Showing papers on "Enthalpy published in 2010"

The cosmological constant and the black hole equation of state

Abstract: The thermodynamics of black holes in various dimensions are described in the presence of a negative cosmological constant which is treated as a thermodynamic variable, interpreted as a pressure in the equation of state. The black hole mass is then identified with the enthalpy, rather than the internal energy, and heat capacities are calculated at constant pressure not at constant volume. The Euclidean action is associated with a bridge equation for the Gibbs free energy and not the Helmholtz free energy. Quantum corrections to the enthalpy and the equation of state of the BTZ black hole are studied.

Computational and Experimental Studies on the Adsorption of CO, N2, and CO2 on Mg-MOF-74

Abstract: Adsorption of carbon monoxide, dinitrogen, and carbon dioxide on the porous metal−organic framework Mg-MOF-74 was investigated by means of a combined methodology comprising variable-temperature infrared spectroscopy and ab initio periodic DFT-D calculations using the CRYSTAL code. Both CO and N2 were found to form nearly linear (Mg2+···CO and Mg2+···NN) adsorption complexes, in contrast with CO2, which forms an angular Mg2+···OCO complex. From IR spectra recorded at a variable-temperature, the standard adsorption enthalpy (ΔH0) was found to be −29, −21, and −47 kJ mol−1 for CO, N2, and CO2, respectively. Calculated values of ΔH0, including an empirical correction for dispersion forces, resulted to be in a reasonably good agreement with those experimentally obtained. Calculations also showed the very significant role played by dispersion forces, which account for about one-half of the adsorption enthalpy for each of the three adsorbates, CO, N2, and CO2. The results are discussed in the broader context of ...

Thermodynamic changes in mechanochemically synthesized magnesium hydride nanoparticles.

Abstract: The thermodynamic properties of magnesium hydride nanoparticles have been investigated by hydrogen decomposition pressure measurements using the Sieverts technique. A mechanochemical method was used to synthesize MgH(2) nanoparticles (down to approximately 7 nm in size) embedded in a LiCl salt matrix. In comparison to bulk MgH(2), the mechanochemically produced MgH(2) with the smallest particle size showed a small but measurable decrease in the decomposition reaction enthalpy (DeltaH decrease of 2.84 kJ/mol H(2) from DeltaH(bulk) = 74.06 +/- 0.42 kJ/mol H(2) to DeltaH(nano) = 71.22 +/- 0.49 kJ/mol H(2)). The reduction in DeltaH matches theoretical predictions and was also coupled with a similar reduction in reaction entropy (DeltaS decrease of 3.8 J/mol H(2)/K from DeltaS(bulk) = 133.4 +/- 0.7 J/mol H(2)/K to DeltaS(nano) = 129.6 +/- 0.8 J/mol H(2)/K). The thermodynamic changes in the MgH(2) nanoparticle system correspond to a drop in the 1 bar hydrogen equilibrium temperature (T(1 bar)) by approximately 6 degrees C to 276.2 +/- 2.4 degrees C in contrast to the bulk MgH(2) system at 281.8 +/- 2.2 degrees C. The reduction in the desorption temperature is less than that expected from theoretical studies due to the decrease in DeltaS that acts to partially counteract the effect from the change in DeltaH.

Solubility and Diffusion of H2S and CO2 in the Ionic Liquid 1-(2-Hydroxyethyl)-3-methylimidazolium Tetrafluoroborate

Abstract: The solubilities and diffusion coefficients of hydrogen sulfide and carbon dioxide gases in the ionic liquid (IL) 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate ([hemim][BF4]), at temperatures ranging from (303.15 to 353.15) K and pressures up to 1.1 MPa, were determined. The solubility data were correlated using the Krichevsky−Kasarnovsky equation, and Henry’s law constants at different temperatures were obtained. From the solubility data, the partial molar thermodynamic functions of solution such as Gibbs energy, enthalpy, and entropy were calculated. The diffusion coefficients were obtained for H2S and CO2 using a semi-infinite volume approach, and a correlation equation with temperature is presented for each gas. A comparison showed that the solubility of H2S was about three times its magnitude, and its diffusion coefficient is of the same order of magnitude as that of CO2 in the IL studied in this work.

Stability and Decomposition of NaBH4

Abstract: We investigate the stability and hydrogen desorption of NaBH4. Dynamic pcT (pressure, concentration, and temperature) measurements under constant hydrogen flows are used to determine thermodynamic parameters of reaction. From the van’t Hoff equation the enthalpy and entropy of reaction, −108 ± 3 kJ mol−1 of H2 and 133 ± 3 J K−1 mol−1 of H2 released, are obtained, respectively. This corresponds to a decomposition temperature of Tdec = 534 ± 10 °C at 1 bar of H2. The decomposition thereby occurs in one step; i.e., only one plateau is visible in the pressure composition isotherms. Elemental Na is identified as the major solid component in the residue by X-ray diffraction. The experimental results are discussed on the basis of theoretical calculations using the density functional theory approach. Starting from the optimized structure of the cubic α-phase of NaBH4, we discuss possible decomposition routes involving elemental Na and B as well as Na−H and Na−B binary compounds as residual products.

Micelle Formation of Long-Chain Imidazolium Ionic Liquids in Aqueous Solution Measured by Isothermal Titration Microcalorimetry

Abstract: Self-association of three long-chain imidazolium ionic liquids (ILs), [C12mim]Br, [C14mim]Br, and [C16mim]Br, in aqueous solution was studied by surface tension measurements over a temperature range from (278.15 to 328.15) K. Effects of temperature and hydrocarbon chain length of the three long-chain ILs on the critical micelle concentration (CMC) were examined. Thermodynamic parameters, ΔmicG, ΔmicH, and ΔmicS, of micellization were determined by applying a mass-action model equation. Isothermal titration microcalorimetry was used to obtain the enthalpy change upon micellization of the three long-chain ILs at 298.15 K. Moreover, the CMCs and the thermodynamic parameters (ΔmicG, ΔmicH, and ΔmicS) were determined based on the isothermal titration microcalorimetry results. These CMC values are approximately equal to the CMCs obtained by surface tension measurement.

Chemical Absorption of Sulfur Dioxide in Room-Temperature Ionic Liquids

Abstract: Gaseous solubilities of sulfur dioxide (SO2) in room-temperature ionic liquids (RTILs), 1-n-butyl-3-methylimidazolium acetate and 1-n-butyl-3-methylimidazolium methyl sulfate, have been measured at four isothermal conditions (about 283, 298, 323, and 348 K) using a gravimetric microbalance. The observed pressure−temperature-composition (PTx) data have been analyzed by use of an equation-of-state (EOS) model, which has been successfully applied for our previous works. Excess thermodynamic functions and Henry’s law constants have been obtained from the observed (PTx) data and our previous measurements of SO2 + 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide using the EOS correlation. All three RTILs show the chemical absorption. The classification of whether the absorption is the physical or chemical type is based on the excess Gibbs and enthalpy functions as well as the magnitude of the Henry’s constant. An ideal association model has been applied in order to interpret those excess thermody...

Mixed Micelle Formation between Amphiphilic Drug Amitriptyline Hydrochloride and Surfactants (Conventional and Gemini) at 293.15−308.15 K

Abstract: The micellization of amphiphilic drug amitriptyline hydrochloride (AMT, an antidepressant) and conventional as well as gemini surfactants has been studied conductometrically in pure and mixed states in aqueous solutions at different temperatures to derive various physicochemical properties such as critical micelle concentration (cmc), ideal cmc (cmcid), counterion dissociation (g), standard Gibbs free energy (ΔGm0), enthalpy (ΔHm0), and entropy of micellization (ΔSm0), micellar mole fractions of surfactant (X1, X1m), mole fraction of surfactant in ideal state (X1id), interaction parameter (β), activity coefficients (f1, f2), and excess free energy of mixing (ΔGex). All the results indicate synergism and attractive interactions in the mixed systems.

Temperature Dependent Thermodynamic Model of the System H+-NH4+-Na+-SO42--NO3--Cl--H2O

Abstract: A thermodynamic model of the system H(+)-NH₄(+)-Na(+)-SO₄²⁻-NO₃⁻-Cl⁻-H₂O is parametrized and used to represent activity coefficients, equilibrium partial pressures of H₂O, HNO₃, HCl, H₂SO₄, and NH₃, and saturation with respect to 26 solid phases (NaCl(s), NaCl·2H₂O(s), Na₂SO₄(s), Na₂SO₄·10H₂O(s), NaNO₃·Na₂SO₄·H₂O(s), Na₃H(SO₄)₂(s), NaHSO₄(s), NaHSO₄·H₂O(s), NaNH₄SO₄·2H₂O(s), NaNO₃(s), NH₄Cl(s), NH₄NO₃(s), (NH₄)₂SO₄(s), (NH₄)₃H(SO₄)₂(s), NH₄HSO₄(s), (NH₄)₂SO₄·2NH₄NO₃(s), (NH₄)₂SO₄·3NH₄NO₃(s), H₂SO₄·H₂O(s), H₂SO₄·2H₂O(s), H₂SO₄·3H₂O(s), H₂SO₄·4H₂O(s), H₂SO₄·6.5H₂O(s), HNO₃·H₂O(s), HNO₃·2H₂O(s), HNO₃·3H₂O(s), and HCl·3H₂O(s)). The enthalpy of formation of the complex salts NaNH₄SO₄·2H₂O(s) and Na₂SO₄·NaNO₃·H₂O(s) is calculated. The model is valid for temperatures < or approximately 263.15 up to 330 K and concentrations from infinite dilution to saturation with respect to the solid phases. For H₂SO₄-H₂O solutions the degree of dissociation of the HSO₄⁻ ion is represented near the experimental uncertainty over wide temperature and concentration ranges. The parametrization of the model for the subsystems H(+)-NH₄(+)-NO₃⁻-SO₄²⁻-H₂O and H(+)-NO₃⁻-SO₄²⁻-Cl⁻-H₂O relies on previous studies (Clegg, S. L. et al. J. Phys. Chem. A 1998, 102, 2137-2154; Carslaw, K. S. et al. J. Phys. Chem. 1995, 99, 11557-11574), which are only partly adjusted to new data. For these systems the model is applicable to temperatures below 200 K, dependent upon liquid-phase composition, and for the former system also to supersaturated solutions. Values for the model parameters are determined from literature data for the vapor pressure, osmotic coefficient, emf, degree of dissociation of HSO₄⁻, and the dissociation constant of NH₃ as well as measurements of calorimetric properties of aqueous solutions like enthalpy of dilution, enthalpy of solution, enthalpy of mixing, and heat capacity. The high accuracy of the model is demonstrated by comparisons with experimentally determined mean activity coefficients of HCl in HCl-Na₂SO₄-H₂O solutions, solubility measurements for the quaternary systems H(+)-Na(+)-Cl⁻-SO₄²⁻-H₂O, Na(+)-NH₄(+)-Cl⁻-SO₄²⁻-H₂O, and Na(+)-NH₄(+)-NO₃⁻-SO₄²⁻-H₂O as well as vapor pressure measurements of HNO₃, HCl, H₂SO₄, and NH₃.