Dynamical properties of a room temperature ionic liquid: Using molecular dynamics simulations to implement a dynamic ion cage model

Maolin Sha; Xiaohang Ma; Na Li; Fabao Luo; Guanglai Zhu; Michael D Fayer

doi:10.1063/1.5126231

Dynamical properties of a room temperature ionic liquid: Using molecular dynamics simulations to implement a dynamic ion cage model

J Chem Phys. 2019 Oct 21;151(15):154502. doi: 10.1063/1.5126231.

Authors

Maolin Sha¹, Xiaohang Ma¹, Na Li², Fabao Luo², Guanglai Zhu³, Michael D Fayer⁴

Affiliations

¹ Department of Physics and Materials Engineering, Hefei Normal University, Hefei 230061, China.
² Department of Chemistry and Chemical Engineering, Hefei Normal University, Hefei 230061, China.
³ Institute of Atomic and Molecular Physics, Anhui Normal University, Wuhu 241000, China.
⁴ Department of Chemistry, Stanford University, Stanford, California 94305, USA.

PMID: 31640381
DOI: 10.1063/1.5126231

Abstract

The transport behavior of ionic liquids (ILs) is pivotal for a variety of applications, especially when ILs are used as electrolytes. Many aspects of the transport dynamics of ILs remain to be understood. Here, a common ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf₂), was studied with molecular dynamics simulations. The results show that BmimNTf₂ displays typical structural relaxation, subdiffusive behavior, and a breakdown of the Stokes-Einstein diffusion relation as in glass-forming liquids. In addition, the simulations show that the translational dynamics, reorientation dynamics, and structural relaxation dynamics are well described by the Vogel-Fulcher-Tammann equation like fragile glass forming liquids. Building on previous work that employed ion cage models, it was found that the diffusion dynamics of the cations and anions were well described by a hopping process random walk where the step time is the ion cage lifetime obtained from the cage correlation function. Detailed analysis of the ion cage structures indicated that the electrostatic potential energy of the ion cage dominates the diffusion dynamics of the caged ion. The ion orientational relaxation dynamics showed that ion reorientation is a necessary step for ion cage restructuring. The dynamic ion cage model description of ion diffusion presented here may have implications for designing ILs to control their transport behavior.