Bao Yifan, Nishiwaki Yuto, Kawano Touma, Utsunomiya Toru, Sugimura Hiroyuki, Ichii Takashi
Department of Materials Science and Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan.
ACS Nano. 2024 Sep 10;18(36):25302-25315. doi: 10.1021/acsnano.4c08838. Epub 2024 Aug 26.
When in contact with charged solid surfaces, ionic liquids (ILs) are known to form solvation structures consisting of alternating cation and anion layers. This phenomenon is considered to originate from the adsorption layer of counterions overcompensating the surface charge, so-called overscreening. However, the response of these layers to surfaces with near-zero or extremely high surface charge density (σ) remains inadequately understood. Here, we probe the solvation structure of ILs on alkali halide surfaces with varied surface orientations: nearly zero-charged RbI(100) and highly charged RbI(111), by employing frequency modulation atomic force microscopy with atomic resolution. Two commonly used ILs are examined in this study: 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Cmpyr][NTf]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cmim][NTf]). On RbI(100) surfaces with near zero σ, we observe alternating cation and anion layers, diverging from the previously proposed monolayer model for IL/alkali halide(100) interfaces. These results support the argument that overscreening occurs under low σ, even approaching zero, and reconcile conflicting experimental conclusions about low σ systems. On RbI(111) surfaces with high σ, we identify solvation structures consisting of two consecutive counterion layers. This structure aligns with the theoretically predicted crowding; a phenomenon rarely observed in commonly used ILs due to typically unreachable σ in electrochemical IL/electrode systems. Our findings indicate that alkali halide(111) surfaces are potentially valuable for exploring the crowding phenomenon in ILs, addressing the current scarcity of experimental observations.
当与带电固体表面接触时，已知离子液体（ILs）会形成由交替的阳离子和阴离子层组成的溶剂化结构。这种现象被认为源于抗衡离子的吸附层对表面电荷的过度补偿，即所谓的过屏蔽。然而，这些层对表面电荷密度（σ）接近零或极高的表面的响应仍未得到充分理解。在这里，我们通过采用具有原子分辨率的调频原子力显微镜，探测离子液体在具有不同表面取向的碱金属卤化物表面上的溶剂化结构：几乎零电荷的RbI(100)和高电荷的RbI(111)。本研究中考察了两种常用的离子液体：1-甲基-1-丙基吡咯烷鎓双（三氟甲基磺酰）亚胺（[Cmpyr][NTf]）和1-乙基-3-甲基咪唑鎓双（三氟甲基磺酰）亚胺（[Cmim][NTf]）。在σ接近零的RbI(100)表面上，我们观察到交替的阳离子和阴离子层，这与先前提出的离子液体/碱金属卤化物(100)界面的单层模型不同。这些结果支持了在低σ甚至接近零的情况下会发生过屏蔽的观点，并调和了关于低σ系统的相互矛盾的实验结论。在σ高的RbI(111)表面上，我们确定了由两个连续的抗衡离子层组成的溶剂化结构。这种结构与理论预测的拥挤现象一致；由于在电化学离子液体/电极系统中通常无法达到σ，这种现象在常用的离子液体中很少观察到。我们的研究结果表明，碱金属卤化物(111)表面对于探索离子液体中的拥挤现象具有潜在价值，解决了目前实验观察的稀缺问题。
