Surprising particle stability and rapid sedimentation rates in an ionic liquid

In this letter we demonstrate that particle suspensions in room temperature ionic liquids differ from aqueous suspensions in some surprising and remarkable ways. Two results are of key importance. First, suspensions of 1 μm diameter silica spheres do not aggregate in pure ethylammonium nitrate (EAN) despite interparticle electrostatic repulsions being completely screened by its 11 M ionic strength. However these dispersions become unstable in the presence of small amounts of water. Using silica colloid probe atomic force microscopy (AFM), optical microscopy and dynamic light scattering we show that this unusual stability is imparted by repulsions between well formed solvation layers, which decrease in number and strength upon addition of water. Second, particle suspensions in pure EAN settle six times more rapidly than predicted by the hindered Stokes equation. This remarkable result is unprecedented in the literature to our knowledge, and could foreshadow interesting lubrication effects for surfaces in EAN.