Prediction of excess enthalpies for ternary aqueous solutions of nonelectrolytes

The excess enthalpiesH _xy ^E of ternary aqueous solutions of nonelectrolytes are used to test the possibility of making predictions for ternary solutions from the properties of the binary solutions only. Two methods are proposed: one is based on the empirical rule (h _xx ·h _yy )^1/2=h _xy . Another leads to the numerical prediction of the overallH _xy ^E . Both are successful for most of the pairs of solutes for whichH _x ^E ,H _y ^E >0.     

Microheterogeneous structure of aqueous solutions of nonelectrolytes X-ray diffraction studies

Journal of Structural Chemistry -     

Hydrophobic effect in aqueous solutions of nonelectrolytes. II. Cross-interactions of alkylureas

The heats of mixing of the aqueous solutions of monomethylurea, monoethylurea, 1,3-dimethylurea, 1,3-diethylurea were determined at 25°C. The excess enthalpies obtained are expressed in virial form, in terms of pair and triplet interaction coefficients. A method previously discussed for predicting the excess enthalpies and the interaction coefficients of ternary solutions has been applied to the alkylureas. The large deviations between the experimental and calculated values, led to the modification of that method, which better accounts for the marked mixed character of these substances.     

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.     

Effect of nonelectrolytes on the gel formation process in aqueous silver cysteine solutions

Electron microscopy and dynamic light scattering are used to investigate the structuring process of a supramolecular system based on the aqueous solutions of L-cysteine and silver nitrate diluted with liquid water-soluble nonelectrolytes. The gel formation mechanism is discussed.     

Aqueous solutions of nonelectrolytes

The liquid state is one of the three principal states of matter and arguably the most important one, the main reasons being the following: (I) the majority of chemical synthesis reactions are liquid-state reactions; (II) separation processes, such as distillation, extraction, and fractional crystallization, are based on vapor–liquid equilibria, liquid–liquid equilibria, and solid–liquid equilibria, respectively, all involving multicomponent mixtures/solutions; (III) when focusing on water as solvent, we note that it is the most abundant substance on the surface of the earth, and being the principal constituent (about 70% by weight) of all living organisms, it is essential for life as we know it. Thus, it is not surprising at all that experimental as well as theoretical work on nonelectrolyte solutions in general, and on aqueous solutions of nonelectrolytes in particular, have held prominent positions in (bio-)physical chemistry for more than a century. The insights thereby gained have contributed decisively to build the formal structure of chemical thermodynamics and have paved the way for the development of practically useful real-solution models needed in chemical engineering. In this review, first the thermodynamic formalism relevant for solubility studies as well as a critical discussion of some popular approximations will be presented concisely. Estimation methods for auxiliary quantities, such as virial coefficients and partial molar volumes at infinite dilution, will be briefly indicated, followed by a summary of rational strategies for data reduction and data correlation. Finally, a few eclectically chosen results obtained for dilute aqueous solutions of nonelectrolytes will be linked to hydrophobic effects, which are generally accepted to play an important role in a wide variety of biological processes.     

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.     

The hydrophobic effect in aqueous solutions of nonelectrolytes. I. Self-interactions of alkylureas

In order to clarify some aspects of the hydrophobic interactions, the enthalpies of dilution of monoethylurea, 1,3-dimethylurea, and 1,3-diethylurea have been determined calorimetrically at 25°C. The calorimetric data, expressed in terms of excess enthalpy, permit the evaluation of the pair and triplet interaction coefficients. The analyses of these and of the analogous coefficientsg _xx andg _xxx, derived from osmotic data, indicate a driving force favorable to the interactions among the hydrated solute molecules. Nevertheless, the positive values of theh _xx andh _xxx coefficients seem to suggest that the source of the effect is a rearrangement of the water molecules rather than a direct association of the solute molecules. There are evidences of a strict correlation between the enthalpic and the entropic effects. Preliminary data were presented at the International Conferences on Chemical Thermodynamics at Baden (1973) and Montpellier (1975). The experimental part was carried out at the Istituto Chimico of the University of Trieste. To whom correspondence should be addressed.     

Hydration and dielectrical properties of aqueous pyrrolidinium trifluoroacetate solutions

Results from microwave measurements of the dielectrical properties of aqueous pyrrolidinium trifluoroacetate solutions at maximum water dispersion frequencies (13–25 GHz) and temperatures of 288, 298, and 308 K are given. The static dielectrical constants, times, and activation parameters of the dielectrical relaxation of solutions are calculated. The enthalpy and time of dielectrical relaxation activation are increased by deceleration of the motion of water molecules in the hydrate shells of ions. The changes in dielectrical parameters are in this case minimal in a series of aqueous solutions of diallylammonium salts with cations of different structures and degrees of substitution. It is shown that pyrrolidinium ions are characterized by weak hydrophobic hydration.     

Hydration of alcohols in aqueous methanol solutions from ultrasonic velocity measurements

Ultrasonic velocity maxima, as a function of alcohol concentration in aqueous methanol solution, have been determined for solutions of a number of mono- and polyhydroxyalcohols. The location of the maximum relative to that in the water-methanol system alone was related to the hydration of the solute molecule. Calculated hydration numbers are linearly correlated with the number of hydroxyl groups in the case of polyhydroxyalcohols, and with the surface area of the non-polar regions in the case of monohydroxyalcohols.     

Hydration numbers of glycine in aqueous sodium chloride and urea solutions

The hydration number of the glycine amino acid in aqueous sodium chloride solution is less than in aqueous urea solution; the difference increases significantly with increasing concentration of the nonaqueous component. Given the same partial volume of water, the hydration numbers of glycine in the two systems are close together (δ ≈ 5%).