Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

No abstract

Colloid probe atomic force microscopy has been used to study the nanotribological properties of the silica-ethylammonium nitrate (EAN)-mica system. Normal force curve measurements reveal a series of steps at separations that are consistent with the size of an EAN ion pair (0.5 nm) due to displacement of structured solvent layers as the two surfaces are brought together. At closest separations, two steps are measured with widths of 0.3 nm and 0.1 nm, which are too small to be due to an ion pair layer. The 0.3 nm step is attributed to a partial displacement of a silica-bound cation-rich layer, with residual cations being removed in the subsequent 0.1 nm step. Lateral force measurements reveal that the frictional response is dependent on the number of ion pair layers between the surfaces. At low forces, when there is more than a single layer of EAN between silica and mica, the lateral force increases relatively steeply with applied load, and is independent of the sliding speed. At intermediate forces, a single layer of cations in an intercalated bilayer structure is present between the surfaces. The friction coefficient (μ) increases logarithmically with sliding speed consistent with an activated, discontinuous sliding process. At high force, μ is small and once again, independent of sliding velocity. The adsorbed cation layer is bound primarily to mica and compressed by the high normal force. This robust layering with a well-defined sliding plane permits the colloid probe to slide easily over the mica surface.

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. Firstly, suspensions of 1 µm diameter silica spheres do not aggregate in pure ethylammonium nitrate (EAN) despite interparticle electrostatic repulsions being completely screened by its 11M 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 decreases in number and strength upon addition of water. Secondly, 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.

No abstract