Heats of Mixing of Aqueous Solutions of Alkali Metal Salts of Substituted Benzenesulfonic Acids

The enthalpy change on mixing aqueous solutions of substituted benzene sulfonic acids and their salts, with salts having a common cation or anion, were measured at constant total ionic strength and at 25°C. The results are qualitatively interpreted in terms of solute–water structural properties and the ion size effect. The heat effects of mixing solutions having common anions obey the sign rule of Young and Smith. In anion–common cation mixings at concentration of 0.5 mol-kg^–1 only exothermic heat effects were observed, whose magnitude increase with the increasing difference in size of the mixed anions. The magnitude of the mixing effect increased with the salt concentration in cation–common anion mixing processes, In anion–common cation mixings the enthalpy of mixing changes sign from negative to positive, indicating a predominantly endothermic effect as concentration increases.     

How polar are ionic liquids? Determination of the static dielectric constant of an imidazolium-based ionic liquid by microwave dielectric spectroscopy

In a pilot study of the dielectric constant of room-temperature ionic liquids, we use dielectric spectroscopy in the megahertz/gigahertz regime to determine the complex dielectric function of five 1-alkyl-3-methylimidazolium salts, from which the static dielectric constant epsilon is obtained by zero-frequency extrapolation. The results classify the salts as moderately polar solvents. The observed epsilon-values at 298.15 K fall between 15.2 and 8.8, and epsilon decreases with increasing chain length of the alkyl residue of the cation. The anion sequence is trifluoromethylsulfonate > tetrafluoroborate approximately tetrafluorophosphate. The results indicate markedly lower polarities than found by spectroscopy with polarity-sensitive solvatochromic dyes.     

Activity Coefficient Prediction for Binary and Ternary Aqueous Electrolyte Solutions at Different Temperatures and Concentrations

The mean spherical approximation (MSA) model, coupled with two hard sphere models, was used to predict the activity coefficients of mixtures of electrolyte solutions at different temperatures and concentrations. The models, namely the Ghotbi-Vera-MSA (GV-MSA) and Mansoori et al.-MSA (BMCSL-MSA), were directly used without introducing any new adjustable parameters for mixing of electrolyte solutions. In the correlation step, the anion diameters were considered to be constant, whereas the cation diameters were considered to be concentration dependent. The adjustable parameters were determined by fitting the models to the experimental mean ionic activity coefficients for single aqueous electrolytes at fixed temperature. The results showed that the studied models predict accurately the activity coefficients for single electrolyte aqueous solutions at different temperatures. In the systems of binary aqueous electrolyte solutions with a common anion, the GV-MSA model has slightly better accuracy in predicting the activity coefficients. Also, it was observed that the GV-MSA model can more accurately predict the activity coefficients for ternary electrolyte solutions with a common anion, especially at higher concentrations.     

Correlation between polarity parameters and dielectric properties of [Na][TOTO]--a sodium ionic liquid

The ionic liquid (IL) [Na][TOTO], with sodium as the cation and an oligoethercarboxylate as the anion, shows properties that differ significantly from conventional ionic liquids, like imidazolium salts. Its polarity, determined in the temperature range of (293-333) K from measurements of Reichardt's E value and the Kamlet-Taft parameters is extraordinarily low and matches the dielectric constant extrapolated from frequency dependent complex permittivity measurements. The dielectric spectra also reveal split dynamics with the dominating slow mode probably associated with the reorientation of -COO(-)...Na(+) ion pairs and the fast mode arising from the flexibility of the oligoethylene moieties of the anion. This assignment, supported by the derived effective dipole moments, argues against our previous hypothesis [O. Zech et al., Chem.-Eur. J., 2009, 15, 1341-1345] that salts, such as [Na][TOTO], have low melting points because of cation complexation in a pseudo crown ether-like fashion and resulting "intra-molecular" charge neutralization. The present data rule out a rigid chelate-like complex as the dominating species. Considering the present findings together with the viscosities of [TOTO](-) salts, a crosslinked structure of the ionic liquid with strong -COO(-)...Na(+) interactions and relatively weaker forces between cations and ether oxygen groups of the anions appears to be plausible.     

Identification of 1,3-dialkylimidazolium salt supramolecular aggregates in solution

The nature of the interactions between 1,3-dialkylimidazolium cations and noncoordinating anions such as tetrafluoroborate, hexafluorophosphate, and tetraphenylborate has been studied in the solid state by X-ray diffraction analysis and in solution by (1)H NMR spectroscopy, conductivity, and microcalorimetry. In the solid state, these compounds show an extended network of hydrogen-bonded cations and anions in which one cation is surrounded by at least three anions and one anion is surrounded by at least three imidazolium cations. In the pure form, imidazolium salts are better described as polymeric supramolecules of the type {[(DAI)(3)(X)](2+)[(DAI)(X)(3)](2-)}(n) (where DAI is the dialkylimidazolium cation and X is the anion) formed through hydrogen bonds of the imidazolium cation with the anion. In solution, this supramolecular structural organization is maintained to a great extent, at least in solvents of low dielectric constant, indicating that mixtures of imidazolium ionic liquids with other molecules can be considered as nanostructured materials. This model is very useful for the rationalization of the majority of the unusual behavior of the ionic liquids.     

Steering the enzymatic activity of proteins by ionic liquids. A case study of the enzyme kinetics of yeast alcohol dehydrogenase

We explore ion-specific effects exerted by ionic liquids (ILs) on the enzyme kinetics of yeast alcohol dehydrogenase. The Michaelis-Menten reaction scheme is used to parameterize the observed kinetics in terms of the apparent dissociation constant of the substrate (Michaelis-Menten constant) K(M), the turnover number k(cat), which reflects the number of product molecules per enzyme molecule per second, and the enzymatic efficiency k(cat)/K(M) of the reaction. Results for fifteen salts are used to deduce Hofmeister anion and cation series. The ion rankings derived from K(M), k(cat) and k(cat)/K(M) differ markedly. Only the results for the enzymatic efficiency correspond to expectations from other phenomena, such as the thermal stability of native proteins. Anion variation has a significantly larger effect on the enzymatic efficiency than cation variation. All ILs decrease k(cat) relative to its value for the IL-free solution, thus driving enzyme deactivation. Enhancements of the enzymatic efficiency by some ions are founded in their effects on the Michaelis-Menten constant. The observed Hofmeister anion and cation series point toward hydrophobic interactions as an important factor controlling ion-specific effects on the enzymatic activity.     

The dielectric response of room-temperature ionic liquids: effect of cation variation

In continuation of recent work on the dielectric response of imidazolium-based ionic liquids (ILs) (J. Phys. Chem. B, 2006, 110, 12682), we report on the effect of cation variation on the frequency-dependent dielectric permittivity up to 20 GHz of ionic liquids. The salts are comprised of pyrrolidinium, pyridinium, tetraalkylammonium, and triethylsulfonium cations combined with the bis-((trifluoromethyl)sulfonyl)imide anion. The dielectric spectra resemble those observed for imidazolium salts with the same anion. In all cases, the major contribution results from a diffusive low-frequency response on the time scale of several 100 ps, which shows a broadly distributed kinetics similar to that of spatially heterogeneous states in supercooled and glassy systems rather than that observed in fluid systems. There is evidence for a weak secondary process near 10-20 ps. Perhaps the most interesting difference to imidazolium salts is founded in the missing portions of the spectra due to processes beyond the upper cutoff frequency of 20 GHz. These are lower than that observed for imidazolium-based salts and seem to vanish for tetraalkylammonium and triethylsulfonium salts. As for imidazolium salts, the extrapolated static dielectric constants are on the order of epsilon(S) congruent with 10-13, classifying these ILs as solvents of moderate polarity.     

Transport of alkali halides through a liquid organic membrane containing a ditopic salt-binding receptor

A ditopic receptor is shown to have an impressive ability to recognize and extract the ion pairs of various alkali halides into organic solution. X-ray diffraction analysis indicates that the salts are bound in the solid state as contact ion pairs. Transport experiments, using a supported liquid membrane and high salt concentration in the source phase, show that the ditopic receptor can transport alkali halide salts up to 10-fold faster than a monotopic cation or anion receptor and 2-fold faster than a binary mixture of cation and anion receptors. All transport systems exhibit the same qualitative order of ion selectivity; that is, for a constant anion, the cation selectivity order is K+ > Na+ > Li+, and for a constant cation, the anion transport selectivity order is I- > Br- > Cl-. The data suggest that with a ditopic receptor, the polarity of the receptor-salt complex can be lowered if the salt is bound as an associated ion pair, which leads to a faster diffusion through the membrane and a higher maximal flux.     

Thermodynamics of electrolyte mixtures. Activity and osmotic coefficients consistent with the higher-order limiting law for symmetrical mixing

The parameters for symmetrical mixing of ions of the same sign in the virial-coefficient (Pitzer) system are evaluated from literature data for 25°C in a manner consistent with the higher-order limiting law of Friedman. Twenty-four systems involve cation mixing with a common anion and fourteen involve anion mixing with a common cation. Heat of mixing data were similarly treated in a recent publication; the results give the temperature coefficients of some of these same parameters. The combined results yield the mixing parameters as functions of temperature on a basis both self-consistent and in accord with the limiting law. The results also yield, for a few systems without a common ion, predicted values in good agreement with experimental data.on leave from Liaoning University, Shenyang, China.     

A High‐Temperature Order–Disorder Phase Transition Coupled With Conformational Change in the Hybrid Material [C6H13NH]2⋅ZnBr4

A new high‐temperature, hybrid, phase‐transition material, 1‐methylpiperidinium tetrabromozincate ( 1 ), that shows a reversible transition at 345 K was synthesized. Differential scanning calorimetry and specific heat capacity measurements confirmed this reversible transformation with a large heat hysteresis of 25 K, which describes a typical first‐order phase transition in 1 . The dielectric constant exhibited a steplike anomaly and showed high and low dielectric states in the high‐ and room‐temperature phases, respectively, and therefore, this hybrid might be considered as a potential switchable dielectric material. The variable‐temperature powder X‐ray diffraction patterns displayed remarkable shifts between the experimental patterns at the two different phases. Single‐crystal X‐ray diffraction analyses at various temperatures revealed that the origin of this transformation could be attributed to disordering of the bromine atoms in the anion and the nitrogen atom of the cation. The cation also assumed a conformational change, which was likely induced by the disordered nitrogen atom. The conformational onset of the transformation of the cation from a planar conformer into a relaxed chair also occurred upon decreasing the temperature below transition point; thus, the combined order–disorder and conformational change induced the structural transformation and the change in symmetry.     

Effects of ion solvation on phase equilibrium and interfacial tension of liquid mixtures

We study the bulk thermodynamics and interfacial properties of electrolyte solution mixtures by accounting for electrostatic interaction, ion solvation, and inhomogeneity in the dielectric medium in the mean-field framework. Difference in the solvation energy between the cations and anions is shown to give rise to local charge separation near the interface, and a finite Galvani potential between two coexisting solutions. The ion solvation affects the phase equilibrium of the solvent mixture, depending on the dielectric constants of the solvents, reflecting the competition between the solvation energy and translation entropy of the ions. Miscibility is decreased if both solvents have low dielectric constants and is enhanced if both solvents have high dielectric constant. At the mean-field level, the ion distribution near the interface is determined by two competing effects: accumulation in the electrostatic double layer and depletion in a diffuse interface. The interfacial tension shows a nonmonotonic dependence on the salt concentration: it increases linearly with the salt concentration at higher concentrations and decreases approximately as the square root of the salt concentration for dilute solutions, reaching a minimum near 1 mM. We also find that, for a fixed cation type, the interfacial tension decreases as the size of anion increases. These results offer qualitative explanations within one unified framework for the long-known concentration and ion size effects on the interfacial tension of electrolyte solutions.     

Significance of a volcano relation of fajans for hydration energies

A volcano-shaped relation between heats of solution of a series of salts and the difference of hydration energy of the ions of the salt was noted by Fajans. This relation has a maximum when the difference of hydration energy of the cation and anion approaches zero. A rationalization of this type of relation is proposed on the basis of sums of reciprocal radii terms which represent the dependence of electrostatic hydration energy on ion size. The general trend of heats of solution with ionic size in series of salts with either a common cation or a common anion is discussed.     

Kinetics of oxidation of some disaccharides in ammoniacal medium

Kinetics of oxidation of lactose, maltose, cellobiose and melibiose has been studied by hexacyanoferrate(III) in the presence of ammonia. A general mechanism involving the formation of intermediate enodiol anion has been suggested. The effects of dielectric constant and salts have been studied in detail. The oxidation products have been characterised by descending paper chromatography. Compensation effects have been observed.     

Salt effects on the phase behavior, structure, and rheology of chromonic liquid crystals

Chromonic liquid crystals are formed by the addition of aromatic molecules such as disodium chromoglycate (cromolyn) to water. In this study, we investigate the addition of salts to the lyotropic nematic phase of cromolyn aqueous solutions. The addition of sodium and potassium salts shifts the isotropic-nematic phase boundary upward by more than 10 degrees C, so that samples that were isotropic at room temperature are transformed into nematic phases. Salt effects are predominantly dictated by the cation, not the anion, and appear to differ based on cation size. In contrast to small, hydrated cations like sodium, large, weakly hydrated cations such as tetraethylammonium and tetrabutylammonium shift the phase boundary downward, thus stabilizing the isotropic phase at the expense of the nematic one. The phase behavior results are highly correlated with viscosity measurements, with an upward shift in the phase boundary correlating with an increase in solution viscosity and vice versa. We also probe the microstructure in cromolyn-salt solutions, both indirectly by small-angle neutron scattering (SANS) and directly by cryo-transmission electron microscopy (cryo-TEM). The cryo-TEM images show the presence of rodlike aggregates that possibly undergo a higher order aggregation into bundles in the presence of salt.     

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.     

Concentration dependence of ionic transport in dilute organic electrolyte solutions

Ion transport is studied in dilute organic liquid electrolyte solutions in which close cation-anion interactions are minimized either through steric hindrance imposed by the bulky tetrabutylammonium cation or by strong solvation of alkali metal cations by DMSO or 1-propanol. In these solutions, the molar conductivity does not appear to depend on either the solvent viscosity or the size of the solvated charge carrier in a manner consistent with Walden's rule. The molar conductivities plotted as a function of the solvent dielectric constant from epsilon = 5.48 to 63.5 appear to lie on a smooth curve for a set of 0.0055 M solutions of tetrabutylammonium trifluoromethanesulfonate in a variety of aprotic solvents. The molar conductivity smoothly increases with increasing dielectric constant to a maximum at roughly epsilon = 33 and then decreases with further increase of the dielectric constant. The conductivity appears to depend only on the dielectric constant and not the specific functional group in this broad family of solvents. A similar plot for a series of linear alcohols as solvents also led to a smooth curve, although the values of the molar conductivity were lower than values in the aprotic solvents by almost an order of magnitude at corresponding values of the solvent dielectric constant.     

Chiral ionic liquids: synthesis, properties, and enantiomeric recognition

We have synthesized a series of structurally novel chiral ionic liquids which have a either chiral cation, chiral anion, or both. Cations are an imidazolium group, while anions are based on a borate ion with spiral structure and chiral substituents. Both (or all) stereoisomeric forms of each compound in the series can be readily synthesized in optically pure form by a simple one-step process from commercially available reagents. In addition to the ease of preparation, most of the chiral ILs in this series are liquid at room temperature with a solid to liquid transformation temperature as low as -70 degrees C and have relatively high thermal stability (up to at least 300 degrees C). Circular dichroism and X-ray crystallographic results confirm that the reaction to form the chiral spiral borate anion is stereospecific, namely, only one of two possible spiral stereoisomers was formed. Results of NMR studies including 1H{15N} heteronuclear single quantum coherence (HSQC) show that these chiral ILs exhibit intramolecular as well as intermolecular enantiomeric recognition. Intramolecularly, the chiral anion of an IL was found to exhibit chiral recognition toward the cation. Specifically, for a chiral IL composing with a chiral anion and a racemic cation, enantiomeric recognition of the chiral anion toward both enantiomers of the cation lead to pronounced differences in the NMR bands of the cation enantiomers. The chiral recognition was found to be dependent on solvent dielectric constant, concentration, and structure of the ILs. Stronger enantiomeric recognition was found in solvent with relatively lower dielectric constants (CDCl3 compared to CD3CN) and at higher concentration of ILs. Also, stronger chiral recognition was found for anions with a relatively larger substituent group (e.g., chiral anion with a phenylmethyl group exhibits stronger chiral recognition compared to that with a phenyl group, and an anion with an isobutyl group has the weakest chiral recognition). Chiral anions were also found to exhibit intermolecular chiral recognition. Enantiomeric discrimination was found for a chiral IL composed of a chiral anion and achiral cation toward another chiral molecule such as a quinine derivative.     

On the mechanism of extraction of trivalent actinides and lanthanides with alkylamine hydrochlorides

The method to study the mechanism of actinides (III) and lanthanides (III) liquid-liquid amine extraction from chloride media has been suggested. The technique is based on the determination of distribution coefficients for extraction of these elements from mixed salting-out agent solutions with common anion. Distribution coefficients were found to be independent of mean ion activity of lithium cation at constant water activity, when extraction of trace amounts of Eu (III) and Cm(III) by tri-n-octylammonium hydrochloride from mixed LiCl−CaCl₂ solutions occurred. It has been concluded that lithium cation does not enter into the complexed extracted.     

烷基磷(膦)酸萃取剂钠盐及其微乳液的NMR,FTIR光谱研究

制备了五种皂化度大于97%的无水烷基磷(膦)酸钠盐,用NMR 谱测定了其微乳体系的H化学位移,测定了酸性萃取剂原液,其固体钠盐以及微乳液的FTIR光谱,观察到微乳液中钠盐阴.阳离子都有水化效应,讨论了水合配位与阴离子结构的关系.     

Static dielectric constant of room temperature ionic liquids: internal pressure and cohesive energy density approach

Measurements of the static dielectric constant (epsilon) of ionic liquids (ILs) are very difficult because of the decay of field by the ionic conductivity of ILs. Herein, we describe an easy method for the prediction of epsilon of various imidazolium-based ILs [C_n mim] from n, i.e. the ratio of internal pressure (P_i) and cohesive energy density (ced). A calibration curve of n vs epsilon for conventional organic solvents (mainly the linear alcohols) has been used to estimate the epsilon of the ILs. Estimated epsilon values for ILs having the anions [Cl]-, [BF 4]-, [PF 6]-, [TfO]-, and [Tf 2N]- showed a very good comparison with the literature results, whereas ILs having the anions [C_n OSO3]- tend to deviate from such correlation. Also, for a series of ILs having a common anion, the epsilon is shown to follow a very good correlation with the molecular volumes. Predicted values show that both the nature of the anion and alkyl chain length of the cation contribute significantly to the epsilon of the ILs. The method developed makes use of properties which can be either experimentally determined or estimated with good accuracy and can be extended to the other categories of ILs with ease and reasonable accuracy.