Structure and dynamics of supercooled and glassy aqueous ionic solutions

Abstract

X-ray diffraction, neutron diffraction isotopic substitution(NDIS), and quasi-elastic neutron scattering (QENS) measurements have been made on concentrated aqueous solutions of lithium halides LiX (XCl, Br, and I) in a temperature range covering the room-temperature liquid, the supercooled liquid, and the glassy state. X-ray radial distribution functions have revealed that the hydration shell of the halide ions is gradually structured with lowering temperature. Another interesting feature is the evolution of water-water interactions centered at 4.3 and 6.9 Å independent of the halide ions, due probably to reinforced hydrogen bonds at low temperatures. The NDIS radial distribution functions related to Li + and Cl − have demonstrated that the orientational correlation of bound water molecules for Cl − is more strongly temperature-dependent than that for Li + At low temperature the formation of a second hydration shell of Li + and Cl − has been established. The QENS data have shown that the translational motion of water molecules in the LiCl solution is more hindered with lowering temperature. The structural and dynamic characteristics of the supercooled and glassy solutions are discussed in connection with nucleation of ice, isotropic reorientational motion of water, and the partial recovery of hydrogen bonds in these solutions.