Phosphorus-31 NMR investigation of the heterogeneous hydrolytic decomposition of zinc(II) bis(O,O-dialkyl dithiophosphate) lubricant additives

Abstract

³¹ P NMR spectroscopy has been used to investigate the heterogeneous-phase hydrolysis of zinc(II) bis(O,O-dialkyl dithiophosphates)(ZnDTP), Zn[S₂P(OR)₂]₂1 where R = 2-propyl 1a, 2-butyl 1b, 4-methyl-2-pentyl 1c and 2-ethylhexyl 1d in a 1:1 mixture of heptane–p-xylene to which a tenfold excess of water (relative to ZnDTP) was added. In each case, the process leads to endproducts identical to those previously found for homogeneous-phase hydrolysis, viz. thiophosphoric acid 4, phosphoric acid 5 and alkyl dihydrogen phosphate 8. All hydrolyses are found to follow firstorder kinetics and the calculated relative rates of hydrolytic breakdown (k_HET= 4.16–0.66 × 10^–5 s^–1) show a significant decrease as the size of alkyl group increases. This is attributed to the ratelimiting step for hydrolysis being the diffusion of the ZnDTP across the solvent–water interface.