Enthalpies Of Transfer Research Articles

Thermochemical study of complexation and solvation in the β-alanine-Cu(NO3)2-water-ethanol system: 2. Enthalpies of solvation of sodium alaninate and copper(II) nitrate in mixed solvents

The heats of mixing of aqueous copper(II) nitrate and sodium β-alaninate with water-ethanol solvents were measured thermochemically at 298.15 K. The enthalpies of transfer of the Ala− alaninate anion and Cu2+ ion in binary water-ethanol mixtures were calculated. The effect of the composition of the binary solvent on the enthalpy of formation of [CuAla]+ and the enthalpies of solvation of the ligand, central ion, and complex ion was studied.

DFT/B3LYP study of the solvent effect on the reaction enthalpies of homolytic and heterolytic O [sbnd]H bond cleavage in mono-substituted chromans

The study of various ortho and meta substituted chroman derivatives was carried out. The reaction enthalpies related to the hydrogen atom transfer (HAT) mechanism and to individual steps of two stepwise mechanisms of phenolic antioxidants action, single electron transfer–proton transfer (SET–PT) and sequential proton loss electron transfer (SPLET) have been calculated using DFT/B3LYP method in solution phase using Polarized Continuum Model (PCM) method. The effects of solvents with various polarities and substituent effect have been investigated. Results show that electron-donating substituents induce a rise in the proton dissociation enthalpy (PDE) and proton affinity (PA), whereas electron-withdrawing groups cause an increase in the bond dissociation enthalpy (BDE), ionization potential (IP) and electron transfer enthalpy (ETE). It has been found that substituents in ortho positions have significantly greater influence on BDE, PA and ETE in comparison with those in meta position. On the contrary, substituents in meta position show higher effect on IP and PDE than substituents in ortho positions. In comparison to gas phase, studied solvents attenuate the substituent effect on the reaction enthalpies of three studied mechanisms. From the thermodynamic point of view, entering SPLET mechanism represents the most probable process in polar solvents like water, DMSO and ethanol, where PA values of studied substituted chromans are considerably lower than BDEs and IPs. However, in benzene, BDEs are lower than PA and IP values, i.e. HAT represents the most probable pathway. Computed results reveal that all dependences of reaction enthalpies on Hammett constants of the substituents in meta position are linear in all solvents.

Solubility of elemental sulfur in pure organic solvents and organic solvent–ionic liquid mixtures from 293.15 to 353.15 K

The solubility of elemental sulfur in pure aromatic and aliphatic hydrocarbons was measured by cloud-point measurements using the laser beam scattering method under atmospheric pressure and temperature ranging from 293.15 to 353.15 K. It was found that the solubility of elemental sulfur in the examined solvents increases with an increase in temperature. Among the solvents tested, sulfur had the highest solubility in toluene. The individual effect of four different imidazolium ionic liquids – 1-butyl-3-methylimidazolium hexafluorophosphate ([C 4mim][PF 6]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([C 6mim][PF 6]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C 4mim][BF 4]), and 1-hexyl-3-methylimidazolium tetrafluoroborate ([C 6mim][BF 4]) on the solubility of sulfur in toluene was also studied. The results show that both the structures of the cations and concentrations of ionic liquids used to play important roles in the solubility of elemental sulfur. The experimental solubility data of elemental sulfur can be well correlated by the modified Apelblat equation. Finally, the free energies of transfer, Δ tr G, entropies of transfer, Δ tr S, and enthalpies of transfer, Δ tr H, of elemental sulfur from toluene to toluene + [C 6mim][PF 6] mixture were determined based on the solubility measurements.

Influence of the composition of aqueous-alcohol solvents on the thermodynamic characteristics of l-phenylalanine dissolution at 298.15 K

The enthalpies of l-phenylalanine dissolution in aqueous methanol, ethanol, 1-propanol and 2-propanol have been determined by calorimetry at 298.15 K and alcohol mole fractions up to x 2 ∼0.4. The standard enthalpies of solution Δ sol H° and transfer Δ tr H° from water to the mixed solvent as well as the enthalpy coefficients of l-phenylalanine–alcohol pair-wise interactions were calculated. The interrelation of the enthalpies of dissolution and transfer for l-phenylalanine with structural features of alcohols has been determined. A comparative analysis of the thermodynamic characteristics of dissolution of l-phenylalanine and some other amino acids (glycine, l-alanine, l-threonine and l-valine) in the mixtures studied has been made.

Luminescence study on the solvation of Cm(III) in binary aqueous solvent mixtures

AbstractThe preferential solvation of Cm(III) in binary aqueous mixtures of MeOH,tBuOH, DMSO, MeCN and acetone is studied at varied solvent composition. Solvation is investigated by time-resolved laser fluorescence spectroscopy and emission spectra and fluorescence lifetime data are obtained. At high mole fractions of organic solvent, preferential solvation of Cm(III) increases in the order: Me2CO <tBuOH ≈ MeOH < MeCN < H2O < DMSO. Thermodynamic data are derived from the spectroscopic results showing small positive standard Gibbs energies for the transfer of Cm(III) into the various solvent mixtures except for DMSO mixtures where negative values are found. The spectroscopically obtained enthalpies of transfer are fitted to a solvation model using the model parameters Δ Δ Hº12and (αn+βN)º, rendering valuable information on the interaction of Cm(III) with the solvent molecules. Cm(III)-solvent interaction increases in the order:tBuOH < MeCN < MeOH < DMSO.

Thermochemical characteristics of the interaction of L-cysteine with glycerol, ethylene glycol, and 1,2-propylene glycol in aqueous solutions

Integral enthalpies of dissolution ΔsolHm of L-cysteine in mixtures of water with glycerol, ethylene glycol, and 1,2-propylene glycol at concentrations of organic solvents of up to 0.37 mole fraction were measured by calorimetry of dissolution. The standard values of the dissolution enthalpies (ΔsolH°) and the transfer enthalpies (ΔtrH°) of amino acid from water to the mixed solvent were calculated. It was shown that the calculated enthalpic coefficients of the pair interactions of L-cysteine with cosolvent molecules have positive values. The obtained data are interpreted from the viewpoint of the prevalence of different types of interactions in the solutions and influence of the cosolvents nature on the thermochemical characteristics of amino acid dissolution.

Dependence of the solution and transfer enthalpies of DL-α-alanyl-DL-α-asparagine on the composition of water-amide binary solvents at 298.15 K

Solution enthalpies of DL-α-alanyl-DL-α-asparagine (AlaAsn) in water-formamide, water-N-methylformamide, water-N,N-dimethylformamide, and water-N,N-dimethylacetamide mixtures were measured in the range of amide mole fractions x 2 = 0–0.3. The standard enthalpies of solution (Δsol H°) and transfer (Δtr H°) of AlaAsn from water to the binary solvent and enthalpy coefficients of pair-wise interactions (h xy ) of AlaAsn with amide molecules were calculated. The influence of the composition of the water-organic mixture on the enthalpy characteristics of AlaAsn is discussed. It is shown that the enthalpy characteristics of solution and transfer of AlaAsn are related to the structure of amides.

DFT/B3LYP study of the substituent effect on the reaction enthalpies of the individual steps of single electron transfer–proton transfer and sequential proton loss electron transfer mechanisms of chroman derivatives antioxidant action

In this paper, the study of various ortho and meta-substituted chroman-6-ol derivatives is presented. The reaction enthalpies related to the individual steps of two stepwise mechanisms of phenolic antioxidants action, single electron transfer–proton transfer (SET–PT) and sequential proton loss electron transfer (SPLET), for studied compounds have been calculated using DFT/B3LYP method in gas-phase and water. Results reveal that electron-donating substituents induce rise in the proton dissociation enthalpy (PDE) and proton affinity (PA) whereas Electron withdrawing groups cause increase in the ionization potentials (IP) electron transfer enthalpy (ETE). Result indicated substituents in ortho have in comparison with the substituents in meta position significantly greater influence on PA and ETE, on the contrary, substituents in meta position cause considerable effect on IP and PDE rather than substituents in ortho positions. Results reveal that water attenuates the substituent effect on IP, PA and PDE values. Computed results indicate that all dependences of reaction enthalpies on Hammett constants of the substituents are linear. From the thermodynamic point of view, SPLET mechanism represents the most promising process in water. Furthermore, results show that calculated enthalpies can be correlated with the length of phenolic C–O and O–H + (bond length of O–H after electron abstraction) bonds and partial charge on the phenoxy radical oxygen atom q(O) of the studied molecules successfully.

Incorporation of carbon nanotubes in a silica HPLC column to enhance the chromatographic separation of peptides: Theoretical and practical aspects

The retention mechanism of a series of peptides on a single-wall carbon nanotube (SWCNT) stationary phase inside an HPLC column was investigated over a wide range of mobile phase compositions. While the similar size C18 column exhibited an efficiency of 11.5 μm, the SWCNT column increased the efficiency, i.e. 7.10 μm at a flow rate of 0.8 mL/min, and significantly affected the separation quality of the peptides. The values of enthalpy (ΔH) and entropy (ΔS(*)) of transfer of the peptides from the mobile to the SWCNT stationary phase were determined. The method studied each factor, i.e. ACN fraction x in the ACN/water mixture and column temperature. The changes in retention factor, ΔH and ΔS(*) as a function of the ACN fraction in the mobile phase were examined. These variations are explained using the organization of ACN in clusters in the ACN/water mixture and on the steric and electronic forces implied in the retention process. The information obtained in this work makes this SWCNT stationary phase useful for peptide research and demonstrated the role of ACN to improve the separation quality.

Synthesis and evaluation of a chiral stationary phase based on quinine: Enantioresolution of dinitrophenyl derivatives of α-amino acids

The natural alkaloid quinine (QN) was immobilized on porous silica particles, and part of the material was subsequently endcapped with n-hexyl hydrocarbon chains. Two synthetic strategies for silanization of the support were first compared. These columns were thoroughly evaluated in order to study the influence of endcapping in the enantiorecognition features. Enantioseparations of twenty N-derivatized 2,4-dinitrophenyl α-amino acids (DNP-amino acids) were studied by changing mobile phase pH, buffer concentration, type of organic solvent in the mobile phase, and column temperature. Maximum retention factors were observed at pH ≈6, at this intermediate pH the tertiary amine of the quinine is protonated to a high degree and therefore available for strong electrostatic interactions with unprotonated anionic DNP-amino acids. The enantioselectivity factors, however, increased as the pH did in the range between 5 and 7. The increase in ionic strength had influence on retention, but not on enantioselectivity, allowing the use of this variable for optimization of retention factors. Finally, the thermodynamic transfer parameters of the enantiomers from the mobile to both CSPs (with and without endcapping, QN-CSP(EC) and QN-CSP, respectively) were estimated from van’t Hoff plots within the range of 10–40 °C. Thus, the differences in the transfer enthalpy, Δ(Δ H°), and transfer entropy, Δ(Δ S°), enabled an investigation of the origin of the differences in interaction energies.

The effect of intramolecular hydrogen bonding on the thermodynamics of partitioning of an isomer of the histamine H2-receptor antagonist metiamide.

The thermodynamics of transfer from water to 1-octanol of the 4 and 5 substituted N-methylimidazole isomers of the H2-receptor antagonist metiamide have been determined. The isomers have similar free energies of transfer but the enthalpy and entropy of transfer of the 4-isomer are substantially greater than those of the 5-isomer. This indicates that the 4-isomer forms an intramolecular hydrogen bond in the polar octanol phase.

Enthalpies of solution of glycylglycylglycine in aqueous solution of amides at 298.15 K

Calorimetric measurements have been carried out at 298.15 K in aqueous solutions of glycylglycylglycine containing formamide, N-methylformamide, N, N-dimethylformamide and N, N-diethylformamide at concentrations of amides as co-solvents up to 0.4 mole fractions. The results obtained have been used to calculate the standard enthalpies of solution (Δ sol H°) and transfer (Δ tr H°) of the glycylglycylglycine from water into the mixtures as well as enthalpy coefficients of pair-wise interaction ( h xy ) of the solute with amide in aqueous media. The h xy values were correlated with the properties of organic solvents using Kamlet–Taft equation. The results of the calorimetric measurements have been discussed with regard to the intermolecular interactions occurring in these systems.

The thermodynamic properties of solutions of L-ascorbic acid in dimethyl sulfoxide and water-dimethyl sulfoxide mixtures

Solutions of L-ascorbic acid (AA) in dimethyl sulfoxide (DMSO) and DMSO-water mixtures were studied by the densitometry, surface tension, and calorimetry methods. The apparent and partial molar volumes of AA in solutions at 298.15 K were calculated. Surface tension insignificantly increased as the concentration of AA in DMSO grew. The enthalpies of solution of AA in the solvents and the enthalpies of transfer of AA from water into DMSO and DMSO-water mixed solvents were calculated. The results obtained were explained by the existence of H-bonds between AA and DMSO molecules.

Effects of Glycerol and Urea on Micellization, Membrane Partitioning and Solubilization by a Non-Ionic Surfactant

We have studied the effect of two cosolvents, urea and glycerol, on the association and interactions of a surfactant, octaethyleneglycol dodecyl ether (C12EO8) and a phospholipid (POPC). We have measured the CMC, the partition coefficient, the effective mole fractions Xesat and Xesol at the onset and completion of the membrane-to-micelle transition (membrane solubilization), and the enthalpies of transfer of surfactant by ITC. Changes in membrane order and dynamics were characterized by time-resolved fluorescence anisotropy measurements of DPH, and micelle sizes and clouding by light scattering. The cosolvents have complex effects that are not governed by the well-known ‘salting in’ or ‘salting out’ effects on the solubility alone. Instead, urea and glycerol alter also the intrinsic curvature (‘effective molecular shape’) of the detergent and the order and packing of the membrane. These curvature effects have an unusual enthalpy/entropy balance and are not additive for lipid and detergent. The results shed light on the phenomena governing lipid-detergent systems in general and have a number of implications for the use of cosolvents in membrane protein studies.

Relative Enthalpy of Formation for Co-Crystals of Small Organic Molecules

Enthalpy of formation values for 1:1 carbamazepine:saccharin (CBZ:SAC), cyheptamide:saccharin (CYH:SAC), and 10,11-dihydrocarbmazepine:saccharin (HCBZ:SAC) co-crystals and relative enthalpy differences between polymorphs of the co-crystal formers—CBZ(P21/c, P1̅), CYH (P21/c, P1̅), and HCBZ (P21/c, Pbca)—were calculated from enthalpy of solution measurements. The results showed that the co-crystals were enthalpically stabilized and the more stable polymorphs of the co-formers were CBZ-P21/c, CYH-P21/c, and HCBZ-Pbca. The differences in enthalpies between polymorphs were related to the differences in the packing of the molecules. Likewise, the heats of formation of the co-crystals were related to differences in molecular and crystal structures. The transfer enthalpies for the components were assessed and included in the enthalpy of formation calculation. The free energy of formation of CBZ:SAC calculated from the ternary CBZ–SAC–methanol phase diagram showed that the co-crystal was the stable thermodynamic form, and the kinetics of crystallization followed the thermodynamics of crystallization.

Density functional study on the bioactivity of ellagic acid, its derivatives and metabolite

The structural and electronic properties of ellagic acid (EA), its derivatives and metabolites are investigated at the density functional level. Analysis shows that the compounds display estrogenic activity due to the type and number of biophores in their molecular structures. This study explores bond dissociation enthalpy, adiabatic ionization potential, proton dissociation enthalpy, proton affinity, and electron transfer enthalpy in order to understand the antioxidant properties of EA. From the thermodynamic point of view, the transfer mechanism of the hydrogen atom is the most important in free radical scavenging progress. A comparison of BDE values for different types of antioxidants shows that EA and its derivatives can be classified as potent natural antioxidants.

THE EFFECT OF THE CHAIN LENGTH OF BONDED PHASE ON RETENTION OF AROMATIC SOLUTES IN RP LC WITH PURE WATER AS MOBILE PHASE

Retention factors (log kw ) in pure water were calculated for a homologous series of alkylbenzenes, PAHs, and aromatic monosubstituted derivatives for column C18, C8, and C4, using the numerical method based on the linear form of the Ościk's equation, which were correlated with the log kw values determined by the linear extrapolation method from the log k dependence on methanol volume fraction in binary eluent. The chromatographic parameters (log kw ), being the hydrophobicity measure, were compared with the experimental partition coefficients in n-octanol/water system (log P). The author examined the contribution of the substituents (‒CH2‒, ‒OH, ‒Cl, ‒NO2, and ‒CHO) in the substance retention in pure water for columns of different hydrocarbon chain length of the bonded phase. Moreover, the transfer thermodynamics of the particular functional groups from water to the stationary phase, as well as the influence of the kind of the stationary phase on the thermodynamics of this process were studied. For this purpose the standard free enthalpy (ΔG o) of the functional groups transfer from water to the stationary phase and from the gas to stationary one was calculated.

Enthalpies of solution of dl-α-alanyl- dl-α-asparagine in aqueous alcohols at 298.15 K

The enthalpies of solution of dl-α-alanyl- dl-α-asparagine (AlaAsn) were measured in aqueous methanol, ethanol, 1-propanol and 2 -propanol with an alcohol mole-fraction content x 2 (from 0 to 0.4) at 298.15 K. The experimental results were used to calculate the enthalpies of transfer of AlaAsn from water to these mixtures as well as the enthalpy coefficients of pair-wise interactions ( h xy ) between AlaAsn and alcohol molecules in water, according to the McMillan–Mayer's model. The h xy values were found to be positive and increasing in a series methanol < ethanol < 1 -propanol < 2 -propanol.

Micellar chromatographic determination of partition coefficients and associated thermodynamic data for pharmaceutical compounds

Micelle/water partition coefficients were obtained for procaine hydrochloride using micellar liquid chromatography (MLC) to illustrate the potential application of this technique to compounds of pharmaceutical interest. Experiments were conducted over a temperature range which led to calculation of a Gibbs free energy, enthalpy and entropy of transfer for procaine hydrochloride. Successful application of this technique was confirmed using a second compound over a range of temperatures, namely caffeine. Overall, this work confirms that MLC can be used to determine precise and accurate partition coefficients that possibly more closely mimic biological membranes than traditional in vitro systems, namely octanol/water.

Structure, electronic properties, and radical scavenging mechanisms of daidzein, genistein, formononetin, and biochanin A: A density functional study

The structural and electronic properties of daidzein, genistein, formononetin, biochanin A and their radicals were investigated at density functional theory (DFT) method employing B3LYP/6-31+G(d,p) and B3LYP/6-31++G(d,p) methods. This study has concerned the determination of the bond dissociation enthalpy (BDE), adiabatic ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA), electron transfer enthalpy (ETE) and relative energies for each O–H groups, highest occupied molecular orbital (HOMO) and Mulliken spin density are also presented. From the molecular structure and thermodynamic point of view, it was revealed that B-ring of isoflavonoids was the active center and the hydrogen atom transfer (HAT) appeared as a major mechanism in antioxidants action. Analysis of the calculated results suggests that the antioxidant activity is in good agreed with that obtained in TEAC assay values and in the following order: genistein > daidzein > biochanina A > formononetin.