Measurement of forces across room temperature ionic liquids between mica surfaces

Ionic liquids (ILs) are liquids that consist only of cations and anions. In spite of a plethora of emerging processes involving ionic liquids, there is no experimental measurement of the structure and dynamics of ILs at the solid-liquid interface comparable to the double layers of electrolyte solutions. Here, we report the direct measurements of the interactions (force-distance functions) between two atomically smooth mica surfaces across three different types of room temperature ILs. The forces are repulsive at all measured distances in all three cases, decaying exponentially with distance. Effective Debye screening lengths are found to be around 1-4 nm, much longer than expected from traditional theories, and strongly dependent on the size and molecular structure of the anion/cation. At small separations, in addition to the exponential repulsions, oscillatory forces are observed, while any monotonically attractive van der Waals forces, if present, are overwhelmed by the repulsive electrostatic and oscillatory forces.