Exact Stoichiometry Far-Off Neutrality Catanionic Nanotubes: Unconventional Self-Assembly of Oppositely Charged Surfactants

Electrostatic energy plays a pivotal role in the minimization of the free energy of self-assembling oppositely charged ions. This is a fundamental principle that guides the formation of aggregates and the separation of phases characterized by a stoichiometric balance between the opposite charges. However, this study unveils the self-assembly of catanionic nanotubes from a mixture of oppositely charged surfactants in aqueous buffer, comprising an anionic bile salt derivative and the cationic cetyltrimethylammonium bromide (CTAB) at an exact strongly unbalanced charge ratio of 9 anions to 1 cation. This anomalous aggregation leads to the formation of highly stable and ordered supramolecular nanotubes with a uniform cross-section of about 20 nm. The key to this behavior lies in the unique, rigid, and facial amphiphilic structure of the bile salt derivative, allocating hydrogen bond donors/acceptors and an aromatic residue, which promotes geometrically constrained derivative-derivative interactions. This facilitates the assembly of the anionic derivative into parallel, helically wrapped, negatively charged ribbons with interspersed CTAB cations, acting to promote their adhesion. Supramolecular nanotubes are highly valued in nanotechnology for applications such as catalysis and tissue engineering due to their rigidity, intrinsic directionality, and capability for dual compartmentalization (inner cavity and outer surface). The catanionic structures reported here are ordered architectures that allow for a high charge and a minimal interference from free monomers, which are essential for optimizing functions such as material loading. A loading ability was demonstrated here by the nanotube interaction with positively charged carbon dots and gold nanorods.