Strong Synergistic Molecular Interaction in Catanionic Surfactant Mixtures: Unravelling the Role of the Benzene Ring

Noncovalent interactions play a crucial role in driving the formation of diverse self-assembled structures in surfactant systems. Surfactants containing a benzene ring structure are an important subset of surfactants. These surfactants exhibit unique colloid and interfacial properties, which give rise to fascinating transformations in the aggregate structures. These transformations are directly influenced by specific noncovalent interactions facilitated by the benzene ring structure including cation-π and π-π interactions. Investigating catanionic surfactant systems that incorporate benzene ring structures provides valuable insights into the distinct noncovalent interactions observed in mixed surfactant systems. Our approach involved studying the enthalpy change ΔH during the titration process, utilizing isothermal titration calorimetry (ITC). Simultaneously, we employed cryogenic transmission electron microscopy (cryo-TEM) to observe the corresponding self-assembly structures. To gain further insight, we delved into the noncovalent interactions of the mixed systems by analyzing the molecular environments variations through chemical shifts of the aggregates using proton magnetic resonance (¹H NMR). The intermolecular interaction was also confirmed by the two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOESY). We conducted a systematic study of the effects of NaCl concentrations, molar ratios, and molecular structures of surfactants on aggregate structures. The existence forms of surfactants are closely linked to the shape of the titration curve and the transition of the aggregate structures. When cationic surfactants were titrated into sodium dodecylbenzenesulfonate (SDBS) micelle solutions, the dominant cation-π interaction leads to the direct formation of vesicle structures. Conversely, when the SDBS system is titrated into benzyldimethyldodecylammonium chloride (DDBAC) micelles, a delicate balance of multiple noncovalent interactions, including cation-π, π-π, hydrophobic, and electrostatic forces, results in a range of aggregate structure transformations such as worm-like micelles and vesicular structures.