There is growing demand for engineering systems and functional materials with improved energy efficiency and longer lifetime through improved friction, wear, and lubrication performance, also known as tribological performance. This demand is driven by economic and societal needs such as minimizing greenhouse gas emissions, ensuring energy security, and improving industrial output and competitiveness. Ionic liquids (ILs) are molten salts with tunable composition and melting points below 100ºC. Their physical and chemical properties make them promising lubricating fluids. However, the high cost and corrosiveness of common ILs combined with a poor understanding of the relationship between IL molecular structure and lubrication performance has limited large-scale utilization of ILs in tribological applications. A class of low-cost, eco-friendly, non-corrosive protic ionic liquids (PILs) called choline amino acid PILs has emerged as a possible alternative to these common ILs. This project seeks to understand how PIL molecular structures affect their arrangement and reaction at solid sliding interfaces. The project will also leverage the complementary expertise and resources of the principal investigators at the University of Texas-Austin (UT-Austin) and Rochester Institute of Technology (RIT) to enhance the quality, size, and diversity of the US engineering workforce through training and education opportunities for undergraduate and graduate students from diverse backgrounds and underrepresented groups in STEM.

The research project will identify links between the molecular structure of choline amino acid PILs – a class of halogen-free, eco-friendly PILs – and their functional performance (i.e., nanoscale and macroscale lubricating behavior). The research will also examine how the functional behavior emerges from the interfacial processes occurring at solid/PILs interfaces in response to spatial confinement and applied normal pressure and shear forces. The working hypothesis is that multiple interfacial processes, including surface adsorption, interfacial phase transformation, and shear-induced mechano-chemical reaction, underpin the promising lubricating properties of choline amino acid PILs; in addition, the kinetics of these processes can be controlled by tailoring the molecular structure of the ions (e.g., length of the alkyl chains). The team’s complementary expertise and instrumentation will be applied to synthesize PILs with systematically-varied structures. These PILs will be employed in nanoscale and macroscale tribological experiments to test the working hypothesis. The project outcomes will inform the design of novel ILs with improved and task-specific tribological properties.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.