Micellar growth in the presence of quaternary ammonium salts A SANS study

Abstract

The effect of the addition of quaternary ammonium bromides, R₄NBr (R=H, CH₃ or C₄H₉) on the growth of sodium dodecyl sulfate (SDS) micelles has been studied by small-angle neutron scattering (SANS) measurements at 30°C. SDS forms ellipsoidal micelles at 0.3 M concentration with an average aggregation number n_s=110. The effect of adding 0.1 M R₄NBr to 0.3 M SDS has been studied and compared with the effect of adding NaBr. The values of n_s are in the order: no salt<NaBr≈(CH₃)₄NBr<NH₄Br<(n-C₄H₉)₄NBr which shows that the effectiveness of the cation (counterion) in increasing n_s is in the order of decreasing hydrated radii. The unusual behaviour of (n-C₄H₉)₄N⁺ is discussed in terms of its possible intercalation in the micelle. At higher [R₄NBr], micellar growth is substantial in the case of (n-C₄H₉)₄NBr which is explained in the light of an increase in the Mitchell–Ninham parameter of the surfactant monomer.