A bistable catanionic system with reinforced thermal stability.

We investigate the structural and thermal behavior of "true" catanionic nanodiscs formed from cetyltrimethylammonium hydroxide and stearic acid in aqueous solution. In this pseudo-ternary system, the molar ratio between the cationic surfactant and the fatty acid governs both the effective surface charge and the internal organization of the aggregates. Dispersions prepared at 1 wt% over a molar ratio range 0.25 ≤ r ≤ 0.75 were characterized by small-angle neutron scattering, static and dynamic light scattering, and differential scanning calorimetry. We demonstrate that nanodisc size and stability are governed by molecular edge-face segregation between the two surfactants, which determines the balance between bilayer packing and rim curvature energy. In the crystalline state, strong compositional asymmetry and chain interdigitation stabilize flat nanodiscs. Upon heating, melting suppresses interdigitation and reduces intraparticle segregation, driving a reversible transition between discs and vesicles. This work establishes, for the first time in a true salt-free catanionic system, a direct link between interdigitation, compositional segregation, and reversible structural bistability. The results provide a thermodynamic framework connecting stoichiometry, molecular packing, and aggregate morphology, enabling predictive design of stable catanionic nanodiscs.