Ionic Mobility within Functionalized Silica Nanopores

Abstract

Molecular dynamics simulations were performed to investigate the structural and dynamical features of an aprotic ionic liquid confined within two types of cylindrical silica pores (hydrophilic and hydrophobic ones) as a function of the pore filling fraction. Analysis of the local density distributions revealed the existence of a dense adsorbed layer in both pores, leading to interfacial ionic liquid densities that resulted between 2 and 3 times larger than bulk. Beyond the characteristics of the surface, it is observed that the nearest-to-the-wall-adsorbed ionic liquid cations accommodate their rings and alkyl chains parallel to the pore wall. Nevertheless, the orientation of the alkyl chain of the more distant cations in the adsorbed layer depends on the functionalization of pore walls, pointing toward the center of the pore for the case of hydrophilic surfaces or toward the pore surface when the wall is covered by hydrophobic moieties. Transport properties were also investigated. The axial translational diffusive dynamics exhibits an overall slowdown upon confinement, being more pronounced in the hydrophilic cavities at low loadings, in agreement with recent experimental results. The ionic conductivity measured in the hydrophilic pores resulted ∼50% lower than in the bulk phase. In contrast, within the hydrophobic pores, the conductivity resulted 30% larger than in hydrophilic cavities and showed weak dependence on loading. The contributions to the collective conductivity, arising from single and distinct components, were analyzed and discussed in terms of microscopic correlations and local densities.