Pumping-out photo-surfactants from an air-water interface using light

We study the adsorption dynamics of an azobenzene-based photo-responsive charged surfactant to investigate how photo-stimulation impacts the dynamics at an air-water interface. The hydrophobic tail of this photo-responsive surfactant photo-converts reversibly from a cis to a trans conformation when the wavelength switches from UV to blue. This change in conformation results in a decrease of the surface tension. Using a kinetically limited model of adsorption, including the electrostatics effects and the competition between the two photo-isomers, we reproduce the dynamics of adsorption of AzoTAB measured experimentally. We find that the cis isomer adsorbs 10 times faster than the trans isomer but the cis conformation also desorbs 300 times faster. As a result, within a few seconds a non-stimulated interface becomes composed of almost 100% trans isomers. We then focus on the competition between the photo-conversion and the adsorption at the interface. Indeed when the interface is stimulated, part of the adsorbed trans isomers rapidly convert to cis. As the latter quickly desorbs, the surface coverage decreases: the light induces a "pumping-out" of the interface. The photo-stimulated interface reaches a stationary state where a vertical gradient of composition is established below the surface. Finally, this study highlights a new way to stimulate a photo-responsive interface: for a solution prepared under blue light, instead of photo-converting the bulk composition by stimulating under UV (which can be quite slow for high absorbance solutions), one can tune and reach a stabilized value of the surface tension in few seconds by stimulating the interface with a blue light with high enough intensity.