Photo-Responsive Asymmetric Gemini Azobenzene Surfactants: Synthesis, Characterization, and Application in Intelligent Foams

Gemini surfactants with stimuli-responsive properties have been demonstrated for a broad range of applications. This study presents the synthesis of a series of photoresponsive asymmetric gemini azobenzene-surfactants (AZO-3(OH)-<i>n</i>, <i>n</i> = 12, 14, 16) and characterizes their structures and photoresponsive properties. Surface tension analysis reveals that the surface activities of AZO-3(OH)-<i>n</i> increase with the degree of asymmetry and decrease significantly after ultraviolet (UV) irradiation. Thermodynamic parameters suggest that the micellization process of the AZO-3(OH)-<i>n</i> molecules is spontaneous. TEM combined with dynamic light scattering confirms that <i>trans</i>-AZO-3(OH)-<i>n</i> with higher asymmetry can assemble into vesicles in aqueous solutions that transform into micelles after UV irradiation. Foaming performance studies show concentration-dependent improvements in both foamability and foam stability. However, the changes in the molecular structure and surface activity after UV irradiation lead to a rapid decrease in the foam half-life. The resulting intelligent foams with rapid photoresponsiveness and excellent cycling stability offer a promising strategy for expanding their applications in industrial production.