Kinetics of Fusion and Fragmentation Nonionic Micelles: Triton X-100

We describe the kinetics of exchange of a pyrene-labeled triglyceride 1 between micelles of Triton X-100, an octylphenol ethoxylate with a mean degree of ethoxylation of 9.5. These micelles have an aggregation number of ca. 110 surfactants/micelle and are capable of solubilizing more than four molecules of 1 per micelle while maintaining Poisson solubilization statistics. When excess surfactant is added to a solution of TX100 micelles containing sufficient 1 that the fluorescence spectrum is dominated by excimer emission, the monomer emission intensity grows in at the expense of excimer emission. Since 1 is insoluble in water, exchange requires fusion of two micelles followed by fragmentation. This process follows second-order kinetics, from which we calculate a second-order rate constant k fus = 1.15 × 10 6 M - 1 s - 1 . Micelle fusion is approximately 5000-fold slower than diffusion controlled. We infer that the barrier to micelle fusion is due primarily to entropic and hydration effects of the ethylene oxide stabilizer chains. This barrier is relatively easily overcome to allow rapid micelle fusion-and-fragmentation to take place in dilute solution at room temperature.