Enthalpies of Mixing and State of Components in Aqueous-Organic Mixtures with Nets of Hydrogen Bonds

The enthalpies of mixing of water with glycerol over the entire composition range were determined at 298.15 K. The partial excess enthalpies of the components of mixtures of water with glycerol, ethylene glycol, 1,2- and 1,3-propylene glycol, and formamide were estimated and used for examining solvation and the state of water and the organic components in the solutions. The composition of the solvation shells of the components of the mixtures were shown to depend on the nature and structure of the organic solvent.     

Development of Abraham model correlations for predicting enthalpies of solvation of nonionic solutes dissolved in formamide

Experimental enthalpy of solution data for 81 solutes dissolved in formamide have been compiled from the published literature and converted to enthalpies of solvation data using standard thermodynamic relationships. Abraham model correlations were derived from the compiled enthalpy of solvation data. The derived mathematical equations describe the observed experimental to within overall standard deviations of 2.3 kJ mol^?1. A principal component analysis based on the calculated equation coefficients, and the coefficients for water and 23 organic solvents previously determined, shows that formamide is both a hydrogen-bond acid and a hydrogen-bond base. Of the solvents that we have studied thus far, formamide is the closest to water in terms of enthalpies of solvation of dissolved solutes.     

The Enthalpies and Heat Capacities of Solution of Tetraethyl- and Tetrabutylammonium Bromides in Methanol,Formamide, and Ethylene Glycol

The heat effects of solution of tetraethyl- and tetrabutylammonium bromides in methanol (MeOH), formamide (FA) and ethylene glycol (EG) were measured at 25 and 40°C at various concentrations. Various calculation methods were used to determine the standard enthalpies of solution. The results were compared with the available data on aqueous systems. The mean standard heat capacities of solution over the temperature range studied were calculated. The behaviors of tetraalkylammonium salts in water and FA were shown to be much less different than in water and alcohols, which was evidence of the possibility of the appearance of solvophobic solvation effects in the solution of nonpolar particles in FA.     

甲酰胺在乙二醇-水混合溶剂中的稀释焓

酰胺是肽的基本结构单元, 而且在蛋白质的二级结构中与酰胺联系的氢键对蛋白质的稳定起着十分重要的作用. 作为蛋白质模型化合物热力学性质研究的一部分, 报道了甲酰胺在乙二醇水溶液中的稀释焓.      

Enthalpy characteristics of dilute solutions of acetonitrile

The enthalpies of mixing of acetonitrile with formamide,N-methylformamide,N,N-dimethylformamide, and hexamethylphosphoric triamide were measured in the temperature interval from 283.15 to 328.15 K. The enthalpy coefficients of binary and ternary interactions were calculated by the methods of the McMillan-Mayer theory. The contributions to the enthalpy of solution due to the formation of a cavity in the solvent, Δ_cav H°, and those due to the interaction of the solute with the solvent, Δ_int H°, were determined. The enthalpies of the specific and non-specific solvation of acetonitrile in the corresponding amides were calculated. Specific interactions were found to contribute the most to the solvation enthalpy of acetonitrile. The obtained values were compared with analogous values for solutions of acetonitrile in water and alcohols. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 289–293, February, 1997.     

Solvation of amides of carboxylic acids in aqueous solutions of ethylene glycol

Enthalpies of solution of amides of formic, acetic, and propionic acids with different degrees of N-substitution in aqueous solutions of ethylene glycol were measured at 298.15 K. The concentration of ethylene glycol did not exceed 4 mol kg^–1. The reasons for increasing endothermic values of the enthalpies characterizing the amide transfer from water to a mixed aqueous-organic solvent on going from primary to tertiary amides and from formamides to the corresponding acetamides are discussed. The enthalpic coefficients of pair interactions between amides and ethylene glycol in water were calculated. The endothermicity of the interaction of the alkyl groups of the amide molecules with ethylene glycol results in positive values of the coefficients. The coefficient values increase with the enhancement of the hydrophobic properties of hydrophilic non-electrolytes (urea, formamide, ethylene glycol) due to an increase in the contribution of the hydrophobic component and a decrease in the contribution from the interaction of the polar groups of amides to the total interaction.     

Heat properties of Bu<Subscript>4</Subscript>NBr solutions in binary mixtures based on formamide

Heat effects of tetrabutylammonium bromide dissolution in mixtures of formamide with methanol and ethylene glycol at 25°C were determined. Partial molar enthalpies of the components of formamide-ethylene glycol mixtures at 25°C were measured by the calorimetric method, and the mixing enthalpies of this system were determined. Within the limits of the second approximation of the Debye-Hueckel theory the standard enthalpies of dissolution Δ_dis H ⁰ were calculated, and the enthalpies of Bu₄NBr transfer from formamide in its mixtures with water methanol and ethylene glycol were found on this basis. The enthalpy parameters of Bu₄NBr pair interactions with the components of the formamide-water, formamide-methanol, and formamide-ethylene glycol mixtures were calculated. The results obtained were compared with the data for the systems containing N-methylformamide and N,N-dimethylformamide.     

Enthalpy characteristics of L-asparagine interaction with glycerol, ethylene glycol, and 1,2-propylene glycol in aqueous solutions

Integral dissolution enthalpies ??_sol H ^m of L-asparagine in the mixtures of water with glycerol, ethylene glycol, and 1,2-propylene glycol at the concentration of organic solvents up to 0.24 mole fraction were measured by the calorimetry method. The standard enthalpies of dissolution (??_sol H ⁰) and transport (??_tr H ⁰) of amino acids from water to the mixed solvents were derived. The calculated enthalpy coefficients of the molecule pair interactions of L-asparagine-cosolvent are positive, except for the amino acid-glycerol-water system. The data obtained are interpreted in terms of prevalence of different types of interactions in solution and effect of the cosolvent nature on the thermochemical characteristics of the L-asparagine dissolution.     

Thermochemical characteristics of the interaction of L-cysteine with glycerol,ethylene glycol,and 1,2-propylene glycol in aqueous solutions

Integral enthalpies of dissolution Δ_sol H ^m of L-cysteine in mixtures of water with glycerol, ethylene glycol, and 1,2-propylene glycol at concentrations of organic solvents of up to 0.37 mole fraction were measured by calorimetry of dissolution. The standard values of the dissolution enthalpies (Δ_sol H°) and the transfer enthalpies (Δ_tr H°) of amino acid from water to the mixed solvent were calculated. It was shown that the calculated enthalpic coefficients of the pair interactions of L-cysteine with cosolvent molecules have positive values. The obtained data are interpreted from the viewpoint of the prevalence of different types of interactions in the solutions and influence of the cosolvents nature on the thermochemical characteristics of amino acid dissolution.     

Enthalpies of solvation of ethylene oxide oligomers CH3O(CH2CH2O)nCH3 (n = 1 to 4) in different H-bonding solvents: Methanol, chloroform, and water. Group contribution method as applied to the polar oligomers

The enthalpies of solution and solvation of ethylene oxide oligomers CH₃O(CH₂CH₂O)_nCH₃ (n = 1 to 4) in methanol and chloroform have been determined from calorimetric measurements at T = 298.15 K. The enthalpic coefficients of pairwise solute–solute interaction for methanol solutions have been calculated. The enthalpic characteristics of the oligomers in methanol, chloroform, water and tetrachloromethane have been compared. The hydrogen bonding of the oligomers with chloroform and water molecules is exhibited in the values of solvation enthalpy and coefficient of solute–solute interaction. This effect is not observed for methanol solvent. The thermochemical data evidence an existence of multi-centred hydrogen bonds in associates of polyethers with the solvent molecules. Enthalpies of hydrogen bonding of the oligomers with chloroform and water have been estimated. The additivity scheme has been developed to describe the enthalpies of solvation of ethylene oxide oligomers, unbranched monoethers and n-alkanes in chloroform, methanol, water, and tetrachloromethane. The correction parameters for contribution of repeated polar groups and correction term for methoxy-compounds have been introduced. The obtained group contributions permit to describe the enthalpies of solvation of unbranched monoethers and ethylene oxide oligomers in the solvents with standard deviation up to 0.6 kJ · mol⁻¹. The values of group contributions and corrections are strongly influenced by solvent properties.     

Thermal properties of tetraalkylammonium bromides in several solvents

The enthalpies of solution of tetraethyl- and tetra-n-hexylammonium bromides have been measured in mixtures of formamide with ethylene glycol at 298.15 and 313.15 K in the whole mole fraction range by the calorimetric method. The standard enthalpies of solution in binary mixtures have been calculated with Redlich–Rosenfeld–Meyer type equation. The enthalpy and heat capacity parameters of pair interaction of organic electrolytes with EG in FA and with FA in EG have been computed and discussed. The enthalpy interaction parameters of single ions with EG in FA medium have been evaluated and compared with those for ion–water and ion–MeOH interaction in FA. The standard heat capacities of solution have been evaluated. The excess enthalpies of solution, Δ_sol H ^E, of Et₄NBr, Bu₄NBr, and Hex₄NBr have been determined. The Δ_sol H ^E values are positive for Et₄NBr and negative for Bu₄NBr and Hex₄NBr and become more negative from Bu₄NBr to Hex₄NBr.     

Thermochemical investigation of interaction of L-serin with glycerol,ethylene glycol,and 1,2-propylene glycol in aqueous solutions

By the method of dissolution calorimetry integral enthalpies of dissolution Δ_sol H ^m of L-serine are measured in the mixtures of water with glycerol, ethylene glycol, and 1,2-propylene glycol at the concentration of the organic solvent up to 0.42 mole fraction. The standard values of enthalpies of dissolution (Δ_sol H ⁰) and transfer (Δ_tr H ⁰) of amino acids from water to mixed solvents are calculated. The calculated values of the enthalpy coefficients of pair interactions of L-serine with the molecules of co-solvents are positive. The data obtained are interpreted in terms of prevalence of different types of interactions in solutions and the influence of nature of co-solvents on the thermochemical characteristics of the dissolved amino acids.     

Solvation of small hydrophobic molecules in formamide: A calorimetric study

The solubility and enthalpy of solution of benzene, cyclohexane, hexane, and heptane in formanide have been determined from titration microcalorimetric experiments at 25°C. The solution enthalpies are significantly more endothermic than in water but still the solubility is much higher. The entropy changes in formamide are small and positive and do not vary significantly with size. The enthalpies of solution of some 1-alkanols, 1-chloro- and 1,5-dichloropentane and pentane-1,5-diol were measured as functions of concentration. The solution enthalpies for 1-alkanols from methanol to decanol increase linearly with chain length. The enthalpic interaction coefficients h_xx are small and negative in formamide while they are large and positive in water. The partial molar heat capacities C _p,2 ^o for 1-propanol, 1-pentanol, benzene and cyclohexane in formamide were determined at 25°C from drop heat capacity measurements. Values of C _p,2 ^o are only slightly larger than the molar heat capacities of the liquid solutes.     

Thermodynamics of solvation and solvophobic effect in formamide

Using semi-adiabatic calorimetry, we measured the enthalpies of solution for various low-polar compounds including alkanes, aromatic hydrocarbons and their halogenated derivatives in formamide at temperature of 298 K. For the same compounds, the values of limiting activity coefficients in formamide were determined using GC headspace analysis at 298 K, and Gibbs free energies of solution and solvation were calculated. Based on these data and the available literature values of the Gibbs free energy of solvation in formamide for a number of other low-polar solutes, a study of the solvophobic effect in this solvent is performed, and its resemblance to the hydrophobic effect in aqueous solutions is demonstrated. It is shown that the contribution of the solvophobic effect into the solvation Gibbs free energy in formamide is much higher than that in aliphatic alcohols, but lower than that in water. Like in water, the magnitude of this contribution for different solutes linearly increases with the solute molecular volume. Solvophobic effect also significantly affects the enthalpies of dissolution in formamide, causing them to be more negative in the case of alkanes and more positive in the case of arenes.     

Thermodynamics of Mixing and Solvation of Ibuprofen and Naproxen in Propylene Glycol + Water Cosolvent Mixtures

By using the van’t Hoff and Gibbs equations, solubility data of ibuprofen (IBP) and naproxen (NAP) determined at several temperatures in propylene glycol (PG) + water (W) cosolvent mixtures were used to evaluate the Gibbs energies, enthalpies, and entropies of solution, mixing and solvation. The solubilities are greater in pure PG and lower in W in both cases at all studied temperatures. These results clearly show that a cosolvent effect is present in these systems. The solvation of these drugs was greater in W compared to PG, and it was lower in the cosolvent mixtures compared with the pure solvents. By means of an enthalpy-entropy compensation analysis, non-linear ΔH _soln^o versus ΔG _soln^o compensation plots were obtained, with negative and positive slopes if all of the composition intervals are considered. Accordingly, it follows that at low PG solvent fractions the dominant factor for the solubility of IBP and NAP in this cosolvent system is the entropy, probably due to loosening of the water structure caused by the addition of the cosolvent.     

Physico-chemical studies in non-aqueous solvents: Part XII. Thermochemical studies of some 1:1 electrolytes in N,N-dimethylacetamide

Heats of solution of alkali metal, silver, ammonium and substituted ammonium perchlorates, lithium chloride and potassium thiocyanate have been measured in the concentration range 1.26–112.8×10^?3 mol l^?1 in N,N-dimethylacetamide (DMA) at 26.9°C. Slopes of the linear plots of heat of solution vs. square root of concentration are two to three times higher than the Debye-Hückel limiting slope indicating thereby complete dissociation of the electrolytes. Standard heats of solution of the electrolytes in DMA are higher than those in formamide and N-methylformamide and are comparable to those in N,N-dimethylformamide. Buckingham's model has been used to determine the solvation enthalpies of various ions in this solvent. The results indicate that ion solvation enthalpies decrease with increase in size of the ions and that the anion solvation enthalpies are higher than cation solvation enthalpies which is consistent with the findings in various solvents.     

Enthalpy characteristics of dilute solutions of amides in acetonitrile

The enthalpy of mixing of formamide,N-methylformamide,N,N-dimethylformamide, and hexamethylphosphoric triamide with MeCN was measured in the 283–328 K range. The enthalpic coefficients of the binary and ternary interactions between the amide molecules are calculated within the framework of the McMillan-Mayer theory. The contributions to the enthalpy of dissolution due to cavity formation in the solvent (Δ_cav H ⁰) and due to solute-solvent interaction (Δ_int H ⁰) were determined. The enthalpies of specific and nonspecific solvation of amides in MeCN were calculated. The main contribution to the enthalpy of solvation of formamide andN-methylformamide is from specific interactions, while forN,N-dimethylformamide and hexamethylphosphoric triamide it is from nonspecific interactions. The values obtained are compared with those for solutions of the amides mentioned in water and methanol. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1730–1735, October, 1993.     

Thermochemical study of the influence of tetraalkylammonium cation sizes on the interaction of their salts with formamide and carbamide in aqueous solutions

Solutions of tetrahexylammonium salts in mixtures of water with additives of hydrophilic substances formamide and carbamide were studied by the thermochemical method at 298.15 K. The enthalpies of solution of tetrahexylammonium perchlorate were measured by the sedimentation method, and the enthalpies of solution of tetrahexylammonium bromide were calculated. The enthalpic coefficients of pair interaction electrolyte—amide in an aqueous solution were determined. The data obtained were compared with those for te tramethyl- tetraethyl- and tetrabutylammonium salts to study the influence of the tetraalkylammonium cation size on its solvation and interaction with amides in an aqueous solution.     

Thermochemistry of Bu<Subscript>4</Subscript>NBr solutions in binary solvents containing formamide

The heats of solution of tetrabutylammonium bromide have been measured in mixtures of formamide (FA) with methanol (MeOH) and ethylene glycol (EG) at 313.15 K by calorimetric method. The standard enthalpies of solution in binary mixtures have been extrapolated to infinite dilution by Redlich–Rosenfeld–Meyer type equation using the literary data at 298.15 K and the present paper data at 313.15 K. The Debye–Hückel limiting law slope A _H required for calculation of the ∆_sol H ⁰ value has been obtained with application the new additive scheme of determination of the physic-chemical characteristics of binaries. The scheme is tested on the example of Bu₄NBr solutions in FA–MeOH mixture at 298.15 K. Its application yields the ∆_sol H ⁰ value very closed on the ones determined with the real (non-additive) characteristics of binaries. The standard enthalpies of solution extrapolated by Redlich–Rosenfeld–Meyer type equation are in a good agreement with the ones computed in terms of the Debye–Hückel theory in the second approximation. The heat capacities characteristics of Bu₄NBr have been calculated in H₂O–FA, MeOH–FA and EG–FA mixtures using the literary and present data. The sequence of solvents H₂O > FA > EG > MeOH located on their ability to solvophobic solvation found by us earlier for enthalpic characteristics is confirmed by the ∆C _p ⁰ values. The comparison of thermochemical characteristics of Bu₄NBr solutions in aqueous and non-aqueous mixtures containing FA has been carried out. The own structure of water remains in the region of small additions of formamide to co-solvents. It considerably differs the H₂O–FA mixture from the investigated non-aqueous systems.