Enthalpies Of Dilution Research Articles

Calorimetric study of bovine serum albumin dilution and adsorption onto polystyrene particles

Titration calorimetry was used to investigate the interaction between a model antigen, bovine serum albumin (BSA), and a model particulate carrier, polystyrene (PS). The binding enthalpy was much higher than reported in the literature for a similar system and did not display a sigmoidal binding curve. These experiments may have accessed low coverage surface sites due to the irreversible nature of protein binding and stepwise titration. An important correction is the heat of dilution of the protein solution. Two regimes were observed: at low concentrations of BSA (below ca. 0.3% (w/v)) an exothermic dilution enthalpy of ca. −100 mJ mg −1 was determined, whereas at higher concentrations of BSA values of ca. −20 mJ mg −1 were obtained. Solution rheological data also showed a change at 0.3% (w/v) BSA, so we hypothesise that the fraction of the BSA as monomers, dimers and polymers in solution changes at approximately 0.3% (w/v).

Enthalpy of dilution of [SO 2 + salt + H 2O] in [salt + H 2O] {salt = (NH 4) 2SO 4 or Na 2SO 4}: Experimental results and modeling

New experimental results are presented for the enthalpy change upon diluting aqueous solutions of sulfur dioxide containing a salt (either ammonium sulfate or sodium sulfate) in aqueous solutions of that salt at about 313 and 352 K. A previously developed thermodynamic model for the vapor–liquid equilibrium of the chemical reacting systems {SO 2 + (NH 4) 2SO 4 + H 2O} and (SO 2 + Na 2SO 4 + H 2O) is extended allowing for the formation of pyrosulfite and for the description of the new experimental results for the enthalpy change upon dilution.

Calorimetric study of the reactions of n-alkylphosphonic acids with metal oxide surfaces

The reaction enthalpies for the solution-phase self-assembly of n-alkylphosphonic acids on the surfaces of TiO 2 and ZrO 2 have been determined using isothermal titration calorimetry at 298 K. The reaction enthalpies were negative (exothermic) for methyl- and n-octylphosphonic acids and positive (endothermic) for n-octadecylphosphonic acid with both metal oxides. The enthalpy/energy analysis showed that the net enthalpy of the formation of self-assembled monolayers (SAMs) at solid–liquid interface can be presented as follows: Δ H r = − D − ( Δ H sol + Δ H dil ) − ( E S − E SAM ) , where D is the binding energy of the SAM molecules with the solid; Δ H sol and Δ H dil are the enthalpies of dissolution and dilution; E S and E SAM are the surface energies of bare solid and SAM, respectively. This equation predicted an increase (and the sign change) of the reaction enthalpy as the alkyl group in n-alkylphosphonic acid increased, which explained the experimental data. Using this equation, the binding energy ( D ) in the SAMs of n-octyl- and n-octadecylphosphonic acids were estimated: 55 ± 5 kJ / mol (for ZrO 2) and 58 ± 7 kJ / mol (for TiO 2).

Enthalpic Interaction of d-Sorbitol and d-Mannitol in Water and Aqueous Sodium Halide Solutions at 313.15 K

The enthalpies of dilution of d-sorbitol and d-mannitol with water and aqueous sodium halide solutions were determined by isothermal microcalorimetry at 313.15 K. Experimental enthalpies of dilutio...

Water-mediated interactions between benzene rings: Calorimetric studies of aromatic model compounds in aqueous solutions at 298 K

Pairwise enthalpic interaction coefficients of the virial expansion of the excess enthalpies were determined at 298 K by measuring the enthalpies of dilution in aqueous solution of binary aqueous solutions containing 4-hydroxyphenylacetic acid, 2-phenylethanol, 3-phenylpropanol, 3-phenylpropionic acid, l-tyrosine and l-phenylalanine. Coefficients obtained are compared with those already reported in the literature for other aromatic substances in aqueous solutions. Not withstanding the similarity of the substances employed, the values of the enthalpic coefficients range from highly negative to highly positive, an indication that the interactions between the benzene rings are largely dependent on the nature of the functional groups. For hydroxylated substances, enhanced hydrophobic interactions are operating, probably for the simultaneous interaction between the benzene rings and the alkyl chains, forced by the hydroxyl group. On the contrary, the strength of hydrophobic interactions in the solutions of the amino acids depend on the pH of the medium and on the presence of hydroxyl group on the aromatic ring. The data are discussed according to an interaction model which assumes the presence of a preferential configuration between two hydrated molecules.

Enthalpy of dilution of (SO 2 + H 2O) and (SO 2 + NH 3 + H 2O) in pure water: experimental results and modeling

New experimental results for the enthalpy change upon diluting aqueous solutions of sulfur dioxide as well as aqueous solutions of sulfur dioxide and ammonia in pure water at about 313 and 352 K are presented. The experimental results are used to modify a previously developed thermodynamic model for the vapor–liquid equilibrium of the chemical reacting system (SO 2 + NH 3 + H 2O). That model is extended allowing on the one side for the formation of pyrosulfite, and on the other side for the description of experimental results for the enthalpy change upon dilution (in pure water) of (NH 3 + H 2O) (taken from the literature), as well as (SO 2 + H 2O) and (SO 2 + NH 3 + H 2O) (from the present work).

Microcalorimetric studies of the effects on the interactions of human recombinant interferon-α2a

The applicability of the physical stability of protein solution monitored by isothermal titration calorimetry (ITC) was evaluated. The second virial coefficient, b 2, derived from the dilution enthalpies of protein solution measured by ITC under various experimental conditions was studied. The protein applied in this work is human recombinant interferon-α2a (hrIFN-α2a), which is a commercial drug applied for the treatment of virus-infected diseases. The results obtained were used to predict the possibility of hrIFN-α2a aggregation, and the prediction can be further confirmed by size-exclusion chromatography (SEC). Various factors affecting the stability of protein solution were investigated, for example, temperature, salts, surfactants, and mechanical stress. Specifically, the results show that the dilution enthalpy of hrIFN-α2a increased with increasing temperature and NaCl concentration, while b 2 decreased, indicating that the attraction between hrIFN-α2a molecules was enhanced under these conditions. On studying the effect of mechanical stress, the data obtained reveals that the introduction of centrifugal or vortex force strengthened the attractive forces between hrIFN-α2a molecules. These implications were supported by SEC data, demonstrating that the amount of aggregated hrIFN-α2a was increased. As a consequence, the methodologies presented in this investigation offer a possibility of monitoring the physical stability of protein solution at various stages of recovery, purification as well as the development of appropriate drug storage formulations.

Calorimetric investigation of the adsorption of cationic dimeric surfactants with a hydrophilic spacer group onto silica

The adsorption of dimeric cationic surfactants, made up of two ammonium species linked via a hydrophilic spacer group, i.e. Br − n-C 12H 25N +Me 2-CH 2(CH 2OCH 2) x CH 2N +Me 2- n-C 12H 25Br −, referred to as 12-EO x -12, onto silica has been studied in aqueous solution at 298 K and free pH. The effect of the molecular structure on the adsorption of these cationic surfactants onto raw silica (SiNa) and HCl-treated (SiH) silica was investigated. For this purpose, batch microcalorimetry measurements of the differential molar enthalpy of dilution and that of adsorption were performed at 298 K. The enthalpy data were supplemented by the classical observations such as the adsorption isotherms. The microcalorimetry results show that the adsorption mechanism and the aggregation at the solid–liquid interface do not depend on the spacer group length. Then, important quantitative differences are observed between the two silica surfaces. The investigated surfactants provide useful molecular probes for the characterization of the surface active sites onto treated and raw silicas. The amounts adsorbed allow the surface sites present in SiNa and SiH to be quantified.

Swelling behaviors, tensile properties and thermodynamic studies of water sorption of 2-hydroxyethyl methacrylate/epoxy methacrylate copolymeric hydrogels

A series of copolymeric hydrogels based on 2-hydroxyethyl methacrylate (HEMA) and epoxy methacrylate (EMA) were synthesized by bulk polymerization. Swelling behaviors and tensile properties of hydrogels were studied. Dynamic swelling behaviors of copolymeric hydrogels indicate that the swelling process of these polymers follows Fickian behavior. The equilibrium water content (EWC) decreased and volume fraction of polymer in hydrogel ( ϕ 2) increased with EMA 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 crosslink density ( v e), increased as EMA content increased or ionic strength of swelling medium increased, attributing to increasing interaction between hydrophobic groups and polymer–polymer interaction with an increase in EMA content or in ionic strength. The polymer–solvent interaction parameter χ reflecting thermodynamic interaction was also studied. As EMA 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 S and χ H , could be explained on the basis of structuring of water through improved hydrogen bonding and hydrophobic interaction.

Enthalpies of dilution of l-cystine in aqueous solutions of sodium hydroxide, potassium hydroxide and hydrochloric acid

The enthalpies of dilution of l-cystine in solutions of two strong alkalis and one strong acid have been determined by isothermal flow-mix calorimetry at the temperatures 298.15 K, 303.15 K, 308.15 K, 313.15 K and 318.15 K. Equations of apparent enthalpies of dilution have been obtained from the experimental data in terms of the improved McMillan–Mayer theory. Enthalpic interaction coefficients, h 2, h 3, and h 4, are obtained and the values of pair-wise enthalpic interaction coefficient, h 2, discussed in the light of solute–solute and solute–solvent interactions.

Enthalpies of dilution of glycine and l-alanine in aqueous 1-propanol solutions at T = 298.15 K

The dilution enthalpies of glycine and l-alanine in aqueous 1-propanol solutions have been measured by a mixing-flow microcalorimeter at T = 298.15 K. The homogeneous enthalpic interaction coefficients of the two zwitterions have been calculated according to the analysis of the excess enthalpy suggested by Friedman [J. Solution Chem. 1 (1972) 387–390]. The enthalpic pairwise interaction coefficients h 2 of glycine are negative and pass through a minimum with increasing 1-propanol content in mixed solvents, while those of l-alanine decrease from positive to negative. The variations of the enthalpic pairwise interaction coefficients are interpreted in terms of solute − solute and solute − solvent interactions.

Group Contribution Values for the Thermodynamic Functions of Hydration at 298.15 K, 0.1 MPa. 2. Aliphatic Thiols, Alkyl Sulfides, and Polysulfides

Group additivity contributions are evaluated for normal and branched thiols (mercaptans), alkyl sulfides, and polysulfides on the basis of a compilation of experimental values of the infinite dilution partial molar Gibbs energy, enthalpy, and heat capacity of hydration, together with molar volumes in aqueous solution at 298.15 K and 0.1 MPa. The thermodynamic data were treated using both first- and second-order group additivity methods. Numerical values of the contributions to each of the thermodynamic properties are obtained for the following first-order S-containing groups: SH, S, S2, and S3 as well as for the C−SH correction for the attachment of the thiol group to the tertiary carbon atom. In the case of the second-order group contribution method, numerical values are determined for the groups C−(S)(H)3, C−(C)(S)(H)2, C−(C)2(S)(H), C−(C)3(S), S−(H)(C), S−(C)2, S−(S)(C), and S−(S)2.

A Monte Carlo simulation study of the ionic liquid 1- n-butyl-3-methylimidazolium hexafluorophosphate: liquid structure, volumetric properties and infinite dilution solution thermodynamics of CO 2

A Monte Carlo simulation study is performed on the ionic liquid 1- n-butyl-3-methylimidazolium hexafluorophosphate. A modified united atom forcefield is developed for the ionic liquid. Volumetric properties such as density, isothermal compressibility and volume expansivity are computed at temperatures ranging from 298 to 343 K. Henry’s constants and infinite dilution enthalpy and entropy of absorption are computed for CO 2 in the ionic liquid. In addition, local structural information is obtained from radial distribution function analysis. Comparison is made between calculated properties and available experimental data. The performance of the new forcefield is assessed by comparison with two other forcefields developed for this compound.

Group Contribution Values for the Thermodynamic Functions of Hydration of Aliphatic Esters at 298.15 K, 0.1 MPa

A compilation of experimental values of the infinite dilution partial molar Gibbs energy, enthalpy, and heat capacity of hydration, together with molar volumes in water at 298.15 K and 0.1 MPa, is presented for aliphatic esters. These data, combined with the related results for aliphatic hydrocarbons, monohydric alcohols, and ketones, are treated in the framework of the first- and second-order group additivity methods. Numerical values of the contributions to each of the thermodynamic properties are obtained by a least-squares procedure for the following first-order groups: CH3, CH2, CH, C, OH, CO, COO, and COOH as well as for the C−OH, C−CO, and C−COO corrections for the attachment of the polar groups to the tertiary carbon atom. For the case of the second-order group contribution method, numerical values are retrieved for the following groups: C−(C)(H)3, C−(C)2(H)2, C−(C)3(H), C−(C)4, C−(C)(H)2(O), C−(C)2(H)(O)alcohol, C−(C)3(O)alcohol, O−(H)(C), CO−(C)2, C−(CO)(H)2(C), C−(CO)(H)(C)2, C−(CO)(C)3, C−(C)2(H)(O)ester, and C−(C)3(O)ester and for the provisional “ester” COO−(C)2 and “methanoate” HCOO−(C) groups.

Enthalpy of Swelling of Potassium Polyacrylate and Polymethacrylate Hydrogels. Evaluation of Excluded‐Volume Interaction

AbstractSummary: The enthalpy of swelling over the entire concentration range in potassium polyacrylate and polymethacrylate hydrogels with varying charge density was measured by means of isothermal Calvet microcalorimetry at 298 K. Concentration plots of the enthalpy of swelling at polymer volume fractions up to 50% were used for evaluation of Flory‐Huggins binary parameter of interaction. Apparent values of χ determined directly from experimental plots revealed extremely strong concentration dependence at high degrees of swelling. This was taken as evidence for the overlap of electrostatic and excluded‐volume interactions. The separate evaluation of these contributions to the enthalpy of swelling was performed using the recent theory developed for the enthalpy of dilution in semi‐dilute polyelectrolyte solutions. The mean value of the parameter χ over a wide range of swelling for the uncharged poly(acrylic acid) and poly(methacrylic acid) gel was found equal to 0.21 and −0.50 respectively. The ionization of the gel network due to the introduction of potassium salt residues resulted in the progressive decrease in the quality of the solvent. At 40–50% of ionization, the binary interaction parameter was about 2.8 both for potassium polyacrylate and polymethacrylate hydrogels. That value is much more than the critical value for liquid‐liquid phase separation in non‐electrolyte polymer solutions. magnified image

Modeling phase equilibria and speciation in mixed-solvent electrolyte systems

A comprehensive mixed-solvent electrolyte model has been applied to calculate phase equilibria and other thermodynamic properties of multicomponent solutions containing salts, acids and bases in wide concentration ranges. The model combines an excess Gibbs energy model with detailed speciation calculations. The excess Gibbs energy model consists of a long-range interaction contribution represented by the Pitzer–Debye–Hückel expression, a short-range term expressed by the UNIQUAC model and a middle-range term of a second-virial-coefficient type for specific ionic interactions. The model accurately represents the thermodynamic behavior of systems ranging from infinite dilution in water to molten salts or pure acids at temperatures from the freezing point to 300 °C. It has been determined that a physically realistic treatment of speciation is important for the simultaneous representation of vapor–liquid equilibria, osmotic coefficients, solid–liquid equilibria, pH, enthalpies of dilution and heat capacities.

Enthalpies of dilution of glycine, l-alanine and l-serine in aqueous potassium chloride solutions

The dilution enthalpies of glycine, l-alanine and l-serine in aqueous potassium chloride solutions of various molalities have been determined using LKB-2277 flow microcalorimetry at 298.15 K. The homogeneous enthalpic interaction coefficients over the whole range of aqueous potassium chloride solutions investigated have been calculated according to the excess enthalpy concept. It is found that pairwise enthalpic interaction coefficients h 2 of glycine and l-serine are all negative and become less negative with increasing of the molalities of potassium chloride, while pairwise enthalpic interaction coefficients h 2 of l-alanine are positive on which the influence of potassium chloride is not obvious. The results are interpreted from the point of view of solute–solute interactions involved by solvent effects.

Studies on the interaction between α-amino acids with polar side-chains and heterocyclic compounds at T=298.15 K

Mixing enthalpies of aqueous l-serine and l-threonine solutions and aqueous tetrohydrofuran (THF) and 1,4-dioxane solutions and their dilution enthalpies have been determined with a Thermometric-2277 Thermal Activity Monitor at T=298.15 K. The h xy values, obtained with the McMillan–Mayer formalism, for the α-amino acids bearing polar side-chain ( l-serine and l-threonine) with THF and 1,4-dioxane are compared with values reported in the literature for the α-amino acids with non-polar side-group (glycine, l-alanine, l-aminobutyric acid and l-valine) with THF and 1,4-dioxane. The results are interpreted in terms of solute–solute interactions. The group additivity concept by Savage and Wood (SWAG) has also been used. Contributions of each amino acid, functional group have been estimated.

Isothermal Titration Calorimetry of the Polyelectrolyte/Water Interaction and Binding of Ca2+: Effects Determining the Quality of Polymeric Scale Inhibitors

The interaction of poly(sodium acrylate) (NaPAA) and poly(sodium styrenesulfonate) (NaPSS) with water over a broad range of concentrations was followed by isothermal titration calorimetry (ITC) and compared to the corresponding low molecular weight salts, sodium acetate and sodium sulfate. Astonishingly, the backbone of NaPSS, known to be more hydrophobic, remains unseen in the dilution enthalpy, and NaPAA and NaPSS show very similar and close to ideal dilution curves. The NaPSS dilution is even more exothermic than Na2SO4. This is related to the specific structure of water screening the hydrophobic polystyrene backbone. Counterintuitively, binding of Ca2+ ions to PAA, a spontaneous process, shows to be highly endothermic; i.e., the binding is solely driven by entropy. This suggests that not Coulomb interactions, but liberation of water molecules from the hydration shells of the components is the driving energy source for the binding of multivalent ions onto polyelectrolytes. Analysis of binding titrations of Ca2+ to three different industrial scale inhibitors (poly(acrylic acid), poly(aspartic acid), poly(acrylic-co-maleic acid) (Sokolan)) allows to qualify their performance. The experiments underline that the understanding of polyelectrolytes and counterion condensation on the basis of Coulomb descriptions, i.e., ions as point charges and water as a homogeneous dielectric media, is at least misleading, as it disregards the predominant thermodynamic effects, the chemical specificity of the components in response to the structure of water.

Enthalpic Interactions of Amino Acids with Saccharides in Aqueous Solutions at 298.15 K

The enthalpies of mixing of three kinds of aqueous amino acid solutions (glycine, l-alanine, and l-serine) with aqueous sorbose solution and aqueous fructose solution and their respective enthalpies of dilution have been determined at 298.15 K using a 2277 flow microcalorimetry. The experiment data have been analyzed in terms of McMillan−Mayer formalism to obtain the enthalpic virial coefficients for heterotactic interaction. The results have been interpreted from the point of view of solute−solute interactions.