Influence of ion structure and surface potential on ionic liquid lubrication

The lubricating properties of ionic liquids can be controlled by varying the molecular structure of the ions1'2 and by applying an electric potential to the sliding contact.3 Results for two systems will be presented. Firstly, the lubricating properties of protic ionic liquids confined between silica and mica surfaces will be examined. Atomic force microscope (AFM) images combined with normal and lateral force curves allows variation in lubricity as a function of normal load to be correlated with differences in the lateral structure of the ions confined between the surfaces. Secondly, results for aprotic ionic liquids confined between silica and gold surfaces will be presented. Friction forces vary with the potential applied to the gold surface because the composition of the ion layer confined between the two surfaces changes from cation-enriched (at negative potentials) to anion-enriched (at positive potentials); whether lubricity is increased or decreased as the composition changes depends on the ion species (c.f. Figure below). This offers a new approach to tuning factional forces reversibly at the molecular level without changing the substrates by employing a self-replenishing boundary lubricant of low vapour pressure.