Heat-capacity changes and partial molal heat capacities of several amino acids in water

Integral enthalpies of solution of several amino acids in water at low concentrations have been determined at 25 and 35°C. These data have been used to derive the heat-capacity change C _p ^o on dissolution at 30°C. Partial molal heat capacities C _p2 ^o have been obtained by combining C _p ^o with C _p2 ^o (heat capacity of pure solid amino acids). The results indicate that the increments in C _p ^o and C _p2 ^o values per CH₂ group increment in the homologous series of -amino acids are constant and in agreement with those found for other homologous series of compounds containing monofunctional groups. However, this is not the case with amino acids having the NH ₃ ⁺ group at the terminal position. The present work also indicates that, as the NH ₃ ⁺ group is shifted away from the COO^– group, hydrophobic hydration decreases, as indicated by a decrease in C _p ^o and C _p2 ^o . the results on various isomers of amino acids show that branching of alkyl groups has no effect on C _p ^o and C _p2 ^o , indicating that hydrophobic hydration is unaffected by branching. The effect of substitution of H by OH and of CH₃ by groups in some amino acids has also been studied and discussed.     

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 heat capacities and volumes of electrolytes and ions in acetonitrile-water mixtures

Apparent molar volumes V and heat capacities C_p, of NaCl, KCl, KNO₃, AgNO₃, KI, NaBPh₄ and Ph₄PCl have been measured in acetonitrile (AN)-water mixtures up to x_AN=0.25 by flow densitometry and flow microcalorimetry. Limited data have also been obtained for NaF, LiCl and KBr up to x _AN =0.15. Single ion volumes and heat capacities of transfer were obtained using the assumption _tX(PH₄P⁺) = _tX(BPh₄-) where X=V or C _p and _tX is the change in X for a species on transfer from H₂O to AN-H₂O mixtures. Volumes and heat capacities for simple salts show relatively little dependence on solvent composition. However, _tX for simple ions show more pronounced variations, exhibiting at least one extremum. These extrema are similar to but much less pronounced than those derived previously for ions in t-butanol-water mixtures. Surprisingly little correlation is found between the present data and other thermodynamic transfer functions. This is attributed to the predominance of ion-solvent over solvent-solvent interactions in AN-H₂O solutions. _tV and _tC_p, for the silver ion differ markedly from those of the alkali metal ions as a result of the well-known specific interaction between Ag⁺ and AN.     

Enthalpies of solution,partial molal heat capacities and apparent molal volumes of sugars and polyols in water

Integral enthalpies of solution of some sugars and polyols in water at low concentrations have been determined calorimetrically at 25 and 35°C. These data have been used to derive heat capacities of solution C°_p at 30°C. Partial molal heat capacities C°_p,2 have been obtained by combining C°_p with C _p,2 ^* , the heat capacity of pure solid compounds. Apparent molal volumes have been obtained from density data. The sugars as well as polyols show significantly high positive C°_p and C°_p,2 values. The results have been explained in terms of a specific hydration model. The effect of substitution of-OH by glycosidic-OCH₃ and of-CHOH by deoxy-CH₂ are also discussed.     

Apparent molar heat capacities and volumes of electrolytes and ions int-butanol-water mixtures

The apparent molar volumes V and heat capacities C _p, of NaCl, LiCl, NaF, KI, NaBPh₄ and Ph₄PCl have been determined in solutions of H₂O containing up to 40 mass% t-butyl alcohol (TBA) by flow densitometry and flow microcalorimetry. Combination of these results with literature data allows calculation of V and C _p, for 16 ions in these mixtures using the assumption that _tX(Ph₄P⁺) = _tX(BPh ₄ ^– ) where X=V or C _p and _tX is the change in X for a species on transfer from H₂O to TBA-H₂O mixtures. These are the first reported single ion values for C _p, in a mixed solvent. While whole electrolyte volumes and heat capacities show relatively smooth changes with solvent composition, _tX(ion) exhibit two well-developed extrema at around 10 and 25 mass% TBA. The shape of the _tX(ion) curves shows considerable uniformity among the alkali metal cations and the halide ions but the extrema become more pronounced with increasing size among the tetraalkylammonium ions. These extrema are analogous to those observed in aqueous organic mixtures of surfactants and are probably indicative of microphase transitions in these strongly interacting solvent mixtures.     

Partial molal enthalpies,excess partial molal heat capacities,and structural effects ofn-Am4NBr in the water-dioxane system

Integral heats of solution of tetra-n-pentylammonium bromide, n-Am₄NBr, in aqueous binary mixtures of dioxane are reported at 25° and 35°C from 0.0 to 0.3 mole fraction of dioxane. The excess partial molal heat capacity c _p ^° at 30°C is derived from the integral heats of solution at infinite dilution at 25° and 35°C. The partial molal enthalpies of n-Am₄NBr exhibit a rather flat maxima at 0.2 mole fraction dioxane. The c _p ^° values suggest a structure-breaking role for dioxane. The results obtained in this study are compared with those obtained with n-Bu₄NBr in the same system in an earlier study.     

Volumes and heat capacities of water andN-methylformamide in mixtures of these solvents

The densities of mixtures ofN-methylformamide (NMF) and water (W) have been measured at 5, 15, 25, 35, and 45°C, and the heat capacities of the same system at 25°C, both over the whole mole-fraction range. From the experimental data the apparent molar volumes (_v) and heat capacities (_c) of NMF and of water are evaluated. The relatively small difference between the partial molar volumes or heat capacities at infinite dilution and the corresponding molar volumes or heat capacities of the pure liquids for both NMF and water suggests that with regard to these quantities replacement of a NMF molecule by a water molecule or vice versa produces no drastic changes. The partial molar volume of water at infinite dilution in NMF is smaller than the molar volume of pure water, but the corresponding partial molar heat capacity is unexpectedly high.     

Computation of heat capacities of solid state benzene,p-oligophenylenes and poly-p-phenylene

Heat capacities (C_p) of solid benzene, biphenyl,p-terphenyl,p-quaterphenyl, and poly-p-phenylene were analyzed using the ATHAS Scheme of computation. The calculated heat capacities based on approximate vibrational spectra of solid benzene and the series of oligomers containing additional phenylene groups were compared to experimental data newly measured and from the literature to identify possible additional large-amplitude motion. The skeletal heat capacity was fitted to the Tarasov equation to obtain the one- and three-dimensional vibration frequencies ₁ and ₃ using a new optimization approach. Their relationship to the number of phenylene groupsn is: ₁=426.0–150.3/n; and ₃=55.4+81.8/n. Except for benzene, the quantitative thermal analyses do not show significant contributions from large-amplitude motion below the melting temperatures.This work was financially supported by the Div. of Materials Res., NSF, Polymers Program, Grant # DMR 90-00520 and Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. for the U. S. Department of Energy, under contract number DE-AC05-96OR22464.     

The effect of carbohydrates on the heat of fusion of water

The freezing of aqueous solutions of carbohydrates has been studied using differential scanning calorimetry. The reduction in the molar heat of fusion of water is attributed to the nonfreezing of a proportion of the water in the presence of the dissolved carbohydrate. The effects of concentration and molecular weight have been investigated.     

Excess heat capacities and orientational order in systems containing hexadecane isomers of different molecular structure

Using the Picker flow microcalorimeter, excess heat capacities have been obtained at 25°C throughout the concentration range for 2,2-dimethylbutane,n-hexane, and cyclohexane each mixed with a series of hexadecane isomers of increasing degrees of orientational order, as determined by depolarized Rayleigh scattering. The isomers are 2,2,4,4,6,8,8-heptamethylnonane, 6-, 4-, and 2-methylpentadecane, andn-hexadecane. Thec _p ^E values are negative, increasing rapidly in magnitude with increase of orientational order, and are not predicted by the Prigogine—Flory theory which neglects order. Values ofc _p ^E are obtained at 10, 25, and 55°C for cyclohexane +6-, 4-, and 2-methylpentadecane which with other literature data lead to the temperature dependence of the thermodynamic excess functions for cyclohexane solutions of the five C₁₆ isomers. The excess enthalpy and entropy vary with the C₁₆ isomer and with temperature, but the corresponding variation of the excess free energy is small, indicating a high degree of enthalpy-entropy compensation. This is consistent with a rapid decrease with temperature of orientational order in the C₁₆ isomers.     

Model systems for hydrophobic interactions: Volumes and heat capacities ofn-alkoxyethanols in water

The densities and heat capacities of the first four members of the 2-n-alkoxyethanols were measured in water over the whole mole fraction range with a flow densimeter and a flow microcalorimeter. The methoxy and n-propoxy homologs were studied at 25°C, ethoxyethanol at 19, 25, and 40C, andn-butoxyethanol at 4, 10, 25, 40, and 55°C. While methoxyethanol behaves as a fairly typical polar nonelectrolyte in water,n-butoxyethanol shows trends in the concentration dependence which resemble micellization; some pseudo-microphase transition occurs at about 0.02 mole fraction, and this transition concentration decreases with increasing temperature. There is no simple relationship between this phenomenon and the existence of a lower critical solution temperature at 49°C since the sharpness of the thermodynamic changes is maximum at the lowest temperature and at 55°C the apparent molal quantities on both sides of the two-phase region appear to fall on the same continuous curve. In the region prior to the pseudo-microphase separation the apparent and partial molal heat capacities decrease regularly but beyond approximately 0.01 mole fraction increase sharply to a maximum, suggesting some type of pre-association. The apparent molal heat capacity of water in the binary solutions is larger than the molar heat capacity of water over the whole mole fraction range. The present data seem to be consistent with a clathrate model for hydrophobic hydration and interactions with these systems.     

Thermodynamic properties of the ethylammonium nitrate + water system: Partial molar volumes,heat capacities,and expansivities

Partial molar volumes at 15, 25, and 45°C and partial molar heat capacities and expansivities at 25°C for ethylammonium nitrate + water mixtures are reported. The results are compared with those for other aqueous cosolvents, particularly hydrazine and ammonium nitrate.     

Thermochemical behavior of mixtures ofN,N-dimethylformamide with dimethylsulfoxide,acetonitrile, andN-methylformamide: Volumes and heat capacities

Densities and molar heat capacities have been measured for mixtures ofN,N-dimethylformamide with dimethylsulfoxide, acetonitrile, andN-methylformamide at 25°C over the complete mole fraction range. From these data the apparent molar volumes and heat capacities have been calculated for both components. These quantities, as a function of the mole fraction, deviate very little from their molar values, indicating that the mixtures can be regarded as almost ideal.     

Apparent molal heat capacities of transfer from H2O to D2O of tetraalkylammonium bromides

The volumetric specific heats (in J-K^–1-cm^–3) of tetraalkylammonium bromides (R ₄ NBr) have been measured at 25°C in the concentration range 0.02 to 0.4 aquamolal in H ₂ O and D ₂ O with a differential flow microcalorimeter. The apparent molal heat capacities _c, calculated from the specific heats and known densities, were fitted with the equation _c= _c ^o +A_cm^1/2+B_cm whereA _c is the Debye-Hückel limiting slope andB _c is an adjustable parameter.The standard heat capacity of transfer C _ptr ^o = _c ^o (D ₂ O- _c ^o (H ₂ O) of R ₄ NBr is positive forR equal ton-propyl andn-butyl and negative for methyl and ethyl. Except for Me ₄ NBr in H ₂ O, allB _c are negative and become more so as the size of the cation increases;B _c is usually more negative in D ₂ O. These results can be interpreted with a two-state model for water and show that a positive C _ptr ^o is evidence that the solute is an overall structure maker, while a negative value indicates a net structure breaker. The negativeB _c is consistent with the existence of strong solute-solute structural (mostly hydrophobic-hydrophobic and hydrophobic-hydrophilic) interactions in the solution.     

Standard molar enthalpies,volumes, and heat capacities of adamantane in cyclohexane,n-hexane,and carbon tetrachloride. Interpretation using the scaled-particle theory

The volumes, heat capacities, and enthalpies of solution of adamantane in cycloxane,n-hexane, and carbon tetrachloride have been measured as a function of concentration at 25°C (15, 25, and 35°C for the volumes). The results extrapolated to infinite dilution have been resolved into cavity formation and interaction terms. The former have been calculated from the equations of the scaled-particle theory. To estimate the contribution from the latter, we have assumed some proportionality between adamantane-solvent and cyclohexane-solvent interactions. This assumption has been verified with the three different solvents for the three studied thermodynamic functions. The diameter of adamantane in solution has been determined to be 6.36 Å.     

Thermodynamics of aqueous gallium chloride. Apparent molar volumes and heat capacities at 25°C

Apparent molar volumes and heat capacities of aqueous GaCl₃ have been measured at 25°C in binary GaCl₃ solutions up to 3 mol-kg^–1, and in ternary GaCl₃-HCl solutions, containing 0.1345 mol-kg^–1 HCl to suppress hydrolysis, up to a concentration of 1 mol-kg^–1 GaCl₃. Using the Pitzer interaction model for the excess properties, and using ridge regression for the derivation of physically meaningful regression parameters, the measurements yield the following results for the standard molar properties and Pitzer parameters at 25°C: V⁰(GaCl₃)=12.85 cm³-mol^–1; ₀ ^v (GaCl₃)=1.10×10^–4 kg-mol^–1–J^–1–cm^–3; _v ¹ (GaCl₃)=2.12×10^–3 kg–mol^–1–J^–1–cm³; Cv(GaCl₃)=1.34×10^–5 kg²–J^–1–cm³; V^o(GaOHCl₂)=13.84 cm³–mol^–1; C _o ^p (GaCl₃)=–480.8 J–K^–1–mol^–1; _J ⁰ (GaCl₃)=–8.02×10^–6 kg–mol^–1–K^–2; _J ¹ (GaCl₃)=0.73×10^–4 kg–mol^–1–K^–2; C_J(GaCl₃)=–2.52×10^–6 kg²-mol^–2-K^–2; C _p ⁰ (GaOHCl₂)=20.4 J-K^–1-mol^–1. The latter parameter has only mathematical significance, its physical meaning is unclear. Comparison of the present experimental results for the standard molar properties of Ga³⁺ with semi-empirical correlations casts doubt upon the general validity of these correlation methods for trivalent cations.     

Volumes and heat capacities of anionic-nonionic surfactant mixtures

Density, heat capacity and surface tension measurements of sodium decylsulfate (NaDeS)-dodecyldimethylamine oxide (DDAO)-water mixtures were carried out as functions of the surfactants total molality m_t at fixed stoichiometric mixture compositions X_NaDeS. From the surface tension data, the critical micelle concentration of NaDeS-DDAO mixtures as a function of X_NaDeS were obtained. From density and heat capacity data, the apparent molar volume V_,2 and heat capacity C_,2 of NaDeS-DDAO mixtures in water were calculated, respectively. At a given mole fraction, V_,2 and C_,2 monotonically increases and decreases, respectively, with increasing mt. However, anomalies were observed at X_NaDeS=0.1 and 0.3 for both V_,2 and C_,2 vs. mt curves. The nonideal contributions to the thermodynamic properties for the formation of surfactant-surfactant mixed micelles in water by mixing aqueous solutions of pure NaDeS and DDAO micelles were calculated at 0.3 mol-kg^–1 for the micellized surfactants mixture. The excess volume V^exc and heat capacity as functions of X_NaDeS are concave and S-shaped curves, respectively. All the properties are compared to those for sodium dodecylsulfate-DDAO mixture. In addition, to clarify the effect of the change in the hydrophobicity of the surfactants mixtures V^exc for the dodecyltrimethylammonium bromide-decyltrimethylammonium bromide mixture were calculated from literature data.     

Thermodynamic properties of alkali halides. II. Enthalpies of dilution and heat capacities in water at 25°C

The enthalpies of dilution and volumetric specific heats of most alkali halides were measured in water at 25°C with flow microcalorimeters in the concentration range 0.01 to 1m. Apparent molal relative enthalpies _L, derived from the enthalpies of dilution, can be represented by a parametric equation in molality. Combining _L with osmotic data, excess entropies can be calculated. Excess free energies, enthalpies, and entropies are compared at 0.5m, and the observed trends are consistent with a model of structural interactions in aqueous alkali halide solutions. The apparent molal heat capacities _C were fitted with the equation _C= _C ^° +A_C(d₀m)^1/2+B _C m. The _C ^° are, in general, additive to better than 1 J-K^–1-mole^–1 and reflect mostly the structural part of ion-solvent interactions. Taking _C ^° (H⁺)=0, conventional ionic _C ^° are obtained. The parameterB _C for different pairs of ions follows approximately the same trends as the corresponding parameterB _V for apparent molal volumes and seems to reflect structural interactions between the ions.     

A group additivity approach for the estimation of heat capacities of organic liquids and solids at 298 K

A group additivity method is described which provides heat capacity estimates of the condensed phase. The data base consists of 810 liquids and 446 solids. Group values for carbon in various common substitution and hybridization states and for 47 functional groups are provided. The standard error of estimation using this approach on this data base is 19.5 (liquids) and 26.9 J/ (mole K) (solids). This can be compared to typical experimental uncertainties of 8.12 and 23,4 J/ (mole K) associated with these measurements, respectively. Experimental uncertainties were estimated from the numerical differences obtained for a given substance from multiple independent literature reports.