Showing papers on "Standard molar entropy published in 2008"

Size Effect on the Thermodynamic Properties of Silver Nanoparticles

Abstract: The Gibbs free energy of silver nanoparticles has been obtained from the calculations of bulk free energy and surface free energy for both the solid and liquid phase. On the basis of the obtained Gibbs free energy of nanoparticles, thermodynamic properties of silver nanoparticles, such as melting temperature, molar heat of fusion, molar entropy of fusion, and temperature dependences of entropy and specific heat capacity have been investigated. Calculation results indicate that these thermodynamic properties can be divided into two parts: bulk quantity and surface quantity, and surface atoms are dominant for the size effect on the thermodynamic properties of nanoparticles. The method that the intersection of the free-energy curves for solid and liquid nanoparticles decide the melting point of nanoparticles demonstrates that the surface free-energy difference between the solid and liquid phase is a decisive factor for the size-dependent melting of nanostructural materials.

First principles based group additive values for the gas phase standard entropy and heat capacity of hydrocarbons and hydrocarbon radicals.

Abstract: In this work a complete and consistent set of 95 Benson group additive values (GAVs) for standard entropies S(o) and heat capacities C(p)(o) of hydrocarbons and hydrocarbon radicals is presented These GAVs include 46 groups, among which 25 radical groups, which, to the best of our knowledge, have not been reported before The GAVs have been determined from a set of B3LYP/6-311G(d,p) ideal gas statistical thermodynamics values for 265 species, consistently with previously reported GAVs for standard enthalpies of formation One-dimensional hindered rotor corrections for all internal rotations are included The computational methodology has been compared to experimental entropies (298 K) for 39 species, with a mean absolute deviation (MAD) between experiment and calculation of 12 J mol(-1) K(-1), and to 46 experimental heat capacities (298 K) with a resulting MAD = 18 J mol(-1) K(-1) The constructed database allowed evaluation of corrections on S(o) and C(p)(o) for non-nearest-neighbor effects, which have not been determined previously The group additive model predicts the S(o) and C(p)(o) within approximately 5 J mol(-1) K(-1) of the ab initio values for 11 of the 14 molecules of the test set, corresponding to an acceptable maximal deviation of a factor of 16 on the equilibrium coefficient The obtained GAVs can be applied for the prediction of S(o) and C(p)(o) for a wide range of hydrocarbons and hydrocarbon radicals The constructed database also allowed determination of a large set of hydrogen bond increments, which can be useful for the prediction of radical thermochemistry

Estimation of physicochemical properties of ionic liquids 1-alkyl-3-methylimidazolium chloroaluminate.

Abstract: The density and surface tension of ionic liquids [C2mim][AlCl4] (1-ethlyl-3-methyl imidazolium chloroaluminate) and [C6mim][AlCl4] (1-hexyl-3-methylimidazolium chloroaluminate) were measured in the temperature range from 283.15 to 338.15 ± 0.05 K. In terms of these experimental results, the estimation of physicochemical properties of 1-alkyl-3-methylimidazolium chloroaluminate ([Cnmim][AlCl4], n = 1−6) was carried out. With the use of the parachor, the values of surface tension of the ILs were predicted. In terms of Glasser's theory, the standard molar entropy, lattice energy, and surface properties of the ILs were estimated. With the use of Kabo's method and Rebelo's method, the molar enthalpy of vaporization of the ILs, ΔlgHm0, was predicted. According to the interstice model, the values of the thermal expansion coefficient of the ILs were also estimated. Since the magnitude order of the thermal expansion coefficient estimated by the model is in good agreement with that measured experimentally, this res...

Humidity-Induced Phase Transitions in Ion-Containing Block Copolymer Membranes

Abstract: The phase behavior of ion-containing block copolymer membranes in equilibrium with humidified air is studied as a function of the relative humidity (RH) of the surrounding air, ion content of the copolymer, and temperature. Increasing RH at constant temperature results in both disorder-to-order and order-to-order transitions. In-situ small-angle neutron scattering experiments on the open block copolymer system, when combined with water uptake measurement, indicate that the disorder-to-order transition is driven by an increase in the partial molar entropy of the water molecules in the ordered phase relative to that in the disordered phase. This is in contrast to most systems wherein increasing entropy results in stabilization of the disordered phase.

Thermal properties of cubic KTa1−xNbxO3 crystals

Abstract: Cubic potassium tantalite niobate [KTa1−xNbxO3 (KTN)] crystals of large size, good quality, and varying Nb concentration have been grown by the Czochralski method and their thermal properties have been systematically studied. The melting point, molar enthalpy of fusion, and molar entropy of fusion of the crystals were determined to be: 1536.9 K, 12 068.521 J mol−1, and 7.85 J K−1 mol−1 for KTa0.67Nb0.33O3; and 1520.61 K, 15 352.511 J mol−1, and 10.098 J K−1 mol−1 for KTa0.67Nb0.33O3, respectively. Based on the data, the Jackson factor was calculated to be 0.994f and 1.214f for KTa0.67Nb0.33O3 and KTa0.63Nb0.37O3, respectively. The thermal expansion coefficients over the temperature range of 298.15−773.15 K are: α=4.0268×10−6/K, 6.4428×10−6/K, 6.5853×10−6/K for KTaO3, KTa0.67Nb0.33O3, and KTa0.63Nb0.37O3, respectively. The density follows an almost linear decrease when the temperature increases=from 298.15 to 773.15 K. The measured specific heats at 303.15 K are: 0.375 J g−1 K−1 for KTaO3; 0.421 J g−1 K−1 for KTa0.67Nb0.33O3, and 0.430 J g−1 K−1 for KTa0.63Nb0.37O3 The thermal diffusion coefficients of the crystals were measured over the temperature range from 303.15−563.15 K. The calculated thermal conductivity values of KTaO3, KTa0.67Nb0.33O3, and KTa0.63Nb0.37O3 at 303.15 K are 8.551, 5.592, and 4.489 W m−1 K−1, respectively. The variation of these thermal properties versus Nb concentration is qualitatively analyzed. These results show that crystalline KTN is a promising material for optical applications.

Adsorption dynamics and thermodynamics of Hb on the Hb-imprinted polymer beads

Abstract: The adsorption of hemoglobin (Hb) onto the Hb-imprinted polymer beads has been dynamically and thermodynamically investigated. The order of Hb adsorption rate was examined to be 0.7-order, and the rate constant k , apparent activation energy E A , and Arrhenius constant A were calculated. Equilibrium modeling of the adsorption showed that the adsorption of MIP beads to the imprinted molecule Hb was fitted well to the Freundlich equation. From the results of the thermodynamic analysis, standard free energy Δ G 0 , standard enthalpy Δ H 0 , and standard entropy Δ S 0 were determined.

Study of the Chemical Equilibrium of the Liquid-Phase Dehydration of 1-Hexanol to Dihexyl Ether

Abstract: The equilibrium constants of the liquid-phase dehydration of 1-hexanol to dihexyl ether (DNHE) and water were determined in the temperature range of (423 to 463) K on Amberlyst 70. The equilibrium constants of the two main side reactions, DNHE decomposition to 1-hexene and 1-hexanol and isomerization of 1-hexene to 2-hexene, were also studied. The etherification reaction proved to be slightly exothermic, with an enthalpy change of reaction of −(9.5 ± 0.2) kJ·mol−1 at 298 K. From this value, the standard formation enthalpy and molar entropy of DNHE were computed to be −(478.6 ± 0.8) kJ·mol−1 and (517.4 ± 0.5) J·K−1·mol−1, respectively. A correction concerning the effect of pressure on the entropy proved to be necessary when computing liquid-phase entropy from gas-phase data. The isomerization of 1-hexene to 2-hexene is exothermic, whereas the decomposition of DNHE is endothermic.

Chemical potentials in aqueous solutions of some ionic liquids with the 1-ethyl-3-methylimidazolium cation.

Abstract: We determined the vapor pressures of aqueous solutions of 1-ethyl-3-methylimidazolium ([C 2mim])-based ionic liquids (IL) with counteranions, tetrafluoroborate (BF 4 (-)), trifluoromethanesulfonate (OTF (-)), and iodide (I (-)). Because in literature the evidence is accumulating and pointing to the fact that ionic liquid ions do not dissociate in aqueous media for the most of the concentration range, we analyzed the vapor pressure data on the basis of binary mixture, and the excess chemical potentials of each component were calculated. From these, the intermolecular interactions in terms of excess chemical potential and hence the concentration fluctuations were evaluated. Though any further discussion into the mixing schemes of the mixture awaits the excess partial molar enthalpy and hence the excess partial molar entropy data, the net interaction in terms of excess chemical potential indicates that the affinity of each IL is ranked in the descending order [C 2mim]I > [C 2mim]OTF > [C 2mim]BF 4. This is consistent with our earlier findings that [C 2mim] (+) is modestly amphiphilic with almost equal hydrophobicity and hydrophilicity, I (-) is a hydrophile, and OTF (-) is amphiphilic, and BF 4 (-) is believed to be strongly hydrophobic.

Thermodynamic Calculation on the Formation of Titanium Hydride

Abstract: A modified Miedema model, using interrelationship among the basic properties of elements Ti and H, is employed to calculate the standard enthalpy of formation of titanium hydride TiHx (1 ≤ x ≤ 2). Based on Debye theories of solid thermal capacity, the vibrational entropy, as well as electronic entropy, is acquired by quantum mechanics and statistic thermodynamics methods, and a new approach is presented to calculate the standard entropy of formation of TiH2. The values of standard enthalpy of formation of TiHx decrease linearly with increase of x. The calculated results of standard enthalpy, entropy, and free energy of formation of TiH2 at 298.16 K are −142.39 kJ/mol, −143.0 J/(molK) and −99.75 kJ/mol, respectively, which is consistent with the previously-reported data obtained by either experimental or theoretical calculation methods. The results show that the thermodynamic model for titanium hydride is reasonable.

Thermodynamic properties of strontium metaniobate SrNb2O6

Abstract: The heat capacity and the enthalpy increments of strontium metaniobate SrNb2O6 were measured by the relaxation method (2-276 K), micro DSC calorimetry (260-320 K) and drop calorimetry (723-1472 K). Temperature dependence of the molar heat capacity in the form C pm=(200.47±5.51)+(0.02937±0.0760)T-(3.4728±0.3115)·106/T 2 J K−1 mol−1 (298-1500 K) was derived by the least-squares method from the experimental data. Furthermore, the standard molar entropy at 298.15 K S m 0 (298.15 K)=173.88±0.39 J K−1 mol−1 was evaluated from the low temperature heat capacity measurements. The standard enthalpy of formation Δf H 0 (298.15 K)=-2826.78 kJ mol−1 was derived from total energies obtained by full potential LAPW electronic structure calculations within density functional theory.

Low T heat capacity measurements and new entropy data for titanite (sphene): implications for thermobarometry of high-pressure rocks

Abstract: The accepted standard state entropy of titanite (sphene) has been questioned in several recent studies, which suggested a revision from the literature value 129.3 ± 0.8 J/mol K to values in the range of 110–120 J/mol K. The heat capacity of titanite was therefore re-measured with a PPMS in the range 5 to 300 K and the standard entropy of titanite was calculated as 127.2 ± 0.2 J/mol K, much closer to the original data than the suggested revisions. Volume parameters for a modified Murgnahan equation of state: V P,T = V 298° × [1 + a°(T − 298) − 20a°(T − 298)] × [1 – 4P/(K 298 × (1 – 1.5 × 10−4 [T − 298]) + 4P)]1/4 were fit to recent unit cell determinations at elevated pressures and temperatures, yielding the constants V 298° = 5.568 J/bar, a° = 3.1 × 10−5 K−1, and K = 1,100 kbar. The standard Gibbs free energy of formation of titanite, −2456.2 kJ/mol (∆H°f = −2598.4 kJ/mol) was calculated from the new entropy and volume data combined with data from experimental reversals on the reaction, titanite + kyanite = anorthite + rutile. This value is 4–11 kJ/mol less negative than that obtained from experimental determinations of the enthalpy of formation, and it is slightly more negative than values given in internally consistent databases. The displacement of most calculated phase equilibria involving titanite is not large except for reactions with small ∆S. Re-calculated baric estimates for several metamorphic suites yield pressure differences on the order of 2 kbar in eclogites and 10 kbar for ultra-high pressure titanite-bearing assemblages.

Solvent electrostriction-driven peptide folding revealed by quasi-Gaussian entropy theory and molecular dynamics simulation.

Abstract: A quantitative understanding of the complex relationship between microscopic structure and the thermodynamics driving peptide and protein folding is a major goal of biophysical chemistry. Here, we present a methodology comprising the use of an extended quasi-Gaussian entropy theory parametrized using molecular dynamics simulation that provides a complete description of the thermodynamics of peptide conformational states. The strategy is applied to analyze the conformational thermodynamics of MR121-GSGSW, a peptide well characterized in experimental studies. The results demonstrate that the extended state of the peptide possesses the lowest partial molar entropy. The origin of this entropy decrease is found to be in the increase of the density and orientational order of the hydration water molecules around the peptide, induced by the "unfolding". While such a reduction of the configurational entropy is usually associated with the hydrophobic effect, it is here found to be mainly due to the interaction of the solute charges with the solvent, that is, electrostriction.

Low-temperature heat capacity of synthetic Fe-and Mg-cordierite : thermodynamic properties and phase relations in the system FeO-Al2O3-SiO2-(H2O)

Abstract: The heat capacity of anhydrous low Fe-cordierite, Fe2Al4Si5O18, was measured for the first time between 5 and 300 K on a milligram-sized synthetic sample using low-temperature heat-pulse calorimetry. The C-p's of anhydrous low Mg-cordierite, Mg2Al4Si5O18, and a hydrous low Mg-cordierite of composition Mg1.97Al3.94Si5.06O18.0.625H(2)O, both previously studied by adiabatic calorimetry (Paukov et al., 2006, 2007), were also determined. At low temperatures around 10 K the C-p data for Fe-cordierite show a small feature that is interpreted as a Schottky anomaly. Using published DSC and adiabatic calorimetry results for anhydrous Fe-cordierite and Mg-cordierite and the results herein, Cp polynomials for both phases were calculated for use at T > 270 K. They are given by: C-p(Fe-Cd) = 91 1.1(+/- 9.7) - 5829.2(+/- 363) x T-0.5 - 13.9424(+/- 2.522) x 10(6) x T-2 + 1470.4(+/- 454.84) x 10(6) x T-3 and C-p(Mg-Cd) = 882.0(+/- 4.9) - 5155.8(+/- 167) x T-0.5 -20.7584(+/- 0.806) x T-2 + 2736.0(+/- 112.73) x 10(6) x T-3, respectively. The standard calorimetric entropy values at 298.15 K, SO, for anhydrous Fe-cordierite, anhydrous Mg-cordierite and hydrous Mg-cordierite are 460.5 +/- 0.5, 406.1 +/- 0.4 and 450.9 +/- 0.5 J/(mol.K), respectively. The latter two values are in good agreement with those determined by adiabatic calorimetry. The lattice (vibrational) and non-lattice contributions to the experimental C-p values for Fe-cordierite were separated by applying the Komada-Westrum model and the values S degrees(vib) = 447.7 J/(mol.K) and S-el(o) = 13.6 J/(mol.K) were obtained for the vibrational and electronic contributions to the standard third-law entropy. Thermodynamic calculations and analysis were carried out in the system FeO-Al2O3-SiO2 with and without H2O. A model C-p polynomial for hydrous Fe-cordierite, Fe2Al4Si5O18 center dot H2O, was derived as: C-p(hFe-Cd) = 967.3(+/- 9.7) - 6070.4(+/- 363) x T-0.5 - 13.9389(+/- 2.522) x 10(6) x T-2 + 1470.4(+/- 454.84) x 10(6) x T-3. The enthalpy of formation from the elements for both hydrous and anhydrous Fe-cordierite and the standard entropy for hydrous Fe-cordierite with one mole of H2O pfu were derived using the experimental phase equilibrium results of Mukhopadhyay & Holdaway (1994) on the reaction 3 Fe-cordierite center dot nH(2)O = 2 almandine + 4 sillimanite + 5 quartz + 3n H2O. For anhydrous Fe-cordierite Delta H-f(o) = -8448.26 kJ/mol was obtained and for hydrous Fe-cordierite, Delta H-f(o) = -8750.23 kJ/mol and S-o = 520.6 J/(mol.K). Phase relations in the FeO-Al2O3-SiO2-(H2O) systems at low pressures are analyzed and isohydrons for H2O in hydrous Fe-cordierite are modelled. H2O contents decrease with increasing temperature and increase with increasing pressure.