Iwahashi Takashi, Ishiyama Tatsuya, Sakai Yasunari, Morita Akihiro, Kim Doseok, Ouchi Yukio
Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo, 152-8552, Japan.
Department of Environmental Applied Chemistry, Faculty of Engineering, University of Toyama, Gofuku, Toyama-shi, Toyama 930-8555, Japan.
Phys Chem Chem Phys. 2020 Jun 14;22(22):12565-12576. doi: 10.1039/d0cp01219j. Epub 2020 May 26.
Room-temperature ionic liquids (RTILs) are being increasingly employed as novel solvents in several fields, including chemical engineering, electrochemistry, and synthetic chemistry. To further increase their usage potential, a better understanding of the structure of their surface layer is essential. Bi-layering at the surfaces of RTILs consisting of 1-alkyl-3-methylimidazolium ([Cmim]; n = 4, 6, 8, 10, and 12) cations and bis(trifluoromethanesulfonyl)amide ([TFSA]) anions was demonstrated via infrared-visible sum-frequency generation (IV-SFG) vibrational spectroscopy and molecular dynamics (MD) simulations. It was found that the sum-frequency (SF) signal from the [TFSA] anions decreases as the alkyl chain length increases, whereas the SF signal from the r mode (the terminal CH group) of the [Cmim] cations is almost the same regardless of chain length. MD simulations show the formation of a bi-layered structure consisting of the outermost first layer and a submerged second layer in a "head-to-head" molecular arrangement. The decrease in the SF signals of the normal modes of the [TFSA] anions is caused by destructive and out-of-phase interference of vibrations of corresponding molecular moieties oriented toward each other in the first and second layers. In contrast, the r mode of [Cmim] does not experience destructive interference because the peak position of the r mode differs marginally at the surface and in the bulk. Our conclusions are not limited to the system presented here. Similar bi-layered structures can be expected for the surfaces of conventional RTILs, which necessitates the consideration of bi-layering in the design and application.
室温离子液体（RTILs）在包括化学工程、电化学和合成化学在内的多个领域正越来越多地被用作新型溶剂。为了进一步提高其使用潜力，深入了解其表面层结构至关重要。通过红外 - 可见和频产生（IV - SFG）振动光谱和分子动力学（MD）模拟，证明了由1 - 烷基 - 3 - 甲基咪唑鎓（[Cmim]；n = 4、6、8、10和12）阳离子和双（三氟甲磺酰）酰胺（[TFSA]）阴离子组成的RTILs表面存在双层结构。研究发现，随着烷基链长度增加，[TFSA]阴离子的和频（SF）信号降低，而[Cmim]阳离子r模式（末端CH基团）的SF信号几乎与链长无关。MD模拟显示形成了由最外层的第一层和“头对头”分子排列的浸没第二层组成的双层结构。[TFSA]阴离子正常模式的SF信号降低是由第一层和第二层中相互取向的相应分子部分振动的相消和异相干涉引起的。相比之下，[Cmim]的r模式不会经历相消干涉，因为r模式的峰值位置在表面和本体中略有不同。我们的结论不限于本文所呈现的体系。对于传统RTILs的表面，预计会有类似的双层结构，这在设计和应用中需要考虑双层结构。
