Ion exchange in reverse micelles

The distribution and dynamics of alkali cations inside Na-AOT reverse micelles have been investigated using Monte Carlo and molecular dynamics simulations. Water is modeled using the extended simple point charge (SPC/E) model. Simulations were carried out for alkali salts of Li+, Na+, K+, and Cs+ placed into the aqueous core of the reverse micelle, for situations corresponding to one and three molecules of added salt. In all cases, we observe that the larger K+ and Cs+ ions exchange with the Na+ counterion; however, the smaller Li+ ion prefers to remains solvated within the core of the reverse micelle. Our study reveals that the oil-water interface of the Na-AOT reverse micelle has the greatest selectivity toward Cs+ followed by K+ and Li+. A model based on enthalpic contributions illustrates that the solvation energies of the different cations in water control the ion-exchange process. The hydration number of the first water shell for Li+ situated in the aqueous core of the reverse micelle with radius R = 14.1 A was similar to that observed at infinite dilution in bulk water.     

Water adsorption in ion-bearing nanopores

Grand canonical Monte Carlo simulations are used to examine the adsorption of water into cylindrical nanopores containing single ions. The isotherms for water adsorbing into nanopores with radii of 0.44, 0.54, 0.64, and 0.74 nm and containing Na+, K+, Ca2+, Cl-, or F- at 298 K are computed. In all cases the nanopores are found to fill at reservoir chemical potentials below the chemical potential of saturated water vapor at 298 K. The threshold chemical potential is found to be sensitive to both the size of the channel and the ion species, with the anion-bearing pores filling at lower chemical potentials. Additionally, the filling threshold chemical potential is found to decrease as the radius of the pores is decreased. Pores with K+ and Cl- are compared, and the Cl- pores are found to exhibit higher water densities in the filled states and a more energetically favorable water structure while yielding lower per particle entropies. Sample simulation configurations are also examined and indicate that at low chemical potentials, the adsorbed water forms a cluster around the ion. Finally, the influence of the choice of water model on the adsorption isotherms is examined.     

Mean force potential of interaction between Na<Superscript>+</Superscript> and Cl<Superscript>−</Superscript> ions in planar nanopores in contact with water under pressure

The mean force potential (MFP) of interaction between counterions Na⁺ and Cl^? in a planar nanopore with structureless hydrophobic walls is calculated via computer simulation under the condition that the nanopore is in contact with water at an external pressure that exceeds the saturation pressure but remains insufficient to fill the nanopore with water. For a nanopore with a liquid phase, the MFP dependence on the interionic distance indicates the dissociation of an ion pair into two hydrated ions in a nanopore that is not completely filled with water. Fluctuations in the number of water molecules drawn into the interionic space decisively influence the dissociation. The attraction between counterions, averaged over thermal fluctuations, depends largely on the pore width and grows as the shielding of the ions’ electric field by water molecules in a narrow pore diminishes. The contributions from energy and entropy to the free energy of hydration are analyzed.     

Photon gated transport at the glass nanopore electrode

The interior surface of the glass nanopore electrode was modified with spiropyran moieties to impart photochemical control of molecular transport through the pore orifice (15-90 nm radius). In low ionic strength acetonitrile solutions, diffusion of a positively charged species (Fe(bpy)(3)(2+)) is electrostatically blocked with approximately 100% efficiency by UV light-induced conversion of the neutral surface-bound spiropyran to its protonated merocyanine form (MEH+). Transport through the pore orifice is restored by either irradiation of the electrode with visible light to convert MEH+ back to spiropyran or addition of a sufficient quantity of supporting electrolyte to screen the electrostatic field associated with MEH+. The transport of neutral redox species through spiropyran-modified glass nanopores is not affected by light, allowing photoselective transport of redox molecules to the electrode surface based on charge discrimination. The glass nanopore electrode can also be employed as a photochemical trap, by UV light conversion of surface-bound spiropyran to MEH+, preventing Fe(bpy)(3)(2+) initially in the pore from diffusing through the orifice.     

Formation and interaction of hydrated alkali metal ions at the graphite-water interface

Ion hydration at a solid surface ubiquitously exists in nature and plays important roles in many natural processes and technological applications. Aiming at obtaining a microscopic insight into the formation of such systems and interactions therein, we have investigated the hydration of alkali metal ions at a prototype surface-graphite (0001), using first-principles molecular dynamics simulations. At low water coverage, the alkali metal ions form two-dimensional hydration shells accommodating at most four (Li, Na) and three (K, Rb, Cs) waters in the first shell. These two-dimensional shells generally evolve into three-dimensional structures at higher water coverage, due to the competition between hydration and ion-surface interactions. Exceptionally K was found to reside at the graphite-water interface for water coverages up to bulk water limit, where it forms an "umbrellalike" surface hydration shell with an average water-ion-surface angle of 115 degrees . Interactions between the hydrated K and Na ions at the interface have also been studied. Water molecules seem to mediate an effective ion-ion interaction, which favors the aggregation of Na ions but prevents nucleation of K. These results agree with experimental observations in electron energy loss spectroscopy, desorption spectroscopy, and work function measurement. In addition, the sensitive dependence of charge transfer on dynamical structure evolution during the hydration process, implies the necessity to describe surface ion hydration from electronic structure calculations.     

Ion pairs in aqueous electrolyte microdrops under conditions of a flat nanopore

The molecular mechanisms of aqueous solvent penetration into a flat nanopore with hydrophobic structureless walls containing a Na⁺Cl^? ion pair with nonfixed distance between ions is studied by computer simulations. A detailed many-body polycenter model of intermolecular interactions calibrated with respect to experimental data for the free energy of attachment of water vapor molecules and quantum-chemical calculations in clusters is used. The ion pair hydration results in its decomposition. Drawing the molecules into the gap between ions makes easier penetration of solvent and filling of the nanopore with electrolyte. The ion-pair dissociation is accompanied by dramatic changes in the chemical potential of molecules and electric properties of the whole system. The thermodynamic characteristics of decomposition are stable as regards variations in the pore width. The post-decomposition electric polarizability demonstrates strong anisotropy associated with the nanopore flatness.     

Dynamics of Simultaneous, Single Ion Transport through Two Single-Walled Carbon Nanotubes: Observation of a Three-State System

The ability to actively manipulate and transport single molecules in solution has the potential to revolutionize chemical synthesis and catalysis. In previous work, we developed a nanopore platform using the interior of a single-walled carbon nanotube (diameter = 1.5 nm) for the Coulter detection of single cations of Li(+), K(+), and Na(+). We demonstrate that as a result of their fabrication, such systems have electrostatic barriers present at their ends that are generally asymmetric, allowing for the trapping of ions. We show that above this threshold bias, traversing the nanopore end is not rate-limiting and that the pore-blocking behavior of two parallel nanotubes follows an idealized Markov process with the electrical potential. Such nanopores may allow for high-throughput linear processing of molecules as new catalysts and separation devices.     

Hydration of sodium, potassium, and chloride ions in solution and the concept of structure maker/breaker

Neutron diffraction data with hydrogen isotope substitution on aqueous solutions of NaCl and KCl at concentrations ranging from high dilution to near-saturation are analyzed using the Empirical Potential Structure Refinement technique. Information on both the ion hydration shells and the microscopic structure of the solvent is extracted. Apart from obvious effects due to the different radii of the three ions investigated, it is found that water molecules in the hydration shell of K+ are orientationally more disordered than those hydrating a Na+ ion and are inclined to orient their dipole moments tangentially to the hydration sphere. Cl- ions form instead hydrogen-bonded bridges with water molecules and are readily accommodated into the H-bond network of water. The results are used to show that concepts such as structure maker/breaker, largely based on thermodynamic data, are not helpful in understanding how these ions interact with water at the molecular level.     

Gibbs energies of transfer of alkali metal cations between mutually saturated water-solvent systems determined from extraction experiments with radiotracer 137Cs

Thermodynamic standard Gibbs energies of transfer of alkali metal cations related to Cs+ cation [DeltatG degrees*,(Cs+)-[DeltatG degrees*,(M+)] between several mutually saturated solvents of the type water-solvent were calculated from determined extraction exchange constants Kexch degrees,*(Cs+/M+). The used liquid-liquid extraction method with radioactive tracing by 137Cs permits attaining higher precision of the values as compared to the methods used up to now. The data for o-nitrophenyloctyl ether, 1,2-dichloroethane, and 1-octanol were compared with literature sources and recommended absolute values of DeltatG degrees,*M+) are reported. For dissociating solvents, the dependences of [DeltatG degrees,*(Cs+) - [DeltatG degrees,*(M+)] on Gibbs energy of hydration of an ion, DeltaGhydr degrees are straight lines either for four cations Cs+, Rb+, K+, and Na+ (nitrosolvents) or for three cations Cs+, Rb+, and K+ (1,2-dichloroethane and 1-octanol). The hydration of Na+ and still more of Li+ in the water-saturated organic phase is apparent from the results. This manifests for high-water-content equilibrium 1-octanol even in a reversal of the values [i.e., DeltatG degrees*,(Li+) being more negative than DeltatG degrees,*(Na+)], although for Cs+, Rb+, and K+, the general trend is conserved. Water-saturated 1-octanol is thus slightly less basic than water, but the overall selectivity is very low. For one studied nondissociating solvent, dioctyl sebacate, the trend of the dependences of log Kexch degrees,*(CsB/M+) on DeltaGhydr degrees is similar to that of Kexch degrees,*(Cs+/M+) for polar solvents, but different for different anions B, thus reflecting ion association in the organic phase.     

Multiple charging of poly(propylene glycol) by binary mixtures of cations in electrospray

Single, double and triple charging of poly(propylene glycol) (PPG) (Mn = 1900 g/mol) in the presence of binary mixtures of cations (Li+, Na+, K+, Cs+, and NH4+) under electrospray ionization (ESI) conditions were investigated. For these studies, sodium ion was selected as the reference cation, and the resulting ion-intensities were evaluated as a function of the [Na+]/[C+] ratio (where C+ is the other cation, i.e., Li+, K+, Cs+ and NH4+). A linear relationship was found between INa+/IC+)and [Na+]/[C+] (INa+ and IC+ stand for the intensity of the singly charged PPG molecules cationized with Na+ and C+ ions, respectively). The slope of the INa+/IC+--[Na+]/[C+] plot (alpha) indicates the binding selectivity of Na+ ions to PPG chains with respect to cation C+. In the case of the doubly charged PPG chains, the INaNa2+/INaC2+ and INaC2+/ICC2+ versus [Na+]/[C+] ratio also yield straight lines with slopes of approximately alpha/2 and 2alpha, respectively (INaNa2+, INaC2+ and ICC2+ are the intensity of the doubly charged PPG chains cationized with two Na+ ions, Na+ and C+ ions, and two C+ ions, respectively). Similarly, linear dependences with the [Na+]/[C+] ratio for the corresponding intensity ratios of the triply charged PPG were found. Based on the value of alpha, the selectivity of the cations was found to increase in the order of Li+ < Cs+ approximately Na+ < K+ approximately NH4+. The observed relative ion intensities are interpreted on the basis of the solution state equilibrium between PPG and the cations. In addition, the investigations showed that the abundances of the doubly and triply charged PPG-containing mixed cations can be optimized in a simple way using the value of alpha.     

Binding of monovalent metal cations by the p-sulfonatocalix[4]arene: experimental evidence for cation-pi interactions in water

Gibbs free energies, enthalpies and entropies for the binding of Na+, K+, Rb+, Cs+, Ag+, Tl+ and NH4+ by the p-sulfonatocalix[4]arene in water are determined by microcalorimetry. Whereas no significant heat effect is detected with Na+ or Ag+, suggesting that these cations are not complexed, weak but selective binding is observed with the other cations. The whole set of thermodynamic parameters, which demonstrate that the cations bind inside the cavity of the calixarene, evidence the importance of the cation-pi interactions for these complexes in water.     

Selective adsorption of poly(ethylene oxide) onto a charged surface mediated by alkali metal ions

Using a surface force balance, we have measured normal and shear interactions between mica surfaces across pure water and across 0.1 M aqueous solutions of LiNO3, NaNO3, KNO3, and CsNO3, both prior to adding polymer and following addition of 1.5 x 10(-4) w/w poly(ethylene oxide) (PEO, Mw = 170 kD) and overnight incubation. Our results reveal that while the PEO adsorbs strongly from the KNO3 and CsNO3 solutions, unexpectedly it does not adsorb at all from the LiNO3 and NaNO3 salt solutions. We attribute this to the different nature of the hydration layers about the alkali metal ions: these favor liganding to the negatively charged mica surface of the etheric -O- group on the ethylene oxide monomer for the case of the more weakly hydrated K+ and Cs+, but not for the case of Na+ or Li+ with their more strongly bound water. A simple model relating the electrostatic energy changes occurring upon such liganding to the experimentally measured hydration energies of the different alkali metal ions supports this attribution.     

Effect of alkali metal cations on adsorption of guar gum onto quartz

The effect of cesium, potassium, sodium, and lithium cations on the adsorption of natural guar gum onto quartz was investigated. The role of these ions was analyzed in terms of their water structure-making or -breaking capabilities. In the presence of structure makers (Na+, Li+) the polymer adsorption density did not change compared to the adsorption levels observed in distilled water. However, in dilute solutions (0.01 N) of structure-breaking cations (Cs+, K+) the adsorption density of guar gum significantly increased, with potassium and cesium producing the same adsorption densities of the polymer. The resulting colloidal aggregation/dispersion equilibria in the quartz-guar gum system were discussed and mechanisms of guar gum-quartz interactions were also suggested. Assuming hydrogen bonding to be the driving adsorption mechanism, it was proposed that guar gum molecules compete with water for silanol surface sites. Structure-breaking cations disturb the interfacial water structure around the quartz particles thus allowing the polymer to more closely approach the quartz surface and interact with the surface groups.     

Solid-state nanopores for ion and small molecule analysis

Solid-state nanopore in analytical chemistry has developed rapidly in the 1990s and it is proved to be a versatile new tool for bioanalytical chemistry. The research field of solid-state nanopore starts from mimicking the biological nanopore in living cells. Understanding the transport mechanism of biological nanopore in vivo is a big challenge because of the experimental difficulty, so it is essential to establish the basic research of artificial nanopores in vitro especially for the analysis of ions and small molecules. The performance of solid-state nanopores could be evaluated by monitoring currents when ions and molecules passed through. The comparison of the two types of nanopores based on current-derived information can reveal the principle of biological nanopores, while the solid-state nanopores are applied into practical bioanalysis. In this review, we focus on the researches of the solid-state nanopores in the fabrication process and in the analysis of ions and small molecules. Fabrication methods of nanopores, ion transport mechanism, small molecule analysis and theoretical studies are discussed in detail.     

Solid-state nanopores for ion and small molecule analysis

Solid-state nanopore in analytical chemistry has developed rapidly in the 1990s and it is proved to be a versatile new tool for bioanalytical chemistry. This review focuses on the analysis of ions and small molecules with nanopores including nanopipettes, polymer film nanopores, Si₃N₄ nanopores, graphene nanopores, MoS₂ nanopores and MOFs.     

Folding of the thrombin aptamer into a G-quadruplex with Sr(2+): stability, heat, and hydration.

It has been shown that the DNA aptamer d(G(2)T(2)G(2)TGTG(2)T(2)G(2)) adopts an intramolecular G-quadruplex structure in the presence of K+. Its affinity for trombin has been associated with the inhibition of thrombin-catalyzed fibrin clot formation. In this work, we used a combination of spectroscopy, calorimetry, density, and ultrasound techniques to determine the spectral characteristics, thermodynamics, and hydration effects for the formation of G-quadruplexes with a variety of monovalent and divalent metal ions. The formation of cation-aptamer complexes is relatively fast and highly reproducible. The comparison of their CD spectra and melting profiles as a function of strand concentration shows that K+, Rb+, NH(4)+, Sr(2+), and Ba(2+) form intramolecular cation-aptamer complexes with transition temperatures above 25 degrees C. However, the cations Li+, Na+, Cs+, Mg(2+), and Ca(2+) form weaker complexes at very low temperatures. This is consistent with the observation that metal ions with ionic radii in the range 1.3-1.5 A fit well within the two G-quartets of the complex, while the other cations cannot. The comparison of thermodynamic unfolding profiles of the Sr(2+)-aptamer and K+ -aptamer complexes shows that the Sr(2+)-aptamer complex is more stable, by approximately 18 degrees C, and unfolds with a lower endothermic heat of 8.3 kcal/mol. This is in excellent agreement with the exothermic heats of -16.8 kcal/mol and -25.7 kcal/mol for the binding of Sr(2+) and K+ to the aptamer, respectively. Furthermore, volume and compressibility parameters of cation binding show hydration effects resulting mainly from two contributions: the dehydration of both cation and guanine atomic groups and water uptake upon the folding of a single-strand into a G- quadruplex structure.     

冠醚配合物的热力学性质的研究 III. 碱金属离子与18C6甲基衍生物配位反应的电导研究

The coordination reaction of Na+, K+, Rb+ and Cs+ with benzo- 15-crown-5, 18-crown-6 and the newly synthesized cyclic polyethers 2, 3-benzo-8, 15-dimethyl-18-crown-6, 2, 3-benzo-8, 11, 15-trimethyl-18-crown-6 in methanol at 25`C has been studied by conductometric titration. The stability constants for the 1:1 coordination compounds were calculated. The marked selectivity of 18-crown-6 toward alkali metal ions was not found in its methyl derivatives. The induction effect of the benzene ring and methyl group on polyether ring reduced the stability of the coordination compounds. In methanol, the stability sequence of te compounds of alkali metal ions with 18-crown-6 was K+>Rb+>Cs+>Na+, that of its dimethyl derivative was K+>Rb+>Na+>Cs+ and that of its trimethyl derivative was K+>Na+>Rb+>Cs+, that is, the methyl substituent had a weaker influence on the stability of Na+ compound than on that of Rb+ or Cs+ compound. In the range of concentration studied, decrease in equivalent conductance is in agreement with the prediction on the basis of the structure of the complexes. The above results may give a clue for modifying the structure of a crown ether for specified selectivity.     

Effect of exchangeable cation on the swelling property of 2:1 dioctahedral smectite--a periodic first principle study

We used both localized and periodic calculations on a series of monovalent (Li+, Na+, K+, Rb+, Cs+) and divalent (Mg2+, Ca2+, Sr2+, Ba2+) cations to monitor their effect on the swelling of clays. The activity order obtained for the exchangeable cations among all the monovalent and divalent series studied: Ca2+ > Sr2+ > Mg2+ > Rb+ > Ba2+ > Na+ > Li+ > Cs+ > K+. We have shown that, in case of dioctahedral smectite, the hydroxyl groups play a major role in their interaction with water and other polar molecules in the presence of an interlayer cation. We studied both type of clays, with a different surface structure and with/without water using a periodic calculation. Interlayer cations and charged 2:1 clay surfaces interact strongly with polar solvents; when it is in an aqueous medium, clay expands and the phenomenon is known as crystalline swelling. The extent of swelling is controlled by a balance between relatively strong swelling forces and electrostatic forces of attraction between the negatively charged phyllosilicate layer and the positively charged interlayer cation. We have calculated the solvation energy at the first hydration shell of an exchangeable cation, but the results do not correspond directly to the experimental d-spacing values. A novel quantitative scale is proposed with the numbers generated by the relative nucleophilicity of the active cation sites in their hydrated state through Fukui functions within the helm of the hard soft acid base principle. The solvation effect thus measured show a perfect match with experiment, which proposes that the reactivity index calculation with a first hydration shell could rationalize the swelling mechanism for exchangeable cations. The conformers after electron donation or acceptance propose the swelling mechanism for monovalent and divalent cations.