Characterization of ionic liquid films in view of tribological properties and development of structure-property relationships by means of electrochemical surface force apparatus

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The goal of this research project is to characterize and examine the tribological behaviour on different layers of thin films of ionic liquids. Tribology, or the study of surfaces in contact sliding against each other is of practical relevance in our everyday life and of utmost economical importance. The measured friction force between two bodies is a result of cumulative interactions of the surfaces on the atomic scale. In particular, the physical andchemical properties of the interface determine the frictional behavior. Due to the unique properties of ionic liquids, including the formation of a layered structure in front of solid surfaces, they are ideal candidates for tomorrows high-performance lubricants. Within this research project experiments at the nano- and micrometer scale will be performed at the University of Oxford in the United Kingdom using a surface force apparatus operating with full electrochemical control of the sample. The advantage of the integration of electrochemical methods is the possibility to alter key parameters such as the roughness of the substrate as well as the stability and arrangement of the ionic liquid layers reversibly and in situ. The primary objective of this project is to understand the constitution of the solid-liquid interface as the sliding plane within the first few nanometers apart from the surface. The mechanical properties will be probed with a high force resolution as required to develop structure-property relationships on the molecular length-scale. Preliminary studies into the frictional properties of ionic liquids have already been carried out at Oxford and by the applicant. Based on these results specific properties of the ionic liquids, including the size of the anions and cations, the charge number and the length of the alkyl chain as well as characteristics of the substrate, e.g. the surface roughness will be varied; furthermore, the controlled contamination of the ionic liquids with water and salts is planned to examine their effect on the friction properties. The fundamental research described in this research proposal is crucial for the establishment of tribological applications of ionic liquids.