Ion Hydration and Association in an Aqueous Calcium Chloride Solution in the GPa Range

Abstract

Neutron diffraction measurements of an aqueous 2 mol dm–3 CaCl2 solutions in D2O have been made at 1 GPa, 298 K as well as 0.1 MPa, 298 K. The experimental structure factors are subjected to Empirical Potential Structure Refinement (EPSR) modeling to reveal the ion hydration and association and solvent water at the atomic level. About seven water molecules surround Ca2+ at the Ca–O and Ca–D distances of 2.44 Å and 3.70 Å, respectively, at both pressures, suggesting no significant pressure effect on the cation hydration. On the other hand, the Cl‐ ion shows a drastic change in water oxygen coordination from ca. 7 at 0.1 MPa to ca. 14 at 1 GPa, accompanied by shortening of Cl–O distance from 3.18 Å to 3.15 Å. However, the number of water hydrogen atoms around Cl– does not change significantly as 6.0–6.7 in spite of shortening Cl–D distance from 2.22 to 2.18 Å on compression. The pressure effect on the solvent water structure is also drastic as an increase in water oxygen atoms of 4.7 at the O–O distance of 2.79 Å at 0.1 MPa to 10.3 at 2.85 Å at 1 GPa. The number of water hydrogen atoms, however, does not change as 1.2 at the O‐D distance of 1.74 Å for both pressures, demonstrating the presence of the O···D hydrogen bonds which are significantly bent at 1 GPa at 298 K. This change of hydrogen bonds in water with pressure probably causes the drastic increase in water oxygen atoms around Cl–.

Pressure-Dependent Structure of Methanol–Water Mixtures up to 1.2 GPa: Neutron Diffraction Experiments and Molecular Dynamics Simulations

“…The uncertainty of the pressure was considered to be less than 10%. The description of the sample holder, along with the experimental setup, appears in a recent publication [ 21 ].…”

“…Raw data were handled by a protocol developed by the instrument scientists of the PLANET instrument (see also in [ 21 ]).…”

Ion solvation and association and water structure in an aqueous cerium (III) chloride solution in the gigapascal pressure range

“…We have recently succeeded in measurements of aqueous electroltye solutions under high pressure and high temperaure, particularly in the gigapascal pressure range, using intense synchrotron radiation and pulsed neutrons combined with high-pressure cells. 6,7 Furthermore, by analyzing X-ray and neutron interference functions with empirical potential structure refinement (EPSR) modeling, 8 the detailed 3D structure of ion solvation of monovalent Na + and Cland divalent Ca 2+ has been elucidated. 6,7 It would be essential to investigate the hydration of trivalent ions in the GPa pressure range to see how much the GPa pressure affects the highly charged ion hydration.…”

“…6,7 Furthermore, by analyzing X-ray and neutron interference functions with empirical potential structure refinement (EPSR) modeling, 8 the detailed 3D structure of ion solvation of monovalent Na + and Cland divalent Ca 2+ has been elucidated. 6,7 It would be essential to investigate the hydration of trivalent ions in the GPa pressure range to see how much the GPa pressure affects the highly charged ion hydration.…”

Neutron Scattering in Coordination Chemistry

“…Many of the contributions in this edition feature data recorded at temperatures well below those available to most bench‐based chemists, opening up the scope for researchers to study molecular magnetism and excitations from true ground state populations. Yamaguchi et al have measured diffraction data at pressures relevant to geological environments (GPa) to understand the hydration properties of calcium chloride under conditions that are similar to what is found in interfacial zones of continental plates and the mantle, which is central to earthquake and volcanic activity …”