Heterogeneous dynamics of ionic liquids in confined films with varied film thickness

Dynamical behavior and characteristics of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) ionic liquid (IL) in confined films with varied film thickness have been investigated using atomistic molecular dynamics simulations. Simulation results indicate that the dynamics of confined ionic groups in interfacial regions is highly heterogeneous and depends strongly on their relative layered positions in the confined IL film. The dynamics and relaxation times of the ionic groups in the bulk region of confined IL films are very similar to that in the corresponding pure bulk simulation. In contrast, the diffusion of the corresponding ionic groups slows down and can be characterized by slaved diffusion while the corresponding relaxation times increase remarkably as these ionic groups come closer to the neutral graphene surface. While at the IL-vacuum interface, the diffusion and relaxation of terminal carbon atoms of butyl chains of [BMIM] cations are much faster than in other layers of the confined IL film and in the bulk region of the simulation system without confinement, due to their librated motion in this interfacial region. The dynamical heterogeneity of the confined ionic groups is intrinsically related to microscopic ionic structures and the orientational preference of [BMIM][PF6] ion pairs in interfacial regions.