Heat Capacities Of Dissolution Research Articles

Unusual behavior of temperature-dependent solvent H/D isotope effects in the enthalpy and heat capacity of hexamethylenetetramine (urotropine) hydration

Based on the identified inconsistencies in the existing literature data on the enthalpies and heat capacities of dissolution (solvation) of hexamethylenetetramine (HMTA) in ordinary (H2O) and heavy (D2O) water, we carried out carefully the calorimetric study of these two binary systems at seven temperatures from (278.15 to 318.15) K. It is established that the considered D2O–H2O enthalpy-isotopic effect exhibits a pronounced exothermic maximum between two sign-inversion points near 293 K and 318 K. The same goes the negative (up to ~305 K) solvent isotope effect (IE) in the partial molar heat capacity of HMTA in water. Such an unusual temperature-dependent behavior of the thermodynamic IEs points out the dualistic character of the HMTA hydration process which is dependent on the initial structure state of the solvent.

Water-DMF-L-α-alanine and water-carbamide-L-α-alanine systems. Thermodynamic properties, solvation, and interspecies interactions at 273–333 K

Thermal effects of dissolution of L-α-alanine in water-N,N-dimethylformamide mixtures (0–0.1 molar parts of DMF) at 283–318 K were measured calorimetrically. Standard enthalpies and heat capacities of dissolution of the amino acid and also thermal variations in entropies and free energies were calculated. The comparison with previously obtained data on the thermodynamics of dissolution of L-α-alamine in the carbamide water solution was performed. Temperature variations in the reduced Gibbs energy in the course of dissolution of L-α-alanine in water-DMF and water-carbamide mixtures have negative values originating from the prevalence of the entropy component. At the increase in temperature both mixtures become less structured. The interaction of L-α-alanine with hydrophilic and hydrophobic amides differs fundamentally: In the first case it is enthalpy-attractive and does not depend on temperature, while in the second case it is repulsive and decreases with the increase in temperature.

Enthalpies and heat capacities of dissolution for calcium chloride and sodium oxalate

The heats of solution at 25 and 40°C in water are measured for various concentrations of calcium chloride and sodium oxalate. The standard enthalpies of solution are determined using the Debye-Huckel second approximation. The heat capacity change upon dissolution and the partial molal heat capacities of electrolytes at infinite dilution are calculated.

Solution Properties of Sodium Perfluoroalkanoates. Heats of Solution, Viscosity B Coefficients, and Surface Tensions

Abstract The heats of solution, the viscosity B coefficients, and the surface activities of sodium perfluoroalkanoates, F(CF2)nCOONa, n=1–4, 6, 7), in water were measured. The changes in the heat capacity for dissolution in water, ΔCp∞, calculated from the heats of solution at 15, 25, and 35 °C, were positive for the salts, and the ΔCp∞ values increased with an increase in n. The increment in the ΔCp∞ values per CF2 group was 93 J K−1 mol−1. The viscosity B coefficients increased with the chain length at a definite temperature, and the increment in the viscosity B coefficients per CF2 group was 0.119 dm3 mol−1 at 25 °C. Further, the viscosity B coefficients for all the salts decreased with the increase in the temperature between 15 and 35 °C. Traube factors, as obtained from the surface tensions, were found to be 3.0 for the homologs from C2 salt to C7 salt. The results are discussed in comparison with those of sodium alkanoates, H(CH2)nCOONa.

Solution Properties of Straight-Chain α-Amino Acids

The viscosity B coefficients, heats of solution and surface activity of straigt chain α-amino acids H (CH2) n-1CH (NH2) COOH (Cn, n=15) in water were measured. The viscosity B coefficient of C1, increased with temperature, while those of C2 and higher homologs decreased with increase in temperature. Changes in heat capacity for dissolution in water, ΔC0p, as calculated from heats of solution at 15, 25, and 35°C, were negative for C1, white ΔC0p values for C2 and higher homologs were positive. C1, and C2 raised the surface tension of water, while C3 and higher homologs reduced it. Based on these results, structural changes in water as solvent with dissolution of α-amino acids and the effects of hydrophobicity of α-amino acids on surface active properties are discussed.

Solution Properties of Dialkyl Phosphate Salts. Apparent Molar Volumes, Viscosity B Coefficients, Heats of Solution, and Osmotic Coefficients

Abstract The apparent molar volumes, viscosity B coefficients, and heats of solution of sodium salts of dialkyl phosphates (methyl, ethyl, and butyl) in water were measured. The limiting apparent molar volumes at 25 °C increased with the number of carbon atoms in the alkyl chains. The viscosity B coefficients of the salts decreased as the temperature was raised from 25 to 35 °C. The changes in heat capacity for dissolution in water, as calculated from the heats of solution at 15, 25, and 35 °C, were positive for the salts. The osmotic coefficients for lithium, sodium, and potassium salts of dimethyl phosphate were also measured. For a given concentration, the osmotic coefficients decreased in the order: Li+<Na+<K+. From the results obtained, the –(–O)2PO2− ionic part may be classified as a structure-maker.

Heats of Solution of Bolaform Electrolytes in Water. II. Polymethylenebis(trimethylammonium) Dibromides

Abstract The heats of solution of polymethylenebis(trimethylammonium) dibromides [(CH3)3N(CH2)nN-(CH3)3]Br2 (n=3–6, 8, 10, and 12) and alkyltrimethylammonium bromides CnH2n+1N(CH3)3Br (n=2–6) in water were measured at 15, 25, and 35 °C. The hydrophobic character of the polymethylene groups was compared with that of alkyl groups on the basis of the changes in the heat capacity for dissolution, which were obtained from the relation between the heats of solution and the temperature.

Enthalpies and heat capacities of dissolution, apparent molar heat capacities, and apparent molar volumes of some mono-, di-, tri-, and tetra-saccharides in water

Molar enthalpies of solution at very low molalities of some mono-, di, tri, and tetra-saccharides in water have been measured at 298.15 and 308.15 K using an isoperibol calorimeter. The molar heat-capacity changes Δ sol C p , m ∞ on dissolution at 303.15 K were derived from molar enthalpies of solution. A Picker flow microcalorimeter and an Anton Paar digital density meter were used to measure apparent molar heat capacities C p, 2, φ and apparent molar volumes V 2, φ of solute at 298.15 K. The various compounds studied show large positive values of Δ sol C p, m ∞ and C p, 2 ∞. The experimental C p, 2 ∞ values are found to be much higher than values estimated using additivity relations and heat capacities for the constituent groups derived from the heat capacities of monofunctional compounds. Results further indicate that the difference in C p, 2 ∞(expt) and C p, 2 ∞(est.) per hydroxyl group increases with an increase in the number of hydroxyl groups in a molecule. An attempt has been made to explain the C p, 2 ∞ values of stereoisomers in terms of the compatibility of the aqueous-solution conformation of the compound and the tetrahedral order in water.

Heats of Solutions of Bolaform Electrolytes in Water

Abstract The heats of solutions of polymethylene-diammonium dichlorides [H3N(CH2)nNH3]Cl2 (n=2–8, 10, and 12) and disodium polymethylene disulfates Na2[O3SO(CH2)nOSO3] (n=2–6, 8, and 10) in water were measured at 15, 25, and 35 °C. The changes in the heat capacity for dissolution, ΔCpo, were obtained from the relation between the heats of soultion and the temperature, and the results were compared with the ΔCpo values for alkylammonium chlorides and sodium alkyl sulfates previously reported.

The Physicochemical Properties of Aqueous Solutions of Sodium Alkanesulfonates. Apparent Molar Volumes, Viscosity B Coefficients, Heats of Solution, and Surface Tensions

Abstract The apparent molar volumes, viscosity B coefficients, heats of solution, and surface activity of sodium alkanesulfonates (RSO3Na, methyl to hexyl) in water were measured. The limiting apparent molar volumes at 25 °C increase with the number of carbon atoms in the alkyl chain; the increment per methylene group is about 16 cm3mol−1. The viscosity B coefficients of MeSO3Na and EtSO3Na increase with the temperature, while those of n-BuSO3Na decrease with an increase in the temperature. The changes in heat capacity for dissolution in water, ΔCpo, as calculated from the heats of solution at 15, 25, and 35 °C, were negative for MeSO3Na and EtSO3Na, while the ΔCpo values for n-PrSO3Na and higher homologs were positive. MeSO3Na raises the surface tension of water, while EtSO3Na and higher homologs reduce it. The structural changes in the solvent water upon the dissolution of sodium alkanesulfonates are discussed in comparison with those of sodium alkyl sulfates previously reported.

Enthalpies and heat capacities of dissolution of some sodium carboxylates in water and hydrophobic hydration

Integral heats of solution ΔHs of sodium carboxylates, CnH2n+1COONa (n=0, 1, 2, 3, 4, 5, and 7) and C6H5(CH2)nCOONa (n=0, 1, 2, and 3), in water at 25 and 35°C have been determined at very low concentrations. The heat capacities of dissolution at infinite dilution, ‡Cpo, of sodium carboxylates have been derived by the integral heat method. The-CH2-increment of ‡Cpo in aliphatic carboxylates has been found to be 14 cal-deg−1-mole−1, which is close to the value derived from other series of compounds, indicating that the interaction of nonpolar moieties with water is independent of the hydrophilic group attached to it. On the other hand, the-CH2-increment for the aromatic sodium carboxylates is much less (about 6 cal-deg−1-mole−1) than for the aliphatic sodium carboxylates, indicating that the hydrophobic interaction is affected by the aromatic end group.