Spontaneous Aggregate Transition in Mixtures of a Cationic Gemini Surfactant with a Double-Chain Cationic Surfactant

Controllable aggregate transitions were realized by mixing two kinds of cationic surfactants, hexylene-1,6-bis(dodecyldimethylammonium bromide) (C12C6C12Br2) and didodecyldimethylammonium bromide (DDAB). It was found that two parameters are the main factors determining the aggregation behavior of the mixed system, the total concentration of DDAB and C12C6C12Br2 (C-T), and the mole fraction of DDAB in the mixtures of DDAB and C12C6C12Br2 (X-DDAB). How these two parameters act on the aggregate transitions was studied in detail by various measurements including surface tension, turbidity, electrical conductivity, zeta potential, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, and H-1 NMR. When C-T was constant, spontaneous vesicle-to-micelle transitions were found with decreasing X-DDAB at high C-T. When X-DDAB was constant, aggregate transitions were generated by gradually increasing C-T, depending on different X-DDAB ranges. At X-DDAB < 0.6, small spherical aggregates formed first and then transferred to vesicles, and finally the vesicles transitioned to micelles. At X-DDAB >= 0.6, the progressive increase in C-T led to aggregate transitions on the order of the arising of vesicles, the continuous growth of vesicles, the disruption of vesicles into micelles, and the final coexistence of vesicles and micelles. The hydrophobic interaction and electrostatic repulsion between DDAB and C12C6C12Br2 together with the related degree of ionization and hydration of the surfactants were gradually adjusted by changing the ratio and the total concentration of these two surfactants, which should be responsible for the complicated aggregation behavior.