Showing papers on "Enthalpy published in 2001"

Thermodynamics of Micelle Formation of Ionic Surfactants: A Critical Assessment for Sodium Dodecyl Sulfate, Cetyl Pyridinium Chloride and Dioctyl Sulfosuccinate (Na Salt) by Microcalorimetric, Conductometric, and Tensiometric Measurements

Abstract: The thermodynamics of micelle formation of ionic surfactants, sodium dodecyl sulfate (SDS), cetyl pyridinium chloride (CPC), and sodium salt of dioctyl sulfosuccinate (Aerosol OT or AOT) have been thoroughly assessed from microcalorimetric, conductometric, and tensiometric measurements, and the results have been rationalized in terms of physicochemical concepts and relations. The past and recent critical micellar concentration (CMC) data on SDS have been considered and compared; the CMCs of SDS, CPC, and AOT determined in this and earlier studies have been processed to evaluate the energetic parameters (free energy, enthalpy, entropy, and heat capacity) of micellization. The effect of the salt, NaCl, on the CMC and energetics of micellization of the surfactants has been also examined.

Thermodynamic properties of lithium bromide-water solutions at high temperatures.

Abstract: Emerging triple-effect LiBr–water absorption chillers operate at higher temperatures and pressures than traditional double-effect chillers. However, there is not enough data about thermodynamic properties of LiBr–water solutions at such high temperatures. Using recently measured data of vapor pressure and heat capacity, we have developed the equations which can calculate the vapor pressure, enthalpy and entropy of LiBr solutions at such high temperatures. The developed equations are valid from concentrations of 40–65 wt.% and also from temperatures of 40–210°C. These equations will be very helpful for the modeling and design of triple-effect LiBr–water chillers.

Temperature Effects on Nickel Sorption Kinetics at the Mineral–Water Interface

Abstract: In recent years, innovative studies have shown that sorption of metals onto natural materials results in the formation of new mineral-like precipitate phases that increase in stability with aging time. While these findings have demonstrated the usefulness of current state-of-the-art molecular-scale methods for confirming macroscopic data and elucidating mechanisms, basic kinetic and thermodynamic parameters for the formation of the metal precipitates have not been examined. This study examined Ni-sorption kinetics on pyrophyllite, talc, gibbsite, amorphous silica, and a mixture of gibbsite and amorphous silica over a temperature range of 9 to 35°C. Using the Arrhenius and Eyring equations, we calculated the energy of activation (E,) and enthalpy (ΔH ) , entropy (ΔS ) , and free energy of activation (ΔG ) , related to the formation of the Ni precipitates. Based on values of E, (93.05 to 123.71 kJ mol -1 ) and ΔS (-27.51 to -38.70 J mol -1 ), Ni sorption on these sorbents was surface-controlled and an associative mechanism. The ΔH values (90.60 to 121.26 kJ mol -1 ) suggest, as indicated by E a values, that an energy barrier was present for the system to overcome in order for the reaction to occur. Additionally, the large, positive ΔG values suggest there is an energy barrier for product formation. Although metal precipitation reactions often occur in the natural environment, this study shows that the rate of these reactions depends strongly on temperature.

Modeling of thermodynamic properties for Bcc, Fcc, liquid, and amorphous iron

Abstract: The thermodynamic properties of pure iron have been analyzed by using models that contain some parameters of physical significance In addition to the lattice, electronic, and magnetic components, a contribution to the heat capacity due to the excitation between two magnetic states has been considered for the fcc phase The liquid and amorphous phases have been treated as one phase with the generalized two-state model The magnetic properties of amorphous iron have been reviewed and taken into account in the modeling A relatively more accurate description of the magnetic effect in the bcc phase has been obtained; this is found to be essential for the accommodation of various experimental measurements on the enthalpy and Gibbs energy differences between bcc and fcc iron

Solubility of C60 Fullerene

Abstract: The most recent and comprehensive set of solubility data of C60 fullerene in various solvents at 298 and 303 K was treated by multivariate stepwise linear regression applied as the linear solvation energy approach. In a few cases where the fullerene formed crystalline solvates, the enthalpy of incongruent melting and the temperature of maximum solubility were used to determine the “hypothetical solubility” of the unsolvated C60, which was then used in the statistical procedure instead of the solubility of the solvate. The “ideal” solubility of the fullerene was estimated, and the “nonideal” part of the Gibbs energy of solution was then related to solute−solvent interactions. Increasing molar volume and solvent polarity (as measured by the Dimroth−Reichardt “general polarity” parameter, ET(30)) diminished the solubility of C60, whereas electron pair donation ability and polarizability enhanced solubility.

Predicting Divalent Metal Sorption to Hydrous Al, Fe, and Mn Oxides

Abstract: Intraparticle diffusion in microporous amorphous oxides of aluminum, iron, and manganese affects contaminant mobility and bioavailability in soils and sediments. This sorption is a lengthy process, as such, predictive methods to assess thermodynamic and transport parameters would be useful. Based on enthalpies observed in recent work, adsorption of Zn, Cd, and Sr to amorphous oxides is a physical type of reaction where the metal ions retain their waters of hydration. Consequently the adsorbate−surface interactions are a function of electrostatic forces of attraction. Accordingly, knowing the hydrated radius and the hydration number of a metal cation, a correlation is used to predict enthalpy and hence affinity. Using the resulting enthalpy and the Polanyi relation, the activation energy was evaluated for Ni and Ca. This Polanyi relationship reveals that for a given metal the activation energies with respect to these oxides are comparable. Additionally, metals of the same periodic group appear to form simi...

Enthalpy and volume changes in lipid membranes I. The proportionality of heat and volume changes in the lipid melting transition and its implication for the elastic constants.

Abstract: Differential scanning calorimetry, pressure calorimetry, and densitometry have been employed to study the relation between volume and enthalpy changes in the melting regime of lipid membranes. We demonstrate a rigid proportional relation between volume expansion coefficient and heat capacity. This result is first shown in densitometric experiments. It implies that calorimetric profiles obey a simple scaling law for the temperature axes in experiments with applied hydrostatic pressure. In a theoretical paper (Heimburg, T. Biochim. Biophys. Acta 1998, 1415, 147-162), we have argued that this relation has far-reaching consequences for the predictiblity of elastic constants from the heat capacity. The proportionality constant between volume and enthalpy changes is found to be independent of the lipid, which has interesting consequences for the calculation of the elastic constants of biological lipid mixtures with unknown composition. We demonstrate this for lung surfactant, which displays a similar relation between volume and enthalpy changes.

Structural Thermodynamics of Hydration

Abstract: A simple two-state structural model of solute hydration has been developed. In this model, both water in the bulk state and water of solute hydration are assumed to consist of two structural species: a high density/high enthalpy species, structurally similar to ice III, and a low density/low enthalpy species, structurally similar to ice I. It is assumed that structural and thermodynamic distinctions between bulk and hydration water originate solely from the differential fractional composition, whereas the two structural species and thermodynamic parameters associated with each species are identical for bulk and hydration water. This model has been used in conjunction with volumetric data reported in the literature to analyze the hydration properties of charged, polar, and nonpolar groups at 25 °C. The equilibrium between the two structural species of water of hydration of charged and polar groups is shifted toward the high density/high enthalpy species. In contrast, the equilibrium between the two specie...

Thermodynamic Aspects of Dicarboxylate Recognition by Simple Artificial Receptors

Abstract: Recognition of dicarboxylates by bis-functional hydrogen-bonding receptors displays divergent thermodynamics in different solvent systems. NMR titration and isothermal titration calorimetry indicated that neutral bis-urea and bis-thiourea receptors form exothermic complexes with dicarboxylates in DMSO, with a near zero entropic contribution to binding. The increased binding strength of bis-guanidinium receptors precluded quantitative measurement of binding constants in DMSO, but titration calorimetry offered a qualitative picture of the association. Formation of these 1:1 complexes was also exothermic, but additional endothermic events occurred at both lower and higher host-guest ratios. These events indicated multiple binding equilibria but did not always occur at a discrete 2:1 or 1:2 host-guest molar ratio, suggesting higher aggregates. With increasing amounts of methanol as solvent, bis-guanidinium receptors form more endothermic complexes with dicarboxylates, with a favorable entropy of association. This switch from association driven by enthalpy to one driven by entropy may reflect a change from complexation involving the formation of hydrogen bonds to that promoted by solvent liberation from binding sites.

Thermodynamic study of the solubility of some sulfonamides in octanol, water, and the mutually saturated solvents

Abstract: The free energy, enthalpy and entropy of solution, were evaluated from solubility data for a group of sulfonamides from 25 to 40°C in octanol, water, and the mutually saturated solvents. In aqueous media, the solubility was determined at the isoelectric point and ionic strength 0.15 mol-L−1. The excess free energy and the activity coefficients of the solutes also were determined. The results are discussed in terms of solute–solvent interactions.

An enthalpic component in cooperativity: the relationship between enthalpy, entropy, and noncovalent structure in weak associations.

Abstract: Attempts to quantify binding interactions of noncovalent complexes in aqueous solution have been stymied by complications arising from enthalpy−entropy compensation and cooperativity. We have extended work detailing the relationship between noncovalent structure and free energy of binding to include the roles of enthalpy and entropy of association. On the basis of van't Hoff measurements of the dimerization of vancomycin type antibiotics, we demonstrate that positive cooperativity manifests itself in a more favorable enthalpy of association and a partially compensating less favorable entropy of association. Finally, we extend these results to rationalize thermodynamic observations in unrelated systems.

Experimental study of phase equilibria and thermodynamic optimization of the Fe-Zn-O system

Abstract: The Fe-Zn-O phase diagram in air was studied over the temperature range from 900 °C to 1500 °C. The compositions of the phases in quenched samples were obtained by electron probe X-ray microanalysis (EPMA). This experimental technique is not affected by zinc losses resulting from vaporization of zinc at high temperatures. The model for the spinel solid solution was developed within the framework of the compound-energy formalism (CEF). The choice of parameters of the CEF and the sequence of their optimization can have a major influence on the predictions in multicomponent phases. These choices can only be made rationally by reference to the specific model being represented in the CEF. This is discussed for the case of the two-sublattice spinel model. In the limiting case, the proposed model reduces to the model by O’Neill and Navrotsky for spinels. When the CEF is used in combination with the equation of Hillert and Jarl to describe the magnetic contribution to thermodynamic functions of a solution, it is necessary to assign certain values of magnetic properties to all pseudocomponents and to magnetic interaction parameters to obtain the most reasonable approximation of the magnetic properties of a solution. It was shown how this can be done based on very limited experimental data. The same equations can be used when the Murnaghan or the Birch-Murnaghan equation is combined with the CEF to describe the pressure dependence of thermodynamic functions. The polynomial model was used to describe the properties of wustite and zincite, and the modified quasichemical model was used for the liquid slag. All thermodynamic and phase-equilibria data on the Fe-O and Fe-Zn-O systems were critically evaluated, and parameters of the models were optimized to give a self-consistent set of thermodynamic functions of the phases in these systems. All experimental data are reproduced within experimental error limits. These include the thermodynamic properties of phases (such as specific heat, heat content, entropy, enthalpy, and Gibbs energy); the cation distribution between octahedral and tetrahedral sites in spinel; the oxygen partial pressure over single-phase, two-phase, and three-phase regions; the phase boundaries (liquidus, solidus, and subsolidus); and the tie-lines.

Solvent Size vs Cohesive Energy as the Origin of Hydrophobicity

Abstract: Two main physical explanations of hydrophobicity seem to be currently competing. The classical, intuitive view attributes it to the fact that interactions between water molecules are much stronger than those between water and nonpolar groups. The second, “heretic” view attributes it to the small size of the water molecule which increases the entropic cost of opening up a cavity to accommodate the solute. Here we examine the solvation of methane in water and in model liquids that lack one or more of water's properties and report a detailed decomposition of the solvation free energy, enthalpy, entropy, and heat capacity in these solvents. The results fully support the classical view. It is found that fluids with strong intermolecular interactions favor expulsion of methane to its pure phase or to CCl4, whereas fluids with weak intermolecular interactions do not. However, the specific thermodynamic signature of the hydrophobic effect (entropy driven at room temperature with a large heat capacity change) is a...

Ferrate(VI) Oxidation of Aqueous Phenol: Kinetics and Mechanism

Abstract: Kinetic and thermodynamic parameters for ferrate(VI) oxidation of phenol have been measured in isotopic solvents, H2O and D2O, using ambient and high-pressure stopped-flow UV−visible spectroscopy. An increase (fast stage) and then a decrease (slow stage) in absorbance at 400 nm are observed when potassium ferrate (K2FeO4) and aqueous phenol solutions are mixed rapidly. This suggests that small amounts of unstable intermediate 4,4‘-biphenoquinone are produced during this redox process. An electron paramagnetic resonance signal for the reaction mixture of ferrate and phenol trapped by spin-trap α-(4-pyridyl-1-oxide)-N-tert-butylnitrone indicates a radical reaction pathway. Gas chromatographic/mass spectrometric measurements show p-benzoquinone is a major organic product, and the red ferric thiocyanate complex formed from addition of potassium thiocyanate to the spent reaction solution indicates that Fe(VI) is reduced to Fe(III). Activation enthalpy, entropy, and volume changes have been determined. There is...

Orthorhombic Intermediate State in the Zinc Blende to Rocksalt Transformation Path of SiC at High Pressure

Abstract: The mechanism of the B3/B1 phase transition of SiC has been investigated by periodic LCAO-DFT least-enthalpy calculations. A new transformation pathway, based on a Pmm2 orthorhombic intermediate state with two SiC units per cell, is found to be energetically favored over the traditional R3m mechanism. The computed activation enthalpy is 0.75 eV/SiC unit at the predicted transition pressure of 92 GPa (B3LYP functional). Activation enthalpy and activation volume vs pressure are analyzed to characterize the kinetic aspects of the transformation.

The assessment of thermodynamic parameters in the Al2O3‐Y2O3 system and phase relations in the Y‐Al‐O system

Abstract: Thermodynamic parameters for solid and liquid phases in the Al 2 O 3 -Y 2 O 3 system are assessed using new calorimetric measurement for the YAG (Y 3 Al 5 O 12 ), YAP (YAlO 3 ) and YAM (Y 4 Al 2 O 9 ) phases. The calculated phase diagram of the Al 2 O 3 -Y 2 O 3 system is in reasonable agreement with experimental data. According to the calculations, the YAP phase melts congruently and is stable down to the low temperatures, while the YAM phase disproportionates to a mixture of YAP and Y 2 O 3 phases at temperatures below 1385 K. The calculated entropy of the YAG phase 300.1 J/(mol.K) is between 2 experimentally determined values 284.8 and 349.1 J/(mol K). However, the difference between calculated and experimental values exceeds uncertainty limits of adiabatic calorimetry data. The enthalpies of melting for the YAG and YAP phases calculated in this study are in reasonable agreement with DTA measurements. The calculated enthalpy of melting for the YAG phase is not consistent with estimates based on solution calorimetric data. New independent measurement of the standard entropy and enthalpy of melting are desirable for the YAG, YAM and YAP phases. The liquidus surface and isothermal section at 2000 K for ternary Al-Y-O system are calculated in this study.

Thermodynamics and kinetics of Zr–Ti–Cu–Ni–Be bulk metallic glass forming liquids

Abstract: The viscosity of the equilibrium melt as well as of the undercooled liquid of bulk metallic glass (BMG) forming Zr–Ti–Cu–Ni–Be alloys is several orders of magnitude higher than in pure metals and other alloys. The temperature dependence of the viscosity is consistent with that of a moderately strong liquid. Entropy, enthalpy and Gibbs free enthalpy of the supercooled liquids are closer to the crystalline mixture than in other alloys. Both, the sluggish kinetics and the relative thermodynamic stabilization are likely to have the same origin. The Zr–Ti–Cu–Ni–Be alloys are dense liquids with a small free volume and a pronounced tendency to develop short-range order. The viscosity can describe the crystallization kinetics at high temperatures, whereas at low temperatures it is controlled by solid like hopping of atoms.

Adsorption of Supercritical Carbon Dioxide on Silica

Abstract: In this work, the adsorption isotherms of carbon dioxide on silica have been measured with a gravimetric method at nine different temperature values (from 312 up to 466 K) and at pressure levels up to 366 bar. The adopted technique allows for a direct evaluation of the density that is measured simultaneously to the adsorption load, thus avoiding possible systematic errors arising from the use of an equation of state to calculate density from pressure and temperature data. The enthalpy changes related to the adsorption process have been obtained from a thermodynamic analysis based only on excess properties.

Calculations of amorphous-forming composition range for ternary alloy systems and analyses of stabilization of amorphous phase and amorphous-forming ability

Abstract: The amorphous-forming composition range (AFCR) was calculated for 338 ternary amorphous alloy systems on the basis of the database given by Miedema's model in order to examine the applicability of the model, to analyze the stability of the amorphous phase, and to determine the dominant factors influencing the ability to form an amorphous phase. The mixing enthalpies of amorphous and solid solution phases were expressed as a function of alloy compositions on the basis of chemical enthalpy. Based on the Eshelby and Friedel model, an elastic enthalpy term was added to the model for the solid solution. Furthermore, an average melting temperature of the constituent elements was added to the model as the topological enthalpy in an amorphous phase. An amorphous phase was assumed to have been formed at the composition where the enthalpy of an amorphous phase was smaller than that of a solid solution. The AFCR was calculated for 335 systems except for the Al-Cu-Fe, Al-Mo-Si and Au-Ge-Si systems. The calculated results are in agreement with the experimental data for Cu-Ni- and Al-Ti-based systems. For typical amorphous alloy systems exemplified by the Zr-, La-, Fe- and Mg-based systems, it was recognized that the calculated AFCR had been overestimated as a result of the model being simplified. We have also shown that the elastic enthalpy term arising in a solid solution phase stabilizes the amorphous phase, and the stabilization mechanism is particularly notable in Mg-based amorphous alloy systems. Short-range order plays an important role in the formation of Al-, Fe- and Pd-metalloid based systems. The following factors have a great influence on amorphous-forming ability: (1) three empirical rules for the achievement of high AFA, (2) melting temperature and viscosity at the melting temperature, (3) elastic enthalpy arising in a solid solution, and (4) short-range order observed in an amorphous phase. The importance of the latter two factors was only identified as a result of the present study.

A Thermometric Titration Study on the Micelle Formation of Sodium Decyl Sulfate in Water

Abstract: The critical micelle concentration (cmc) and the enthalpy of micelle formation (ΔmicH of sodium n-decyl sulfate (SDeS) in aqueous solutions were determined in the temperature range 15–45°C by isothermal titration microcalorimetry. An empirical sigmoidal expression providing high accuracy and internal consistency was used for the evaluation of the titration curves. As a function of increasing temperature, the cmc passes through a minimum at 29.5°C, where ΔmicH changes sign from positive to negative. The isosteric enthalpies calculated by means of the van't Hoff relation agree very well with the directly measured calorimetric enthalpies. The cmc, enthalpy, Gibbs energy, entropy, and heat capacity data of SDeS micellization are reported and compared with the results of previous studies on the same system. It was found that the presence of n-decanol, which appears to be a product of the natural hydrolysis of SDeS, significantly lowers the cmc. ΔmicH is less appreciably influenced by this impurity.

A Langmuir–Hinshelwood model for a hydrogen transfer mechanism in the selective hydrogenation of acetylene over a Pd/γ-Al2O3 catalyst prepared by the sol–gel method

Abstract: This study represents an effort to fit a Langmuir–Hinshelwood-type mechanism which involves a proposed hydrogen transfer step from a carbonaceous deposit with observed rate data, spectroscopic data, and reported literature results on a supported Pd/γ-Al 2 O 3 catalyst prepared by the sol–gel method. Our data was taken in a differential reactor as the temperature was varied from 100 to 225°C in six increments. A change in the apparent reactant orders and activation energies support a multiple mechanism rate expression. Our data correlates best with an expression composed of two mechanisms: a competitive, low-temperature mechanism (mechanism I) with an activation energy of 29 kJ/mol, and a non-competitive, high-temperature mechanism (mechanism II) with an activation energy of 168 kJ/mol. Both mechanisms were fit using an enthalpy of adsorption of −124 kJ/mol for acetylene. The enthalpy of the hydrogen transfer was found to be 133 kJ/mol, indicating an endothermic process. Mechanism II is predicted to emerge and dominate at high temperatures.