Ionic Liquids (ILs) are a subset of molten salts, distinguished by having melting points below 100 °C. Their low melting points are brought about by weakening electrostatic interactions between the ions and hindering their packing into a crystal lattice. Electrostatic forces are reduced by engineering their molecular structure so that at least one of the ions is large and organic, which increases the distance between neighbouring charged centres, and by delocalising the ionic charge over a large molecular volume. 

High resolution amplitude modulated atomic force microscope (AFM) images  will be used to demonstrate how IL nanostructure changes at a solid surface with the ion structure, and the effect of dissolved solutes.^1-3The effect of applying a potential to a conducting solid surface on the IL interfacial nanostructure will also be discussed, and recent results for the spontaneous exfoliation of graphene (c.f. Figure 1) into an ionic liquid will be described, shown schematically below.⁹

Ionic liquids nanoscale properties make them potentially excellent lubricants: as the ions interact strongly with oppositely charged surfaces, they resist ‘squeeze out’ as surfaces are compressed, meaning a lubricating film will remain in place up to higher forces than for a comparable molecular liquid. The lubricating properties of ionic liquids can be controlled by varying the molecular structure of the ions and by applying an electric potential to the sliding contact.^4-5AFM reveals that tribotronic control of friction using an external potential applied to a gold surface is possible for ionic liquid concentrations as low as 5 mol % in hexadecane.^6-7For IL concentrations less than 5 mol % friction does not vary with surface potential, but for 5 mol % and above changing the potential changes to the composition of the IL boundary layer from cation-enriched (negative potentials) to anion-enriched (positive potentials). As the lubricities of the cation-rich and anion-rich boundary layers differ, the enables active control of friction in oil-based lubricants.

References

1.               Elbourne, A.; Voitchovsky, K.; Warr, G. G.; Atkin, R., Ion structure controls ionic liquid near-surface and interfacial nanostructure. Chemical Science 2015,6(1), 527-536.

2.               Page, A. J.; Elbourne, A.; Stefanovic, R.; Addicoat, M. A.; Warr, G. G.; Voitchovsky, K.; Atkin, R., 3-Dimensional atomic scale structure of the ionic liquid-graphite interface elucidated by AM-AFM and quantum chemical simulations. Nanoscale 2014,6(14), 8100-8106.

3.               Elbourne, A.; McDonald, S.; Voïchovsky, K.; Endres, F.; Warr, G. G.; Atkin, R., Nanostructure of the Ionic Liquid–Graphite Stern Layer. ACS Nano 2015,9(7), 7608-7620.

4.               Sweeney, J.; Hausen, F.; Hayes, R.; Webber, G. B.; Endres, F.; Rutland, M. W.; Bennewitz, R.; Atkin, R., Control of Nanoscale Friction on Gold in an Ionic Liquid by a Potential-Dependent Ionic Lubricant Layer. Physical Review Letters 2012,109(15), 155502.

5.               Li, H.; Rutland, M. W.; Atkin, R., Ionic liquid lubrication: influence of ion structure, surface potential and sliding velocity. Physical Chemistry Chemical Physics 2013,15(35), 14616-14623.

6.               Cooper, P. K.; Li, H.; Rutland, M. W.; Webber, G. B.; Atkin, R., Tribotronic control of friction in oil-based lubricants with ionic liquid additives. Physical Chemistry Chemical Physics 2016,18(34), 23657-23662.

7.               Li, H.; Cooper, P. K.; Somers, A. E.; Rutland, M. W.; Howlett, P. C.; Forsyth, M.; Atkin, R., Ionic Liquid Adsorption and Nanotribology at the Silica–Oil Interface: Hundred-Fold Dilution in Oil Lubricates as Effectively as the Pure Ionic Liquid. The Journal of Physical Chemistry Letters 2014,5(23), 4095-4099.