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.     

Solvation of Tetraethyl- and Tetrabutylammonium Bromides in Aqueous Acetone and Aqueous Hexamethyl Phosphoric Triamide Mixtures in the Water-Rich Region

The enthalpies of solutions of tetraethylammonium and tetrabutylammonium bromides in the water-rich region of the water–acetone and water–hexamethyl phosphoric triamide mixed solvents have been measured at 25°C using a precise calorimetry system. The enthalpies of electrolyte solutions at infinite dilution were calculated using the Debye–Hückel theory. The enthalpies of solute transfer from water to the mixtures with acetone and hexamethyl phosphoric triamide were calculated. The enthalpy coefficients of solute–pair interactions with hexamethyl phosphoric triamide and acetone in aqueous solution were obtained using the McMillan–Mayer formalism. The values obtained were compared with those for other organic cosolvents. It was found that in aqueous solution the solutes show a strong tendency for hydrophobic interaction with cosolvent molecules, particularly in the water–hexamethyl phosphoric triamide system.     

Thermodynamic properties of aqueous solutions of tetraalkylammonium salts: Dependence of the hydrophobic hydration effect on the cation size

The heats of solution of tetrahexylammonium and tetraheptylammonium bromides in water were measured at 318.15 and 328.15 K. The standard enthalpies and specific heats of solution and the temperature changes in the free energy and entropy of solution were calculated. A comparison of the thermodynamic properties of solutions for the homologous series of tetraalkylammonium salts demonstrated that the enthalpic, entropic, and specific heat characteristics of solution are positive and increase almost linearly up to tetrapentylammonium bromide. On passing to larger homologues, these parameters decrease, suggesting that the hydrophobic hydration effect is substantially weaker in solutions of tetraalkylammonium salts with larger cations. Original Russian Text ? A.V. Kustov, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 2, pp. 368–373.     

Enthalpies of solution and of crystallization of lithium nitrate and of lithium nitrate trihydrate in water at 25°C

The enthalpies of crystallization of LiNO₃ and LiNO₃–3H₂O from aqueous solutions at 25°C, measured by a calorimetric method and determined from the previously published data on the concentration dependence of the enthalpy of solution, are reported. The results are compared with the values obtained from the concentration dependences of the activity coefficients and from the temperature dependences of the solubilities. The enthalpy of solution at infinite dilution and the enthalpy of hydration are given.     

Partial molar enthalpies of solution as indicators of interactions in mixtures of 2-butoxyethanol and 2-butanol with water

Calorimetric measurements have been made of differential enthalpies of solution of both components in the binary system 2-butoxyethanol-water and of 2-butanol in the system 2-butanol-water as a function of composition at three different temperatures. The heat capacity changes for dissolution were calculated from the temperature variation of the solution enthalpies. Drastic changes of the solution properties are seen with increasing solute concentration in water-rich solutions. In the 2-butoxyethanol-water system, which could be studied over the whole composition range, four different regions can be identified. At extreme dilution in water, the solute is fully hydrated with a primary hydration layer of monolayer thickness involved in long-range secondary hydration. In dilute solutions the primary hydration layer is unchanged but the secondary hydration diminishes with increasing solute concentration. In semi-dilute solution the primary hydration layer breaks down and the particular hydrophobic characteristics of hydrocarbon groups in aqueous solution disappear. At higher solute content the mixtures show no hydrophobic character but the behavior of regular mixtures of polar solutes.     

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.     

Effect of Temperature on the Solubility of Short-Chain Carboxylic Acids in Water

The aqueous solubilities of pentanoic and hexanoic acids were measured between 298.15 and 333.15 K at intervals of 5.00 K. Nonlinear dependences were found for the variation of solubility with temperature. The temperature dependences of the solubility of the acids in water are described by nonlinear van’t Hoff plots. Experimental data were fitted by the method of least-squares to a second-order polynomial equation, which was then used to determine the differential solution enthalpies at selected temperatures. The values for the apparent Gibbs energy and the apparent entropy of solution are calculated.     

Hydrophobic hydration and hydrophobic interaction in tetraalkylammonium salt solutions: The cation size effect

The heats of solution of tetrahexylammonium and tetraheptylammonium bromides in water and aqueous solutions of formamide and hexamethylphosphoric triamide have been measured at 318 and 338 K. The standard enthalpies and heat capacities of solution and the enthalpic and heart capacity parameters of amide-electrolyte pair interaction have been determined. It has been confirmed that the standard heat capacities of solution and the amide-electrolyte interaction parameters as a function of the cation size pass through an extremum. The hydrophobic hydration effect is weaker for large tetraalkylammonium cations.     

Heat capacities of crystalline tetraalkylammonium salts

The behavior of crystalline tetraalkylammonium salts at 290–350 K was studied by differential scanning calorimetry. For tetraethyl- and tetrabutylammonium bromides (Et₄NBr and Bu₄NBr), the experimental heat capacities agreed well with the literature values. For tetrahexyl-, tetraheptyl-, and tetraoctylam-monium bromides (Hex₄NBr, Hep₄NBr, and Oct₄NBr), phase transitions were found between crystal modifications whose characteristic temperatures depended significantly on the size of the cation. Empirical equations for the temperature dependences of the heat capacities of the salts within the ranges of homogeneous equilibrium phases were derived.     

Ein isoperiboles kalorimeter zur bestimmung der lösungsenthalpien von salzen

An isoperibolic calorimeter for determining the enthalpies of solution of salts was constructed. The enthalpies of solution of lanthanum (III) ions in water were determined. The calculated enthalpies of hydration reveal an effect very similar to the tetrad effect observed in extraction equilibria of rate earth(III) compounds     

Vapor pressure and activity of components of melts formed by manganese chloride and chlorides of alkaline-earth metals

The boiling points of melts were measured at different pressures over the melts for certain concentrations of components of the liquid phase in binary systems constituted by manganese chloride and chlorides of alkaline-earth metals. The temperature dependences of the saturated vapor pressure, normal boiling points, and enthalpies of evaporation were calculated. The concentration dependences of the thermodynamic activities of the components of four binary systems were analyzed.     

Concentration Dependence of the Activation Energy of Conductivity in Aqueous Sodium Selenite and Potassium Tellurite

The temperature and concentration dependences of conductivity have been studied for aqueous solutions of sodium selenite and potassium tellurite. The corresponding empirical equations have been derived. The activation energies of conductivity in both electrolytes are calculated for different solution concentrations. The ascertained dependence is interpreted in the light of Samoilov's theory on positive and negative hydration of ions. Analysis of the results suggests that the larger the positive hydration of ions, the higher the activation energies of conductivity for the salts.     

Correlations between the Basicity and Proton Affinity of Substituted Anilines

The enthalpies and free energies of gas-phase proton affinity (PA) of aniline and 62 derivatives with different electron donor and electron acceptor substituents in the aromatic ring and at the nitrogen atom were calculated by the PM3 technique. Linear correlations between the shapes of pK_a and PA have been found. The deviation of data from the above dependences for ortho-substituted anilines are discussed in the light of possible hydrophobic hydration of molecular fragments near the protonation reaction center.     

Calculation of the Thermodynamic Properties of the Copper-Zinc System from Measured Boiling Point Values

The temperature dependences of the saturated vapor pressure, the enthalpies of vaporization, standard boiling points, enthalpies, entropies, Gibbs energies, and component activities were calculated for liquid alloys of the copper-zinc system from the measured boiling points at various pressures. The calculation results were compared with the previous data obtained for this system by other methods.     

Evaluation of the contribution to hydration of nonelectrolytes from the hydrophobic effect

A new method was suggested for estimating the hydrophobic effect of contributions to the Gibbs energies and enthalpies of hydration of hydrocarbons, inorganic gases and rare gases. In accordance with this method the hydrophobic effect contribution to the Gibbs energy was evaluated from the difference between the hydration Gibbs energy of a solute and the non hydrophobic contribution. To estimate the latter value, the known dependence connecting the Gibbs energies of solvation of a solute in a number of aprotic solvents to the Hildebrand solubility parameter for these solvents was used. The non hydrophobic contribution to the Gibbs energy of hydration was calculated for various solutes from such dependences extended to water as solvent. The Hildebrand solubility parameter for water used in the calculation was corrected for the effect of association through hydrogen bonding. This correction was made by subtraction of the water self-association enthalpy from the enthalpy of vaporization of water. The evaluated Gibbs energies of the hydrophobic effect are positive for saturated hydrocarbons, inorganic gases and rare gases and linearly depend on the solute molecular refraction. The hydrophobic contribution to the hydration enthalpies of the solutes was calculated in the same manner as was made to calculate the hydrophobic contribution to Gibbs energies of hydration. Enthalpies of the hydrophobic effect for the solutes under study are negative.     

Enthalpies of solution in water of urotropine as function of concentration and temperature

Enthalpies of solution of Hexamethylenetetramine in water at several molal concentrations and at four temperatures (278.15, 288.15, 298.15, and 308.15) K were determined. The values of enthalpy of solution at infinite dilution were combined with the enthalpy of sublimation to get the corresponding enthalpies of hydration. The change in heat capacity at infinite dilution was calculated.     

Temperature and Concentration Dependence of the Electrical Conductance,Diffusion, and Kinetics Parameters of the Ions in Aqueous Solutions of Sulfuric Acid,Selenic Acid,and Potassium Tellurate

The temperature and concentration dependences of the electrical conductance of aqueous solutions of sulfuric acid, selenic acid, and potassium tellurate were studied. The coefficients of the corresponding empirical equations were determined, and the values of equivalent conductances of the anions were evaluated at infinite dilution at the experimental temperatures. The values of the coefficients in the Fuoss and Onsager equation were evaluated for the three electrolytes at 298 K. The values of the molecular and ionic coefficients of self-diffusion at infinite dilution were calculated in the temperature range 288–318 K. The change of the translational energy Δ E∘_tr. of water molecules in the ionic hydration sphere was determined. The number of water molecules participating in the ionic hydration sphere at 298 K and the changes of Gibbs free energy, enthalpy, and entropy of activation of ionic conductance were calculated. The results obtained were interpreted according to the Samoylov’s theory of positive and negative hydration of ions. The differences observed in the temperature dependences of the mentioned parameters were explained in terms of the different radii and hydration numbers of the ions.     

Enthalpies of Transfer of Tetraalkylammonium Bromides and CsBr from water to Aqueous DMF at 298.15 K

Enthalpies of transfer of tetraalkylammonium bromides and CsBr from water to aqueous DMF mixtures are reported and analyzed in terms of a new solvation theory. It was found that a previous equation could not reproduce these data over the whole range of solvent compositions. Using a new solvation theory to model the enthalpies of transfer shows excellent agreement between experimental and calculated values over the entire range of solvent compositions. The analyses show that tetrapropylammonium bromide, Pr₄NBr, and tetrapentylammonium bromide, Pen₄NBr, are preferentially solvated by water; in contrast tetrabutylammonium bromide, Bu₄NBr, is preferentially solvated by DMF. The solvation of tetramethylammonium bromide, Me₄NBr, and cesium bromide, CsBr, is random. The extent to which the tetraalkylammonium bromides disrupt solvent–solvent bonds increases systematically in going from Me₄NBr to Pen₄NBr.     

The thermodynamic characteristics of sublimation of fenamate molecular crystals

The temperature dependences of vapor pressure were determined by the transpiration method and the thermodynamic functions of sublimation were calculated for six molecular crystals from the group of nonsteroidal antiinflammatory drugs, for niflumic, flufenamic, tolfenamic, mefenamic, and N-phenylanthranylic acids and diphenylamine. The influence of substituents on the enthalpies of sublimation of compounds of this class was studied. A correlation was observed between the enthalpies of sublimation under standard conditions and the temperatures of fusion.     

The diffusion and solubility of Irganox 1098 in polyamide 6

Diffusion coefficients (D) and solubilities (S) of a phenolic antioxidant (Irganox^® 1098) in polyamide 6 were determined in the temperature range of 139-180 °C. D and S show Arrhenius and van’t Hoff dependences on temperature respectively, with discontinuities at the melting temperature of the antioxidant. The activation energies for diffusion and enthalpies of solution are lower above than below the melting temperature. Based on the determined parameters for diffusion and solubility, the blooming of Irganox^® 1098 as a function of temperature and time was calculated.