Licentate Thesis: Temperature-dependent X-ray Scattering of Liquid Water

by Jonas A. Sellberg (Stockholm University, Department of Physics)

Thursday 31 May 2012, 10:00 → 12:00 Europe/Stockholm

SLAC, sal 321, byggnad 137

This thesis presents joint experimental and theoretical x-ray scattering studies of liquid water at near atmospherical pressures. X-ray scattering probes the local structure of disordered condensed matter, which for water changes drastically over the wide temperature range reviewed in this thesis. Above the melting temperature, wide-angle x-ray scattering measurements at Stanford Synchrotron Radiation Lightsource (SSRL) were performed at 7 ºC, 25 ºC, and 66 ºC. Scattering data were recorded up to 16 Å-1, which rendered it possible to obtain the intermolecular pair-correlation function (PCF) of the liquid directly through a Fourier transform of the molecular structure factor. The high quality of the data also revealed weak fourth and fifth hydration shells, resolving shell structure out to 12 Å. Interestingly, the temperature dependence of the hydration shells differed, as the first and fourth peaks were less sensitive to a temperature change, whereas especially the second and fifth peaks significantly increased in magnitude upon cooling. Large-scale molecular dynamics (MD) simulations using the TIP4P/2005 force-field reproduced the experimental shell-structure and the temperature change observed. The Shiratani-Sasai local structure index (LSI) identified that an interplay between a minority of tetrahedrally coordinated molecules and a majority of molecules with disordered local structure gave rise to the peculiar temperature dependence of the intermediate-range oscillations in the O-O PCF, with the second and fifth peaks as fingerprints of a growing tetrahedral ordering upon cooling. Below the freezing point, x-ray scattering from evaporatively cooled 10 and 50 µm in diameter droplets of supercooled pure bulk water was performed at the Linac Coherent Light Source (LCLS). The rapid cooling in vacuo and ultra-short and ultra-bright characteristics of the x-ray laser enabled us to probe the metastable liquid down to previously unexplored temperatures of -50 ± 3 ºC. Scattering data were recorded up to 3 Å-1 on a static pixel array detector, limiting the information about the PCF at these temperatures compared to the high-temperature study. Nevertheless, valuable knowledge about the second hydration shell could be inferred with the assistance of MD simulations using the same force-field. A remarkable increase of structures with local tetrahedral coordination was observed upon supercooling water, implying a continuous transition towards a low-density liquid. At -50 ± 3 ºC where the low-density liquid structure dominates, the number of droplets containing ice nuclei increased rapidly, but a fraction of the probed volume within the droplets remained liquid over millisecond timescales. These results demonstrate a novel approach to study the structure of deeply supercooled liquids.