Dynamics of hydration water on rutile studied by backscattering neutron spectroscopy and molecular dynamics simulation

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 a much slower component related to the translational jumps of the fully hydrogen-bonded water molecules. The temperature dependence of the relaxation times associated with the localized dynamics remains Arrhenius down to at least 195 K, whereas the slow translational component shows non-Arrhenius behavior above about 205 K. Thus, an Arrhenius-type behavior of the faster localized dynamic component extends below the temperature of the dynamic transition in the slow translational component. We suggest that the qualitative difference in the character of the temperature dependence between these slow and fast components may be due to the fact that the latter involves motions that require breaking fewer hydrogen bonds.