Conservative and Dissipative Interactions of Ionic Liquids in Nanoconfinement

By applying a small-amplitude (∼200 pm) oscillation to an atomic force microscopy probe during force–distance spectroscopy we are able to separate the “resistance to squeeze” of an ionic liquid nanoconfined between the probe and a mica sheet into its conservative and dissipative components. The interaction stiffness of the ionic liquid increases to a solid-like value as the mica is approached, while the “effective local viscosity” dramatically increases above the bulk value due to adjacent layers of ion-pairs interlocking as they slide over each other.