Study of association of alcohols in aprotic solvents by multinuclear NMR

Binary mixtures of alcohols(ethyl,n-butyl and n-amyl)with several aprotic sol-vents,such as acetone,dioxane,THF,DMSO and DMF have been studied systematically by~1H,~(13)C,~(15)N and ~(17)O NMR measurements.The concentration dependence of chemical shift ofthe solvent was used to evaluate equilibrium constants of the complexation of alcohols withthe solvents.A relationship between the proton shift of alcohol uncombined with the solventand its concentration was found,and the fraction of unassociated hydroxyl groups was thusquantitatively described.The effect of solvent on self association and complexation of thealcohol is discussed on the basis of the electron donicity of the solvents.     

A multinuclear NMR study on the microscopic ionization constants of adenosine-5′-triphosphate in aqueous solution

A ¹H, ¹³C and ³¹P NMR study of adenosine-5′-triphosphate has been performed as a function of pH. The examination of the chemical shifts allowed the proposal of an ionization scheme in which the third deprotonation takes place both on the adenine ring and on the phosphate group in a ratio of about 2:1. Furthermore, an analysis of carbon relaxation times supports this mechanism of ionization and confirms that the adenine ring binds the proton both on the NH₂ group and the N₇ nitrogen atom.     

Solubility and preferential solvation of phenacetin in methanol + water mixtures at 298.15 K

The mole fraction solubility of phenacetin (PNC) in methanol + water binary solvent mixtures at 298.15 K was determined along with density of the saturated solutions. All these solubility values were correlated with the Jouyban–Acree model. Preferential solvation parameters of PNC by methanol (δx_1,3) were derived from their thermodynamic solution properties using the inverse Kirkwood–Buff integrals (IKBI) method. δx_1,3 values are negative in water-rich mixtures but positive in methanol mole fraction of >0.32. It is conjecturable that in the former case the hydrophobic hydration around non-polar groups of PNC plays a relevant role in the solvation. The higher solvation by methanol in mixtures of similar cosolvent compositions and methanol-rich mixtures could be explained in terms of the higher basic behaviour of methanol.     

Elucidating structure and dynamics of crystalline α-zirconium phosphates intercalated with water and methanol by multinuclear solid-state MAS NMR: A comprehensive NMR approach

Solid-state NMR experiments on ²H, ³¹P, ¹³C, and ¹H nuclei, including ³¹P T₁, ¹H T₁, and ¹H T_1ρ measurements, as well as on the kinetics of proton-phosphorus cross-polarization have been performed to characterize the crystalline and amorphous α-zirconium phosphates, which were intercalated with D₂O and/or CD₃OD. The ¹³C{¹H} CP MAS NMR experiment performed for compound 1-CD ₃ OD (Zr (HPO₄)₂ ^. 0.2CD₃OD) with carbon cross-polarization via protons of phosphate groups has provided a prove that the methanol was intercalated into the interlayer spaces of this compound. The variable-temperature ²H solid-echo MAS NMR spectra of intercalated compounds demonstrated that the methanol molecules, in contrast to the mobile water, were immobile, keeping, however, free CD₃ rotations around the C₃-axis. It has been demonstrated that the intercalated species, D₂O and CD₃OD, do not affect the high-frequency motions of the phosphate groups. By utilizing local structural models that satisfy the constraints of the experimental data, it has been suggested that the immobile methanol molecules are located in the cavity between two neighboring layers of the zirconium phosphates. Thus, the present work illustrates the reliable criteria in a comprehensive NMR approach to structural and dynamic studies of such systems.     

Investigation of nanodispersion in polystyrene–montmorillonite nanocomposites by solid-state NMR

Nanocomposites result from combinations of materials with vastly different properties in the nanometer scale. These materials exhibit many unique properties such as improved thermal stability, reduced flammability, and improved mechanical properties. Many of the properties associated with polymer–clay nanocomposites are a function of the extent of exfoliation of the individual clay sheets or the quality of the nanodispersion. This work demonstrates that solid-state NMR can be used to characterize, quantitatively, the nanodispersion of variously modified montmorillonite (MMT) clays in polystyrene (PS) matrices. The direct influence of the paramagnetic Fe³⁺, embedded in the aluminosilicate layers of MMT, on polymer protons within about 1 nm from the clay surfaces creates relaxation sources, which, via spin diffusion, significantly shorten the overall proton longitudinal relaxation time (T). Deoxygenated samples were used to avoid the particularly strong contribution to the T of PS from paramagnetic molecular oxygen. We used T as an indicator of the nanodispersion of the clay in PS. This approach correlated reasonably well with X-ray diffraction and transmission electron microscopy (TEM) data. A model for interpreting the saturation-recovery data is proposed such that two parameters relating to the dispersion can be extracted. The first parameter, f, is the fraction of the potentially available clay surface that has been transformed into polymer–clay interfaces. The second parameter, ϵ, is a relative measure of the homogeneity of the dispersion of these actual polymer–clay interfaces. Finally, a quick assay of T is reported for samples equilibrated with atmospheric oxygen. Included are these samples as well as 28 PS/MMT nanocomposite samples prepared by extrusion. These measurements are related to the development of high-throughput characterization techniques. This approach gives qualitative indications about dispersion; however, the more time-consuming analysis, of a few deoxygenated samples from this latter set, offers significantly greater insight into the clay dispersion. A second, probably superior, rapid-analysis method, applicable to oxygen-containing samples, is also demonstrated that should yield a reasonable estimate of the f parameter. Thus, for PS/MMT nanocomposites, one has the choice of a less complete NMR assay of dispersion that is significantly faster than TEM analysis, versus a slower and more complete NMR analysis with sample times comparable to TEM, information rivaling that of TEM, and a substantial advantage that this is a bulk characterization method. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3188–3213, 2003     

Large-scale and narrow dispersed latex formation in batch emulsion polymerization of styrene in methanol–water solution

This work is an extension of previous research results reported by our team (Colloid Polym Sci 291:2385–2389, 2013), where monodisperse, large-scale, and high-solid-content latexes of poly(n-butyl acrylate) were obtained with the particle coagulation method induced by the electrolyte. However, large-scale polystyrene latex particle is difficult to synthesize with this approach; moreover, demulsification phenomena easily take place especially in high solid content. In this article, a new approach to prepare large-scale polystyrene latex particle was proposed. Methanol was added to aqueous phases to decrease the interfacial tension between the polymer particle surface and continual phases, further decreasing interfacial free energy. Consequently, the surfactant molecules would loosely pack on the polymer particle surface, which is favored by particle coagulation. Experimental investigations showed that the final polystyrene particle scale only reaches to 93.5 nm when the methanol/water ratio is equal to 0:100, but the particle size attains 270 nm when the methanol/water ratio is equal to 30:70. These results indicated that polystyrene particle coagulation can be induced by methanol by varying the surfactant molecule adsorption on the particle surface. This investigation also provided a new simple approach to prepare large-scale, stable latex particles.     

Structure of С@Al<Subscript>2</Subscript>O by multinuclear solid-state NMR spectroscopy

The ²⁷Al, ¹H MAS NMR method is used to study initial nanosized metastable aluminum oxides of a pseudoboehmite series (γ- and δ-Al₂O₃) after being coated with graphene (С@Al₂O₃) and annealed in the air (С@Al₂O₃-Т). It is demonstrated that aluminum nanoparticles coated with graphene and annealed at high temperatures (to 750°C for γ and 1180°C for δ) preserve their phase composition but differ from initial oxides by a very low concentration of defects (ОН groups). After the annealing of the graphene coating the hydroxyl cover of oxides is reduced, however, the set of ОН groups differs greatly from that of the initial oxides. Only one type of terminal ОН groups with a ~0.2 ppm shift and one type of bridging μ²-ОН groups with a 1.8 ppm shift for γ-Al₂O₃ from OH-μ²-AlVAl_n and 2.1 ppm for δ-Al₂O₃ OH-μ²-AlIVAl_n are observed. The data obtained make it possible to characterize in detail the δ-Al₂O₃ pure phase.     

The Effect of Ni2+ and Cu2+ on the Photocatalytic Degradation of Dyes by the Chitosan–TiO2 Complex

The present research combines biosorption and photocatalysis in a functional TiO(2)-immobilized chitosan adsorbent (CTA). CTA can degrade organic pollutants and adsorb metal ions simultaneously. Target pollutants were dyes of cationic (rhodamine B, Rh.B) and anionic (methyl orange, MO) nature, with Ni(2+) and Cu(2+) selected as heavy metals. The presence of Ni(2+) or Cu(2+) improved the degradation ability of CTA for MO, but inhibited the degradation of Rh.B, with Cu(2+) exhibiting stronger effects than Ni(2+). There was no significant difference in CTA activity when the metal ions were pre-adsorbed or when they coexisted in the solution with the organic dyes. Protons in the reaction system affected the degradation performance in a similar way for Ni(2+) and Cu(2+) leading to a different effect on the degradation for MO and Rh.B. An X-ray photoelectron spectroscopy analysis of the binding energies of the metal ions on the surface in the presence of the cationic or anionic dyes explained the different behaviors. Since anionic and cationic dyes possess chromogenic groups of different charges, they adversely affect the production of OH? radicals when coexisting with Cu(2+) or Ni(2+).     

Changes in the composition of the solvation shell of lithium ions in γ-butyrolactone upon addition of 15-crown-5 as studied by quantum chemistry

Quantum chemical modeling of Li⁺ ion transfer from the solvation shell of γ-butyrolactone (GBL) as the solvent to the cavity of 15-crown-5 (15C5) macrocyclic ligand was carried out. Calculations were performed using the PBE nonempirical density functional and an extended basis for the SBK pseudopotential. The solvation energy was included in the framework of the polarizable continuum model. The calculated geometric parameters of GBL and 15C5 molecules are in good agreement with experimental X-ray data. The energies and structures of the Li(GBL) _n ⁺ (n = 1–5) complexes and Li(GBL) _m (15C5)⁺ (m = 0–3) mixed complexes were calculated. The binding energy of the fifth GBL molecule is low; therefore, the Li⁺ ion is mainly surrounded by four GBL molecules. The formation of mixed complexes by consecutive displacement of GBL molecules from the solvation shell of the lithium ion leads to structures with the coordination number 5. The equilibrium constants of these processes were used to determine the dependence of the composition of the solvation complexes on the concentration of 15C5 in the system. The concentrations of the Li(15C5)⁺ and Li(GBL)(15C5)⁺ complexes appeared to be comparable. The revealed structural features of the Li⁺ solvation complexes in the GBL-15C5 system were used to analyze the operating efficiency of lithium power sources.     

Application of phosphotungstic acid–nickel composite-modified carbon paste electrode for electrocatalytic oxidation of methanol in alkaline solution

Phosphotungstic acid (PWA) was used for accumulation of nickel ions at the carbon paste electrode for preparation of PWA-modified CPE (PWA/CPE). The PWA was evenly mixed with graphite powder and paraffin oil. Then, for preparation of Ni/PWA/CPE, Ni ions were included onto the PWA/CPE surface through immersion method at open circuit condition. The scanning electron microscopy (SEM), energy-dispersive spectroscopy and electrochemical methods were used to verify the prepared electrodes. The SEM images reveal that morphology of the CPE was influenced by PWA addition. Application of the Ni/PWA/CPE for methanol oxidation was explored by various electrochemical techniques. Electrochemical response of methanol oxidation at the surface of Ni/PWA/CPE was 2.5 times higher than that Ni/CPE. The obtained results indicated that the modified electrode exhibited high electrocatalytic activity toward methanol oxidation. Then, catalytic rate constant was found to be 8.25 × 10⁴ cm³ mol ^?1 s^?1 using chronoamperometry method. Furthermore, the effects of several parameters, such as PWA loading, NiSO₄ concentration, accumulation time and methanol concentration toward methanol oxidation at the surface of this modified electrode as well as stability, have been investigated.     

Combinatorial screening of ternary Pt–Ni–Cr catalysts for methanol electro-oxidation

Methanol electro-oxidation activity of ternary Pt–Ni–Cr system was studied by using a combinatorial screening method. A Pt–Ni–Cr thin-film library was prepared by sputtering and quickly characterized by a multichannel multielectrode analyzer. Among the 63 different composition thin-film catalysts, Pt₂₈Ni₃₆Cr₃₆ showed the highest methanol electro-oxidation activity and good stability. This new composition was also studied in its powder form by synthesizing and characterizing Pt₂₈Ni₃₆Cr₃₆/C catalyst. In chronoamperometry testing, the Pt₂₈Ni₃₆Cr₃₆/C catalyst exhibited “decay-free” behavior during 600 s operation by keeping its current density up to 97.1% of its peak current density, while the current densities of Pt/C and Pt₅₀Ru₅₀/C catalysts decreased to 14.0% and 60.3% of their peak current densities, respectively. At 600 s operation, current density of the Pt₂₈Ni₃₆Cr₃₆/C catalyst was 23.8 A g_{noble metal}⁻¹, while that of those of the Pt/C and Pt₅₀Ru₅₀/C catalysts were 2.74 and 18.8 A g_{noble metal}⁻¹, respectively.     

Pt–Ni carbon-supported catalysts for methanol oxidation prepared by Ni electroless deposition and its galvanic replacement by Pt

Pt–Ni particles supported on Vulcan XC72R carbon powder have been prepared by a combination of crystalline Ni electroless deposition and its subsequent partial galvanic replacement by Pt upon treatment of the Ni/C precursor by a solution of chloroplatinate ions. The Pt-to-Ni atomic ratio of the prepared catalyst has been confirmed by EDS analysis to be ca. 1.5:1. No shift of Pt XPS peaks has been observed, indicating no significant modification of its electronic properties, whereas the small shift of the corresponding X-ray diffraction (XRD) peaks indicates the formation of a Pt-rich alloy. No Ni XRD peaks have been observed in the XRD pattern, suggesting the existence of very small pockets of Ni in the core of the particles. The surface electrochemistry of electrodes prepared from the catalyst material suggests the existence of a Pt shell. A moderate increase in intrinsic catalytic activity towards methanol oxidation in acid has been observed with respect to a commercial Pt catalyst, but significant mass specific activity has been recorded as a result of Pt preferential confinement to the outer layers of the catalyst nanoparticles.     

Thermodynamics of cation exchange in illite: K → Ni and Sr → Ni

Exchange equilibria involving Ni²⁺ ions on K- and Sr-illites were studied using the exchange isotherms and derived thermodynamic parameters. The equilibrium was attained within 3 hr and maximum adsorption (in the vicinity of the BEC) of Ni²⁺ ions occurred at pH 5.5. The exchange at pH 5.5 and an equilibrium time of 3 hr yielded isotherms, K and ΔG₀ values which were indicative of higher affinity of Ni²⁺ ions for the illite surface than K⁺ or Sr²⁺ ions. The affinity varied with temperature. Enthalpy changes indicated a stronger binding of Ni²⁺ ions on Sr-illite than on K-illite. These inferences were also supported by entropy changes. The surface phase activity coefficients and the excess thermodynamic parameters were indicative of a non-ideal heterogeneous exchange in which the mixture of cations on the illite surface was more stable and more tightly bound with significant differences in the hydration states of the cations in the mixture from the homo-ionic form.     

The crystal structure of a valinomycin•2Ca2+ complex and the multi-step solution equilibria in acetonitrile characterised by 1H NMR,UV–Vis,and mass spectrometry

Valinomycin is a well-known potassium-selective ion carrier, but the nature of its direct interaction with calcium ion, another biologically important cation, is also of considerable interest. We have determined the first crystal structure of a valinomycin–calcium complex, which crystallises exclusively as a 1:2 valinomycin:Ca²⁺ complex, [VCa₂(OTf)₂(H₂O)₂]²⁺ in the solid state (V = valinomycin, OTf = triflate). Along with the 1:2 complex, the solution phase 1:1 and 2:1 complexes have been characterised in acetonitrile by ¹H NMR and UV–Vis titration experiments. The molar absorptivity curves and binding equilibrium constants for these complexes have been determined by the global analysis of the UV–Vis titration data using the program SIVVU?.     

Ni–Al composite hydroxides fabricated by cation–anion double hydrolysis method for high-performance supercapacitor

Chemical doping of nickel hydroxide with other cations(e.g. Al~(3+)) is an efficient way to enhance its electrochemical capacitive performances. Herein, a simple cation–anion(Ni~(2+)and AlO_2) double hydrolysis method was developed toward the synthesis of nickel–aluminum(Ni–Al) composite hydroxides. The obtained composite hydroxides possesses a porous structure, large surface area(121 m~2/g) and homogeneous element distribution. The electrochemical test shows that the obtained composite hydroxides exhibits a superior supercapacitive performances(specific capacitance of 1670F/g and rate capability of 87% from 0.5 A/g to 20 A/g) to doping-free nickel hydroxide(specific capacitance of 1227 F/g and rate capability of 47% from 0.5 A/g to 20 A/g). Moreover, the galvanostatic charge/discharge test displays that after 2000 cycles at large current density of 10 A/g, the composite hydroxides achieves a high capacitance retention of 98%, indicative of an excellent electrochemical cycleability.     

Stability of Ag@SiO_2 core–shell particles in conditions of photocatalytic overall water-splitting

Core–shell nanoparticles containing plasmonic metals(Ag or Au) have been frequently reported to enhance performance of photo-electrochemical(PEC) devices. However, the stability of these particles in water-splitting conditions is usually not addressed. In this study we demonstrate that Ag@SiO_2 core–shell particles are instable in the acidic conditions in which WO_3-based PEC cells typically operate, Ag in the core being prone to oxidation, even if the SiO_2 shell has a thickness in the order of 10 nm. This is evident from in situ voltammetry studies of several anode composites. Similar to the results of the PEC experiments, the Ag@SiO_2 core–shell particles are instable in slurry-based, Pt/ZnO induced photocatalytic water-splitting. This was evidenced by in situ photodeposition of Ag nanoparticles on the Pt-loaded ZnO catalyst, observed in TEM micrographs obtained after reaction. We explain the instability of Ag@SiO_2 by OH-radical induced oxidation of Ag, yielding dissolved Ag+. Our results imply that a decrease in shell permeability for OH-radicals is necessary to obtain stable, Ag-based plasmonic entities in photo-electrochemical and photocatalytic water splitting.