It is not a rule that the first aggregation state of an ionic surfactant consists of spherical micelles. The case of tetradecyltrimethylammonium salicylate

Ionic surfactants can assemble into wormlike micelles (WLM) at high concentrations, forming supramolecular structures that exhibit similarities to polymeric solutions. Although the rheology of these supramolecular aggregates is well understood, experimental thermodynamic studies at low concentrations are still in their early stages. In this study, we employed tetradecyltrimethylammonium salicylate (TTASal) to investigate the driving forces behind WLM formation for the first time, using isothermal titration calorimetry, electrical conductivity measurements, and cryogenic transmission electron microscopy. Our findings indicate that TTASal initially aggregates into WLM rather than spherical micelles, demonstrating that WLM formation is influenced by surfactant ion-counterion interactions rather than concentration alone. Notably, the enthalpy change associated with the aggregation process emerges as a key determinant in dictating the aggregation of free monomers into spherical or WLM.