pH and light dual stimuli-responsive wormlike micelles with a novel Gemini surfactant

Stimuli-responsive surfactant systems have attracted much interest recently. In this work, we studied a novel pH and light dual-responsive Gemini surfactant N3C14AZOC14N3, in which two single-tailed amino surfactants were covalently connected by an azobenzene moiety. This surfactant showed typical adsorption and micellization behaviors of Gemini surfactants, the high surface activity and strong micellization tendency with a low critical micelle concentration (cmc). The self-assembly behaviors of N3C14AZOC14N3 in aqueous solutions were highly depended on pH, but concentration of surfactant also affected its physicochemical properties apparently such as the viscosity of the sample. For example, wormlike micelles were the preferred morphologies in the neutral conditions, and the increased concentration benefited the enhancement of viscosity dramatically. Moreover, wormlike micelles formed by this Gemini surfactant alone showed pH and light dual-responses, which were transformed into rodlike micelles through decreasing pH or irradiating by UV-light as confirmed by cryo-TEM and rheology measurements. pH titration and spectra techniques including both UV-Vis spectra and nuclear magnetic resonance (NMR) were employed to study the morphological transition mechanism caused by pH and light, respectively. The effect of pH on the morphological transition of micelles was mainly caused by the protonation variation of amino headgroups, whereas the light-induced reversible transition was attributed to the isomerization between trans- and cis-azobenzene instead. [Display omitted]