A new method to determine micellar aggregation numbers: positron annihilation lifetime spectroscopy

Abstract

A new method was developed to measure micellar aggregation numbers (N_ag) by using positron annihilation lifetime spectroscopy (LS), in a similar way as is done in fluorescence quenching experiments. Triplet positronium (o-Ps) is taken as a surrogate to the fluorescent probe, while the quencher is duroquinone, in 0.28 M sodium dodecyl sulfate (SDS) aqueous solution. On the basis of a Poisson distribution of the quencher molecules over the micelles, it is expected that the intensity of those o-Ps atoms decaying in micelles where no quencher molecules are present should decrease exponentially as a function of the quencher concentration and aggregation number, whereas their lifetime, τ₄⁰, should remain constant. The high oxidation power of the quencher towards Ps is first established in pure dodecane, yielding a reaction rate constant k′=16.2 l mol⁻¹ ns⁻¹. Quantitatively, however, this value would correspond to a solute radius about twice too large, whether the time dependence of the encounter rate coefficient is considered or not. Contrary to expectation, the lifetime of o-Ps trapped in the SDS micelles appears to decrease with increasing duroquinone concentration. This decrease indicates a much smaller overall oxidation rate constant than in dodecane, owing to strong localization of duroquinone near the interface layer. Nevertheless, taking an integral of the LS decay spectra from a time long enough to eliminate the components of lifetimes shorter than τ₄⁰ leads to N_ag=65±3, in good agreement with previous determinations.