Photoswitchable Arylazopyrazole Surfactants at the Water-Air Interface: A Microscopic Perspective

Surfactants play an important role in modifying the properties of water-air interfaces. Here, we combine information from molecular dynamics simulations, surface tensiometry, and vibrational sum-frequency generation spectroscopy to study the interfacial behavior of photoswitchable arylazopyrazole (AAP) surfactants. This combination of the experimental techniques allows a direct relation between surface tension and surface concentration rather than just the bulk concentration. Specifically, we conducted a comparison between two derivatives, one with an octyl terminal group (O-AAP) and the other without this group (H-AAP), focusing on their respective E and Z isomers. From the simulations of these four systems, we see that those with a stronger cluster formation, likely resulting from higher intermolecular attractive interactions, display higher surface tensions for the intermediate surface excess. In some cases, even a small but noticeable maximum in the surface tension isotherm is observed for systems with strong cluster formation. Such a maximum is not observed in the experiments, although such an observation would be compatible with the general properties of the Frumkin isotherm. We exclude that the peak is due to the finite width of the simulation box. Apart from this effect, the general features of the surface tension are consistent between the experiment and simulation. Evidence is also provided that it is primarily the interaction of the aromatic rings that determines the strength of the surfactant interactions.