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Isotropic ordering of ions in ionic liquids on the sub-nanometer scale

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Abstract

This article investigates structures of ionic liquids. Using a high-powered multiple-mode two dimensional infrared spectroscopic technique, we measure the anisotropy of interionic vibrational coupling in ionic liquids. Such anisotropy reports the relative orientation between cations and anions. Surprisingly, opposite to the well-propagated idea of ion pairing, a random orientation between the nearest non-spherically symmetric cation and anion is observed in ionic liquids. On the one hand, numerous previous experiments and theoretical calculations have shown that ionic liquids are highly ordered at the mesoscale, forming a bicontinuous nanostructure of ionic domains and hydrophobic domains. On the other hand, our results clearly prove that the ion ordering within the ionic domains is essentially random. Such an ordering at the larger scale (nanometers) and a lack of ordering at the smaller scale (sub-nanometer) is very rare for most liquids, or may even be unique to ionic liquids. Herein, we propose that ionic liquids may be regarded as 3D interconnecting nanocomposites of molten-salt-like domains and molecular-liquid-like domains. Such unique structuring could explain the fact that ionic liquids, like composite materials, often possess favorable properties of both “ionic” and “molecular” components.

Graphical abstract: Isotropic ordering of ions in ionic liquids on the sub-nanometer scale

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Publication details

The article was received on 06 Dec 2017, accepted on 22 Dec 2017 and first published on 22 Dec 2017


Article type: Edge Article
DOI: 10.1039/C7SC05184K
Citation: Chem. Sci., 2018, Advance Article
  • Open access: Creative Commons BY license
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    Isotropic ordering of ions in ionic liquids on the sub-nanometer scale

    H. Chen, X. Chen, J. Deng and J. Zheng, Chem. Sci., 2018, Advance Article , DOI: 10.1039/C7SC05184K

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