Mechanochemistry of phosphate esters confined between sliding iron surfaces
CA Latorre and JE Remias and JD Moore and HA Spikes and D Dini and JP Ewen, COMMUNICATIONS CHEMISTRY, 4, 178 (2021).
The molecular structure of lubricant additives controls not only their adsorption and dissociation behaviour at the nanoscale, but also their ability to reduce friction and wear at the macroscale. Here, we show using nonequilibrium molecular dynamics simulations with a reactive force field that tri(s-butyl)phosphate dissociates much faster than tri(n-butyl)phosphate when heated and compressed between sliding iron surfaces. For both molecules, dissociative chemisorption proceeds through cleavage of carbon-oxygen bonds. The dissociation rate increases exponentially with temperature and stress. When the rate-temperature- stress data are fitted with the Bell model, both molecules have similar activation energies and activation volumes and the higher reactivity of tri(s-butyl)phosphate is due to a larger pre-exponential factor. These observations are consistent with experiments using the antiwear additive zinc dialkyldithiophosphate. This study represents a crucial step towards the virtual screening of lubricant additives with different substituents to optimise tribological performance. Antiwear additives are crucial to ensuring the reliable operation of lubricated machine components, but virtual screening to obtain new additives has hardly been explored. Here, the authors use nonequilibrium molecular dynamics simulations with a reactive force field to investigate the mechanochemical dissociation of phosphate esters with different alkyl substituents to inform future molecular design.
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