Application of the Self-Assembled Monolayer (SAM) Model to Dithiophosphate and Dithiocarbamate Engine Wear Inhibitors

In previous studies of dithiophosphate [DTP = S_2P(OR)_2] wear inhibitors bound to an oxidized iron surface, we found that the cohesive energy of the self-assembled monolayers (SAM) for DTP molecules with various organic R groups correlates with the wear inhibition observed in full engine experiments. In this paper we expand these calculations to consider dynamics at 500 K and then use the SAM model to predict new candidates for wear inhibitors. Using molecular dynamics (MD) simulations at 500 K, we show that the SAM has one DTP per two surface Fe sites of iron oxide. At this coverage we find that the cohesive energy of the SAM at 500 K is in the sequence 2-alkyl > 1-alkyl > aryl (e.g., iPr > iBu > Ph) which again correlates with wear inhibitor performance observed in engine tests. We then considered 7 novel DTPs and predict that R = cyclo-hexyl, nPr, and benzyl may perform as well as iPr. We then used the SAM wear inhibitor model to assess the likely performance of 11 novel classes of potential wear inhibitors. On the basis of this model we selected dithiocarbamates (DTC) as the best candidate to supplement DTP. We then considered a number of possible alkyl substitutions for DTC. The SAM model suggests that iC_5 and nC_3 are the best candidates, followed closely by iC_3.