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

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 degrees 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 approximate to (CH3)(4)NBr < NH4Br < (n-C4H9)(4)NBr 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)(4)N+ is discussed in terms of its possible intercalation in the micelle. At higher [R4NBr], micellar growth is substantial in the case of (n-C4H9)(4)NBr which is explained in the light of an increase in the Mitchell-Ninham parameter of the surfactant monomer.