On the Influence of Pore Size and Pore Loading on Structural and Dynamical Heterogeneities of an Ionic Liquid Confined in a Slit Nanopore

Molecular dynamics simulations were performed to investigate the structural and dynamical properties of varying amounts of the ionic liquid (IL) [EMIM+]­[TFMSI–] confined inside slit-like graphitic pores of different widths, H. The ions distributed in layers inside the slit pores, with the number of layers depending on pore size. A reduction in pore loading leads to the formation of regions of high and low density of ions in the center of the pore. Variations in pore size and pore loading seem to induce only slight changes in the local liquid structure of [EMIM+]­[TFMSI–] in the different layers, as compared with the liquid structure of the bulk IL. This finding, when combined with our previous work for a different IL (Singh, R.; Monk, J.; Hung, F. R. J. Phys. Chem. C 2011, 115, 16544–16554), suggests that confinement inside slit-like nanopores may or may not induce changes in the local liquid structure depending on the specific IL. However, pore size and pore loading have a marked effect on the dynamics of confined [EMIM+]­[TFMSI–]. The overall dynamics of the confined ions become faster with increasing pore size. The local dynamics of the IL are heterogeneous, with the ions exhibiting slower dynamics in the layers closer to the walls. When ρ = ρ bulk, the ions in the first layers (closest to the pore walls) and in the second layers of a pore of H = 5.2 nm have faster dynamics than those in the same layers of a pore of H = 2.5 nm; the ions in the center of a pore of H = 5.2 nm have dynamics similar to that of the bulk IL. For varying amounts of [EMIM+]­[TFMSI–] inside a pore of H = 5.2 nm, slight differences in the dynamics of the ions in the first and second layers are observed. In contrast, the dynamics of the ions in the center of the pore change markedly, with the fastest dynamics observed when ρ = 0.8ρ bulk (even faster than those of a bulk system). Marked deviations from Gaussian behavior (e.g., large secondary peaks) arise in the self-part of the van Hove correlation function with reductions in pore loading, which suggest that the local dynamics become more complex as regions of high and low density form in the center of the pore when pore loading is reduced.