The quest for ion pairing in supercritical aqueous electrolytes

Structural and dynamical properties of concentrated alkali- and alkaline-earth metal chloride aqueous solutions

Concentrated ionic aqueous electrolytes possess a diverse array of applications across various fields, particularly in the field of energy storage. Despite extensive examination, the intricate relationships and numerous physical mechanisms underpinning diverse phenomena remain incompletely understood. Molecular dynamics simulations are employed to probe the attributes of aqueous solutions containing LiCl, NaCl, KCl, MgCl2, and CaCl2, spanning various solute fractions. The primary emphasis of the simulations is on unraveling the intricate interplay between these attributes and the underlying physical mechanisms. The configurations of cation-Cl- and Cl--Cl- pairs within these solutions are disclosed. As the solute fraction increases, consistent trends manifest regardless of solute type: (i) the number of hydrogen bonds formed by the hydration water surrounding ions decreases, primarily attributed to the growing presence of counter ions in proximity to the hydration water; (ii) the hydration number of ions exhibits varying trends influenced by multiple factor; and (iii) the diffusion of ions slows down, attributed to the enhanced confinement and rebound of cations and Cl- ions from the surrounding atoms, concurrently coupled with the changes in ion vibration modes. In our analysis, we have, for the first time, clarified the reasons behind the slowing down of the diffusion of the ions with increasing solute fraction. Our research contributes to a better understanding and manipulation of the attributes of ionic aqueous solutions and may help designing high-performance electrolytes.

Specific ion effects in aqueous eletrolyte solutions confined within graphene sheets at the nanometric scale

The underlying mechanisms of specific ion effects on structure and dynamics of aqueous solutions have been long debated. On the other hand, the role of polarization at hydrophobic interfaces when aqueous electrolytes are present is of great importance, as it has been observed at the air-vapor interface. In this work, we have explored influence of ionic species on microscopical properties of aqueous sodium halide solutions constrained inside a double layer graphene channel, as a model for a realistic hydrophobic interface. Our systems have been simulated by molecular dynamics techniques, explicitly including polarization in water molecules and ions. Water and ionic density profiles showed the tendency of ionic species to occupy the whole space available, in good agreement with spectroscopic experimental data. The exception to this general behavior was fluoride, which preferred to stay away from interfaces. Two main regions were defined: interfaces and the central part of the slab, the bulklike region. Ionic hydration numbers at interfaces were lower than those at the bulklike area by about one to two units. We have also analyzed water-ion orientations and polarization distributions and obtained a marked dependence on ionic concentration. Residence time of anions suffered important fluctuations and tended to be largest at interfaces. Large variations of the static permittivity between interfacial and bulklike regions were observed. Ionic diffusion was found to be between 10(-5) and 10(-6) cm(2) s(-1) and showed to be mainly dependent on the concentration, whereas the type of anion considered and the polarizability had significantly less relevance. Conductivities were found to be dependent on ionic concentrations and the polarizabilities of anions, as well as on the spatial direction considered.

Molecular Simulation Analysis and X-ray Absorption Measurement of Ca2+, K+and Cl-Ions in Solution

This paper presents recent advances in the use of molecular simulations and extended X-ray absorption fine structure (EXAFS) spectroscopy, which enable us to understand solvated ions in solution. We report and discuss the EXAFS spectra and related properties governing solvation processes of different ions in water and methanol. Molecular dynamics (MD) trajectories are coupled to electron scattering simulations to generate the MD-EXAFS spectra, which are found to be in very good agreement with the corresponding experimental measurements. From these simulated spectra, the ion-oxygen distances for the first hydration shell are in agreement with experiment within 0.05-0.1 A. The ionic species studied range from monovalent to divalent, positive and negative: K+, Ca2+, and Cl-. This work demonstrates that the combination of MD-EXAFS and the corresponding experimental measurement provides a powerful tool in the analysis of the solvation structure of aqueous ionic solutions. We also investigate the value of electronic structure analysis of small aqueous clusters as a benchmark to the empirical potentials. In a novel computational approach, we determine the Debye-Waller factors for Ca2+, K+, and Cl- in water by combining the harmonic analysis of data obtained from electronic structure calculations on finite ion-water clusters, providing excellent agreement with the experimental values, and discuss how they compare with results from a harmonic classical statistical mechanical analysis of an empirical potential.

Hydration and contact ion pairing of Ca2+ with Cl− in supercritical aqueous solution

X-ray absorption fine structure (XAFS) spectroscopy was used to measure the first-shell structure about Ca2+ in high-temperature aqueous solution. XAFS spectra were acquired at the Ca K edge at temperatures up to 400 degrees C and pressures up to 350 bars. For the system at 400 degrees C, both Ca (4038.5 eV) and Cl (2822.4 eV) K-edge data were acquired and a global model was used to fit the two independent sets of XAFS data. Measurements were made at the bending magnet beamline (sector 20) at the Advanced Photon Source, Argonne. Above 250 degrees C, a significant number of Ca2+-Cl- direct contact ion pairs form in agreement with existing thermodynamic data for this system. For a 1 m CaCl2 solution at 400 degrees C, the mean coordination structure about Ca2+ contains 3.2+/-0.6 water molecules at an average Ca-O distance of 2.356+/-0.026 A and 1.8+/-0.7 Cl- at a Ca-Cl distance of 2.677+/-0.007 A. An evaluation of the Ca and Cl preedge and near-edge (x-ray absorption structure) spectra provided further confirmation of the change in the Ca2+ first-shell structure and symmetry. Overall these measurements provide a structural basis for understanding solvation of Ca2+ in hydrothermal systems. These results also provide important new insights into the structural aspects of Ca2+ ion pairing that are the basis of many biological processes under ambient conditions.