Unexpected monolayer-to-bilayer transition of arylazopyrazole surfactants facilitates superior photo-control of fluid interfaces and colloids†

Abstract

E/ Z photo-isomerization of a new surfactant causes substantial changes in interfacial properties, which are a prerequisite for responsive and adaptive material control on a molecular level.

Abstract

Interfaces that can change their chemistry on demand have huge potential for applications and are prerequisites for responsive or adaptive materials. We report on the performance of a newly designed n-butyl-arylazopyrazole butyl sulfonate (butyl-AAP-C ₄S) 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 (Δ γ = 27 mN m ^–1) and surface excess (Δ Γ > 2.9 μmol m ^–2) demonstrate superior performance of the butyl-AAP-C ₄S amphiphile to that of existing ionic surfactants. Neutron reflectometry and vibrational 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 dramatic consequences at larger length scales as highlighted by applications like the light-triggered collapse of aqueous foam which is tuned from high (>1 h) to low (<10 min) stabilities and light-actuated particle motion via Marangoni flows.

Notes

†Electronic supplementary information (ESI) available: Synthesis and further characterization, videos of Marangoni flow. See DOI: 10.1039/c9sc05490a