| Abstract
| - We describe experiments that examine the rates of solute exchange in micelles formed from the surfactantsSynperonic A7 and Synperonic A50. These surfactants are linear alkane (mixed C13 and C15) ethoxylateswith mean degrees of ethoxylation of 6.5 for A7 and 53.7 for A50. The solute is gycerol-1,2-distearate-3-pyrenebutyrate, 1. Solutions containing on average 0.4 molecules of 1 per micelle show both excimer andmonomer fluorescence. Excimer emission originates from micelles containing at least two pyrenechromophores. In A50, 1 forms aggregates that do not exchange on a time scale of weeks. When solutionsof A7 are treated with an excess of empty micelles, the excimer fluorescence decays with a pseudo-first-order rate that depends on the concentration of empty micelles. Because 1 is so insoluble in water, itsexchange involves either fusion of two micelles to form a short-lived supermicelle, or fragmentation of amicelle into submicelles, which then grow back to form normal micelles. The corresponding rate constantsare k2 = 8.7 × 104 M-1s-1 for the second-order process and k1 = 0.85 s-1 for the first-order process. Theserates are more than an order of magnitude slower than the rates of micelle fusion and fragmentation forTriton-X100 micelles (Rharbi et al. J. Am. Chem. Soc. 2000, 122, 6242) determined by the same technique.We discuss possible sources for these differences.
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