Enthalpy Data Research Articles

Excess enthalpies of the reactive system ethanol + acetic acid + ethyl acetate + water for chemically equilibrium states at 313.15 K

The data on excess molar enthalpies, $$ H_{\text{m}}^{\text{E}} $$ , for binary mixtures (ethanol + water and acetic acid + ethyl acetate) and quaternary system (acetic acid + ethanol + ethyl acetate + water) at 313.15 K and standard pressure (101.303 kPa) were obtained using the Setaram C80 calorimeter. The results on excess enthalpies data were correlated by Redlich–Kister equations and NRTL model.

Crystallinity of block copolymer controlled by cyclodextrin

We report a differential scanning calorimetry study to investigate the effect of cyclodextrins (CD) on the crystallinity of a copolymer. Tetronics was selected as copolymer with star-like shape formed by four polyethylene oxide flanked by four polypropylene oxide blocks linked to ethylenediamine central group. The use of CD with different cavity size was exploited for a block selective inclusion. A model for supramolecular association was considered for a quantitative description of the enthalpy data. The polymer chain incorporation into the CD cavity generates a loss of crystallinity. The stoichiometry of the CD/copolymer aggregates can be tuned by changing the CD cavity size. The investigated systems represent interesting necklace-like supramolecular structures with tunable crystallinity.

Thermodynamic assessment of Ag–Cu–In

The Ag–Cu–In thermodynamic material system is of interest for brazing alloys and chalcopyrite thin-film photovoltaics. To advance these applications, Ag–Cu–In was assessed and a Calphad model was developed. Binary Ag–Cu and Cu–In parameters were taken from previous assessments, while Ag–In was re-assessed. Structure-based models were employed for β-bcc(A2)-Ag3In, γ-Ag9In4, and AgIn2 to obtain good fit to enthalpy, phase boundary, and invariant reaction data for Ag–In. Ternary Ag–Cu–In parameters were optimized to achieve excellent fit to activity, enthalpy, and extensive phase equilibrium data. Relative to the previous Ag–Cu–In assessment, fit was improved while fewer parameters were used.

Solubility and hydrolysis of U(VI) in 0.5 mol/kg NaCl solutions at T = 22 and 80 °C

Abstract In this study, the solubility and hydrolysis of U(VI) was investigated at elevated temperature. Experiments were performed in solutions of NaCl 0.51 mol/kg of water in the pHm range 4–13.4 (with pHm = –log[H+]) at T = 22 ± 3 and 80 ± 5 °C under N2/Ar atmosphere. Two solid phases were used as starting U(VI) materials: UO3·2H2O(cr) equilibrated in solutions with 4 ≤ pHm ≤ 5 and Na2U2O7·H2O(cr) (a synthetic clarkeite-like phase) equilibrated in solutions with 7 ≤ pHm ≤ 13.4. The solubility of UO3·2H2O(cr) at T = 80 °C in the acidic pHm-range is decreased by a factor of ten with respect to the U(VI)-phase studied at room temperature. Solid phase characterization by XRD, SEM–EDS and quantitative chemical analysis indicate that this decrease is correlated to the transformation of UO3·2H2O(cr) into a sodium-containing U(VI) solid phase. The solubility of Na2U2O7·H2O(cr) in the alkaline pHm-range is drastically enhanced at T = 80 °C. The main contribution to this effect results from the increased acidity of water at elevated temperature, although an enhanced hydrolytic activity of U(VI) or a decreased stability of Na2U2O7·H2O(cr) are also considered, in order to explain the solubility data under these experimental conditions. Solid phase characterization after completing the solubility experiments confirms that Na2U2O7·H2O(cr) retains its stoichiometry and structure at T = 80 °C within the timeframe of the study (t ≤ 290 days). A thermodynamic model, comprising conditional stability constants and enthalpy data has been developed, describing reliably the processes of hydrolytic dissolution of Na2U2O7·H2O(cr) in alkaline NaCl solutions up to 0.51 mol/kg and their temperature dependence in the range 22–80 °C. This work represents the first comprehensive study on U(VI) solubility and hydrolysis at elevated temperatures extending to alkaline pHm conditions relevant in the context of nuclear waste disposal. It provides robust solubility upper limits to be considered in source term estimations, but also thermodynamic data for both solid phases and aqueous species that can be implemented in thermodynamic databases and related geochemical calculations.

Experimental Investigation of the Enthalpy and Heat Capacity of Liquid Cesium

The enthalpy of liquid cesium in the temperature range from 431 to 976 K was measured by a drop method on an isothermal calorimeter. The approximation equation of enthalpy was obtained and the isobaric heat capacity was determined. The estimated errors in the enthalpy and heat capacity data are 0.5% and 1.5%, respectively. The obtained results are compared with the literature data. The position of the minimum heat capacity of liquid cesium was clarified (733 K). Tables of recommended values of caloric properties in the investigated temperature range were developed.

Aqueous solutions of [EMIM] 1,1,2,2-tetrafluoroethanesulfonate and [EMIM] trifluoromethanesulfonate: A thermodynamic study

Abstract While ionic liquids (ILs) based on a trifluoromethanesulfonate ([OTF] − ) anion and their mixtures with other compounds have been relatively well explored, the information about structurally similar 1,1,2,2-tetrafluoroethanesulfonate-based ([TFES] − ) ILs is either very scarce or, as in the case of their mixtures with water, totally lacking. We herein thus present a broad thermodynamic characterization of aqueous solutions of [EMIM][TFES] and [EMIM][OTF], with the study of aqueous solutions of the latter serving as a benchmark. First, the peculiar solid-liquid phase transition behavior of the neat [EMIM][TFES] was examined and its density was determined over a temperature range encompassing both the subcooled and the stable liquid. For the aqueous solutions of the two ILs, we performed extensive measurements at several temperatures in the range from (288.15 to 318.15) K of water activity ( a w ) across the entire composition range, mixing enthalpy and density specifically in the highly dilute IL region, and the solution freezing temperatures at water mole fractions greater than 0.6. The ample a w data were fitted ( i ) by a two-step procedure and ( ii ) simultaneously with the excess enthalpy data using an extended NRTL-type model. Although both treatments provide adequate representations of the data and prove to yield reasonable estimates of derivative thermal properties, the latter approach to the global description of energetics of the systems is preferred for its handiness in practical calculations. We duly proved its reliability in extrapolation and/or prediction against experimental data on various properties, among others, SLE. The analysis of volumetric data by the Redlich-Meyer equation allowed us to obtain accurate values of infinite dilution partial molar volumes of the ILs and revealed the nature of their hydration as rather hydrophobic. The solution behavior of the systems studied was found to be essentially non-ideal: their small S-shaped excess Gibbs energy ( G E ) actually results from the compensation of their large positive excess enthalpy ( H E ) and entropy ( S E ). Thus, despite being completely miscible with water, the two ILs are not typically hydrophilic, but intermediate on an imaginary (hydrophobicity/hydrophilicity) scale. The structural variation in the anion, though influencing H E and S E considerably, is only subtly reflected in G E and activity coefficients. Due to their somewhat lower values and a weaker decrease of apparent molar volume with increasing molality observed for [EMIM][TFES], [TFES] − is identified to be slightly more hydrophilic than [OTF] − .

Vapor-liquid equilibria and excess enthalpies of the binary systems 1-pentanol or 2-pentanol and 1-hexene or 1,2,4-trimethylbenzene for the development of biofuels

Accurate experimental data of vapor-liquid equilibria (VLE) and excess enthalpies are reported for four binary systems: (1-pentanol + 1-hexene), (2-pentanol + 1-hexene), (1-pentanol + 1,2,4-trimethylbenzene) and (2-pentanol + 1,2,4-trimethylbenzene). An isothermal total pressure cell was used for measuring VLE at T = 313.15 K. The data were fitted using Margules, Wilson and NRTL equations. Excess enthalpies were measured at two different temperatures T = (298.15 and 313.15) K using an isothermal flow calorimeter and were correlated by the Redlich-Kister equation. All systems present a positive deviation from the Raoult's Law. An azeotropic behavior with maximum pressure is observed for the mixtures 1-pentanol or 2-pentanol with 1,2,4-trimethylbenzene. In addition, an endothermic behavior, which increases with temperature, is obtained when the alcohols are mixed with these hydrocarbons.

Evaluations of the accuracies of DMol3 density functionals for calculations of experimental binding enthalpies of N2, CO, H2, C2H2 at catalytic metal sites

The density functionals that can be used with numerical basis sets in the density functional methodology of the programme DMol3 are evaluated by comparison with experimental data. The objective is to identify and validate the most accurate and computationally efficient density functional to be used in DMol3 simulations that use large molecular models (many hundreds of atoms) of the reaction steps involved in the chemical mechanisms of metalloenzymes and synthetic metallocatalysts. The experimental systems focus on enthalpy data for reactions analogous to those involved in the chemical mechanism of nitrogenase, including coordination of N2, CO, H2 and C2H4 and hydrogen bonding. Some geometric and vibrational frequency data are included in the 19 test systems. The conclusion is that the gradient-corrected functionals PBE, PW91 and BP provide acceptably accurate results, with the best functional being PBE, which yields reaction energies within or very close the experimental error range. These functionals are also the most computationally efficient.

Multiple Thermal Gelation of Hydroxypropyl Methylcellulose and Kappa-Carrageenan Solutions and their Interaction with Salts

The multiple gelation behavior of aqueous solutions of kappa-carrageenan (KCG) and low molecular weight hydroxypropyl methylcellulose (HPMC) was studied with the presence of various salts. Multiple gelation behavior of aqueous solutions of HPMC/KCG/salt mixture were found. The shear viscosity of HPMC/KCG blend increased by one orders of magnitude, while the viscosity of HPMC/KCG/potassium chloride (KCl) mixture increased by three orders of magnitude as compared to HPMC solution at temperatures below apparent gelation. The dynamic elastic modulus of HPMC/KCG blend increased by two orders of magnitude, while the elastic modulus of HPMC/KCG/potassium chloride (KCl) mixture increased by three orders of magnitude as compared to HPMC solution at temperatures below apparent gelation temperature. The gel elastic modulus of the solution blend of HPMC/KCG/salt mixture decreased in the order of KCl > NaCl > CaCl2. Thermal analysis revealed a linear relationship between the depression of melting temperature and the salt concentrations, which is independent of KCG. The free water content computed by enthalpy data showed that free water content decreased with increasing salt concentrations. The secondary peak which typically associated with bound water appeared in the mixture of HPMC and KCG in the presence of KCl. As the concentrations of KCl salt increased, the bound water peak also amplified and lifted to a higher temperature.

Determination of solid/liquid fraction of three aluminium binary alloys using a new single-pan scanning calorimeter

There is an increased demand for data with higher precision for the enthalpy changes and the fraction of solid/liquid temperatures of materials. Therefore, continuous efforts are often devoted to design calorimeters that can accurately measure materials’ thermophysical properties. In this study, a new single-pan scanning calorimeter was used to measure the transition temperature and enthalpy change of three aluminium binary alloys. Measured results also were compared with the calculated results using thermodynamic software. The measured high accuracy enthalpy data were used to determine transient temperature. It is concluded that the new instrument is a promising device that can achieve reliable and reproducible materials’ thermophysical data.

Size driven thermodynamic crossovers in phase stability in zirconia and hafnia

AbstractHafnia (HfO2) and zirconia (ZrO2) are of great interest in the quest for replacing silicon oxide in semiconductor field effect transistors because of their high permittivity. Both exhibit extensive polymorphism and understanding the energetics of their transitions is of major fundamental and practical importance. In this study, we present a systematic thermodynamic summary of the influence of particle size on thermodynamic phase stability in hafnia and zirconia using recently measured enthalpy data from the literature. The amorphous phase is found to be the most energetically stable above 165 and 363 m2/g of surface area for HfO2 and ZrO2, respectively. Below 16 and 20.3 m2/g of surface area, respectively, the monoclinic phase is the most energetically stable for HfO2 and ZrO2. At intermediate sizes there are closely balanced energetics among monoclinic, tetragonal, and cubic phases. The energy crossovers reflect decreasing surface enthalpy in the order monoclinic, tetragonal, cubic and amorphous for both hafnia and zirconia.

Thermodynamic properties of misfit cobaltite [Bi2-xCa2O4][CoO2]1.7

We synthetized a misfit structure cobaltite [Bi2-xCa2O4][CoO2]1.7 and studied its thermal behavior by simultaneous thermal analysis (STA), low temperature heat capacity measurement in physical property measurement system (PPMS), differential scanning calorimetry (DSC) to measure heat capacity and drop calorimetry to obtain the relative enthalpy increments. The oxygen non-stoichiometry determined by thermogravimetry (STA data combined with weight loss measurement of the material reduced in hydrogen atmosphere) was analyzed in terms of point defect model resulting in two independent reactions, oxygen release and cobalt disproportionation, described by the respective enthalpy and entropy changes. The applied model satisfactorily describes two regimes of oxygen release/incorporation associated with the respective Co valency changes Co4+/Co3+ and Co3+/Co2+. The temperature dependence of the molar heat capacity above room temperature was derived by simultaneous least squares method from DSC and enthalpy data considering the contribution due to oxygen release and deviation from ideal stoichiometry. The low temperature heat capacity was analyzed in terms of a combined Debye–Einstein model.

Estimation of Solvation Quantities from Experimental Thermodynamic Data: Development of the Comprehensive CompSol Databank for Pure and Mixed Solutes

The Gibbs energy of solvation measures the affinity of a solute for its solvent and is thus a key property for the selection of an appropriate solvent for a chemical synthesis or a separation process. More fundamentally, Gibbs energies of solvation are choice data for developing and benchmarking molecular models predicting solvation effects. The Comprehensive Solvation—CompSol—database was developed with the ambition to propose very large sets of new experimental solvation chemical-potential, solvation entropy, and solvation enthalpy data of pure and mixed components, covering extended temperature ranges. For mixed compounds, the solvation quantities were generated in infinite-dilution conditions by combining experimental values of pure-component and binary-mixture thermodynamic properties. Three types of binary-mixture properties were considered: partition coefficients, activity coefficients at infinite dilution, and Henry’s-law constants. A rigorous methodology was implemented with the aim to select data at appropriate conditions of temperature, pressure, and concentration for the estimation of solvation data. Finally, our comprehensive CompSol database contains 21 671 data associated with 1969 pure species and 70 062 data associated with 14 102 binary mixtures (including 760 solvation data related to the ionic-liquid class of solvents). On the basis of the very large amount of experimental data contained in the CompSol database, it is finally discussed how solvation energies are influenced by hydrogen-bonding association effects.

Thermal decomposition of europium sulfates Eu2(SO4)3·8H2O and EuSO4

Reactions of europium sulfates Eu2(SO4)3·8H2O and EuSO4 complete decomposition were studied by Simultaneous Thermal Analysis. It was revealed that one-step dehydratation of Eu2(SO4)3·8H2O crystallohydrate is accompanied by the formation of amorphous anhydrous europium sulfate Eu2(SO4)3. Crystallization of amorphous europium (III) sulfate occurs at 381.1°С (in argon) and 391.3°C (in air). The average enthalpy values for dehydratation reaction of Eu2(SO4)3·8H2O (ΔH° = 141.1kJ/mol), decomposition reactions of Eu2(SO4)3 (ΔH = 463.1kJ/mol), Eu2O2SO4 (ΔH = 378.4kJ/mol) and EuSO4 (ΔH = 124.1kJ/mol) were determined. The step process mechanisms of thermal decomposition of europium (III) sulfate in air and europium (II) sulfate in inert atmosphere were established and justified. The kinetic parameters of complete thermal decomposition of europium (III) sulfate octahydrate were calculated by Kissinger model. The standard enthalpies of compound formation were calculated using thermal effects and formation enthalpy data for binary compounds.

Structural phase transition of BeTe: an ab initio molecular dynamics study.

Beryllium telluride (BeTe) with cubic zinc-blende (ZB) structure was studied using ab initio constant pressure method under high pressure. The ab initio molecular dynamics (MD) approach for constant pressure was studied and it was found that the first order phase transition occurs from the ZB structure to the nickel arsenide (NiAs) structure. It has been shown that the MD simulation predicts the transition pressure P T more than the value obtained by the static enthalpy and experimental data. The structural pathway reveals MD simulation such as cubic→tetragonal→orthorhombic→monoclinic→orthorhombic→hexagonal, leading the ZB to NiAs phase. The phase transformation is accompanied by a 10% volume drop and at 80GPa is likely to be around 35GPa in the experiment. In the present study, our obtained values can be compared with the experimental and theoretical results. Graphical abstract The energy-volume relation and ZB phase for the BeTe.

Vapor–Liquid Equilibrium and Excess Enthalpy Data for Systems Containing N,N-Dimethylacetamide

Isothermal P–x vapor–liquid equilibrium and excess enthalpy (HE) data were experimentally determined for the 10 binary systems hexane, heptane, 1-octene, benzene, toluene, ethanol, methanol, ethyl acetate, dibutyl ether, and dichloromethane with N,N- dimethylacetamide using the static synthetic method and isothermal flow calorimetry, respectively. The experimental data from this work together with data taken from literature were used for the extension of the group contribution method Modified UNIFAC (Dortmund).

Determination of sublimation enthalpies of substituted benzophenones, fluorenes and diphenyl ethers by solution calorimetry approach

An approach for the determination of solvation enthalpies of substituted aromatic compounds is developed. In this work the solvation enthalpy of a reference unit (benzophenone, fluorene and diphenyl ether) is calculated as the difference between solution enthalpy and sublimation enthalpy at 298K. An additive approach for the determination of solvation enthalpies of substituted aromatic compounds was used. Solvation enthalpies of sixteen substituted benzophenones, fluorenes and diphenyl ethers were calculated by this approach. Thermochemical data of solution enthalpies of benzophenones, fluorenes and diphenyl ether were measured by a precision solution calorimeter at 298K. Then sublimation enthalpies of the studied compounds were derived by a solution calorimetry (SC) approach at 298K.We have also found that in a number of cases instead of the solution enthalpy in benzene at 298K the fusion enthalpy at the melting temperature can be used. A comparison between literature data and the calculated vaporization and sublimation enthalpies demonstrates a satisfactory performance of the proposed approach.

Synthesis and surface enthalpy of MgGa2O4 spinel

Surface enthalpy for magnesium gallate, MgGa2O4 spinel, was directly measured using water adsorption microcalorimetry. The method relies on the Gibbs adsorption theory, using the heat involved when water molecules are adsorbed on the surface of the oxide to derive accurate surface enthalpy data. For this method to be efficient, high amounts of surfaces need to be probe, requiring the usage of nanoparticles. Therefore, in this study we initially propose a new method to synthesize MgGa2O4 nanoparticles by controlling the pH of the precipitating solution during co-precipitation of Mg,Ga hydroxides, followed by calcination. The nanoparticles showed the spinel phase and were characterized in terms of composition before subjected to calorimetry. The surface energy of the studied MgGa2O4 nanoparticles was found to be 1.44±0.1J/m2, consistently with surface energy data reported for other spinels.

High temperature heat capacity of (U, Am)O2±x

Mixed uranium and americium dioxides (U, Am)O2±x are candidates as possible transmutation targets for generation IV reactors. In this work, the enthalpy increments of this solid solution were measured in the 470–1750 K temperature range by drop calorimetry for Am/(Am + U) ratios equal to 0.32, 0.39, 0.49, 0.58 and 0.68. Then, the heat capacity functions were obtained by derivation of the enthalpy data. The results of this work were compared to the heat capacity and enthalpy functions reported in the literature for the UO2 [1] and AmO2 [2] binary oxides and for the U0.9Am0.1O2±x, U0.8Am0.2O2±x mixed oxides [3]. From the obtained trend, it was found out that an excess contribution to the enthalpy increment appears for T > 1100 K in the compositions with Am/(Am + U)≥0.4 and a possible explanation attributing this effect to oxygen hypostoichiometry is provided. Finally, to verify the hypothesis, thermodynamic computations based on the CALPHAD method were performed for AmO2-x under air and the results confirmed that the source of the excess contribution is the formation of oxygen vacancies.

Thermodynamic properties for calcium molybdate, molybdenum tri-oxide and aqueous molybdate ion

The thermodynamic properties for CaMoO4(cr), MoO3(cr) and MoO42−(aq) were investigated. CaMoO4(cr) is the one of the host crystals of the yellow phases which are known as hygroscopic harmful phases in the nuclear fuel glasses. MoO3(cr) is the main component of the host crystals. MoO42−(aq) is the aqueous ion present in underground water after dissolution of the yellow phases. The standard molar entropies, Δ0TSm∘, at 298.15K for CaMoO4(cr) and MoO3(cr) were determined by measuring their isobaric heat capacities, Cp,m∘, from 2K. Their standard Gibbs energies of formation, ΔfGm∘, were determined by combining the Δ0TSm∘ data with the reference data of the standard enthalpies of formation, ΔfHm∘. The standard electrochemical potential, E°, of MoO42−(aq) was determined from the thermodynamic cycle on the basis of thermodynamic properties obtained for CaMoO4(cr) and MoO3(cr). The unknown standard Gibbs energies of solution, ΔslnGm∘, at 298.15K for the actinide molybdates ThMo2O8(cr) and UMoO6(cr) were predicted from the presently obtained thermodynamic data for CaMoO4(cr), MoO3(cr) and MoO42−(aq). The obtained thermodynamic values are as follows:Δ0TSm∘(CaMoO4(cr), 298.15K)/(JK−1mol−1)=122.23±1.22.Δ0TSm∘(MoO3(cr), 298.15K)/(JK−1mol−1)=75.43±0.75.Δ0TSm∘(MoO42−(aq), 298.15K)/(JK−1mol−1)=32.25±4.41.ΔfGm∘(CaMoO4(cr), 298.15K)/(kJmol−1)=−1437.78±1.11.ΔfGm∘(MoO3(cr), 298.15K)/(kJmol−1)=−667.20±0.63.ΔfGm∘(MoO42−(aq), 298.15K)/(kJmol−1)=−836.61±1.02.E∘(MoO42−(aq), 298.15K)/V=4.34±0.01.ΔslnGm∘(ThMo2O8(cr), 298.15K)/(kJmol−1)=184.84±42.48.ΔslnGm∘(UMoO6(cr), 298.15K)/(kJmol−1)=68.33±34.47.The present obtained data are expected to be useful for geo-chemical simulation of diffusion of radio-active elements through underground water.