Effect of Anionic Alkyl Chain Length on Tribological Properties of Ionic Liquids: Molecular Dynamics Simulations

Ionic liquids (ILs) are widely adopted as lubricating materials in engineering fields for steel sliding contacts, and the adsorption structure and kinematic state of friction surfaces are crucial for understanding the improvement of tribological properties in experiments. In this study, we employed molecular dynamics methods to examine the structure and shear dynamics of five ILs with the same cationic triethanolamine paired with carboxylate anions of different alkyl chain length, confined between two crystalline iron surfaces. The results show that the chain length of anions influence the quantity of hydrogen bonds formed, the distribution on the iron surfaces, the thickness of the adsorption layers during the sliding process and the overall motion state. Under elastohydrodynamic lubrication conditions, ILs with longer alkyl chain exhibit less friction on a macroscopic scale due to the weaker hydrogen bonds between the anions and cations, the formation of thicker adsorption layers between sliding surfaces, and the overall pronounced layering phenomenon. These atomic insights into the structure and state of motion during friction can help promote the use of ILs as lubricating materials in engineering applications.