Combined SANS and SAXS studies on alkali metal dodecyl sulphate micelles

Combined small-angle neutron scattering (SANS) and small-angle x-ray scattering (SAXS) studies have been performed on charged micelles of anionic surfactants of alkali metal dodecyl sulphate (MDS, M = Li, Na, Rb and Cs) in aqueous solution. The dimensions of the micelles have been obtained using SANS as the neutrons are mostly scattered by the hydrogenous core of the micelles. The contrast for the head groups and the counterions is very small, so they are not seen with the neutrons. On the other hand, x-rays are scattered by the electron density fluctuation, which is proportional to the atomic number, and thus x-rays have been used to see the high-Z components of the micelles directly. SAXS from LiDS gives the headgroup thickness, and the counterion condensation is measured directly by the scattering from the high-Z counterions of MDS (M = Na, Rb and Cs) micelles. Based on the results of these two techniques, the role of the different counterions in deciding the structure of micelles has been explained in terms of counterion condensation and the thickness over which they are condensed around the charged micelle. It is found that as the hydrated size of the counterion decreases, the counterion condensation increases and counterions are condensed over a smaller thickness, which results in larger charge neutralization and hence a larger size of the micelles