Theoretical Study on the Structure and Lubricating Properties of Disulfide Compounds as Extreme Pressure Additive

The molecular geometries optimization and electronic structures for twelve disulfide compounds were investigated making use of density functional theory (DFT) and ab initio method at the 6-31G basis set level. The active atoms and reaction bonds of the disulfides with respect to iron atom cluster were determined making use of frontier molecular orbital theory. The interaction pattern between the organic disulfide compounds and iron atom cluster was discussed based on the approximate rule of orbital energy. Some parameters characterizing the action strength between the organic disulfide compounds and iron atom cluster, including the bonding strength, reactive strength, and static action strength, were analyzed using frontier electron density, super de-localizability, and net atomic charge as criteria. The results indicated that the S-S and C-S bonds of the compounds would be preferentially broken in interacting with the metal surface. The anti-wear ability of the disulfide compounds as the additives was strengthened and the extreme pressure performance weakened with increasing carbon chain. The prediction results based on quantum chemistry calculations were in good accordance with the relevant friction and wear test results, which might provide reliable reference data for the molecular design of novel high-performance antiwear and extreme pressure additives.