Showing papers on "Enthalpy published in 1998"

The Proton's Absolute Aqueous Enthalpy and Gibbs Free Energy of Solvation from Cluster-Ion Solvation Data

Abstract: A method is presented to determine the absolute hydration enthalpy of the proton, ΔHaq°[H+], from a set of cluster-ion solvation data without the use of extra thermodynamic assumptions. The absolute proton hydration enthalpy has been found to be ∼50 kJ/mol different than traditional values and has been more precisely determined (by about an order of magnitude). Conventional ion solvation properties, based on the standard heat of formation of H+(aq) set to zero, have been devised that may be confusing to the uninitiated but are useful in thermochemical evaluations because they avoid the unnecessary introduction of the larger uncertainties in our knowledge of absolute values. In a similar strategy, we have motivated the need for a reassessment of ΔHaq°[H+] by the trends with increased clustering in conventional cluster-ion solvation enthalpy differences for pairs of oppositely charged cluster ions. The consequences of particular preferred values for ΔHaq°[H+] may be evaluated with regard to cluster-ion prop...

Physisorption and Chemisorption of Alkanethiols and Alkyl Sulfides on Au(111)

Abstract: The energetics and kinetics of the adsorption of a variety of sulfur-containing hydrocarbons onto Au(111) have been explored using helium beam reflectivity and temperature-programmed desorption (TPD) techniques. Simple alkanethiols as well as dialkyl sulfides, dialkyl disulfides, and other sulfur-containing organics were found to adsorb with a low coverage physisorption enthalpy about 20% greater than the heat of vaporization in the bulk. In contrast to the dialkyl sulfides that only physisorb, alkanethiols and dialkyl disulfides also interact chemically with the gold with a chemisorption enthalpy of 126 kJ/mol that is independent of alkyl chain length. The presence of sterically hindering substituent groups on the carbon atom adjacent to the sulfur atom produces, however, a reduction in the chemisorption enthalpy of up to 15%. Temperature-programmed desorption of nonequilibrated, high exposure layers of alkanethiols with eight carbon atoms or longer displayed a second, higher energy, chemisorption peak a...

Enthalpy and heat capacity changes for the proton dissociation of various buffer components in 0.1 M potassium chloride

Abstract: Enthalpy and heat capacity changes for the deprotonation of 18 buffers were calorimetrically determined in 0.1 M potassium chloride at temperatures ranging from 5 to 45 degrees C. The values of the dissociation constant were also determined by means of potentiometric titration. The enthalpy changes for the deprotonation of buffers, except for the phosphate and glycerol 2-phosphate buffers, were found to be characterized by a linear function of temperature. The enthalpy changes for the second dissociation of phosphate and glycerol 2-phosphate where divalent anion is formed on dissociation were fitted with the second order function of temperature rather than the first order. Temperature dependence of buffer pH calculated by using the enthalpy and heat capacity changes obtained was in good agreement with the temperature variation of the pH values actually measured in the temperature range between 0 and 50 degrees C for all the buffers studied. On the basis of the results obtained, a numeric table showing the temperature dependence of pK values for the 18 buffers is presented.

A Simple Model of Water and the Hydrophobic Effect

Abstract: We use a simple computational model, proposed originally by Ben-Naim, to study the anomalous properties of water and the hydrophobic effect. Water molecules are modeled as two-dimensional (2D) Lennard-Jones disks, with three orientation-dependent hydrogen-bonding arms, arranged as in the Mercedes Benz (MB) logo. Phase space is explored using NPT Monte Carlo simulations. For pure water, the MB model qualitatively predicts the density anomaly (and the related negative thermal expansion coefficient at low temperature), the minimum in the isothermal compressibility as a function of temperature, the large anomalous heat capacity, and freezing to the 2D model analogue of ice, a low-density hexagonal crystal phase. For the solvation of nonpolar solutes (disks without H bonds), the model predicts the experimental trends with temperature of the free energy, entropy, enthalpy, molar volume, and heat capacity. A unique feature of these simulations is that they provide well-converged heat capacities of transfer, an i...

A Helmholtz Free Energy Formulation of the Thermodynamic Properties of the Mixture {Water + Ammonia}

Abstract: A thermodynamic model incorporating a fundamental equation of state for the Helmholtz free energy of the mixture {water+ammonia} is presented which covers the thermodynamic space between the solid–liquid–vapor boundary and the critical locus. It is also valid in the vapor and liquid phases for pressures up to 40 MPa. It represents vapor–liquid equilibrium properties with an uncertainty of ±0.01 in liquid and vapor mole fractions. Typical uncertainties in the single-phase regions are ±0.3% for the density and ±200 J mol−1 for enthalpies. Details of the data selection and the optimization process are given. The behavior of the fundamental equation of state is discussed in all parts of the thermodynamic space.

Heat capacity of poly(trimethylene terephthalate)

Abstract: Thermal analysis of poly(trimethylene terephthalate) (PTT) has been carried out using standard differential scanning calorimetry and temperature-modulated differential scanning calorimetry. Heat capacities of the solid and liquid states of semicrystalline PTT are reported from 190 K to 570 K. The semicrystalline PTT has a glass transition temperature of about 331 K. Between 460 K and 480 K, PTT shows an exothermic ordering. The melting endotherm occurs between 480 K and 505 K with an onset temperature of 489.15 K (216 C). The heat of fusion of typical semicrystalline samples is 13.8 kJ/mol. For 100% crystalline PTT the heat of fusion is estimated to be 28--30 kJ/mol. The heat capacity of solid PTT is linked to an approximate group vibrational spectrum, and the Tarasov equation is used to estimate the skeletal vibrational heat capacity ({Theta}{sub 1} = 542 K and {Theta}{sub 3} = 42 K). A comparison of calculation and experimental heat capacities show agreement of better than {+-}2% between 190--300 K. The experimental heat capacity of liquid PTT can be expressed as a linear function of temperature: C{sub p} {sup L}(exp) = 211.6 + 0.434 T J/(K mol) and compares well with estimations from the ATHAS data bank using groupmore » contributions of other polymers with the same constituent groups ({+-} 0.5%). The change of heat capacity at T{sub g} of amorphous PTT has been estimated from the heat capacities of liquid and solid to be 86.4 J/(K mol). Knowing C{sub p} of the solid, liquid, and the transition parameters, the thermodynamic functions: enthalpy, entropy and Gibbs function were obtained.« less

Chromatographic Study of Adsorption of n-Alkanes on Zeolites at High Temperatures

Abstract: Henry constants, adsorption enthalpies, and entropies of C5−C10 n-alkanes were determined using the chromatographic method on BEA*, TON, MOR, and FAU type zeolites in the temperature range 200−375 °C. On all zeolites, the Henry constants of the linear alkanes increase exponentially with the alkane chain length. Adsorption enthalpies and entropies are related to the pore geometry and have an opposite effect on the Henry adsorption constant: smaller pore sizes lead to a strong increase of the adsorption enthalpy with the carbon number, which is favorable for the adsorption equilibrium. However, this increase of −ΔH0 with CN is accompanied by a high loss of degrees of freedom in small pores, leading to a strong increase of the adsorption entropy with chain length and counterbalancing the positive effect of the enthalpy on the Henry constants. On large-pore zeolites, the opposite effect is observed. The strongest increase of the Henry constants with the carbon number occurs on zeolites in which an intermedia...

Critical Evaluation and Selection of Standard State Thermodynamic Properties for Chromium Metal and Its Aqueous Ions, Hydrolysis Species, Oxides, and Hydroxides

Abstract: This review critically evaluates the reported thermodynamic data on chromium metal, oxides, hydroxides, free aqueous ions, and hydrolysis species. Several discrepancies and inconsistencies have been uncovered and resolved to improve equilibrium calculations for chemical modeling and related engineering purposes. A revised set of data is derived from evaluation of electrochemical measurements, silver chromate solubility measurements, and auxiliary post-1980 data, reevaluation of earlier data, and reconsideration of the path for the thermodynamic network. The recommended thermodynamic values for Cr(cr), C , C , Cr , Cr2 , Cr2O3(cr), CrO3(cr), FeCr2O4(cr), CrCl2(cr), CrCl3(cr), and KFe3(CrO4)2(OH)6(cr) at 25 °C, 1 bar, and infinite dilution are given.

Thermodynamics of High-Pressure Adsorption of Argon, Nitrogen, and Methane on Microporous Adsorbents

Abstract: Adsorption equilibria of argon, nitrogen, and methane on the 13X molecular sieve and the AS activated carbon were measured at five temperatures over a wide pressure range from 0.1 to 20 MPa using a microbalance. From experimental adsorption isotherms, which are excess functions, the absolute isotherms were calculated using equations of state for a real gas phase. Grand canonical Monte Carlo simulations and density functional theory calculations were carried out in order to explain specific features of the resulting isotherms. Different thermodynamic functions evaluated from the excess and absolute adsorption isotherms were analyzed over a wide pressure range at the average temperature of the range studied.

Thermodynamic Properties of Water: Tabulation from the IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use | NIST

Abstract: Tables are provided for the density, enthalpy, entropy, and volume of water and steam calculated from the IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use. This formulation is the current international standard for water’s thermodynamic properties, and is implemented in NIST Standard Reference Database 10. The properties are tabulated along the vapor-liquid saturation curve as a function of both temperature and pressure. They are also tabulated for single-phase states on a grid of temperatures and pressures extending to 2000 oC and 1000 MPa.

Enthalpy relaxation in binary amorphous mixtures containing sucrose.

Abstract: Purpose. To compare the enthalpy relaxation of amorphous sucrose and co-lyophilized sucrose-additive mixtures near the calorimetric glass transition temperature, so as to measure the effects of additives on the molecular mobility of sucrose.

A Study of the Aggregation Behavior of Hexyltrimethylammonium Bromide in Aqueous Solution

Abstract: The self-association of n-hexyltrimethylammonium bromide (C6TAB) in aqueous solution has been examined as a function of temperature and electrolyte concentration. The critical micelle concentration (CMC) and the degree of counterion binding (β) were determined by conductivity measurement at temperatures over the range 288.15–318.15 K. Ultrasound velocity measurements were used to obtain the CMC in water and in a range of concentrations of electrolyte (0.1 to 0.6 mol kg−1NaBr) and static light scattering to obtain the aggregation number and the degree of counterion binding in water at 298.15 K. The enthalpy change on micellization in water was measured by microcalorimetry. Apparent adiabatic compressibilities were calculated from a combination of density and ultrasound velocity measurements. Changes in the thermodynamic properties on micellization were determined by applying the mass action model; good agreement was found between experimental and theoretical enthalpy changes. From comparison with the properties of other n-alkyltrimethylammonium bromides it has been shown that the CMC of C6TAB in water is lower than that predicted from the linear relationships between CMC and the number of carbon atoms in the alkyl chain. Similarly, the standard Gibbs energy of micellization is less negative than predicted, and the degree of counterion binding is much lower than for other CnTABs. It is suggested that the anomalous behavior of C6TAB is a consequence of the more highly organized core of the aggregates of very low aggregation number (3–4) and the high degree of exposure of the micellar components to the aqueous environment.

Thermodynamic analysis of protein interactions with biosensor technology.

Abstract: A methodology using biosensor technology for combined kinetic and thermodynamic analysis of biomolecular interactions is described. Rate and affinity constants are determined with BIAcore. Thermodynamics parameters, changes in free energy, enthalpy and entropy, are evaluated from equilibrium data and by using rate constants and transition state theory. The methodology using van't Hoff theory gives complementary information to microcalorimetry, since only the direct binding is measured with BIAcore whereas microcalorimetry measures all components, including e.g. hydration effects. Furthermore, BIAcore gives possibilities to gain new information by thermodynamic analysis of the rate constants.

Thermodynamics of Membrane Partitioning for a Series of n-Alcohols Determined by Titration Calorimetry: Role of Hydrophobic Effects†

Abstract: Recent studies have shown that the traditional paradigm relying on hydrophobic effects is not adequate to describe membrane partitioning of amphiphilic solutes. To elucidate the thermodynamics and determine the role of the hydrophobic effect in the partitioning of small amphiphilic molecules into lipid bilayers, we have used titration calorimetry to directly measure the enthalpy, partition coefficients, and heat capacity change for the partitioning of a series of n-alcohols into lipid bilayers of several lipid compositions. The incremental thermodynamic quantities have been compared with model compound data for partitioning into bulk hydrocarbon solvents. We have found that there is a large negative heat capacity change upon partitioning, indicating a major contribution from the dehydration of nonpolar solute moieties; however, these hydrophobic effects also involve changes in lipid interactions with water in the interfacial region of the bilayer. In addition, we have found that the enthalpy effects are l...

Adsorption of proteins at the liquid/air interface.

Abstract: Thermodynamic equations for describing protein adsorption layers at liquid/fluid interfaces are derived as a generalization of a theory published recently [J. Colloid Interface Sci. 1996, 183, 26]. In this new theory the nonideality of enthalpy (Flory−Huggins' parameter) and entropy of mixing are taken into account, and also the effect of the electric charge of the protein molecules on surface pressure is considered. The model is verified by experimental dynamic and equilibrium surface tension data for HSA solutions obtained from pendent drop experiments (ADSA). The derived isotherm is in good agreement with the experimental data. The values of the isotherm parameters surface area per molecule, electric charge of the HSA molecule, and the adsorption layer thickness are close to values obtained by other methods in literature. It follows that HSA molecules undergo almost no denaturation at the solution/air interface and occupy a surface area of about 50 nm2, independent of the packing in the adsorption laye...

Kinetics of γ → α phase transformation in Fe-Mn alloys containing low manganese

Abstract: The kinetics of the γ → α phase transformation in Fe-Mn alloys with low Mn contents was studied using differential scanning calorimetry. Analysis of the measured heat capacity yielded both the austenite fractions as a function of temperature as well as the enthalpy difference between ferrite and austenite which decreases with increases in both the temperature and the Mn concentration. The transformation was modelled using an interface controlled growth model. It was assumed that the interface velocity is proportional to the chemical potential difference of the Fe lattice. The interface mobility has a simple exponential temperature dependence. Taking into account the austenite grain size distribution, the activation energy and the pre-exponential factor for the interface mobility were estimated from the heat effects at 140 kJ mol−1 and 58 mm mol J−1 S−1 respectively.

Pressure and temperature control of product chirality in asymmetric photochemistry. Enantiodifferentiating photoisomerization of cyclooctene sensitized by chiral benzenepolycarboxylates

Abstract: Pressure effects upon asymmetric photosensitization have been investigated for the first time in the enantiodifferentiating Z−E photoisomerization of cyclooctene (1), sensitized by chiral aromatic esters (2−7). The product's enantiomeric excess (ee) and E/Z ratio were critical functions of the applied pressure, exhibiting an unprecedented switching of the product chirality. Depending upon the chiral sensitizer employed, the differential activation volume (ΔΔV⧧) varies widely from −3.7 to +5.6 cm3 mol-1, which is unexpectedly large for an enantiodifferentiation in the excited state. However, the ΔΔV⧧ values obtained do not correlate with the differential activation enthalpy (ΔΔH⧧) or entropy (ΔΔS⧧) obtained from temperature-dependence studies, indicating that pressure and temperature function as independent perturbants for the photoenantiodifferentiation process. Further investigations on the pressure dependence of ee at low temperatures enable us to construct the first three-dimensional diagram that corre...

Protein stability and folding : a collection of thermodynamic data

Abstract: 1 Evaluation of the approaches for the determination of conformational stability and related thermodynamic quantities.- 2 Comments on the tabulated data.- 3 Why to prefer complex thermodynamic functions.- References.- Table 1. Gibbs energy change-molarvalues.- Table 2. Enthalpy and heat capacity changes-molar values.- Table 3. Enthalpy and heat capacity changes-specific values.- References (Tables 1-3).- Index of Proteins.

New method to calculate thermodynamic and transport properties of a multi-temperature plasma : application to n2 plasma

Abstract: Multi-temperature thermal plasmas have often to be considered to account for the nonequilibrium effects. Recently Andre et al. have developed the calculation of concentrations in a multi-temperature plasma by artificially separating the partition functions into a product by assuming that the excitation energies are those of the lower levels (electronic, vibration, and rotation). However, at equilibrium, differences, increasing with temperature, can be observed between partition functions calculated rigorously and with their method. This paper presents a modified method where it has been assumed that the preponderant rotational energy is that of the vibrational level v=0 of the ground electronic state and the preponderant vibrational energy is that of the ground electronic state. The internal partition function can then be expressed as a product of series expressions. At equilibrium for N 2 and N 2 + partition functions the values calculated with our method differ by less than 0.1% from those calculated rigorously. The calculation has been limited to three temperatures: heavy species Th , electrons Te , and vibrational T v temperatures. The plasma composition has been calculated by minimizing the Gibbs free enthalpy with the steepest descent numerical technique. The nonequilibrium properties have been calculated using the method of Devoto, modified by Bonnefoi and Aubreton. The ratio θ=Te/Th was varied between 1 and 2 as well as the ratio θ v =T v /T h for a nitrogen plasma. At equilibrium the corresponding equilibrium transport properties of Ar and N 2 are in good agreement with those of Devoto and Murphy except for T>10,000 K where we used a different interaction potential for N–N + . The effects of θv and θe on thermodynamic and transport properties of N 2 are then discussed.