Mass Distribution and Diffusion of [1-Butyl-3-methylimidazolium][Y] Ionic Liquids Adsorbed on the Graphite Surface at 300-800 K

The structure and diffusion behavior of 1-butyl-3-methylimidazolium ([bmim](+)) ionic liquids with [Cl](-), [PF6](-), and [Tf2N](-) counterions near a hydrophobic graphite surface are investigated by molecular dynamics simulation over the temperature range of 300-800 K. Near the graphite surface the structure of the ionic liquid differs from that in the bulk and it forms a well-ordered region extending over 30 angstrom from the surface. The bottom layer of the ionic liquid is stable over the investigated temperature range due to the inherent slow dynamics of the ionic liquid and the strong Coulombic interactions between cation and anion. In the bottom layer, diffusion is strongly anisotropic and predominantly occurs along the graphite surface. Diffusion perpendicular to the interface (interfacial mass transfer rate k(t)) is very slow due to strong ion-substrate interaction. The diffusion behaviors of the three ionic liquids in the two directions all follow an Arrhenius relation, and the activation barrier increases with decreasing anion size. Such an Arrhenius relation is applied to surface-adsorbed ionic liquids for the first time. The ion size and the surface electrical charge density of the anions are the major factors determining the diffusion behavior of the ionic liquid adjacent to the graphite surface.