Neutron Diffraction Study on the Structure of Aqueous LiNO3 Solutions

Structure and size of complete hydration shells of metal ions and inorganic anions in aqueous solution

The structures of nine hydrated metal ions in aqueous solution have been redetermined by large angle X-ray scattering to obtain experimental data of better quality than those reported 40-50 years ago. Accurate M-OI and M-(OI-H)⋯OII distances and M-OI(H)⋯OII bond angles are reported for the hydrated magnesium(II), aluminium(III), manganese(II), iron(II), iron(III), cobalt(II), nickel(II), copper(II) and zinc(II) ions; the subscripts I and II denote oxygen atoms in the first and second hydration sphere, respectively. Reported structures of hydrated metal ions in aqueous solution are summarized and evaluated with emphasis on a possible relationship between M-OI-OII bond angles and bonding character. Metal ions with high charge density have M-OI-OII bond angles close to 120°, indicative of a mainly electrostatic interaction with the oxygen atom in the water molecule in the first hydration shell. Metal ions forming bonds with a significant covalent contribution, as e.g. mercury(II) and tin(II), have M-OI-OII bond angles close to 109.5°. This implies that they bind to one of the free electron pairs in the water molecule. Comparison of M-O bond distances of hydrated metal ions in the solid state with one hydration shell, and in aqueous solution with in most cases at least two hydration shells, shows no significant differences. On the other hand, the X-O bond distance in hydrated oxoanions increases by ca. 0.02 Å in aqueous solution in comparison with the corresponding X-O distance in the solid state. A linear correlation is observed between volume, calculated from the van der Waals radius of the hydrated ion, and the ionic diffusion coefficient in aqueous solution. This correlation strongly indicates that monovalent metal ions, except lithium and silver(I), and singly-charged monovalent oxoanions have a single hydration shell. Divalent metal ions, bismuth(III) and the lanthanoid(III) and actinoid(III) ions have two hydration shells. Trivalent transition and tetravalent metal ions have two full hydration shells and portion of a third one. Doubly charged oxoanions have one well-defined hydration shell and an ill-defined second one.

Experimental Determination of Relationship between Intramolecular O-D Bond Length and Its Stretching Vibrational Frequency of D2O Molecule in the Liquid State

Experimental evidence has been obtained for the structure-spectra relationship of hydrogen bonds in aqueous solutions. Intramolecular O-D distance, r_OD, has been determined by the least-squares fitting analysis of the neutron interference term in the high-Q region observed for pure D₂O and concentrated aqueous solutions. The average O-D stretching frequency, ν_OD, has been obtained from the position of the center of gravity of the observed ATR-IR O-D stretching band. The linear relationship between r_OD and ν_OD has been confirmed in the liquid state. The slope of dν_OD/dr_OD is evaluated to be -21 000 ± 1000 cm^-1 Å^-1.

Liquid Structure with Nano-Heterogeneity Promotes Cationic Transport in Concentrated Electrolytes

Using molecular dynamics simulations, small-angle neutron scattering, and a variety of spectroscopic techniques, we evaluated the ion solvation and transport behaviors in aqueous electrolytes containing bis(trifluoromethanesulfonyl)imide. We discovered that, at high salt concentrations (from 10 to 21 mol/kg), a disproportion of cation solvation occurs, leading to a liquid structure of heterogeneous domains with a characteristic length scale of 1 to 2 nm. This unusual nano-heterogeneity effectively decouples cations from the Coulombic traps of anions and provides a 3D percolating lithium-water network, via which 40% of the lithium cations are liberated for fast ion transport even in concentration ranges traditionally considered too viscous. Due to such percolation networks, superconcentrated aqueous electrolytes are characterized by a high lithium-transference number (0.73), which is key to supporting an assortment of battery chemistries at high rate. The in-depth understanding of this transport mechanism establishes guiding principles to the tailored design of future superconcentrated electrolyte systems.