Enthalpies Of Dilution Research Articles

Excess enthalpies and apparent molar volumes of aqueous solutions of crown ethers and cryptand(222) at 25�C

The enthalpies of dilution and densities of aqueous solutions of 12-crown-4, 15-crown-5, 18-crown-6, 1,10-diaza-18-crown-6 and cryptand (222) were measured at 25°C. The excess enthalpies and enthalpic coefficients of solute-solute interactions were calculated by the McMillan-Mayer theory formalism. Values for the apparent molar volumes at infinite dilution were determined by extrapolation. The contributions of the-CH2CH2O-group to values of h2 and to the limiting partial molar volume were calculated for the series of crown ethers studied. It is concluded that the hydrophobic hydration and the hydrophobic solute-solute interaction are predominant in the solutions investigated.

Thermodynamics of ionization of aqueous sulfamic acid, an almost-strong electrolyte

We have measured enthalpies of dilution of aqueous sulfamic acid solutions at 25°C and used results of these measurements to calculate the standard enthalpy of ionization of sulfamic acid. The average ΔH1on0 obtained in this work was 735±200 J-mol−1. We have also measured enthalpies of solution of crystalline sulfamic acid in water at 25°C. The results from both measurements are combined with some earlier results from Wu and Hepler to obtain a ‘best value’ for the standard enthalpy of solution, ΔHS0 =19.2±0.2 kJ-mol−1.

N,N,N-Alkyloctyldimethylammonium Chlorides in Water: A Thermodynamic Investigation

Specific conductivities, densities, heat capacities and enthalpies of dilution at 298 K and osmotic coefficients at 310 K were measured for N,N,N -octylbutyldimethylammonium, N,N,N -octylpentyldimethylammonium, and N,N -dioctyldimethylammonium chlorides in water as functions of concentration. From the specific conductivity data, the CMC and the degree of the counterion dissociation have been calculated. It is shown that additional CH 2 groups in the variable alkyl chain affect both CMC and β less than they do in the single chain surfactants. The partial molar volumes, heat capacities, relative enthalpies, nonideal free energies, and entropies at 298 K were derived as functions of the surfactant concentration. By increasing the tail of the variable alkyl chain, the apparent molar volume and heat capacity vs concentration curves are shifted regularly towards larger and smaller values, respectively. As well, in the micellar region, by increasing the alkyl chain length while the nonideal entropy vs log m/CMC curves are shifted regularly towards more positive values, those for free energy and enthalpy are not. The standard partial molar volumes and heat capacities linearly increase with the alkyl chain length slopes being slightly larger and smaller, respectively, than those for single chain surfactants. The partial molar volumes and heat capacities in the micellized form are also linearly related to the number of methylene groups with CH 2 contributions which are close and larger, respectively, than those for single chain surfactants. However, the heat capacity for the longer homologues deviates from the linearity. Since this peculiarity has been observed for most of the investigated properties, it has been ascribed to a different location of this chain in the micellar structure with respect to the shorter chains. In addition, thermodynamic properties of N,N,N-octylbutyldimethylammonium chloride are compared with those of N,N-octylmethylpiperidinium chloride which can be considered to be derived from the fusion of a methyl and the butyl groups in the N,N,N-octylbutyldimethylammonium chloride.

Effect of the Carboxylic Group on Thermodynamic Properties of Aqueous Micellar Solutions of Amphiphilic Betaine Ester Derivatives

The results of thermodynamic study at 313 K in aqueous solutions of some amphiphilic betaine chloride ester derivatives (C n-BEC, n = 10, 12, 14), showing biological activity, are here reported for the first time. Dilution enthalpies were measured by means of a flow type microcalorimeter, the density (at 298 and 313 K) was measured by means of a vibrating tube densimeter, and the osmotic coefficients by means of a vapor pressure osmometer. Apparent and partial molar quantities were obtained from the experimental data and expressed as a function of molalities, assuming the infinite dilution as standard state. On the basis of pseudo-phase separation model, the values of enthalpies and volumes of micellization were graphically evaluated. Using the calculated enthalpies of micellization at 313 K and previously reported values for 298 K, the heat capacities of micellization were also obtained. The general thermodynamic trend as a function of chain length is comparable with that of other homologous series of cationic surfactants, in spite of the presence of the carboxyl group. For this class of surfactants, too, the additivity rule seems to hold: for all the properties studied, the -CH 2- group contributions are comparable with those reported in the literature for other classes of cationic surfactants, despite the presence of the carboxylic group between the charged head and the hydrophobic tail. In micellar solutions, the -COO- group contribution to volume is 22.4 cm 3 mol -1 instead of 16.3 cm 3 mol -1 for each -CH 2-. The comparison of the trends of thermodynamic quantities of C 10-BEC with those of dodecyltrimethylammonium chloride (DTAC) supports the hypothesis that two methylene groups of DTAC and -CH 2-COO- group of betaine ester derivatives are not incorporated into the micelle core.

Dilution enthalpies of alkanols in concentrated aqueous solutions of urea at 25�C

Enthalpies of dilution of some aliphatic alcohols were determined at 25°C in aqueous 7M urea solutions by flow microcalorimetry. The excess enthalpies were expressed as power expansion series in molalities referred to 1 kg of constant composition urea-water mixture. This urea-water mixture was utilized throughout as a mixed solvent. The values of the second enthalpic virial coefficients were all found to be positive and generally lower than the corresponding values in water. Large differences were encountered, as in water, by comparing normal and branched isomeric propanols and butanols. For one system it was possible to measure the third coefficients, which were also positive. The second enthalpic coefficients were found to increase with the molecular weight of the alkanols. These facts suggest that in the presence of a large concentration of urea, the excess enthalpies are mainly determined by apolar interactions. This is surprising and potentially rich in consequences for a better understanding of the interactions among amino acid residues distantly situated in the primary sequences but topologically near in the loops of globular proteins. An analysis, carried out using the Savage-Wood additivity group method, shows that the enthalpic contributions (that appear to play a crucial role in water in making the polar interaction to be favorable) become essentially unfavorable in urea-water solvent. The hypothesis that the peptide-peptide interactions are prevented by the preferential solvation of urea is also discussed.

Aqueous solutions containing amino acids and peptides: Part 29. The enthalpies of dilution of some amino acids at 25°c

The enthalpies of dilution of aqueous solutions at 25° C have been determined for the amino acids β-alanine, α-aminobutyric acid, γ-aminobutyric acid, ϵ-aminocaproic acid, α-aminovaleric acid (norvaline) and threonine. The results have been treated using the excess function concept and homotactic interaction coefficients have been obtained. These are briefly discussed in terms of intermolecular interactions between the hydrated solute species.

Thermodynamic and19F NMR studies of antimony trifluoride in water

Densities, specific heat capacities per unit volume and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for antimony trifluoride in water as functions of concentration. From the first three properties the apparent and partial molar volumes, heat capacities and relative enthalpies were derived. As well, pH measurements in water at 25°C and19F NMR spectra in water and methanol at 33°C were also carried out. All the thermodynamic properties together with the chemical shifts abruptly change in the very dilute concentration region (<0.1m) and, then, tend to a constant value. These trends have been rationalized through a simple model based on an equilibrium of dissociation of SbF3 into two ionic species. From the simulation of all the data it is derived that two concomitant equilibria are present in solution: the hydrolysis process of SbF3 which explains the pH values and the ionic dissociation of SbF3 which accounts for the19F NMR data.

Properties of aqueous solutions of electrolytes

Foreword. EXPERIMENTAL VALUES OF PHYSICO-CHEMICAL PARAMETERS. Density. Viscosity. Specific Heat Capacity. Thermal Conductivity. Saturated Vapor Pressure. Electrolitic Conductivity. Diffusion Coefficients. Freezing Points. Boiling Point. Integral Dissolution Enthalpy. Dilution Enthalpy. Thermal Constants of Substances. Mean Ionic Activity Coefficients. METHODS FOR CALCULATION OF PHYSICO-CHEMICAL PARAMETERS. Mean Ionic Activity Coefficients. Water Activity. Density. Viscosity. Heat Capacity. Thermal Conductivity. Boiling Point. Freezing Point. Surface Tension. Diffusion Coefficients. Molar Electrical Conductivity. Integral Enthalpy of Dissolution. Differential Dissolution Enthalpy. Hydration Enthalpy. References.

Integral enthalpies of mixing for water + lithium bromide + lithium iodide and water + lithium chloride + lithium nitrate at 298.15 K

The analyses of absorption refrigerating machines, absorption heat pumps, and absorption heat transformers require thermal and physical properties for working medium + absorbent systems. The enthalpy of mixing data for working fluids are one of the most important basic properties for the research and the design of absorption refrigeration and heat pump systems. Differential enthalpies of dilution and differential enthalpies of solution for water + lithium bromide + lithium iodide (salt mole ratio 4:1) and water + lithium chloride + lithium nitrate (salt mole ratio 2.8:1) were measured with a twin isoperibol calorimeter at 298.15 K. Integral enthalpies of mixing for these two ternary systems at 298.15 K were measured as a function of mass fraction, and were exothermic in both ternary systems. The results were correlated by means of Redlich-Kister-type polynomials. The average absolute deviation between the experimental and the calculated values from these equations was 0.57 % for water + lithium bromide + lithium iodide and 0.52% for water + lithium chloride + lithium nitrate.

Thermodynamic properties of some N-alkyl-N-methylpiperidinium chlorides and N-alkylpiperidine hydrochlorides in water

Densities and heat capacities at 25[degrees]C were measured for N-octyl-, N-decyl- and N-dodecyl-N-methylpiperidinium chlorides and for N-octyl- and N-dodecylpiperidine hydrochlorides in water as functions of concentration. Enthalpies of dilution at 25[degrees]C and osmotic coefficients at 37[degrees]C of the N-methyl-N-alkylpiperidinium chlorides were also measured as functions of concentration. The partial molar volumes, heat capacities, relative enthalpies, nonideal Gibbs energies and entropies at 25[degrees]C were derived as functions of the surfactant concentration. By increasing the alkyl chain length of the surfactant, both the apparent molar volume vs. concentration curves are shifted toward greater values while the corresponding ones for the heat capacity are moved toward more negative values. These results are consistent with the higher hydrophobicity the longer the alkyl chain of the surfactant is. In the micellar region, the entropy and enthalpy vs. log m/m[sub cmc] curves increase in a parallel manner by decreasing the alkyl chain length of the surfactarnt. Consequently, the negligible effect of the hydrophobicity of the surfactant on the Gibbs energy vs. log m/m[sub cmc] trends is due to the enthalpy-entropy compensative effect. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. The absolute values of both the volume andmore » heat capacity of micellization increase with an increasing number of carbon atoms in the alkyl chain (n[sub c]). The enthalpy and entropy of micellization vs. n[sub c] are convex curves. Comparisons are also made between the present data and those of some alkylpyridinium chlorides reported elsewhere.« less

Thermodynamics of aqueous guanidinium hydrochloride solutions in the temperature range from 283.15 to 313.15 K

The densities, apparent molar volumes, apparent molar heat capacities, enthalpies of dilution, and activity coefficients of aqueous guanidinium hydrochloride solutions over a wide molality range and over the temperature range 298.15-313.15 K are reported

Role of functional-group position in determining the interaction between structure-making and structure-breaking solutes. A microcalorimetric study of the interactions in aqueous solutions between positional isomers of alkan-m-ols and urea or biuret

Enthalpies of dilution of ternary aqueous solutions containing biuret or urea and positional isomers of alkan-m-ols have been determined by flow microcalorimetry at 298.15 K. Pairwise cross-interaction coefficients of the virial expansion of the excess enthalpies were evaluated. The values of the coefficients are positive, depending in a very complex manner on the length of the alkyl chain and on the position occupied by the hydroxy group along the alkyl chain. The behaviour of these systems seems to be determined by a balance between favourable hydrophilic–hydrophilic and repulsive hydrophilic–hydrophobic interactions. When the hydroxy group occupies more central positions, unfavourable interactions prevail and the cross coefficient for the alkanol bearing the longest alkyl chain is almost zero.

Self-Association Data for di-Triethanolamine 1,10-Decanedicarboxylate from Calorimetric Heats of Dilution in Aqueous Solution

Two separate transition concentrations are observed in the partial molar enthalpies of dilution of aqueous solutions of 1,10-decanedicarboxylic acid neutralized with triethanolamine. The first transition is believed to be caused by dimer or lower oligomer aggregation accompanied by changes in proton dissociation equilibria, while the second discontinuity in partial molar enthalpies vs concentration data is expected to correspond to micelle formation. Like other ionic surfactants, the enthalpy of dimer or oligomer formation from monomers as well as the enthalpy of micelle formation from surfactant dimers or oligomers at their respective critical concentrations are found to be close to zero at 25°C. Critical aggregational concentrations from the present calorimetric data are in good agreement with results obtained at 22°C with other techniques. Possible reasons for pH and concentration dependent slopes in plots of relative partial molar enthalpies vs concentration at subaggregational concentrations are discussed.

The enthalpy of dilution of calcium chloride in aqueous ethanol at the temperatures 288.15 K, 298.15 K, and 308.15 K

Molar enthalpies of dilution of CaCl 2 in { xC 2H 5OH + (1- x)H 2O} solvents, measured at the temperatures 288.15 K, 298.15 K, and 308.15 K in a flow-microcalorimeter, are reported for various mole fraction x. Excess molar enthalpies were also measured over the entire composition range for the binary (ethanol + water) solvent at the same three temperatures. An extended form of the Debye-Hückel limiting law was used to correlate the experimental results and to obtain molar enthalpies of dilution at infinite dilution of CaCl 2 for fixed compositions of the mixed solvent, and to estimate relative apparent molar enthalpies.

Role of hydrophilic interactions in sustaining preferential orientations: calorimetric and nuclear magnetic resonance studies of ternary aqueous systems containing a destructuring agent and hydroxylated compounds at 298.15 K

Enthalpies of dilution of ternary aqueous solutions containing biuret or urea and alkane-n, m-diols have been determined by flow microcalorimetry at 298.15 K. The pairwise cross-interaction coefficients of the virial expansion of the excess enthalpies were determined. The coefficients are positive and depend in a complex manner on the length of the alkyl chain of the hydroxylated substances. A balance between favourable hydrophilic–hydrophilic and repulsive hydrophilic–hydrophobic interactions is the factor that determines the behaviour of these systems. NMR relaxation time measurements have also been performed: however, this technique does not show the complexity unravelled by calorimetry. For binary solutions of α,ω-diols, a correlation is presented between the relaxation times and the enthalpic self-interaction coefficients.

Some aspects of the enthalpy of dilution of biological polyelectrolytes

It is shown that calorimetric experiments on the dilution process of polyelectrolytes, i.e. on the excess enthalpy of electrostatic origin, can give information on the conformational state of the polymer. Analytical expression are obtained relating the enthalpy of dilution to the charge density on the polymer, from Manning's theory. The experimental data reported on some ionic biopolymers are interpreted on the basis of the theoretical prediction of the electrostatic contribution to the enthalpy of dilution in water of a polyelecrolytic salt and of a weak polyacid.

Thermodynamic studies of octyltrimethylammonium chloride in water

Densities, heat capacities, enthalpies of dilution at 298 K and osmotic coefficients at 310 K of octyltrimethylammonium chloride were measured as functions of concentration. From the experimental data, the partial molar volumes, heat capacities, relative enthalpies, nonideal free energies and entropies at 298 K were derived as functions of concentration. A comparison between the above data and those of dodecyltrimethylammonium chloride reported in the literature shows that the increase of the alkyl chain length shifts the apparent molar volumevs. concentration curves towards greater values and the heat capacity, relative enthalpy and free energyvs. concentration curves towards smaller values. By assuming the pseudo-phase transition model the properties of micellization (ΔYm) were graphically evaluated. TheΔYm values of OTAC compared with those of DTAC are consistent with the increase of the hydrophobicity by increasing the alkyl chain length.

Volumes, Dilution Enthalpies and Dielectric Polarization of Water-Sulpholane Mixtures at Different Temperatures

Densities and dielectric constants of water-sulpholane mixtures have been measured at 25, 30 and 40°C over the complete mole fraction range. Excess molar volumes and apparent molar volumes of the two components were derived from density data. The temperature and concentration dependences of these quantities are discussed in terms of the structural modification of water imposed by sulpholane. Dielectric constant data, analyzed in terms of Kirkwood theory, show that the short range intermolecular interactions between the components in the mixtures are conform to the depolymerization of the water hydrogen bonded network promoted by the mixing with sulpholane. The changes of the water structure have been further investigated by measuring the enthalpies of dilution of the sulpholane at 30°C. The data were analyzed with a polynomial equation to derive the relative apparent molal enthalpies. The resulting values of these quantities are in good agreement with the volumetric and dielectric constant results.

Enthalpies of interaction of urea in mixtures of water and N,N-dimethylformamide at 25�C

Enthalpies of dilution of urea dissolved in mixtures of water and N,N-dimethylformamide with mole fractions of water 0.33, 0.50 and 0.67, respectively, have been measured calorimetrically at 25°C. From these, enthalpic interaction coefficients were obtained. The present results are compared with those of urea dissolved in pure water and in pure N,N-dimethylformamide.

A model for the interaction between hydrophilic and hydrophobic solutes - aqueous solutions containing biuret or urea and hydroxylated substances

Enthalpies of dilution of ternary aqueous solutions containing biuret or urea and alkan-1-ols, alkan-l,2-diols and alkan-l,2,3-triols have been determined at 298.15 K by means of a flow microcalorimeter. The cross pairwise coefficients of the virial expansion of the excess enthalpies were determined. These coefficients are positive and depend in a complex manner on the length of the alkyl chain of the alkanols. The behaviour of these systems was interpreted in terms of preferential interactions between hydrophilic groups and of repulsive interactions between hydrophobic and hydrophilic groups.