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 Ni2+ 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 Ni2+-Cl− at 473 K. The chloride ions in lithium chloride and caesium chloride solutions are hydrogen bonded with water molecules at Cl-D (D2O) 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 nCl-D of 3.9 ± 0.5, as also evidence from an X-ray diffraction study.