Photolysis of ketones in oxygen-saturated micellar solution: oxygen scavenging of C-centered radicals in microheterogeneous media

Abstract

The oxygen scavenging of C-centered radicals in microheterogenous media was studied by investigation of the photolysis of several ketones (dibenzyl ketone (DBK), o- and p-methyldibenzyl ketone (o-MeDBK and p-MeDBK) and d,1-2,4-diphenylpentan-3-one (d,1-DPP)) in oxygen-saturated sodium dodecylsulfate (SDS) micellar solution. The acyl- and benzyl-type radicals initially formed during the photolysis of these ketones can undergo geminate micellar and random bulk aqueous recombination or can react with oxygen in the bulk or micellar phases to yield peroxy radicals. The efficiency of radical scavenging by oxygen was calculated on the basis of three experimental parameters, i.e. the recombination probability Pr, of a primary acyl-benzyl radical pair (RP), the secondary cage effect for benzyl-benzyl RP recombination and the chemical yield of dibenzyls. Under a partial pressure of 1 atm, oxygen was found to scavenge only those radicals which enter the bulk aqueous phase if the lifetime of the geminate radicals inside the micelles is relatively short (about 20 ns). However, for micellized geminate radicals with a longer lifetime (about 70–80 ns), scavenging by oxygen inside the micelles successfully competes with geminate radical recombination. These results are compared with other investigations involving the dynamics and partitioning of oxygen in SDS micelles.