LASER PHOTOLYSIS STUDIES OF SINGLET MOLECULAR-OXYGEN IN AQUEOUS MICELLAR DISPERSIONS

The kinetics of singlet oxygen in aqueous (D2O and H2O) micellar systems were examined using laser flash photolysis with 1,3-diphenylisobenzofuran (DPBF) as the reactive monitor. The use of two different sensitizer types (2-acetonaphthone, solubilized in the interior of the micelles, and methylene blue, present in bulk aqueous phase) demonstrated that both the natural decay rate of 1O2* and its bimolecular rate constant for reaction with DPBF are insensitive to the site of singlet oxygen production in the micellar solution. Singlet oxygen lifetimes in solutions of ionic surfactant (sodium dodecyl sulfate (SDS), cetyl trimethylammonium bromide (CTAEI), and sodium laurate) were longer than any values previously reported in D2O: 53 ± 5 μs. This value is proposed as a minimum value of the singlet oxygen lifetime in D2O. Lifetimes measured in nonionic surfactant solution (Brij 35, Igepal CO 630, and Igepal CO 660) were considerably shorter: 21-26 μs. This effect is probably due to the loss of electronic excitation of 1O2* (1Δg) to vibrational modes of the terminal hydroxyl groups of these nonionic surfactants. This quenching action appears to be related to the aggregation of the surfactant in aquesous media, since quenching by these surfactants was not observed in organic solvents. The bimolecular rate constant for reaction of singlet oxygen with DPBF (kr) was approximately 6.5 × 108 L mol-1 s-1 for the cationic surfactant CTAB and the three nonionic surfactants. However, k, was found to be approximately 60% higher in the two anionic surfactants SDS and sodium laurate. The lifetime of singlet oxygen in surfactant-H2O solution was estimated by extrapolation from H2O-D2O mixtures. The values obtained for two surfactants were 4.0 (CTAB) and 3.5 μs (Igepal CO 630). © 1979 American Chemical Society.