Colloids in lubrication: Development of amphiphiles from molecular structure to tribological performance

Lubrication is often applied at interfaces where friction occurs upon relative motion. In boundary lubrication regimes, amphiphilic lubricant additives spontaneously adsorb onto the solid/liquid interfaces, tailoring surface properties and decisively dictating macroscopic lubrication performance. This process is essential for the proper functioning of machinery and for prolonging its service life. The molecular structure of amphiphilic compounds is a key determinant of their interfacial adsorption behavior, which directly influences lubrication quality. Recent advances in characterization techniques, particularly neutron reflection (NR) and small-angle neutron scattering (SANS), have enabled more precise analysis of the nanostructure self-assembled at interfaces from the amphiphilic molecules. These developments have provided new insights into the relationship between molecular structure, interfacial adsorption, and lubrication behavior, stimulating the growing interest in the design of lubrication systems that are novel, efficient, cost-effective, and environmentally friendly. This review summarizes recent experimental progress on the role of amphiphilic molecular structure in interfacial adsorption in both oil-based and water-based lubrication systems and its impact on lubrication. First, we discuss the interactions of amphiphilic molecules with particle surfaces in oil-based systems, including over-based detergent/dispersants, nanoparticles, and emulsion droplets. Next, we review the packing and structure-function relationship of these molecules at solid-liquid interfaces under friction and at gas-liquid interfaces in lubricant foams within oil-based systems. We also describe the behavior of amphiphilic molecules in water-based lubrication systems, including various surfactants and their experimentally revealed lubrication mechanisms. Finally, we outline current challenges and future directions for the development of amphiphilic lubrication additives.