β-Cyclodextrin induced hierarchical self-assembly of a cationic surfactant bearing an adamantane end group in aqueous solution.

A cationic surfactant with adamantane as the end group, 1-[11-((adamantane-1-carbonyl)oxy)-undecyl]pyridinium bromide (AP), has been synthesized. Its β-cyclodextrin (β-CD) induced hierarchical self-assembling behaviors in aqueous solution were investigated using transmission or scanning electron microscopy methods and small-angle X-ray scattering measurements. Like conventional single chain surfactants, micelles could be formed by AP itself in dilute solutions. However, the dramatic phase transitions of these micelles occurred when host-guest inclusions between AP and β-CD were sequentially produced at different host/guest molar ratios (R), corresponding to the supramolecules with different chemical structures. The AP micelles could be changed into spherical unilamellar vesicles by adding β-CD to reach an R value of 1 : 1. Such vesicles then evolved into multi-wall nanotubes or hydrogels when the β-CD amount was further increased to obtain an R value of 2 : 1. The unique structural characteristics of these supramolecular aggregates come from their "monolayer-like" walls, which have rarely been reported in the past for CD/surfactant inclusion complexes. The interesting results obtained here not only enrich the β-CD/surfactant aggregation systems, but also provide a novel and facile strategy to tune the morphology and structure of aggregates.