pH Drives the Ice Nucleation Mechanism of Perfluorooctanoic Acid

Perfluorooctanoic acid (PFOA) is a pervasive global pollutant, persisting in the atmosphere without natural degradation pathways and posing significant ecological and health risks. Using two surface-specific techniques, sum frequency generation (SFG) spectroscopy and surface excess coverage, we investigated changes in monolayers of PFOA and the non-halogenated analogue, octanoic acid (OA), at room and near-freezing temperatures. SFG spectra showed that OA monolayers were influenced only by bulk concentration, remaining less impacted by the temperature. PFOA monolayers are acidic at lower temperatures, regardless of the concentration. The temperature did not significantly impact the surface excess concentrations; however, changing the sample pH greatly influenced surface propensity, with acidic PFOA samples having twice the surface concentration as the deprotonated analogue, perfluorooctanoate (PFO^–), samples at any temperature. These results reveal that pH plays a significant role in low-temperature studies and gives potential insight into the ice nucleation efficiency of PFOA.