Small-angle neutron scattering study of sodium alkyl sulfate mixed micelles

Sodium (octyl + decyl) and (octyl + dodecyl) sulfate mixed surfactant solutions containing the two solutes in a 1:1 proportion were studied by small-angle neutron scattering. The measurements were carried out at different concentrations and temperatures. For mathematical reasons it was not possible to determine the composition of mixed micelles through a least-squares fitting procedure, and therefore Clint's theory assuming ideal mixing of the components was used to calculate the monomer concentrations and the composition of micelles. With the calculated composition, two-shell spherical and ellipsoidal shape models were fitted to the experimental scattering curves. Except for the (octyl + dodecyl) sulfate micelles at higher concentrations, the best fit was obtained by a spherical model. The aggregation number in the mixed systems obtained from the fit lies between the aggregation numbers of the two single-component constituent systems, but closer to that of the longer chain and shows strong concentration and temperature dependence. The fractional charge of the mixed micelles was found to be smaller than the fractional charge of the single-component ones.