Showing papers on "Enthalpy published in 1992"

Thermodynamic Properties of the NaCl+H2O System. II. Thermodynamic Properties of NaCl(aq), NaCl⋅2H2(cr), and Phase Equilibria

Abstract: Equations that described the thermodynamic properties of the NaCl+H2O system were obtained from a fit to experimental results for this system. The experimental results included in the fit spanned the range of temperature of approximately 250 to 600 K and, where available, the range of pressure from the vapor pressure of the solution to 100 MPa. New equations and/or values for the following properties are given in the present work: 1) ΔfG0m and ΔfH0m, for formation from the elements, for NaCl(cr) for 298.15 K and 0.1 MPa, 2) ΔfG0m and ΔfH0m from the elements, as well as S0m and C0p,m, all for 298.15 K, 0.1MPa, for NaCl⋅2H2O(cr), 3) the change in chemical potential for both NaCl and H2O in NaCl(aq) as a function of temperature, pressure, and molality, valid from 250 to 600 K and, where available, from the vapor pressure of the solution to 100 MPa. Comparison of the accuracies of experimental methods, where possible, has also been performed.

Use of liquid hydrocarbon and amide transfer data to estimate contributions to thermodynamic functions of protein folding from the removal of nonpolar and polar surface from water.

Abstract: This extension of the liquid hydrocarbon model seeks to quantify the thermodynamic contributions to protein stability from the removal of nonpolar and polar surface from water. Thermodynamic data for the transfer of hydrocarbons and organic amides from water to the pure liquid phase are analyzed to obtain contributions to the thermodynamics of folding from the reduction in water-accessible surface area. Although the removal of nonpolar surface makes the dominant contribution to the standard heat capacity change of folding (delta C0fold), here we show that inclusion of the contribution from removal of polar surface allows a quantitative prediction of delta C0fold within the uncertainty of the calorimetrically determined value. Moreover, analysis of the contribution of polar surface area to the enthalpy of transfer of liquid amides provides a means of estimating the contributions from changes in nonpolar and polar surface area as well as other factors to the enthalpy of folding (delta H0fold). In addition to estimates of delta H0fold, this extension of the liquid hydrocarbon model provides a thermodynamic explanation for the observation [Privalov, P. L., & Khechinashvili, N. N. (1974) J. Mol. Biol. 86, 665-684] that the specific enthalpy of folding (cal g-1) of a number of globular proteins converges to a common value at approximately 383 K. Because amounts of nonpolar and polar surface area buried by these proteins upon folding are found to be linear functions of molar mass, estimates of both delta C0fold and delta H0fold may be obtained given only the molar mass of the protein of interest.(ABSTRACT TRUNCATED AT 250 WORDS)

Calculation of the thermodynamic properties of aqueous species at high pressures and temperatures. Effective electrostatic radii, dissociation constants and standard partial molal properties to 1000 °C and 5 kbar

Abstract: Within the framework of the revised HKF (H. C. Helgeson, D. H. Kirkham and G. C. Flowers, Am. J. Sci., 1981, 281, 1249) equations of state (J. C. Tanger IV and H. C. Helgeson, Am. J. Sci., 1988, 288, 19), prediction of the standard partial molal thermodynamic properties of aqueous ions and electrolytes at high pressures and temperatures requires values of the effective electrostatic radii of the ions (re), as well as provision for the temperature and pressure dependence of the relative permittivity of the solvent, H2O. Values of the relative permittivity of H2O, together with the Born functions needed to compute the standard partial molal Gibbs free energy, enthalpy, entropy, heat capacity and volume of solvation were calculated as a function of temperature and density from a modified version of the Uematsu–Franck equation (M. Uematsu and E. U. Franck, J. Phys. Chem. Ref. Data, 1980, 9, 1291). The temperature/pressure dependence of re is described in terms of a solvent function designated by g, which was evaluated in the present study at temperatures and pressures to 1000 °C and 5 kbar by regressing experimental standard partial molal heat capacities and volumes of NaCl reported in the literature together with published dissociation constants for NaClo at supercritical temperatures and pressures using the revised HKF equations of state for aqueous species. The calculated values of re decrease substantially with increasing temperature at constant pressure ⩽2 kbar, and with decreasing pressure at constant temperature 400 °C. The equations and parameters summarized below permit calculation of the standard partial molal properties of aqueous species from the revised HKF equations of state over a much more extensive range of temperature than was previously possible.

High‐temperature elastic constant data on minerals relevant to geophysics

Abstract: The high-temperature measurements of elastic constants and related temperature derivatives of nine minerals of interest to geophysical and geochemical theories of the Earth's interior are reviewed and discussed. A number of correlations between these parameters, which have application to geophysical problems, are also presented. Of especial interest is α, the volume coefficient of thermal expansion, and a section is devoted to this physical property. Here we show how α can be estimated at very high temperatures and how it varies with density. An estimate of α for Mg-perovskite at deep-mantle conditions is made. The formula for the Gruneisen ratio γ as a function of V and T is presented, including plots of the numerical values of γ over a wide T and V range. An example calculation of γ for MgO is made. The high-T-high-P values of γ calculated here agree well with results from the ab initio method of calculation for MgO. The use of the thermoelastic parameters is reviewed, showing application to the understanding of thermal pressure, thermal expansivity, enthalpy, and entropy. We review an extrapolation formula to determine Ks, the adiabatic bulk modulus, at very high T. We show that the thermal pressure is quite linear with T up to high temperatures (∼1800 K), and, as a consequence, the anharmonic contribution to the Helmholtz free energy is sufficiently small, so that it can and should be ignored in thermodynamic calculations for mantle conditions.

In search of a thermodynamic description of biomass yields for the chemotrophic growth of microorganisms.

Abstract: Correlations for the prediction of biomass yields are valuable, and many proposals based on a number of parameters (Y(ATP), Y(Ave), eta(o), Y(c), Gibbs energy efficiencies, and enthalpy efficiencies) have been published. This article critically examines the properties of the proposed parameters with respect to the general applicability to chemotrophic growth systems, a clear relation to the Second Law of Thermodynamics, the absence of intrinsic problems, and a requirement of only black box information. It appears that none of the proposed parameters satisfies all these requirements. Particularly, the various energetic efficiency parameters suffer from major intrinsic problems. However, this article will show that the Gibbs energy dissipation per amount of produced biomass (kJ/C-mod) is a parameter which satisfies the requirements without having intrinsic problems. A simple correlation is found which provides the Gibbs energy dissipation/C-mol biomass as a function of the nature of the C-source (expressed as the carbon chain length and the degree of reduction). This dissipation appears to be nearly independent of the nature of the electron acceptor (e.g., O(2), No(3) (-), fermentation). Hence, a single correlation can describe a very wide range of microbial growth systems. In this respect, Gibbs energy dissipation is much more useful than heat production/C-mol biomass, which is strongly dependent on the electron acceptor used. Evidence is presented that even a net heat-uptake can occur in certain growth systems.The correlation of Gibbs energy dissipation thus obtained shows that dissipation/C-mol biomass increases for C-sources with smaller chain length (C(6) --> C(1)), and increases for both higher and lower degrees of reduction than 4. It appears that the dissipation/C-mol biomass can be regarded as a simple thermodynamic measure of the amount of biochemical "work" required to convert the carbon source into biomass by the proper irreversible carbon-carbon coupling and oxidation/reduction reactions. This is supported by the good correlation between the theoretical ATP requirement for biomass formation on different C-sources and the dissipation values (kJ/C-mol biomass) found. The established correlation for the Gibbs energy dissipation allows the prediction of the chemotrophic biomass yield on substrate with an error of 13% in the yield range 0.01 to 0.80 C-mol biomass/(C)-mol substrate for aerobic/anaerobic/denitrifying growth systems.

Properties of Aqueous Solutions of Electrolytes

Abstract: Foreword. EXPERIMENTAL VALUES OF PHYSICO-CHEMICAL PARAMETERS. Density. Viscosity. Specific Heat Capacity. Thermal Conductivity. Saturated Vapor Pressure. Electrolitic Conductivity. Diffusion Coefficients. Freezing Points. Boiling Point. Integral Dissolution Enthalpy. Dilution Enthalpy. Thermal Constants of Substances. Mean Ionic Activity Coefficients. METHODS FOR CALCULATION OF PHYSICO-CHEMICAL PARAMETERS. Mean Ionic Activity Coefficients. Water Activity. Density. Viscosity. Heat Capacity. Thermal Conductivity. Boiling Point. Freezing Point. Surface Tension. Diffusion Coefficients. Molar Electrical Conductivity. Integral Enthalpy of Dissolution. Differential Dissolution Enthalpy. Hydration Enthalpy. References.

Ab initio studies of the water dimer using large basis sets: The structure and thermodynamic energies

Abstract: Ab initio calculations with various large basis sets have been performed on the water dimer in order to study the structure, energetics, spectra, and electrical properties. As a reference system, the calculations of the water monomer were also performed. The second order Mo/ller‐Plesset perturbation theory (MP2) using a large basis set (O:13s,8p,4d,2f/H:8s,4p,2d) well reproduces various water monomer experimental data except for the somewhat underestimated absolute energy and hyperpolarizability. The monomer energy calculated with the fourth‐order Mo/ller–Plesset perturbation theory (MP4) with the above basis set is −76.407 hartrees, which is only 0.073 hartree above the experimental energy. To compare the theoretical and experimental dimer structures and thermal energies accurately, we summarized the quantum statistical thermodynamic quantities with corrections for anharmonic vibration, rotation, rotation–vibration coupling, and internal rotation. With the correction for the anharmonic binding potential and rotation, the predicted interoxygen distance of the dimer is 2.958 A, which is so far the closest to the experimental value ∼2.976 A. The predicted dimer dipole moment is 2.612 D, which is the first agreement with experiment (2.60–2.64 D). The predicted frequency shift of the dimer with respect to the monomer is in good agreement with experiment. With the MP2 calculation using the large basis set, the basis set superposition error correction (BSSEC) of the dimer is only 0.33 kcal/mol, which is by far the smallest among the MP2 results reported. Without BSSEC, the predicted binding energy, enthalpy, free energy, and entropy are all in good agreement with experiment within the error bounds, whereas with BSSEC, some of them seem to be slightly off the experimental error bounds. Nevertheless, the results with BSSEC can be more reliable than those without BSSEC.

Relationship between the glass transition temperature and the interaction parameter of miscible binary polymer blends

Abstract: An equation that relates the gh transition temperature, TE, and a binary interaction parameter, x, for miscible binary polymer blends was derived. The equation including no adjustable parameters was based on a thermodynamic mixing formalism using enthalpy as the thermodynamic parameter. The en- thalpy of mixing was written as a van Laar expression, and the T, was formally treated as a second-order Ehrenfest transition. The equation satisfactorily predicts TK-composition curves for miscible bw polymer blends that exhibit either positive or negative deviation from a linear mixing rule, depending on the strength of the interaction. Good agreement was found between calculated x and values experimentally determined by equilibrium melting point depression and inverse gas chromatography.

Multiple enantioselective retention mechanisms on derivatized cyclodextrin gas chromatographic chiral stationary phases.

Abstract: 2,6-di-O-pentyl-3-O-(trifluoroacetyl) (DP-TFA) derivatized, {beta}- and {gamma}-cyclodextrins are able to resolve a wide variety of volatile racemic compounds. The enantiomeric recognition mechanism of these phases was investigated. The retention behavior of homologous series showed that lengthening the side alkyl chain increases the retention time but does not affect enantioselectivity. The thermodynamic parameters, free energy, enthalpy, and the difference in free energy, and enthalpy, and entropy between enantiomers were evaluated for 24 enantiomeric pairs. From this data, it appears that the compounds can be arranged in two groups. One group has high values for enthalpy, entropy, free energy, and the corresponding difference parameters between enantiomers. The second group has significantly lower values for all parameters. It is shown that compounds belonging to the second group follow an enthalpy-entropy compensation regression (in k{prime} vs {Delta}H) while the group I compounds do not. Also on a given column the mass capacity for group II compounds is significantly higher than that for group I compounds. A small difference was found In the mass transfer behavior of the two groups of compounds. It is believed that there may be at least two different chiral recognition mechanisms with the derivatized cyclodextrin gas chromatographic stationary phases. It ismore » postulated that one mechanism involves cyclodextrin (CD) inclusion complex formation and the other does not. Currently, more enantiomers seem to resolve through external or multiple association than through an inclusion process. The fact that derivatized CD chiral stationary phases can resolve different enantiomers via different mechanisms (and probably by combination or intermediate mechanisms) provides the practical benefit of Increasing the number and types of compounds resolved on the columns.« less

Computer program for calculating and fitting thermodynamic functions

Abstract: A computer program is described which (1) calculates thermodynamic functions (heat capacity, enthalpy, entropy, and free energy) for several optional forms of the partition function, (2) fits these functions to empirical equations by means of a least-squares fit, and (3) calculates, as a function of temperture, heats of formation and equilibrium constants The program provides several methods for calculating ideal gas properties For monatomic gases, three methods are given which differ in the technique used for truncating the partition function For diatomic and polyatomic molecules, five methods are given which differ in the corrections to the rigid-rotator harmonic-oscillator approximation A method for estimating thermodynamic functions for some species is also given

Energetics of a hexagonal-lamellar-hexagonal-phase transition sequence in dioleoylphosphatidylethanolamine membranes.

Abstract: The phase diagram of DOPE/water dispersions was investigated by NMR and X-ray diffraction in the water concentration range from 2 to 20 water molecules per lipid and in the temperature range from -5 to +50 degrees C. At temperatures above 22 degrees C, the dispersions form an inverse (HII) phase at all water concentrations. Below 25 degrees C, an HII phase occurs at high water concentrations, an L alpha phase is formed at intermediate water concentrations, and finally the system switches back to an HII phase at low water concentrations. The enthalpy of the L alpha-HII-phase transition is +0.3 kcal/mol as measured by differential scanning calorimetry. Using 31P and 2H NMR and X-ray diffraction, we measured the trapped water volumes in HII and L alpha phases as a function of osmotic pressure. The change of the HII-phase free energy as a function of hydration was calculated by integrating the osmotic pressure vs trapped water volume curve. The phase diagram calculated on the basis of the known enthalpy of transition and the osmotic pressure vs water volume curves is in good agreement with the measured one. The HII-L alpha-HII double-phase transition at temperatures below 22 degrees C can be shown to be a consequence of (i) the greater degree of hydration of the HII phase in excess water and (ii) the relative sensitivities with which the lamellar and hexagonal phases dehydrate with increasing osmotic pressure. These results demonstrate the usefulness of osmotic stress measurements to understand lipid-phase diagrams.

Heat capacities of Fe 2 O 3 -bearing silicate liquids

Abstract: Direct measurements of liquid heat capacity, using a Setaram HT1500 calorimeter in step-scanning mode, have been made in air on six compositions in the Na2O-FeO-Fe2O3-SiO2 system, two in the CaO-FeO-Fe2O3-SiO2 system and four of natural composition (basanite, andesite, dacite, and peralkaline rhyolite). The fitted standard deviations on our heat capacity measurements range from 0.6 to 3.6%. Step-scanning calorimetry is particularly useful when applied to iron-bearing silicate liquids because: (1) measurements are made over a small temperature interval (10K) through which the ferric-ferrous ratio of the liquid remains essentially constant during a single measurement; (2) the sample is held in equilibrium with an atmosphere that can be controlled; (3) heat capacity is measured directly and not derived from the slope of enthalpy measurements with temperature. Liquid compositions in the sodic and calcic systems were chosen because they contain large concentrations of Fe2O3 (up to 19 mol%), and their equilibrium ferric-ferrous ratios were known at every temperature of measurement. These measurement have been combined with heat capacity (Cp) data in the literature on iron-free silicate liquids to fit Cp as a function of composition. A model assuming no excess heat capacity (linear combination of partial molar heat capacities of oxide components) reproduces the liquid data within error (±2.2% on average). The derived partial molar heat capacity of the Fe2O3 component is 240.9 ±7.9 J/g.f.w.-K, with a standard error reduced by more than a factor of two from that in earlier studies. The model equation, based primarily on simple, synthetic compositions, predicts the heat capacity of the four magmatic liquids within 1.8% on average.

A black box mathematical model to calculate auto‐ and heterotrophic biomass yields based on Gibbs energy dissipation

Abstract: On the basis of the estimated Gibbs energy dissipation per C-mol biomass produced and a convenient black box description of microbial growth, a general equation for the calculation of the yield of biomass on electron donor has been obtained. This black box model defines four formal electron donating reactions for biomass, carbon source, electron donor, and electron acceptor. The proposed description leads to a simple equation which gives the biomass yield on electron donor for chemotrophic growth systems under carbon and energy limitation for which biomass is the only anabolic product. The variables involved are the degrees of reduction and the Gibbs energy characteristics of the four compounds, and the required Gibbs energy dissipation per C-mol produced of biomass. It appears that biomass yields on electron donor for auto- and heterotrophic growth under aerobic, denitrifying, and fermentative conditions can be estimated with 10-15% error in a range of Y(DX)-values of 0.01-0.80 C-mol/(C)-mol electron donor. Also, simple regularities in the Gibbs energy and enthalpy of organic substrates are found. Furthermore, simple relations are derived to calculate the thermodynamic maximal biomass yield, conditions required for growth to occur, heat production, biomass yield on electron acceptor, and anaerobic product yield. Finally a new definition of thermodynamic efficiency is derived.

Solubility of hydrogen in metals under high hydrogen pressures: Thermodynamical calculations

Abstract: The solubility of hydrogen was calculated up to high hydrogen pressures for ten metals having small solubilities under normal pressures. For this purpose, the equation of state of hydrogen was calculated by the procedure of Hemmes et al., and the values obtained for the molar volume, the enthalpy, the Gibbs free energy and the entropy are tabulated in the range 0.1 MPa ⩽ p ⩽ 100 GPa and 400 ⩽ T ⩽ 2000 K. For solubility calculations, the chemical potential of hydrogen in solid solution was estimated from heats of solution observed at low hydrogen pressures and concentrations, by including a pvH term (vH ≊ 2.5 × 10−3nm3/H atom), and concentration-dependent terms arising from interactions between hydrogen atoms. The solubilities obtained for f.c.c. metals Al, γ-Fe, Ni, Cu, Ag, Pt, Au and b.c.c. metals Cr, α-Fe, Mo, W are shown in the form of Arrhenius plots with pressures as a parameter. Strong enhancements of the solubility over Sievert's law are found at high hydrogen pressures (p ⪆1 GPa).

Structure and thermodynamic properties of water–methanol mixtures: Role of the water–water interaction

Abstract: Thermodynamic properties and structures of water–methanol mixtures at various temperatures have been investigated by means of Monte Carlo simulations and subsequent analyses. The OPLS model by Jorgensen was used for the methanol–methanol interaction and both the Caravetta–Clementi (CC) potential and TIP4P potential by Jorgensen et al. were used for the water–water interaction. We show that the role of water–water interaction is very important in discussing aqueous solutions of alcohols, and examine the origin of the exothermic mixing processes. We have investigated the sensitivity of the temperature dependence of the enthalpy of mixing to the water–water interaction. The CC potential is able to reproduce the temperature dependence observed in experiments, although the absolute values of the mixing enthalpy were larger than the experimental ones. While the TIP4P potential results in better agreement for the excess enthalpy and volume near room temperature, the temperature dependence of the excess enthalpy did not agree with experiment. The difference in the magnitude of the exothermic hydration for different water–water interactions is explained in terms of the energetic stability of the clathrate hydrate compared with ice, on the basis that the structure of water in the vicinity of a methanol molecule is similar to the clathrate hydrate. It is found that the energetic stability of the clathrate hydrate for the CC model is higher than that for TIP4P, and this is responsible for the larger exothermic hydration. The higher stability of the clathrate hydrate structure for the CC potential, in turn, arises from the difference in the pair interaction energy surface between two kinds of potential functions; the minimum energy structure and the flexibility of the hydrogen bonded pair.

Thermodynamic re-evaluation of the CuNi system

Abstract: This re-evaluation of the Cu-Ni system includes the influence of magnetic effects on the thermodynamic properties. A consistent set of coefficients has been achieved, which allows both, the equilibrium phase diagram and the thermodynamic functions, to be calculated. Comparison with experimental data shows good agreement, except for the lower temperature region in the solid, where measurements are extremely difficult to execute.

Prediction of thermodynamic properties of fluid mixtures by molecular dynamics simulations: methane-ethane

Abstract: NPT and NVT molecular dynamics simulation results are reported for the fluid methane-ethane mixture. Methane is modelled as Lennard-Jones fluid and ethane as a two-centre Lennard-Jones fluid with parameters obtained some time ago by fitting WCA-type perturbation theory results to two experimental vapour pressures and one bubble density for each substance. It has also already been shown that these effective pair potentials yield good predictions of the pressures and internal energies of the pure substances in the whole fluid region. In order to obtain an effective potential between the unlike molecules, previously derived fluctuation formulas for the dependence of the mixture excess properties on the unlike molecule interaction parameters are used. A test of these fluctuation formulas shows their statistical uncertainties to be small. Hence, the unlike interaction parameters can be determined from one mixture simulation and a fit to one experimental excess volume and one excess enthalpy. Then second virial...

Thermodynamics of ribonuclease T1 denaturation.

Abstract: Differential scanning calorimetry has been used to investigate the thermodynamics of denaturation of ribonuclease T1 as a function of pH over the pH range 2-10, and as a function of NaCl and MgCl2 concentration. At pH 7 in 30 mM PIPES buffer, the thermodynamic parameters are as follows: melting temperature, T1/2 = 48.9 +/- 0.1 degrees C; enthalpy change, delta H = 95.5 +/- 0.9 kcal mol-1; heat capacity change, delta Cp = 1.59 kcal mol-1 K-1; free energy change at 25 degrees C, delta G degrees (25 degrees C) = 5.6 kcal mol-1. Both T1/2 = 56.5 degrees C and delta H = 106.1 kcal mol-1 are maximal near pH 5. The conformational stability of ribonuclease T1 is increased by 3.0 kcal/mol in the presence of 0.6 M NaCl or 0.3 M MgCl2. This stabilization results mainly from the preferential binding of cations to the folded conformation of the protein. The estimates of the conformational stability of ribonuclease T1 from differential scanning calorimetry are shown to be in remarkably good agreement with estimates derived from an analysis of urea denaturation curves.

Thermodynamic study of the marked differences between acetonitrile/water and methanol/water mobile-phase systems in reversed-phase liquid chromatography

Abstract: A thermodynamic approach which considers partitioning of solute molecules between the mobile phase and stationary phase and relates the enthalpy, ΔH, and entropy, ΔS, of solute transfer to the solute partial molar excess quantities is described for the evaluation of the relative contributions of the two phases to solute retention in reversed-phase liquid chromatography (RPLC). The experimental results for ΔH and ΔS of several alkylbenzenes in methanol/water and acetonitrile/water mobile-phase systems are discussed in light of this approach