Heat Capacity Difference Research Articles

Enthalpy of formation of 5-fluoro-1,3-dimethyluracil: 5-Fluorouracil revisited

In the present work, a re-determination of thermochemical data for 5-fluorouracil was performed and a new determination of thermochemical parameters for 5-fluoro-1,3-dimethyluracil are presented. The standard (po=0.1MPa) molar enthalpies of formation, in the crystalline phase, of 5-fluorouracil and 5-fluoro-1,3-dimethyluracil, at T=298.15K, were derived from the standard molar energies of combustion in oxygen, measured by rotating bomb combustion calorimetry. For these compounds, the standard molar enthalpies of sublimation, at T=298.15K, were determined from the temperature-vapour pressure dependence, obtained by the Knudsen mass-loss effusion method. Using the values for the heat capacity differences between the gas and the crystalline phases of the compounds studied, the standard (po=0.1MPa) molar enthalpies, entropies and Gibbs free energies of sublimation, at T=298.15K, were derived. From the experimentally determined values, the standard molar enthalpies of formation, in the gas phase, at T=298.15K, of 5-fluorouracil and 5-fluoro-1,3-dimethyluracil were calculated as −(454.5±1.6) and −(478.5±1.3)kJ·mol−1, respectively. These values were compared with estimates obtained from very accurate theoretical calculations using the G3(MP2)//B3LYP composite method and appropriately chosen reactions.

Enthalpy of sublimation of natural aromatic amino acids determined by Knudsen’s effusion mass spectrometric method

The sublimation process of aromatic amino acids, l-phenylalanine, l-tyrosine and l-tryptophan, were investigated by Knudsen’s effusion method with mass spectrometric vapor composition measurement. Fragmentation of molecules under electron ionization was discussed. For these compounds, the molar enthalpies of sublimation were derived from the Clausius–Clapeyron equation. Using values for the heat capacity differences between the gas and crystal phases of the compounds the molar standard enthalpies of sublimation at T=298.15K, were obtained. The molar enthalpies of solvation in water at infinite dilution (i.d.) were derived from the molar enthalpies of solution of amino acids in water at i.d. and the molar standard enthalpies of sublimation obtained. Correlations between the enthalpies of sublimation/solvation and some molecular descriptors of amino acids were discussed.

Nonlinear-optical refraction of silver nanoparticle composites

In this paper, the experimental data on nonlinear refraction of silver nanoparticle composites using a standard Z-scan technique are presented. It was found that the colloids of silver nanoparticles of various size possess a defocusing ability. Based on general considerations, one can conclude thermal lens nature of the nonlinear refraction of the colloids. Significantly different magnitudes of the nonlinear refractive index of silver nanoparticles suspended in water and in glycerol can be explained by differences in the specific heat capacity of mentioned fluids. The effective thickness for nonlinear-optical interaction of light with a two-dimensional silver nanoparticle array was estimated.

Molar heat capacities of aqueous 2-amino-2-ethyl-1,3-propanediol solutions and their ternary mixtures containing piperazine or lithium salts

The molar heat capacities of a primary sterically hindered amine 2-amino-2-ethyl-1,3-propanediol (AEPD) and its aqueous solutions, in the whole range of concentrations, were measured at temperatures from 303.15K to 353.15K by heat flow differential scanning calorimetry. The molar heat capacities of aqueous ternary solutions of AEPD with PZ or salt (LiCl or LiBr) at different amine and salt concentrations were also presented. The excess molar heat capacities of the binary solutions were determined, and the data were correlated with temperature and amine mole fraction using a Redlich–Kister-type equation. For the ternary systems, the heat capacity differences were obtained, and the heat capacity data were represented as a function of temperature and solute concentration using the Sohnel and Novotný equation. The applied correlations well represented the experimental data heat capacities of the studied solvents as suggested by the low values of average absolute deviation, which are 0.2%, 0.1%, and 0.2% for the pure, binary, and ternary systems, respectively.

Solid–liquid equilibrium of dicyandiamide in different solvents

Solubilities of dicyandiamide in five different solvents including water, methanol, ethanol, glycol and acetone were determined at temperature ranging from 273.15 to 338.95K at atmospheric pressure using a gravimetric method. Fusion enthalpy, ΔHf, melting temperature, Tm, and the difference in molar heat capacity of the liquid and solid form of dicyandiamide, ΔCp, were determined by differential scanning calorimetry. On this basis, the ideal solubility of dicyandiamide was calculated, and validity of heat capacity assumptions (specifically, ΔCp=0) in predicting the ideal solubility has been evaluated. Further, the measured solubility data was used with the calculated ideal solubility to obtain activity coefficients, which were then fitted to a van’t Hoff like regular solution equation.

A single n-heptane droplet behavior in rapid compression machine

This study investigated the evaporation characteristics of a single n-heptane droplet inside a rapid compression machine (RCM). The RCM is an experimental apparatus used to simulate a single compression stroke of an internal combustion engine to observe the combustion phenomena in homogeneous charge compression ignition (HCCI) environments. During the compression stroke, the temperature inside the RCM increases and the fuel droplet evaporates under unsteady conditions. In this study, a single n-heptane droplet was placed at the center of the reaction chamber. The droplet was suspended from the tip of a fine thermocouple (50μm), and the transient bulk temperature of the droplet was measured. The evaporation process was recorded by a high-speed charge-coupled device (CCD) camera with a frame rate of 500frames per second. The initial droplet diameter (450–700μm) operation time (170ms and 210ms), and the compression ratio (14 and 18) were set as experimental parameters. To observe the pure evaporation of the droplet, the reaction chamber was filled with N2 gas. Experimental results showed that the droplet diameter decreased with the constant evaporation rate during the evaporation process, even under time-varying temperature and pressure surrounding the droplet. The evaporation rate of the droplet rose as the diameter of the droplet increased, due to the differences in droplet heat capacity. As the compression rate increased, temperature inside the reaction chamber increased and increase in droplet evaporation rate was observed. The operation time did not appear to affect the droplet’s evaporation rate.

Heat capacity measurements of various biomass types and pyrolysis residues

This study aims at measuring heat capacity on a large range of biomass types and on pyrolysis chars obtained under conditions representative of industrial reactors for further use of these data in thermal conversion models. As expected, linear increase could be observed with temperature for the 21 biomasses tested between 313K and 353K. The relative difference of heat capacity between the different biomasses was significant but lower than 20%, with values ranging from 1300 to 2000Jkg−1K−1. The sample form, either in block or powder, did not seem to have any influence. Values of about 1000Jkg−1K−1 were measured on chars, whatever the pyrolysis temperature. These values are in the same order of magnitude as those extrapolated from literature correlations. As expected, linear increase could be observed with temperature, except in the range 353–513K, in which an exothermal phenomenon of recombination seems to occur inside the char.

Thermal stability of matrix protein from Newcastle disease virus

The thermal stability of the matrix protein (M protein) of Newcastle disease virus (NDV) has been investigated using high-sensitivity differential scanning calorimetry (DSC) at pH 7.4. The thermal folding/unfolding of M protein at this pH value is a reversible process involving a highly cooperative transition between folded and unfolded monomers with a transition temperature (Tm) of 63°C, an unfolding enthalpy, ΔH(Tm), of 340kcalmol−1, and the difference in heat capacity between the native and denatured states of the protein, ΔCp, of 5.1kcalK−1mol−1. The heat capacity of the native state of the protein is in good agreement with the values calculated using a structure-based parameterization, whereas the calculated values for the hypothetical fully-unfolded state of the protein is higher than those determined experimentally. This difference between the heat capacity of denatured M protein and the heat capacity expected for an unstructured polypeptide of the same sequence, together with the data derived from the heat-induced changes in the steady-state fluorescence of the protein, indicates that the polypeptide chain maintains a significant amount of residual structure after thermal denaturation.

Surface air temperature variability in global climate models

AbstractNew results from the Coupled Model Intercomparison Project phase 5 (CMIP5) and multiple global reanalysis datasets are used to investigate the relationship between the mean and standard deviation (SD) in the surface air temperature (SAT) at intra‐ and inter‐annual timescales. A combination of a land–sea mask and orographic filter was used to investigate the geographic region with the strongest correlation and in all cases this was found to be for low‐lying over‐land locations. This result is consistent with the expectation that differences in the effective heat capacity of the atmosphere are an important factor in determining the SAT response to forcing.

Thermodynamics of fenoxycarb in solution

The solubility of fenoxycarb has been determined between 278 and 318K in several organic solvents. The solid phase at equilibrium and some indication of polymorphism has been properly examined by powder XRD, DSC, Raman and ATR-FTIR spectroscopy, solution 1H NMR and SEM. Using literature data the activity of the solid phase within a Raoult’s law definition has been calculated, based on which solution activity coefficients have been estimated. In ethyl acetate, the van’t Hoff enthalpy of solution is constant over the temperature range and equals the melting enthalpy. However, it is shown that the solution is slightly non-ideal with the heat capacity difference term compensating for the activity coefficient term. In toluene, the van’t Hoff enthalpy of solution is constant as well but clearly higher than the melting enthalpy. In methanol, ethanol and isopropanol, van Hoff curves are strongly non-linear, the slope however clearly approaching the melting enthalpy at higher temperatures. In all solvents, positive deviations from Raoult’s law are prevailing. The activity coefficients follow a decreasing order of isopropanol>ethanol>methanol>toluene>ethyl acetate, and in all solvents decrease monotonically with increasing temperature. The highest activity coefficient is about 18 corresponding to about 2.5kJ/mol of deviation from ideality.

Thermodynamics and Kinetics of the Interactions of Calmodulin with Orai

Calcium dependent inactivation (CDI) is a key regulator of calcium release‐activated channels. Calmodulin (CaM) has been shown to interact with Orai1, evoking CDI. Though the calmodulin‐binding sequence (CBS) of Orai has been identified, the mechanism is not clear. Here we generated recombinant CaM and characterized its interaction with a peptide corresponding Orai CBS (Orai‐Pep) using isothermal titration calorimetry (ITC) and fluorescence. The peptide binding to CaM increases the extent of hydrophobicity as indicated by the increase of ANS fluorescence. ITC revealed that such a binding is moderate with Ka= 9.5 × 105M−1 and the reaction is exothermic with ΔH = −15.8 kJ/mol. To better define how CaM promotes CDI, we focused on the interaction of CaM and troponin C (TnC) chimeras with Orai‐Pep. 1TnC, in which the 1st EF‐hand of CaM is replaced with the corresponding EF‐hand of TnC, and 2TnC have one‐order lower binding affinity while 3TnC and 4TnC show similar binding strength to that of CaM. This data indicates that the 1st and 2nd EF‐hands of CaM are more important for binding, possibly through their interaction with Trp76 of Orai, in which its accessibility to acrylamide and KI decreases in the complex of 1TnC and 2TnC. Currently we are investigating the kinetics and heat capacity difference of CaM and chimeras upon peptide binding. This work is supported in part by National Science Foundation.

Osmotic Stress and Membrane Phase Changes During Freezing of Stallion Sperm: Mode of Action of Cryoprotective Agents1

The aim of this study was to determine how different membrane-permeable and -impermeable cryoprotective agents modulate tolerance of stallion sperm to osmotic stress and stabilize membranes during cryopreservation. Special emphasis was on hydroxyl ethylene starch (HES), which exposes cells to minimal osmotic stress due to its large molecular weight. Percentages of motile sperm post-thaw were found to be similar when glycerol, sucrose, and HES were used at their optimal concentrations. Percentages of plasma membrane intact sperm after return to isotonic medium were highest for HES. Fourier transform infrared spectroscopy studies were carried out to study subzero membrane phase and permeability behavior. Cryoprotectants were shown to decrease the initial rate of membrane dehydration during freezing, decrease the activation energy for water transport, and increase the total extent of freezing-induced dehydration. Freezing studies with liposomes as a model system showed that only the membrane-permeable cryoprotective agents glycerol and ethylene glycol protected membranes against leakage, whereas egg yolk, sucrose, and HES did not. Differential scanning calorimetry studies showed that sucrose and HES raise the glass transition temperature of the freezing extender and the difference in heat capacity associated with the glass transition. This indicates that these compounds enable formation of a stable glassy matrix at higher subzero temperatures. Sperm cryosurvival rates can be increased by combining different cryoprotectants with different protective functions; membrane permeable cryoprotective agents stabilize membranes and modulate the rate of cellular dehydration, whereas di- and polysaccharides increase the glass transition temperature and facilitate storage and handling at higher subzero temperatures.

A Method to Predict the Equilibrium Solubility of Drugs in Solid Polymers near Room Temperature Using Thermal Analysis

A method is presented for determining the equilibrium solubility of a drug in a solid polymer at or near room temperature, which represents a typical storage temperature. The method is based on a thermodynamic model to calculate the Gibbs energy change ΔG(SS) associated with forming a binary drug-polymer solid solution from the unmixed polymer and solid drug. The model includes contributions from heat capacity differences between the solid solution and the corresponding unmixed components, breaking up of the solid drug structure, and drug-polymer mixing. Calculation of ΔG(SS) from thermal analysis data is demonstrated, and it is shown that minima of plots of ΔG(SS) versus the dissolved drug concentration represent the equilibrium drug solubility in the polymer. Solid solutions were produced for drug-polymer systems (griseofulvin, indomethacin, itraconazole; PVP K30, Eudragit L100, Eudragit E100) in drug weight fractions up to ∼25%. At 25°C, it was seen that heat capacity effects were important in determining the drug solubility. It was concluded that drug solubilities in solid polymers can be determined using thermal analysis, and must include heat capacity effects when evaluated near room temperature.

Solid–liquid phase diagram of the two Ionic Liquids EMIMCl and BMIMCl

Ionic Liquids (ILs) are subject of intensive research for industrial applications due to their versatile properties. Efforts are made to obtain liquids at moderate temperatures by choosing an appropriate combination of cation and anion or by adding solvents to reduce the melting point. An alternative for melting point depression is the addition of another Ionic Liquid while the desired functionalities, like non-volatility or the aprotic character of the IL, are preserved. In the current work the solid–liquid equilibrium (SLE) of the solvent-free model system of the two Ionic Liquids 1-ethyl-3-methylimidazolium chloride (EMIMCl) and 1-butyl-3-methylimidazolium chloride (BMIMCl) are investigated. The solid–liquid phase behavior is found to be of eutectic type with a eutectic composition xEMIMCl,eu=0.5 and a eutectic temperature Teu=315K. Compared to the pure component melting point of EMIMCl a melting point depression ΔT=46K is achievable by adding BMIMCl. The SLE data can be described by the Schröder–Van-Laar equation in the simplest form neglecting differences in heat capacity, assuming ideal liquid phase behavior and the crystallization of pure components. This gives evidence that the system of the two Ionic Liquids BMIMCl–EMIMCl exhibits an ideal eutectic solid–liquid phase behavior.

Vapor pressures, standard molar enthalpies, entropies Gibbs energies of sublimation and heat capacities of 2,5- and 3,5-dibromobenzoic acids

The vapor pressures of solid and liquid 2,5- and 3,5-dibromobenzoic acids were determined by torsion–effusion and thermogravimetry (under both isothermal and non-isothermal conditions), respectively. The molar enthalpies of sublimation ΔcrgHm°(Tm) and vaporization ΔlgHm°(Tm) were calculated, respectively, at the middle temperature (Tm) of the respective temperature intervals from the temperature dependence of vapor pressure derived by the experimental torsion–effusion and thermogravimetric data. The melting temperatures and the molar enthalpies of fusion of these compounds were determined by d.s.c. as well as the molar heat capacities (in the temperature range from 288.2K to 326.2K). The molar enthalpies and entropies of sublimation and vaporization obtained by torsion–effusion and thermogravimetry, respectively, were adjusted to the reference temperature of 298.15K using the estimated heat capacity differences between gas and liquid for vaporization experiments and the estimated heat capacity differences between gas and solid for sublimation experiments. Therefore, the averages of the standard (p°=0.1MPa) molar enthalpies, entropies and Gibbs energies of sublimation at 298.15K, have been derived:CompoundΔcrgHm°(298.15K) (kJ/mol)ΔcrgSm°(298.15K) (J/(molK))ΔcrgGm°(298.15K), kJ/mol2,5-Dibromobenzoic acid (cr)133.0±3.0263.6±8.054.4±5.03,5-Dibromobenzoic acid (cr)120.4±3.0209.1±8.058.0±5.0

Complementary explanation of temperature response in the lower atmosphere

There are a number of asymmetries in the surface air temperature response to forcing, including polar amplification and changes to the diurnal and seasonal temperature ranges. We propose that such spatial–temporal signatures of climate change can, in part, be explained by differences in the effective heat capacity of the atmosphere. We have demonstrated that predictions arising from this hypothesis are simultaneously satisfied through the analysis of temperature records from daily to inter-decadal timescales using observational and reanalysis datasets. This mechanism can help to explain why we see the largest temperature trends in the winter months (0.42 K/decade in winter compared to 0.18 K/decade in summer) and why the diurnal temperature range decreases in a warming world, having decreased by ∼0.4 K since 1950.

Experimental and computational study on the energetics of the cyclic anhydrides of glycine and alanine

The standard (p°=0.1MPa) molar enthalpies of formation, in the crystalline phase, of the cyclic anhydrides of glycine and DL-alanine, at T=298.15K, were derived from the standard molar energies of combustion in oxygen, measured by static bomb combustion calorimetry. For these compounds, the standard molar enthalpies of sublimation, at T=298.15K, were determined from the temperature-vapour pressure dependence, obtained by the Knudsen mass-loss effusion method. Through the values for the heat capacity differences between the gas and the crystalline phases of the studied compounds, the standard (po=0.1MPa) molar enthalpies, entropies and Gibbs energies of sublimation, at T=298.15K, were derived. From the experimental values, the standard molar enthalpies of formation, in the gaseous phase, at T=298.15K, of glycine anhydride and DL-alanine anhydride were calculated as –(321.0±1.9)kJ·mol−1 and –(382.7±2.0)kJ·mol−1, respectively. A theoretical study at the G3 and G4 levels has been carried out, and the calculated enthalpies of formation have been compared with the experimental values.

Correlator product state study of molecular magnetism in the giant Keplerate Mo72Fe30

We have studied the properties of the giant Keplerate molecular magnet ${\mathrm{Mo}}_{72}{\mathrm{Fe}}_{30}$, as a function of applied magnetic field, using the correlator product state (CPS) tensor network ansatz. The magnet is modeled with an $S=5/2$ antiferromagnetic Heisenberg Hamiltonian on the 30-site icosidodecahedron lattice, a model for which exact diagonalization is infeasible. The CPS ansatz produces significant improvements in variational energies relative to previous studies using the density matrix renormalization group, a result of its superior ability to handle strong correlation in two-dimensional spin systems. The CPS results reaffirm that the ground-state energies adhere qualitatively to the parabolic progression of the rotational band model (RBM), but show important deviations near 1/3 of the saturation field. These deviations predict anomalous behavior in the differential magnetization and heat capacity that can not be explained by the RBM alone. Finally, we show that these energetic deviations originate from a qualitative change in the ground state that resembles a finite-size analog of a phase transition.

Structure-property relationships in ILs: A study of the alkyl chain length dependence in vaporisation enthalpies of pyridinium based ionic liquids

The enthalpies of vaporization for the series of pyridinium-based ionic liquids with bis(trifluoromethylsulfonyl)imide anion [C n Py][NTf2] ( n = 2, 3, 4, 5, and 6) have been determined with the quartz crystal microbalance technique combined with the Langmuir evaporation. The linear dependence of vaporization enthalpies on the chain length has been revealed. New approach based on volumetric, surface tension, and speed of sound measurements has been developed for estimation of heat capacity differences between gas and liquid phase, which were required for adjustment of measured vaporization enthalpies to the reference temperature 298 K.

Effect of the stefan flux on coefficients of heat- and mass transfer (a binary system)

On the basis of analyzing equations of motion, mass transfer, and energy of two-component flux there were determined conditions under which the analogy between heat exchange and mass exchange on their separate and combined occurrence, taking account of the Stefan flux, is valid. It has been established that the difference in heat capacity of flow components has an influence on heat transport by the Stefan flux.