Turning the solubility and lubricity of ionic liquids by absorbing CO₂

Ionic liquids (ILs) attract high interest as lubricants or lubricant additives due to their special physicochemical characteristics. CO₂ is a widely distributed gas. In many situations, its influence on lubricants cannot be avoided. In this work, three ILs are synthesized from choline and amino acids of glycine, L-proline and lysine, respectively. The influence of CO₂ absorption on their solubility and lubricity is investigated. In general, it is interesting to find that the solubility is decreased and their lubricity is obviously improved after absorbing CO₂, which is strongly related to their functional group (amino group) interactions with CO₂. The formation of carbamate groups greatly increases the viscosity resulting in less solid contacts, and strengthens the interfacial adhesion between ILs and solids.