Exploring the Photoswitching Dynamics of Azobenzene Salts and their Role in Inducing Wormlike Micelle Formation in Cationic Surfactants

Small organic salts, such as the sodium salt of salicylic acid and naphthoic acid, are well-established as classical additives that promote wormlike micelle (WLM) formation in cationic surfactant systems by minimizing electrostatic repulsions and enhancing headgroup interactions. Extending this concept toward responsive soft materials, two azo-containing structural analogues of salicylate- and naphthoate-based additives, AZOS and AZON, respectively, were synthesized. Physical characterization linked molecular design with the nature of hydrogen bonding and π-π interactions and demonstrates how these features influence photoresponsivity and aggregate growth in cationic surfactants such as cetyltrimethylammonium bromide (CTAB). UV-vis analysis shows transient photoswitching for AZOS, consistent with a predominantly azo configuration stabilized by intramolecular hydrogen bonding, while AZON remained photoinactive in aqueous media. Aggregation, hydrogen bonding, and tautomerism affected the photoactivity of the synthesized additives. Density functional theory (DFT) calculations indicate that solvent-mediated hydrogen bonding, along with enhanced tendency of π-π stacking, suppressed photoswitching of AZON in water. Rheology and small-angle neutron scattering (SANS) studies of CTAB micelles revealed that AZOS promotes micellar elongation via charge screening and predominantly intramolecular hydrogen bonding, whereas AZON formed denser and more elastic networks through enhanced π-π stacking promoted by intermolecular hydrogen bonding. Hence, these results demonstrate how structural tuning in azobenzene-based salts enables simultaneous control of optical and rheological behavior in surfactant systems.