Showing papers on "Enthalpy published in 2003"
TL;DR: In this paper, the removal of heavy metals such as Mn, Co, Ni, and Cu from aqueous solution is studied using a raw kaolinite. And the results show that heavy metal removal is an endothermic process and the process of adsorption is favored at high temperatures.
707 citations
TL;DR: It is shown that linear relationships exist between dissociation activation energies and enthalpy changes and merely from knowledge of adsorption energies the barriers to catalytic elementary reaction steps can be estimated.
Abstract: The activation energy to reaction is a key quantity that controls catalytic activity. Having used ab inito calculations to determine an extensive and broad ranging set of activation energies and enthalpy changes for surface-catalyzed reactions, we show that linear relationships exist between dissociation activation energies and enthalpy changes. Known in the literature as empirical Bronsted−Evans−Polanyi (BEP) relationships, we identify and discuss the physical origin of their presence in heterogeneous catalysis. The key implication is that merely from knowledge of adsorption energies the barriers to catalytic elementary reaction steps can be estimated.
507 citations
TL;DR: In this paper, the authors performed density-functional theory (DFT) calculations of carbon dissolution and diffusion in iron, the latter being a typical example of interstitial diffusion, and they found that a supercell with 128 Fe atoms and one C atom is sufficient for describing dilute concentrations of carbon in bcc Fe.
Abstract: We perform density-functional theory (DFT) calculations of carbon dissolution and diffusion in iron, the latter being a typical example of interstitial diffusion. The Kohn-Sham equations are solved with periodic boundary conditions and within the projector-augmented-wave formalism, using the generalized gradient approximation for electron exchange and correlation. With the solution enthalpy as an indication of cell size convergence, we find a supercell with 128 Fe atoms and one C atom is sufficient for describing dilute concentrations of carbon in bcc Fe. The solution enthalpy of carbon in an octahedral site in ferrite is predicted to be 0.74 eV, i.e., the dissolution of carbon in bcc ferromagnetic (FM) Fe is an endothermic process. Using the Fe128C1 periodic cell, we find that the minimum-energy path (MEP) of carbon diffusion from one octahedral site to another (via a tetrahedral site) has a barrier of 0.86 eV, in excellent agreement with the experimental value of 0.87 eV. This encouraging benchmark result prompted us to investigate carbon diffusion in austenite, whose electronic structure is less well characterized experimentally. Cell size convergence results show that a supercell with 32 Fe atoms and one C atom is sufficient. The calculated solution enthalpy is $\ensuremath{-}0.17$ eV, which indicates that the dissolution of carbon in fcc Fe is exothermic, consistent with the known greater solubility of C in austenite compared to ferrite. The MEP shows that carbon moves linearly from an octahedral site to another, contrary to the common notion of an off-plane diffusion path. The diffusion barrier is calculated to be 0.99 eV. Since we model austenite with the FM high-spin phase, the diffusion barrier we obtain is not directly comparable to the experiments in which austenite is usually paramagnetic. However, this prediction is relevant for C incorporation into Fe thin films, since FM high-spin fcc Fe can be obtained by epitaxial growth of thin Fe films on a Cu substrate.
379 citations
TL;DR: In this article, the moisture sorption isotherms of potato were determined using a gravimetric static method at 30, 45 and 60 °C, and over a range of relative humidities.
277 citations
TL;DR: In this article, high temperature oxide melt solution calorimetry has been used to measure the enthalpy as a function of polymorphism and surface area for oxides of Al, Ti, and Zr.
Abstract: Many oxides tend to form different structures (polymorphs) for small particles. High temperature oxide melt solution calorimetry has been used to measure the enthalpy as a function of polymorphism and surface area for oxides of Al, Ti, and Zr. The results confirm crossovers in polymorph stability at the nanoscale. The energies of internal and external surfaces of zeolitic silicas with open framework structures are an order of magnitude smaller than those of oxides of normal density.
265 citations
TL;DR: In this paper, the authors measured the capacity of 1-butyl-3-methylimidazolium hexafluorophosphate (C4mim][PF6) in a range of temperatures from (5 to 550) K, based on measurements by calorimetry.
Abstract: Thermodynamic functions for 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) are reported in a range of temperatures from (5 to 550) K, based on new measurements by calorimetry. Heat capacities of the crystal, glass, and liquid phases for [C4mim][PF6] were measured with a pair of calorimeters. A vacuum-jacketed adiabatic calorimeter was used at temperatures between (5 and 310) K, and a heat bridge-scanning calorimeter was used from (300 to 550) K. With the adiabatic calorimeter, the fusion Tfus = 283.51 K, = 19.60 kJ·mol-1, and the glass transition Tg = 190.6 K were observed. The [C4mim][PF6] test sample was determined to have a mole fraction purity of 0.9956 by a fractional melting analysis. Densities of the liquid were measured in a range of temperatures from (298 to 353) K with a pycnometer equipped with a capillary neck. An unexpected endothermal transition, with a very small enthalpy change of 0.25 J·g-1 (0.071 kJ·mol-1), was observed in a range of temperatures from (394 to 412) K. He...
255 citations
TL;DR: In this article, the kinetics of pyrolysis of rice husk were studied under non-isothermal heating in air or nitrogen media, and the results were best described by the equation of Ginstling-Brounshtein valid for diffusion-controlled reactions starting on the exterior of spherical particles with uniform radius.
231 citations
TL;DR: In this article, the adsorption behavior of Ni(II) onto bentonite was studied as a function of temperature under optimized conditions of shaking time, amount of adsorbent, pH, and concentration of the adsorbate.
227 citations
TL;DR: In this article, the measurements of the solubility of carbon dioxide and oxygen in a commonly used room temperature ionic liquid, butyl methyl imidazolium tetrafluoroborate ([bmim][BF4]), are reported as a function of temperature between 303 K and 343 K and at pressures close to atmospheric.
Abstract: The measurements of the solubility of carbon dioxide and oxygen in a commonly used room temperature ionic liquid, butyl methyl imidazolium tetrafluoroborate ([bmim][BF4]), are reported as a function of temperature between 303 K and 343 K and at pressures close to atmospheric. A new experimental apparatus, based on a saturation method, is presented. The solubility is expressed in terms of molarities, mole fractions, and Henry's law coefficients. From their variation with temperature, the partial molar thermodynamic functions of solvation such as the standard Gibbs energy, the enthalpy, and the entropy are calculated. The precision of the experimental data, considered as the average absolute deviation of the Henry's law coefficients from appropriate smoothing equations, is 4% for oxygen and 3% for carbon dioxide.
186 citations
TL;DR: In this paper, three independent methods (sublimation, solubility and solution calorimetry) were used to study the dissolution and solvation processes of diflunisal (DIF) and flurbiprofen (FBP).
186 citations
TL;DR: In this article, the metastable phase Al9Ni2 was investigated in order to characterize the thermodynamics and kinetics of its formation, and it was observed as the first phase to form in a series of sputterdeposited Al/Ni multilayer foils, but it did not form in foils with a very small bilayer period.
TL;DR: In the framework of the two-dimensional non-ideal solution model, surface layer equations of state, adsorption isotherms and functions of the distribution of protein molecules in respect to different molar area were derived and the model reflects the well-known differences between proteins and ordinary surfactants.
TL;DR: In this article, the enthalpies of the reactions in which carbon dioxide hydrate is dissociated to carbon dioxide vapor and either water or ice are determined by an analysis with the Clapeyron equation.
TL;DR: In this paper, the phase ratio changes, if they occur, can cause deviations from linearity in a van't Hoff plot, and the phase ratios influence can be eliminated by considering the molecular difference between two solutes instead of the solutes themselves.
Abstract: Potential temperature is used in oceanography as though it is a conservative variable like salinity; however, turbulent mixing processes conserve enthalpy and usually destroy potential temperature. This negative production of potential temperature is similar in magnitude to the well-known production of entropy that always occurs during mixing processes. Here it is shown that potential enthalpy—the enthalpy that a water parcel would have if raised adiabatically and without exchange of salt to the sea surface—is more conservative than potential temperature by two orders of magnitude. Furthermore, it is shown that a flux of potential enthalpy can be called “the heat flux” even though potential enthalpy is undefined up to a linear function of salinity. The exchange of heat across the sea surface is identically the flux of potential enthalpy. This same flux is not proportional to the flux of potential temperature because of variations in heat capacity of up to 5%. The geothermal heat flux across the o...
TL;DR: In this article, the authors used a phenomenological approach based on a "polymer-like" G(E) model coupled with the statistical-mechanical theory of isotope effects.
Abstract: Liquid-liquid phase splitting in ternary mixtures that contain a room-temperature ionic liquid and an alcohol aqueous solution-namely, [bmim] [PF6] + ethanol + water and [bmim] [NTf2] + 2-methylpropanol + water-is studied. Experimental cloud-point temperatures were obtained up to pressures of 400 bar, using a He-Ne laser light-scattering technique. Although pressurization favors mutual miscibility in the presence of high concentrations of alcohols, the contrary occurs in water-rich solutions. Both ternary mixtures exhibit a very pronounced water-alcohol co-solvent effect. Solvent isotope effects are also investigated. Phase diagrams are discussed using a phenomenological approach based on a "polymer-like" G(E) model coupled with the statistical-mechanical theory of isotope effects. The combined effect of a red shift of -15 cm(-1) for the O-H deformation mode of ethanol with a blue shift of +35 cm(-1) for the O-H stretching mode, both of which occurring after liquid infinite dilution in the ionic liquid, rationalizes the observed isotope effect in the phase diagram. Predicted excess enthalpy (H-E) values are inferred from the model parameters. Furthermore, using the Prigogine-Defay equation, an estimation of the excess volumes (V-E) is obtained.
TL;DR: The design and synthesis of receptors containing a Cu(II) binding site with appended ammonium groups and guanidinium groups, along with thermodynamics analyses of anion binding, are reported and provide a picture of the roles of the host, guest, counterions, and solvent.
Abstract: The design and synthesis of receptors containing a Cu(II) binding site with appended ammonium groups (1) and guanidinium groups (2), along with thermodynamics analyses of anion binding, are reported. Both receptors 1 and 2 show high affinities (10(4) M(-1)) and selectivities for phosphate over other anions in 98:2 water:methanol at biological pH. The binding of the host-guest pairs is proposed to proceed through ion-pairing interactions between the charged functional groups on both the host and the guest. The affinities and selectivities for oxyanions were determined using UV/vis titration techniques. Additionally, thermodynamic investigations indicate that the 1:phosphate complex is primarily entropy driven, while the 2:phosphate complex displays both favorable enthalpy and entropy changes. The thermodynamic data for binding provide a picture of the roles of the host, guest, counterions, and solvent. The difference in the entropy and enthalpy driving forces for the ammonium and guanidinium containing hosts are postulated to derive primarily from differences in the solvation shell of these two groups.
TL;DR: In this paper, isothermal titration calorimetry was used to determine the binding constant, stoichiometry, enthalpy, and entropy of beta-lactoglobulin/low and high-methoxyl pectin (beta-lg-LM- and HM-pectin) complexes at 22 degrees C and at pH 4.
Abstract: Isothermal titration calorimetry (ITC) was used to determine the binding constant, stoichiometry, enthalpy, and entropy of beta-lactoglobulin/low- and high-methoxyl pectin (beta-lg-LM- and HM-pectin) complexes at 22 degrees C and at pH 4. The binding isotherms revealed the formation of soluble intrapolymer complexes (C1) further followed by their aggregation in interpolymer complexes (C2). The interaction between beta-lg and LM- or HM-pectin in C1 and C2 occurred spontaneously with a Gibbs free energy around -10 kcal/mol. The C1 were enthalpically driven, whereas enthalpic and entropic factors were involved in the C2 formation. Because ITC did not allow the dissociation of different enthalpic contributions, the values measured as pectin and beta-lg interacted could partially be attributed to conformational changes. The C1 had a binding stoichiometry of 8.3 and 6.1 beta-lg molecules complexed per LM- or HM-pectin molecule, respectively. The C2 had about 16.5 and 15.1 beta-lg molecules complexed per LM- and HM-pectin, respectively.
TL;DR: In this article, the entropy and enthalpy of graphite intercalation into graphite were measured using open-circuit voltages (OCV) in half-cells with graphite electrodes.
TL;DR: In this article, the binding isotherms were obtained for the proteins with butyl-sepharose and octyl-sapharose resins at various temperatures and salt concentrations.
Abstract: This study investigated the binding mechanism between proteins and hydrophobic adsorbents from a thermodynamics aspect. The proteins investigated here included lysozyme, myoglobin, and α-amylase. The binding isotherms were obtained for the proteins with butyl-sepharose and octyl-sepharose resins at various temperatures and salt concentrations. The binding isotherms were then analyzed using the van't Hoff equation, and the binding enthalpy and entropy of each step of the binding process are discussed. Moreover, the effects of ligand chain length and protein characteristics were also investigated. Notably, the preferential interaction model has been adopted in the discussion of the dehydration step in the binding mechanism. The analytical results show that the binding process is entropy-dominated at higher temperature and the amount of released water molecules increases with temperature. Furthermore, the binding enthalpy (ΔHvH) and heat capacity were calculated by the van't Hoff and Kirchoff equations and w...
TL;DR: In this article, the authors use existing experimental measurements of the entropy, speci2c heat, and enthalpy outside this temperature range to construct a possible form of entropy in the "cult-to-probe" region, and estimate the excess entropy Sex of the liquid over the crystal within relatively narrow limits.
Abstract: The behavior of thermodynamic and dynamic properties of liquid water at atmospheric pressure in the temperature range between the lower limit of supercooling (TH ≈ 235 K) and the onset of the glassy state at Tg has been the focus of much research, and many questions remain about the properties of water in this region. Since direct measurements on water in this temperature range remain largely infeasible, we use existing experimental measurements of the entropy, speci2c heat, and enthalpy outside this range to construct a possible form of the entropy in the “di4cult-to-probe” region. Assuming that the entropy is well-de2ned in extreme metastable states, and that there is no intervening discontinuity at atmospheric pressure, we estimate the excess entropy Sex of the liquid over the crystal within relatively narrow limits. We 2nd that our approximate form for Sex shows atypical behavior when compared with other liquids: using a thermodynamic categorization of “strong” and “fragile” liquids, water appears to be fragile on initial cooling below the melting temperature, and strong in the temperature region near the glass transition. This thermodynamic construction can be used, with appropriate reservations, to estimate the behavior of the dynamic properties of water by means of the Adam–Gibbs equation—which relates con2gurational entropy Sconf to dynamic behavior. Although the Adam– Gibbs equation uses Sconf rather than Sex as the control variable, the relation has been used successfully in a number of experimental studies with Sconf replaced by Sex. This is likely a result of a proportionality between Sconf and Sex, which we con2rm for simulations of a model
TL;DR: The results demonstrate that combined temperature--pressure-dependent studies can help delineate the free-energy landscape of proteins and hence help elucidate which features and thermodynamic parameters are essential in determining the stability of the native conformational state of proteins.
Abstract: We studied the thermodynamic stability of a small monomeric protein, staphylococcal nuclease (Snase), as a function of both temperature and pressure, and expressed it as a 3D free-energy surface on the p,T-plane using a second-order Taylor expansion of the Gibbs free-energy change delta G upon unfolding. We took advantage of a series of different techniques (small-angle X-ray scattering, Fourier-transform infrared spectroscopy, differential thermal analysis, pressure perturbation calorimetry and densitometry) in the evaluation of the conformation of the protein and in evaluating the changes in the thermodynamic parameters upon unfolding, such as the heat capacity, enthalpy, entropy, volume, isothermal compressibility and expansivity. The calculated results of the free-energy landscape of the protein are in good agreement with experimental data of the p,T-stability diagram of the protein over a temperature range from 200 to 400 K and at pressures from ambient pressure to 4000 bar. The results demonstrate that combined temperature--pressure-dependent studies can help delineate the free-energy landscape of proteins and hence help elucidate which features and thermodynamic parameters are essential in determining the stability of the native conformational state of proteins. The approach presented may also be used for studying other systems with so-called re-entrant or Tamman loop-shaped phase diagrams.
TL;DR: In this article, the binodal and spinodal lines and second virial coefficient of the protein at 275 < T < 295 K were determined independently, and the protein chemical potential and osmotic pressure for concentrations as high as 320 mg mL-1 in the above temperature range.
Abstract: For insight into the thermodynamics and phase behavior in concentrated protein solutions, we study the liquid−liquid phase separation with lysozyme. We determine independently the binodal and spinodal lines, and the second virial coefficient of the protein at 275 < T < 295 K. From these data, we determine the protein chemical potential and osmotic pressure for concentrations as high as 320 mg mL-1 in the above temperature range. We find that for this protein, the enthalpy of the liquid−liquid separation vanishes at the critical temperature, Tc, and is comparable to and may exceed the crystallization enthalpy (∼65 kJ mol-1) at lower Ts. The enthalpy of the pair interactions averaged over all polar angles is significantly lower; this comparison suggests structuring of the dense liquid. We propose that the pair of parameters (molecular volume, second virial coefficient) may be an adequate predictor of the phase behavior of solutions of proteins with relatively simple interaction potentials.
TL;DR: In this article, a thermodynamic characterisation of the adsorption process of dihydrogen on the zeolite Li-ZSM-5 was carried out by means of variable-temperature infrared spectroscopy, with the simultaneous measurement of temperature and equilibrium pressure.
TL;DR: In this article, the activity coefficients at infinite dilution for both polar and nonpolar solutes in an ionic liquid, 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM+][BF4-]), have been determined by gas−liquid chromatography at T = (298.15 K and 323.15 k).
Abstract: The activity coefficients at infinite dilution, , for both polar and nonpolar solutes in an ionic liquid, 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM+][BF4-]), have been determined by gas−liquid chromatography at T = (298.15 K and 323.15 K). This work is part of our research focus on ionic liquids. The selectivity values have been calculated at T = 298.15 K, and the results indicate that the ionic liquid, [HMIM+][BF4-], should be a good solvent for separation of benzene and alkanes. The partial molar excess enthalpy values at infinite dilution have also been determined at T = 298.15 K and have been discussed in terms of intermolecular interactions. The results have been discussed in terms of measurements of , using other ionic liquids, taken from the recent literature.
TL;DR: In this article, high-sensitivity differential scanning calorimetry was applied for the first time to study the micellization of bovine β-casein in aqueous solutions, complemented by analytical ultracentrifugation data.
Abstract: High-sensitivity differential scanning calorimetry was applied for the first time to study the micellization of bovine β-casein in aqueous solutions, complemented by analytical ultracentrifugation data. The micellization was described as a single endothermic heat capacity peak located in the temperature range of 0−50 °C. The transition originated at temperatures very close to 0 °C. The position of the heat capacity peak was strongly dependent on the protein concentration and the solvent composition. Thermodynamic parameters of the micellization, the transition temperature and enthalpy, were determined over a wide range of protein concentration. The observed thermal behavior of β-casein was well fitted by the thermodynamic shell model of micellization proposed by Kegeles (J. Phys. Chem. 1979, 83, 1728). The effects of different cosolutes (inorganic salts, urea, Tris, and ethanol) on the thermodynamic parameters of micellization of β-casein underline the hydrophobic character of the transition.
TL;DR: In this paper, a tensimetric measurement of the CO2(g) pressure over the biphasic mixture of La2O2CO3(s) at different temperatures was performed.
TL;DR: In this article, the effect of counterions on micelle concentration and enthalpy of micellization was investigated by using isothermal titration calorimetry (ITC).
Abstract: Thermodynamics of micelle formation of anionic surfactants was investigated by using isothermal titration calorimetry (ITC). Highly purified decyl and dodecyl sulfates have been used to analyze the effect of counterions (Li+, Na+, K+, and Cs+) on critical micelle concentration (cmc) and enthalpy of micellization (ΔHmic) determined between 10 and 60 °C. The enthalpy of micellization decreases strongly with increasing temperature and passes trough zero (endothermic to exothermic processes), while the cmc versus temperature exhibits a minimum. At a given temperature and for a fixed chain length, the decrease of cmc and ΔHmic in the order Li+ > Na+ > K+ > Cs+ is related to the increase of the binding of counterions to micelles. The electrostatic repulsions between ionic headgroups, which prevent the aggregation, are progressively screened as the ionic character decreases with the size of the counterion. The heat of dilution of micelles is markedly dependent on temperature and is correlated with the temperatur...
TL;DR: In this paper, an analysis of the temperature and composition dependence of the excess thermodynamic properties for oligomers of poly(ethylene oxide) (PEO) in aqueous solution obtained from molecular dynamics simulations was performed.
Abstract: Analysis of the temperature and composition dependence of the excess thermodynamic properties for oligomers of poly(ethylene oxide) (PEO) in aqueous solution obtained from molecular dynamics simulations reveals that the lower critical solution temperature (LCST) behavior of PEO/water solutions is enthalpy-driven. At lower temperatures, the formation of ether−water hydrogen bonds results in a very favorable ΔG EX consistent with the consolubility of PEO and water. Increasing temperature leads to a dramatic reduction in favorable ether−water interactions due to break-up of ether−water hydrogen bonding, while relatively persistent water−water hydrogen bonding maintains the energetic penalty associated with disrupted water−water interactions upon insertion of the ether. The entropy gain associated with the break-up of ether−water hydrogen bonds and reduction in the structure of hydrating water with increasing temperature is insufficient to offset the unfavorable enthalpic effects associated with the break-up ...
TL;DR: In this article, the effect of heat on lactoperoxidase activity in bovine milk was studied over a range of 68 to 76 °C and the energy of activation was investigated.
Abstract: The effect of heat on lactoperoxidase activity in bovine milk was studied over a range of 68 to 76 °C. Values of residual enzymatic activity after different treatments were studied by kinetic analysis, obtaining D-values and the Z-value (3.1 °C). Denaturation of lactoperoxidase, measured by loss in activity, can be described as a 1st-order reaction. Rate constants were calculated, as was the energy of activation, which was 737.69 kJ/mol. Thermodynamic parameters were also calculated. The high value obtained for the variation in enthalpy of activation indicates that a high amount of energy is required to initiate denaturation, probably due to the molecular conformation of lactoperoxidase.