| Summary: | <p>Ionic liquids (ILs) have gained notoriety within the field of liquid matter, specifically as a lubricant. Fascination with ILs as lubricants is due to their physio-chemical stability and strong interactions with solid surfaces, marking an improvement over industrial lubricants. IL lubricants can therefore operate in high-pressure regimes without compromising the integrity of the surfaces.</p>
<p>While studies have demonstrated that pure ILs are excellent lubricants, using large amounts is costly and unfeasible for industry. A route around this problem is to use trace amounts of IL as an additive to make an industrially efficient lubricant. </p>
<p>This thesis explores the interfacial structure and frictional response across a range of IL-solvent mixtures to explore their performance as lubricant additives. Using the surface forces balance (SFB), the structural and frictional forces at the nanoscale can be simultaneously measured while the applied load is controlled. Macroscale friction behaviour is also studied using a ball-on-disc tribometer.</p>
<p>Both nanoscale and macroscale measurements across a range of ILs, mixed with either a polar solvent or a model base oil, showed that friction is influenced by the liquid film structure. Using the SFB, we were able to understand the molecular mechanisms underlying the friction, as we could probe the simultaneous confined structure. Varying the composition of these mixtures revealed that while macroscale performance could be improved with trace additions of ILs, the nanoscale behaviour was not as reliable.</p>
<p>The SFB measurements of oil-soluble ILs showed a lack of layering and ineffective lubrication when compared to the base oil, while the water-soluble IL showed reliable and superlubric performance across both length scales, with the nanoscale studies showing that a switch in the structural forces was reflected in the frictional forces. These results indicate that while ILs can become lubricant additives, a wide-scale systematic study over multiple length scales and surfaces is required. </p>
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