Phase Behaviors of Dialkyldimethylammonium Bromide Bilayers

Dialkyldimethylammonium bromide (2C_nBr) surfactants in bilayer exhibit polymorphic phases and rich self-assembly behaviors. However, the effects of alkyl chain length on the phase behavior of the 2C_nBr bilayer are still not completely understood. Herein, we investigate the 2C_nBr bilayers by performing all-atom molecular dynamics simulations, taking into consideration the influence of temperature and initial interdigitated degree. Our findings indicate that DOAB (2C₈Br) is challenging to remain in a bilayer structure, while DDAB (2C₁₂Br) and DPAB (2C₁₆Br) maintain bilayers in different phases. DDAB bilayers exhibit an interdigitated gel phase, and this phase structure enhances the stability and rigidity of the bilayer membrane. In contrast, DPAB bilayers show a ripple gel phase, which has better softness and ductility. The differences in phase behaviors can be attributed to a competition between the rigidity of the 2C_nBr surfactants and the hydrophobic interaction of the alkyl tails. The results reveal the crucial role of the bilayer phase in determining the rigidity of bilayer membranes.