Abstract: Quantum mechanical methods were employed to calculate the intermolecular interaction energies between lithium stearate (Li-ST) thickener and three synthetic base oils, namely polyalphaolefin (PAO3.6), alkyl naphthalene (AN3), and diisooctyl sebacate (DOS). The results showed that the interaction between PAO3.6 and Li-ST molecules was dominated by dispersion forces. For AN3 and DOS, a synergistic coupling of hybrid orbital interactions and dispersion forces was formed with the Li-ST thickener via π-electron delocalization and ester group coordination, respectively. The interaction energies between the three synthetic base oils and Li-ST increase in the order of PAO3.6 < AN3 < DOS. Frontier molecular orbital analysis confirmed that electron transfer occurs for the thickener in polar systems, accompanied by a gradual reduction in the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The performance characterization results of the as-prepared synthetic lubricating greases revealed that higher intermolecular interaction energy corresponds to superior colloidal stability and tribological performance of the greases. Accordingly, the overall performance of the three synthetic lubricating greases ranked from low to high as follows: PAO3.6-based lubricating grease < AN3-based lubricating grease < DOS-based lubricating grease.

Key words: lubricating grease, quantum mechanics, density functional theory, intermolecular interactions, frontier orbital theory