Correspondence Comment on “Experimental pKa Determination for Perfluorooctanoic Acid (PFOA) and the Potential Impact of pKa Concentration Dependence on Laboratory-Measured Partitioning Phenomena and Envrionmental Modeling” In Burns et al. (1) and more recently in Ellis and Webster (2), Ellis and co-workers present a structural and electronic basis for their measured pKa value of 3.8 for PFOA, as well as more details and supporting arguments regarding a mechanism they postulated for the mediated volatilization of PFOA from the surface of water. Here we would like to address that the theoretical basis they present for these points contains serious shortcomings and inconsistencies, as well as misinterpretations of the cited literature. (1) In ref 1 Ellis and co-workers reinforce their hypothesis of aerosol-mediated water-air transfer, stating: “When PFO is protonated at the surface of the water droplet, the neutral acid (PFOA) will readily partition to the gas phase. In short, PFO in bulk water is transported to the atmosphere via aerosol generation, where it is protonated and then rapidly released to the gas phase as PFOA”. This implies that there is a higher degree of protonation of PFOA occurring at the water surface than in the bulk water. And indeed consistent with this Ellis and Webster claimed more recently (2) that the water surface exhibits a different pH value than bulk water does. However, Jungwirth and co-workers, who are the authors of the papers cited by Ellis and Webster for proving the existence of a pH gradient at the surface, only postulate an elevated hydronium ion concentration, not a difference in hydronium activity and thus pH (3). In a personal communication (4), Jungwirth has confirmed to us that they do not claim a gradient in the hydronium activity to exist between the bulk phase and the surface. Hence, the postulated increased protonation of PFO/ PFOA (or other bases) in the surface layer as compared to the bulk layer and the inferred mechanistic explanation for the water surface-air transfer cannot exist. (Side remark: this postulated elevated hydronium ion concentration is considered a highly controversial hypothesis and has been marked as controversial by the authors of this hypothesis (3)). (2) Burns et al. claim that the pKa of PFOA is 3.8 at low concentrations and not close to 0 as had been postulated before (5, 6). Most of the literature that Ellis and co-workers cite as in favor of their pKa value of 3.8 are actually not in favor and are often misinterpreted. The paper of Moroi et al. (7) is a case in point. In ref 1, Ellis and co-workers, referring to their postulated suppression of pKa due to aggregation, state “This was also consistent with the observations of Moroi et al., who interpolated a pKa of ∼2.5 for colloidal PFOA from measurements conducted on long and short-chain PFCA homologues.” However, Moroi et al. reported that aggregation in fact increases the pKa and does not decrease it as Ellis infers. Further, as Moroi et al. measured the pKa for longchain PFCAs in the aggregated form they conclude regarding the monomeric form that “the acidity constants of the homologues longer than the C3 or C4 acid clearly decrease with increasing carbon number of the acids” (7); thus as they reported pKa values in the range of 0.6-0.4 for short-chain acids, this implies that they foresee the pKa of long-chain 5150
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 13, 2009
PFOA being near zero. This later point is further in direct contrast to Ellis and Webster’s comment to Mader (2), where it is stated that the work by Moroi et al. is in agreement with a pKa for monomeric PFOA being “much higher than zero”. As to some of the other literature cited in favor of a pKa of 3.8 in the same reference: Guo et al. (8) do not present a quantitative pKa but they state that “The agreement between the two sets of data confirms that perfluoroheptanoic acid is a strong acid which essentially ionizes completely...”. Lopez-Fontan et al. (9) derive a pKa of 1.31 from their measurements which is much closer to 0 than to 3.8 and Ylinen et al. (10) do not do any such measurements but simply report a value of 2.5 without giving any source. Thus, these collective references are arguably in a closer agreement to a value near zero than 3.8. (3) As a minor point, there appears to be a possible inconsistency in the reported results in Burns et al. (1) that is not explained, at least according to how the data were presented. The interpolated pKa (cosolvent free) of 5 mM PFOA from the NMR experiments is said to be 2.3 (see text and Figure S5); however, according to Yasuda-Shedlovsky experiments (in Figure 2), one reads a pKa of 3.2 at this concentration. This inconsistency of nearly 1 order of magnitude was not discussed in the text, and casts doubt on either pKa value. (4) In Burns et al. (1), it is unclear how the Hartree-Fock and density functional theory calculations confirmed their hypothesis of how chain length influences the electronic environment of the carboxylic hydrogen, and by how much. However, in ref 2 the authors use the ACD software for proving that there is a pKa difference of 0.97 between the linear and a branched PFOA. We found that ACD software (version 12.0) used by Ellis predicts a pKa of 0.5 for linear PFOA. It is unclear why they trust ACD to differentiate a pKa difference of 1 order of magnitude between a linear and branched isomer, even though the ACD-predicted pKa value for the linear isomer is over 3 orders of magnitude lower than their measured value. It is inconsistent to claim that the predicted relative difference in pKa values is realistic, but the absolute pKa values are not, as the same underlying algorithms are used by ACD to determine all pKa values. Thus, the ACD software does not reveal a “loss of inductive withdrawal of electron density at the carboxylate head by fluorine” for longer chain PFCAs that is extensive enough to increase the pKa by 3 orders of magnitude, as it does, in fact predict a pKa of 0.5 for PFOA. (5) Burns et al. (1) suggest that molecular modeling techniques such as “those used by Goss (5), are often very inaccurate” (and presumably ACD as well) for predicting pKa values. This statement is inappropriate. The general performance of the COSMOtherm software (version C2.1 release 01.07; COSMOlogic GmbH &Co. KG: Leverkusen, Germany, 2007) in predicting pKa values shows a standard deviation of 0.5 log units (11). In ref 5 COSMOtherm has been evaluated specifically for 5 different fluorinated acids for which reliable and undisputed pKa values are available and the predictions were within the general performance; thus fluorinated acids are considered to be within the chemical application domain. It should further be stressed that the helical conformation of perfluorinated alkyl chains, which Burns et al. argue is a crucial feature that influences the pKa, is indeed considered in the COSMOtherm calculations (see Figure 1 for the 3D structure of PFOA used in these calculations). Thus, as the behavior of perfluorinated alkyl 10.1021/es900451s CCC: $40.75
2009 American Chemical Society
Published on Web 05/28/2009
(4)
(5) (6) (7)
FIGURE 1. 3-D conformation of PFOA as it was used for the calculation of the pKa using COSMOtherm. groups are well accounted for by COSMOtherm, we see very little likelihood that the molecular modeling is off by more than 3 orders of magnitude from the real value. Further, the close agreement of COSMOtherm predictions with those of SPARC (5) and ACD software, only reinforce the COSMOtherm predictions. Thus, we conclude that a convincing mechanistic explanation for a mediated transfer of PFOA from water/oceans into the atmosphere is still missing and the controversy about the correct pKa of monomeric PFOA is still unsolved. Until these issues are resolved, we recommend that modellers account for the wide range of reported pKa values in their models, currently from 0 to 4, as done recently by Armitage et al. (12).
Literature Cited (1) Burns, D. C.; Ellis, D. A.; Li, H.; McMurdo, C. J.; Webster, E. Experimental pKa Determination for Perfluorooctanoic Acid (PFOA) and the Potential Impact of pKa Concentration Dependence on Laboratory-Measured Partitioning Phenomena and Environmental Modeling. Environ. Sci. Technol. 2008, 42, 9283–9288. (2) Ellis, D. A.; Webster, E. Response to Comment on “Aerosol Enrichment on the Surfactant PFO and Mediation of the Water-Air Transport of Gaseous PFOA”. Environ. Sci. Technol. 2009, 43 (4), 1234–1235. (3) Vacha, R.; Buch, V.; Milet, A.; Devlin, P.; Jungwirth, P. Autoionization at the surface of neat water: Is the top layer
(8) (9) (10)
(11)
(12)
pH neutral, basic, or acidic? Phys. Chem. Chem. Phys. 2007, 9, 4736–4747. Jungwirth, P. Personal communication, “You are right that pH is defined in terms of activity while we are talking about hydronium concentration. Therefore, we explicitely mention our operational definition of pH in terms of concentration already in the abstract. But I admit that this may be confusing, therefore, I am solely talking about hydronium concentration now.” Goss, K.-U. The pKa values of PFOA and other highly fluorinated carboxylic acids. Environ. Sci. Technol. 2008, 42, 456–458. Goss, K.-U. The pKa values of PFOA and other highly fluorinated carboxylic acids. Environ. Sci. Technol. 2008, 42, 5032–5032. Moroi, Y.; Yano, H.; Shibata, O.; Yonemitsu, T. Determination of Acidity Constants of Perfluoroalkanoic Acids. Bull. Chem. Soc. Jpn. 2001, 74, 667–672. Guo, W.; Brown, T. A.; Fung, B. M. Micelles And Aggregates Of Fluorinated Surfactants. J. Phys. Chem. 1991, 95, 1829– 1836. Lopez-Fontan, J. L.; Sarmiento, F.; Schulz, P. C. The aggregation of sodium perfluorooctanoate in water. Colloid Polym. Sci. 2005, 283, 862–871. Ylinen, M.; Kojo, A.; Hanhijarvi, H.; Peura, P. Disposition Of Perfluorooctanoic Acid In The Rat After Single And Subchronic Administration. Bull. Environ. Contam. Toxicol. 1990, 44, 46– 53. Klamt, A.; Eckert, F.; Diedenhofen, M.; Beck, M. E. First principles calculations of aqueous pK(a) values for organic and inorganic acids using COSMO-RS reveal an inconsistency in the slope of the pK(a) scale. J. Phys. Chem. A 2003, 107, 9380–9386. Armitage, J. M.; MacLeod, M.; Cousins, I. T. Modeling the Global Fate and Transport of Perfluorooctanoic Acid (PFOA) and Perfluorooctanoate (PFO) Emitted from Direct Sources Using a Multispecies Mass Balance Model. Environ. Sci. Technol. 2009, 43 (4), 1134–1140.
Kai-Uwe Goss* UFZ Helmholtz Center for Environmental Research, UFZ, Permoserstr. 15, 04318 Leipzig, German
Hans Peter H. Arp Department of Environmental Engineering, Norwegian Geotechnical Institute (NGI), P.O. Box 3930 Ullevål Stadion, N-0806, Oslo, Norway ES900451S
VOL. 43, NO. 13, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
5151
