Dynamics of Hydration Water on Rutile Studied by Backscattering Neutron Spectroscopy and Molecular Dynamics Simulation E. Mamontov,* ,† D. J. Wesolowski, ‡ L. Vlcek, § P. T. Cummings, §,| J. Rosenqvist, ‡ W. Wang, ⊥ and D. R. Cole ‡ Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6473, Chemical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6110, Department of Chemical Engineering, Vanderbilt UniVersity, NashVille, Tennessee 37235-1604, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6496, and EnVironmental Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6036 ReceiVed: December 20, 2007; ReVised Manuscript ReceiVed: June 4, 2008 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 (TiO 2 ) 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. 1. Introduction Quasielastic neutron scattering (QENS) was instrumental in the recent observations of a dynamic transition in confined water when supercooled below its homogeneous nucleation tempera- ture. 1–3 Below the transition temperature of ≈220 K, the temperature dependence of the measured relaxation time in water changes from a high-temperature non-Arrhenius-type to a low- temperature Arrhenius-type. The original explanation for this dynamic transition, which was supported by molecular dynamics (MD) simulations, 4 is that the crossover in the water dynamics corresponds to the “fragile”-to-“strong” liquid transition pre- dicted a decade ago. 5 In this interpretation, the dynamic transition reflects the structural changes due to a transformation from a high-density, high-temperature, liquid phase to a low- density, low-temperature, liquid phase having a much more developed hydrogen bond network. More recently it has been argued that the observed dynamic transition could also be due to a confinement-induced vanishing of the R-relaxation in water, which leaves only a -relaxation that is characterized by an Arrhenius behavior. 6 While the nature of the dynamic transition is actively debated, its presence has been found in QENS experiments on water confined in various systems, such as carbon nanotubes, 7 oxide nanopowder surfaces, 8,9 and hydration water in lysozyme 10 and DNA. 11 The latter experiments appear to support a conjecture that it is the change of mobility in the hydration water that triggers the onset of the dynamic transition and the related bioactivity in proteins and other biomolecules. 10,11 Oxide surfaces are attractive for studying the dynamics of hydration water because of their relative simplicity compared to biosurfaces, which allows detailed molecular dynamics simulations for interpretation of the experimental data. For example, in our recent QENS-MD study 9 we have identified three distinct hydration layers on the (110) crystal surface of rutile (R-TiO 2 ), L 1 ,L 2 , and L 3 , characterized by different structures and dynamics (see Figure 1). The diffusion dynamics of hydration water on rutile on the nanosecond time scale was studied on the High Flux Backscattering Spectrometer (HFBS) at the NIST Center for Neutron Research (NCNR) and exhibited a transition from high-temperature non-Arrhenius behavior between 220 and 210 K. 9 Faster dynamics, on the time scale of tens of picoseconds or less, were investigated on the Disk Chopper Spectrometer (DCS) at the NCNR and exhibited two separate components, both of which appeared Arrhenius down to the lowest measurement temperature of 250 K. 9 These faster dynamics could not however be investigated in the DCS experiment below 250 K because of insufficient energy resolu- tion. Likewise, these faster dynamics could not be adequately * To whom correspondence should be addressed. E-mail: mamontove@ ornl.gov. Phone: (865) 574-5109. Fax: (865) 574-6080. † Spallation Neutron Source, Oak Ridge National Laboratory. ‡ Chemical Sciences Division, Oak Ridge National Laboratory. § Vanderbilt University. | Center for Nanophase Materials Sciences, Oak Ridge National Laboratory. ⊥ Environmental Sciences Division, Oak Ridge National Laboratory. J. Phys. Chem. C 2008, 112, 12334–12341 12334 10.1021/jp711965x CCC: $40.75  2008 American Chemical Society Published on Web 07/17/2008