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) nano-powder down to a temperature of 195 K. We attribute the dynamics measured on the BASIS on the time scale of tens to hundreds of picoseconds 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 extending to 50 nanoseconds. The results suggest that the scattering experiments probe two types of molecular motion in the surface layers, namely (1) a fast component that involves dynamics of the outermost hydration layer with unsaturated hydrogen bonds and localized motions of all water molecules and (2) a slower component, representing translational jumps of the fully hydrogen-bonded water molecules. The temperature dependence of the relaxation times associated with the fast dynamic component remains Arrhenius down to at least 195 K, whereas the slow translational component shows a dynamic transition to non-Arrhenius behavior above about 210K. Thus, an Arrhenius-type behavior of the fast 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 the slow and fast component may be due to the fact that the latter involves motions that require breaking fewer hydrogen bonds.
