Effects of Pore Size and Pore Loading on the Properties of Ionic Liquids Confined Inside Nanoporous CMK-3 Carbon Materials

We report molecular dynamics simulations of the structural and dynamical properties of 1,3-dimethylimidazolium chloride, [dmim(+)] [Cl-], confined inside the ordered mesoporous carbon CMK-3. This material exhibits interconnected nanopores with heterogeneities in pore shape, pore size, and pore surface roughness. Our results indicate that variables such as pore size, pore loading, and pore morphology have a profound influence on the structural and dynamical properties of the confined IL. Significant layering is observed in all the systems, with the number of layers and the relative positions of the density peaks of the ions varying with pore size and pore loading. At low pore loadings, the density profile along the axial direction becomes highly heterogeneous, indicating the presence of regions where rho approximate to rho(bulk) and regions partially depleted of IL for which rho approximate to rho(bulk). The radial distribution functions suggest that the structure of the confined IL is similar to that of a bulk IL; however, important variations in the height of the peaks are observed as the pore loading changes. Our results indicate that the dynamics of the confined IL are significantly slower than those observed in bulk systems. The mean squared displacements (MSDs) of the confined ions in the three directions are of similar magnitude, in contrast to what was observed when the same IL is confined inside a slit-like pore of similar size. For pore loadings similar to rho(bulk), the MSDs in the axial direction increase monotonically with pore size. For fixed pore sizes, the axial MSDs decrease monotonically as the pore loading increases above rho(bulk) but below rho(bulk) we observe nonmonotonic variations in the axial MSDs. Our results indicate that, for any given pore size, the axial diffusivity of the confined cations reach maxima at pore loadings below rho(bulk), possibly due to the large heterogeneities observed in the axial density profile at these low pore loadings.