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Abstract— The laser flash photolysis method has been used to determine the bimolecular rate constants for the reaction between O2(1Δ9) and several lipid‐soluble and water‐soluble substrates. Values for lipid‐soluble substrates have been obtained using aqueous dispersions of surfactants above the critical micelle concentration with 1,3 diphenylisobenzofuran as monitor of singlet oxygen. Under these conditions the hydrophobic substances are solubilized by the micellar phase. For substrates which are water‐soluble, 9,10‐anthracene dipropionic acid disodium salt was used as singlet oxygen monitor. For several substances, the values obtained are comparable to the values found in homogeneous nonaqueous solutions. In cases where significant differences have been found these have been rationalized according to the individual case. The only major unexpected result concerned β‐carotene which, in micellar dispersion, failed to react at all with O2(1Δ9) This may be due to multi‐molecular aggregations occurring in the polar medium. The work described herein shows clearly that, under appropriate conditions, singlet oxygen kinetics can be effectively followed in aqueous solutions by time resolved methods. The indiscriminate use of β‐carotene as a quencher of O2(1Δ9)in mainly aqueous media is questioned.

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The kinetics of singlet oxygen in aqueous (D20 and H20) 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 J02* 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 (CTAB), and sodium laurate) were longer than any values previously reported in D20: 53 ± 5 ps. This value is proposed as a minimum value of the singlet oxygen lifetime in D20. Lifetimes measured in nonionic surfactant solutions (Brij 35, Igepal CO 630, and Igepal CO 660) were considerably shorter: 21-26 ps. This effect is probably due to the loss of electronic excitation of x02* ^Ag) 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 aqueous 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 X 108 L mol-1 s"1 for the cationic surfactant CTAB and the three nonionic surfactants. However, kT was found to be approximately 60% higher in the two anionic surfactants SDS and sodium laurate. The lifetime of singlet oxygen in surfactant-H20 solution was estimated by extrapolation from H20-D20 mixtures. The values obtained for two surfactants were 4.0 (CTAB) and 3.5 ps (Igepal CO 630).

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