The molecular scale of switchable wetting

分子间界面相互作用共同决定了宏观润湿性质，但在分子水平上对润湿的深入了解一直是缺乏的。在这里，我们建议使用基于螺吡喃/花菁异构化的光开关表面来瞬时切换表面的润湿性质，并实时跟踪水的分子响应。在封闭的螺吡喃形式中，分子是非极性的，而在开放的花菁形式中是两性离子的。在紫外光照射下，得到花菁形式；可见光将分子切换回闭合形式。众所周知，具有螺吡喃/花菁对官能化的表面对于螺吡喃来说表现为疏水，而对于花菁形式则表现为亲水。使用超短激光脉冲切换这些表面的疏水性的能力--比分子重新定向的时间更短--提供了一种独特的方法来跟踪水对疏水表面性质阶跃变化的响应。通过这种方式，我们的目标是获得关于可切换衬底上的(去)润湿动力学的分子水平的信息，并将水组织的分子水平细节与宏观润湿特性相关联。在光开关之前、期间和之后，将使用和频率产生(SFG)光谱来研究水的结构和界面处的有机涂层。在SFG中，红外激光脉冲和可见光激光脉冲在界面处重叠。如果红外激光脉冲与分子振动共振，信号就会强烈增强。由于其选择规则，SFG专门探测界面层，看不到大块水。振动频率提供了有关氢键网络强度的信息，而信号的强度是水排列数量的衡量标准。此外，我们还可以从聚合物的CH振动中获得有关其有序性的信息。通过将SFG探针法与光脉冲相结合来引发疏水表面和亲水表面的转变，我们可以获得亚皮秒时间尺度上的动力学信息，以探索水分子适应新表面结构的分子时间尺度。我们要解决的典型问题是：-水的氢键网络和疏水和亲水结构的水取向有什么不同？-光开关切换的速度有多快，其余聚合物改变有序和取向的速度有多快？-水适应新情况的速度有多快？-接触线扩散的动力学是什么？这项研究将在分子水平上对润湿现象提供前所未有的见解，不仅有望为更好地了解基础知识开辟道路，也将为设计更好的活性表面开辟道路。

Intermolecular interfacial interactions co-determine macroscopic wetting properties, yet insights into wetting at the molecular level have been lacking. Here, we propose to use photoswitchable surfaces based on spiropyran/merocyanine isomerization to instantaneously switch the surface’s wetting properties, and follow the molecular response of water in real-time. In the closed spiropyran form, the molecule is nonpolar, while it is zwitterionic in the open merocyanine form. Under UV light irradiation the merocyanine form is obtained; visible light switches the molecule back to the closed form. It is well-known that surfaces functionalized with the spiropyran/merocyanine pair behave hydrophobic for the spiropyran case, but hydrophilic for the merocyanine form. The ability to switch the hydrophobicity of these surfaces using ultrashort laser pulses – shorter than the timescales on which molecules reorient –provides a unique way to follow the response of water to a step change in the hydrophobic surface properties. In this manner, we aim to obtain molecular-level information about the (de)wetting dynamics at switchable substrates and to correlate molecular-level details on the water organization with macroscopic wetting properties. The structure of water and the organic coating at the interface will be investigated before, during, and after photoswitching using sum frequency generation (SFG) spectroscopy. In SFG, an infrared laser pulse and a visible laser pulse are overlapped at the interface. If the infrared laser pulse is in resonance with a molecular vibration, the signal is strongly enhanced. Due to its selection rules, SFG probes specifically the interfacial layers and does not see the bulk water. The vibrational frequency provides information about the strength of the hydrogen bond network, while the intensity of the signal is a measure for the amount of water alignment. Furthermore, we can obtain information about the ordering of the polymer from its CH vibrations. By combining the SFG probe method with an optical pulse to initiate the transition between hydrophobic and hydrophilic surface, we can obtain dynamical information on sub-picosecond timescales, to probe the molecular timescales on which the water molecules adapt to the new surface structure. Typical questions we aim to address are: - What are differences in the hydrogen bond network of water and the water orientation for the hydrophobic and hydrophilic structure? - How fast does the photoswitch switch and how fast does the rest of the polymer change ordering and orientation? - How fast does the water adapt to the new situation?- What is the dynamics of the contact line spreading? This study will provide unprecedented insights into wetting phenomena at a molecular level, expected to open avenues not only for a better fundamental understanding, but also for designing superior active surfaces.