Zinc dithiophosphates (ZnDTPs) are ubiquitous lubricating oil additives in today's passenger car motor oils, providing the important functions of wear and oxidation inhibition. However, the molecular-level mechanism by which these materials reduce wear is not understood. As a first step in developing an understanding of this mechanism, we used ab initio quantum chemical methods to examine the structures, vibrations, and energetics of these systems. The results show that the two phosphorus-sulfur bonds of the dithiophosphate of ZnDTPs are equiValent and have character intermediate between single and double bonds. This contrasts with the paradigm of one double bond (PdS) and one single bond (P-S) often used. Vibrational studies of DTP systems lead to a strong IR transition at about 650 cm -1 and a weak transition at about 530 cm -1 . We find modes in good agreement with experiment, where the high-frequency mode is antisymmetric PS stretch (not PdS), while the lower mode is symmetric PS stretch (not P-S). On the basis of the ab initio calculation results, we used the biased Hessian method to develop a vibrationally accurate force field (FF) for ZnDTPs. This FF can be used to examine the binding of DTPs to metal and metal oxide surfaces.
Zinc dithiophosphate (DTP) molecules have long been used as wear inhibitor oil additives for automotive engines. In order to obtain an atomistic understanding of the mechanism by which these molecules inhibit wear, we examined the geometries, energetics, and vibrations of an oxidized iron surface [(001) surface of α-Fe2O3] using the MSX force field (FF) based on ab initio quantum chemistry (QC) calculations. The DTP molecules studied include (RO)2PS2 with R = methyl, isobutyl, isopropyl, and phenyl. The α-Fe2O3 surface is described using the generalized valence bond (GVB) model of bonding. The geometries, binding energies, and vibrational frequencies from ab initio calculations on simple clusters are used with the biased Hessian method to develop the MSX FF suitable for describing the binding of DTP molecules to the surfaces. We find that the cohesive energies for the self-assembled monolayers (SAM) of the DTP molecules on the Fe2O3 surface correlate with the antiwear performance observed in experimental engine tests. This suggests that the search for more effective and environmentally benign wear inhibitors can use the cohesive energies for SAM formation as a criterion in selecting and prioritizing compounds for experimental testing.
In previous studies of dithiophosphate [DTP = S2P(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 iC5 and nC3 are the best candidates, followed closely by iC3.
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