Heterogeneity in the Dynamics of the Ionic Liquid [BMIM+][PF6 –] Confined in a Slit Nanopore

Molecular dynamics simulation was used to investigate the dynamics of the ionic liquid [BMIM+][PF6 –] when confined inside an uncharged slit-like graphitic pore of width H = 5.4 nm, in the temperature range 300–400 K. Our results indicate that the dynamics of the confined ions are highly heterogeneous and depend strongly on the distance of the ions with the pore walls. The ions in the center regions of the pore have dynamics and relaxation times that are very similar to those observed in a bulk IL at the same temperature. In contrast, the dynamics slow down appreciably as the ions get closer to the pore walls, and their relaxation times increase markedly. The overall diffusivities in the parallel (x, y) direction for the confined ions are found to be always smaller than the bulk diffusivities. Our results also suggest that these dynamical heterogeneities are linked to differences in structural properties. Radial distribution functions suggest that the structure of the ions in the center of the pore is similar to that observed in the bulk IL; however, significant structural differences are observed between the ions near the pore walls and those in the center of the pore.