Who’s in charge? Electro-responsive QCM Studies of Ionic Liquid as an Additive in Lubricant Oils

Abstract: Electrochemical quartz crystal microbalance has been employed to investigate electro-responsiveness of an ionic liquid as an additive in lubricant oils on a gold surface. Polarisation of the surface reveals changes in frequency where an increase in magnitude amplified the observed response, corresponding to a controllable alternation of the ionic liquid configuration on the surface as a function of applied potential. The frequency changes are due to different packing of the anion and cation, respectively, on the surface as their mass densities and geometries are different. Relaxation of the system was reversible to the application of a potential and it was also found to be diffusion dependent, where the ratio between the ion diffusivities could be extracted from the results. Measurement of the system relaxation reveals a potential decay of that of a discharging capacitor, with an internal resistance inducing an initial potential drop due to the resistivity of the oil medium. The discharge behaviour was also proven to show high internal reproducibility validity within experiments. This newly discovered insight in responsive differences of ion packing is of importance, not only for ionic liquid additives in tribology, but for understanding and exploiting ionic liquids in an array of electrochemical applications.