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Surface of room temperature ionic liquid [bmim][PF6] studied by polarization- and experimental configuration-dependent sum frequency generation vibrational spectroscopy

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Abstract

Understanding and control of the surface properties such as molecular orientations are of great importance in numerous applications of ionic liquids. However, there remain discrepancies among the previous experimental and theoretical studies on the surface orientation and structures of room temperature ionic liquids (RTIL) systems. In this article, the orientation of 1-butyl-3-methylimidazolium ([bmin]) cation at the air/liquid interface of a characteristic RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), was investigated by the sum frequency generation vibrational spectroscopy (SFG-VS). Detailed polarization and experimental configuration analyses of the SFG-VS spectra showed the possibility of a small spectral splitting in the CH3 symmetric stretching region, which can be further attributed to the probable existence of multiple orientations for the interfacial [bmim] cations. In addition, the (N)-CH3 vibrations were absent, ruling out the prediction by several recent molecular dynamics simulations which state that portions of the [bmim] cations orient with a standing-up (N)-CH3 group at the ionic liquid surface. Hence, new realistic theoretical models have to be developed to reflect the complex nature of the ionic liquid surface.

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Authors and Affiliations

  1. Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, 230026, China

    Ganghua Deng & Shilin Liu

  2. Nanjing University of Aeronautics and Astronautics, Nanjing, 211106, China

    Youqi Guo

  3. Beijing National Laboratory for Molecular Sciences; State Key Laboratory of Molecular Reaction Dynamics; Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China

    Xia Li, Zhen Zhang, Zhou Lu & Yuan Guo

Authors
  1. Ganghua Deng
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  2. Youqi Guo
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  3. Xia Li
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  4. Zhen Zhang
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  5. Shilin Liu
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  6. Zhou Lu
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  7. Yuan Guo
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Correspondence to Zhou Lu or Yuan Guo.

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Deng, G., Guo, Y., Li, X. et al. Surface of room temperature ionic liquid [bmim][PF6] studied by polarization- and experimental configuration-dependent sum frequency generation vibrational spectroscopy. Sci. China Chem. 58, 439–447 (2015). https://doi.org/10.1007/s11426-014-5241-5

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  • DOI: https://doi.org/10.1007/s11426-014-5241-5

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