Structure and Dynamics of Water Interacting with Hydrophilic, Nanostructured CuO Coatings¹

We have used neutron scattering and electron microscopy to investigate how the structure, dynamics, and phase transitions of water near a bare copper surface are affected by coating it with strongly hydrophilic CuO nanostructures. Our high-energy-resolution elastic neutron scattering measurements show the abrupt freezing transition of water near a bare Cu surface is spread into a continuous transition spanning a temperature range of ~80 K upon coating with the CuO nanostructures. From these elastic scans, we infer the presence of at least two distinct water populations, differing in their freezing behavior and their proximity to the nanostructures. Quasielastic neutron scattering measurements support this interpretation by providing evidence of three water components diffusing on different time scales. Our environmental-SEM images reveal micron-size water droplets wet to the nanostructures. In addition, neutron diffraction measurements indicate that the water closest to the CuO nanostructures freezes into an amorphous solid, while more distant water freezes into crystalline ice. The presence of the two forms of ice is consistent with vibrational spectra observed by inelastic neutron scattering.