41: Shaking the surface: How sonication rewrites PFAS interfacial structure

Abstract: Understanding how molecules organize at interfaces is essential for explaining processes that control chemical transport, environmental fate, and interfacial reactions. This is particularly important for per and polyfluoroalkyl substances (PFAS), a class of persistent surfactants that readily accumulate at air-water interfaces and influence interfacial chemistry in natural and engineered systems. The molecular structure adopted by PFAS at these interfaces can strongly affect processes such as adsorption, transport, and interactions with surrounding water molecules. In this work, we investigate the interfacial behavior of perfluorotetradecanoic acid using vibrational sum frequency generation spectroscopy, a surface-sensitive technique that probes molecular structure specifically at interfaces. We compare two interfacial architectures: a Langmuir monolayer prepared by spreading molecules directly at the interface and an interfacial film that forms from a sonicated bulk solution. These preparation routes represent two distinct pathways through which PFAS molecules can reach and organize at an interface. The vibrational spectra of the carboxylic acid headgroup reveal clear differences between these systems, indicating distinct interfacial environments and molecular ordering. These results demonstrate that the pathway of film formation can significantly alter the interfacial structure of perfluoroalkyl carboxylic acids. More broadly, this work highlights how differences in film preparation influence spectroscopic interpretation of PFAS interfaces and provide new insight into the molecular behavior of environmentally relevant fluorinated surfactants.