Dynamics of Hydration Water on Rutile Studied by Backscattering Neutron Spectroscopy and Molecular Dynamics Simulation

Abstract

The high energy resolution, coupled with the wide dynamic range, of the new backscattering spectrometer (BASIS) at the Spallation Neutron Source, Oak Ridge National Laboratory, has made it possible to investigate the diffusion dynamics of hydration water on the surface of rutile (TiO2) nanopowder down to a temperature of 195 K. The dynamics measured on the BASIS on the time scale of tens of picoseconds to more than a nanosecond can be attributed to the mobility of the outer hydration water layers. The data obtained on the BASIS and in a previous study using the backscattering and disk-chopper spectrometers at the NIST Center for Neutron Research are coupled with molecular dynamics simulations extended to 50 ns. The results suggest that the scattering experiments probe several types of molecular motion in the surface layers, namely a very fast component that involves dynamics of water molecules with unsaturated hydrogen bonds, a somewhat slower component due to localized motions of all water molecules, and...