Department of Chemistry, Carnegie Mellon University , Pittsburgh, Pennsylvania 15213, United States.
School of Computational Sciences, Korea Institute for Advanced Study , Seoul 02455, Republic of Korea.
J Phys Chem B. 2017 Feb 9;121(5):1100-1107. doi: 10.1021/acs.jpcb.6b10678. Epub 2017 Jan 26.
The structure of ionic liquids (ILs) surrounding solute dyes and the effects of solvent structure on solute diffusion have been investigated using molecular dynamics (MD) and the experimental tools of confocal and fluorescence correlation spectroscopies. Although confocal microscopy and simulations show that the local environment around solutes in ILs is heterogeneous and that the structural heterogeneity is rather long-lived, the local polarity and the diffusion constant were found to be uncorrelated. Moreover, the complex diffusion observed experimentally is not due to the structural heterogeneity of the IL but rather due to the dynamic heterogeneity arising from the viscous glassy nature of the IL environment. MD simulations show that the degree of dynamic heterogeneity depends on the first nonvanishing electric multipole moment of the solute. The dynamics of a cationic solute are the least heterogeneous, whereas those of a solute without an electric multipole moment are the most heterogeneous. This indicates that the length scale over which the solute-solvent interactions occur, and thus the number of solvent degrees of freedom that couple to the solute, are the key factors governing the dynamic heterogeneity of the solute.
使用分子动力学（MD）和共焦及荧光相关光谱学这两种实验工具，研究了离子液体（ILs）中溶质染料的周围结构以及溶剂结构对溶质扩散的影响。尽管共焦显微镜和模拟表明，溶质在 ILs 中的局部环境是不均匀的，并且结构不均匀性相当持久，但发现局部极性与扩散常数之间没有相关性。此外，实验中观察到的复杂扩散不是由于 IL 的结构不均匀性引起的，而是由于 IL 环境的粘性玻璃态引起的动态不均匀性引起的。MD 模拟表明，动态不均匀性的程度取决于溶质的第一个非零电多极矩。阳离子溶质的动力学最不均匀，而没有电多极矩的溶质的动力学最不均匀。这表明，溶质-溶剂相互作用发生的长度尺度，以及与溶质耦合的溶剂自由度的数量，是控制溶质动态不均匀性的关键因素。
