Visualization of the 3D Structure of Subcritical Aqueous Ca(NO3)2 Solutions at 25~350 °C and 40 MPa by Raman and X-Ray Scattering Combined with Empirical Potential Structure Refinement Modeling

Raman scattering measurements were performed on 1 mol dm−3 aqueous calcium nitrate (Ca(NO3)2) and sodium nitrate (NaNO3) solutions containing 4% (w/w) D2O in a temperature range from 25 to 350 °C and pressure of 40 MPa. As the temperature increased, the N–O symmetric stretching vibrational band (ν1) of NO3− at 1045–1047 cm−1 shifted to a lower wavenumber by 5~6 cm−1. The band analysis using one Lorentzian component showed that the full-width at half maximum (FWHM) did not change significantly below 175 °C but increased rapidly above 200 °C for both solutions. The peak area for an aqueous Ca(NO3)2 solution showed a breakpoint between 225 and 250 °C, suggesting a change in the coordination shell of NO3− at 175~250 °C. The OD symmetric stretching vibrational band of HDO water was deconvoluted into two Gaussian components at 2530 and 2645 cm−1; the former component has high temperature dependence that is ascribed to the hydrogen bonds, whereas the latter one shows less temperature dependence due to the non-hydrogen bonds of water. X-ray scattering measurements were performed on a 1 mol dm−3 aqueous Ca(NO3)2 solution at 25 to 210 °C and 40 MPa. Empirical potential structure refinement (EPSR) modeling was used to analyze the X-ray scattering data. Ca2+ forms a rigid coordination shell consisting of about seven water molecules at 2.48 Å and one NO3− at 25~170 °C, with further water molecules substituted by NO3− at 210 °C. NO3− is surrounded by 13~14 water molecules at an N–Ow distance of 3.6~3.7 Å. The tetrahedral network structure of solvent water pertains from 25 to 170 °C but is transformed to a dense packing arrangement at 210 °C.