Ionic Solvation in Aqueous and Nonaqueous Solutions

 The history of studies on ionic solvation is briefly reviewed, and structural and dynamic properties of solvated ions in aqueous and nonaqueous solutions are discussed. An emphasis is placed on ionic solvation in nonaqueous mixed solvents in which preferential solvation of ions takes place. A new parameter for expressing the degree of preferential solvation of an ion is proposed.     

Study on Preferential Solvation of Water by Electrochemical Method

The preferential solvation of water plays an important role in ferrocene research which is a subject of current interest. Voltammetric investigations were carried out for Au electrode in acetonitrile/water, showing preferential solvation of water. In our work, the preferential solvation of water in acetonitrile/water was studied by electrochemical methods including cyclic volitammetry, electrochemical impedance spectra and double‐step chronoamperometry. Ferrocenemethanol (FcCH₂OH) molecules as a solute spontaneously adsorb on the electrode surface in anhydrous acetonitrile, resulting from acetonitrile molecules tend to form an acetonitrile solvent layer on the surface of the electrode and acetonitrile solvent layer has a lower energy barrier than the aqueous solvent layer, which has been obtained by modeling solvation. The solvent strongly influences electrochemical behavior of solute. Once there is an amount of water in acetonitrile solvent, FcCH₂OH that adsorbed on the electrode surface desorb. This is because water preferentially solvate with FcCH₂OH in term of intermolecular forces between solvent and solute. Moreover, hydrogen bond between water molecules and FcCH₂OH molecules is stronger than dipole‐dipole interaction between acetonitrile molecules and FcCH₂OH molecules in solvation effect. Through electrochemical behavior of FcCH₂OH changing, preferential solvation of water is analyzed by electrochemical methods.     

The Standard Partial Molar Volumes of Ions in Solution. Part 3. Volumes in Solvent Mixtures Where Preferential Solvation Takes Place

Standard partial molar volumes of 1:1 salts in aqueous mixtures of ethanol (EtOH), dimethyl sulfoxide (DMSO) and acetonitrile (MeCN) at 298.15 K were obtained from the literature. In such mixtures there is evidence that preferential solvation occurs in the solvent shell around the ion where electrostriction takes place. Specifically, the anions are better solvated by the water whereas the cations are generally solvated by both the water and the nonaqueous component of the mixtures to various extents. There are no clear-cut criteria for how the measured volumes are to be apportioned between the ions in such mixtures. Various solvation models were used to estimate the volumes of the salts by calculation of the electrostriction around the ions. Only the taking into account of the preferential solvation of the ions in the solvation shell yielded calculated results of the standard partial molar volumes of the salts in agreement with the experimental data.     

The effect of properties of water-organic solvent mixtures on the solvation enthalpy of 12-crown-4, 15-crown-5, 18-crown-6 and benzo-15-crown-5 ethers at 298.15 K

Crown ethers are preferential solvated by organic solvents in the mixtures of water with formamide, N-methylformamide, acetonitrile, acetone and propan-1-ol. In these mixed solvents the energetic effect of the preferential solvation depends quantitatively on the structural and energetic properties of mixtures. The energetic properties of the mixtures of water with hydrophobic solvents (N,N-dimethylformamide, dimethylsulfoxide, N,N-dimethylacetamide, hexamethylphosphortriamide) counteract the preferential solvation of the crown ether molecules. The effect of the hydrophobic and acid-base properties of the mixture of water with organic solvent on the solvation of 12-crown-4, 15-crown-5, 18-crown-6 and benzo-15-crown-5 ethers was discussed. The solvation enthalpy of one -CH₂CH₂O- group in water, N,N-dimethylformamide and hexamethylphosphortriamide is equal to −24.21, −16.04 and −15.91 kJ/mol, respectively. The condensed benzene ring with 15-crown-5 ether molecule brings about an increase in the exothermic effect of solvation of the crown ether in the mixtures of water with organic solvent.     

Preferential Solvation in Mixed Solvents X. Completely Miscible Aqueous Co-Solvent Binary Mixtures at 298.15 K

Summary.  The Kirkwood-Buff integrals for 18 completely miscible aqueous co-solvent binary mixtures have been recalculated from thermodynamic data, and the volume-corrected preferential solvation parameters derived from them are presented. Also presented are these latter quantities for 15 additional such mixtures, for which the volume correction has not been applied previously. The self-interaction of the water, the mutual interaction of the water and the co-solvent, and the self-interaction of the co-solvent at infinite dilution derived from these integrals and parameters are then discussed. The systems studied include aqueous hydrogen peroxide, methanol, ethanol, 1- and 2-propanol, 2-methyl-2-propanol, 2,2,2-trifluoroethanol, 1,1,1,3,3,3-hexafluoro-2-propanol, ethane-1,2-diol, glycerol, 2-methoxyethanol (at 313 and 343 K), 2-ethoxyethanol, 2-butoxyethanol, 2-aminoethanol, N-methyl- and N,N-dimethyl-2-aminoethanol, tetrahydrofuran, 1,4-dioxane, acetone, formic, acetic, and propanoic acids, piperidine, pyridine, acetonitrile, formamide, N-methyl- and N,N-dimethylformamide, N-methylacetamide, N-methylpyrrolidin-2-one (at 303 K), hexamethyl phosphoric triamide, dimethylsulfoxide, and tetramethylenesulfone (at 303 K). Received January 10, 2001. Accepted (revised) February 20, 2001     

Solvatochromic Parameters and Preferential Solvation Behavior for Binary Mixtures of 1,3-Dialkylimidazolium Ionic Liquids with Water

Binary mixtures of 1,3-dialkylimidazolium based ionic liquids (ILs) and water were selected as solvent systems to investigate the solute-solvent and solvent-solvent interactions on the preferential solvation of solvatochromic indicators at 25℃. Empirical solvatochromic parameters, dipolarity/polarizability (π^*), hydrogen-bond donor acidity (α), hydrogen-bond acceptor basicity (β), and Reichardt's polarity parameters (E_T^N) were measured from the ultraviolet-visible spectral shifts of 4-nitroaniline, 4-nitroanisole, and Reichardt's dye. The solvent properties of the IL-water mixtures were found to be influenced by IL type and IL mole fraction (x_IL). All these studied systems showed the non-ideal behavior. The maximum deviation to ideality for the solvatochromic parameters can be obtained in the xIL range from 0.1 to 0.3. For most of the binary mixtures, the π^* values showed the synergistic effects instead of the E_T^N, α and β values. The observed synergy extent was dependent on the studied systems, such as the dye indicator and IL type. A preferential solvation model was utilized to gather information on the molecular interactions in the mixtures. The dye indicator was preferentially solvated on the following trend: IL >IL-water complex >water.     

On-line preferential solvation studies of polymers by coupled chromatographic-Fourier transform infrared spectroscopic flow-cell technique

Qualitative and quantitative comparison between liquid chromatography (LC) and LC coupled with Fourier transform infrared spectroscopy (LC-FTIR) to evaluate preferential solvation phenomenon of polymers in a mixed solvent has been performed. These studies show that LC-FTIR technique leads to detailed structural information without the requirement for determination of additional parameters for quantitative analysis except calibration. Appropriate experimental conditions for preferential solvation study have been established by variation of polymer concentration, molar mass and eluent content.     

Preferential Solvation of Acetic Acid in Binary Mixtures of Carbon Tetrachloride–Cyclohexane and Carbon Tetrachloride–1,2-Dichloroethane

The partition of acetic acid between aqueous solutions and various binary mixtures of carbon tetrachloride + cyclohexane and carbon tetrachloride + 1,2-dichloroethane were carried out at 30.0 °C. The nonlinear dependence of both the monomer partition coefficient and the dimerization constant on the mole fraction composition has been rationalized in terms of preferential solvation of the various solvation sites of the involved species. Two and four polar sites were identified for the monomer and dimer forms, respectively. The solvation preference is controlled by the relative polarities of the binary solvent components, being, in general, higher for the monomer sites compared to the dimer ones. The dimer is almost equally stable in various carbon tetrachloride + 1,2-dichloroethane mixtures, and the variation of dimerization constants with changing binary composition arises mainly from the monomer solvation stabilization.     

Solution viscosity behavior of polystyrene-based cationic ionomers: Effects of quaternary-group structure and counter ion

The dilute solution viscosity was investigated for several polystyrene-based cationic io-nomers. It was found that intramolecular aggregation among the ionic groups was strongly dependent on the sizes of quaternary onium groups and counter anions. The extent of the aggregation was controled by the solvent polarity and the solvation to the ionic groups. When there was a strong selective soivation to small counter ions, the structure of onium groups shows a minor effect on the viscosity behavior, indicating little aggregation among the ionic groups. A strong solvation to small quaternary cations also eliminates the intra-molecular aggregation and the influence of counter ions was barely observable. When the selective solvation to counter ions was disabled by the enlarged size of the counter ions, however, the viscosity depended on onium group structure or spacer chain length. If the solvent solvates neither counter ions nor quaternary groups, the smaller the sizes of onium cation and counter anion, the lower the reduced viscosity due to an enhancement of the ionic aggregate formation. © 1995 John Wiley & Sons, Inc.     

Study of Solvent Effects on the Protonation of Functional Group of Disubstituted Anilines: Factor Analysis Applied to the Correlation between Protonation Constants and Solvatochromic Parameters in Ethanol–Water Mixtures

Summary. The stoichiometric protonation constants (log β) of some disubstituted aniline derivatives in ethanol–water mixtures (0–90% ethanol by volume) at 25.0 ± 0.1°C were firstly submitted to factor analysis in order to obtain the number factors which affect the variation of the whole data sets and, afterwards, submitted to target factor analysis to identify these factors. The influence of solvatochromic parameters in the interactions between aniline derivatives and the solvent studied was identified and quantified. The general equation of Kamlet and Taft was reduced for these mixtures to two terms using combined factor analysis (FA) and target factor analysis (TFA): the independent term and the hydrogen-bond donating ability, α (HBD), solvatochromic parameters. Further, the quasi-lattice quasi-chemical (QLQC) theory of preferential solvation has been applied to quantify the preferential solvation by water of electrolytes in ethanol–water mixtures. The effects of the substituents on the protonation constants, the additivities of these effects, and the applicability of the Hammett equation to the behavior of substituents are also discussed. Further, Hammett’s reaction constant for the protonation of disubstituted anilines has been obtained for all the solvent mixtures and correlates well with α (HBD) of the solvent.     

The pH-dependence of preferential solvation as studied by intermolecular homo- and heteronuclear NOE measurements of adenosine in water–trifluoroethanol mixtures

A study of the state of solvation of the adenine ring in adenosine and adenosine 5-monophosphate disodium salt in water and in a (7.5:2.5) water–TFE mixture has been carried out by measurement of homo- and heteronuclear intermolecular NOE enhancements between water or TFE and the aromatic protons of these compounds. The results give evidence of site specificity in solute–solvent interaction for both solvent systems and preferential solvation of the solute by TFE in the water–TFE mixture. Significant pH dependence of these interactions has been discovered.     

Application of Covington's nonstatistical distribution of solvate species theory to the preferential solvation of Tl+ in various binary solvent systems

Covington's nonstatistical distribution of solvate species theory of preferential solvation is used to analyze ²⁰⁵ Tl chemical shift data for 0.005M TlNO ₃ in nine binary solvent systems. The theory quite adequately fits the experimental data, and equilibrium constants and free energies of preferential solvation are obtained. The effect of choosing different values for the solvation number is investigated and found to be slight except for systems involving DMSO, ³ where decreasing the solvation number from 6 to 3 improves the fit to experimental data.     

Enthalpies of Transfer of Tetraalkylammonium Bromides and CsBr from water to Aqueous DMF at 298.15 K

Enthalpies of transfer of tetraalkylammonium bromides and CsBr from water to aqueous DMF mixtures are reported and analyzed in terms of a new solvation theory. It was found that a previous equation could not reproduce these data over the whole range of solvent compositions. Using a new solvation theory to model the enthalpies of transfer shows excellent agreement between experimental and calculated values over the entire range of solvent compositions. The analyses show that tetrapropylammonium bromide, Pr₄NBr, and tetrapentylammonium bromide, Pen₄NBr, are preferentially solvated by water; in contrast tetrabutylammonium bromide, Bu₄NBr, is preferentially solvated by DMF. The solvation of tetramethylammonium bromide, Me₄NBr, and cesium bromide, CsBr, is random. The extent to which the tetraalkylammonium bromides disrupt solvent–solvent bonds increases systematically in going from Me₄NBr to Pen₄NBr.     

Solute-Solvent Interaction Effects on Protonation Equilibrium of Substituted N-Benzylidene-2-hydroxyanilines in Aqueous Ethanol: The Application of Factor Analysis to Solvatochromic Parameters and Protonation Equilibria

Summary. In this study, the stoichiometric protonation constants, logK_OH and logK_NH, of sixteen substituted N-benzylidene-2-hydroxyanilines have been determined potentiometrically in ethanol-water mixtures of varying composition (10–80% ethanol by volume) at 25.0±0.1°C. The values of the constants, logK, were submitted to factor analysis in order to obtain the number of factors which affect the variation of the whole data sets of protonation constants and, afterwards, to target factor analysis to identify these factors. The influence of solvatochromic parameters in the interactions between Schiff bases derivatives and the solvent studied was identified and quantified. Kamlet and Taft general equations allow calculation of the logK values of Schiff bases studied in any ethanol-water mixtures up to 80% (v/v) and thus provide the knowledge of the acid-base behaviour in these solvent media. Further, the quasi-lattice quasi-chemical (QLQC) theory of preferential solvation has been applied to quantify the preferential solvation by water of electrolytes in ethanol-water mixtures.     

Solvation of Cobalt(II) Ion in N,N-Dimethylformamide–Methanol Mixed Solvent: Calorimetry and VIS Spectroscopy Studies

The enthalpies of solution of Co(II) and Na(I) trifluoromethanesulfonates (triflates) in N,N-dimethylformamide (DMF)–methanol (MeOH) mixtures have been measured over the whole range of solvent composition. From these data the enthalpies of transfer of Co(II) and triflate ions from methanol to the mixed solvent have been determined usingliterature values of the enthalpies of transfer of the Na⁺ ion. The results have been analyzed by means of the theory of preferential solvation. The analysis revealed the preference of DMF for solvating the Co(II) ion in the MeOH-rich region of solvent composition and the lack of preference of any component in the DMF-rich region. Visible absorption spectra of the Co(II) ion in DMF–MeOH mixtures have been also measured in the whole range of solvent composition and analyzed using the partial least-squares method. The mean composition of the solvation sphere of the Co(II) ion versus solvent composition has been determined on the basis of both analyses. The results were found to be consistent with each other and with those obtained previously from FT-IR spectra.     

Ionic and dipolar solvation dynamics in liquid water

The solvation time correlation function for solvation in liquid water was measured recently. The solvation was found to be very fast, with a time constant equal to 55 fs. In this article we present theoretical studies on solvation dynamics of ionic and dipolar solutes in liquid water, based on the molecular hydrodynamic approach developed earlier. The molecular hydrodynamic theory can successfully predict the ultrafast dynamics of solvation in liquid water as observed from recent experiments. The present study also reveals some interesting aspects of dipolar solvation dynamics, which differs significantly from that of ionic solvation. Dedicated to Prof. C N R Rao on his 60th birthday     

Ionic components of the real and chemical Gibbs energy of transport of potassium and chloride ions from water to water-methanol mixtures

With the method of volta-potential differences at 298.15 K the ionic components of the standard real and chemical thermodynamic properties of the resolvation of potassium and chloride ions were determined in mixtures of water and methanol (MeOH). The value of the surface potential at the methanol/gas phase boundary (Δχ^MeOH = ?0.18 V) is obtained. The characteristics of the solvation of these ions in a water-methanol medium are identified, and a comparison with literature data for other nonaqueous mixtures is carried out.     

Preferential solvation effects in the electrochemistry and charge-transfer spectra of cyanoiron(II) complexes

The energies of the charge-transfer bands and the redox potentials of substituted cyanoiron complexes are strongly influenced by preferential solvation effects in water-acetonitrile mixed solvents, exhibiting a linear dependence with respect to the acceptor number scale. The dependence increases with the number of cyanide ligands in the complexes.     

Preferential solvation in three component systems: Evaluation of Kirkwood-Buff parameters

The extent of local excess or deficiency of a component solvent near the solute in a mixed binary solvent has been calculated using the Hall formalism for the Kirkwood-Buff equation. The possibility of calculation of the two solute-solvent Kirkwood-Buff parameters using the values is discussed. A model calculation using literature data for preferential solvation in mixed binary solvents is presented. The solute-solvent and solvent-solvent interactions and the relative size of the solvents are also shown to be relevant factors in determining the values.     

Thermochemical behaviour of crown ethers in the mixtures of water with organic solvents: Part VII. Enthalpy of solution of 15-crown-5 and benzo-15-crown-5 in the mixtures of water with acetone at 298.15 K

Enthalpy of solution of crown ethers (15-crown-5 and benzo-15-crown-5) in water-acetone mixtures have been measured within the whole range of mole fraction at 298.15 K. The obtained data have been compared with those of the solution enthalpy of both crown ethers in the mixtures of water with dimethyl sulfoxide. The replacement of SO group with CO in the molecule of the organic solvent brings about an increase in the exothermic effect of the solution of 15-crown-5 and benzo-15-crown-5 ethers, especially in the mixtures with a medium water content. The observed effect is connected with the preferential solvation of the molecules of both crown ethers by acetone molecules in the water-acetone mixtures. The process of preferential solvation of 15-crown-5 and benzo-15-crown-5 ethers does not take place in the water-dimethyl sulfoxide mixture.