structure of these polymers was carried out through powder X‐ray diffraction analysis. CONCLUSION: PANI‐S exhibited lower conductivity due to the presence of less emeraldine base form, lower crystallinity, greater d‐spacing and greater inter‐chain separation than PANI‐C. The hydrophilicity and thermal stability of PANI‐S were higher than those of PANI‐C. The unit cell volume of PANI‐S was much higher, resulting in a larger crystallite size and a greater number of atoms in the unit cell than PANI‐C. Copyright © 2009 Society of Chemical Industry     

Crystallization-induced reactions of copolymers: 6. Reorganization of polyesters in the liquid crystal state

A series of random thermotropic poly(4-oxybenzoate-co-ethylene terephthalate), P(OB/ET), and poly(3-chloro-4-oxybenzoate-co-ethylene terephthalate), P(COB/ET), copolymers were prepared and their thermal properties were determined. The crystallization induced reactions (CIR) of some of these copolymers were investigated in the liquid crystal state. The effects of reaction parameters such as reaction time, temperature, type and amount of catalyst, and molecular weight of the initial sample were examined for yields, compositions, properties and sequence distributions of the reorganized products. The initial liquid crystal copolymers formed a mixture of soluble and insoluble products after the CIR process, and the latter are believed to be multiblock, crystalline oxybenzoate copolymers which are infusible above an average oxybenzoate sequence length of approximately four units.     

活性添加剂对硫酸钙结晶过程的影响

研究反应温度、酸浓度、溶剂浓度及搅拌速度对硫酸钙结晶过程的影响;在溶剂中加入活性添加剂后,硫酸钙的溶解度增大,晶形由细长瓦楞状变成了粗大棱柱状.     

纳米晶粒磷酸锌锂固相反应合成的研究

以聚乙二醇-400（PEG-400）为模板，用硫酸锌和磷酸二氢铵为原料经低热固相反应合成得到磷酸锌铵[NH4Zn2（POP4）（HPO4）]，再以磷酸锌铵和氢氧化锂为反应体系进行了固相反应合成研究，并进行了红外光谱、X射线衍射及热分析研究，结果证明，反应体系在700℃保温2h后进行固相合成反应，可获得比较纯的纳米晶粒磷酸锌锂，合成工艺简便，为企业生产提供了一种新的途径。