A Dual pH- and Light-Responsive Spiropyran-Based Surfactant: Investigations on its Switching Behavior and Remote Control over Emulsion Stability

A cationic surfactant containing a spiropyran unit is prepared exhibiting a dual-responsive adjustability of its surface-active characteristics. The switching mechanism of the system relies on the reversible conversion of the non-ionic spiropyran (SP) to a zwitterionic merocyanine (MC) and can be controlled by adjusting the pH value and via light, resulting in a pH-dependent photoactivity: While the compound possesses a pronounced difference in surface activity between both forms under acidic conditions, this behavior is suppressed at a neutral pH level. The underlying switching processes are investigated in detail, and a thermodynamic explanation based on a combination of theoretical and experimental results is provided. This complex stimuli-responsive behavior enables remote-control of colloidal systems. To demonstrate its applicability, the surfactant is utilized for the pH-dependent manipulation of oil-in-water emulsions.