Molecular Rheology of a Nanometer-Confined Ionic Liquid

Molecular mechanisms in the shear of an ionic liquid in nanometer-scale confinement were investigated by atomic force microscopy with a laterally oscillating tip. On a single-crystal gold electrode, the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf 2 ]) exhibits molecular ordering in a simple cubic structure upon confinement by the nanometer-sized asperity. The strength of shear resistance decreases exponentially with increasing number of confined molecular layers. The dependence of lateral forces and of dissipation on the applied electrochemical potential confirms a mechanism in the electrolubrication by ionic liquids, namely, the change of the slippage plane from the interface with the electrode into the ionic liquid for increasing surface potential.