Alkali Halide Ions Research Articles

The structure of ionic aqueous solutions at interfaces: An intrinsic structure analysis

We investigate the interfacial structure of ionic solutions consisting of alkali halide ions in water at concentrations in the range 0.2-1.0 molal and at 300 K. Combining molecular dynamics simulations of point charge ion models and a recently introduced computational approach that removes the averaging effect of interfacial capillary waves, we compute the intrinsic structure of the aqueous interface. The interfacial structure is more complex than previously inferred from the analysis of mean profiles. We find a strong alternating double layer structure near the interface, which depends on the cation and anion size. Relatively small changes in the ion diameter disrupt the double layer structure, promoting the adsorption of anions or inducing the density enhancement of small cations with diameters used in simulation studies of lithium solutions. The density enhancement of the small cations is mediated by their strong water solvation shell, with one or more water molecules "anchoring" the ion to the outermost water layer. We find that the intrinsic interfacial electrostatic potential features very strong oscillations with a minimum at the liquid surface that is ∼4 times stronger than the electrostatic potential in the bulk. For the water model employed in this work, SPC/E, the electrostatic potential at the water surface is ∼-2 V, equivalent to ∼80 k(B)T (for T = 300 K), much stronger than previously considered. Furthermore, we show that the utilization of the intrinsic surface technique provides a route to extract ionic potentials of mean force that are not affected by the thermal fluctuations, which limits the accuracy of most past approaches including the popular umbrella sampling technique.

SIZE DEPENDENCE OF SOLVATION STRUCTURE AND DYNAMICS OF IONS IN LIQUID N-METHYLACETAMIDE: A MOLECULAR DYNAMICS SIMULATION STUDY

The solvation structure and dynamics of alkali metal (Li+, Na+, K+, Rb+, Cs+) and halide (F-, Cl-, Br-, I-) ions in liquid N -methylacetamide (NMA) are calculated at two different temperatures T = 313 K and 453 K, by using classical molecular dynamics simulations. We have also considered [Formula: see text] and some larger cations such as I +, Me 4 N +, and Et4N+ in this study to investigate the size dependence solvation structure and dynamics of ions in liquid NMA. With the increase of ion size, the self-diffusion coefficients of cations are found to increase and the maximum is observed at Me4N+ , whereas for halide ions the increase of diffusion coefficient with ion size continues up to I- and no maximum is observed. However, the relative increase of the diffusion coefficients of larger ion compared to those of Li+ and F+ are found to be significantly higher at low temperature. Results are very good in agreement with experimental observation.

Diffusion of ions in supercritical water: Dependence on ion size and solvent density and roles of voids and necks

We have performed molecular dynamics simulations of alkali metal (Li +, Na +, K +, Rb +, Cs +) and halide (F −, Cl −, Br −, I −) ions in supercritical water at 673 K. The calculations were done for water at three different densities of 1.0, 0.7 and 0.35 g cm −3 to investigate the effects of solute size on the diffusion of ions in supercritical water. On increase of ion size, we observe a maximum for diffusion of ions in supercritical water of higher densities (1.0 and 0.7 g cm −3). However, no such maximum is found for ion diffusion in the supercritical water of low density (0.35 g cm −3) or for diffusion of neutral solutes at all densities. These results are analyzed in terms of passage through voids and necks present in supercritical water. Correlations of the observed diffusion behavior with the sizes of ions and voids present in the systems are discussed.

Ditopic Binding of Alkali Halide Ions to Trimethylboroxine

Trimethylboroxine (TMB) is a six-membered ring compound containing Lewis acidic boron and Lewis basic oxygen atoms that can bind halide anion and alkali metal cation, respectively. We employed Fourier transform ion cyclotron resonance spectroscopy to study the gas-phase binding of LiBrLi + and F � (KF)2 to TMB. TMB forms association complexes with both LiBrLi + and F - (KF)2 at room temperature, providing direct evidence for the ditopic binding. Interestingly, the TMB·F � (KF)2 anion complex is formed 33 times faster than the TMB·Li + BrLi cation complex. To gain insight into the ditopic binding of an ion pair, we examined the structures and energetics of TMB·Li + , TMB·F - , TMB·LiF (the contact ion pair), and Li + ·TMB·F - (the sep- arated ion pair) using Hartree-Fock and density functional theory. Theory suggests that F - binds more strongly to TMB than Li + and the contact ion-pair binding (TMB·LiF) is more stable than the separated ion-pair binding (Li + ·TMB·F - ).

BH4− Self-Diffusion in Liquid LiBH4

The hydrogen dynamics in solid and in liquid LiBH4 was studied by means of incoherent quasielastic neutron scattering. Rotational jump diffusion of the BH4- subunits on the picosecond scale was observed in solid LiBH4. The characteristic time constant is significantly shortened when the system transforms from the low-temperature phase to the high-temperature phase at 383 K. In the molten phase of LiBH4 above 553 K, translational diffusion of the BH4- units is found. The measured diffusion coefficients are in the 10(-5)cm2/s range at temperatures around 700 K, which is in the same order of magnitude as the self-diffusion of liquid lithium or the diffusion of ions in molten alkali halides. The temperature dependence of the diffusion coefficient shows an Arrhenius behavior, with an activation energy of Ea = 88 meV and a prefactor of D0 = 3.1 × 10(-4)cm2/s.

Ion Interactions with the Carbon Nanotube Surface in Aqueous Solutions: Understanding the Molecular Mechanisms

We study the molecular mechanisms of alkali halide ion interactions with the single-wall carbon nanotube surface in water by means of fully atomistic molecular dynamics simulations. We focus on the basic physical-chemical principles of ion-nanotube interactions in aqueous solutions and discuss them in light of recent experimental findings on selective ion effects on carbon nanotubes.

Molecular Dynamics Study of Interfacial Confinement Effects of Aqueous NaCl Brines in Nanoporous Carbon

In this paper, studies of aqueous electrolyte solutions in contact with a family of porous carbon geometries using classical molecular dynamics simulations are presented. These simulations provide an atomic scale depiction of ion transport dynamics in different environments to elucidate power of aqueous electrolyte supercapacitors. The electrolyte contains alkali metal and halide ions, which allow for the examination of size trends within specific geometries as well as trends in concentration. The electrode pores are modeled as planar graphite sheets and carbon nanotubes with interstices ranging from one to four nanometers. Ordered layers form parallel to the carbon surface, which facilitates focused ion motion under slightly confining conditions. As a result, the ion’s diffusivities are enhanced in the direction of the slit or pore. Further confining the system leads to decreased ion diffusivities. The ions are fully hydrated in all but the smallest slits and pores with those sizes showing increased ion pairing. There is strong evidence of charge separation perpendicular to the surface at all size scales, concentrations, and ion types, providing a useful baseline for examining differential capacitance behavior and future studies on energy storage. These systems show promise as high-power electrical energy storage devices.

Importance of Accurate Dynamic Polarizabilities for the Ionic Dispersion Interactions of Alkali Halides

Ab initio quantum mechanical calculations of the dynamic polarizability of alkali metal and halide ions are performed as a function of imaginary frequency. Electron correlation is shown to provide a significant correction to ionic polarizabilities. Ab initio ion-surface dispersion coefficients are compared with single- and multimode London approximations. The commonly employed single-mode model with the characteristic frequency taken from the ionization potential of the ion is shown to be inadequate, underestimating dispersion forces with an average error around 40% or as high as 80% for halide ions. Decomposition of the polarizability data into five modes covers the major modes of each ion adequately (four modes for Li(+)). Illustrative calculations of surface potentials at the mica surface in aqueous alkali halide electrolytes are made. Charge reversal is obtained with the more polarizable cations, K(+) and Rb(+). The error in the single-mode ionization potential models is seen as a strong shift in the surface potential from negative toward positive values.

Ion pairing in molecular simulations of aqueous alkali halide solutions.

Using classical molecular dynamics simulations, we study ion-ion interactions in water. We study the potentials of mean force (PMF) for the full set of alkali halide ion pairs, and in each case, we test different parameter sets for modeling both the water and the ions. Altogether, we compared 300 different PMFs. We also calculate association equilibrium constants (KA) and compare them to two types of experiments. Of additional interest here was the proposition of Collins called the "law of matching water affinities", where the relative affinity of ions in solution depends on the matching of cation and anion sizes. From observations on the relative depths of the free energies of the contact ion pair (CIP) and the solvent-shared ion pair (SIP), along with related solvent structure analyses, we find a good correlation with this proposition: small-small and large-large should associate in water, and small-large should be more dissociated.

Perturbations of Water by Alkali Halide Ions Measured using Multivariate Raman Curve Resolution

Polarized Raman spectroscopy, combined with multivariate curve resolution (MCR), is used to measure the influence of dilute alkali halide ions on the OH stretch vibrational band of water. The frequency and integrated intensity of the resulting hydration shell OH bands are found to increase with increasing anion size. Comparisons of results obtained from salt solutions in H(2)O and HOD/D(2)O imply that ion-water interactions reduce the influence of resonance coupling on the OH stretch band of H(2)O. Polarized Raman results indicate that the hydration shell of F(-) gives rise to a nearly perfectly polarized OH stretch band, while large anions produce larger depolarization.

Unraveling Water’s Entropic Mysteries: A Unified View of Nonpolar, Polar, and Ionic Hydration

[Figure: see text]. Most chemical processes on earth are intimately linked to the unique properties of water, relying on the versatility with which water interacts with molecules of varying sizes and polarities. These interactions determine everything from the structure and activity of proteins and living cells to the geological partitioning of water, oil, and minerals in the Earth's crust. The role of hydrophobic hydration in the formation of biological membranes and in protein folding, as well as the importance of electrostatic interactions in the hydration of polar and ionic species, are all well known. However, the underlying molecular mechanisms of hydration are often not as well understood. This Account summarizes and extends emerging understandings of these mechanisms to reveal a newly unified view of hydration and explain previously mystifying observations. For example, rare gas atoms (e.g., Ar) and alkali-halide ions (e.g., K+ and Cl-) have nearly identical experimental hydration entropies, despite the significant charge-induced reorganization of water molecules. Here, we explain how such previously mysterious observations may be understood as arising from Gibbs inequalities, which impose rigorous energetic upper and lower bounds on both hydration free energies and entropies. These fundamental Gibbs bounds depend only on the average interaction energy of a solute with water, thus providing a deep link between solute-water interaction energies and entropies. One of the surprising consequences of the emerging picture is the understanding that the hydration of an ion produces two large but nearly perfectly cancelling, entropic contributions: a negative ion-water interaction entropy and a positive water reorganization entropy. Recent work has also clarified the relationship between the strong cohesive energy of water and the free energy required to form an empty hole (cavity) in water. Here, we explain how linear response theory (whose roots may also be traced to Gibbs inequalities) can provide remarkably accurate descriptions of the process of filling aqueous cavities with nonpolar, polar, or charged molecules. The hydration of nonpolar molecules is well-described by first-order perturbation theory, which implies that turning on solute-water van der Waals interactions does not induce a significant change in water structure. The larger changes in water structure that are induced by polar and ionic solutes are well-described by second-order perturbation theory, which is equivalent to linear response theory. Comparisons of the free energies of nonpolar and polar or ionic solutes may be used to experimentally determine electrostatic contributions to water reorganization energies and entropies. The success of this approach implies that water's ability to respond to solutes of various polarities is far from saturated, as illustrated by simulations of acetonitrile (CH 3CN) in water, which reveal that even such a strongly dipolar solute only produces subtle changes in the structure of water.

Determination of alkali and halide monovalent ion parameters for use in explicitly solvated biomolecular simulations.

Alkali (Li+, Na+, K+, Rb+, and Cs+) and halide (F−, Cl−, Br−, and I−) ions play an important role in many biological phenomena, roles that range from stabilization of biomolecular structure, to influence on biomolecular dynamics, to key physiological influence on homeostasis and signaling. To properly model ionic interaction and stability in atomistic simulations of biomolecular structure, dynamics, folding, catalysis, and function, an accurate model or representation of the monovalent ions is critically necessary. A good model needs to simultaneously reproduce many properties of ions, including their structure, dynamics, solvation, and moreover both the interactions of these ions with each other in the crystal and in solution and the interactions of ions with other molecules. At present, the best force fields for biomolecules employ a simple additive, nonpolarizable, and pairwise potential for atomic interaction. In this work, we describe our efforts to build better models of the monovalent ions within the pairwise Coulombic and 6-12 Lennard-Jones framework, where the models are tuned to balance crystal and solution properties in Ewald simulations with specific choices of well-known water models. Although it has been clearly demonstrated that truly accurate treatments of ions will require inclusion of nonadditivity and polarizability (particularly with the anions) and ultimately even a quantum mechanical treatment, our goal was to simply push the limits of the additive treatments to see if a balanced model could be created. The applied methodology is general and can be extended to other ions and to polarizable force-field models. Our starting point centered on observations from long simulations of biomolecules in salt solution with the AMBER force fields where salt crystals formed well below their solubility limit. The likely cause of the artifact in the AMBER parameters relates to the naive mixing of the Smith and Dang chloride parameters with AMBER-adapted Åqvist cation parameters. To provide a more appropriate balance, we reoptimized the parameters of the Lennard-Jones potential for the ions and specific choices of water models. To validate and optimize the parameters, we calculated hydration free energies of the solvated ions and also lattice energies (LE) and lattice constants (LC) of alkali halide salt crystals. This is the first effort that systematically scans across the Lennard-Jones space (well depth and radius) while balancing ion properties like LE and LC across all pair combinations of the alkali ions and halide ions. The optimization across the entire monovalent series avoids systematic deviations. The ion parameters developed, optimized, and characterized were targeted for use with some of the most commonly used rigid and nonpolarizable water models, specifically TIP3P, TIP4PEW, and SPC/E. In addition to well reproducing the solution and crystal properties, the new ion parameters well reproduce binding energies of the ions to water and the radii of the first hydration shells.

Quantized Hydration Energies of Ions and Structure of Hydration Shell from the Experimental Gas-Phase Data

With previous data on alkali metal and halide ions included [Rais, J.; Okada, T. Anal. Sci. 2006, 22, 533], we analyzed rather broad data on ionic hydration from the point of view of gaseous cluster energetics. We have now added alkaline earth cations, Zn(2+), H(+), OH(-), Cu(+), Ag(+), Bi(+), Pb(+), and alkylammonium cations. The present analysis revealed the octa-coordinated nature of alkaline earth cations, which is not fully pronounced for Be(2+) and Zn(2+), existence of Eigen protonium complex, which is trigonally hydrated, and particular property of the first OH-, H(2)O cluster. Whereas these findings are generally in accordance with theoretical model calculation studies, we have foreseen in addition tetrahedral hydration for halide anions and Rb(+) and Cs(+), as well as for alkylammonium ions. The obtained picture of the quantized solvation of ions is mirrored in the ionization potentials of outer electrons of pertinent atoms. This is a second independent phenomenon, and together, they invoked a common pattern formation ("Aufbau") obeying tetra- and octa-coordinated principles.

Evaluation of individual ionic partial molar volumes in aqueous solutions

In a recent H 2O–D 2O molar volume isotope effect study on the alkali metal halide solutions, individual ionic standard partial molar volume contributions ( V ¯ ion ∘ ( H 2 O ) - V ¯ ion ∘ ( D 2 O ) = Δ V ¯ ion ∘ ( H–D ) ) have been derived, dependent only on the chosen (Pauling) ionic radii, r ion. It has been concluded that the isotope effect of the cations (except for Li +) and that of the chloride anion exhibit a linear function of 1/ r ion, with Δ V ¯ ion ∘ ( H − D ) vanishing when 1/ r ion approaches zero. On the other hand, the directly measurable hydrodynamic “cosphere” volumes of the ions, V ¯ cosph ∘ , derived from the kinetic ionic (Stokes) radii, show a similar pattern, here also the cationic and anionic r ion relations cross or meet at V ¯ cosph ∘ ( Cs + ) ≅ V ¯ cosph ∘ ( Cl - ) . Because both Δ V ¯ ion ∘ ( H–D ) and V ¯ cosph ∘ originate in the hydration volumes of the ions, ( V ¯ hydr ∘ = V ¯ ion ∘ - V ¯ intr ∘ ) determined by electrostriction, this r ion pattern of the V ¯ hydr ∘ (ion) provides an “extra-thermodynamic” assumption to evaluate the individual cationic and anionic molar volume contributions in normal water solution. As a basis for the evaluation, this r ion pattern is applied to the conventional cationic and anionic V ¯ hydr ∘ (ion) values (related to V ¯ ion ∘ ( H + ,aq . ) = 0 cm 3 · mol - 1 ) , calculated from literature V ¯ ion ∘ data, assuming that the V ¯ intr ∘ (ion) = V ¯ cryst ∘ (ion). To adjust the V ¯ hydr ∘ ( Cl - ) to the V ¯ hydr ∘ ( Na + , K + , Rb + , Cs + ) versus r ion relation at T = 298.15 K, the conventional anionic V ¯ hydr ∘ (ion) data have to be increased, the cations have to be decreased by 4.2 cm 3 · mol −1. Similar adjustments at five different temperatures between T = (273 and 323) K, result in V ¯ ion ∘ ( H + ,aq . ) = - ( 4.2 ± 0.2 ) cm 3 · mol - 1 at T = 298.15 K, with an increment of −0.02 cm 3 · K −1 · mol −1. Related to these parameters, relations of V ¯ ion ∘ versus temperature for the alkali metal and halide ions were calculated. The cationic and anionic V ¯ hydr ∘ (ion) values, when related to V ¯ ion ∘ ( H + ,aq . ) = - 4.2 cm 3 · mol - 1 , show strong r ion correlation with V ¯ cosph ∘ , their ratio versus r ion offers a linear equation to compare the influence of the ions on the solvent structure on a well defined “quantitative” scale. The value of the derived V ¯ ion ∘ ( H + ,aq . ) , while being consistent with that widely accepted ca. − (5 ± 2) cm 3 · mol −1, has a higher precision with a reasonable basis of the applied “extra-thermodynamic” assumption. It can serve to verify other partitioning procedures used in non-aqueous solvents.

Determination of the absolute values of Gibbs energy of hydration or solvation ΔG0 abs(M+ or X-)h or s of monovalent (alkali metal and halide) ions and other thermodynamic parameters in aqueous and non-aqueous solvents using a single standard state and analysis of ΔG0 abs(M+ or X-)h or s values based on cluster-ion solvation method

or X - ) h or s and other thermodynamic parameters in aqueous and non-aqueous solvents (methanol, ethanol, n-propanol, isopropanol, n-butanol, ethylene glycol, propylene carbonate (PC), N-methyl formamide (NMF), acetone, tetrahydrofuran (THF), 1,4-dioxan, acetonitrile) were determined directly using modified Born equation and a single standard state [i.e. AG° and ΔH 0 (but not ΔS 0 ) of H 2 gas and other elements to be zero in their elementary standard states at 1 atm. pressure (1 bar) and 298 K]. The ΔH 0 abs (M + or X - ) h or s and ΔG 0 (M + or X - ) h or s values are calculated considering ion-dipole, ion-quadrupole interactions and without considering the interaction terms. Very few data are available in non-aqueous solvents for comparison. Most of the single ion values (particularly for the anions) suffer from limitations associated with the erroneous principle of division of solvation energies ΔG 0 h or s (MX) or ΔΗ 0 h or s (MX) of electrolytes into single ion values. ΔG 0 (M + or X - ) h values determined using cluster-ion solvation data have also been presented. The values have been claimed to be most accurate in recent years. However, the method involves a number of assumptions of doubtful validity and the values cannot be regarded to be equivalent to ΔG 0 (Μ + or X - ) h or determined by other workers using different methods. Coupling the values of ΔG 0 (M + or X - ) h or s and ΔH 0 (M + or X - ) h or s with ΔG 0 (M + or X - ) g and ΔH 0 (M + or X - ) g values in the gaseous state, ΔG 0 (M + or X - ) Water or org. solvent' ΔH 0 (Μ + or X - ) water or org. solvent and ΔS 0 (M + or X - ) water or org. solvent [values in water and in organic solvents] are determined.

Photoluminescence of Au− formed in 12CaO · 7Al2O3 single crystal by Au+-implantation

Au+ ion implantation with fluences from 1×1014 to 3×1016cm−2 into 12CaO·7Al2O3 (C12A7) single crystals was carried out at a sample temperature of 600°C. The implanted sample with the fluence of 1×1015cm−2 exhibited photoluminescence (PL) bands peaking at ∼3.1 and ∼2.3eV at ⩽150K when excited by He–Cd laser (325nm). This was the first observation of PL from C12A7. These two PL bands are possibly due to intra-ionic transitions of an Au− ion having the electronic configuration of 6s2, judged from their similarities to those reported on Au− ions in alkali halides. However, when the concentration of the implanted Au ions exceeded the theoretical maximum value of anions encaged in C12A7 (∼2.3×1021cm−3), surface plasmon absorption appeared in the optical absorption spectrum, suggesting Au colloids were formed at such high fluences. These observations indicate that negative gold ions are formed in the cages of C12A7 by the Au+ implantation if an appropriate fluence is chosen.

Erratum to: Three Empirical Correlations Connecting Gaseous Cluster Energies and Solvation Energies of Alkali Metal and Halide Ions

Erratum to: Three Empirical Correlations Connecting Gaseous Cluster Energies and Solvation Energies of Alkali Metal and Halide Ions

Three Empirical Correlations Connecting Gaseous Cluster Energies and Solvation Energies of Alkali Metal and Halide Ions

This paper gives two empirical correlations of formation Gibbs energies of gaseous clusters DeltaG(f)n as function of number of solvent molecules attached to the ion, n, and one correlation connecting the DeltaG(f)n for each individual cluster with the total DeltaG(o)hydr value. The experimental ratios of DeltaG(f)2/DeltaG(f)1 and DeltaG(f)3/DeltaG(f)1 for both alkali metal and halide ions are on average equal to 0.75 and 0.5, respectively. DeltaG(f)n values for n > or = 4 are correlated with n as DeltaG(f)n = [a/(n - 1)] DeltaG(f)1 + b DeltaG(f)1. For all available data on cluster energies and each individual cluster, the DeltaG(f)n's are straight-line functions of DeltaG(o)hydr. This well corresponds to another empirical rule stating that the Gibbs energies of transfer of ions between two solvents are often as well straight-line functions of DeltaG(o)(hydr) [J. Rais and T. Okada, J. Phys. Chem. A, 2000, 104, 7314]. Tentative models of the found behavior are proposed. A full data set of the gaseous cluster energies of formation based on inclusion of new, usually not used entries from the literature is provided.

Solute size effects on the solvation structure and diffusion of ions in liquid methanol under normal and cold conditions

We have performed a series of molecular dynamics simulations of alkali metal (Li+, Na+, K+, Rb+, and Cs+) and halide (F-, Cl-, Br-, and I-) ions in liquid methanol at two different temperatures to investigate the effects of ion size on the hydration structure and diffusion of ions in methanol under normal and cold conditions. Simulations are also carried out for some of the larger cations such as I+, (CH3)4N+, and (C2H5)4N+ and also neutral alkali metal atoms in methanol at both temperatures. With the increase of ion size, the diffusion coefficients of both positive and negative ions are found to show anomalous behavior. For cations, it is found that the maximum of the diffusion coefficient versus ion size curve occurs at the rather large cation of (CH3)4N+ unlike in water where the maximum occurs at the relatively smaller ion of Rb+. For halide ions, the anomalous behavior, i.e., the increase of diffusion with ion size, continues up to iodide ion and no maximum is observed. These results are in good agreement with experimental observations. The diffusion coefficients of neutral atoms are found to be greater in methanol than that in water and they decrease monotonically with solute size, whereas the diffusion coefficients of the corresponding ions are found to be smaller in methanol. Accordingly, an ion experiences a smaller Stokes friction and a higher dielectric friction in methanol than in water. These contrasting effects are believed to be responsible for the shift of the maximum of ion diffusion toward a larger ion size when compared with similar anomalous size dependence in liquid water.

Solute–Solvent Charge Transfer in Aqueous Solution

H bonds as main electron-transfer vehicles: DFT molecular dynamics simulations on the GlyGly peptide in water indicate that solute–-solvent interactions appear to involve a significant amount of intermolecular charge transfer at the charged termini (see picture). Similar results are obtained for alkali halide ions in solution. The electronic charge mostly distributes uniformly throughout the H bonds, and it arranges electrostatically when they are absent.