Estimation of the Acid Dissociation Constant of Perfluoroalkyl Carboxylic Acids through an Experimental Investigation of their Water-to-Air Transport

The acid dissociation constants (pK_as) of perfluoroalkyl carboxylic acids (PFCAs) have been the subject of discussion in the literature; for example, values from −0.2 to 3.8 have been suggested for perfluorooctanoic acid (PFOA). The dissociated anionic conjugate bases of PFCAs have negligible air–water partition coefficients (K_AWs) and do not volatilize from water. The neutral acids, however, have relatively high K_AWs and volatilization from water has been demonstrated. The extent of volatilization of PFCAs in the environment will depend on the water pH and their pK_a. Knowledge of the pK_as of PFCAs is therefore vital for understanding their environmental transport and fate. We investigated the water-to-air transfer of PFCAs in a novel experimental setup. We used ∼1 μg L^–1 of PFCAs in water (above environmental background concentrations but below the concentration at which self-association occurs) at different water pH (pH 0.3 to pH 6.9) and sampled the PFCAs volatilized from water during a 2-day experiment. Our results suggest that the pK_as of C_4–11 PFCAs are <1.6. For PFOA, we derived a pK_a of 0.5 from fitting the experimental measurements with a volatilization model. Perfluoroalkane sulfonic acids were not volatilized, suggesting that their pK_as are below the investigated pH range (pK_a <0.3).