Unexpected Monolayer-to-Bilayer Transition of Arylazopyrazol Surfactants Facilitates Superior Photo-Control of Fluid Interfaces and Colloids
Interfaces that can change their chemistry on demand have huge potential for applications and are prerequisite for responsive or adaptive materials. We report on the performance of a newly designed n-butyl-arylazopyrazole butyl sulfonate (butyl-AAP-C4S) surfactant that can change its structure at the air-water interface by E/Z photo-isomerization in an unprecedented way. Large and reversible changes in surface tension (∆γ=27mN/m) and surface excess (∆Γ>2.9µmol/m²) demonstrate superior performance of the butyl-AAP-C4S amphiphile to that of existing ionic surfactants. Neutron reflectometry and sum-frequency generation spectroscopy reveal that these large changes are caused by an unexpected monolayer-to-bilayer transition. This exceptional behavior is further shown to have also dramatic consequences at larger length scales as highlighted by two application like the light-triggered collapse of aqueous foam which is tuned from high (>1h) to low (<10min) stabilities and light-actuated particle motion via Marangoni flows.