Potentiometric Cross-Sensitive Sensors Based on Perfluorinated Membranes Treated at Different Relative Humidity for Codetermination of Cations and Anions in Alkaline Solutions of Amino Acids

The results of studying characteristics of potentiometric DP-sensors (sensors with the Donnan potential as their analytical signal) are presented for alkaline solutions of a sulfur-containing amino acid with perfluorosulfonic cation exchange membranes subjected to thermal treatment and mechanical deformation at different relative humidity. Correlation between the distribution of sensitivity of DP-sensors towards cations and anions and diffusion permeability of membranes was found. A multisensor system including two DP-sensors based on membranes with optimized properties and a glass electrode for codetermination of potassium cations and amino acid anions and zwitterions in solutions at pH >7 are developed.     

Mechanisms of ion transport in lithium salt-doped polymeric ionic liquid electrolytes at higher salt concentrations

We used atomistic simulations to study the mechanisms of ion transport in salt-doped polymeric ionic liquid systems at higher salt concentrations. Consistent with the experimental observations, our simulations indicate that at higher salt concentrations, the anion mobilities become lower than that of the lithium cations. Further, the anion mobilities become relatively insensitive to the salt concentration, while the mobilities of lithium increase with increasing salt concentration. We rationalize the results for the anion mobilities by considering the fractions of anions which are exclusively coordinated with the polycations (Type1); co-coordinated with cations and lithium (Type2); and those exclusively coordinated with lithium (Type3). By considering the coordination characteristics of the different types of anions and their hopping motions, we demonstrate that the net anion mobilities results from a compensation effect of the salt concentration dependence of the mobilities of the different anions. With respect to the mobilities of the lithium ions, we demonstrate that the latter moves primarily by a structural diffusion mechanism involving refreshing of the solvation shell during hopping. Further, for the majority of the lithium ions, the solvation shell is shown to be comprised of co-coordinated Type2 anions, and that the number of polycations and the unique polymer chains involved in such coordination decreases with increasing salt concentration. Such changes are shown to weaken the solvation shell around the lithium, thereby facilitating faster ion motion. Together, our results suggest that systems in which the anion which exhibits a stronger coordination to the polycation in comparison to that of the lithium can facilitate higher transference numbers without a concomitant reduction in the mechanical strength.     

Membrane potential in charged porous membranes

For charged porous membranes, the separation efficiency to charged particles and ions is affected by the electrical properties of the membrane surface. Such properties are most commonly quantified in terms of zeta-potential. In this paper, it is shown that the zeta-potential can be calculated numerically from the membrane potential. The membrane potential expression for charged capillary membranes in contact with electrolyte solutions at different concentrations is established by applying the theory of non-equilibrium thermodynamic to the membrane process and considering the space-charge model. This model uses the Nernst–Planck and Navier–Stokes equations for transport through pores, and the non-linear Poisson–Boltzmann equation, which is numerically solved, for the electrostatic condition of the fluid inside pores. The integral expressions of the phenomenological coefficients coupling the differential flow (solute relative to solvent) and the electrical current with the osmotic pressure and the electrical potential gradients are established and calculated numerically. The mobilities of anions and cations are individually specified. The variations of the membrane potential (or the apparent transport number of ions in the membrane pores) are studied as a function of different parameters: zeta-potential, pore radius, mean concentration in the membrane, ratio of external concentrations and type of ions.     

Investigation of the effect of ionic strength of Tris-acetate background electrolyte on electrophoretic mobilities of mono-, di-, and trivalent organic anions by capillary electrophoresis

The effect of ionic strength of the background electrolyte (BGE) composed of tris(hydroxymethyl)aminomethane (Tris) and acetic acid on the electrophoretic mobility of mono-, di- and trivalent anions of aliphatic and aromatic carboxylic and sulfonic acids was investigated by capillary zone electrophoresis (CZE). Actual ionic mobilities of the above anions were determined from their CZE separations in Tris-acetate BGEs of pH 8.1 to 8.2 in the 3 to 100 mM ionic strength interval at constant temperature (25 degrees C). It was found that the ionic strength dependence of experimentally determined actual ionic mobilities does not follow the course supposed by the classical Onsager theory. A steeper decrease of actual ionic mobilities with the increasing ionic strength of BGE and a higher estimated limiting mobility of the anions than that found in the literature could be attributed to the specific behavior of the Tris-acetate BGEs. Presumably, not only a single type of interaction of anionic analytes with BGE constituents but rather the combination of effects, such as ion association or complexation equilibria, seems to be responsible for the observed deviation of the concentration dependence of the actual ionic mobilities from the Onsager theory. Additionally, several methods for the determination of limiting ionic mobilities from CZE measured actual ionic mobilities were evaluated. It turned out that the determined limiting ionic mobilities significantly depend on the calculation procedure used.     

Low-lying isomers of Si(n)(+) and Si(n)(-) (n=31-50) clusters

We carry out a systematic search for the atomic structures of silicon cluster cations and anions in the size range n=31-50 using density functional theory in the generalized-gradient approximation. The obtained lowest-energy candidates feature cagelike structures. We find that the computed binding energies and the dissociation pathways as well as the mobilities of our lowest-energy isomers of the cations are all in good agreement with the measured data from experiments. Furthermore, based on these isomers, we reveal that the steplike feature appearing in the measured high-resolution mobilities can be correlated with the corresponding fullerenes explicitly, which strongly support the notion that endohedral silicon fullerenelike structures are the most favored growth pattern for silicon clusters in the range n=31-50. Our calculation and analysis suggest that the proposed isomers are probably very close to the major-abundance isomers observed in experiments.     

Insights into the Complexity of Weak Intermolecular Interactions Interfering in Host–Guest Systems

The recognition properties of heteroditopic hemicryptophane hosts towards anions, cations, and neutral pairs, combining both cation–π and anion–π interaction sites, were investigated to probe the complexity of interfering weak intermolecular interactions. It is suggested from NMR experiments, and supported by CASSCF/CASPT2 calculations, that the binding constants of anions can be modulated by a factor of up to 100 by varying the fluorination sites on the electron‐poor aromatic rings. Interestingly, this subtle chemical modification can also reverse the sign of cooperativity in ion‐pair recognition. Wavefunction calculations highlight how short‐ and long‐range interactions interfere in this recognition process, suggesting that a disruption of anion–π interactions can occur in the presence of a co‐bound cation. Such molecules can be viewed as prototypes for examining complex processes controlled by the competition of weak interactions.     

Determination of sulfur-containing anions in alkaline solutions using arrays of DP-sensors based on hybrid perfluorinated membranes with proton-donor dopants

The influence of proton-donor properties and concentration of dopant nanoparticles introduced into Nafion and MF-4SС perfluorosulfonic cation-exchange membranes on the characteristics of cross-sensitive DP-sensors (sensors whose analytical signal is the Donnan potential) in alkaline solutions of sulfur-containing organic compounds were studied. The dopants were acid salts of heteropoly acids (HPAs) and hydrated silica SiO₂ and zirconia ZrO₂ surface-modified with sulfur-containing groups and an acid HPA salt. A correlation was revealed between the DP-sensor sensitivity to anions (and zwitter-ions) in alkaline solutions, size and proton-donor ability of the added particles, and diffusion permeability of hybrid membranes. Optimum compositions of membranes for arrays of cross-sensitive DP-sensors ensuring the simultaneous determination of cations and anions (and zwitter-ions) in the test solutions with an error of less than 18% were selected.     

Analytical derivative techniques for TDDFT excited-state properties: Theory and application

We review our recent work on the methodology development of the excited-state properties for the molecules in vacuum and liquid solution.The general algorithms of analytical energy derivatives for the specific properties such as the first and second geometrical derivatives and IR/Raman intensities are demonstrated in the framework of the time-dependent density functional theory(TDDFT).The performance of the analytical approaches on the calculation of excited-state energy Hessian has also been shown.It is found that the analytical approaches are superior to the finite-difference method on the computational accuracy and efficiency.The computational cost for a TDDFT excited-state Hessian calculation is only 2–3 times as that for the DFT ground-state Hessian calculation.With the low computational complexity of the developed analytical approaches,it becomes feasible to realize the large-scale numerical calculations on the excited-state vibrational frequencies,vibrational spectroscopies and the electronic-structure parameters which enter the spectrum calculations of electronic absorption and emission,and resonance Raman spectroscopies for medium-to large-sized systems.     

Formation constants for complexes of transition-metal cations with O- and N-donor ligands in aqueous solutions

A method has been developed for the theoretical calculation of stability constants for transitionmetal complexes proceeding from molecular properties. An analytical form of the cation-ligand interaction potential including the partial bond covalence has been developed. The potential is expressed through molecular parameters and a covalence parameter; the latter may be determined from experimentally measured or theoretically calculated gas-phase data. The stability constants have been calculated for 1: 1 complexes of divalent metal cations with hydroxide anion, acetic, glycolic, and lactic acid anions, and ammonia in aqueous solutions. The covalence parameters have been calculated for the bonds of metal cations with O-and N-donor ligands.     

Magnetic hyperpolarizabilities in a cubic response formulation

We demonstrate the applicability of a recently derived cubic response function formalism for performing fully analytical calculations of hypermagnetizabilities, their anisotropy and their dispersion. The calculations involve the noble gas atoms, He, Ne, and Ar, and the corresponding isoelectronic halogen anions and alkali cations.     

Anion-promoted cation motion and conduction in zeolites

The motion of sodium cations in sodalite and cancrinite has been investigated by force field calculations, solid-state NMR, and impedance spectroscopy. Special emphasis is dedicated to the influence of anions on sodium mobilities. Local cation motion is promoted when they interact with anions. However, not all systems with high local mobilities exhibit good ion conductivities, as cooperativity of the motion appears to be an important factor, as well. The activation barrier for local sodium motion (calculations) and long-range transport (dc conductivities) is lowered in sodalite when halogenide anions, Cl(-), Br(-), or I(-), are present. The activation barriers increase with increasing size of the anion and decreasing coordination in the transition state. On the basis of (23)Na solid-state NMR data, all the sodium ions in the dense sodalite structure are rather rigid up to 470 K. All the cations in chromate sodalite, and Na(+) in the small cancrinite epsilon-cages without anion interactions, show a restricted local motion at higher temperatures. There is a selective high local motion of Na(+) in the neighborhood of chromate anions in the more open channel system of cancrinite. These results suggest that sodium migration can be enhanced, at least locally, in open channel systems by anion interactions. A dynamics coupling between anion reorientation and cation mobility was not observed.     

Hydrogen-Bonding-Driven Ion-Pair Formation in Protic Ionic Liquid Aqueous Solution

Protic ionic liquids (PILs) in solution especially in water have attracted more and more attention due to their unique properties. The solvation of PILs in water is important to their properties and applications. To explore the solvation of bio-based PILs in water, acidity of 49 [AA]X amino acid ionic liquids (AAILs) consisting of 7 different cations and 7 different anions was studied as a favorable probe. The pK_a values for [AA]X PILs containing same cations were obtained and discussed. The acidity strength of the [AA]X PILs varies with both cation and anion which does not follow the conventional assumption that the acidity for PILs is independent of anions. The acidic discrepancy of [AA]X PILs aqueous solution is probably mediated by the formation of ion pairs according to a revised solvation model of PILs. Quantum-chemistry calculation was employed to unpuzzle anion's different effects on the acid balance of cations where cation-anion hydrogen bonds play an important role. Such difference in acidity allows us to understand the formation of solvated ion pairs. This work provides an insight into the fundamental solvation of PILs from acid perspective and their influence on acidity properties for the first time.     

Possible origin of the inverse and direct Hofmeister series for lysozyme at low and high salt concentrations

Protein solubility studies below the isoelectric point exhibit a direct Hofmeister series at high salt concentrations and an inverse Hofmeister series at low salt concentrations. The efficiencies of different anions measured by salt concentrations needed to effect precipitation at fixed cations are the usual Hofmeister series (Cl(-) > NO(3)(-) > Br(-) > ClO(4)(-) > I(-) > SCN(-)). The sequence is reversed at low concentrations. This has been known for over a century. Reversal of the Hofmeister series is not peculiar to proteins. Its origin poses a key test for any theoretical model. Such specific ion effects in the cloud points of lysozyme suspensions have recently been revisited. Here, a model for lysozymes is considered that takes into account forces acting on ions that are missing from classical theory. It is shown that both direct and reverse Hofmeister effects can be predicted quantitatively. The attractive/repulsive force between two protein molecules was calculated. To do this, a modification of Poisson-Boltzmann theory is used that accounts for the effects of ion polarizabilities and ion sizes obtained from ab initio calculations. At low salt concentrations, the adsorption of the more polarizable anions is enhanced by ion-surface dispersion interactions. The increased adsorption screens the protein surface charge, thus reducing the surface forces to give an inverse Hofmeister series. At high concentrations, enhanced adsorption of the more polarizable counterions (anions) leads to an effective reversal in surface charge. Consequently, an increase in co-ion (cations) adsorption occurs, resulting in an increase in surface forces. It will be demonstrated that among the different contributions determining the predicted specific ion effect the entropic term due to anions is the main responsible for the Hofmeister sequence at low salt concentrations. Conversely, the entropic term due to cations determines the Hofmeister sequence at high salt concentrations. This behavior is a remarkable example of the charge-reversal phenomenon.     

Determination of glycine,alanine, and leucine at different solution ph with the aid of donnan potential sensors based on hybrid membranes

The cross sensitive sensors whose analytical signal is the Donnan potential (PD-sensors) were developed for the determination of the amino acids glycine, alanine, and leucine in acidic and alkaline solutions. Hybrid materials based on perfluorinated sulfo cation-exchange membranes Nafion and MF-4SC with incorporated zirconium dioxide and silicon dioxide nanoparticles, including those with modified surfaces containing proton-acceptor groups, were used in the PD-sensors. The sensitivity of the PD-sensors to hydronium ions, which interfere with the determination of amino acids at pH < 7, was considerably decreased due to the use of the membranes obtained by an in situ method that contained silicon dioxide nanoparticles with amine-containing groups. The greatest sensitivity of the PD-sensors to the anions of amino acids at pH > 7 and the smallest sensitivity to the cations K⁺ were observed in hybrid membranes, which combined an increased rate of anion transfer and a low moisture capacity. The use of the PD-sensors based on hybrid membranes makes it possible to determine the cations, anions, and zwitterions of amino acids over a wide range of pH with a sufficiently high accuracy.     

The influence of hydrogen bonding on the physical properties of ionic liquids

Potential applications of ionic liquids depend on the properties of this class of liquid material. To a large extent the structure and properties of these Coulomb systems are determined by the intermolecular interactions among anions and cations. In particular the subtle balance between Coulomb forces, hydrogen bonds and dispersion forces is of great importance for the understanding of ionic liquids. The purpose of the present paper is to answer three questions: Do hydrogen bonds exist in these Coulomb fluids? To what extent do hydrogen bonds contribute to the overall interaction between anions and cations? And finally, are hydrogen bonds important for the physical properties of ionic liquids? All these questions are addressed by using a suitable combination of experimental and theoretical methods including newly synthesized imidazolium-based ionic liquids, far infrared spectroscopy, terahertz spectroscopy, DFT calculations, differential scanning calorimetry (DSC), viscometry and quartz-crystal-microbalance measurements. The key statement is that although ionic liquids consist solely of anions and cations and Coulomb forces are the dominating interaction, local and directional interaction such as hydrogen bonding has significant influence on the structure and properties of ionic liquids. This is demonstrated for the case of melting points, viscosities and enthalpies of vaporization. As a consequence, a variety of important properties can be tuned towards a larger working temperature range, finally expanding the range of potential applications.     

Charge-selective recognition at fibroin-modified electrodes for analytical application

A novel fibroin-modified electrode with charge recognition is reported. The characteristics of silk fibroin membranes have been exploited for analytical applications. The membrane, with an isoelectric point of pH 4.5, was applied to graphite and carbon-fiber electrodes. The modified electrode was negatively charged in solutions of pH>4.5, and so rejected anions and attracted cations. In solutions of pH<4.5 the electrode was positively charged, and so rejected cations and attracted anions. The pH-responsive charge recognition of the modified electrode was investigated for some neurocompounds. A fibroin carbon-fiber electrode was used for in-vivo determination of the concentration of the cationic neurotransmitter dopamine (DA).This revised version replaces the article published online on April 2005.     

The semiclassical and the quasiclassical partition function of a quartic anharmonic oscillator

After a substitution a known Laplace-type integral is used to derive quantum corrections to the classical partition function of a quartic anharmonic oscillator in the framework of the Wigner—Kirkwood perturbation expansion. By straightforward calculations results are given in a closed form allowing the analytical formulation of the thermodynamic functions H, E, S, C_υ. The numerical results agree for arbitrary anharmonicity and for high and intermediate temperatures with the numerical partition function calculated from the Hioe—Montroll eigenvalues. Furthermore, the same integral type is used for the analytical calculation of a “quasiclassical” partition function and of “quasiclassical” moments. In the trace formulation of the partition function all commutators are neglected. The harmonic oscillator density matrix is applied to the evaluation of the truncated trace expressions. The “quasiclassical” partition function is an exact upper bound and lies always below the classical partition function.     

Carminic acid as a reagent for the spectrofluorimetric determination of molybdenum and tungsten-I Development of procedures

Carminic acid forms red fluorescent complexes with molybdenum(VI) and tungsten(VI) which provide spectrofluorimetric methods for the determination of these metals in the range 0.1–0.9 ppm and 0.4–0.36 ppm at pH 5.2 and 4.6 respectively. The optimum conditions for the analytical procedures have been established, and the effects of a wide range of cations and anions have been investigated. The reagent forms 1:1 complexes with both molybdenum and tungsten, and the conditional stability constants have been evaluated.     

Micellar electrokinetic chromatography systems for the separation of mixtures of charged and uncharged compounds

In this study, the migration behavior of charged and uncharged analytes was investigated under different conditions. Effective mobilities - electrophoretic mobilities under the influence of micelles - of cations, anions, and neutrals were measured at neutral, basic, and acidic pH (7.5, 11, and 2.2) using background electrolytes containing different sodium dodecyl sulfate (SDS) concentrations (0-90 mM) and acetonitrile (ACN) proportions (0-75%). SDS concentration and ACN proportion were found to have a tremendous effect on the effective mobilities and migration order of the model compounds. Although the SDS micelles preferably interact with neutrals and cations, hydrophobic bonds can also occur with anions. Cations, anions, and neutrals having rather different migration behaviors, it is possible to considerably enhance the selectivity of the method by adjusting properly the SDS concentration and the ACN proportion. These observations confirm the interest of using micellar electrokinetic chromatography not only for the separation of neutral substances but also to analyze charged compounds.     

Sulfur speciation by capillary electrophoresis with indirect spectrophotometric detection: In search of a suitable carrier electrolyte to maximize sensitivity

Various probes have been evaluated as alternative ions to chromate, which is most frequently used in the analysis of small inorganic anions by capillary electrophoresis with indirect UV detection. Sulfur species (S₂O₃²⁻, SO₄²⁻, S₄O₆²⁻, S(−II)) have been determined. The optimization of the method was particularly focused on S(−II) since this species rapidly yields S₂O₃²⁻ and SO₄²⁻ in the presence of oxidizing agents. Therefore, it could not be analysed by capillary electrophoresis with chromate as the background electrolyte. The alternative probe ions all contain aromatic rings (benzene or naphthalene) to provide the intrinsic background absorbance for indirect detection. They are all fully ionized at the pH chosen for this application (TRIS buffer, pH=8) and the range of their mobilities is large enough to suit the analytes mobilities. They have no oxidizing properties. Transfer ratios have been determined experimentally and compared to calculated values derived from the Kohlrausch regulation function. All experimental values were lower than expected from the calculations, which proves the limitations of the Kohlrausch theory concerning the configuration (electrolyte/analyte) of this study. However, maximizing (z_A/z_E)·ε_E (with z_A and z_E being the charges of the analyte and the probe, respectively, and ε_E the molar absorptivity of the probe) and keeping the mobility of the probe close to those of the analytes will give a good hint for the choice of the most suitable UV-absorbing probe. Pyromellitate and naphthalenetrisulfonate, the mobilities of which are close to that of S(−II), give the best sensitivity for this species, with good resolution and sensitivity for all other species.