Ionic liquids (ILs) have been considered as alternative electrolytes for energy storage in electrochemical double-layer capacitors, where the electrolyte is in contact with the nanoporous electrodes. Inserting ILs inside nanoporous materials is one step in the synthesis of optically-active and magnetic nanomaterials (nanorods, nanowires, nanotubes) based on ILs. These nanomaterials hold promise in photovoltaics, analytical chemistry and biomedical applications. Relevant macroscopic properties of these systems (e.g., electrical capacitance and resistance, magnetic and optical properties) are determined by the molecular-level properties of the confined ILs. However, a fundamental understanding of the behavior of nanoconfined ILs is still in its infancy.  We report classical molecular dynamics simulation results of typical ILs ([bmim+][PF6-], [emim+][NTf2-]) confined inside carbons with pores of different geometries (multi-walled carbon nanotubes, slit graphitic pores). Our results suggest that variables such as pore size and pore loading affect the structural properties of the confined IL (local density, ion orientation and radial distribution functions). We discuss how pore size and loading affect the dynamical properties of the confined IL (mean squared displacements, diffusivities). Our results indicate that the dynamics of the confined ions are 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 similar to those observed in the bulk IL at the same temperature. In contrast, the dynamics slow down appreciably as the ions get closer to the pore walls. Our results also suggest that these dynamical heterogeneities are linked to variations in local structural properties. 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.

Structure and dynamics of ionic liquids confined in nanoporous carbons

243rd National Meeting (2012)

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