  We used single-molecule tracking to probe the local rheology of interfacial
water. Fluorescent rhodamine molecules were tracked on silica surfaces as a
function of ambient relative humidity, which controlled the thickness of
condensed water nanofilms. At low humidity, the molecules exhibited confined
diffusion in the vicinity of isolated adsorption sites characterized by a broad
distribution of binding stiffness constants; subsequent chemical or physical
surface passivation selectively eliminated stiffer binding sites. At increased
humidity, molecularly thin water films condensed, permitting near-surface
transport of rhodamine molecules. Motion was subdiffusive, with an anomalous
exponent increasing with the nanofilm thickness. Molecular trajectories were
temporally anticorrelated, ergodic, but also featured transient binding and
intermittent diffusion. Statistical modeling demonstrated that this complex
motion in water nanofilms had the characteristics of fractional Brownian motion
combined with a continuous time random walk. This was consistent with diffusion
within viscoelastic nanofilms, suggesting persistent molecular structuring in
the vicinity of the silica surface.
