State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, 55 East Zhongguancun Road, P.O. Box 2735, Beijing 100190, China.
J Phys Chem B. 2013 May 2;117(17):5102-12. doi: 10.1021/jp311017r. Epub 2013 Apr 18.
In many applications, ionic liquids (ILs) work in a nonequilibrium steady state driven by an external electric field. However, how the electric field changes the structure and dynamics of ILs and its underlying mechanism still remain poorly understood. In this paper, coarse-grained molecular dynamics simulations were performed to investigate the structure and dynamics of 1-ethyl-3-methylimidazolium nitrate ([EMIm][NO3]) under a static electric field. The ion cage structure was found to play an essential role in determining the structural and dynamic properties of the IL system. With a weak or moderate electric field (0-10(7) V/m), the external electric field is too weak to modify the ion cage structure in an influential way and thus the changes of structural and dynamic properties are negligible. With a strong electric field (10(7)-10(9) V/m) applied, ion cages expand and deform apparently, leading to the increase of ion mobility and self-diffusion coefficient with electric field, and the self-diffusion of ions along the electric field becomes faster than the other two directions due to the anisotropic deformation of ion cages. In addition, the Einstein relation connecting diffusion and mobility breaks down at strong electric fields, and it also breaks down for a single ion species even at moderate electric fields (linear-response region).
在许多应用中，离子液体（ILs）在外电场的驱动下处于非平衡稳态。然而，电场如何改变离子液体的结构和动力学及其潜在机制仍知之甚少。本文采用粗粒化分子动力学模拟研究了在静态电场下 1-乙基-3-甲基咪唑硝酸盐([EMIm][NO3])的结构和动力学。发现离子笼结构对于确定 IL 体系的结构和动力学性质起着至关重要的作用。在外电场较弱或中等强度（0-10(7) V/m）时，外电场太弱，无法以有影响的方式改变离子笼结构，因此结构和动力学性质的变化可以忽略不计。施加强电场（10(7)-10(9) V/m）时，离子笼明显膨胀和变形，导致离子迁移率和自扩散系数随电场增加，并且由于离子笼的各向异性变形，离子沿电场的自扩散速度比其他两个方向更快。此外，连接扩散和迁移率的爱因斯坦关系在强电场下失效，即使在中等电场（线性响应区域）下，对于单个离子物种也失效。
