Enthalpies Of Dilution Research Articles

Thermodynamic Characterization of Polyanetholesulfonic Acid and Its Alkaline Salts

Experimental and theoretical results for the thermodynamic properties of polyanetholesulfonic acid and its lithium, sodium, and cesium salts in aqueous solution at 298 K are presented. The osmotic pressure was measured using membrane and vapor pressure apparatus in the concentration range c(m) = 0.001-0.30 monomoles/dm(3). The osmotic coefficients obtained from these measurements were low, from 0.2 to 0.45 in this concentration range, indicating a strong interaction between counterions and polyions. The osmotic coefficients of the polyacid and its lithium and sodium salts appeared to be equal within experimental error, but the results for the cesium salt were lower. This indicates a somewhat stronger binding of cesium ions to the polyanion. In addition, enthalpies of dilution, DeltaH(D), from a certain concentration, m(m), to m(m) = 0.0044 monomoles/kg were measured. The measured heats of dilution were exothermic, with the acid producing the strongest and the cesium salt the weakest effect. These results were compared with previously published data for polyelectrolytes of similar structure, namely, polystyrenesulfonic acid and its alkaline salts. The osmotic pressure results indicate that polystyrenesulfonates bind the counterions more strongly than polyanetholesulfonic acid and its salts. Consistent with this finding, the enthalpies of dilution reveal that more heat is released upon dilution of polyanetholesulfonates (stronger exothermic effect) in comparison with the corresponding solutions of polystyrenesulfonic acid in its alkaline salts. These findings can be explained in terms of the structural differences between the two polyions. The experimental results were analyzed in relation to popular electrostatic theories such as the Manning condensation theory and the Poisson-Boltzmann cell model approach, where the polyion is pictured as a uniformly charged line or cylinder. In addition, we performed Monte Carlo simulations for a model polyanetholesulfonic anion having discrete charges. In all of the calculations, the solvent was treated as a continuum with the dielectric constant of pure water under the conditions of measurement. The theoretical considerations mentioned above yield results in semiquantitative agreement with the measured quantities.

Enthalpic Pairwise Interactions between Some Amino Acids and 2-Butanone in Aqueous Solutions at 298.15 K

The enthalpies of dilution of aqueous solutions of six kinds of amino acids (glycine, l-α-alanine, l-γ-aminobutyric acid, l-α-valine, l-α-serine, and l-α-threonine) with 2-butanone and enthalpies of mixing were measured at 298.15 K, using a mixing-flow microcalorimeter. The experimental data were treated according to the McMillan−Mayer theory, to obtain the enthalpic interaction coefficients. Combining the enthalpic pairwise interaction coefficients (hxy) of amino acids with cyclohexanone from the literature, the experimental results have been discussed from the point of view of solute−solute and solute−solvent interactions.

Concentration effect of sodium chloride on enthalpic interaction coefficients of D-mannitol and D-sorbitol in aqueous solution

The dilution enthalpies of D-mannitol and D-sorbitol in aqueous sodium chloride solution at various concentrations have been determined by isothermal microcalorimetry at 298.15 K. The homogeneous enthalpic interaction coefficients over a quite large range of concentration of aqueous sodium chloride solutions have been calculated according to the excess enthalpy concept. The results show that enthalpic pairwise interaction coefficients (h2) of D-mannitol and D-sorbitol are positive in aqueous sodium chloride solution and become more positive with increase of the concentration of sodium chloride. The results are interpreted in terms of the different conformations of the two polyols, solute-solute and solute-solvent interactions involved by solvent effects.

Thermodynamic properties of the LiCl–H 2O system at vapor–liquid equilibrium from 273 K to 400 K

A formulation of the thermodynamic properties of the LiCl–H 2O system at vapor–liquid equilibrium is presented in the form of separate Gibbs energy equations for the vapor and solution phases. Explicit thermodynamically consistent equations for density, isobaric heat capacity, enthalpy, entropy, enthalpy of dilution and osmotic coefficient are given. The pressure at vapor–liquid equilibrium is calculated from the condition of phase equilibrium. The description of the properties is valid from 273 K or from the crystallization line up to 400 K in temperatures and for solution composition from 0 to 50 wt% of LiCl in the solution, which is the region covewred by available experimental data. The thermodynamic properties of the gas phase are approximated by the properties of pure water vapor computed from the IAPWS formulation 1995. The Gibbs energy equation for solution is based upon a body of experimental data that have been critically assessed. Within the present study, 136 experimental works have been collected containing a total of 2921 data points on various thermodynamic properties of the LiCl–H 2O solutions. Gaps in the database are shown to give experimenters orientation for future research. The uncertainties associated with correlation are estimated to be ±0.4% for density, ±2.0% for pressure and ±2.4% for isobaric heat capacity. The uncertainty in values of enthalpy is estimated to be less than ±10 kJ kg −1 and less than ±0.03 kJ kg −1 K −1 for entropy. Values of the particular properties generated by the representative equations are provided to assist with the confirmation of computer implementation of the calculation procedure.

Studies on the Enthalpic Interactions of Formamide and N,N-Dimethylformamide with myo-Inositol in Aqueous Sodium Chloride Solutions at 298.15 K

The mixing enthalpies of formamide and N,N-dimethylformamide with myo-inositol and their respective enthalpies of dilution in aqueous sodium chloride solution have been determined by using flow-mix isothermal microcalorimetry at 298.15 K. These experimental results have been used to determine the heterotactic enthalpic interaction coefficients (hxy, hxxy, and hxyy) according to the McMillan−Mayer theory. It has been found that the heterotactic enthalpic pairwise interaction coefficients (hxy) between formamide and myo-inositol in aqueous sodium chloride solution are negative and become less negative with an increase in the concentration of sodium chloride. In contrast, the values of hxy between N,N-dimethylformamide and myo-inositol are positive and become more positive with increasing concentration of sodium chloride solutions. The results are discussed in terms of solute−solute interaction and solute−solvent interaction.

Enthalpies of Dilution for myo-Inositol in Aqueous Alkali Metal Salt and Alkaline Earth Metal Salt Solutions

The enthalpies of dilution of myo-inositol in aqueous solutions of alkali metal salts and alkaline earth metal salts have been determined with flow-mix isothermal microcalorimetry at 298.15 K. The enthalpic interaction coefficients in the range of salt solutions have been calculated according to the McMillan−Mayer theory. The results show that enthalpic pairwise interaction coefficients h2 of myo-inositol are negative in aqueous alkali metal salt and alkaline earth metal salt solutions. The absolute value of h2 becomes small with an elongation of the radius of the alkali metal cation while it becomes large with an elongation of the radius of the alkaline earth metal cation. The results are discussed in terms of the different metallic ions, interactions of solute with solute and solute with solvent.

Studies on the Interactions of l-Valine and l-Threonine with Saccharides in Aqueous Solutions at 298.15 K

Mixing enthalpies of aqueous l-valine and l-threonine solutions with aqueous sorbose solution and aqueous fructose solution and their dilution enthalpies have been determined with a Thermometric-2277 thermal activity monitor at 298.15 K. The experimental data have been analyzed in terms of McMillan−Mayer formalism to obtain the enthalpic virial coefficients for heterotactic interaction. The enthalpic pairwise interaction coefficients hxy of l-valine with saccharides are positive and those of lthreonine are negative. The variations of the enthalpic pairwise interaction coefficients are interpreted in terms of solute−solute interactions.

Enthalpic interaction coefficients of glycine in aqueous sodium halide solutions at 298.15 K

The enthalpies of dilution have been measured for glycine in aqueous NaCl and NaBr solutions at 298.15 K using a flow microcalorimetry. The enthalpic interaction coefficients of glycine in the studied aqueous sodium halide solutions have been calculated according to the McMillan–Mayer model. Combining the previous studies for other sodium halides, the effect of the anions on enthalpic pairwise interaction coefficients between the glycine molecules has been rationalized in terms of the concepts of electrostatic interaction and structural interaction. The experimental data of the present work and those of the literature are combined, to put forward the general chemical principal, namely the more strongly solutes are solvated the less will be their tendency to interact with other solutes.

Thermodynamic and aggregation properties of aza- and imino-substituted gemini surfactants designed for gene delivery

Improving the efficiency of gene delivery by using non-viral vectors is currently an area of considerable research interest. Novel derivatives of gemini surfactants having aza- (12-5N-12, 12-7N-12, 12-8N-12) and imino- (12-7NH-12) substituted spacer groups and C12 tails have been designed to improve DNA transfection. Physicochemical characterization of micelle and interfacial properties of these cationic compounds are reported. The effect of these substitutions on the aggregation properties of the gemini surfactants is discussed in the context of results for the 12-s-12 and 12-EOx-12 gemini series, previously reported in the literature. Aza substitution results in a spacer of intermediate hydrophobicity to the above series, reflected by the magnitude of both the critical micelle concentrations and head group areas. Enthalpy and apparent molar volume of micellization data illustrate the differences in the aggregation properties that result from the bulkier and more hydrophobic aza-substituent in the spacer as compared to an ether oxygen (for the 12-EOx-12 series) containing spacer. The 12-7N-12 and 12-8N-12 compounds show aberrant features in the surface tension and enthalpy of dilution results that are not observed for the 12-5N-12 and 12-7NH-12 compounds. Premicelle association is considered to be a source of this behaviour.

Studies on swelling behaviors, mechanical properties, network parameters and thermodynamic interaction of water sorption of 2-hydroxyethyl methacrylate/novolac epoxy vinyl ester resin copolymeric hydrogels

A series of the highly cross-linked hydrogels has been developed by the syntheses of bulk copolymers based on 2-hydroxyethyl methacrylate (HEMA) and novolac epoxy vinyl ester resin (NEVER). The resulting hydrogels were investigated on their equilibrium water content (EWC), swelling behaviors and tensile properties. Dynamic swelling behaviors of copolymeric hydrogels indicate that the swelling process of these polymers follows Fickian behavior. The EWC decreased and volume fraction of polymer in hydrogel ( ϕ 2) increased with NEVER content increasing due to its hydrophobicity. The increase of ionic strength of swelling medium or temperature results in a decrease in EWC and an increase in values of ϕ 2. Young’s modulus and tensile strength of hydrogels, as well as effective cross-link density ( v e), increased as NEVER content increased or ionic strength of swelling medium increased, attributing to increasing interaction between hydrophobic groups and polymer–polymer interaction with an increase in NEVER content or in ionic strength. The polymer–solvent interaction parameter χ reflecting thermodynamic interaction was also studied. As NEVER content, ionic strength of swelling medium or temperature increased, the values of χ increased. The values of χ and its two components χ H and χ S varied with increasing T. The negative values and trend of the enthalpy and entropy of dilution derived from values of χ H and χ S, could be explained on the basis of structuring of water through improved hydrogen bonding and hydrophobic interaction.

Thermodynamic properties of aqueous micellar solutions of some new acetylated gluco-cationic surfactants

Thermodynamic properties of the aqueous solutions of a newly synthesized homologous series of N-[2-(2,3,4,6-tetra- O-acetyl-β- d-glucopyranosyl)ethyl]- N, N-dimethyl- N-alkylammonium bromides with hydrophobic tails of 8, 12 and 16 carbon atoms, determined as a function of concentration by means of direct methods, are here reported for the first time. Dilution enthalpies, densities and sound velocities were measured at 298 K, allowing for the determination of apparent and partial molar enthalpies, volumes and compressibilities. Changes in thermodynamic quantities upon micellization were derived using a pseudo-phase transition approach. The trends of apparent and partial molar enthalpies versus m confirm the increase in global hydrophobicity of the molecule, already suggested by the value of the cmc of C 12AGCB, lower than that of DTAB, and show that, also in the case of surfactants with a bulky substitution on the polar head, the group contribution of the –CH 2– is additive both in the plateau (−1.3 kJ mol −1 group −1) and in the micellization enthalpy changes (−1.5 kJ mol −1 group −1). The determination of apparent molar volumes and compressibilities presents some experimental problems, due to the strong tendency of this class of surfactants to adsorb on the glass surfaces. Even so, the group contribution of the methylene group results as being additive and comparable to that of other cationic and anionic surfactants. From the comparison with DTAB, the volume of (2,3,4,6-tetra- O-acetyl-β- d-glucopyranosyl)ethyl group results as 291 cm 3 mol −1.

Enthalpic interactions of some α-amino acids with 1,2-ethanediol in aqueous solutions at 298.15 K

The enthalpies of mixing of six kinds of aqueous amino acid solutions (glycine, l-alanine, l-valine, l-serine, l-threonine and l-proline) and aqueous 1,2-ethanediol solution and their respective enthalpies of dilution have been measured at 298.15 K using flow microcalorimetry. The experimental data have been analyzed in terms of the McMillan–Mayer formalism to obtain the heterotactic enthalpic interaction coefficients. The heterotactic enthalpic pairwise interaction coefficients, h xy , have been discussed from the points of view of solute–solute interactions.

The dilution enthalpies of formamide in mixtures of water and propanol at 298.15 K

The dilution enthalpies of formamide in aqueous n-propanol and i-propanol solutions have been determined using a CSC-4400 isothermal calorimeter at 298.15 K. The homogeneous enthalpic interaction coefficients in the range of propanol concentration (0%–30%) have been calculated according to the excess enthalpy concept. The results are interpreted in terms of solute–solute and solute–solvent interactions. Hydroxyl groups present different efficiency in promoting hydrophobic interactions in dependence of their position in the interacting molecules.

Enthalpies of transfer of Mebicarum from water to aqueous solutions of carbamide and its methyl-substituted derivatives at 298.15 K

The standard molar enthalpies of solution of Mebicarum (MbCA) in aqueous solutions of carbamide (CA), 1,3-dimethylcarbamide (1,3-DMCA), and 1,1,3,3-tetramethylcarbamide (TMCA) of various molalities, as well as the enthalpies of dilution of solutions of these compounds, were measured at 298.15 K. The enthalpy of transfer of MbCA from water to aqueous solutions of 1,3-DMCA exhibits an unusual dependence on the concentration of 1,3-DMCA. An analysis of the McMillan-Mayer enthalpy parameters of pair interactions revealed that the hydration of MbCA should be regarded as a superposition of the hydrophobic and hydrophilic mechanisms, with the latter one being predominant.

A new equation for calculating the enthalpies of dilution of solutions of nonassociated electrolytes

An equation for determining the enthalpies of dilution of solutions of nonassociated electrolytes was derived on the basis of the results obtained by Kuznetsova. The enthalpies of dilution calculated by this equation are compared with those obtained using the known equation based on the Debye-Huckel theory.

Towards an understanding of the molecular mechanism of solvation of drug molecules: A thermodynamic approach by crystal lattice energy, sublimation, and solubility exemplified by paracetamol, acetanilide, and phenacetin

Temperature dependencies of saturated vapor pressure for the monoclinic modification of paracetamol (acetaminophen), acetanilide, and phenacetin (acetophenetidin) were measured and thermodynamic functions of sublimation calculated (paracetamol: ΔGsub298 = 60.0 kJ/mol; ΔHsub298 = 117.9 ± 0.7 kJ/mol; ΔSsub298 = 190 ± 2 J/mol · K; acetanilide: ΔGsub298 = 40.5 kJ/mol; ΔHsub298 = 99.8 ± 0.8 kJ/mol; ΔSsub298 = 197 ± 2 J/mol · K; phenacetin: ΔGsub298 = 52.3 kJ/mol; ΔHsub298 = 121.8 ± 0.7 kJ/mol; ΔSsub298 = 226 ± 2 J/mol · K). Analysis of packing energies based on geometry optimization of molecules in the crystal lattices using diffraction data and the program Dmol3 was carried out. Parameters analyzed were: (a) energetic contribution of van der Waals forces and hydrogen bonding to the total packing energy; (b) contributions of fragments of the molecules to the packing energy. The fraction of hydrogen bond energy in the packing energy increases as: phenacetin (17.5%) < acetanilide (20.4%) < paracetamol (34.0%). Enthalpies of evaporation were estimated from enthalpies of sublimation and fusion. Activity coefficients of the drugs in n‐octanol were calculated from cryoscopic data and by estimation of dilution enthalpy obtained from solubility and calorimetric experiments (for infinite dissolution). Solubility temperature dependencies in n‐octanol and n‐hexane were measured. The thermodynamic functions of solubility and solvation processes were deduced. Specific and nonspecific solvation terms were distinguished using the transfer from the “inert” n‐hexane to the other solvents. The transfer of the molecules from water to n‐octanol is enthalpy driven for paracetamol; for acetanilide and phenacetin, entropy driven. © 2006 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 95:2158–2169, 2006

Thermodynamics of aqueous solutions of dodecyldimethylethylammonium bromide

The thermodynamic properties of the aqueous solutions of dodecyldimethylethylammonium bromide (DEDAB) were determined as a function of concentration by means of direct methods. Dilution enthalpies at 298 and 313 K, densities and sound velocities at 298 K were measured, allowing the determination of apparent and partial molar enthalpies, volumes, heat capacities and compressibilities. Changes in thermodynamic quantities upon micellization were derived using a pseudo-phase transition approach. These data allow for the determination of the effect of the CH 2 group, when added to the polar head of alkyltrimethylammonium bromides. The properties mainly affected by this addition are the enthalpies and, as a consequence, the entropies. The lowering of the charge density on the quaternary nitrogen due to the inductive effect of the ethyl group, greater than that of the methyl one, raises the plateau value of apparent and molar enthalpy by a quantity similar to that due to the removing of a methylene group from the hydrophobic chain. This effect does not play a great role in the value of the cmc (i.e. on the free energy of micelle formation), since the small decrease in cmc of DEDAB compared to DTAB reflects the increase in the overall hydrophobicity of the molecule. Volumes of DEDAB are greater than those of DTAB by about 15 cm 3 mol −1, both at infinite dilution and at micellar phase, a value in agreement with that generally accepted for a methylene group. The trends of apparent molar heat capacities and compressibilities vs m are the same as for DTAB: in fact, these quantities are related to the number of water molecules involved in the hydrophobic processes in solution, not very greatly affected by the substitution of a methyl group by an ethyl one on the polar head. In summary, this substitution affects to a significant extent the first derivatives of the free energy, but does not affect the second derivatives.

Group Contribution Values for the Thermodynamic Functions of Hydration at 298.15 K, 0.1 MPa. 4. Aliphatic Nitriles and Dinitriles

A compilation of experimental values of the infinite dilution partial molar Gibbs energy, enthalpy, and heat capacity of hydration together with partial molar volumes in water at 298.15 K and 0.1 MPa is presented for aliphatic nitriles and dinitriles. These data are treated in the framework of the first- and second-order group additivity methods. Thermodynamic properties are determined for the first-order CN group and for the following second-order groups: C-(CN)(C)(H)2, C-(CN)(C)2(H), and CN-(C). The relatively long-range dipole−dipole interactions of two terminal nitrile groups require additional corrections, {CN-(CH2)2-CN}corr and {CN-(CH2)3-CN}corr, for lower dinitriles. New experimental studies of aqueous branched mononitriles and lower dinitriles (particularly propanedinitrile) are required to expand the usefulness and accuracy of group contribution models for aqueous nitriles.

Enthalpic interactions between some amino acids and cyclohexanone in aqueous solutions at 298.15 K

The enthalpies of mixing of glycine, l-α-alanine, l-γ-aminobutyric acid, l-α-valine, l-α-serine and l-α-threonine with cyclohexanone in aqueous solutions and their respective enthalpies of dilution have been measured by calorimetry at 298.15 K. Experimental enthalpies of dilution and mixing have been correlated with the virial expansion equation that was obtained with the McMillan–Mayer theory. The enthalpic interaction parameters h xy , h xxy and h xyy of the amino acids studied with cyclohexanone in aqueous solutions have been evaluated, and the heterotactic enthalpic pair interaction coefficients ( h xy ) are discussed in terms of solute–solute interactions.

Enthalpies of Dilution of D-p-Hydroxyphenylglycine in Buffer Solutions at Different pH

The effect of pH on the dilution enthalpies of D-p-hydroxyphenylglycine in phosphate buffer solutions has been investigated by isothermal titration microcalorimetry at 298.15 K. The corresponding homogeneous enthalpic interaction coefficients have been calculated according to the excess enthalpy concept. The results show that the enthalpies of dilution of D-p-hydroxyphenylglycine in phosphate buffer solutions at different pH are all positive. The overall trend is that enthalpies of dilution become more positive with an increase of pH, but there is a minimum of the enthalpy of dilution at pH = 7.0. The enthalpic pair interaction coefficients, h2, all have negative values. The results are interpreted from the point of view of solute-solute and solute-solvent interactions involved in the solvent effects.