The critical role of interfacial Coulomb force in the orientation alignment behavior of lubricant molecules

The non-bonding interactions of four interfaces with typical polar interfacial characteristics are simulated using molecular dynamics methods. The Coulomb and Van der Waals terms in the optimized potentials for liquid simulations all-atom (OPLS_AA) force field are distinguished and their effects on the arrangement behavior of the lubricant molecules are investigated. A mathematical model is proposed to quantitatively characterize the influence on the tilt degree of lubricant molecules (PAO4 and PEG200) near the nano-silica surfaces. The tilt degree of the lubricant molecules at the interface is determined by the molecular chain tilt factor γ, which is calculated by combining the "polar characteristics of the solid surface", the characteristics of the "polar units" within the liquid molecules, and the linking modes between them.