P-31 NMR INVESTIGATION OF THE HETEROGENEOUS HYDROLYTIC DECOMPOSITION OF ZINC(II) BIS(O,O-DIALKYL DITHIOPHOSPHATE) LUBRICANT ADDITIVES

P-31 NMR spectroscopy has been used to investigate the heterogeneous-phase hydrolysis of zinc(II) bis(O,O-dialkyl dithiophosphates) (ZnDTP), Zn[S2P(OR)(2)](2) 1 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 first order kinetics and the calculated relative rates of hydrolytic breakdown (k(HET) = 4.16-0.66 x 10(-5) s(-1)) show a significant decrease as the size of alkyl group increases. This is attributed to the rate-limiting step for hydrolysis being the diffusion of the ZnDTP across the solvent-water interface.