Thermodynamic Properties of Peptide Solutions. Part 17. Partial Molar Volumes and Heat Capacities of the Tripeptides GlyAspGly and GlyGluGly,and Their Salts K[GlyAspGly] and Na[GlyGluGly] in Aqueous Solution at 25 °C

The partial molar volumes, V^o₂, and partial molar heat capacities, C_p,2^o, at infinite dilution have been determined for the two tripeptides glycylaspartylglycine (glyaspgly) and glycylglutamylglycine (glyglugly), and also for their salts K[glyaspgly] and Na[glyglugly], in aqueous solution at 25 °C. The ionization constants at 25 °C for the aspartyl and glutamyl side-chains have also been determined. These new thermodynamic results have been combined with literature data for electrolytes to obtain the volume and heat capacity changes upon ionization of the acidic side-chains of the peptides. The results are compared with those for other carboxylic acid systems. The partial molar heat capacities and volumes have also been used to calculate the contributions of the acidic amino acid side-chains to the thermodynamic properties.     

Thermodynamic properties of peptide solutions 3. Partial molar volumes and partial molar heat capacities of some tripeptides in aqueous solution

Partial molar volumes, V ₂ ^o , and partial molar heat capacities, C _p,2 ^o , of the tripeptides glycylglycylglycine, glycylglycylalanine, glycylalanylglycine and alanylglycylglycine have been determined in aqueous solution at 25°C. For the three alanyl-containing tripeptides, the data indicate that the tripeptide-water interaction is influenced by the side chain position within the molecule. The results have been rationalized in terms of likely solutesolvent interactions. The V ₂ ^o and C _p.2 ^o data have also been used to calculate the contribution to these properties of a-CH₃ side chain.     

Thermodynamic and19F NMR studies of antimony trifluoride in water

Densities, specific heat capacities per unit volume and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for antimony trifluoride in water as functions of concentration. From the first three properties the apparent and partial molar volumes, heat capacities and relative enthalpies were derived. As well, pH measurements in water at 25°C and¹⁹F NMR spectra in water and methanol at 33°C were also carried out. All the thermodynamic properties together with the chemical shifts abruptly change in the very dilute concentration region (<0.1m) and, then, tend to a constant value. These trends have been rationalized through a simple model based on an equilibrium of dissociation of SbF₃ into two ionic species. From the simulation of all the data it is derived that two concomitant equilibria are present in solution: the hydrolysis process of SbF₃ which explains the pH values and the ionic dissociation of SbF₃ which accounts for the¹⁹F NMR data.     

Thermodynamic properties of peptide solutions 4. Partial molar volumes and heat capacities of aqueous solutions of some glycyl dipeptides

Partial molar volumes, V ₂ ^o and partial molar heat capacities C _p,2 ^o have been determined in aqueous solution at 25°C for the dipeptides glycyl-L-asparagine, glycyl-DL-threonine, glycyl-DL-serine and glycyl-DL-phenylalanine. These results, along with those for some other dipeptides of sequence Gly-X, were used to estimate side chain contributions to V ₂ ^o and C _p,2 ^o . For these dipeptides both V ₂ ^o and C _p,2 ^o were found to be a linear function of the respective thermodynamic property for the amino acid X. The contributions of the glycyl units to V ₂ ^o and C _p,2 ^o of the dipeptide are discussed.     

Excess heat capacities of ternary systems containing chlorobenzene or chloronaphthalene

Densities and heat capacities of ternary systems were determined at 25°C. The ternary systems consisted of: a polar molecule (component 1) + a mixture of alkanes (components 2 and 3) of different sizes and shapes. Five such systems were studied: chlorobenzene + cyclohexane + n-heptane; chlrobenzene + cyclohexane + n-hexadecane; chlorobenze + cyclohexane + isooctane; chlorobenzene + isooctane + n-heptane; 1-chloronaphthalene + isooctane + n-heptane. The excess molar volumes and heat capacities were obtained along dilution lines by component 1 (chlorobenzene or 1-chloronaphthalene) of mixtures of components 2 and 3 (at fixed component 2 mole fraction X₂). Unexpectedly the excess heat capacities C _p1(23) ^E of the pseudo-binaries {1+(2+3)} do not always fall between the two (limiting) curves of C _p12 ^E and C _p13 ^E corresponding to the two binaries {1+2} and {1+3}. Instead, especially for {chlorobenzene + cyclohexane + an n-alkane} the C _p1(23) ^E curves are displaced toward less negative values, even beyond the limiting values corresponding to the binaries. This correlates semi-quantitatively with the negative C _p23 ^E of the binary {2+3}.Presented at the Symposium, 76^th CSC Congress, Sherbrooke, Quebec, May 30-June 3, 1993, honoring Professor Donald Patterson on the occasion of his 65^th birthday.     

The state of dissolved benzene in aqueous solution

Henry's law constants for benzene in water have been measured by bringing water layers into equilibrium with solutions of benzene in carbon tetrachloride or in cyclohexane. The mole fraction of benzene in the aqueous layer was determined by ultraviolet absorption spectrophotometry, and its fugacity was taken as equal to that in the nonaqueous phase, reliable data for the C₆H₆–CCl₄ and C₆H₆–c–C₆H₁₂ systems being available in the literature. Measurements were made at 5^o intervals from 10 to 30°C inclusive, at mole fractions down to from 10% to 20% of saturation. In no case did Henry's law constant depart significantly from constancy, and it was in reasonable agreement with some representative literature values based on saturated solubility. The constancy and the magnitudes of our K_h values indicate that appreciable dimerization does not occur in the temperature range examined here. This conclusion contrasts with the suggestion of Reid, Quickenden, and Franks that their calorimetrically measured heat of solution of benzene in water is different enough from the van't Hoff heat to imply possible dimerization of the solute; it also contrasts with the hydrophobic-bond-forming tendency which Ben-Naim, Wilf, and Yaacobi ascribe to benzene on the basis of their studies of the solubilities of benzene and biphenyl. The results of the latter study, when combined with the known second virial coefficient of benzene vapor, predict that more than 20% of the benzene in saturated aqueous solution at 25°C should be present as dimer, in clear contradiction to the results of the present work.     

The apparent molar heat capacities and volumes of aqueous NaCl from 0.01 to 3 mol kg−1 in the temperature range 274.65 to 318.15 K

The heat capacities per unit volume of aqueous solutions of NaCl were measured with a flow microcalorimeter. The molality and temperature range covered were 0.01 to 3 mol kg^?1 and 274.65 to 318.15 K. The derived apparent molar heat capacities C₂, φ, when extrapolated to infinite dilution, give standard partial molar heat capacities C₂^o which are in excellent agreement with those of Criss and Cobble. The excess apparent molar heat capacities (C₂, φ - C₂^o) can be used to predict the temperature dependence of (H₂, φ - H₂^o), the excess apparent molar enthalpy. The calculated values of ΔH₂, φ agree within experimental uncertainty with the integral enthalpies of dilution of Ensor and Anderson and of Messikomer and Wood up to 323.15 K. Above this temperature significant differences are observed. The densities of the solutions were also remeasured in the same range of temperature and molality with a flow densimeter, and the derived apparent molar volumes agree with the literature values.     

Thermodynamics of aqueous EDTA systems: Apparent and partial molar heat capacities and volumes of aqueous strontium and barium EDTA

Apparent molar heat capacities and volumes have been determined for Na₂SrEDTA (aq) and Na₂BaEDTA (aq). Standard state partial molar heat capacities and volumes have been calculated as well as the partial molar properties at 0.1 m ionic strength that are needed for various thermodynamic calculations. Selected enthalpies and stability constants from the literature have been combined with out heat capacities to generate predicted stability constants to 200°C.     

Molar Volumes and Heat Capacities of Electrolytes and Ions in Nonaqueous Solvents: 1. Formamide

Apparent molar volumes and heat capacities of 27 electrolytes have been measured as a function of concentration in formamide at 25°C using a series-connected flow densimeter and Picker calorimeter system. These data were extrapolated to infinite dilution using the appropriate Debye–Hückel limiting slopes to give the corresponding standard partial molar quantities. Ionic volumes and heat capacities at infinite dilution were obtained by an appropriate assumption based on the reference electrolyte Ph₄PBPh₄ (TPTB). The ionic volumes, but not the heat capacities, agree reasonably well with previously published statistically based predictions. The values obtained are discussed in terms of simple models of electrolyte solution behavior and a number of interesting features are noted, including, possible dependencies of ionic volumes on solvent isothermal compressibility and of ionic heat capacities on solvent electron acceptor abilities.     

Thermodynamic properties of N-octyl-, N-decyl- and N-dodecylpyridinium chlorides in water

Densities, heat capacities and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for N-octyl-, N-decyl- and N-dodecyl-pyridinium chlorides in water over a wide concentration region. Conductivity measurements were performed in order to evaluate the cmc and the degree of counterion dissociation. Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25°C were derived from the experimental data as functions of the surfactant concentration. The changes with concentration of these properties are quite regular with the exception of the heat capacities which display anomalies at about 0.9, 0.25 and 0.12 mol-kg^–1 for the octyl, decyl and dodecyl compounds, respectively. At these concentrations there were also changes in the slopes of the specific conductivity and of the product of the osmotic coefficients and the molality vs. concentration. These peculiarities can be ascribed to micelle structural transitions. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. These data have been compared to those for alkyltrimethylammonium bromides and alkylnicotinamide chlorides. It is shown that the introduction of the hydrophilic CONH₂ group lowers the hydrophilic character of the pyridinium ring.     

Solvation of amino acid residues in water and urea-water mixtures: Volumes and heat capacities of 20 amino acids in water and in 8 molar urea at 25°C

The limiting partial molar volumes V ^o and heat capacities C _p ^o of 20 amino acids have been determined in water and in 8 molar urea at 25.0°C using flow calorimetry and flow densimetry. The side chain contributions to V ^o and C _p ^o were obtained as the difference between the properties of the various amino acids and those of glycine, both in water and in 8M urea. The solvent accessible surface area of the amino acid residues were obtained using a method developed by Hermann, and the total surface areas were separated into their hydrophobic A _Hb and hydrophilic components. In water, C _p ^o values for the various residues C _p ^o (R) were found well correlated with A _Hb , though much less so in the urea solution. Hence, C _p ^o (R) values, in water yield a good estimate of side chain hydrophobicity, but the (waterurea) transfer heat capacities appear strongly affected by specific solvation effects in the urea solution.Presented at the sixth Italian meeting on Calorimetry and Thermal Analysis (AICAT) held in Naples, December 4–7, 1984.     

Flow microcalorimeter for heat capacities of solutions

A new type of flow microcalorimeter for measuring heat capacities at constant pressure of liquids and solutions was constructed. This calorimeter is the similar in design to Picker's except for the flow system, which consists of two syringe type of pumps and two flowing paths in each flow cell. It was found that the magnitude of heat loss from cells depended on liquids themselves used and the flow rates of sample liquids. The molar heat capacities, C_p of benzene and ethanol were determined relative to those of cyclohexane and water, respectively. The excess molar heat capacities, C_p(E) for the systems of benzene + cyclohexane and water + ethanol were also determined at 298.15K by the direct mixing method. An inaccuracy for C_p(E) was estimated to be within ± 1%.     

Effect of temperature on ionic volumes and heat capacities in methanol

A flow densimeter and flow heat capacity calorimeter have been employed to measure precision densities and specific heats of selected electrolytes and nonelectrolytes in methanol at 20, 25, and 40°C. Apparent molar volumes and heat capacities have been calculated and the corresponding standard state functions, V ^o and C _p, ^o , evaluated. The data have been used, along with known volumes and heat capacity data at 25°C for numerous alkanes, to generate volumes and heat capacities of model compounds for organic electrolytes. Comparison of the thermodynamic functions for the model compounds with those of the corresponding electrolytes at the respective temperatures has made it possible to assign single ion values and to establish the temperature effects of ionic charges on the volumes and heat capacities of solutes.     

Thermodynamic investigations on derivatives of pyrimidine nucleic acid bases: Joint use of calorimetric, volumetric and structural data for the description of properties of pyrimidine nucleic acid bases and their derivatives

The experimental enthalpies of solution Δ_solH_m^∞, van’t Hoff enthalpies of sublimation Δ_s^gH_m⁰ of solid compounds, partial molar volumes V₂⁰, and partial molar heat capacities C_p,2⁰ of aqueous solutions of pyrimidine nucleic acid bases and their derivatives, determined previously and collected here, are discussed in terms of calculated structural parameters. Relations have been established between the calorimetric and volumetric properties. Correlations have been developed to relate both the enthalpies of solvation and the partial molar heat capacities to the polar and apolar parts of the accessible molecular surface areas.     

Apparent molar heat capacities and volumes of some alkylated derivatives of uracil and adenine in aqueous solution at 25°C

Densities and apparent molar heat capacities of some alkylated derivatives of uracil and adenine: 1-methyluracil, 1,3-dimethyluracil, 1,3-diethylthymine, 5,6-trimethylene-1,3-dimethyluracil, 5,6-tetramethylene-1,3-dimethyluracil, 5,6-pentamethylene-1,3-dimethyluracil, 2,9-dimethyladenine, 2-ethyl-9-methyladenine, 2-propyl-9-methyladenine, 8-ethyl-9-methyladenine, 6,8,9-trimethyladenine and 8-ethyl-6,9-dimethyladenine were determined using flow calorimetry and flow densimetry at 25°C. It was found that the partial molar volumes and heat capacities correlate linearly with the number of substituted methylene groups-CH ₂ -as well as to the number of hydrogen atoms, n _H , belonging to the skeleton of the molecule. In the case of alkylated uracils a difference was observed in the values at infinite dilution V ₂ ^o and C _p2 ^o , depending on the substitution of alkyl and cyclooligomethylene groups.     

Thermodynamic properties of biologically active substances: 3-acetyl-9-methoxy-2-phenyl-11H-indolizino[8,7-b]indole and 8-acetylharmine

The enthalpies of solution of 3-acetyl-9-methoxy-2-phenyl-11H-indolizino[8,7-b]indole and 8-acetylharmine in dimethyl sulfoxide were measured by isothermal calorimetry at solute: solvent molar ratios of 1: 9000, 1: 18000, and 1: 36000. From the data obtained, the standard enthalpies of solution of the compounds in dimethyl sulfoxide at infinite dilution were calculated. The heat capacities of 8-acetylharmine were determined by dynamic calorimetry in the interval 298.15–673 K, and the C _p ^o = f(T) equations were obtained. The standard enthalpies of combustion of the compounds were estimated by approximate methods, and their heats of melting were calculated. From the data obtained, using Hess cycle, the standard enthalpies of formation of the compounds were calculated.     

Thermodynamics of ionic solvation inn-propanol from −50 to 50°C

Enthalpies of the solution of 1–1 electrolytes (LiBr, LiI, NaI, NaBPh₄, R₄NBr (R=Me, Et, Pr, Bu, Am), Et₄NCl, Bu₄NI, Ph₄PBr) in n-propanol were determined experimentally from–50 to 50°C. Standard enthalpies of solution of the electrolytes studied were determined on the basis of the extrapolation equation of Debye-Hueckel theory and taking ionic association into account. The reference salt Ph₄PBPh₄ was used to obtain the ionic solvation properties. Partial molar heat capacities of ions in propanol were calculated and compared to our previously reported data for ethanol. The changes in the ionic heat capacities with temperature are discussed.     

Apparent molar volumes,heat capacities,and molecular excluded volumes of methylated,hydroxy and methoxy derivatives of 1-methylcytosine in aqueous solutions at 25°C

Densities and specific heat capacities of aqueous solutions: 1-methylcytosine; 1-methyl-N⁴-hydroxycytosine; 1,5-dimethylcytosine; I,N⁴-dimethylcytosine; 1,5-dimethyl-N⁴-hydroxycytosine; 1-methyl-N⁴-methoxycytosine; 1,N⁴,N⁴-trimethylcytosine, 1,5-dimethyl-N⁴-methoxycytosine were determined using flow calorimetry and flow densimetry at 25°C. Apparent molar volumes and heat capacities were then determined. Molecular excluded volumes were evaluated. A relationship was found between the values of the increments in partial molar values and the kind of groups substituted. Four types of contributions were distinguished: substitution of hydrogen on C, N, and O (in OH group on N4) atoms by CH₃ group and replacement of hydrogen on N4 atom by OH group. The correlation between the experimental partial molar volumes and calculated molecular excluded volumes was also elaborated.     

DISQUAC predictions of phase equilibria,molar and standard partial molar excess quantities for 1-alkanol+cyclohexane mixtures

Literature data for phase equilibria: vapor-liquid VLE, liquid-liquid LLE, and solid-liquid SLE; molar excess Gibbs energies G ^E , molar excess enthalpies H ^E ; activity coefficients _i and partial molar excess enthalpies H _i ^E,o at infinite dilution for 1-alkanol (1)+cyclohexane (2) mixtures are examined by the DISQUAC group contribution model. For a more sensitive test of DISQUAC, the azeotropes, obtained from the reduction of the original isothermal VLE data, are also examined for systems characterized by hydroxyl, alkane and cyclohexane groups. The alkane/cyclohexane and alkane/hydroxyl interaction parameters have been estimated previously. The cyclohexane/hydroxyl interaction parameters are reported in this work. The first dispersive parameters increase regularly with the size of the alkanol; from 1-octadecanol they are constant; an opposite behavior is encountered for the third dispersive parameters, which are constant from 1-dodecanol. The second dispersive parameters decrease as far as 1-propanol and then increase regularly; from 1-octadecanol they are constant. The quasichemical parameters are equal to those for the alkane/hydroxyl interactions. Phase equilibria, the molar excess functions, and activity coefficients at infinite dilution are reasonably well reproduced. Poor results are found for H _i ^E,o and DISQUAC predictions for H _i ^E,o are strongly dependent on temperature.     

Thermal and volumetric properties of chloroform+benzene mixtures and the ideal associated solution model of complex formation

In the ideal associated solution model, activity coefficients of all species (labelled A, B, and AB here) at equilibrium are taken to be unity at all compositions and temperatures. We have applied this model to an analysis of thermodynamic properties (vapor pressures, excess enthalpies, partial molar enthalpies of solution, excess heat capacities, and excess volumes) of the chloroform+benzene system in terms of K, H, C _p , and V for the equilibrium represented by A+B=AB. It is demonstrated that there is reasonably good consistency between this simple model and all of the thermodynamic data, which shows that the model is realistic enough to be useful in assessing the properties of the not-very-stable AB complex in the chloroform+benzene system. New thermal (partial molar enthalpies of solution and excess heat capacities) and volumetric properties of the chloroform+benzene system have been measured, with results presented here.