43: Forever chemicals and Langmuir films: Irreversible adsorption of perfluorooctanoic acid to stearic acid monolayers

Abstract: Langmuir trough surface pressure measurements were used to examine how the soluble PFAS contaminant, PFOA (perfluorooctanoic acid), affects the properties of insoluble stearic acid (SA) films adsorbed to the air-water interface. Surface pressure-area (π-A) isotherms were obtained for stearic acid films on subphases containing varying concentrations of PFOA, and each film’s isothermal compressibility was calculated to evaluate the PFOA-SA interactions. Previous studies have shown that PFOA significantly alters the phase behavior of DPPC monolayers, motivating this work examining PFOA influence on simpler, model Langmuir films. The results show that PFOA interacts strongly with stearic acid, shifting the π-A isotherms to larger molecular areas, indicating disruptions in packing, phase behavior and mechanical properties at the air-water interface. PFOA sub-phase concentrations as low as 200 nM significantly alter film properties, suggesting that PFOA inserts between stearic acid molecules, loosening packing and permitting easier deformation of the monolayer. The presence of PFOA leads to two collapse events observed in the mixed monolayer isothermal compressibility instead of a single steep collapse observed for pure SA. Films containing PFOA also exhibit higher pressures at larger molecular areas, indicating a more expanded and fluid-like monolayer. These findings provide molecular-level insight into how perfluorinated contaminants alter intermolecular interactions and interfacial properties of Langmuir monolayers with consequences for how this regulated pollutant affects more complicated systems such as pulmonary surfactant.