Phase Behavior and Aggregation in a Catanionic System Dominated by an Anionic Surfactant Containing a Large Rigid Group

The phase behavior and aggregates of a catanionic system have been investigated. The anionic surfactants in the mixed systems were sodium N-alkylmaleimidepimaric carboxylate (C-n-MPA-Na, n=12, 14, 16), which were prepared from rosin and contain a large rigid skeleton and a flexible alkane chain, and the cationic surfactant was cetyltrimethylammonium bromide (CTAB). The phase behavior of the C-14-MPA-Na/CTAB system transformed sequentially from a viscoelastic solution to an aqueous surfactant two-phase system (ASTP), an aqueous surfactant three-phase system (AS3P), and an anisotropic homogeneous phase as the concentration of C-14-MPA-Na was continuously increased from 10 to 35 mM. The C-16-MPA-Na/CTAB system showed similar phase behavior, whereas the C-12-MPA-Na/CTAB system did not form the AS3P system. The corresponding microstructures in the different phases were investigated by using rheology and cryogenic transmission electron microscopy (Cryo-TEM). The aggregates in the viscoelastic solutions are thread-like, annular, and worm-like micelles. The microstructures in the upper phase of the ASTP are worm-like micelles, and in the lower phase are spherical and rod-like micelles. The aggregates in the upper and lower phases of the AS3P are worm-like micelles and spherical and rod-like micelles, respectively. The aggregates in the middle phase of the AS3P and the anisotropic homogeneous phase are sponge-like micelles. The clear Cryo-TEM images of the sponge-like micelles are presented.