Surface Structure of Ionic Liquids Using High-Resolution RBS

Elemental depth profiles of typical ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethane-sulfonyl) imide ([CnC1Im][Tf2N], n=2, 6, 10), were measured using high-resolution Rutherford backscattering spectroscopy (HRBS) and high-resolution elastic recoil detection analysis. The obtained depth profiles deviate from the uniform stoichiometric composition in the surface region, showing preferential orientations of ions at the surface. The results were well reproduced by molecular dynamics (MD) simulations, demonstrating that the state-of-the-art MD simulations are a reliable method to study surface structures of ILs. The surface structures of 11 equimolar mixtures were also studied using HRBS. A general tendency that larger IL is enriched at the surface was found.