Photoswitchable arylazopyrazoles surfactants at the water-air interface: a microscopic perspective

Surfactants play an important role in modifying the properties of water-air interfaces. However, information on the microscopic behavior of the mechanisms is scarce. Here, we combine information from molecular dynamics simulations, surface tensiometry, and vibrational sum frequency generation spectroscopy to study the behavior of arylazopyrazole (AAP) as a surfactant. This combination of the experimental techniques allows a direct relation between surface tension and surface excess rather than just bulk concentration. Specifically, we compare the case with (O-AAP) and without (H-AAP) the addition of an octyl carbon chain, as well as for two different isomers (E and Z). From the simulations of these four systems, we see that a stronger cluster formation is accompanied by a weaker reduction of the surface tension for intermediate surface excesses. In some cases, even a small but noticeable maximum in the surface tension isotherm is observed. This correlation is expected from the general properties of the Frumkin isotherm. Both the addition of the carbon chain and the consideration of the E isomer compared to the Z isomer increase this effect. Although the experimental data of the surface tension vs. surface excess do not show a maximum, the general features of the surface tension isotherm are consistent between experiment and simulation. Evidence is also provided that it is primarily the interaction of the aromatic benzene rings that determines the strength of the surfactant interactions.