Size of sodium dodecyl sulphate micelles in aqueous NaCl solutions as studied by positron annihilation lifetime spectroscopy Original Research Article

The changes in size and shape of the micelles formed by 0.28 M sodium dodecyl sulphate in aqueous solution are studied as a function of added sodium chloride concentration at 303 K, by using positron annihilation lifetime spectroscopy (LS). As in a previous work, the trapping of ortho-positronium (o-Ps) from the aqueous to the organic micellar subphase results in the appearance of two o-Ps states with distinct lifetimes. The LS spectra are analysed on the basis of a time-dependent trapping rate coefficient, which involves two parameters characteristic of the micellar system: the mean core radius, Rcore, and the mean aggregation number, Nag. On selecting a set of Nag values at various NaCl concentrations from two previous independent experimental works, the LS data provide Rcore values. A good agreement is observed between the latter values and those expected either from the theoretical geometrical considerations or from the previous experimental data on the mean micelle diffusion coefficient. The agreement is obtained only based on the assumption that Ps exists in the solution in a bubble state and not as a quasi-free particle.