Zwitterionic heterogemini surfactants containing ammonium and carboxylate headgroups 2: aggregation behavior studied...

The aggregation behavior in aqueous solution of zwitterionic heterogemini surfactants, N,N-dimethyl-N-[2-(N'-alkyl-N'-β-carboxypropanoylamino)ethyl]-1-alkylammonium bromides (2C(n)AmCa, in which n represents hydrocarbon chain lengths of 8, 10, 12, and 14), with nonidentical headgroups containing ammonium and carboxylate was investigated through small-angle neutron scattering (SANS), dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM) techniques. We found that the aggregation behavior of 2C(n)AmCa strongly depended on the hydrocarbon chain length and the surfactant concentration. 2C(8)AmCa forms spherical micelles with radius of approximately 2 nm in solution. 2C(10)AmCa forms rod-like micelles at low concentration in the solution, and the structure changes to vesicles with increasing concentration. The membrane thickness of the vesicle is independent of the surfactant concentration. Aggregation shape transitions were also observed for 2C(12)AmCa. In addition, it appears that the vesicles of 2C(12)AmCa coexist with the rod-like micelles at a wide range of surfactant concentrations. Interestingly, 2C(14)AmCa forms vesicles with averaged membrane thickness of 2.32 nm in solution, even at the extremely low concentration of 10 times the cmc. Thus, it was concluded that 2C(n)AmCa exhibits unique aggregation behavior, such as the formation of spherical micelle→rod-like micelle→rod-like micelle+vesicle (coexistence)→vesicle with increasing hydrocarbon chain length and surfactant concentration. We also found that the membrane structure is an interdigitated bilayer when the vesicles are formed. This formation of interdigitated structures is related to the origins of the effective properties of 2C(n)AmCa in solution.