I came across an interesting article today:
Finite Temperature Structure and Dynamics of Zinc Dialkyldithiophosphate Wear Inhibitors: A Density Functional Theory and ab Initio Molecular Dynamics Study
by Nicolas J. Mosey and Tom K. Woo at the University of Western Ontario.
Published in the Journal of Physical Chemistry A-Pages in 2003 (volume 107, p.5058-5070).
Here's the conclusions section:
Quote:
The structures, energies, and finite temperature dynamics of ZDDP monomers, dimers, and isomers with a wide variety of substituents have been examined with static DFT calculations and through ab initio molecular dynamics simulation. The investigation of the structures of these species indicated that at finite temperature the ZDDP additives likely exist in the monomeric form. Furthermore, it was shown that at finite temperature this species undergoes facile dissociation of one or two of the Zn-S bonds and that this dissociation has an effect on the electron-accepting abilities of the monomer. An examination of the relative energetics of the monomer and its isomer, LI-ZDDP, revealed that these two species are of similar stability.
The finite temperature simulations of both the ZDDP monomer and LI-ZDDP isomer allowed for the identification of several distinct thermal decomposition pathways involving the loss of radicals, olefins, and sulfides. Alkyl-substituted ZDDPs underwent the preferential loss of alkyl radicals, whereas aryl-substituted species decomposed through the elimination of the alkoxy radicals. Both processes result in the formation of (thio)phosphate species that may be precursors to the antiwear film, but those resulting from the loss of alkoxy radicals have a decreased oxygen content and may result in the formation of a film of poor quality. This difference in the decomposition pathways highlights a difference between aryl- and alky-lsubstituted ZDDPs and may provide a possible atomic level explanation for experimental results that indicate that films generated from aryl-substituted ZDDPs offer significantly less antiwear protection than those derived from alkyl-substituted additives.
The loss of olefins was also observed to occur through either of two distinct mechanisms involving radical intermediates or concerted processes. Both of these elimination mechanisms involved the transfer of a beta-hydrogen from the alkyl group and led to possible precursors to the antiwear films. Thus, for substituents that contain a greater number of these atoms, the formation of the antiwear film is more easily achieved, which is consistent with experimental observations. Furthermore, this process is not likely to occur for aryl-substituted ZDDPs and may further contribute to the reduced antiwear abilities of films derived from aryl-substituted additives.
Finite Temperature Structure and Dynamics of Zinc Dialkyldithiophosphate Wear Inhibitors: A Density Functional Theory and ab Initio Molecular Dynamics Study
by Nicolas J. Mosey and Tom K. Woo at the University of Western Ontario.
Published in the Journal of Physical Chemistry A-Pages in 2003 (volume 107, p.5058-5070).
Here's the conclusions section:
Quote:
The structures, energies, and finite temperature dynamics of ZDDP monomers, dimers, and isomers with a wide variety of substituents have been examined with static DFT calculations and through ab initio molecular dynamics simulation. The investigation of the structures of these species indicated that at finite temperature the ZDDP additives likely exist in the monomeric form. Furthermore, it was shown that at finite temperature this species undergoes facile dissociation of one or two of the Zn-S bonds and that this dissociation has an effect on the electron-accepting abilities of the monomer. An examination of the relative energetics of the monomer and its isomer, LI-ZDDP, revealed that these two species are of similar stability.
The finite temperature simulations of both the ZDDP monomer and LI-ZDDP isomer allowed for the identification of several distinct thermal decomposition pathways involving the loss of radicals, olefins, and sulfides. Alkyl-substituted ZDDPs underwent the preferential loss of alkyl radicals, whereas aryl-substituted species decomposed through the elimination of the alkoxy radicals. Both processes result in the formation of (thio)phosphate species that may be precursors to the antiwear film, but those resulting from the loss of alkoxy radicals have a decreased oxygen content and may result in the formation of a film of poor quality. This difference in the decomposition pathways highlights a difference between aryl- and alky-lsubstituted ZDDPs and may provide a possible atomic level explanation for experimental results that indicate that films generated from aryl-substituted ZDDPs offer significantly less antiwear protection than those derived from alkyl-substituted additives.
The loss of olefins was also observed to occur through either of two distinct mechanisms involving radical intermediates or concerted processes. Both of these elimination mechanisms involved the transfer of a beta-hydrogen from the alkyl group and led to possible precursors to the antiwear films. Thus, for substituents that contain a greater number of these atoms, the formation of the antiwear film is more easily achieved, which is consistent with experimental observations. Furthermore, this process is not likely to occur for aryl-substituted ZDDPs and may further contribute to the reduced antiwear abilities of films derived from aryl-substituted additives.
