Thermodynamic parameters of solvation of nonelectrolytes in aqueous solutions of amides of carboxylic acids

Interaction and reorganization contributions to solvation enthalpies of nonelectrolytes in aqueous solutions of amides of carboxylic acids with different degree of N-substitution and N-methylpyrrolidone are calculated. The data are discussed using structurally thermodynamic characteristics of water-amide systems obtained by us previously. It is found that the type of concentration dependence of the solvation enthalpy of nonelectrolytes in all solutions investigated is determined by the type of reorganization component. It is shown that the highest solvation exothermicity of nonelectrolytes in water is due to the lowest value of the reorganization contribution in spite of that nonelectrolytes interact weaker with water than with non aqueous components.     

Thermodynamics of solute transfer between chloroform and water

The apparent enthalpies, free energies and entropies of transfer from water saturated chloroform to chloroform saturated aqueous buffer (pH 7) were determined for five primary alcohols and six other organic nonelectrolytes using an isoperibol flow microcalorimeter. A linear relationship between the enthalpies and free energies of transfer is found for the homologous series of alcohols indicating that the occurrence of enthalpy-entropy compensation in solute transfer is not restricted to solvent systems of low mutual solubility. The apparent thermodynamics of transfer from chloroform to aqueous buffer were compared with those from 2,2,4-trimethylpentane to aqueous buffer and were rationalized in terms of solvation interactions.     

Thermodynamic and structural properties of aqueous linear diol solutions

Thermodynamic characteristics of aqueous linear diol solutions are calculated. These data are used to identify regularities in the variations of the structural properties of the mixtures being studied. The correlation between the entropy and enthalpy characteristics of water-diol systems with excess packing coefficients is evidence that the structural and energy properties of aqueous linear diol solutions are determined by universal interactions. The form of the concentration dependences of the solvation enthalpies and entropies of noble gases in water-linear diols mixtures is determined by the reorganization component and is attributed to the destruction of the H bond network of water, which results in the formation of the most densely packed solutions in the medium range of compositions.     

Evaluating the contribution of solvophobic effects to the Gibbs energy of solvation in methanol

A method for analyzing the thermodynamical manifestations of solvophobic effects is proposed on the basis of considering the relationship between the Gibbs energy and solvation enthalpy of nonelectrolytes. It is demonstrated that, for solutions in nonassociated solvents, there is a linear isoequilibrium dependence between them, and the coefficients of linear dependence are almost equivalent for various dissolved substances and solvents. It is determined that the deviations from this dependence observed in the case of associated solvents are always positive, and the consequences of the manifestations of solvophobic effects are considered. The contributions from the solvophobic effect to the Gibbs energy of solvation of various nonpolar compounds in methanol are determined on the basis of a thermodynamic model of solvation suggested earlier. It is shown that in both methanol and aqueous solutions, the values of these contributions correlate linearly with the characteristic molecular volume of the dissolved substance.     

A Study on Hydration of Urea and Urotropin by Density and Speed of Sound Measurements in Aqueous Solutions

Experimental data on the speed of propagation of ultrasound waves, density, and isobaric heat capacity in aqueous solutions of urea and urotropin have been considered. The findings have been used for calculating the molar isentropic compressibilities of solutions of the investigated substances over the temperature range 278.15 to 308.15 K. Invoking a theoretical solvation model based on the isentropic compressibility, which takes into account compressibilities of the hydrated complexes, their structural characteristics have been determined in aqueous solutions of nonelectrolytes: hydration numbers h, molar isentropic compressibility of hydrated complexes ?? _h V _h , molar volumes of water in a hydration shell V _1h , molar volumes of the solute without its hydration environment V _2h , and many other properties. The possibility of hydrophobic solvation has been shown for urotropin solutions and hydrophilic solvation for urea solutions.     

Hydration numbers and the state of water in hydration spheres of magnesium chloride and magnesium sulfate solutions

An equation for determining hydration numbers and molar adiabatic compressibilities has been derived by correct thermodynamic processing of a data set comprising ultrasound velocity, density, and isobaric heat capacity in aqueous magnesium chloride and sulfate solutions for concentrations ranging from very dilute solutions to the complete solvation boundary and temperatures of 273.15?C323.15 K. One basic innovation of the derived equation consists in that it accounts for the dependence of solvation numbers on solution concentration and in that all changes experienced by the solution are associated with the solvation of a stoichiometric mixture of electrolyte ions or nonelectrolytes molecules.     

New approach to free energy of solvation applying continuum models to molecular dynamics simulation

A new approach to the calculation of the free energy of solvation from trajectories obtained by molecular dynamics simulation is presented. The free energy of solvation is computed as the sum of three contributions originated at the cavitation of the solute by the solvent, the solute-solvent nonpolar (repulsion and dispersion) interactions, and the electrostatic solvation of the solute. The electrostatic term is calculated based on ideas developed for the broadly used continuum models, the cavitational contribution from the excluded volume by the Claverie-Pierotti model, and the Van der Waals term directly from the molecular dynamics simulation. The proposed model is tested for diluted aqueous solutions of simple molecules containing a variety of chemically important functions: methanol, methylamine, water, methanethiol, and dichloromethane. These solutions were treated by molecular dynamics simulations using SPC/E water and the OPLS force field for the organic molecules. Obtained free energies of solvation are in very good agreement with experimental data.     

Anisotropic inertial term of the solvation energy in binary solutions

An expression based on the Fröhlich theorem is given for the anisotropic inertial solvation potential of solutions. The principle of the additivity of the anisotropic inertial solvation potentials of solution components is put forward and substantiated. A model thermodynamic function of the anisotropic inertial solvation potential of a binary solution is suggested. The effect of formation of 1:1 complexes and bimolecular associates on the anisotropic inertial solvation potential of a binary solution is analyzed. The composition dependences of the anisotropic inertial solvation potentials of binary solutions of nitrobenzene, acetonitrile, nitromethane, and tetrachloromethane in associated and nonassociated polar and nonpolar solvents and in water are determined. The dependences obtained are compared to the corresponding model functions. Changes in the contribution of specific intermolecular interactions to the anisotropic inertial term of the Helmholtz energy of solvation of binary solutions are revealed by this method. Previously unknown anisotropic inertial solvation potentials are obtained for associated and polar nonassociated liquids in relation to their content in hexane. Conclusions on the magnitude and character of changes in the microstructure of solutions are made. The transformation of the anisotropic inertial to isotropic noninertial term of the Helmholtz energy of solvation is noted by the example of a solution with the ethanol volume fraction in hexane of 0.13.     

The enthalpies of solution of water, <Emphasis Type="Italic">o</Emphasis>-xylene,and triton X-100 in isopropanol

The enthalpies of solution of water, o-xylene, and Triton X-100 in 2-propanol were measured calorimetrically at 288.15, 298.15, and 313.15 K. The results obtained and literature data were used to calculate the heats of condensation, solution, and solvation of these nonelectrolytes. The influence of the nature of nonelectrolytes and temperature on the enthalpies of nonspecific and specific solvation is discussed.     

用NMR研究D-葡萄糖在碱性溶液内的行为

葡萄糖在碱性溶液中存在异构化、解离和溶剂化三种作用,但葡萄糖的中性、碱性溶液~(13)CNMR图谱几乎没有变化。我们测定了葡萄糖、果糖和甘露糖在不同pH的~(13)CNMR;也测定了葡萄糖的中性、碱性水溶液的~1HNMR,用INDOR法确定其H_((1)),H_((2))峰,并和葡萄糖分子、离子和水合离子的EHMO计算结果对照分析,说明了葡萄糖图谱不变的原因。     

Thermochemistry of dissolution of aniline in mixtures of water with methanol andtert-butyl alcohol

The enthalpies of dissolution {ie2391-1} of aniline in mixtures of water with methanol andtert-butyl alcohol were determined in the whole concentration range of mixed solvents at 25 °C. Maxima of endothermicity were found in both systems at −0.3 and −0.06 molar fractions, respectively. The data obtained were compared with the thermochemical characteristics of the nonelectrolytes studied previously (dimethylsulfoxide, nitromethane, formamide,etc). The conclusion was drawn that the behavior of aniline in water-alcohol mixtures is mainly determined by its proton-donating ability and solvation of the benzene ring. A comparison of acoustic data and spectral and thermodynamic characteristics showed that thermochemical data are the most sensitive indicator for energy and structural changes occurring upon variation of the composition of a mixed solvent. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2471–2478, December, 1998.     

Thermodynamic quantities of surface formation of aqueous electrolyte solutions. VI. Comparison with typical nonelectrolytes, sucrose and glucose

To demonstrate an important distinction between the electrolytes and nonelectrolytes, surface tension of aqueous solutions of typical nonelectrolytes, sucrose and glucose, was measured as a function of temperature and concentration. The presence of sucrose or glucose molecules in the surface region affects the surface tension in the same way as the presence of an ion does. There is, however, a difference in the temperature coefficient of the surface tension between typical nonelectrolyte solutions, sucrose and glucose, and alkali halide solutions. The entropy of surface formation of sucrose and glucose solutions is the same as that of pure water, while that of alkali halide solutions decreases with concentration. The relation between this entropy change and the formation of electric double layers was discussed.     

Solvation and Enthalpy Coefficients of Interaction of Carboxylic Acid Amides with 1,2-Diols in Water

The enthalpies of solution of formamides, acetamides, and propionamides with various extents of N-substitution in aqueous solutions of propylene glycol and 1,2-butanediol (concentration 4 mol kg^{- 1}) at 298.15 K were measured. The enthalpies of transfer of the amides from water to mixed aqueous-organic solvents were determined and discussed in combination with data on the enthalpies of transfer of amides from water to aqueous ethylene glycol solutions. The enthalpy coefficients of pair interactions of amides with 1,2-diols in water were calculated and used for evaluating the group components on the basis of the principle of group additivity of contributions. As the hydrophobic properties of polyfunctional nonelectrolytes are enhanced, the coefficients increase, which is due to the growth of the hydrophobic component of the interaction.     

The dependence of the osmotic coefficient on the composition of multicomponent solutions

The thermodynamic theory of binary aqueous solutions of electrolytes taking into account the electrostatic interaction of ions and their hydration and association was extended to multicomponent solutions. Equations for calculating the osmotic coefficient of multicomponent solutions from parameter estimates (hydration and association numbers under standard conditions) determined for the corresponding binary subsystems were substantiated. Interval parameter estimates were used to calculate the osmotic coefficients for several three-five-component aqueous solutions containing both nonelectrolytes and electrolytes. A comparison of the results with the literature data showed that cross interactions between components could be ignored for the multicomponent solutions studied.     

A method for calculating the Gibbs energy of nonspecific solvation

A method for calculating the Gibbs energy of nonspecific solvation of nonelectrolytes was suggested. The new equation for the Gibbs energy of nonspecific solvation contains one solvent parameter that characterize nonspecific solvent-solute interactions and two experimental Gibbs energies of solvation in two standard solvents. The method is applicable to a wide range of solutes and solvents. It was successfully used to describe some 800 Gibbs energies of solvation for systems without specific solvent-solute interactions.     

Ab initio aqueous thermochemistry: application to the oxidation of hydroxylamine in nitric acid solution

Ab initio molecular orbital calculations were performed and thermochemical parameters estimated for 46 species involved in the oxidation of hydroxylamine in aqueous nitric acid solution. Solution-phase properties were estimated using the several levels of theory in Gaussian03 and using COSMOtherm. The use of computational chemistry calculations for the estimation of physical properties and constants in solution is addressed. The connection between the pseudochemical potential of Ben-Naim and the traditional standard state-based thermochemistry is shown, and the connection of these ideas to computational chemistry results is established. This theoretical framework provides a basis for the practical use of the solution-phase computational chemistry estimates for real systems, without the implicit assumptions that often hide the nuances of solution-phase thermochemistry. The effect of nonidealities and a method to account for them is also discussed. A method is presented for estimating the solvation enthalpy and entropy for dilute aqueous solutions based on the solvation free energy from the ab initio calculations. The accuracy of the estimated thermochemical parameters was determined through comparison with (i) enthalpies of formation in the gas phase and in solution, (ii) Henry's law data for aqueous solutions, and (iii) various reaction equilibria in aqueous solution. Typical mean absolute deviations (MAD) for the solvation free energy in room-temperature water appear to be ~1.5 kcal/mol for most methods investigated. The MAD for computed enthalpies of formation in solution was 1.5-3 kcal/mol, depending on the methodology employed and the type of species (ion, radical, closed-shell) being computed. This work provides a relatively simple and unambiguous approach that can be used to estimate the thermochemical parameters needed to build detailed ab initio kinetic models of systems in aqueous solution. Technical challenges that limit the accuracy of the estimates are highlighted.     

The bulk properties of dioxane solutions in ethylene glycol at 25–75°C

The densities of ethylene glycol solutions of dioxane were measured at 0–20 mol % dioxane at five temperatures ranging from 25 to 75°C. The apparent molar volumes of dioxane in the solutions were studied. The results were compared with similar characteristics of the aqueous solutions of dioxane and ethylene glycol solutions of tetrahydrofuran. In contrast to these systems, the ethylene glycol-dioxane system did not show a minimum of the apparent molar volumes of dissolved nonelectrolytes. The conclusion was drawn that the solvophobic effects were absent in the bulk properties of the ethylene glycol solutions of dioxane because of the low lability of the spatial net of H-bonds of ethylene glycol and peculiarities of the molecular structure of dioxane.     

The Nature of Aqueous Solutions: Insights into Multiple Facets of Chemistry and Biochemistry from Freezing‐Point Depressions

Contrary to current widely held beliefs, many concentrated aqueous solutions of electrolytes and nonelectrolytes behave ideally. For both, the same simple equation yields mole fractions of water that are equal to the theoretical activities of water. No empirical activity coefficients or ad hoc parameters are needed. Thermodynamic hydration numbers and the number of particles produced per mole of solute are found by searching freezing‐point depression measurements, as if asking the water, “How much available water solvent is left and how many solute particles are there?” The results answer questions currently under debate: Do solutes alter the nature of water outside their immediate surroundings? What is the number of ion pairs formed by various electrolytes and what affects extents of their formation? What are some factors that cause precipitation of proteins, latexes, and so forth from aqueous solutions upon addition of other solutes (Hofmeister series)? Which nonelectrolytes form aggregates in water and what are the implications? Why do different solutes affect viscosity differently? How do ion‐selective channels in cell membranes function at the molecular level?