Ruthenium nanoparticles in ionic liquids: structural and stability effects of polar solutes

Ionic liquids are a stabilizing medium for the in situ synthesis of ruthenium nanoparticles. Herein we show that the addition of molecular polar solutes to the ionic liquid, even in low concentrations, eliminates the role of the ionic liquid 3D structure in controlling the size of ruthenium nanoparticles, and can induce their aggregation. We have performed the synthesis of ruthenium nanoparticles by decomposition of [Ru(COD)(COT)] in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C(1)C(4)Im][NTf(2)], under H(2) in the presence of varying amounts of water or 1-octylamine. For water added during the synthesis of metallic nanoparticles, a decrease of the solubility in the ionic liquid was observed, showed by nanoparticles located at the interface between aqueous and ionic phases. When 1-octylamine is present during the synthesis, stable nanoparticles of a constant size are obtained. When 1-octylamine is added after the synthesis, aggregation of the ruthenium nanoparticles is observed. In order to explain these phenomena, we have explored the molecular interactions between the different species using (13)C-NMR and DOSY (Diffusional Order Spectroscopy) experiments, mixing calorimetry, surface tension measurements and molecular simulations. We conclude that the behaviour of the ruthenium nanoparticles in [C(1)C(4)Im][NTf(2)] in the presence of 1-octylamine depends on the interaction between the ligand and the nanoparticles in terms of the energetics but also of the structural arrangement of the amine at the nanoparticle's surface.     

On the thermodynamic stability and structural transition of clathrate hydrates

Gas mixtures of methane and ethane form structure II clathrate hydrates despite the fact that each of pure methane and pure ethane gases forms the structure I hydrate. Optimization of the interaction potential parameters for methane and ethane is attempted so as to reproduce the dissociation pressures of each simple hydrate containing either methane or ethane alone. An account for the structural transitions between type I and type II hydrates upon changing the mole fraction of the gas mixture is given on the basis of the van der Waals and Platteeuw theory with these optimized potentials. Cage occupancies of the two kinds of hydrates are also calculated as functions of the mole fraction at the dissociation pressure and at a fixed pressure well above the dissociation pressure.     

Thermal vacancy effects on the thermodynamic properties and stability of fullerites

We study the influence of thermal vacancies on equilibrium thermodynamic properties of the high-temperature phase of fullerites taking into account the strong anharmonicity of the lattice vibrations. Treating a crystal with point defects as a quasi-multicomponent system and using the correlative method of the unsymmetrized self-consistent field and the Girifalco interaction potential for the molecular subsystem, we have obtained the vacancy formation parameters for the C(60) fullerite. We also take into account the divacancies. The influence of the lattice defects on the specific heats of fullerites is negligible, since a dominant contribution to them is given by intramolecular degrees of freedom. We have calculated the contributions of vacancies to various thermodynamic properties, the volume thermal expansion coefficient, the isothermal bulk modulus, and the components of the isothermal elastic tensor, depending on the temperature and pressure. Near the estimated triple point, these contributions run to more than 10% and increase still further at a metastable region. We also discuss the influence of defects on the thermodynamic stability of fullerites.     

The scaled-particle theory and solvent isotope effects on the thermodynamic properties of inert solutes in water and methanol

The scaled-particle theory has been applied to the calculation of the thermodynamic changes associated with the formation of a cavity in several isotopic varieties of liquid water and methanol. From these results, the thermodynamic functions for the transfer of a cavity (or a hard-sphere solute) have been computed for the following solvent pairs: H₂OD₂O, H₂OH₂ ¹⁸O, H₂ ¹⁸OD₂ ¹⁸O, D₂OD₂ ¹⁸O, CH₃OHCH₃OD. For the last two of these solvents, density measurements required for the calculations were carried out as a function of temperature. The calculated deuterium solvent isotope effect on the heats and entropies of hard-sphere solutes in water is much greater than the¹⁸O isotope effect; the former also exhibits a more pronounced temperature dependence. The transfer functions computed for hard-sphere solutes are compared to experimental data on the transfer of various solutes from H₂O to D₂O and from CH₃OH to CH₃OD. In most of the cases examined, the cavity effect accounts for a large part of the transfer quantities measured for rare gases, hydrocarbons, and solutes containing a significant hydrocarbon substituent.     

Simultaneous determination of kinetic and thermodynamic parameters from fast-reaction kinetic measurements

A combined stopped-flow temperature-jump apparatus interfaced with a dedicated microcomputer has been used to study the complexation reaction of iron(III) with thiocyanate in aqueous solution. Kinetic rate-constants (k(f) = 143 l.mole(-1) .sec(-1) from T-jump, k(f) = 150 l.mole(-1) .sec(-1) from stopped flow), equilibrium constants (K = 143 from T-jump, K = 150 from stopped flow) and the thermodynamic enthalpy change (DeltaH(c) = -6.7 kJ/mole) could be independently determined from the simultaneous application of the two techniques.     

Oxidative thermal degradation of LDPE and the determination of some thermodynamic quantities

The pyrolysis of polyolefin wastes is one of the possible ways to obtain chemical feedstocks. In this work, the thermal degradation of low density polyethylene, (LDPE), which is a major product within plastics, was investigated in a semi-batch reactor system. First-order rate kinetics approach was chosen and reaction rate coefficients, k, and some thermodynamic quantities determined such as activation energy, reaction enthalpy, free activation enthalpy, and entropy of degradation of LDPE for different air flow rates. We found that the maximum value of some thermodynamic quantities, such as reaction rate coefficient is 0.0243 min⁻¹ at 600 mL min⁻¹ air flow rate and the free activation enthalpy (ΔG^≠) is 148.66 kJ mol⁻¹ at 450 mL min⁻¹ air flow rate and the reaction enthalpy (ΔH^≠) is 57.65 J mol⁻¹ at 623 K temperature conditions. Moreover, we found that the oxidative degradation of LDPE is not spontaneous and has lower energy necessary (for degradation) than non-oxidative degradation processes.     

X-ray structural parameters and the thermal stability of 1,2-dioxetanes

The structural parameters and thermal activation data for 1,2-dioxetanes are reported, showing that the degree of puckering of the peroxide ring does not influence the thermal stability of these “high energy” molecules.     

Effects of nonpolar solutes on the thermodynamic response functions of aqueous mixtures

We investigate the effect of adding nonpolar solutes at atmospheric pressure on water's temperature of maximum density, isothermal compressibility, and isobaric heat capacity, using a statistical mechanical model of water solutions [H. S. Ashbaugh, T. M. Truskett, and P. G. Debenedetti, J. Chem. Phys. 116, 2907 (2002)]. We find that the temperature of maximum density increases with solute hydrophobicity, as characterized by its size, and decreases with its van der Waals attractive parameter a, in agreement with experiment. We predict similar trends for the addition of solutes on the isothermal compressibility and isobaric heat capacity: solute hydrophobicity causes an upward shift in water's anomalies, whereas dispersive interactions as measured by the solute's van der Waals attractive parameter shift the anomalies to lower temperatures. The locus along which the competing contributions of solute size sigma and interaction strength a to the shift in water's response functions balance each other obeys the scaling relationship sigma6 approximately a.     

Simultaneous determination of carbohydrates and products of carbohydrate metabolism in fermentation mixtures by HPLC

An improved procedure for separating and quantitating carbohydrates, alcohols, and organic acids in fermentation mixtures metabolized by intestinal microflora is described. The high-pressure liquid chromatographic method is efficient, reproducible, and sensitive. A column packed with cation-exchange resin in the hydrogen form, eluted isocratically with 0.028 M H2SO4 at 40 degrees C separates the compounds of interest. The eluate is monitored with ultraviolet and refractive index detectors in series. On-line acquistion and storage of detector output by a computer allows post-analysis data manipulation and quantitation. Using this method, the metabolic profiles for the fermentation of glucose, fructose, lactose, and sucrose by several intestinal microorganisms are characterized and compared.     

Simultaneous determination of nitrogen and lithium by thermal neutron activation analysis

The determination of lithium and nitrogen in a variety of materials by thermal neutron activation is described. The nuclear reactions used are ¹⁴N(n,p)¹⁴C and ⁶ Li(n,α)³H. Radionuclides. ¹⁴C and ³H for counting are isolated by fusion of the irradiated sample in a vacuum system. Data are presented on lithium and nitrogen concentrations in several terrestrial standards. The new method allows reliable measurements on 10–50-mg samples.     

Simultaneous determination of stoichiometry, degree of condensation and stability constant A generalization of the molar-ratio method

A generalization of the molar-ratio method is proposed, which allows study of relatively weak complexes. The method is based on treatment of the data from a molar-ratio saturation curve. From the mathematical expression derived, the stoichiometry, degree of condensation and stability constant are easily evaluated. Graphical representations of the results can be used advantageously.     

A rare porous zinc phosphonocarboxylate framework with high thermal stability and interesting structural transformation

A rare porous metal-phosphonocarboxylate framework with ultrahigh thermal stability over 500℃ was obtained, which can be transformed into three different cluster-based frameworks with the same CaF₂-type topology.     

Structural effects of DNA on thermodynamic stability of DNA-dendronized polymer nanoclusters and nanoclusteration process

We studied the means by which DNA-dendronized polymer nanoclusters and the nanoclusteration process are affected by structural properties of the nanocluster components, including the length of dendronized polymer, wrapping radius of DNA, and surface charge densities on the DNA and dendronized polymer, by calculating the total free energy of the system and free energy of the nanoclusteration process. The most thermodynamically stable nanocluster conformation was then predicted based on the values of the free energies. It was found that the nanoclusters with longer dendronized polymers, shorter DNA wrapping radius, and larger surface charge density on both the DNA and dendronized polymers are more stable.     

A thermodynamic model for the shape and stability of twinned nanostructures

Although thermodynamically metastable, planar defects are often observed in many faceted nanomaterials including nanocrystals, nanorods, and nanowires, even after annealing. These planar defects include contact twins and (intrinsic or extrinsic) stacking faults, and are usually neglected by most analytical models. For example, many bulk metals have the face-centered cubic structure, but small nanocrystals and nanorods of the same material often exhibit various structural and morphological modifications such as single or multiple symmetric twinning, as well as 5-fold cyclic twinning resulting in decahedral and truncated decahedral nanostructures. Presented here is a general analytical model for the investigation of nanomaterials of arbitrary shape, and with any configuration of planar defects. The model is tested for the case of twinning in unsupported gold nanocrystals and nanorods, and is shown to give results in excellent agreement with experimental and computational studies reported in the literature.     

The effects of mechanical properties of DNA on the thermodynamic stability of the DNA-dendronized polymer nanocluster

The effects of the mechanical properties of DNA, such as bending, twisting, and bending-twisting interactions on the thermodynamic stability of DNA-dendronized polymer nanoclusters with different conformations in terms of free energy are investigated. The effect of temperature on the free energy is also studied and the values of enthalpy and entropy of the solution of the nanocluster are predicted. The obtained thermodynamic quantities help us to have a better understanding about the mechanical properties of DNA and the stability of the nanocluster in gene therapy.     

A review on the thermal stability of calcium apatites

High temperature processing is essential for the preparation of apatites for biomaterials, lighting, waste removal and other applications. This requires a good understanding of the thermal stability and transitions upon heating. The most widely used is hydroxyapatite (HAp), but increasing interest is being directed to fluorapatite (FAp) and chlorapatite (ClAp). The structural modifications for substitutions are discussed to understand the temperature processing range for the different apatites. This is based on a review of the literature from the past few decades, together with recent research results. Apatite thermal stability is mainly determined by the stoichiometry (Ca/P ratio and structural substitutions) and the gas composition during heating. Thermal stability is lowered the most by a substitution of calcium and phosphate, leading to loss in phase stability at temperatures less than 900?°C. The anions in the hexagonal axis, OH in HAp, F in FAp and Cl in ClAp are the last to leave upon heating, and prevention of the loss of these groups ensures high temperature stability. The information discussed here will assist in understanding the changes of apatites during heating in calcination, sintering, hydrothermal processing, plasma spraying, flame pyrolysis, and other high-temperature processes.     

Simultaneous thermal and surfactant-induced Marangoni effects in thin liquid films

The deformation of a thin liquid film in the presence of a surfactant monolayer, varying temperature distributions, and limited mass flux is considered. Use of lubrication theory yields a coupled pair of partial differential equations for the film height and surfactant surface monolayer concentration. The long-wave stability of the isothermal film is examined over a wide range of parameter values. It is shown that droplet patterns are obtained under certain thermal conditions for both an isothermal and nonisothermal underlying substrate. For the case of a localized thermal gradient initially imposed at the air-liquid interface, severe film thinning beneath the heat source was observed, which was not accompanied by droplet formation; pseudo steady states are observed in this case. In all situations the surfactant is found to rigidify the air-liquid interface, retarding thermally driven flow, while evaporation (condensation) acts to destabilize (stabilize) the film.     

Enhanced thermal stability and structural characteristics of different MMT-Clay/epoxy-nanocomposite materials

Epoxy-clay nanocomposites, HDTMA-BDGE, HDTMA-BPDG, HDTMA-BBDG, HDTMA-TGDDM and HDTPP-BDGE were synthesized using hexadecylammonium clay and hexadecylphosphonium clay, respectively. The Montmorillonite (MMT) clay was modified with quaternary ammonium salt and with triphenylphosphonium salt which was intercalated into the interlayer region of MMT-Clay. The epoxy-clay systems were cured by using diaminodiphenylsulphone as a curing agent. The X-ray diffraction patterns obtained for the systems confirmed the nanodispersion of MMT-Clay in the epoxy networks. The ammonium clay-modified systems displayed appreciable mechanical and glass-transition temperature properties while, the phosphonium clay-modified system exhibited highest thermal resistance properties compared with unmodified epoxy systems. The T_g decrease observed in all the clay-modified epoxy systems, may be compromised with their advantage of requiring the filler content very low (5wt%), when compared to the conventional epoxy systems whose filler quantity is normally required from 25 to 30 wt%.     

The effect of TiO2 additives on the structural stability and thermal properties of yttria fully-stabilized zirconia

TiO₂(0–20 mol%)-8 mol% YSZ (8YSZ) ceramics were synthesized by a traditional solid-state reaction method. A cubic single phase was observed for 8YSZ, 4 mol% TiO₂-8YSZ and 8 mol% TiO₂-8YSZ. Tetragonal and cubic mixed phases were observed for 12–20 mol% TiO₂-8YSZ ceramics. The sintering temperature was 1,700 °C for 8YSZ and 4 mol% TiO₂-8YSZ ceramics, whereas it was 1,500 °C for 8–20 mol% TiO₂-8YSZ. The thermal conductivity at room temperature decreased in proportion to increasing TiO₂ content, from 3.0 to 2.3 W/m K. The specific heat of TiO₂-8YSZ ceramics was unaltered as the TiO₂ content changed.