Insights into the chemical behavior of zinc dialkyldithiophosphate anti-wear additives in their isomeric and decomposed forms through molecular simulation

Ab initio molecular dynamics simulations and static quantum chemical calculations at the density functional level of theory have been applied to study zinc dialkyldithiophosphate (ZDDP) engine oil anti-wear additives in their parent, isomeric and decomposed forms to identify differences in the basic finite temperature chemical behavior of these species that may be related to the formation of ZDDP anti-wear films. It was found that the isomers, which contain Zn–O bonds, exhibited finite temperature behavior leading to the dissociation of these bonds, while the parent ZDDP molecule, which contains only Zn–S bonds, did not. The dissociation of the Zn–O bonds improves the abilities of these molecules to participate in film formation reactions as electron donors and hence the transformation of the ZDDP molecule into species with Zn–O bonds, such as the isomers and decomposed species, may aid in anti-wear film formation. It was observed that the dissociation of the Zn–O bonds allowed the molecules to attain rearranged structures where the coordination at the zinc atom had significantly changed and it was found that these rearranged structures could decompose through the elimination of metathiophosphates (MTPs). The results of static density functional theory calculations on all species indicated that the elimination of MTPs was energetically favorable at high temperatures, however, the ability of the parent molecule to participate in rearrangement determines whether decomposition will occur. The fact that these species readily participate in reactions that lead to MTP formation is relevant within the context of ZDDP anti-wear film formation since the MTP molecules possess properties that make them ideal precursors to the thiophosphate networks that are thought to comprise these films. Overall, the results of this study indicate that differences between the ZDDP molecule, its isomers and decomposition products exist that may affect the abilities of these species to participate in the formation of anti-wear films. This information will be useful within the development of an overall mechanism for the formation of ZDDP anti-wear films.