The effect of functional groups on the sphere-to-wormlike micellar transition in quaternary ammonium surfactant solutions

To investigate the effects of functional groups on the formation of wormlike micelles, a cationic surfactant, N-tetradecyl-N-(2-hydroxyethyl)dimethylammonium bromide (DM), was synthesized in this work. The viscoelastic behaviors of DM aqueous solution mixed with sodium salicylate (NaSal), sodium benzoate (NaBen) and sodium laurate (NaLau), respectively, have been studied with rheological measurements. Cryogenic transmission electron microscopy (cryo-TEM) and 1 H NMR technology have been employed to investigate the morphology and arrangement of wormlike micelles. The results show that the effects of NaSal and NaLau on wormlike micellar growth are significantly higher than that of NaBen. Superficially, such a great difference in the rheology properties arises from the effect of hydroxyl group in NaSal because this is the sole difference between the NaSal and NaBen. By comparing with NaBen in DM solution, the long hydrocarbon chain of NaLau induces greater hydrophobic interactions between Lau − anions and DM surfactants than aromatic phenyl group of NaBen, and hydrophobic interactions play an important role in the growth of micelles.