Thermodynamic parameters of solvation of nonelectrolytes in aqueous solutions with hydrogen bond networks

The contributions from interaction and reorganization to the enthaplies of solvation of polar and nonpolar nonelectrolytes in aqueous solutions of formamide, ethanediol, and 1,2-propanediol, forming 3D-associated structures in the entire composition range, were calculated. The enthalpy terms of the solvation of nonelectrolytes in aqueous solutions of methyl-and dimethylformamide were estimated. The data were compared considering the thermodynamic characteristics of these aqueous systems that we determined previously. It was found that the shape of the concentration dependences of the enthalpies of solvation of nonelectrolytes in all the examined solutions is determined by the reorganization term. The fact that the solvation of nonelectrolytes in water is the most exothermic compared to the aqueous-organic systems under consideration is due to the lowest value of the reorganization term in water, despite the fact that nonelectrolytes interact with water more weakly than with the nonaqueous components.     

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.     

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.     

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.     

Effects of ion concentration on the hydrogen bonded structure of water in the vicinity of ions in aqueous NaCl solutions

Molecular dynamics simulations of dilute and concentrated aqueous NaCl solutions are carried out to investigate the changes of the hydrogen bonded structures in the vicinity of ions for different ion concentrations. An analysis of the hydrogen bond population in the first and second solvation shells of the ions and in the bulk water is done. Although essentially no effect of ions on the hydrogen bonding is observed beyond the first solvation shell of the ions for the dilute solutions, for the concentrated solutions a noticeable change in the average number of water-water hydrogen bonds is observed in the second solvation shells of the ions and even beyond. However, the changes in the average number of hydrogen bonds are found to be relatively less when both water-water and ion-water hydrogen bonds are counted. Thus, the changes in the total number of hydrogen bonds per water are not very dramatic beyond the first solvation shell even for concentrated solutions.     

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.     

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.     

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.     

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?     

Hydration of nonelectrolytes in binary aqueous solutions

Literature data on the thermodynamic properties of binary aqueous solutions of nonelectrolytes that show negative deviations from Raoult’s law due largely to the contribution of the hydration of the solute are briefly surveyed. Attention is focused on simulating the thermodynamic properties of solutions using equations of the cluster model. It is shown that the model is based on the assumption that there exists a distribution of stoichiometric hydrates over hydration numbers. In terms of the theory of ideal associated solutions, the equations for activity coefficients, osmotic coefficients, vapor pressure, and excess thermodynamic functions (volume, Gibbs energy, enthalpy, entropy) are obtained in analytical form. Basic parameters in the equations are the hydration numbers of the nonelectrolyte (the mathematical expectation of the distribution of hydrates) and the dispersions of the distribution. It is concluded that the model equations adequately describe the thermodynamic properties of a wide range of nonelectrolytes partly or completely soluble in water.     

Solvation of amides of formic and acetic acids in water-glycerol mixture. Additivity of thermochemical characteristics of solutions

Values of the solution enthalpy of are measured and values of solvation enthalpy are calculated for formamide and N,N-two-substituted methyl-and ethylamides of formic and acetic acids in the mixed solvent: water-glycerol. Enthalpy coefficients of pair interactions between amides and glycerol in aqueous solutions are calculated. The influence of mixture composition and also of a structure and properties of the dissolved compounds on enthalpy characteristics is considered. Within the frames of the offered additive scheme the contributions from the structural fragments of molecules of amides to enthalpy characteristics of solutions are established. It has allowed us to analyze quantitatively the role of nonspecific and specific solvation of amides in solution, to predict the enthalpy of evaporation, solution, solvation, the enthalpy coefficients of pair interactions of experimentally unstudied N-methylformamide, N-ethylformamide, N-methyl-N-ethylformamide, N-methylacetamide, N-ethylacetamide, and N-methyl-N-ethylacetamide in the mixtures of water-glycerol, and also to evaluate the donor numbers of these specified amides.     

Different Views on the Stability of Protein Conformations and Hydrophobic Effects

Apparently contradictory statements about the thermodynamics of aqueous protein solutions and of hydrophobic effect are quoted and discussed. Some credibility is found in the divergent points of view and it is pointed out that they focus attention on different aspects of the complicated conditions in aqueous solutions, some of which are more important than others for the stability of protein conformations.The importance of characteristics of solvent water is emphasized, in particular (1) the strong mutual cohesion of water molecules, and (2) structural changes of water induced by (nonpolar) solute molecules. It is stressed that consideration of only one of these effects and an inexpedient choice of standard states are origins of confusion in the literature about aqueous systems. A simple approach to hydrophobic effects considering both of the above mentioned effects, is proposed.     

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.     

The Effect of Temperature on the Rate of Alkaline Hydrolysis of Esters in Concentrated Aqueous Solutions of Et4NCl

The rate of alkaline hydrolysis of 4-nitrophenyl toluenesulfonate, 4-nitrophenyl diethyl phosphate, and 4-nitrophenyl dimethylcarbamate (with Et₄NOH as reagent) in concentrated aqueous solutions of Et₄NCl (0-4.5 M) at 15, 25, 40, and 60 °C was studied. The nature of the effect of Et₄NCl on the reaction rate was explained by the action of the electrolyte on the structure of the water. It was shown that the increase in the rate results from change in the solvation component of the free energy of activation.     

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.     

The dynamics of water at DNA interfaces: computational studies of Hoechst 33258 bound to DNA

Together, spectroscopy combined with computational studies that relate directly to the experimental measurements have the potential to provide unprecedented insight into the dynamics of important biological processes. Recent time-resolved fluorescence experiments have shown that the time scales for collective reorganization at the interface of proteins and DNA with water are more than an order of magnitude slower than in bulk aqueous solution. The molecular interpretation of this change in the collective response is somewhat controversial some attribute the slower reorganization to dramatically retarded water motion, while others describe rapid water dynamics combined with a slower biomolecular response. To connect directly to solvation dynamics experiments of the fluorescent probe Hoechst 33258 (H33258) bound to DNA, we have generated 770 ns of molecular dynamics (MD) simulations and calculated the equilibrium and nonequilibrium solvation response to excitation of the probe. The calculated time scales for the solvation response of H33258 free in solution (0.17 and 1.4 ps) and bound to DNA (1.5 and 20 ps) are highly consistent with experiment (0.2 and 1.2 ps, 1.4 and 19 ps, respectively). Decomposition of the calculated response revealed that water solvating the probe bound to DNA was still relatively mobile, only slowing by a factor of 2-3, while DNA motion was responsible for the long-time component (approximately 20 ps).     

The Study of Water Structure in Aqueous Solutions of Butoxyethanol by Enthalpy of Mixing Measurements

In order to confirm the existence of regions I and II in aqueous solutions of butoxyethanol(BE), the concentration and temperature dependences of enthalpies of mixing of aqueous BE solutions with some organic solvents were measured. It has been found that the increments of apparent enthalpies of mixing per mole of water with respect to the mole fraction of BE change irregularly at a certain concentration. This concentration nearly corresponds to the reported boundary between regions I and II. Although similar behavior has also been observed in aqueous solutions of iso-butoxyethanol, aqueous solutions oftert-butoxyethanol have shown no anomalous behaviors. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Excess enthalpies of solution of some primary and secondary alcohols in sodium dodecylsulfate micellar solutions

The exces enthalpies of solution of some primary and secondary alcohols in aqueous sodium dodecylsulfate micellar solutions were measured and the results were explained by considering the distribution of alcohols between aqueous and micellar phases. The distribution constant and the enthalpy of transfer (and the standard free energy and entropy of transfer) were obtained. The thermodynamic parameters for the transfer of secondary alcohols from the aqueous to the sodium dodecylsulfate (NaDS) micellar phase differ slightly from those of the corresponding primary alcohols. For both series of alcohols the additivity rule holds for free energies of transfer whereas enthalpies and entropies display convex curves. The present data are compared to those for the transfer of the same solutes from the aqueous to the dodecyldimethylamine oxide (DDAO) and dodecyltrimethylammonium bromide (DTAB) micellar phases. The role of the hydrophilic interactions between the OH group and the micelles' head groups is formulated. The thermodynamics of the branched methyl group were determined. Furthermore, the thermodynamics of solvation of primary alcohols in water, in NaDS micelles, and in octane have been calculated using reference states based on the assumption that the empty space around alcohols in the initial and final states is the same. It is shown that the solvation of alcohols in NaDS micellar phase is enthalpy driven and that the thermodynamic properties of solvation vs. the length of the alcohol tail is the same for water and NaDS micelles whereas it is different for octane. A possible explanation for this difference is that the alkyl chain of alcohols folds in octane.