Effect of Ag addition on crystallization kinetics and thermal stability of As–Se chalcogenide glasses

Chalcogenide glasses of (As₅₀Se₅₀)_100?xAg_x (0 ≤ x ≤ 25) were prepared using the melt quenching technique under non-isothermal conditions. Differential scanning calorimetry curves measured at different heating rates (5 ≤ β ≤ 40 K min^?1) are used to characterize the as-quenched samples. The thermal stability was monitored through the calculation of the temperature difference T _c ? T _g, stability parameter S and crystallization rate factor K _p. The glass-forming ability (GFA) was investigated on the basis of Hurby parameter H _r which is a strong indicator of GFA. In addition, the activation energy of glass transition E _t, activation energy of crystallization E _c and Avrami exponent n of the studied compositions were determined. The mechanism of crystallization was found to be a combination of two- and three-dimensional crystal growth.     

Ligand exchange method for determination of mole ratios of relatively weak metal complexes: a comparative study

Ligand exchange method is introduced as an alternative to Job’s and mole ratio methods for studying the stoichiometry of relatively weak metal complexes in solutions. The method involves adding varying amounts of a ligand (L) to an excess constant amount of a colored complex (MX) with appropriate stability and molar absorptivity. The absorbance of each solution is measured at the λ_max of the initial complex, MX, and plotted against the concentration of the studied ligand, L. If the newly formed complex ML does not absorb at the λ_max of the initial complex, then attenuation of the absorbance of the initial complex on adding varying quantities of the investigational ligand gives an inverse calibration line that intersects with the calibration curve of initial complex at a given point. If a line parallel to the ordinate is drawn from this point to the x-axis, the ratio of the two parts of the x-axis to the left and to the right (α/β) gives the metal to ligand molar ratio in the complex formed, ML. The new method has been applied to the study of the composition of iron (III) complexes with three bisphosphonate drugs: alendronate, etidronate, and ibandronate. The mole ratio was found to be 1:1 with the three investigated bisphosphonates and results were further confirmed by Job’s and mole ratio methods. The ligand exchange method is simpler, quicker, easier to perform and more accurate than Job’s and mole ratio methods for studying weak and relatively weak complexes.

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The molecular structure of tetra-tert-butyldiphosphine: an extremely distorted, sterically crowded molecule

The molecular structure of tetra-tert-butyldiphosphine has been determined in the gas phase by electron diffraction using the new DYNAMITE method and in the crystalline phase by X-ray diffraction. Ab initio methods were employed to gain a greater understanding of the structural preferences of this molecule in the gas phase, and to determine the intrinsic P-P bond energy, using recently described methods. Although the P-P bond is relatively long [GED 226.4(8) pm; X-ray 223.4(1) pm] and the dissociation energy is computed to be correspondingly small (150.6 kJ mol(-1)), the intrinsic energy of this bond (258.2 kJ mol(-1)) is normal for a diphosphine. The gaseous data were refined using the new Edinburgh structure refinement program ed@ed, which is described in detail. The molecular structure of gaseous P(2)Bu(t)(4) is compared to that of the isoelectronic 1,1,2,2-tetra-tert-butyldisilane. The molecules adopt a conformation with C(2) symmetry. The P-P-C angles returned from the gas electron diffraction refinement are 118.8(6) and 98.9(6) degrees, a difference of 20 degrees, whilst the C-P-C angle is 110.3(8) degrees. The corresponding parameters in the crystal are 120.9(1), 99.5(1) and 109.5(1) degrees. There are also large deformations within the tert-butyl groups, making the DYNAMITE analysis for this molecule extremely important.     

Thermoanalytical investigations of the decomposition course of copper oxysalts

The thermal decomposition course of copper acetate monohydrate (CuAc) was examined on heating up to 600°C at various rates, by TG, DTA and DSC. Non-isothermal kinetic and thermodynamic parameters were determined in air or nitrogen. SEM was used to describe the decomposition course and the solid products were identified by IR and XRD analysis. The results indicated that CuAc was dehydrated at 190°C and then partially decomposed at 220°C, giving rise to CuO in addition to a minor portion of Cu₂O and Cu₄O₃. The last two oxides seemed to facilitate the decomposition of the rest of the anhydrous acetate. Cu₂O and Cu₄O₃ were oxidized in air at >400°C, in a process that did not occur in nitrogen.It is a pleasure to thank the Queen's University of Belfast, particularly the staff of the Electron Microscope Unit for assistance in obtaining the electron micrographs. Thanks are also due to the Egyptian Government for the granted fellowship.     

Copper(II) and nickel(II) complexes of N,N′-bis(2-cyanoethyl)-5, 7-dioxo-1,4,8,11-tetra-azacyclotetradecane

Summary Reaction of 5,7-dioxo-1,4,8,11-tetra-azacyclotetradecane with acrylonitrile gives the dicyanoethylated ligand (L). The Cu^II complex [CuLH_-2]·2H₂O has been isolated from basic solution where the macrocycle is deprotonated and acts as a dinegative quadridentate ligand. The ligand L is protonated in acidic solution and the ionisation equilibria can be summarised as LH _inf2 ^sup2+ LH⁺ +H⁺; K₁ LH⁺ L + H⁺; K₂ where pK₁ = 3.05 and pK₂ = 5.94 at 25 °C and I = 0.1 mol dm^-3 (NaNO₃). Complexation with Cu^II can be represented by the equilibria at 25 °C. Cu²⁺ + L [CuLH_-1]⁺ + H⁺; log_{11 – 1} = -3.43 Cu²⁺ + L [CuLH_-2] + 2H²⁺; log_{11 – 2} = -9.18 For Ni^II only the single equilibrium is of importance. Ni²⁺ + L [NiLH_-2] + 2H²⁺; log_{11 – 2} = -14.45     

Non-isothermal decomposition of silver maleate dihydrate and anhydrous silver fumarate

The non-isothermal decompositions of silver maleate dihydrate (C₄H₂O₄Ag₂2H₂O) and anhydrous silver fumarate (C₄H₂O₄Ag₂) were studied up to 500°C, in a dynamic atmosphere of air, by means of TG and DTA measurements. Both compounds showed some sublimation (at 120°C for silver maleate and at 180°C for silver fumarate) prior to the onset of decomposition (at 170°C for silver maleate and at 280°C for silver fumarate). The gaseous decomposition products of both compounds were found, using IR spectroscopy, to be dominated by maleic anhydride and CO₂. Minor proportions of ethylene, ethyl alcohol, acetone, methane and isobutene were also identified. Metallic silver was the final solid product, as identified by X-ray diffractometry. NMR analysis was used to monitor the isomerization of the maleate radical into the more stable fumarate above 230°C. Kinetic parameters (E _a and lnA) were calculated from the effect of heating rate, (2, 5, 10, and 20 deg min^?1) on the DTA measurements. A mechanism is suggested for the decomposition pathways of these compounds, on basis of the results obtained and, also, on similarities with analogous systems.     

Dielectric method for detection of phase separation and its kinetics in TMPC/PS blends

A new method for the detection of phase separation and its kinetics through real-time measurements is presented using the dielectric technique. The kinetics of phase separation were determined for a blend of tetramethyl bisphenol-A polycarbonate TMPC and polystyrene PS at different temperatures. The temperature dependence of the rate constant of phase separation was determined. The activation energy of phase separation process is found to be equal to 46 kcal/mole. In addition, it was possible to determine the variation in the composition of the TMPC-rich phase with time. The results obtained were compared with the literature data and were found to be in good agreement.     

Kinetics and mechanism of the oxidation of [N-(2-hydroxy-ethyl)ethylenediamine-N,N′,N′-triacetatocobalt(II)] by vanadate ion

The kinetics of oxidation of Co^IIHEDTA {HEDTA = N-(2-hydroxyethyl)ethylenediamine-N,N,N-triacetic acid} by vanadate ion have been studied in aqueous acid in the pH range 0.75–5.4 at 43–57 °C. The reaction exhibits second-order kinetics; first-order in each of the reactants. The reaction rate is a maximum at pH = 2.1. A mechanism is proposed in which the species [Co^IIHEDTA(H₂O)]^– and VO₂ ⁺ react to form an intermediate which decompose slowly to give pentadentate Co^IIIHEDTA(H₂O) and V^IV as final products. The rate law was derived and the activation parameters calculated: H* = 26.96 kJ mol^–1 and S* = –311.08 JK^–1 mol^–1.