Wang Bingchen, Li Lei
Department of Chemical & Petroleum Engineering, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA.
Nanoscale. 2021 Nov 4;13(42):17961-17971. doi: 10.1039/d1nr05437f.
Since the interface between ionic liquids (ILs) and solids always plays a critical role in important applications such as coating, lubrication, energy storage and catalysis, it is essential to unravel the molecular structure and dynamics of ILs confined to solid surfaces. Here we report direct observation of a unique double-layering quantized growth of three IL ( [Emim][FAP], [Bmim][FAP] and [Hmim][FAP]) nanofilms on mica. AFM results show that the IL nanofilms initially grow only by covering more surface areas at the constant film thickness of 2 monolayers (ML) until a quantized increase in the film thickness by another 2 ML occurs. Based on the AFM results, we propose a double-layering model describing the molecular structure of IL cations and anions on the mica surface. The interesting double-layering structure can be explained as the result of several competing interactions at the IL-mica interface. Meanwhile, the time-dependent AFM results indicate that the topography of IL nanofilms could change with time and mobility of the nanofilm is lower for ILs with longer alkyl chains, which can be attributed to the stronger solvophobic interaction. The findings here have important implications on the molecular structure and dynamics of ILs confined to solid surfaces.
由于离子液体（ILs）与固体之间的界面在诸如涂层、润滑、能量存储和催化等重要应用中始终起着关键作用，因此弄清单层固体表面上离子液体的分子结构和动力学至关重要。在此，我们报告了在云母上对三种离子液体（[Emim][FAP]、[Bmim][FAP]和[Hmim][FAP]）纳米薄膜独特的双层量化生长的直接观察结果。原子力显微镜（AFM）结果表明，离子液体纳米薄膜最初仅通过在2个单层（ML）的恒定膜厚度下覆盖更多表面积来生长，直到膜厚度出现另一个2 ML的量化增加。基于AFM结果，我们提出了一个描述云母表面上离子液体阳离子和阴离子分子结构的双层模型。这种有趣的双层结构可以解释为离子液体 - 云母界面处几种竞争相互作用的结果。同时，随时间变化的AFM结果表明，离子液体纳米薄膜的形貌会随时间变化，并且对于具有较长烷基链的离子液体，纳米薄膜的迁移率较低，这可归因于更强的疏溶剂相互作用。此处发现对于单层固体表面上离子液体的分子结构和动力学具有重要意义。
