On the stability of (MX6)3− complexes in halide melts of three-valent metals

ABSTRACT

The effects of volume change at dissociation equilibrium of possible anion complexes of the (MX₆)^3? type in halide melts of trivalent metals MX3 within the statistical thermodynamics based on the MSA approximation were analysed. With the help of a simplified Akdeniz–Tosi model of a mixture of charged hard spheres of different diameters and valencies, we obtained the full system of equilibrium equations including the mass action law (MAL) and the equation of state (EoS). It was shown that the simplest approximation of the complex diameter as a tripled diameter of simple ions leads to a significant overestimation of the effects of volume changes at dissociation. It was found that the complex dissociation should be accompanied by a significant increase in density in a narrow temperature interval. It can be associated with the specific manifestation of electrolytic dissociation in the case of the trivalent metal halides’ auto-complexation.     

Features of the chemical equilibrium of dimerization in a system of solid spheres

Statistical and thermodynamic analyses of the equilibrium of dimerization in solid-phase systems is performed for a model of the Van der Waals type and the Perkus-Yevik approximation. For the model of Van der Waals type, the simple equation γ = exp[p ₀(2?λ³)] is obtained for an average activity coefficient (p ₀ is reduced pressure and λ is bond length in dimer) that describes both positive and negative deviations from the ideal, depending on the change in volume after the elementary act of chemical reaction. It is found that the Perkus-Yevik approximation predicts similar results with more pronounced deviations from the ideal, and the activity coefficient depends on the degree of dissociation as well.     

Rigidity of colloidal crystals of silica spheres modified with polymers on their surfaces in organic solvents

Rigidity (G) of colloidal crystals in organic solvents of acetonitrile and nitrobenzene has been measured by reflection spectroscopy in sedimentation equilibrium. The colloidal spheres used are the silica spheres (136 nm in diameter) modified on their surfaces with polymers, poly(maleic anhydride-co-styrene) [P(MA-ST)], poly(methyl methacrylate) (PMMA), or polystyrene (PST). Log G increases linearly with the slope of unity as log N (number density of colloidal spheres) increases. The mean values of the b-factor, which is the fluctuation parameter in crystal lattices and should be smaller than 0.1 according to the Lindeman's rule, are 0.045±0.003, 0.039±0.007, and 0.038±0.003 for P(MA-ST)/SiO₂, PMMA/SiO₂, and PST/SiO₂, respectively. These values are larger than that of colloidal crystals of mother silica spheres in the deionized aqueous suspension, 0.028. These results support the important role of the excluded volume effects from the polymer layers formed around the silica surfaces. However, contribution of the excluded volume effects from the electrical double layers formed around the spheres in the organic solvents is also effective in the colloidal crystallization. Electronic Publication     

Equilibria of formation and dehydration of the carbinolamine intermediate in the reaction of benzaldehyde with hydrazine

Measurement of polarographic limiting currents at equilibria made it possible at pH 3-7 to simultaneously determine concentrations of benzaldehyde, of its hydrazone and of the carbinolamine derivative. The dependence of concentration of carbinolamine at equilibrium on pH indicated presence of its di-, mono-, and unprotonated forms. Acid dissociation constants of the formation (pK_a¹≈3.2) of the diprotonated form and of the dissociation of the monoprotonated form of carbinolamine (pK_a²≈4.7) were estimated. The equilibrium constants of formation (K₁) and dehydration (K₂) of the carbinolamine intermediate were determined.     

Formation of autocomplexes in halide melts of trivalent metals

Chemical equilibrium with respect to the dissociation of charged autocomplexes (MX₆)^3? in ionic melts of the MX₃ type is analyzed. The chemical equilibrium M³⁺ + 6X^? = (MX₆)^3? in salt melts of trivalent metal halides shifts strongly toward dissociation, due to the electrostatic interactions between charged particles in the melts.     

Giant colloidal crystals of fluorine-containing polymer spheres in exhaustively deionized aqueous suspensions and in the presence of sodium chloride

 Gigantic colloidal single crystals (2–6 mm) are formed for fluorine-containing polymer spheres (120–210 nm in diameter) in exhaustively deionized aqueous suspensions. The spheres used are poly(tetrafluoroethylene) (PTFEA and PTFEB), copolymer of tetrafluoroethylene and perfluorovinylether (PFA) and copolymer of tetrafluoroethylene and perfluoropropylene (PTP). The phase diagrams of these spheres are obtained in the deionized suspensions and also in the presence of sodium chloride for PFA. The critical sphere concentrations of crystal melting (φ _c) for these spheres are around 0.0006 in volume fraction, which are close to, but slightly larger than, those of monodispersed polystyrene spheres (φ _c ≈ 0.00015) and colloidal silica spheres(φ _c = 0.0002–0.0004) reported previously. The crystals are largest when the sphere concentrations are a bit higher than the φ _c value and their size decreases as the sphere concentration increases. Reflection spectra are taken in sedimentation equilibrium as a function of the height from the bottom of the suspension. The static elastic modulus is estimated to be 10.8 and 28.7 Pa for PTFEA and PTP spheres at the sphere concentrations 0.00325 and 0.00322 in volume fraction, respectively. Received: 27 October 1999 Accepted in revised form: 16 November 1999     

Structural and Thermodynamic Characteristics of Compounds X2MYH2 (M = Al,Ga, In; X = F,Cl, Br,I; Y = N,P, As) Formed by Hydrogen Halide Elimination from Donor-Acceptor Complexes X3MYH3

Geometry and thermodynamic characteristics of complexes X₂MYH₂ (M = Al, Ga, In; X = F, Cl, Br, I; Y = N, P, As) and their components were found by the B3LYP density functional method with the LANL2DZ(d,p) basic set. The nitrogen complexes X2MNH2 have a planar structure, whereas the phosphorus and arsenic complexes are pyramidal. Upon HX elimination, the dissociation energy of the M-Y bonds considerably increases (by 150-270 kJ mol^{- 1}), which makes the dissociation of X2MYH2 into components thermodynamically unfeasible even at temperatures about 1000°C. A linear correlation between the dissociation enthalpies of M-Y bonds in the X₃MYH₃ and X₂MYH₂ complexes was found for each central atom M, which makes it possible to estimate the dissociation enthalpies of coordination-unsaturated compounds of the Group IIIa elements from the dissociation enthalpies of their coordination-saturated analogs. The enthalpies of dimerization of X₂MYH₂ fall in the range from 40 (Y = P, As) to 260 kJ mol^{- 1} (Y = N), which makes the process X₃MNH₃ = [X₂MNH₂]₂ + HX with the retention of the metal-nitrogen bond more favorable than the dissociation of the initial complex into the components. Thus, dimers [X₂MNH₂]₂ can be intermediates in chemical deposition of nitrides from the gas phase of donor-acceptor complexes.     

On the Nitroxide Quasi‐Equilibrium in the Alkoxyamine‐Mediated Radical Polymerization of Styrene

Summary: The range of validity of two popular versions of the nitroxide quasi‐equilibrium (NQE) approximation used in the theory of kinetics of alkoxyamine mediated styrene polymerization, are systematically tested by simulation comparing the approximate and exact solutions of the equations describing the system. The validity of the different versions of the NQE approximation is analyzed in terms of the relative magnitude of (dN/dt)/(dP/dt). The approximation with a rigorous NQE, k_c[P][N] = k_d[P N], where P, N and P N are living, nitroxide radicals and dormant species respectively, with kinetic constants k_c and k_d, is found valid only for small values of the equilibrium constant K (10⁻¹¹–10⁻¹² mol · L⁻¹) and its validity is found to depend strongly of the value of K. On the other hand, the relaxed NQE approximation of Fischer and Fukuda, k_c[P][N] = k_d[P N]₀ was found to be remarkably good up to values of K around 10⁻⁸ mol · L⁻¹. This upper bound is numerically found to be 2–3 orders of magnitude smaller than the theoretical one given by Fischer. The relaxed NQE is a better one due to the fact that it never completely neglects dN/dt. It is found that the difference between these approximations lies essentially in the number of significant figures taken for the approximation; still this subtle difference results in dramatic changes in the predicted course of the reaction. Some results confirm previous findings, but a deeper understanding of the physico‐chemical phenomena and their mathematical representation and another viewpoint of the theory is offered. Additionally, experiments and simulations indicate that polymerization rate data alone are not reliable to estimate the value of K, as recently suggested.

Validity of the rigorous nitroxide quasi‐equilibrium assumption as a function of the nitroxide equilibrium constant.     

Negative thermal expansibility change for dissociation of lysozyme variant amyloid protofibril

A disulfide‐deficient variant of hen lysozyme, 0SS, is known to form an amyloid protofibril spontaneously, and to dissociate into monomers at high hydrostatic pressure. We carried out native PAGE at various temperatures (20–35°C) and pressures (0.1–200 MPa), to characterize the dissociation equilibrium of disulfide‐deficient variant of hen lysozyme amyloid protofibril. Based on the density profiles, the partial molar volume and thermal expansibility changes for dissociation, Δv_D and Δe_D, were obtained to be ?74 cm³/mol at 25°C and ?2.3 cm³ mol^?1 K^?1, respectively. The dissociation of amyloid fibril destroys the cross β‐structure, and such conformational destruction in native protein fold rarely accompanies negative thermal expansibility change. We discussed the negative thermal expansibility change in terms of hydration and structural packing of the amyloid protofibril.     

Dielectric properties of a liquid of dipole hard spheres

The Kirkwood factor g _K of a model polar liquid of dipolar hard spheres (DHSs) was approximated by analytical equations using the approximation of the interaction of the second neighbors within the hindered rotation model. The derived equations describe the temperature and density dependences of the dielectric functions of the DHS liquid.     

Microwave‐Assisted Solvothermal Synthesis of VO2 Hollow Spheres and Their Conversion into V2O5 Hollow Spheres with Improved Lithium Storage Capability

Monodispersed hierarchically structured V₂O₅ hollow spheres were successfully obtained from orthorhombic VO₂ hollow spheres, which are in turn synthesized by a simple template‐free microwave‐assisted solvothermal method. The structural evolution of VO₂ hollow spheres has been studied and explained by a chemically induced self‐transformation process. The reaction time and water content in the reaction solution have a great influence on the morphology and phase structure of the resulting products in the solvothermal reaction. The diameter of the VO₂ hollow spheres can be regulated simply by changing vanadium ion content in the reaction solution. The VO₂ hollow spheres can be transformed into V₂O₅ hollow spheres with nearly no morphological change by annealing in air. The nanorods composed of V₂O₅ hollow spheres have an average length of about 70 nm and width of about 19 nm. When used as a cathode material for lithium‐ion batteries, the V₂O₅ hollow spheres display a diameter‐dependent electrochemical performance, and the 440 nm hollow spheres show the highest specific discharge capacity of 377.5 mAhg^?1 at a current density of 50 mAg^?1, and are better than the corresponding solid spheres and nanorod assemblies.     

Design and characterization of Ni2+ and Co2+ decorated Porous Magnetic Silica spheres synthesized by hydrothermal-assisted modified-Stöber method for His-tagged proteins separation

The complete elimination of enzymes from the reaction mixture and the possibility of its recycling for several rounds result in great benefits, allowing the reduction of the enzyme consumption and their usability in continuous processes. In this work, it is evaluated the capture of a H6-tagged green fluorescence protein (GFP-H6) on porous magnetic spheres using the Co²⁺ and Ni²⁺ affinity adsorption as a possible cost-effective and up-scaled alternative way for the immobilization of His-tagged proteins. For this purpose, Porous Magnetic Silica (PMS) spheres were synthesized by one-step hydrothermal-assisted modified-Stöber method. The obtained spheres have a homogenous size distribution of 400 nm diameter. The γ-Fe₂O₃ nanoparticles are homogenously distributed in the silica matrix. The obtained PMS spheres have a saturation magnetization of about 10 emu/g. Magnetophoresis measurements show a total separation time of 16 min at 60 T/m. The obtained PMS spheres were successfully and homogenously decorated with Co²⁺ and Ni²⁺ and then evaluated for the capture of a GFP-H6 protein. The results were compared with the performance of the commercial beads Dynabeads® His-Tag Isolation &; Pulldown.     

反应性模板法合成硫化银中空球-银纳米粒子异质结构

以Cu₂S中空球为反应性模板, 通过在水溶液中与银离子的阳离子交换和氧化还原反应制备了大小均匀的Ag₂S中空球-Ag纳米粒子异质结构, 即Ag₂S-Ag异质中空球. 该异质结构中每个Ag₂S中空球的直径约为600 nm, 壁厚约20–30 nm, 其表面均附着一个Ag纳米粒子. 采用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、X射线衍射(XRD)和能量色散X射线谱(EDS)对所得Ag₂S-Ag异质中空球的结构和组成进行了表征. 若以CuS中空球为反应性模板, 在相似转化条件下则主要得到不含Ag粒子的Ag₂S中空球. 该结果表明, Cu₂S中的Cu(I)的还原性在Ag₂S-Ag异质中空球的形成中发挥了重要作用. 通过对所制备的Ag₂S-Ag异质中空球进行二次生长, 还可以得到Ag₂S中空球的半球表面均被Ag膜所包覆的Ag₂S-Ag异质中空球.     

Structure of the CO bond on supported Pd particles: influence of size and surface state

We have investigated the surface of supported palladium particles by static secondary ion mass spectrometry (SSIMS). Pd particles were grown in situ on alumi na (oxide layer and sapphire surfaces) and stabilized by heating treatment. The particle size, density and crystallographic structure were determined in previous studies by transmission electron microscopy and diffraction (TEM and TED). Various ionic species are detected by SSIMS analysis which makes it possible to characterize the CO absorbed layer: Pd _n CO⁺ (n=1, 2) for molecular adsorption and Pd _n C⁺ for CO dissociation. The size dependence of the bonding state of CO (linear, bridge, ...) was monitored by: PdCO⁺/σ _n Pd _n CO⁺ signal ratio over various size particles (mean diameter in the 2–9 nm range). Investigations were performed as a function of CO coverage (adsorption at room temperature) and also under CO dissociation conditions: heating under CO atmosphere or CO+O₂ (catalysis). The data analysis shows that on clean Pd particles smaller than 3 nm the CO molecules give rise mainly to PdCO⁺ species. We have interpreted this result by the adsorption of CO on two palladium atoms, the carbon end being tightly bonded to a low coordination Pd atom and the oxygen end weakly bonded to a neighbour Pd atom. These couples of Pd atoms form the specific sites for CO dissociation, the density of which depends on the roughness of the particle surface.     

Ion Product of Pure Water Characterized by Physics-Based Water Model

Pure water has been characterized for nearly a century, by its dissociation into hydronium (H₃O)¹⁺ and hydroxide (HO)^1- ions. As a chemical equilibrium reaction, the equilibrium constant, known as the ion product or the product of the equilibrium concentration of the two ion species, has been extensively measured by chemists over the liquid water temper-ature and pressure range. The experimental data have been nonlinear least-squares fitted to chemical thermodynamic-based equilibrium equations, which have been accepted as the industrial standard for 35 years. In this study, a new and statistical-physics-based water ion product equation is presented, in which, the ions are the positively charged protons and the negatively charged proton-holes or prohols. Nonlinear least squares fits of our equation to the experimental data in the 0-100 ℃ pure liquid water range, give a factor of two better precision than the 35-year industrial standard.     

Reduction of silver cation in the cavity of diaza-18-crown-6: A theoretical study

The structure and stability of diaza-18-crown-6 (1) complex with silver cation was studied by the density functional method with the PBE functional. The reduction of the cation in the macrocycle cavity was simulated and possible stability of the resulting van der Waals complex of the crown ether 1 with silver atom was analyzed. It is shown that, after electron capture, two equilibrium conformers of Ag⁺·1 give two structures locally stable with respect to the dissociation into the silver atom and the crown ether in its nearest equilibrium conformation. One of the neutral structures, that of the C _s symmetry, corresponds to a global minimum on the potential energy surface of the Ag⁰—1 system. It ensures the thermodynamic stability of the reduced complex with respect to dissociation. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 395–399, March, 2007.     

Urea-Induced Dissociation of Nerve Growth Factor from Venom of Chinese Cobra by SEC

The urea-induced dissociation of nerve growth factor from venom of Chinese cobra (_cNGF) was studied by intrinsic fluorescence emission spectra, SEC, urea-gradient polyacrylamide gel electrophoresis, assays of biological activity and thermodynamic parameters. The results showed that when urea concentration was lower than or equal to 4.0 mol L⁻¹ or higher than or equal to 8.0 mol L⁻¹, _cNGF existed only in native homodimer form or monomer form, respectively; whereas when urea concentration was higher than 4.0 mol L⁻¹ and lower than 8.0 mol L⁻¹, they existed simultaneously in the native homodimer and monomer forms and the former decreased, while the latter increased with the increase in urea concentration. Based on the association–dissociation equilibrium between _cNGF and urea molecules, an equation, which includes two characteristic dissociation parameters K and ∆m, was presented to describe the urea-induced dissociation process of _cNGF. As the reaction temperature increased from 15 to 35 °C, positive enthalpy and entropy changes were observed, and the parameter K increased from 2.72 × 10⁻¹³ to 5.18 × 10⁻¹² (L mol⁻¹), while the parameters ∆m and ∆G, respectively, decreased from 10.18 to 8.42 and from −10.27 to −18.67 (kJ mol⁻¹), which means that the urea-induced dissociation of _cNGF was spontaneous and entropy-driven and the higher temperature was favorable for the dissociation process. Using the procedures and equations mentioned in the paper, the urea-induced dissociation of _cNGF is first comprehensively described. Furthermore, this work presents a useful method for people to study the dissociation of dimer or multimer proteins induced by denaturants, inducers, pH, etc.

     

Facile and Controllable Synthesis of Monodisperse CaF2 and CaF2:Ce3+/Tb3+ Hollow Spheres as Efficient Luminescent Materials and Smart Drug Carriers

Highly uniform and well‐dispersed CaF₂ hollow spheres with tunable particle size (300–930 nm) have been synthesized by a facile hydrothermal process. Their shells are composed of numerous nanocrystals (about 40 nm in diameter). The morphology and size of the CaF₂ products are strongly dependent on experimental parameters such as reaction time, pH value, and organic additives. The size of the CaF₂ hollow spheres can be controlled from 300 to 930 nm by adjusting the pH value. Nitrogen adsorption–desorption measurements suggest that mesopores (av 24.6 nm) exist in these hollow spheres. In addition, Ce³⁺/Tb³⁺‐codoped CaF₂ hollow spheres can be prepared similarly, and show efficient energy transfer from Ce³⁺ to Tb³⁺ and strong green photoluminescence of Tb³⁺ (541 nm, ⁵D₄→⁷F₅ transition of Tb³⁺, the highest quantum efficiency reaches 77 %). The monodisperse CaF₂:Ce³⁺/Tb³⁺ hollow spheres also have desirable properties as drug carriers. Ibuprofen‐loaded CaF₂:Ce³⁺/Tb³⁺ samples still show green luminescence of Tb³⁺ under UV irradiation, and the emission intensity of Tb³⁺ in the drug‐carrier system varies with the released amount of ibuprofen, so that drug release can be easily tracked and monitored by means of the change in luminescence intensity. The formation mechanism and luminescent and drug‐release properties were studied in detail.     

Urea-Induced Dissociation of Nerve Growth Factor from Venom of Chinese Cobra by SEC

The urea-induced dissociation of nerve growth factor from venom of Chinese cobra (_cNGF) was studied by intrinsic fluorescence emission spectra, SEC, urea-gradient polyacrylamide gel electrophoresis, assays of biological activity and thermodynamic parameters. The results showed that when urea concentration was lower than or equal to 4.0 mol L^?1 or higher than or equal to 8.0 mol L^?1, _cNGF existed only in native homodimer form or monomer form, respectively; whereas when urea concentration was higher than 4.0 mol L^?1 and lower than 8.0 mol L^?1, they existed simultaneously in the native homodimer and monomer forms and the former decreased, while the latter increased with the increase in urea concentration. Based on the association–dissociation equilibrium between _cNGF and urea molecules, an equation, which includes two characteristic dissociation parameters K and ?m, was presented to describe the urea-induced dissociation process of _cNGF. As the reaction temperature increased from 15 to 35 °C, positive enthalpy and entropy changes were observed, and the parameter K increased from 2.72 × 10^?13 to 5.18 × 10^?12 (L mol^?1), while the parameters ?m and ?G, respectively, decreased from 10.18 to 8.42 and from ?10.27 to ?18.67 (kJ mol^?1), which means that the urea-induced dissociation of _cNGF was spontaneous and entropy-driven and the higher temperature was favorable for the dissociation process. Using the procedures and equations mentioned in the paper, the urea-induced dissociation of _cNGF is first comprehensively described. Furthermore, this work presents a useful method for people to study the dissociation of dimer or multimer proteins induced by denaturants, inducers, pH, etc.