Showing papers on "Enthalpy published in 2007"

High-enthalpy hydrogen adsorption in cation-exchanged variants of the microporous metal-organic framework Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2.

Abstract: Exchange of the guest Mn2+ ions in Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2 (1-Mn2+; BTT = 1,3,5-benzenetristetrazolate) with selected cations results in the formation of isostructural framework compounds 1-M (M = Li+, Cu+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+). Similar to the parent compound, the new microporous materials are stable to desolvation and exhibit a high H2 storage capacity, ranging from 2.00 to 2.29 wt % at 77 K and 900 torr. Measurements of the isosteric heat of adsorption at zero coverage reveal a difference of 2 kJ/mol between the weakest and strongest H2-binding materials, which is attributed to variations in the strength of interaction between H2 molecules and unsaturated metal centers within each framework. The Co2+-exchanged compound, 1-Co2+, exhibits an initial enthalpy of adsorption of 10.5 kJ/mol, the highest yet observed for a microporous metal−organic framework.

Surface tensions of imidazolium based ionic liquids: anion, cation, temperature and water effect.

Abstract: This work addresses the experimental measurements of the surface tension of eight imidazolium based ionic liquids (ILs) and their dependence with the temperature (288–353 K) and water content. The set of selected ionic liquids was chosen to provide a comprehensive study of the influence of the cation alkyl chain length, the number of cation substitutions and the anion on the properties under study. The influence of water content in the surface tension was studied for several ILs as a function of the temperature as well as a function of water mole fraction, for the most hydrophobic IL investigated, [omim][PF6], and one of the more hygroscopic IL, [bmim][PF6]. The surface thermodynamic functions such as surface entropy and enthalpy were derived from the temperature dependence of the surface tension values.

Heat of Absorption of Carbon Dioxide (CO2) in Monoethanolamine (MEA) and 2-(Aminoethyl)ethanolamine (AEEA) Solutions

Abstract: A method is proposed for measuring the enthalpy of absorption of CO2 (ΔHabs) that is differential in temperature and semi-differential in loading. A 2-L reaction calorimeter was used to measure the enthalpy of absorption of CO2 with aqueous solutions of monoethanolamine (MEA) and (2-aminoethyl)ethanolamine (AEEA) over the temperature range of 40−120 °C. By feeding CO2 in several steps and keeping the change in loading between each new equilibrium rather low (∼0.03−0.07 mol CO2/mol amine), it was possible to obtain values of ΔHabs that were semi-differential in loading. Experiments conducted at different temperature levels gave enthalpy values that were differential in temperature. Measured ΔHabs values were compared with integral values found in the literature and with differential values calculated from solubility measurements. The experimental results indicate that the use of the Gibbs−Helmholtz equation in an aggregate form does not necessarily give an accurate description of the ΔHabs values.

Vapourisation of ionic liquids.

Abstract: Eight common imidazolium based ionic liquids have been successfully evaporated in ultra-high vacuum, their vapours analysed by line of sight mass spectrometry and their heats (enthalpy) of vapourisation determined. They were found to evaporate as ion pairs, with heats of vapourisation which depend primarily on the coulombic interactions within the liquid phase and the gas phase ion pair. An electrostatic model is presented relating the heats of vapourisation to the molar volumes of the ionic liquids.

The Gaseous Enthalpy of Formation of the Ionic Liquid 1-Butyl-3-methylimidazolium Dicyanamide from Combustion Calorimetry, Vapor Pressure Measurements, and Ab Initio Calculations

Abstract: Ionic liquids are attracting growing interest as alternatives to conventional molecular solvents. Experimental values of vapor pressure, enthalpy of vaporization, and enthalpy of formation of ionic liquids are the key thermodynamic quantities, which are required for the validation and development of the molecular modeling and ab initio methods toward this new class of solvents. In this work, the molar enthalpy of formation of the liquid 1-butyl-3-methylimidazolium dicyanamide, 206.2 ± 2.5 kJ·mol-1, was measured by means of combustion calorimetry. The molar enthalpy of vaporization of 1-butyl-3-methylimidazolium dicyanamide, 157.2 ± 1.1 kJ·mol-1, was obtained from the temperature dependence of the vapor pressure measured using the transpiration method. The latter method has been checked with measurements of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, where data are available from the effusion technique. The first experimental determination of the gaseous enthalpy of formation of the ion...

The Nature of Water Nucleation Sites on TiO2(110) Surfaces Revealed by Ambient Pressure X-ray Photoelectron Spectroscopy

Abstract: X-ray photoelectron spectroscopy at ambient conditions of pressure (up to 1.5 Torr) and temperature (265K

The Nature of Water Nucleation Sites on Tio (2) (110) Surfaces Revealed By Ambient Pressure X-Ray Photoelectron Spectroscopy

Abstract: X-ray photoelectron spectroscopy at ambient conditions of pressure (up to 1.5 Torr) and temperature (265 K < T < 800 K) was used to study the adsorption of water on rutile TiO{sub 2}(110) under conditions of thermodynamic equilibrium. It was found that OH groups in bridging positions, normally present in small amounts due to residual O-vacancies, act as nucleation sites for subsequent water adsorption. The adsorption enthalpy of water binding to these sites is {approx}70 kJ/mol, much stronger than that in the bulk liquid (45 kJ/mol). A model is proposed that relates the structure of the oxide surface to its hydrophilic character.

Enthalpy Relaxation and Embrittlement of Poly(l-lactide) during Physical Aging

Abstract: The enthalpy relaxation behavior of poly(l-lactide) (PLLA) below glass transition temperature (Tg) and its effects on the mechanical properties were studied using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), and tensile test. Both the enthalpy loss (δH) and peak endothermic temperature (Tp) in the glass transition region show a linear increase with the logarithm of aging time (ta). On the basis of DSC results, the kinetics of the enthalpy relaxation process were analyzed, and the kinetic parameters of PLLA were obtained: relaxation rate of enthalpy at 40 °C βH = 1.77 J g-1 per decade, and apparent activation energy Δh* = 1107 kJ mol-1. Besides, analysis on the kinetic of enthalpy relaxation for poly(dl-lactide) (PDLLA) shows that the Δh* value of PDLLA is similar to that of PLLA. Moreover, it was found that the strength and modulus of PLLA increase gradually; on the contrary, the toughness reduces dramatically during the physical aging. The yield strength and tensi...

The dependence of enzyme activity on temperature : determination and validation of parameters

Abstract: Traditionally, the dependence of enzyme activity on temperature has been described by a model consisting of two processes: the catalytic reaction defined by ΔGDaggercat, and irreversible inactivation defined by ΔGDaggerinact. However, such a model does not account for the observed temperature-dependent behaviour of enzymes, and a new model has been developed and validated. This model (the Equilibrium Model) describes a new mechanism by which enzymes lose activity at high temperatures, by including an inactive form of the enzyme (Einact) that is in reversible equilibrium with the active form (Eact); it is the inactive form that undergoes irreversible thermal inactivation to the thermally denatured state. This equilibrium is described by an equilibrium constant whose temperature-dependence is characterized in terms of the enthalpy of the equilibrium, ΔHeq, and a new thermal parameter, Teq, which is the temperature at which the concentrations of Eact and Einact are equal; Teq may therefore be regarded as the thermal equivalent of Km. Characterization of an enzyme with respect to its temperature-dependent behaviour must therefore include a determination of these intrinsic properties. The Equilibrium Model has major implications for enzymology, biotechnology and understanding the evolution of enzymes. The present study presents a new direct data-fitting method based on fitting progress curves directly to the Equilibrium Model, and assesses the robustness of this procedure and the effect of assay data on the accurate determination of Teq and its associated parameters. It also describes simpler experimental methods for their determination than have been previously available, including those required for the application of the Equilibrium Model to non-ideal enzyme reactions.

Excess Magnitudes for Ionic Liquid Binary Mixtures with a Common Ion

Abstract: Densities, isobaric thermal expansivities, and excess enthalpies were determined for the binary systems formed by ionic liquids with a common anion, 1-hexyl-3-methylimidazolium tetrafluoroborate + 1-ethyl-3-methylimidazolium tetrafluoroborate, and 1-hexyl-3-methylimidazolium tetrafluoroborate + 1-butyl-3-methylimidazolium tetrafluoroborate, and by those with a common cation, 1-butyl-3-methylimidazolium tetrafluoroborate + 1-butyl-3-methylimidazolium methylsulfate and 1-butyl-3-methylimidazolium tetrafluoroborate + 1-butyl-3-methylimidazolium hexafluorophosphate. Volumetric properties were obtained by means of a vibrating tube densimeter, whereas Calvet microcalorimetry was used to determine the excess enthalpies. Due to the high viscosity of the samples, nonstandard experimental techniques had to be used to obtain the results. The experimental data are interpreted on the basis of the chemical structure of the ionic liquids.

Influence of substrate temperature on the stability of glasses prepared by vapor deposition.

Abstract: Physical vapor deposition of indomethacin (IMC) was used to prepare glasses with unusual thermodynamic and kinetic stability. By varying the substrate temperature during the deposition from 190 K to the glass transition temperature (Tg=315 K), it was determined that depositions near 0.85Tg (265 K) resulted in the most stable IMC glasses regardless of substrate. Differential scanning calorimetry of samples deposited at 265 K indicated that the enthalpy was 8 J/g less than the ordinary glass prepared by cooling the liquid, corresponding to a 20 K reduction in the fictive temperature. Deposition at 265 K also resulted in the greatest kinetic stability, as indicated by the highest onset temperature. The most stable vapor-deposited IMC glasses had thermodynamic stabilities equivalent to ordinary glasses aged at 295 K for 7 months. We attribute the creation of stable IMC glasses via vapor deposition to enhanced surface mobility. At substrate temperatures near 0.6Tg, this mobility is diminished or absent, resulting in low stability, vapor-deposited glasses.

Effect of temperature on the removal of lead(II) by adsorption on China clay and wollastonite

Abstract: The removal of Pb(II) from aqueous solution by adsorption on china clay and wollastonite is an attractive process. The amount of Pb(II) removed by adsorption is highly dependent on the temperature of the adsorbate solution and favours low temperature. The equilibrium times were noted, i.e. 90 min for china clay and 120 min for wollastonite. The various rate parameters of the adsorption process have been determined at different temperatures. The activation energies were determined and found to be −5.345 kJ mol−1 and −8.730 kJ mol−1 for Pb(II)-china clay and Pb(II)-wollastonite systems, respectively. The adsorption isotherm was measured experimentally at various temperatures. The negative values of enthalpy change (ΔH = −77.95 kJ mol−1 and −16.40 kJ mol−1 for china clay and wollastonite, respectively) indicate the exothermic nature of the adsorption processes for both systems. The isosteric heats of the adsorption process have been determined at various surface coverages of the adsorbents used. The various thermodynamic parameters have been calculated to elucidate the mechanism involved in the adsorption process.

Adsorption of volatile organic compounds onto carbon nanotubes, carbon nanofibers, and high-surface-area graphites.

Abstract: The adsorption of different alkanes (linear and cyclic), aromatics, and chlorohydrocarbons onto different nonmicroporous carbons—multiwalled carbon nanotubes (CNTs), carbon nanofibers (CNFs), and high-surface-area graphites (HSAGs)—is studied in this work by inverse gas chromatography (IGC). Capacity of adsorption was derived from the isotherms of adsorption, whereas thermodynamic properties (enthalpy of adsorption, surface free energy characteristics) have been determined from chromatographic retention data. HSAGs present the highest adsorption capacity, followed by CNTs and CNFs (although CNTs present an intermediate surface area between the two HSAG studied). Among the different adsorbates tested, benzene exhibits the highest adsorption capacity, and the same trend is observed in the enthalpy of adsorption. From surface free energy data, enthalpies of adsorption of polar compounds were divided into dispersive and specific contributions. The interactions of cyclic (benzene and cyclohexane) and chlorinated compounds (trichloroethylene, tetrachloroethylene, and chloroform) with the surfaces are mainly dispersive over all the carbons tested, CNTs being the material with the highest dispersive contribution, as was deduced also from the entropy parameter. Adsorption parameters were correlated with morphological and chemical properties of the materials.

Temperature dependence of thermodiffusion in aqueous suspensions of charged nanoparticles.

Abstract: Measurements of particle flows driven by temperature gradients are conducted as a function of temperature on aqueous suspensions of polystyrene nanoparticles and proteins of T4 lysozyme and mutant variants of T4 lysozyme. The thermodiffusion coefficients are measured using a microfluidic beam deflection technique on suspensions with particle concentrations on the order of 1 vol %. At T or ~ 50 degrees C, the thermodiffusion coefficient is positive with a value consistent with the predictions of a theoretical model originally proposed by Derjaguin that is based on the enthalpy changes due to polarization of water molecules in the double layer.