Heat Capacities Of Solutions Research Articles

Prediction Partial Molar Heat Capacity at Infinite Dilution for Aqueous Solutions of Various Polar Aromatic Compounds over a Wide Range of Conditions Using Artificial Neural Networks

Artificial neural networks (ANNs), for a first time, were successfully developed for the prediction partial molar heat capacity of aqueous solutions at infinite dilution for various polar aromatic compounds over wide range of temperatures (303.55-623.20 K) and pressures (0.1-30.2 MPa). Two three-layered feed forward ANNs with back-propagation of error were generated using three (the heat capacity in T = 303.55 K and P = 0.1 MPa, temperature and pressure) and six parameters (four theoretical descriptors, temperature and pressure) as inputs and its output is partial molar heat capacity at infinite dilution. It was found that properly selected and trained neural networks could fairly represent dependence of the heat capacity on the molecular descriptors, temperature and pressure. Mean percentage deviations (MPD) for prediction set by the models are 4.755 and 4.642, respectively.

Corrections to the simple effectiveness-NTU method for counterflow cooling towers and packed bed liquid desiccant–air contact systems

Analytical expressions for the corrections to the simple effectiveness-NTU method are developed using perturbation technique for cooling towers and liquid desiccant–air contact systems. The developed model takes into consideration the effect of nonlinearities of humidity ratio and enthalpy of air in equilibrium with water or desiccant solutions. The model also takes into consideration the effect of water loss by evaporation and the effect of variation of the specific heat capacity of water or solution. The comparison with numerical integration of the dimensional heat and mass transfer equations shows that the analytical results are generally satisfactory and the improvement over the simple ε-NTU method is significant.

Origin of heat capacity changes in a "nonclassical" hydrophobic interaction.

Origin of heat capacity changes in a "nonclassical" hydrophobic interaction.

Thermal behavior of n-alcohols, benzene, and toluene in water urea and water ethyleneglycol systems

Relations are suggested for calculating the heat characteristics of alcohols, benzene, and toluene in aqueous solutions of urea and ethylene glycol. The solution heat of an alcohol shows greater dependence on the glycol concentration, while the heat capacity of solution, on the urea concentration. For n-butanol in aqueous glycol, the characteristic temperature at which the solution heat of the alcohol is zero decreases as the glycol concentration increases. The dependence of the characteristic temperature of 1-butanol on the urea concentration has a maximum.

Thermodynamics of glycine solution in aqueous urea. Rule $$\sqrt m $$

The solution heats of glycine in aqueous urea have been determined by calorimetry at 298 K (molality 0–13) and 313 K (molality 0–22). The solution heat of the amino acid is described by the linear dependence of this quantity on the square root of the urea molality. The enthalpy, entropy, and Gibbs energy of the transfer of glycine from water to aqueous urea, as well as the heat capacity, entropy variation, and Gibbs energy of glycine solution have been calculated for the temperature range 273–323 K. It is found that urea additions to water reverse the sign of the heat capacity of solution.

Liquid heat capacity of aqueous sulfolane + 2-amino-2-methyl-1-propanol solutions

Heat capacities of aqueous solutions of sulfolane with 2-amino-2-methyl-1-propanol (AMP) were measured over the temperature range from 303.15 to 353.15 K with a differential scanning calorimeter. Twelve solutions of sulfolane + AMP + water that cover the mole fractions of water from 0.6 to 0.8 were studied. The liquid heat capacities of sulfolane alone and of binary mixtures such as sulfolane + water and sulfolane + AMP were also studied. The heat capacities of sulfolane and sulfolane + water were found to be in good agreement with those values reported in the literature. A Redlich-Kister type equation was applied to represent the composition dependence of the heat capacities of binary and ternary systems. For 132 data points of sulfolane + AMP + water, the fitted results of heat capacity calculations (overall average absolute percentage deviation (AAD%)) were 0.3 and 7.7% for the molar heat capacity and the excess molar heat capacity, respectively. The heat capacities of aqueous mixtures of sulfolane with AMP presented in this study can be used to estimate the head load of absorbents in acid gas capture process when using sulfolane + AMP + water as the absorbent.

Standard Gibbs energy of formation of MgLa determined by solution calorimetry and heat capacity measurement from near absolute zero kelvin

The standard Gibbs energy of formation, Δ f G T ° , of MgLa in the temperature range from near absolute 0 to 525 K were determined by calorimetry. The heat capacities, C p , from 2 K to 525 K were measured by the relaxation method and DSC. Also, a thermal anomaly at 5.9 K, which appeared to be a superconductive phase transition, was found in the obtained C p values. The Δ f G T ° ( MgLa ) values were determined by combining the C p data with the standard enthalpy of formation at 298 K which was measured by the Calvet-type calorimeter using hydrochloric acid solution. From 2 to 300 K, the Δ f G T ° increases gradually, and it can be evaluated as a linear function of temperature above 300 K as follows: Δ f G T ° ( MgLa ) ( kJ mo l − 1 ) = − 39.236 − 6.9832 × 10 − 3 T + 2.1016 × 10 − 3 T log T + 1.9114 × 10 − 5 T 2 − 0.81004 T − 1 ± 7.40 ( 2 − 300 K ) , Δ f G T ° ( MgLa ) ( kJ mo l − 1 ) = − 41.100 + 9.9974 × 10 − 3 T ± 7.40 ( 300 − 525 K ) . This result is expected to be useful as basic thermodynamic data of Mg-based alloys.

Composition dependence and the nature of endothermic freezing and exothermic melting

The heat capacity, C(p), and enthalpy and entropy change of alpha-cyclodextrin, H(2)O, and 4-methylpyridine solutions have been studied during their freezing on heating, isothermal freezing, and the solid's melting on cooling. Freezing occurs in several endothermic steps on heating to 383 K and alpha-cyclodextrin rich solutions freeze in four steps. The melting rate becomes slower with decrease in temperature and its steps merge. Decreasing the amount of alpha-cyclodextrin decreases the C(p) change on freezing. The endothermic freezing phenomenon differs from freezing of a pure liquid and is attributed to formation of a solid inclusion compound and its incongruent way of exothermic melting.

Extension of the Dense System Equation of State to Electrolyte Solutions

In this work we have applied the Dense System Equation of State (DSEOS) to electrolyte solutions. We have found that this equation of state can predict the density of electrolyte solutions very accurately. It has been tested for different electrolytes solutions at different temperatures and compositions. A hypothetical binary model has been applied to find the dependencies of parameters of this equation of state on solution temperature and composition. Using such a simple model the heat capacity of NaCl solution was calculated for which the absolute percent deviation is less than 2 %. The DSEOS is tested for the following electrolytes: Na2SO4, MgCl2, MgSO4, KCl, NaCl, and NaBr. We found that the DSEOS predicts the density of aqueous electrolyte solutions mentioned above accurately so that its percent error in density is less than 0.04.

Molar Heat Capacity of Various Aqueous Alkanolamine Solutions from 303.15 K to 353.15 K

The molar heat capacities of aqueous 2-((2-aminoethyl)amino)ethanol (AEEA), 3-amino-1-propanol (AP), 2-(methylamino)ethanol (MAE), 1-amino-2-propanol (MIPA), and N,N-dimethylethanolamine (DMEA) solutions are reported at 11 different temperatures in the range (303.15 to 353.15) K. The percentage average absolute deviation (% AAD) of the measured molar heat capacities of MEA and MDEA is found to be 0.23 % when compared with the literature values. Molar excess heat capacities are calculated from the measurement results and correlated as a function of the mole fractions using the Redlich−Kister equation. The excess partial molar heat capacities ( − ) are also reported.

A new tool for an old job: Using fixed cell scanning calorimetry to investigate dilute aqueous solutions

The development of fixed twin cell temperature scanning calorimeters has enabled the more efficient determination of heat capacities of dilute aqueous solutions with a precision comparable to that of the Picker flow heat capacity calorimeters developed nearly 40 years ago. Experiments require less than 0.5 cm 3 of solution, and results can be obtained routinely over the temperature range (278 to 395) K at pressures up to a few bars. Multiple scanning of samples by both increasing and decreasing temperature allows assessment of instrument drift, solute stability, and reproducibility of results. Chemical calibration is essential to take full advantage of the precision and sensitivity of the calorimeters. The calorimetric output is a direct measure of the difference in heat capacity per unit volume of a solution and of a reference liquid, usually water. Thus, densities of the solution and reference liquid are needed to transform the results into heat capacities per unit mass of solution. Examples of solutes that have been investigated include a variety of inorganic and organic compounds that dissolve to give simple ionic or neutral species, or that produce complexes or species that exist in equilibrium distributions that can change as the temperature is scanned. Appropriate selection of the results from experiments on combinations of solutes allows calculation of standard state (zero concentration) thermodynamic quantities for chemical processes and reactions over the ranges of temperature scanned at the solution compositions investigated. Results for a few specific systems are presented and discussed for some representative classes of solutes that have been investigated in our laboratory since 1998.

Standard Gibbs Energy of Formation of Mg<SUB>3</SUB>La Determined by Solution Calorimetry and Heat Capacity Measurement from Near Absolute Zero Kelvin

The standard Gibbs energy of formation, Δ f G° Τ , of Mg 3 La in the temperature range from near absolute zero Kelvin to 525 K were determined by calorimetry. The heat capacities, Cp, from 2 K to 525 K were measured by the relaxation method and DSC. The A f G° T (Mg 3 La) values were determined by combining the Cp data with the standard enthalpy of formation at 298 K which was measured by the Calvet-type calorimeter using hydrochloric acid solution. From 2 to 350 K, the Δ f G° Τ increases gradually, and it can be evaluated as a linear function of temperature above 350 K as follows: Δ f G° Τ (Mg 3 La)/kJ mo -1 =-76.204-2.5304 x 10 -2 T+ 1.2160 x 10 -2 T log T + 1.1814 x 10 -5 T 2 - 0.80332 T -1 ± 17.0 (2-350K) Δ f G° T (Mg 3 La)/kJmol -1 =-79.652+1.9567 x 10 -2 T ± 17.0 (350-525K). This result is expected to be useful as basic thermodynamic data of Mg-based alloys.

Heat Capacities of Aqueous Solutions of Acetone; 2,5-Hexanedione; Diethyl Ether; 1,2-Dimethoxyethane; Benzyl Alcohol; and Cyclohexanol at Temperatures to 523 K

The heat capacities of aqueous solutions of acetone, 2,5-hexanedione, diethyl ether, 1,2-dimethoxyethane, benzyl alcohol and cyclohexanol at concentrations of 0.1 to 1.0 mol⋅kg−1 were determined at temperatures of 298.15, 423.15, 473.15 and 523.15 K and pressures up to 28 MPa. The measurements were performed at ambient conditions using the commercial Picker differential flow calorimeter and at high temperatures and pressures with a customized Picker type calorimeter constructed at the Blaise Pascal University, Clermont-Ferrand. Standard molar heat capacities were obtained by weighted extrapolation to the infinite dilution limit. The contributions of –CO–, –O– and –OH groups to the standard molar volume and standard molar heat capacity were determined from the newly determined and literature data. The variation of the three oxygen-containing group contributions with temperature and molecular structure is examined qualitatively.

Standard Thermodynamic Properties of H3PO4(aq) over a Wide Range of Temperatures and Pressures

The densities and heat capacities of solutions of phosphoric acid, 0.05 to 1 mol kg-1, were measured using flow vibrating tube densitometry and differential Picker-type calorimetry at temperatures up to 623 K and at pressures up to 28 MPa. The standard molar volumes and heat capacities of molecular H3PO4(aq) were obtained, via the apparent molar properties corrected for partial dissociation, by extrapolation to infinite dilution. The data on standard derivative properties were correlated simultaneously with the dissociation constants of phosphoric acid from the literature using the theoretically founded SOCW model. This made it possible to describe the standard thermodynamic properties, particularly the standard chemical potential, of both molecular and ionized phosphoric acid at temperatures up to at least 623 K and at pressures up to 200 MPa. This representation allows one to easily calculate the first-degree dissociation constant of H3PO4(aq). The performance of the SOCW model was compared with the other approaches for calculating the high-temperature dissociation constant of the phosphoric acid. Using the standard derivative properties, sensitively reflecting the interactions between the solute and the solvent, the high-temperature behavior of H3PO4(aq) is compared with that of other weak acids.

Hydration heat capacities of hydrophobic solutes at 248–373 K

A new equation is suggested to define the temperature dependence of the Gibbs energy of hydration of hydrophobic substances: ΔG 0 = b 0 + b 1 T + b 2lnT. According to this equation, the hydration heat capacity is in inverse proportion to temperature. Consistent values of hydration heat capacity of nonpolar solutes have been obtained for different temperatures using data on solubility and dissolution enthalpy. The contributions of the hydrocarbon radicals and OH group to the heat capacity of hydration of the compounds were found for the temperature range 248–373 K. The hydration heat capacity of the hydroxyl group has a weak dependence on temperature and increases by only 12 J/(mol·K) in the specified temperature interval. Changes in the hydration entropy of hydrophobic and OH groups are calculated for the temperature increasing from 248 K to 373 K.

Apparent molar volumes and apparent molar heat capacities of aqueous magnesium nitrate, strontium nitrate, and manganese nitrate at temperatures from 278.15 K to 393.15 K and at the pressure 0.35 MPa

Apparent molar volumes V ϕ and apparent molar heat capacities C p, ϕ were determined at the pressure 0.35 MPa for aqueous solutions of magnesium nitrate Mg(NO 3) 2 at molalities m = (0.02 to 1.0) mol · kg −1, strontium nitrate Sr(NO 3) 2 at m = (0.05 to 3.0) mol · kg −1, and manganese nitrate Mn(NO 3) 2 at m = (0.01 to 0.5) mol · kg −1. Our V ϕ values were calculated from solution densities obtained at T = (278.15 to 368.15) K using a vibrating-tube densimeter, and our C p, ϕ values were calculated from solution heat capacities obtained at T = (278.15 to 393.15) K using a twin fixed-cell, differential, temperature-scanning calorimeter. Empirical functions of m and T were fitted to our results, and standard state partial molar volumes and heat capacities were obtained over the ranges of T investigated.

The lattice heat capacity of binary compounds in the approximation of three-dimensional debye sublattices

An analytic description of the temperature dependence of the heat capacity of crystals in the model of interacting three-dimensional Debye sublattices is suggested. The parameters of the model are the characteristic temperatures Θ2 and Θ3 of the sublattices and temperature Θ1 determined by the interaction between the sublattices. Satisfactory applicability of the model is substantiated for the examples of the temperature dependences of the heat capacity of gallium and indium arsenide solid solutions.

Heat Capacities and Thermodynamic Properties of a H2O + Li2B4O7 Solution in the Temperature Range from 80 to 356 K

The molar heat capacities of an aqueous Li2B4O7 solution were measured with a precision automated adiabatic calorimeter in the temperature range from 80 to 356 K at a concentration of 0.3492 mol⋅kg−1. The occurrence of a phase transition was determined based on the changes in the curve of the heat capacity with temperature. A phase transition was observed at 271.72 K corresponding to the solid-liquid phase transition; the enthalpy and entropy of the phase transition were evaluated to be Δ H m = 4.110 kJ⋅mol−1 and Δ S m = 15.13 J⋅K−1⋅mol−1, respectively. Using polynomial equations and thermodynamic relationship, the thermodynamic functions [H T −H 298.15] and [S T −S 298.15] of the aqueous Li2B4O7 solution relative to 298.15 K were calculated in temperature range 80 to 355 K at intervals of 5 K. Values of the relative apparent molar heat capacities of the aqueous Li2B4O7 solution, C p,φ, were calculated at every 5 K in temperature range from 80 to 355 K from the experimental heat capacities of the solution and the heat capacities of pure water.

Stabilization of yeast hexokinase A by polyol osmolytes: Correlation with the physicochemical properties of aqueous solutions

Osmolytes of the polyol series are known to accumulate in biological systems under stress and stabilize the structures of a wide variety of proteins. While increased surface tension of aqueous solutions has been considered an important factor in protein stabilization effect, glycerol is an exception, lowering the surface tension of water. To clarify this anomalous effect, the effect of a series of polyols on the thermal stability of a highly thermolabile two domain protein yeast hexokinase A has been investigated by differential scanning calorimetry and by monitoring loss in the biological activity of the enzyme as a function of time. A larger increase in the T m of domain 1 compared with that of domain 2, varying linearly with the number of hydroxyl groups in polyols, has been observed, sorbitol being the best stabilizer against both thermal as well as urea denaturation. Polyols help retain the activity of the enzyme considerably and a good correlation of the increase in T m (Δ T m) and the retention of activity with the increase in the surface tension of polyol solutions, except glycerol, which breaks this trend, has been observed. However, the Δ T m values show a linear correlation with apparent molal heat capacity and volume of aqueous polyol solutions including glycerol. These results suggest that while bulk solution properties contribute significantly to protein stabilization, interfacial properties are not always a good indicator of the stabilizing effect. A subtle balance of various weak binding and exclusion effects of the osmolytes mediated by water further regulates the stabilizing effect. Understanding these aspects is critical in the rational design of stable protein formulations.

Thermodynamics of solution of histidine

The enthalpies of solution of l-histidine in water at 288.15–318.15 K and 0.003–0.15 mol kg −1 were measured. The enthalpies of solution were found to be independent of the solute molality up to ∼0.1 mol kg −1. Standard enthalpies and heat capacities of solution were computed. Free energies and entropies of solution have been estimated in the temperature range studied using literature solubility data and the results of the present study. The temperature increase was found to result in a pronounced rise of the l-histidine solubility due to the significant increase of the TΔ S values. The characteristic temperatures for the thermodynamic properties of histidine aqueous solutions were estimated.