Molecular Dynamics Study of Ionic Liquid Film Based on [emim][Tf2N] and [emim][TfO] Adsorbed on Highly Oriented Pyrolytic Graphite

Molecular dynamics simulation was used to study the ionic liquid(IL) crystalline film based on 1-ethyl-3-methylimidazolium bistrifluoromethylsulfonyl]imide(emim]Tf₂N]) and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate(emim]TfO]) on the graphite surface. Our results show that the cations are parallelly distributed to the surface in the 1/2 monolayer(ML) crystalline film. The Tf₂N]^- anions are parallel to the surface with the oxygen atoms at the bottom, whereas the TfO]^- anions are perpendicularly distributed to the surface also with the oxygen atoms at the bottom in the 1/2 ML crystalline film. It has been found that the IL-vapor interface strongly influences the arrangement of ions at the interface. The anions in the top layer with the oxygen atoms outmost turn over to make themselves with the F atoms outmost so as to form C-H···O hydrogen bonds with the cations. The calculated orientational ordering shows that in the outmost layer at the IL-vapor interface, the cation rings present either parallel or perpendicular to the surface at 350 K.