Chen Hailong, Chen Xin, Deng Jingwen, Zheng Junrong
Beijing National Laboratory for Condensed Matter Physics , CAS Key Laboratory of Soft Matter Physics , Institute of Physics , Chinese Academy of Sciences , Beijing 100190 , China.
College of Chemistry and Molecular Engineering , Beijing National Laboratory for Molecular Sciences , Peking University , Beijing 100871 , China . Email:
Chem Sci. 2017 Dec 22;9(6):1464-1472. doi: 10.1039/c7sc05184k. eCollection 2018 Feb 14.
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.
本文研究离子液体的结构。我们使用高功率多模二维红外光谱技术，测量了离子液体中离子间振动耦合的各向异性。这种各向异性反映了阳离子和阴离子之间的相对取向。令人惊讶的是，与广为流传的离子配对观点相反，在离子液体中观察到最近的非球形对称阳离子和阴离子之间呈随机取向。一方面，许多先前的实验和理论计算表明，离子液体在中尺度上高度有序，形成了离子域和疏水域的双连续纳米结构。另一方面，我们的结果清楚地证明，离子域内的离子排列基本上是随机的。对于大多数液体来说，在较大尺度（纳米）上的这种有序性以及在较小尺度（亚纳米）上缺乏有序性是非常罕见的，甚至可能是离子液体所特有的。在此，我们提出离子液体可被视为类熔盐域和类分子液体域的三维互连纳米复合材料。这种独特的结构可以解释这样一个事实，即离子液体像复合材料一样，通常具有“离子”和“分子”成分的良好性质。
