Partial Molar Heat Capacities Research Articles

Thermodynamics of hydrocarbon solutions using g.l.c. n-Hexane, n-heptane, benzene, and toluene as solutes each at infinite dilution in n-hexadecane, in n-octadecane, and in n-eicosane

Infinite-dilution activity coefficients were measured by g.l.c. over a range of 50 K for n-hexane, n-heptane, benzene, and toluene in n-hexadecane, n-octadecane, and n-eicosane; measurements for cyclohexane and tetrachloromethane in n-hexadecane are also discussed. The experimental results were fitted to an adaptation of the equation of Clarke and Glew, thus calculating partial molar excess enthalpies and heat capacities. The results are compared with those obtained by extrapolation from static results at finite mole fractions.

Partial molar heat capacities of the side chains of some amino acid residues in aqueous solution

Partial molar heat capacities of the side chains of some amino acid residues (Ala, Val, Leu, Ile, Ser) have been determined over a broad temperature range from calorimetric heat capacity measurements of the corresponding tripeptides and cyclodipeptides. The data obtained are compared with those determined earlier from the heat capacities of analog compounds. It is shown that in amino acids and even tripeptides of the Gly-X-Gly type, the influence of the end charges on the heat capacity of the side chain is rather significant even in buffered solutions of high ionic strength.

Heat capacity of proteins: I. Partial molar heat capacity of individual amino acid residues in aqueous solution: Hydration effect

The partial molar heat capacities of various peptides and various organic compounds that model the amino acid side-chains or their parts in aqueous solution have been determined by precise scanning microcalorimetry in the temperature range from 5 to 125 degrees C. This provides an estimate of the partial molar heat capacity of the peptide--CHCONH--group and the side-chains of all amino acid residues. The values obtained are compared with the values found for these substances in the gaseous phase, in order to define the hydration effect. It has been shown that the partial heat capacity of the non-polar groups is positive at low temperature (5 degrees C) and decreases with increasing temperature, while for the polar and charged groups it is negative at low temperature, becomes zero at room temperature and increases further with increasing temperature. This leads to a hydrophobicity scale of the amino acid side-chains based upon the temperature dependences of their heat capacities. Due to the observed specificity in the temperature dependence, at room temperature, the heat capacities of amino acid side-chains correlate well with the non-polar surface areas.

Heat capacity of proteins: II. Partial molar heat capacity of the unfolded polypeptide chain of proteins: Protein unfolding effects

Using the heat capacity values for amino acid side-chains and the peptide unit determined in the accompanying paper, we calculated the partial heat capacities of the unfolded state for four proteins (apomyoglobin, apocytochrome c, ribonuclease A, lysozyme) in aqueous solution in the temperature range from 5 to 125 degrees C, with an assumption that the constituent amino acid residues contribute additively to the integral heat capacity of a polypeptide chain. These ideal heat capacity functions of the extended polypeptide chains were compared with the calorimetrically determined heat capacity functions of the heat and acid-denatured proteins. The average deviation of the experimental functions from the calculated ideal ones in the whole studied temperature range does not exceed the experimental error (5%). Therefore, the heat-denatured state of a protein, in solutions with acidic pH preventing aggregation, approximates well the completely unfolded state of this macromolecule. The heat capacity change caused by hydration of amino acid residues upon protein unfolding was also determined and it was shown that this is the major contributor to the observed heat capacity effect of unfolding. Its value is different for different proteins and correlates well with the surface area of non-polar groups exposed upon unfolding. The heat capacity effect due to the configurational freedom gain by the polypeptide chain was found to contribute only a small part of the overall heat capacity change on unfolding.

Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Standard partial molal properties of organic species

The standard partial molal properties of organic aqueous species at high pressures and temperatures can be predicted using an adaptation of a revised equation of state for inorganic aqueous ions and electrolytes (Tanger and Helgeson, 1988), together with correlations among equation of state parameters (SHOCK and Helgeson, 1988). These correlations include a charge-dependent relation between Born coefficients and the standard partial molal entropies of aqueous species at 25°C and 1 bar (SHOCK. et al., 1989). Thermodynamic calculations indicate that in the liquid phase the standard partial molal volumes (V̄°), heat capacities (C̄° p), and entropies (S̄°), as well as the apparent standard partial molal enthalpies of formation (ΔH°) of aqueous electrolytes with organic anions maximize with increasing temperature at Psat ∗ ∗ P sat represents pressures corresponding to liquid-vapor equilibrium for the system H 2O, except at temperature < 100°C where it refers to the reference pressure of 1 bar. and approach — ∞ at the critical point of H 2O. In contrast, the corresponding properties of neutral organic aqueous species in the liquid phase minimize with increasing temperature Psat and approach ∞ at the critical point of H 2O. Predicted equilibrium constants for alkane solubilities and carboxylic acid dissociation reactions at elevated pressures and temperatures are in close agreement with experimental data reported in the literature, which supports the validity and generality of the equations of state as well as the predictive algorithms used in the calculations. As a consequence, high temperature/ pressure standard partial molal properties, as well as equilibrium constants and other reaction properties, can be predicted for reactions involving a wide variety of organic aqueous species for which little or no experimental data are available at temperatures > 25°C. Present capabilities permit such predictions to be made for hydrothermal and magmatic conditions at pressures and temperatures as high as 5 kb and 1000°C

Thermodynamic properties of N-octyl- and N-dodecylnicotinamide chlorides in water

Densities, heat capacities and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for N-octyl- and N-dodecylnicotinamide chlorides in water over an extended concentration region. Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25°C were derived as a function of the surfactant concentration. For both surfactants, plots of volumes, enthalpies and free energies vs. concentration are regular whereas those of heat capacities and entropies present anomalies at about 0.8 and 0.1m for the octyl and dodecyl compounds, respectively. Changes in the slope of a plot of osmotic coefficients times molality vs. molality were also observed at these same concentrations. These peculiarities are ascribed to micelle structural transitions. The nonideal free energies do not seem to depend on the alkyl chain length when they are plotted vs. m/Ccmc. Also, a plot of the nonideal free energy vs. logm/Ccmc is roughly independent of the nature of the surfactant because of the constant activity of surfactants in micellar solutions. Nonideal free energies, enthalpies and entropies have been calculated at 15 and 35°C. At each concentration the nonideal free energy is temperature independent as a result of a compensatory effect between enthalpy and entropy. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. These data suggest that the nicotinamide group possesses less hydrophilic character than the ammonium group.

Organic solutes in paraffin solvents: Influence of the size of the solvent molecule on solution data. Dedicated to (Dr.) Gu¨nther Ohloff on the occasion of his 65th birthday.

Solution properties at ideal dilution were measured for 124 solute probes by gas chromatography on five pure paraffins of well defined molecular weight at temperatures of 110–190°C. Standard chemical potential differences, related to the molal Henry coefficient, were calculated from retention data. Their temperature dependence allowed the calculation of partial molar solution enthalpy, entropy and heat capacity differences between the ideal gas phase and the ideal dilute solution. It was found that the standard chemical potential difference depended linearly on the variable ζ = 1000/ M l, where M l is the molecular weight of the paraffin stationary phase. The linear dependence was best explained by assuming that it is due to the linear dependence of the partial molar solution entropy of the solute on the variable ζ. Consequently, on a molal basis, a solute forms equithermal solutions in any of the liquid paraffins. Comparison with literature data suggested that these results have general validity. Approximate relationships are given to calculate the coefficient of variation of the retention index with temperature and with the variable ζ from thermodynamic data.

Some thermodynamic properties of crown ethers in aqueous solution

Enthalpies of dissolution in water for 12-crown-4, 15-crown-5, and 18-crown-6 were determined calorimetrically in the temperature range 288.15 to 308.15 K. Heat capacities for the three pure compounds were determined at 298.15 K, using a drop-calorimetric technique, and the enthalpy of sublimation of 18-crown-6 was determined calorimetrically at 298.15 K. Values for enthalpies of solvation and for partial molar heat capacities at 298.15 K are calculated for infinitely dilute solutions. Discussion of the results is based on an empirical additivity relation.

Thermodynamic properties of peptide solutions. Part 6.—The amino acid side-chain contributions to the partial molar volumes and heat capacities of some tripeptides in aqueous solution

Limiting partial molar volumes, V02, and partial molar heat capacities, C0p, 2, have been determined for the tripeptides glycyl-L-valylglycine, glycyl-DL-serylglycine and the D and L isomers of glycylleucylglycine in aqueous solution at 298.15 K. These V02 and C0p, 2 results, in conjunction with those for glycylglycylglycine, were used to estimate the contribution of the side chains to the thermodynamic properties. The results obtained are compared with those estimated using V02 and C0p, 2 data for amino acids and dipeptides. The comparison suggests that the tripeptides of sequence glycyl-X-glycine are reasonable models for estimating the contribution to the thermodynamic properties of proteins made by solvent-accessible side chains.

Partial molar heat capacities and volumes of some nucleic acid bases, nucleosides and nucleotides in aqueous urea solutions at 298.15 K

Partial molar heat capacities (text-decoration:overlineC0p2) and volumes (text-decoration:overlineV02) of some nucleic acid bases, nucleosides and nucleotides in 1, 3 and 6 mol kg–1 aqueous urea solutions have been measured using a dynamic-flow microcalorimeter and a vibrating-tube digital densimeter, respectively. The corresponding heat capacity and volume transfer parameters from water to aqueous urea solutions have been used to understand the nature of interactions between the constituents of nucleic acids and urea molecules. It has been inferred that urea molecules have weaker interactions with the nucleic acid bases at low concentrations of urea. However, at high concentrations of urea, the significantly positive values of heat capacities (text-decoration:overlineC0p2, tr) and volumes of transfer (text-decoration:overlineV02, tr) indicate stronger hydrophilic–hydrophilic/ionic group interactions of the nucleic acid bases, nucleosides and nucleotides with urea and which may be responsible for the denaturation of the nucleic acid helix in high concentration aqueous urea solutions.

Partial molar heat capacities and volumes of transfer of nucleic acid bases, nucleosides and nucleotides from water to aqueous solutions of sodium and calcium chloride at 25�C

Partial molar heat capacities\(\overline C _{p2}^o \) and volumes\(\overline V _2^o \) of some nucleic acid bases, nucleosides and nucleotides have been measured in 1m aqueous NaCl and CaCl2 solutions using Picker flow microcalorimeter and a vibrating tube digital densimeter. The partial molar heat capacities of transfer and volumes of transfer from water to the electrolyte solutions were calculated using earlier data for these compounds in water. The values of these transfer parameters are positive. The higher values for transfer to aqueous CaCl2 solutions reflect the stronger interactions of the constituents of the nucleic acids with Ca+2 ions than with the Na+ ions.

Thermodynamics of sulphuric acid: apparent and partial molar heat capacities and volumes of aqueous HSO–4 from 10–55 °C and calculation of the second dissociation constant to 350 °C

Apparent molar heat capacities and volumes of aqueous solutions containing H2SO4 have been determined from 10 to 55°C. These results are complicated by the presence of bisulphate and sulphate ions as well as effects due to chemical relaxation. Consideration of these contributions has led to derived standard-state partial molar heat capacities and volumes for HSO–4 over the stated temperature range. The heat capacity and volume data, which are related to Gibbs free energies by derivatives with respect to temperature and pressure, have been used along with a semi-theoretical model to calculate equilibrium constants for the dissociation of bisulphate to 350°C. Results of these calculations are in good agreement with the best equilibrium data presently available.

Heat capacity of alcohols in aqueous solutions in the temperature range from 5 to 125�C

Using a precise technique of scanning microcalorimetry the heat capacity differences between water and dilute aqueous solutions of ethanol, n-propanol, n-butanol and n-pentanol were measured from 5 to 125°C and the partial molar heat capacities of these substances in water were determined. It was found that the heat capacity increment for alcohol disolved in water is proportional to the number of the-CH 2 − groups and decrease with a temperature increase. The heat capacity increment of hydration of non-polar groups is shown to be positive and large at room temperature and decreases in magnitude as the temperature increases. In contrast, the heat capacity increment of hydration of polar groups is negative at room tempreature and increases as the temperature increases. From the temperature dependence of the heat capacity increment one can assume that the water molecules solvated by the non-polar groups of the alcohols behave in a non-cooperative manner.

Heat capacities and volumes of some polybasic carboxylic acids in water at 298.15 K

Heat capacities divided by volume of aqueous solutions of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, dl-malic acid, l-(+)-tartaric acid, and citric acid have been measured using flow calorimetry at 298.15 K. Employing densities of the above mentioned solutions, determined with a vibrating-tube densimeter, we obtained partial molar heat capacities at infinite dilution.

The thermodynamics of the adsorption of xenon and argon on zeolite NaX at high pressures

The heats of adsorption of xenon and argon in the region of the highly nonideal gas phase depend significantly on temperature. The partial molar isosteric heat capacity of the adsorbate increases sharply at high concentrations of the adsorbed molecules in the cavities. Most studies on the thermodynamic properties of adsorption systems at high pressures have been based on the analysis of adsorption isosteres (1.-3)o This approach, however, involves significant difficulties due to the necessity of the correct consideration of the nonideal nature of the equilibrium phase and the adsorption deformation of the adsorbent. Thus, the heat capacity of the adsorbate, which is a term proportional to the second deriva- tive of the initial adsorption isotherms, is found with about 30% error. Nevertheless, in previous work (2), we showed that the partial molar heat capacities of the adsorbate in microporous adsorbents at high occupancies and pressures have maxima. This finding apparent ~- ly indicates that the structure of the adsorbate is altered under certain conditions. A direct calorimetric study at high pressures would help elucidate the behavior of the ther- modynamic functions of the adsorbate. Work in this area has recently been undertaken (4-6). The thermodynamic properties of xenon and krypton adsorbed on zeolite NaX was calcu- lated using the adsorption isosteres (7, 8). In previous work (6), we directly measured the absolute heats of adsorption of Xe and Kr on zeolite NaX at 304.5 K and from I0 kPa to I0 MPa. These heats are called absolute since the uniform definition of adsorption as the complete content of the adsorbate in the adsorption field is used in the measurement of the adsorption itself. The measured heats of adsorption and the heats calculated from the iso- steres are in accord within experimental error. Curve I in Fig~ I shows the approximation of the dependence of the calorimetric heats of xenon adsorption in zeolite NaX on the adsorption at 304.5 K, curve 3 shows the dependence of the isosteric heat of adsorption at 165 K, and curves 2 and 4 show the analogous curves for krypton in zeolite NaXo Figure i implies that the heats of adsorption depend significantly on the temperature~ This finding was noted previously for these systems (7, 8). Although the curves for the heats of adsorption at 165 K for xenon and at 120 K for krypton in Fig. i were obtained from isosteres at pressures less than at- mospheric and the curves at 304.5 K were obtained calorimetrically , their comparison is still possible since the isosteric heats at low pressures are close to the calorimetric values. This circumstance permits us to determine the behavior of the other thermodynamic parameters of Xe and Kr adsorbed on zeolite NaX.

Apparent and partial molar heat capacities and volumes of aqueous HClO4 and HNO3 from 10 to 55 °C

Apparent molar heat capacities and volumes of aqueous solutions containing HClO4 and HNO3 have been determined from 10 to 55 °C. The temperature dependences of the standard state heat capacities and volumes of ClO4− (aq) and NO3− (aq) from 10 to 55 °C were found to be well represented by the following equations:[Formula: see text]Combination of the experimental results with semiempirical equations for ion–solvent interaction has led to predictions of the standard state volumes and heat capacities for these ionic species at higher temperatures. Keywords: heat capacities, volumes, nitric acid, perchloric acid.

Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Standard partial molal properties of inorganic neutral species

Consideration of interactions between neutral aqueous species and H 2O dipoles in terms of effective Born coefficients permits extension of the revised HKF (Helgeson, Kirkham and Flowers, 1981) equations of state (Tanger and Helgeson, 1988) for the standard partial molal properties of ionic species at high pressures and temperatures to include inorganic gases, acids, and other neutral aqueous species. Correlation algorithms similar to those used to estimate equation of state parameters for ions and electrolytes ( shock, and Helgeson, 1988) have also been developed for neutral aqueous species. Calculation of the standard partial molal thermodynamic properties of dissolved inorganic gases as well as other neutral aqueous species as a function of pressure and temperature indicates that the standard partial molal volume (V̄ 0), heat capacity (C̄ 0 p), and entropy (S̄ 0), together with the apparent standard partial molal enthalpy of formation (ΔH̄ 0) of many of these species in the liquid phase minimize with increasing temperature at P SAT ∗ ∗ P SAT represents pressures corresponding to liquid-vapor equilibrium for the system H 2O, except at temperatures <100°C where it refers to the reference pressure of 1 bar. and approach ∞ at the critical point of H 2O. In the case of other neutral aqueous species such as SiO 2(aq), V ̄ 0 , C ̄ 0 p , S ̄ 0 , and Δ H ̄ 0 behave as functions of temperature and pressure like those of electrolytes in the liquid phase and maximize with increasing temperature at P SAT, approaching — ∞ at the critical point of H 2O. Which of these types of behavior is exhibited by V ̄ 0 , C ̄ ) p , S ̄ 0 , and Δ H ̄ 0 for a given neutral aqueous species depends in part on the relative volatility of the aqueous species and the effect of the species on solvent dipole-dipole interaction. Close agreement between predicted and experimentally determined equilibrium constants for gas solubility and inorganic acid dissociation reactions at high temperatures and pressures supports the validity and generality of the equations of state and the predictive algorithms. High temperature/pressure equilibrium constants can be predicted for reactions involving a wide variety of neutral aqueous species for which few or no experimental data are available at temperatures > 25°C. Present capabilities permit such predictions to be made for hydrothermal and magmatic conditions at pressures and temperatures to 5 kb and 1000°C.

Thermodynamics of some nucleic acid bases and nucleosides in water, and their transfer to aqueous glucose and sucrose solutions at 298.15 K.

The partial molar heat capacities and volumes of some of the constituents of nucleic acids have been determined in water and 1 molal aqueous glucose and sucrose solutions in order to elucidate the nature of interactions occurring between various nucleic acid bases, nucleosides and the sugar (glucose and sucrose) molecules. The results have been explained in terms of the contributions from hydrophobic interactions, hydrophilic interactions and the hydrogen bonding between the solute and solvent molecules. The results have also been compared with those of amino acids and peptides in aqueous glucose and sucrose solutions.

Thermodynamics of aqueous uranyl ion: Apparent and partial molar heat capacities and volumes of aqueous uranyl perchlorate from 10 to 55°C

Apparent molar heat capacities and volumes of aqueous solutions containing UO2(ClO4)2 in dilute (≈0.06 molal) HClO4 have been determined from 10 to 55°C. These results have been analyzed using Young's rule to obtain apparent molar volumes and heat capacities for the solute UO2(ClO42). The temperature dependences of the conventional standard-state heat capacity and volume functions for UO22+ (aq) are well represented by the following equations: V̄°/cm(30 mo1P-1) −80.94 + 0.6091 T - 0.001063T2 andC̄/p0/(J K−1 mol−1) = 350.5 — 0.8722T — 5308/T —90) that are valid from 10 to 55°C. These results differ substantially from results of earlier measurements employing different uranyl salts. These differences are likely due to the presence of ion-paired or hydrolyzed U (VI) species in the earlier studies.

A new microcalorimetric vessel for dissolution of slightly soluble gases enthalpies of solution in water of carbon tetrafluoride and sulphur hexafluoride at 288.15, 298.15, and 308.15 K

A new flow-microcalorimetric vessel has been designed for the determination of the enthalpy of solution of slightly soluble gases. The vessel was a modification of our ordinary titration vessel that fitted into our four-channel microcalorimetric system. The calorimeter was calibrated by dissolution of oxygen in water and has been tested by dissolution of helium and propane in the same solvent. Enthalpies of solution of carbon tetrafluoride and sulphur hexafluoride in water were determined at 288.15, 298.15, and 308.15 K, and values for the partial molar heat capacities in water were derived. A new calorimetric enthalpy of solution of oxygen in water at 308.15 K was determined.