Autoxidation in micelles. Synergism of vitamin C with lipid-soluble vitamin E and water-soluble Trolox

Abstract

A study was made of the effect of the inhibitors ascorbic acid (C), .alpha.-tocopherol (E), and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylate (Trolox, T) on the autoxidation of linoleic acid in 0.50 M sodium dodecyl sulfate (SDS) micelles at pH 7.0 in phosphate buffer. Reactions were thermally initiated at 30.degree. C in the SDS micelles by a micelle-soluble initiator, di-tert-butylhyponitrite (DBHN). Although water-soluble C alone is an inefficient inhibitor, when combined with micelle-soluble E, it acts synergistically with the latter to extend the efficient antioxidant action of E beyond the sum of the induction periods of C and E acting separately. Similarly C acts synergistically with the water-soluble antioxidant, T. Quantitative studies of these effects under controlled rates of initiation (Ri) reveal that C functions to regenerate a mole of E (or T) per mole of C used. Kinetic studies show that the rate of autoxidation is 1st order in micellar linoleic acid and one-half order in micellar DBHN concentrations. Therefore, the classical rate law, -dO2/dt = kp[R.sbd.H] (Ri)1/2 is followed. The higher oxidizability (kp/2kt1/2 = 4.48 .times. 10-2 M-1/2 s-1/2) of linoleate in micelles compared to that in homogeneous solution in chlorobenzene (kp/2kt1/2 = 2.30 .times. 10-2 M-1/2 s-1/2) is interpreted in terms of the effect of the polar interfacial region of the micelles on a dipolar transition state, R.sbd.O.hivin.O: .**GRAPHIC**. of the propagation reaction.