pH-Regulated Molecular Self-Assemblies in a Cationic−Anionic Surfactant System: From a “1−2” Surfactant Pair to a “1−1” Surfactant Pair

With the aid of pH variation, direct transformation of a "1-1" cationic-anionic surfactant pair to a "1-2" cationic-anionic surfactant pair was attained in the system of cetyltrimethylammonium bromide and n-decylphosphoric acid. Owing to the transformation of a "1-1" pair to a "1-2" pair, diverse microstructures and peculiar phase behavior in this cationic-anionic surfactant mixture was obtained at different pH. It is proposed that pH can be manipulated for effectively tailoring the self-assembled organization in this cationic-anionic surfactant system, including spherical micelle, wormlike micelle, vesicle, and lamellar structure. In contrast to the conventional "1-1" surfactant pair, the "1-2" cationic-anionic surfactant pair exhibits unexpectedly weak aggregating ability. It is suggested that the hydrated volume of surfactant headgroup should be taken into consideration to better elucidate the self-assembly behavior of these "1-2" cationic-anionic surfactant mixtures.