Nanometric ionic-liquid films on silica: a joint experimental and computational study

Atomic force microscopy images for [bmim][Tf2N] films deposited at ambient conditions by drop-casting show a population of terraced islands of mesoscopic area (1-100 mu(2)) and similar to 50 nm height. The regularity of terraces and steps, stiff mechanical properties and a fragile fracture mode all suggest that the islands are solid-like, even though bulk [bmim][Tf2N] is liquid at the temperature of the experiment. Molecular dynamics simulations for a homogeneous [bmim][Tf2N] film 4 nm thick on silica also display marked layering in proximity to silica of periodicity closely matching the experimental estimate of the step height. The density modulation of the simulated sample, however, decays into an approximatively homogeneous and fluid-like density distribution similar to 2 nm from the solid surface. The detailed comparison of experiments and simulations is contained in the closing section of the paper.