Ionic Liquids Confined in Hydrophilic Nanocontacts: Structure and Lubricity in the Presence of Water

We have investigated the influence of ambient humidity on the nanoconfined structure and response to shear of ionic liquids. Three ionic liquids (ILs) were selected, namely, 1-ethyl-3-methyl imidazolium ethylsulfate ([EMIM][EtSO4]), 1-ethyl-3-methyl imidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM] [FAP]), and 1-hexyl-3-methyl imidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIM][FAP]), to investigate the influence of hygroscopic and hydrophobic anions, as well as different alkyl chain lengths. We employed an extended surface forces apparatus (eSFA) to ascertain the structure of the confined films, whereas colloidal-probe lateral force microscopy (CPM) was used to measure shear forces in the nanosized contact between mica and a silica sphere. The presence of water, the anion, and the alkyl chain length of the imidazolium cation were found to influence the equilibrium structure of the nanoconfined film, as well as its dynamic properties. Adsorbed water appears to change both the ion-pair orientation and the slip condition for film-thickness transitions, that is, the resistance of the IL layers to being squeezed out from the contact. Three lubrication regimes have been identified: a boundary-film lubrication regime with the lowest friction, an intermediate lubrication regime that is highly dependent on the IL anion, and an isoviscous rigid hydrodynamic lubrication regime (with Newtonian fluid-film behavior). It is shown how IL composition and water both influence speed and load dependence of shear forces at the nanoscale. Understanding the response to shear provides further insight into the properties of nanoconfined IL films.