Clusters in Polymer−Surfactant AOT Microemulsions Probed by Excited State Quenching Kinetics

Abstract

Excited-state quenching kinetics were applied to sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) reversed micelles with poly(oxy)ethylene (POE) of molecular weight 35 000 Da where clusters were previously detected [Laia,C. A. T.; Brown ,W.; Almgren, M.; Costa, S. M. B. Langmuir 2000, 16, 465]. The data obtained in AOT reversed micelles, with a fixed value of the molar ratio of water to surfactant (w 0 = [H 2 O]/[AOT]) w 0 = 20, were compared in the absence and presence of the polymer. Two reaction time scales were investigated with different probes: 1,3,6,8-pyrenetetrasulfonic acid (PTSA) in the nanosecond range and Cr(bpy) 3 3+ in the microsecond range, using, respectively, time-resolved fluorescence with picosecond resolution and laser flash photolysis techniques. The experimental decays were analyzed assuming two components, one due to polymer-free micelles described by a Poisson distribution model kinetics and another associated with the polymer-induced micelle clusters, analyzed according to a model based on the concept of random walks in regular compact lattices. A step frequency, k w = 10 7 s - 1 , was extracted from the nanosecond results for the quencher's random walk within the cluster. By contrast, the triplet absorption quenching studies did not distinguish between free micelles and clusters and afforded an aggregation number for the former in the range 260 ′ 40, in good agreement with literature data. Decay simulations confirm the validity of the data treatment used in the nanosecond range, but the short time behavior predicted in the microsecond range was not observed within the equipment time resolution.