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Ion-Cage Interpretation for the Structural and Dynamic Changes of Ionic Liquids under an External Electric Field
journal contribution
posted on 2013-05-02, 00:00 authored by Rui Shi, Yanting WangIn 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 (0–107 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 (107–109 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).