The ubiquity of perfluoroalkyl carboxylic acids (PFCAs) in both atmospheric and environmental water is well-documented. Nonetheless, huge discrepancies exist in the reported pKa values for PFCAs, often spanning three to four units. These disparities stem from an incomplete understanding of how pH influences the ionized state of PFCA molecules in bulk solution and at the air-water interface. Using pH titration and surface tension measurements, we show that the pKa of the PFCA adsorbed at the air-water interface differs from the bulk. We systematically explore how the length of the fluoroalkyl tail of PFCAs influences their surface-pKa and interfacial activity across a broad pH range. Our study reveals a crucial consequence of PFCA accumulation at the air-water interface: a reduction in the evaporation rate of microdroplets, which would further impact the stability of atmospheric aqueous aerosols containing PFCAs. This study underscores the potential significance of pH in directing the dynamics of atmospheric aerosol droplets containing PFCAs and prompts the inclusion of pH as a key determinant in the predictions of their fate.
