Ionic Liquids Confined in a Realistic Activated Carbon Model: A Molecular Simulation Study

Abstract

Classical molecular dynamics simulations were performed to study the structure and dynamics of the ionic liquid (IL) [emim+][NTf2–] inside a slit graphitic nanopore and a realistic model of a coconut shell activated carbon (CSAC), which was generated using a reverse Monte Carlo protocol and matches the experimental radial distribution function from the real adsorbent. The CSAC model material consists of semigraphitic carbon sheets with different sizes and shapes, which form irregularly connected pores of roughly rectangular shape. In general, the ions inside the CSAC model material form layers parallel to the walls, as observed for the IL inside slit pores; however, the distribution of pore sizes and the complex pore geometry of the CSAC model materials cause the density profiles and the orientation of the ions to depart significantly from the uniform behavior observed for these properties of the IL inside slit pores. The presence of interconnected pores with a distribution of sizes in the CSAC model materials also causes confinement effects to be weaker than in slit pores of the same size; as a result, the ions inside CSAC model materials have a liquid structure similar to that of the bulk IL and have faster dynamics than those of ions inside slit pores of the same size. The ions near the pore walls of the CSAC model material move slower than the ions that are farther away from the walls, as observed for the IL inside slit pores; however, the complex pore geometry and variations of pore size with position within the CSAC model material cause the dynamics of the confined IL to exhibit significant spatial heterogeneities and depart significantly from the uniform, regular behavior observed in slit nanopores. Our results suggest that the structure and dynamics of ILs confined inside porous materials having heterogeneities in pore size, pore shape, and pore interconnectivity can depart significantly from the properties of ILs confined inside ideal pores of simple geometries.