Aggregate morphology transition of an adamantane-containing surfactant via the host-guest interaction with β-cyclodextrin

(11-((Adamantane-1-carbonyl) oxy)-N,N-dimethylundecan-1-benzylammonium bromide (C11AD), a novel cationic surfactant containing an adamantane group and a hydrophobic chain, had been synthesized. Experiments were carried out to understand aggregate morphology transition via the host-guest interaction with β-cyclodextrin. Two groups of inclusion complexes were prepared by mixing C11AD and β-cyclodextrin (β-CD) in aqueous solution with 1:1 or 1:2 stoichiometry, denoted as C11AD/nβ-CD (n = 1 or 2). Results showed that C11AD could self-assemble into spherical-like micelles in aqueous solution, and the micelles of C11AD transformed into spherical vesicles, straight nanotubes or networks hydrogels with β-CD in different stoichiometries. The aggregation behaviors of surfactant C11AD by the host-guest interaction with β-cyclodextrin (β-CD) were investigated by transmission electron microscopy (TEM), Cryogenic Transmission electron microscopy (Cryo-TEM), scanning electron microscopy (SEM) and dynamic light scattering (DLS) measurement. A possible mechanism to explain aggregate transitions was suggested by the results of FT-IR, 1H NMR and 1H-1H 2D ROESY NMR spectra. Data reflected that β-CD first combined with the adamantane group in C11AD/1β-CD system to form spherical vesicles. Then increasing the β-CD amount, superfluous cyclodextrin would combine with the hydrophobic chain in C11AD/2β-CD system. Thus the hydrophobic interaction was destroyed and the hydrogen bonds, the main driving force, were the prime mover in the process of nanotubes and hydrogels formation.