Ionic liquid lubrication : influence of ion structure, surface potential and sliding velocity

Colloid probe atomic force microscopy (AFM) has been employed to investigate the nanotribology of the ionic liquid (IL)–Au(111) interface. Data is presented for four ILs, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM] FAP), 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([BMIM] FAP), 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIM] FAP) and 1-butyl-3-methylimidazolium iodide ([BMIM] I), at different Au(111) surface potentials. Lateral forces vary as a function of applied surface potential and ion structure because the composition of the confined ion layer changes from cation-enriched (at negative potentials) to mixed (at 0 V), and to anion-enriched (at positive potentials). ILs with FAP− anions all exhibit similar nanotribology: low friction at negative potentials and higher friction at positive potentials. [BMIM] I displays the opposite behaviour, as an I⁻ anion-enriched layer is more lubricating than either the [BMIM]⁺ or FAP^- layers. The effect of cation charged group (charge-delocalised versus charged-localised) was investigated by comparing [BMIM] FAP with 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([Py] FAP). [BMIM] FAP is less lubricating at negative potentials, but more lubricating at positive potentials. This indicated that even at positive potentials the cation concentration in the boundary layer is sufficiently high to influence lubricity. The influence of sliding velocity on lateral force was investigated for the [EMIM] FAP–Au(111) system. At neutral potentials the behaviour is consistent with a discontinuous sliding process. When a positive or negative potential bias is applied, this effect is less pronounced as the colloid probe slides along a better defined ion plane.