Spiropyran Sulfonates for Photo and pH Responsive Air-Water Interfaces and Aqueous Foam

Abstract

Responsive foams and interfaces, are interesting for active materials that respond to external stimuli. We have used the photochromic reaction of a spiropyran surfactant to render interfacial, rising bubble as well as foaming properties active to light stimuli. In order to address the air-water interface, we have applied sum-frequency generation (SFG) which has provided qualitative information on the surface excess and the interfacial charging state as a function of light irradiation and pH. Under blue light, the surfactant forms a closed ring spiro form (SP), whereas under dark conditions the ring opens and the merocyanine (MC) form is generated. Using SFG, we show that different pH conditions of the bulk solution lead to changes in the interfacial charging state. We have exploited the fact that the MC surfactant's O-H group can be deprotonated with pH, and used that to tune the molecules net charge at the interface. In fact, SFG shows that with increasing pH the intensity of the OH stretching band from interfacial water molecules increases which we associate to an increase in surface net charge. At a pH of 5.3, irradiation with blue light leads to a reversible decrease of OH intensities, whereas the CH intensities were unchanged compared to the corresponding intensities under dark conditions. These results are indicative of changes in the surface net charge with light, which are also expected to influence the foam stability via changes in the electrostatic disjoining pressure. In fact, measurements of the foam stabilities are consistent with this hypothesis with higher foam stability in the dark. At pH 2.7 this behavior is reversed. This is corroborated by rising bubble experiments, which demonstrated an unprecedented reduction of ~30% in bubble velocity when the bubbles were irradiated with blue light compared to the velocity of bubbles with the surfactants in the dark state.