Hydration Of Ions In Aqueous Solutions Research Articles

Hydration of Alkali Metal and Halide Ions from Static and Dynamic Viewpoints.

Ion hydration in aqueous solutions plays a paramount role in many fields. Despite many studies on ion hydration, the nature of ion hydration is not consistently understood at the molecular level. Combining neutron scattering (NS), wide-angle X-ray scattering (WAXS), and molecular dynamics (MD), we quantify the ionic hydration degree (hydration ability) systematically for a series of alkali metal and halide ions based on static and dynamic hydration numbers. The former is based on the orientational correlation of water molecules bound to an ion derived from the positional information from NS and WAXS. The latter is defined as the mean number of water molecules remaining in the first coordination shell of an ion over a residence time of bound water molecules around the ion from MD. The static and dynamic hydration numbers distinguish hydration from coordination and quantify the ionic hydration degree, which provides a valuable reference for understanding various phenomena in nature.

Hydration numbers of ions in aqueous solutions of KOH, KCl, KI, and KIO3 according to refractometric data

Ion hydration in aqueous solutions of KOH, KCl, KI, and KIO3 was studied by refractometry. The sum of the hydration numbers of the potassium ion and each anion in these compounds and the hydration numbers of OH−, Cl−, I−, and IO3− anions were determined by the Lorentz-Lorenz equation at different salt concentrations. The model suggests that the polarizability of the hydrated ion is proportional to the cube of the ion radius and the volume of the hydrated ion equals the sum of the nonhydated ion and hydration shell volumes. The hydration numbers of the anions calculated by the proposed procedure increase with their radii in the sequence: OH−, Cl−, I−, and IO3−.

Ion hydration in aqueous solutions of lithium chloride, nickel chloride, and caesium chloride in ambient to supercritical water

Neutron diffraction with an isotopic substitution technique was used to determine the structure (bond distance, hydration number, and orientational correlation of water) of ion hydration in concentrated aqueous solutions of lithium chloride, nickel chloride and caesium chloride in ambient to supercritical water (648 K, 169 MPa). The lithium ion has an average hydration number of 4.1 ± 0.3 at 298 ~ 470 K, but 2.8 ± 0.3 at 647 K. The Li–O bond distance was 2.02 ± 0.01 Å over the temperature range measured, whereas the Li–D bond length decreased from 2.60 ± 0.01 Å at 298 K to 2.49 ± 0.01 Å at 647 K, indicating a weakening of the orientational correlation of water molecules around Li + in supercritical water. The Ni–O and Ni–D distances were 2.10 ± 0.01 Å and 2.68 ± 0.01 Å, respectively, irrespective of temperature at 298 ~ 473 K. The hydration number of Ni 2+ was found to be 6.4 ± 0.3 at 298 and 373 K, but decreased to 5.0 ± 0.2 at 473 K, suggesting the ion-pairing of Ni 2+–Cl − at 473 K. The chloride ions in lithium chloride and caesium chloride solutions are hydrogen bonded with water molecules at Cl–D (D 2O) distances of 2.28 ~ 2.33 Å over a temperature range measured. With an increase in temperature, the number of hydrogen bonded Cl–D interactions in the LiCl solutions changed from 5.6 ± 0.1 at 298 ~ 350 K, 4.5 ± 0.1 at 470 K, to 2.5 ± 0.1 at 648 K, showing that there is a marked lessening of the degree of orientational correlation of water molecules around Cl − in high-temperature liquid, in particular, in supercritical water. In the caesium chloride solution, on the other hand, there is a tendency for formation of contact ion-pair Cs +–Cl − at 473 K due to a decrease in n Cl–D of 3.9 ± 0.5, as also evidence from an X-ray diffraction study.

Hydroxide Ion Hydration in Aqueous Solutions

Hydroxide ion hydration was studied in aqueous solutions of selected alkali metal hydroxides by means of Fourier transform infrared (FTIR) spectroscopy of HDO isotopically diluted in H2O. The quantitative difference spectra procedure was applied for the first time to investigate such systems. It allowed removal of bulk water contribution and separation of the spectra of solute-affected HDO. The obtained spectral data were confronted with ab initio calculated structures of small gas-phase and polarizable continuum solvation model (PCM) solvated aqueous clusters, OH-(H2O)n, n = 1-7, to establish the structural and energetic states of hydration spheres of the hydrated hydroxide anion. This was achieved by comparison of the calculated optimal geometries with the interatomic distances derived from HDO band positions. The energetic state of water in OH- hydration shells, as revealed by solute-affected HDO spectra, is similar to that of an isoelectronic F- anion. No evidence was found for the existence of stable hydroxide dimer, H3O2-, in an aqueous solution. Spectral data do confirm, however, existence of a weak interaction with a single water molecule at the hydrogen site of OH-.