Solute Exchange in synperonic surfactant Micelles

International audienceWe describe experiments that examine the rates of solute exchange in micelles formed from the surfactants Synperonic A7 and Synperonic A50. These surfactants are linear alkane (mixed C13 and C15) ethoxylates with 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 and monomer fluorescence. Excimer emission originates from micelles containing at least two pyrene chromophores. In A50, 1 forms aggregates that do not exchange on a time scale of weeks. When solutions of 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, its exchange involves either fusion of two micelles to form a short-lived supermicelle, or fragmentation of a micelle into submicelles, which then grow back to form normal micelles. The corresponding rate constants are k2 = 8.7 × 104 M-1s-1 for the second-order process and k1 = 0.85 s-1 for the first-order process. These rates are more than an order of magnitude slower than the rates of micelle fusion and fragmentation for Triton-X100 micelles (Rharbi et al. J. Am. Chem. Soc. 2000, 122, 6242) determined by the same technique. We discuss possible sources for these differences