Length shortening and surfactant mixing behavior of nonionic/ionic mixed cylindrical micelle

Cylindrical micelles, which are surfactant self-assembled structures with nm scale, usually grow in length as surfactant concentration increases. Small angle neutron scattering of nonionic/ionic (C12E5/DTAB) mixed cylindrical micellar solution showed the shape of aggregates maintained the cylindrical geometry while the micellar length shortened as the fraction of ionic surfactant increased. Unexpectedly, we observed, for the first time, the micellar length shortened as total surfactant concentration increased at constant DTAB mole fraction. This observation suggests that strong non-ideal mixing of the surfactants in the cylindrical micelles, leading to an end-cap energy lowering with increasing concentration, is responsible for the length shortening.