二氧化硅表面水纳米膜的时间反相关亚扩散表明了近表面粘弹性
Temporally Anticorrelated Subdiffusion in Water Nanofilms on Silica Suggests Near-Surface Viscoelasticity
摘要：
我们使用单分子追踪来探测界面水的局部流变性。荧光罗丹明分子被跟踪在二氧化硅表面上作为环境相对湿度的函数，其控制冷凝水纳米膜的厚度。在低湿度下，分子表现出局限的扩散在附近的孤立的吸附位点，其特征在于由广泛分布的结合刚度常数;随后的化学或物理表面钝化选择性地消除刚性结合位点。在湿度增加时，分子薄的水膜凝结，允许罗丹明分子近表面运输。运动是亚扩散，与纳米膜厚度的异常指数增加。分子轨迹是时间相关的，遍历，但也具有短暂的结合和间歇性扩散。统计建模表明，这种复杂的运动在水纳米膜具有分数布朗运动的特点结合连续时间的随机行走。这与粘弹性纳米膜内的扩散一致，表明二氧化硅表面附近存在持久的分子结构。
英文摘要：
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.
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