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

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–. Empirical potential structure refinement modeling combined with neutron diffraction data of a 2 mol dm–3 CaCl2 aqueous solution in D2O show a drastic change in the coordination shell of a structure breaking Cl– ion and solvent water, but no significant change in the coordination shell of a structure making Ca2+ ion on compression to 1 GPa, 298 K.