Comment/Reply pubs.acs.org/jced
Reply to Comment on “The Effect of Surface Adsorption and Molecular Geometry on the Determination of Henry’s Law Constants for Fluorotelomer Alcohols” Yaoxing Wu and Victor W.-C. Chang* School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore
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n the comments on our previous work,1 Dr. Rayne challenged the interpretation of experimental results involving zigzag conformation of FTOHs with less than eight CF2 units by stating: “...the zigzag conformation of C4 and longer perfluoroalkyl chains is a transition state between leftand right-handed helical global minima. As such, the geometrical explanation invoking a zigzag global minimum conformation for n:2 (n < 8) FTOH does not appear to be valid.” The statement is mainly based upon Rayne and Forest’s interpretation of the computationally derived pKa values of perfluoroalkyl carboxylic acids [PFCAs] in their original work.2 As no significant change in the estimated aqueous monomeric pKa was observed with perfluoroalkyl chain length (C1 to C9), they conclude: “At all levels of theory, the perfluoroalkyl chain begins to adopt a twisted (helical) geometry at ≥ 4 to minimize repulsive interactions between adjacent −CF2− moieties as the chain sequentially lengthen” and “Increasing chain length does not substantially influence the structure or electronic character of the carboxylic acid head group”.2 Responding to the comment, we would like to draw attention to the following aspects: First, the expected linear dependence of Henry’s law constants of short-chain n:2 FOTHs (n ≥ 4) on perfluoroalkyl chain length was not observed either in our work or other similar studies.3,4 There must be some influencing mechanism(s) that leads to these results. We suggest that the conformational change of the FTOH molecule may play a role, but it may not be the sole controlling factor. Some other factors such as interfacial adsorption, which has been elaborated in our work,1 might also contribute to the unusual air−water partitioning behavior of FTOHs.5,6 Second, it is worth to note that there has been certain debate on the computationally derived and experimentally determined pKa values of longer chain PFCAs,7−10 and Rayne et al. also state in another work that “...all computational methods used to date appear to only agree with the established experimental data for the shorter straight chain C2 through C5 compounds. As the perfluoroalkyl chain length increases beyond C6, all approaches underestimate the experimental pKa. This deviation between experimental and estimated values increases from about one pKa unit for C6 to about three pKa units for C12....our current computational tools may be incapable of reliably estimating pKa values for the longer-chain PFCAs.”8 Thus, the uncertainty in PFCA acidity affects the understanding regarding the mechanisms and results in different interpretation regarding the molecular geometry of perfluorocarbons. Finally, the coexistence of planar zigzag and helical conformation of small perfluorocarbon molecules has been widely observed in perfluorocarbon single crystal structure data, © 2013 American Chemical Society
and the conformational equilibria appears to be strongly dependent on the temperature and molecular length.11−15 For example, Krafft and Riess state in their review of chemistry of molecular fluorocarbon−hydrocarbon diblocks: “The thread of the helix depends somewhat on the number of CF2 units (with the need for a helical structure increasing with chain length) and tends to become lesser when temperature or pressure increases...The planar trans zigzag configuration was thus determined to be the most stable for C4F10 and C6F14.”11 Wang et al. also state the following: “From DSC, FTIR, and Xray analysis [of brominated (F-alkyl)alkanes], it is observed that, below eight CF2 units, the fluorocarbon segments mainly form a zig-zag conformation. Above eight CF2 units, slight distortion at each CF2 leads to a helix conformation....This structural modification gives the longer CF2 segments rigid-rod characteristics...”13 Although the perfluoroalkyl chain has a very simple structure, many basic questions remain unclear because of the uncertain interpretation of the experimental and computational data. We appreciate the comment from Dr. Rayne. It is important to note that the actual partitioning behavior of FTOHs is the combination of various influencing factors. More research efforts are needed to further deconvolute these effects.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: (65) 6790 4773. Fax: (65) 6792 1650. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Wu, Y.; Chang, V. W. C. The effect of surface adsorption and molecular geometry on the determination of henry′s law constants for fluorotelomer alcohols. J. Chem. Eng. Data 2011, 56 (8), 3442−3448. (2) Rayne, S.; Forest, K. Theoretical studies on the pKa values of perfluoroalkyl carboxylic acids. J. Mol. Struct. (THEOCHEM) 2010, 949 (1−3), 60−69. (3) Goss, K. U.; Bronner, G.; Harner, T.; Hertel, M.; Schmidt, T. C. The partition behavior of fluorotelomer alcohols and olefins. Environ. Sci. Technol. 2006, 40 (11), 3572−3577. (4) Lei, Y. D.; Wania, F.; Mathers, D.; Mabury, S. A. Determination of vapor pressures, octanol−air, and water−air partition coefficients for polyfluorinated sulfonamide, sulfonamidoethanols, and telomer alcohols. J. Chem. Eng. Data 2004, 49 (4), 1013−1022.
Received: March 28, 2013 Accepted: April 8, 2013 Published: April 17, 2013 1418
dx.doi.org/10.1021/je400300k | J. Chem. Eng. Data 2013, 58, 1418−1419
Journal of Chemical & Engineering Data
Comment/Reply
(5) Lei, Y. D.; Shunthirasingham, C.; Wania, F. Comparison of headspace and gas-stripping techniques for measuring the air−water partitioning of normal alkanols (C4 to C10): Effect of temperature, chain length, and adsorption to the water surface. J. Chem. Eng. Data 2007, 52 (1), 168−179. (6) Shunthirasingham, C.; Ying, D. L.; Wania, F. Evidence of bias in air−water Henry′s law constants for semivolatile organic compounds measured by inert gas stripping. Environ. Sci. Technol. 2007, 41 (11), 3807−3814. (7) Moroi, Y.; Yano, H.; Shibata, O.; Yonemitsu, T. Determination of acidity constants of perfluoroalkanoic acids. Bull. Chem. Soc. Jpn. 2001, 74 (4), 667−672. (8) Rayne, S.; Forest, K.; Friesen, K. J. Computational approaches may underestimate pKa values of longer-chain perfluorinated carboxylic acids: Implications for assessing environmental and biological effects. J. Environ. Sci. Health., Part A 2009, 44 (4), 317−326. (9) 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 (24), 9283−9288. (10) Henne, A. L.; Fox, C. J. Ionization constants of fluorinated acids. J. Am. Chem. Soc. 1951, 73 (5), 2323−2325. (11) Krafft, M. P.; Riess, J. G. Chemistry, physical chemistry, and uses of molecular fluorocarbon- hydrocarbon diblocks, triblocks, and related compounds-unique “apolar” components for self-assembled colloid and interface engineering. Chem. Rev. 2009, 109 (5), 1714− 1792. (12) Campos-Vallette, M.; Rey-Lafon, M. Vibrational spectra and rotational isomerism in short chain n-perfluoroalkanes. J. Mol. Struct. 1983, 101 (1−2), 23−45. (13) Wang, J.; Ober, C. K. Solid state crystalline and liquid crystalline structure of semifluorinated 1-bromoalkane compounds. Liq. Cryst. 1999, 26 (5), 637−648. (14) Masetti, G.; Cabassi, F.; Morelli, G.; Zerbi, G. Conformational order and disorder in poly(tetrafluoroethylene) from the infrared spectrum. Macromolecules 1973, 6 (5), 700−707. (15) Boerio, F. J.; Koenig, J. L. Crystal field splitting in Raman spectra of polytetrafluoroethylene. J. Chem. Phys. 1971, 54 (9), 3667− 3669.
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