Disordering and Reordering of Ionic Liquids under an External Electric Field

The previous study (Wang and Voth, J. Am. Chem. Soc. 2005, 127, 12192) has shown that the nonpolar alkyl cationic tail groups of ionic liquids can aggregate and form nanoscale spatial heterogeneity. To facilitate applications of ionic liquids under nonequilibrium conditions, it is of great interest to see how this spatial heterogeneity of ionic liquids changes when an external electric field is applied. The nonequilibrium molecular dynamics simulations indicate that, when the external electric field strength increases, the ionic liquid structure is first disordered from spatially heterogeneous to spatially homogeneous, and is then reordered to nematic-like. Accordingly, both the ionic self-diffusion and the system volume increase in the spatially homogeneous region and decrease in the nematic-like region. This is understood by the competition between the external electric field and the electrostatic interactions between ions.