Photoswitchable\nArylazopyrazole Surfactants at the\nWater–Air Interface: A Microscopic Perspective

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