Finite Temperature Structure and Dynamics of Zinc Dialkyldithiophosphate Wear Inhibitors:  A Density Functional Theory and ab Initio Molecular Dynamics Study

Abstract

The thermal decomposition of several zinc dialkyldithiophosphate (ZDDP) antiwear additives has been explored with both finite temperature gas-phase ab initio molecular dynamics (MD) simulations and static quantum chemical calculations at the density functional (DFT) level of theory. Calculations have been performed on the ZDDP monomers (Zn(S2P(OR)2)2), ZDDP dimers, and the corresponding linkage isomers (LI-ZDDPs) with a variety of substituents (R = H, Me, Et, i Pr, t Bu, Ph). The results show that the monomeric form of ZDDP likely dominates at finite temperatures for all substituents considered and that the LI-ZDDP isomer is nearly thermoneutral with respect to the parent ZDDP monomer. Optimized geometries of the ZDDP monomer give 4-coordinate Zn structures that are consistent with previously reported theoretical calculations. However, ab initio molecular dynamics simulations of the ZDDP monomers at elevated temperatures show that 2- and 3-coordinate complexes are instead favored and that the decrease in coordination number has a significant effect on the electronic structure of the molecule that may affect the reactivity of ZDDPs with other chemical species present in engine oils. The ab initio MD simulations also provide insight into several decomposition pathways of the various ZDDP species that include the loss of either alkyl or alkoxy radicals as well as the elimination of olefins and sulfides from the ZDDP molecule. The results are discussed in terms of how the observed processes will affect the overall abilities of the antiwear film.

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…Examples of this include CarParrinello MD (CP-MD) [287][288][289][290][291] [214,[292][293][294][295], which have given important insights into the reactivity of common friction modifier (molybdenum disulphide, molybdenum dithiocarbamate) and anti-wear (zinc dialkyldithiophosphate) additives. Ab initio NEMD simulations have also been performed on additive systems subjected to confinement and shear.…”

Interpretation of experiments on ZDDP anti-wear films through pressure-induced cross-linking

“…Nonetheless, it is important to understand the decomposition processes in order to justify the model system used in the calculations and gain fundamental insight into processes that occur during the initial stages of film formation. This has been the focus of previous simulations [30][31][32][33]. In those studies, several decomposition routes accessible to ZDDPs were identified and explored at the molecular level.…”

Neutral zinc(II) O,O‐di‐alkyldithiopho‐ sphates—variable temperature 31P NMR and quantum chemical study of the ZDDP monomer–dimer equilibrium

“…This intermediate was first reported using an elevated temperature gas-phase ab initio molecular dynamics simulation of ZDDP. [24] Figure. is shown in Fig. 9.…”

“…[24] The dimerization energies, calculated for the monomer-dimer equilibrium, equaled 2-24.7 kcal/mol, depending on the alkyl substituent.…”