A Computational Study of the Behavior of the Ionic Liquid [BMIM+][PF6-] Confined Inside Multiwalled Carbon Nanotubes

We have performed molecular dynamics to study the structural and dynamical properties of the ionic liquid (IL) [BMIM+][PF6-] confined inside multiwalled carbon nanotubes (MWCNTs) with inner diameters ranging between 2.0 and 3.7 nm. Our results indicate that the diameter of the MWCNT and the pore loading have a profound influence on the structural and dynamical properties of the confined IL. Regarding the structural properties, significant layering is observed in the mass density profiles of the cations and anions in the radial direction. The cations close to the pore walls tend to align with their imidazolium ring parallel to the surface. Regions of high and low density are observed in both the radial and the axial directions upon reduction in the pore loading. Regarding the dynamics, the confined cations move faster than the anions, in analogy to bulk systems, but the dynamics are much slower in confinement than in the bulk. The confined ions spend a larger time in the cage regime before finally reaching the Fickian (diffusive) regime. Our results also suggest that the cations in the center of the pore tend to move faster than those close to the pore walls; the anions exhibit a similar behavior, although the differences in dynamics are not as evident as those observed between layers of cations. Our results also suggest that, for some pore sizes, the axial mean square displacement (MSD) increases with decreasing pore filling; however, for some pore sizes, the axial MSD seems to have a nonmonotonic dependence with pore loading. Our results also suggest a nonmonotonic dependence of the axial MSD with pore size and similar pore loading. These nonmonotonic behaviors are possibly due to the presence of local variations in the axial density profile of the IL as the pore loading decreases for a given pore size. In analogy to bulk systems, there are large differences in the characteristic time scales for the translational and rotational motions of the confined cations.