Structure and dynamics of an ionic liquid confined inside a charged slit graphitic nanopore

Molecular dynamics simulations were used to study the structure and dynamics of the ionic liquid (IL) [emim ⁺][NTf ₂ ^-] confined inside a slit graphitic nanopore of width H = 5.2 nm, where the pore walls have different densities of electrical charges. Previous simulation studies of similar systems have mainly focused on the structural and electrostatic properties of the confined IL, without studying in detail the dynamics of these systems. Our results indicate that the density of electrical charges σ affects the structure and the dynamics of the confined IL, especially in the layer of ions close to the electrically charged walls. Increases in σ in the pore walls lead to increases in the local density of counterions, as well as to important changes in the liquid structure of the ions, especially in the layers close to the pore walls. In particular, important differences in the g(r) functions are observed, and in the layer of ions near negatively charged walls, the ethyl groups of the cation cluster together to form small, nonpolar domains. Increases in σ in the pore walls also induce important changes in the dynamics of the confined ions. In particular, as σ is raised, important reductions in the z-component of the mean square displacements (MSDs) of the counterions in the layers near the pore walls are observed. The ions in these layers exhibit slower relaxation times and strong deviations from Gaussian dynamics. In general, the ions inside the pore move slower than the ions in the bulk IL; however, as σ is raised, the parallel component of the MSD of the ions near the pore wall or in the center of the pore becomes comparable to and even larger than the MSD of the bulk ions.