Structural Properties and Vibrational Spectra of Ethylammonium Nitrate Ionic Liquid Confined in Single-Walled Carbon Nanotubes

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• 作者：Guobing Zhou ; Yunzhi Li ; Zhen Yang ; Fangjia Fu ; Yiping Huang ; Zheng Wan ; Li Li ; Xiangshu Chen ; Na Hu ; Liangliang Huang
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：March 10, 2016
• 年：2016
• 卷：120
• 期：9
• 页码：5033-5041
• 全文大小：536K
• ISSN：1932-7455

文摘

The structures and relevant vibrational spectra of an ethylammonium nitrate (EAN) ionic liquid (IL) confined in single-walled carbon nanotubes (SWCNTs) with various diameters have been investigated in detail by using classical molecular dynamics simulation. Our simulation results demonstrate that the EAN IL confined in larger SWCNTs can form well-defined multishell structures with an additional cation chain located at the center. However, a different single-shell hollow structure has been found for both the cations and the anions in the 1 nm SWCNT. For the cations confined in SWCNTs, the CH₃ groups stay closer to the nanotube walls because of their solvophobic nature, while the NH₃⁺ groups prefer to point toward the central axis. Accordingly, the NO₃^– anions tend to lean on the SWCNT surface with three O atoms facing the central axis to form hydrogen bonds (HBs) with the NH₃⁺ groups. In addition, in the 1 nm SWCNT, the CH₃ groups of cations exhibit an obvious blue shift of around 16 cm^–1 for the C–H stretching mode with respect to the bulk value, and the N–H stretching mode of NH₃⁺ groups is split into two characteristic peaks with one peak appearing at a higher frequency. Such a blue shift is attributed to the existence of more free space for the C–H bonds of confined CH₃ groups, while the splitting phenomenon is due to the fact that more than 60% of the confined NH₃⁺ groups have one dangling N–H bond. For the anions confined in the 1 nm SWCNT, the N–O stretching mode of NO₃^– has a maximum red shift of around 24 cm^–1 with respect to the bulk value, which is attributed to enhanced HBs between anions and cations. Our simulation results reveal a molecular-level correlation between confined structural configurations and the corresponding vibrational spectra changes for the ILs confined in nanometer scale environments.