Interfacial Activity and Surface pKa of Perfluoroalkyl Carboxylic Acids (PFCAs)

Perfluoroalkyl carboxylic acids (PFCAs) are widely used synthetic chemicals that are known for their exceptional stability and interfacial activity. Despite their industrial and environmental significance, 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 the bulk solution and at the air–water interface. Using pH titration and surface tension measurements, we show that the pKa values of the PFCAs adsorbed at the air–water interface differ from the bulk. Below the equivalence point, the undissociated and dissociated forms of the PFCAs exist in equilibrium, driving to the spontaneous adsorption and reduced air–water surface tension. Conversely, above the equivalence point, the complete ionization of the headgroup into the carboxylate form renders PFCAs highly hydrophilic, resulting in reduced interfacial activity of the molecules. The distinction in the chemical environments at the interface and bulk results in differences in the pKa of PFCA molecules in the bulk phase and at the air–water interface. We explore the effects of the fluoroalkyl tail length of PFCAs on their surface pKa and interfacial activity across a broad pH range. We further demonstrate the influence of pH-dependent ionized state of PFCAs on their foamability and the rate of microdroplet evaporation, understanding of which is crucial for optimizing their industrial applications and developing effective strategies for their environmental remediation. This study underscores the potential significance of pH in directing the interfacial activity of PFCAs and prompts the inclusion of pH as a key determinant in the predictions of their fate and potential risks in the environment.

In a recent study, Allen and co-workers presented a model relating the surface tension and surface-pKa for medium chained fatty acids 4 .This is modification on a model originally proposed by Cratin for the adsorption of fatty acid on oil-water interface 5 .Here we present key equations and assumptions of the model, which allows us to assess the surface-pKa of the PFCA molecules.At a pH below the equivalence point (at the air-water interface), following equilibrium exists: where HA refers to the PFCA molecule.The corresponding equilibrium constant is given as and p " = − log  " .The mole fraction of HA molecules is given as: Combining equations ( 1) and (2) yields Here pKa(s) is surface-pKa of the PFCA molecules at the air-water interface.If the total activity (a) is assumed to be the sum of dissociated and undissociated PFCAs, its value is given by Note that equation (4) holds true under the assumption that the PFCA species present at the interface coexist independently.
In our case, we assume the reference state of PFCA as its least interfacial active state i.e. max surface tension represented as max(γ).The change in surface tension of a surfactant is given by Δγ From evaluation of plot of Δγ as a function of pH (Supplementary Note Fig. 2.), we can infer that for these systems,  50 = 0 in the low pH regime and  0 % = 1 in the high pH regime, just as reported by Allen and coworkers 4 .
Under such assumption 4,6,7 , the activity is given as Here Δγ 8'9 = Δγ i.e. maximum in Δγ with respect to the least interfacial active state which in the case of PFCA is at the pH at equivalence point (pHeq).This assumption sets bounds on the numerical value of .At low pH, molecules exhibit minimum surface activity, thus Δγ~ Δγ 8'9 , rendering  ~ 0. In the high pH regime, molecules are highly surface active, thus Δγ~ 0, rendering  ~ 1.Using equations 4 and 6, we can write Assuming at sufficiently low pH all PFCAs exist in their undissociated form HA i.e.  50 = 1 and Δγ 8'9 = Δγ, and at neutralization point Δγ ~ 0, and  50 = 0.Under these two assumptions,  50 = 0 and  0 % = 1 which can be used to rewrite equation 7 as which simplifies to Using equation ( 3) and (9) we can get to the final relation, Δγ Δγ 8'9 = 1 1 + 10 (65%63"( 7)) (10) This relation assumes that lowering of surface tension with decreasing pH is the result of protonation of the PFCA (i.e.formation of HA), which has significantly higher interfacial activity (and lower surface tension) than the reference state (A -) which is true for PFCAs.For estimating surface-pKa, we use equation ( 10) to fit our experimental data (Δγ/ Δγ 8'9 vs pH) with pKa(s) as the only free-fit parameter.

Fig. S2 .
Fig. S2.Binary diagram representing the change in solubility of PFOA in water across a wide pH range.The circles represent the insoluble state of the PFOA in water and the squares represent the soluble state.

Fig. S3 .
Fig. S3.The effect of the change in pH on the contact angle of a droplet containing 6 mM PFOA onto a polyethylene surface.Increase in contact angle upto pHeq indicates high interfacial activity of the undissociated acid headgroups, while reduction in contact angle thereafter, denotes reduction in interfacial activity due to desorption of molecules from the interface into the bulk of the solution.

Fig. S4 .
Fig. S4.Measured contact angle,  of a water droplet on the superhydrophobic substrate used for the droplet drying experiments.The initial droplet volume was 4 µL, and the contact angle was nearly constant throughout the drying process.

Fig. S5 .
Fig. S5.Box plot showing the droplet drying times of DI water, clean rain and acid rain.The error bars represent the standard deviation from replicate experiments.There are considerable differences in the total drying time of the three droplets at the p < 0.1 level.Post-hoc comparisons using the Tukey HSD test reveal that mean drying time for the acid rain is significantly different than clean rain (p = 0.07) and DI water (p < 0.001), and that clean rain is significantly different than DI water (p=0.002).All data sets passed Shapiro-Wilk test for normality.