Multiple modulations for supramolecular hydrogels of bola-form surfactants bearing rigid and flexible groups

A series of bola-form surfactants with two identical azobenzene ends separated by a flexible chain but different cationic heads were synthesized. These amphiphilic molecules exhibited rich self-assembly properties in aqueous solutions. The physical characterizations demonstrated that the cationic heads showed a decisive influence on both the gelation behavior and the gel strength. The surfactant with a trimethylammonium head group did not form a hydrogel even at a higher concentration, yet it promoted the gelation of pyridinium and methylimidazolium head bearing surfactants, which formed hydrogels but could not boost each other's gelation capability. Besides the heating induced reversible gel-sol transition, the UV light irradiation also triggered the change of gel to sol while the irradiation with visible light led to a reverse process. Through the addition of cyclodextrins, the hydrogels transformed into sols, while the sols reverted to gels accompanied by the addition of competitive guest molecules. The surfactant hydrogels displayed a stimulus-response to both anionic and cationic additives based on different mechanisms, and reverse order of addition of the external guest molecules and host cyclodextrins also resulted in the modulation of the gel-sol transition. By the combination of photo- and chemical-stimuli, multiple responses on the transformations between the gel and sol were realized.