Gibbs Energies Of Transfer Research Articles

Standard Gibbs energies of transfer of potassium tetraphenylborate from water to (water + t -butyl alcohol) mixtures and the primary medium effect

Standard Gibbs energies of transfer of potassium tetraphenylborate from water to (water +t -butyl alcohol) (TBA) mixtures at T= 298.15 K have been studied. The mole fractions of TBA in the mixed solvent are 0.00, 0.01, 0.02, 0.03, 0.04, 0.045, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.20, 0.40, 0.60, 0.80, and 1.00, respectively. The primary medium effect of potassium tetraphenylborate in the mixed solvent is discussed.

The Response Behavior of PAA–B12C4 Electrode to Li+, Mg2+, and Ca2+ in Some Aprotic Solvents and Its Thermodynamic Application

Abstract The potentiometric response behavior of an electrode, which consists of a non-plasticized carboxyl-modified polyacrylamide polymer (PAA) coupled to benzo-12-crown-4 (PAA–B12C4) to Li+, Mg2+, and Ca2+ in such aprotic solvents as acetonitrile (AN), propylene carbonate (PC), and γ-butyrolactone (GBL), were examined. The electrode showed a Nernstian response to Li+ and Ca2+ in AN, and Li+, Mg2+, and Ca2+ in PC and a near-Nernstian response to Mg2+ in GBL. From the response to Ca2+ it could be confirmed that the membrane potential was mainly established at the polymer membrane–solution interface. The electrode was applied to a study of their complexing with such other basic aprotic solvent molecules (D) as N,N-dimethylformamide, dimethyl sulfoxide, and hexamethylphosphoric triamide. The successive complex formation constants (βi) and Gibbs energies of transfer (ΔGtr(j)) of Li+ in AN and Mg2+ in PC in relation to such D were obtained.

SOLVENT EFFECTS ON CRYPTAND (222) COMPLEXATION

The formation of TI+,Na+ and K+ cryptates with the ligand (222) in 10 nonaqueous solvents has been investigated by cyclic voltammetry at 25°C. Based on the results obtained the dependencies of stability constants of resulting 1 : 1 complexes on Gutmann donor numbers (DN) of the solvents and on standard Gibbs transfer energies of the metal ions from water to given solvent have been analyzed. It was found that DN describe the medium effect far better than the corresponding ΔGt o values. It can be deduced, therefore, that the stability constant changes are essentially limited by the cation desolvation free energies, but the residual solvation of the cation encapsulated in the ligand cavity is also important.

Electron-Transfer Reactions in SDS Micelles: Reactivity of Pyrene and Tris(2,2‘-bipyridyl)ruthenium(II) Excited States Investigated by Time-Resolved Luminescence Quenching

The effects of micellization on the rate constants and Gibbs energy of electron transfer (ΔGet) are studied by time-resolved luminescence quenching of tris(2,2‘-bipyridyl)ruthenium(II) (RUBIPY) and pyrene (PY) by electron acceptors and donors in sodium dodecylsulfate (SDS) micelles. For RUBIPY, which is bound to the SDS micelles and accessible to water, ΔGet is considerably smaller than what is found in organic solutions, though the spectral properties of RUBIPY and the value of diffusion-controlled quenching rate constant are typical for the micellar interior. For this system, dissolution in micelles enables a change of the local reactant concentration and mobility, retaining their higher reactivity in aqueous solution. In contrast, for PY, which is known to be localized in the palisade layer of an SDS micelle, the quenching rate constants coincide well with those found in acetonitrile.

Gibbs energy of transfer and conductivity properties of NaBr solutions in mixtures of water with propan-1-ol at 298.15 K

Abstract The electrical conductivity of NaBr solutions and the emf of the cell: glass electrode (Na)/vbNaBr/vbAgBr, Ag in the mixtures of water with propan-l-ol in the range 0–80 mol% have been measured at 298.15 K. The values of limiting molar conductance (Λ0), association constants (KA), and parameter of distance of ions (R) were obtained with the Fuoss-Justice equation. From the conductivity data obtained, the values of the Walden products (Λ0η) were also calculated. The values of the Gibbs energy of transfer (Δ1G0) of NaBr from water to mixtures of water with propan-l-ol mixtures were obtained from the emf data. The dependencies of limiting molar conductance (Λ0), Walden products (Λ0η), association constants (KA) and Gibbs energy of transfer (Δ1G0) of NaBr versus the mixed solvent composition have been discussed. The obtained results have been compared with earlier findings for sodium chloride and iodide.

Preferential Solvation of Silver (I) Cryptand 2,2,2 Perchlorate Complex in Water-Pyridine and Water-Dmso Mixtures

The preferential solvation of silver (I) cryptand 2,2,2 perchlorate complex has been studied in water-dimethylsulphoxide and water-pyridine mixtures by Gibbs transfer energy and solvent transport number measurements. While the Gibbs transfer energy of silver (I) cryptand 2,2,2 cation continuously decreases, that of the perchlorate ion increases with the addition of organic component in both solvent mixtures. the solvent transport number of the salt passes through a maximum, ΔPy = 10.4 and ΔDMSO = 2.5 at X Py = 0.35 and XΔDMSO = 0.45 respectively. These results indicate a heteroselective solvation of the salt with the complex cation being selectively solvated by the organic component and the perchlorate ion by water.

Ionic partition diagrams of ionisable drugs: pH-lipophilicity profiles, transfer mechanisms and charge effects on solvation

The transfer of a series of weak acids, bases and ampholytes has been studied by cyclic voltammetry at the water ∣ 1,2-dichloroethane interface in order to determine their lipophilicity. Physicochemical parameters such as the standard Gibbs energy of transfer and the standard partition coefficients of the ionised forms I of these compounds (log P 0′ I ) were measured as a function of aqueous pH, and the dissociation constants in the organic phase were deduced from these data. The results obtained are presented in the form of ionic partition diagrams which define the domains of predominance of all the species present in both phases. This representation affords both a precise description of the mechanisms governing the transfer from one phase to the other, and a reliable assessment of pH lipophilicity profiles. The various partition coefficients are compared to evaluate qualitatively the lipophilicity of each species, and the differences between the partition coefficients of the charged and the neutral form of a compound are interpreted in terms of their chemical structure, showing an increase in organic phase affinity for ions with intramolecular charge stabilisation. This study demonstrates also that both charged and neutral species can penetrate into the organic phase and shows the importance of the passive transfer of organic ions for drug disposition and pharmacokinetics.

Aqueous Solubilities, Infinite Dilution Activity Coefficients and Octanol-Water Partition Coefficients of Tricyclic Analogs of Acyclovir*

Solubilities of tricyclic analogs of acyclovir have been determined in water at 25, 35, and 45°C and in octanol, water-saturated octanol, and octanol-saturated water at 25°C. Octanol-water partition coefficients were determined at 25°C. Melting temperatures and molar enthalpies of fusion were measured. Activity coefficients in water, octanol, and in aqueous octanol solutions were determined and are discussed. The effect of hydrophilic and hydrophobic substituents in the tricyclic analogs on their thermodynamic properties are discussed. The standard Gibbs energy of transfer between the saturated phases were found to correlate with known values of the melting point of the solvents and the solubilities of the solute. For a number of the compounds examined, correlations between the minimum inhibitory concentration against the herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2), varicella-zoster virus (VZV), thymidine kinase-deficient (TK−) strains of VZV and \(\Delta G_{{\text{W}} \to {\text{0}}}^0 ;\Delta G_{{\text{tr}}}^0\) were established. Detailed conclusions have been derived concerning the relationships between the structure and the thermodynamic parameters of the compounds examined.

STABILITY OF 15-CROWN-5 AND BENZO-15-CROWN-5 COMPLEXES WITH ALKALI METAL CATIONS IN NONAQUEOUS MEDIA

The formation of alkali metal cation complexes with 15-crown-5 and benzo-15-crown-5 in ten organic solvents has been investigated by cyclic voltammetry at 25°C. Stability constants of the resulting 1 : 1 complexes have been determined by monitoring the shift in reduction potentials of the cations against ligand concentration. Based on the results obtained the dependences of stabilities of the complexes on Gutmann donicity of the solvents and on standard Gibbs transfer energies of the metal ion from water to a given solvent have been analyzed and discussed.

SOLVENT EFFECTS ON THE AQUATION OF CATIONIC, ANIONIC AND NONIONIC CHLOROCOBALT(III) COMPLEXES

The kinetics of aquation of α- and β- cis[Co(edda)Cl2]−, α- cis[Co(edda)(H2O)Cl] and α- cis[Co(trien)Cl2]+ were investigated in aqueous mixtures of t-BuOH, MeOH, DMSO, ethylene carbonate and urea. The transfer Gibbs energy values for the reactants, evaluated from the solubility of the complexes, confirm the decisive role of the reactant solvation on the aquation rate.

Effects of the Hydrophobicity of the Reactants on Diels−Alder Reactions in Water

To assess the importance of the hydrophobicity of different parts of diene and dienophile on the aqueous acceleration of Diels-Alder reactions, second-order rate constants have been determined for the reactions of cyclopentadiene (1), 2,3-dimethyl-1,3-butadiene (4), and 1,3-cyclohexadiene (6) with N-methyl-, N-ethyl-, N-propyl-, and N-butylmaleimide (2a-d) in different solvents. All these reactions are accelerated in water relative to organic solvents as a result of enhanced hydrogen bonding and enforced hydrophobic interactions during the activation process. The beneficial influence of water as compared to 1-propanol on the rate of the Diels-Alder reaction of 4 with 2a-d increases linearly with the length of the alkyl chain of 2. In contrast, for the reaction of both 1 and 6 with 2a-d, no such effect was observed. This difference can be explained by a hydrophobic interaction between the methyl groups of 4 and the N-alkyl group of 2 during the activation process. In the reactions of 1 and 6, lacking the methyl substituents, this interaction is not possible and elongation of the alkyl chain from ethyl onward does not result in an additional acceleration by water. The enhanced hydrophobicity near the reaction center of dienes 4 and 6 compared to 1 results in an increased aqueous acceleration of the Diels-Alder reactions of the former dienes with 2a. These data indicate that an increase in the hydrophobicity close to the reaction center in the diene has a much more pronounced effect on the rate acceleration in water than a comparable increase in hydrophobicity in the dienophile further away from the reaction center. The Gibbs energies of transfer of initial state and activated complex of the Diels-Alder reactions under study have been determined. As expected, for all reactions the initial state in water is destabilized compared to that in 1-propanol. This destabilization becomes more pronounced when the nonpolar character of diene (close to the reaction center) or dienophile (distant from the reaction center) is increased. Likewise, an increase in the nonpolar character of 2 results in a destabilization of the activated complex. In contrast, addition of methyl or methylene units to the diene is not accompanied by a significant destabilization of the activated complex in water as compared to 1-propanol. We conclude that hydrophobic groups near the reaction center seem to lose their hydrophobic character completely in the activated complex of the Diels-Alder reaction, whereas more distant groups retain their nonpolar character throughout the reaction.

Ethanol-induced conformational transitions in holo-alpha-lactalbumin: spectral and calorimetric studies.

Conformational transitions of holo-alpha-lactalbumin in a hydro-ethanolic cosolvent system was studied by spectrofluorescence, CD in near- and far-uv regions, and high-sensitivity differential scanning calorimetry. Experimental results allow us to propose that in isothermal conditions alpha-lactalbumin undergoes a number of conformational transitions with increasing ethanol concentration: N<=>I<=>D<=>H. The existence of I-state was deduced from spectrofluorometric and near-uv CD data. In this state the aromatic chromophores of the amino acid side chains are more accessible to the solvent displaying higher local mobility. The H-state was detected from far-uv CD spectra as a state corresponding to the content of alpha-helices higher than originally found in native protein. However, calorimetric measurements provide data revealing only the two-state mechanism of alpha-lactalbumin unfolding in both water and in aqueous ethanol solutions. This indicates that the energy levels of N- and I-states as well as of D- and H-states are similar. Thermodynamics of the unfolding of alpha-lactalbumin in hydroethanolic solutions was analyzed with the help of the linear model of solvent denaturation. Unfolding increments of enthalpy, entropy, and Gibbs energy of transfer of the protein from a reference aqueous solution to hydro-ethanolic solutions of different concentrations were determined from the calorimetric data. They are linear functions of molar ethanol fraction. The slope of the unfolding increment of Gibbs energy of transfer was calculated from data on transfer of amino acid residues taking into account the average solvent accessibility of amino acid residues in the native structure of small globular proteins, using the additive group contribution method.

Single-ion transfer properties: a measure of ion-solvation in solvents and solvent mixtures

Single-ion transfer properties such as Gibbs energies, entropies and enthalpies may be successfully employed to account for the interactions of ions with solvents. Several electrochemical methods such as polarography or potentiometry in combination with extrathermodynamic assumptions, e.g. the assumption of a reference redox system, the assumption of a negligible liquid junction potential or the assumption of a negligible thermal diffusion potential permit the estimation of single-ion transfer properties. These data are a basis for the understanding of the interactions of ions with solvents and solvent mixtures. Gibbs energies of transfer may be used to learn about the nature of the chemical bond between ions and solvent molecules and to quantize the donor and the acceptor properties of solvents. Entropies of transfer ( TΔ t S 0-values) yield information of the effects of the solvated ions on the surrounding solvent matrix. The contributions from the Gibbs energies and the TΔ t S 0-values allow to perceive the enthalpies of transfer. Single-ion transfer properties into solvent mixtures show the effects of preferential solvation.

Simple kinetic models of ion transfer across an interface between two immiscible electrolyte solutions.: A comparison with experimental data

The Nernst–Planck equation is used to examine the kinetic behavior of three simple models of ion transfer, which all assume that the potential of mean force acting on the ion can be obtained as a superposition of the solvation potential and the electrical double layer potential. A comparison is made with the experimental kinetic data for 21 ion transfer reactions at the water/nitrobenzene interface. It is shown that only the models which account in an explicit way for the effect of the electrical double layer can provide a plausible description of experimental behavior. In particular, the absence of a correlation between the apparent standard rate constant or the apparent charge transfer coefficient and the standard Gibbs energy of ion transfer across the water/nitrobenzene interface can be due to the mutual compensation of the effect of electrical double layer and the effect of the difference in solvation energy on the ion transfer rate. It is concluded that a double layer correction is necessary to obtain true kinetic parameters of ion transfer across the interface.

Osmolyte-driven contraction of a random coil protein.

The Stokes radius characteristics of reduced and carboxamidated ribonuclease A (RCAM RNase) were determined for transfer of this "random coil" protein from water to 1 M concentrations of the naturally occurring protecting osmolytes trimethylamine N-oxide, sarcosine, sucrose, and proline and the nonprotecting osmolyte urea. The denatured ensemble of RCAM RNase expands in urea and contracts in protecting osmolytes to extents proportional to the transfer Gibbs energy of the protein from water to osmolyte. This proportionality suggests that the sum of the transfer Gibbs energies of individual parts of the protein is responsible for the dimensional changes in the denatured ensemble. The dominant term in the transfer Gibbs energy of RCAM RNase from water to protecting osmolytes is the unfavorable interaction of the osmolyte with the peptide backbone, whereas the favorable interaction of urea with the backbone dominates in RCAM RNase transfer to urea. The side chains collectively favor transfer to the osmolytes, with some protecting osmolytes solubilizing hydrophobic side chains as well as urea does, a result suggesting there is nothing special about the ability of urea to solubilize hydrophobic groups. Protecting osmolytes stabilize proteins by raising the chemical potential of the denatured ensemble, and the uniform thermodynamic force acting on the peptide backbone causes the collateral effect of contracting the denatured ensemble. The contraction decreases the conformational entropy of the denatured state while increasing the density of hydrophobic groups, two effects that also contribute to the ability of protecting osmolytes to force proteins to fold.

Amperometric detection of alkali metal ions on micro-fabricated composite polymer membranes

Transfer reactions of alkali metal ions, either direct or facilitated by the ionophores dibenzo-18-crown-6 and valinomycin, have been studied at the water∣2-nitrophenyloctylether-polyvinyl chloride gel micro-interface. The polarised micro-hole array interface between water and the composite polymer membrane is used as a selective transducer for the amperometric detection of alkali metal ions. The formal Gibbs transfer energies of ion transfer from the aqueous phase to the 2-nitrophenyloctylether-polyvinyl chloride gel phase are evaluated from voltammetric measurements. The association constants are calculated to determine the selectivity imparted by the ionophores such as dibenzo-18-crown-6 and valinomycin on transferring the alkali metal ion at the gel micro-interface.

Conductivity properties and potentiometric studies of NaBr solutions in mixtures of water with propan-2-ol at 298.15 K

Abstract The electrical conductivity of NaBr solutions and the emf of the cell: glass electrode (Na)|NaBr|AgBr|Ag in mixtures of water with propan-2-ol in the range 0–80 wt.% have been measured at 298.15 K. The values of limiting molar conductance (Λ0), association constants (KA), and the parameter of distance of ions (R) were obtained with the Fuoss–Justice equation. From the conductivity data obtained, the values of the Walden products (Λ0η) were also calculated. The values of the Gibbs energy of transfer (ΔtG0) of NaBr from the water to the mixtures of water with propan-2-ol mixtures were obtained from the emf data. The dependencies of viscosity (η), limiting molar conductance (Λ0), Walden products (Λ0η), association constants (KA) and the Gibbs energy of transfer (ΔtG0) of NaBr versus the mixed solvent composition have been discussed. The obtained results were compared with those obtained earlier for sodium chloride and iodide.

Facilitated alkali ion transfer at the water|1,2-dichloroethane interphase: ab initio calculations concerning alkaline metal cation–1,10-phenanthroline complexes

Abstract A series of calculations on the energetics of complexation of alkaline metal cations with 1,10-phenanthroline are presented. It is an experimental fact that the ordering of the Gibbs energy of transfer across the water|1,2-dichloroethane interphase is governed by the charge/size ratio of the different cations; the larger cations showing the lower Gibbs energy of transfer. This ordering of the Gibbs energies of transfer is reverted in the presence of 1,10-phenanthroline in the organic phase. We have devised a thermodynamic cycle for the transfer process and, by means of ab initio calculations, have drawn the conclusion that in the presence of phenanthroline the Gibbs energy of transfer is governed by the stability of the phenanthroline/M+ complex, which explains the observed tendency from a theoretical point of view.

Standard Gibbs energies of transfer of rubidium tetraphenylborate from water to straight‐chain n‐alkanols

AbstractSolubilities of ribidium tetraphenylborate in water, methanol, ethanol, 1‐propanol, 1‐butanol, 1‐pentanol, 1‐hexanol and l‐octanol at 288.15, 298.15, 308.15, 318.15, 328.15 K have been determined by spectrophotometry. The standard Gibbs energy of transfer of ribidium tetraphenylborate from water to straight‐chain n‐alkanols and the primary medium effect of rubidium tetraphenylborate from 288.15 to 328.15 K have been calculated. Furthermore, the contribution of microscopic interactions to the standard Gibbs energy of transfer of rubidium tetraphenylborate in calculated and discussed.

Non-Bornian Theory of the Gibbs Energy of Ion Transfer between Two Immiscible Liquids

A non-Bornian theory has been proposed for the standard Gibbs free energy of ion transfer at the organic solvent (O)/water (W) interface. In this theory the ionic charge (z)-dependent term of , which has so far been considered as describing electrostatic (i.e., long-range) ion−solvent interactions, is treated as describing the specific (short-range) interactions. A quantum chemical consideration has shown that the short-range interaction energy can be given by a quadratic function of the surface field strength (E) of an ion. In the present theory it should also be noted that a hydrophilic ion, being preferentially hydrated in the O phase, is assumed to transfer across the interface as the hydrated ion. The new, non-Bornian theory has been applied to 34 ions in the nitrobenzene (NB)/W system. In the data analyses the ions have been classified into five groups, including hydrated cations, nonhydrated cations, hydrated anions, nonhydrated anions, and polyoxometalate anions (nonhydrated), where “hydrated” or “nonhydrated” means that the ion is associated with some water molecules in NB or not. Regression analyses have shown that in accord with the theory, the z-dependent term of can be given by a quadratic function of E for each group of ions. Furthermore, the present theory has been found to be useful for the prediction of for some hydrophilic ions.