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

The changes in size and shape of the micelles formed by 0.28 M sodium dodec yl sulphate in aqueous solution are studied as a function of added sodium c hloride concentration at 303 K, by using positron annihilation lifetime spe ctroscopy (LS). As in a previous work, the trapping of ortho-positronium (o -Ps) from the aqueous to the organic micellar subphase results in the appea rance of two o-Ps states with distinct lifetimes. The LS spectra are analys ed on the basis of a time-dependent trapping rate coefficient, which involv es two parameters characteristic of the micellar system: the mean core radi us, R-core, and the mean aggregation number, N-ag. On selecting a set of N- ag. On values at various NaCl concentrations from two previous independent experimental works, the LS data provide R-core values. A good agreement is observed between the latter values and those expected either from the theor etical geometrical considerations or from the previous experimental data on the mean micelle diffusion coefficient. The agreement is obtained only bas ed on the assumption that Ps exists in the solution in a bubble state and n ot as a quasi-free particle. (C) 2000 Elsevier Science B.V. All rights rese rved.