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Experimental Determination of Air/Water Partition Coefficients for 21 Per- and Polyfluoroalkyl Substances Reveals Variable Performance of Property Prediction Models
Open accessAbstract
Per- and polyfluoroalkyl substances (PFAS) are a group of chemicals of high environmental concern. However, reliable data for the air/water partition coefficients ( K aw ), which are required for fate, exposure, and risk analysis, are available for only a few PFAS. In this study, K aw values at 25 °C were determined for 21 neutral PFAS by using the hexadecane/air/water thermodynamic cycle. Hexadecane/water partition coefficients ( K Hxd/w ) were measured with batch partition, shared-headspace, and/or modified variable phase ratio headspace methods and were divided by hexadecane/air partition coefficients ( K Hxd/air ) to obtain K aw values over 7 orders of magnitude (10 –4.9 to 10 2.3 ). Comparison to predicted K aw values by four models showed that the quantum chemically based COSMO therm model stood out for accuracy with a root-mean-squared error (RMSE) of 0.42 log units, as compared to HenryWin, OPERA, and the linear solvation energy relationship with predicted descriptors (RMSE, 1.28–2.23). The results indicate the advantage of a theoretical model over empirical models for a data-poor class like PFAS and the importance of experimentally filling data gaps in the chemical domain of environmental interest. K aw values for 222 neutral (or neutral species of) PFAS were predicted using COSMO therm as current best estimates for practical and regulatory use.
Overview
AI snapshot
Written from the paper’s full text.
TL;DR
This study experimentally determined air/water partition coefficients (Kaw) at 25 °C for 21 neutral PFAS using a hexadecane/air/water thermodynamic cycle, yielding log Kaw values from −4.89 to 2.28. COSMOtherm was the most accurate of four tested prediction models, and it was used to predict Kaw for 222 neutral (or neutral species of) PFAS as current best estimates.
Key findings
- New experimental log Kaw values at 25 °C spanned −4.89 to 2.28, over 7 orders of magnitude, for 21 neutral PFAS.
- COSMOtherm had the lowest error (RMSE 0.42 log units) compared with HenryWin, OPERA, and LSER-IFSQSAR (RMSE 1.28–2.23); COSMOtherm showed slight underprediction (mean error −0.34 log units), and correcting by +0.34 reduced RMSE to 0.26 log units.
- Log Kaw increased by 0.43 ± 0.02 log units per CF2 and by 0.14 ± 0.04 log units per CH2; N-methyl substitution of perfluoroalkane sulfonamides increased Kaw by 2 log units, while further hydroxyethyl substitution decreased it by 1.5 log units.
- COSMOtherm-predicted log Kaw for 222 neutral (or neutral species of) PFAS ranged from −10.0 to 9.7, with 200 (90%) between −5 and 5.
Methods
- KHxd/w was measured for 21 PFAS by batch partition (18 PFAS), shared-headspace (9 PFAS), and/or modified variable phase ratio headspace (VPR-HS; 5 FTOHs) methods at 25 °C; sulfonamides used 1 mM HCl, and analysis was by LC/MS or GC/MS.
- Kaw was derived from the hexadecane/air/water thermodynamic cycle as Kaw = KHxd/w / KHxd/air, using KHxd/air values from the authors’ previous study; direct VPR-HS Kaw measurements were also made for five PFAS for comparison.
- Four structure-only prediction models were evaluated against the new experimental data: HenryWin, OPERA, LSER-IFSQSAR, and COSMOtherm; COSMOtherm was then used to predict log Kaw for 222 PFAS.
Limitations
- Stated: the batch partition method is applicable only up to log KHxd/w ≈ 3.6 because hexadecane microdroplets in water bias apparent KHxd/w; the modified VPR-HS method may be limited to weakly sorptive compounds due to adsorption; the shared-headspace method requires sufficient volatility and hydrophobicity; direct VPR-HS Kaw can have substantial errors if mass balance assumptions fail; further model validation is desirable given diverse PFAS structures; COSMOtherm is not a free tool.
- Evident from the text: the experimental evaluation covers only 21 neutral PFAS, so model performance and structure–property relationships are demonstrated only within this chemical domain; the 222 COSMOtherm-predicted log Kaw values are not experimentally validated.
Model-written summary. Check it against the paper before relying on it.
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Claims the paper makes
1 of 44 cited sources checked · 0 supported · 0 flagged · accuracy not yet measured
- Findingown data
Kaw values at 25 °C were determined for 21 neutral PFAS by using the hexadecane/air/water thermodynamic cycle.
“Kaw values at 25 °C were determined for 21 neutral PFAS by using the hexadecane/air/water thermodynamic cycle.”
Quote in the paperSubstantial claim - Findingreasoning
By combining the batch partition and shared-headspace methods, the authors measured log KHxd/w from −1.15 to 5.66 for PFAS with different molecular structures.
“by combining these two methods, we were able to measure log KHxd/w from −1.15 to 5.66 for PFAS with different molecular structures.”
Quote in the paperSubstantial claim - Findingnone given
In the shared-headspace method, the water phase is free of hexadecane.
“In the shared-headspace method, the water phase is free of hexadecane.”
Quote in the paperSubstantial claim - Findingown data
Particularly large errors were found for PFOSA (error, +4.9), PFHxSA (error, +4.4), PFBSA (error, +3.9), and 6:1 FTI-7H (error, +3.0), with plus sign indicating overprediction.
“Particularly large errors were found for PFOSA (error, +4.9), PFHxSA (error, +4.4), PFBSA (error, +3.9), and 6:1 FTI-7H (error, +3.0), with the plus sign in the parentheses indicating overprediction.”
Quote in the paperSubstantial claim - Findingown data
log KHxd/w of 6:1 FTI-7H is 1.0 log unit lower than that of 6:1 FTI.
“Another finding is that log KHxd/w of 6:1 FTI-7H is 1.0 log unit lower than that of 6:1 FTI”
Quote in the paperSubstantial claim
- Findingreasoning
The data plot for the modified VPR-HS method did not always follow the relationship between PA and VHxd expected by eq 2.
“However, the data plot did not always follow the relationship between PA and VHxd expected by eq 2 (Figure S1).”
Quote in the paperSubstantial claim - Findingreasoning
The confidence intervals for the modified VPR-HS method were large, particularly for 4:4 and 5:2s FTOHs.
“As a result, the CIs were large, particularly for 4:4 and 5:2s FTOHs.”
Quote in the paperSubstantial claim - Findingown data
The batch and shared-headspace methods resulted in practically identical log KHxd/w values for 6:2 and 5:2s FTOHs.
“The two methods resulted in practically identical values for 6:2 and 5:2s FTOHs.”
Quote in the paperSubstantial claim - Findingown data
The values for 3:1, 3:3, and 4:2 FTOHs from different sources agree well with each other, mostly within 0.1-0.2 log units.
“The values for 3:1, 3:3, and 4:2 FTOHs from different sources agree well with each other, mostly within 0.1–0.2 log units.”
Quote in the paperNot a substantial claim - Findingown data
Comparison to predicted Kaw values by four models showed that COSMOtherm stood out for accuracy with an RMSE of 0.42 log units, compared to HenryWin, OPERA, and the LSER with predicted descriptors (RMSE, 1.28-2.23).
“Comparison to predicted Kaw values by four models showed that the quantum chemically based COSMOtherm model stood out for accuracy with a root-mean-squared error (RMSE) of 0.42 log units, as compared to HenryWin, OPERA, and the linear solvation energy relationship with predicted descriptors (RMSE, 1.28–2.23).”
Quote in the paperSubstantial claim - Findingown data
Deviations from experimental values for HenryWin increased with increasing chain length.
“Deviations from experimental values increased with increasing chain length.”
Quote in the paperNot a substantial claim - Findingown data
HenryWin (bond contribution model) suffered from a high RMSE (2.2 log units) with strong biases to overprediction.
“HenryWin (bond contribution model) suffered from a high RMSE (2.2 log units) with strong biases to overprediction.”
Quote in the paperSubstantial claim - Findingreasoning
If the water phase contained 0.006 vol % hexadecane droplets and the shared-headspace log KHxd/w is accurate, the apparent log KHxd/w values measured by the batch method can be reproduced.
“Indeed, if we assume that the water phase contained 0.006 vol % hexadecane droplets and that log KHxd/w determined by the shared-headspace method is accurate, we can reproduce the “apparent log KHxd/w” values measured by the batch method (see SI-4 for the calculation).”
Quote in the paperSubstantial claim- The authors state this as an assumption: In the batch partition method, KHxd/w can be calculated from the measured water phase concentration under the mass conservation assumption.
- The authors state this as an assumption: In the calculation reproducing the batch method apparent log KHxd/w values, the water phase contained 0.006 vol % hexadecane droplets and the shared-headspace log KHxd/w is accurate.
- Findingown data
Using eq 1, the authors obtained log Kaw values at 25 °C from −4.89 to 2.28, over 7 log units.
“Using eq 1, we obtained log Kaw values (25 °C) from −4.89 to 2.28, thus over 7 log units (Table 2).”
Quote in the paperSubstantial claim - Findingown data
KHxd/w values were measured for 21 PFAS.
“KHxd/w values were measured for 21 PFAS; their names, abbreviations, and CAS registry numbers are listed in Table 1.”
Quote in the paperSubstantial claim - Findingown data
Log KHxd/w values measured by both batch and shared-headspace methods are available for seven compounds.
“Log KHxd/w values measured by both batch and shared-headspace methods are available for seven compounds.”
Quote in the paperSubstantial claim - Findingreasoning
Kaw values for 222 neutral (or neutral species of) PFAS were predicted using COSMOtherm as current best estimates for practical and regulatory use.
“Kaw values for 222 neutral (or neutral species of) PFAS were predicted using COSMOtherm as current best estimates for practical and regulatory use.”
Quote in the paperSubstantial claim - Findingown data
Preliminary experiments with PFBSA and PFHxSA with pure water resulted in log KHxd/w values that were 0.5-0.9 log units lower than those determined with 1 mM HCl solution later.
“Preliminary experiments with PFBSA and PFHxSA with pure water resulted in log KHxd/w values that were 0.5–0.9 log units lower than those determined with 1 mM HCl solution later.”
Quote in the paperSubstantial claim - Findingown data
The log KHxd/w values measured by the batch method were significantly lower than those by the shared-headspace method for 4:2 FTI, 6:1 FTI, 6:1 FTI-7H, 6:2 FTAC, and 4:2 FTMAC, which are all relatively hydrophobic.
“However, the log KHxd/w values measured by the batch method were significantly lower than those by the shared-headspace method for the remaining five compounds (i.e., 4:2 FTI, 6:1 FTI, 6:1 FTI-7H, 6:2 FTAC, 4:2 FTMAC), which are all relatively hydrophobic.”
Quote in the paperSubstantial claim- The authors state this as an assumption: In the batch partition method, KHxd/w can be calculated from the measured water phase concentration under the mass conservation assumption.
- Findingown data
Hexadecane/water partition coefficients (KHxd/w) were measured with batch partition, shared-headspace, and/or modified variable phase ratio headspace methods.
“Hexadecane/water partition coefficients (KHxd/w) were measured with batch partition, shared-headspace, and/or modified variable phase ratio headspace methods and were divided by hexadecane/air partition coefficients (KHxd/air) to obtain Kaw values over 7 orders of magnitude (10–4.9 to 102.3).”
Quote in the paperSubstantial claim - Findingown data
The modified VPR-HS method resulted in log KHxd/w values comparable to those measured by the batch partition method.
“The modified VPR-HS method resulted in log KHxd/w values comparable to those measured by the batch partition method (Table 2).”
Quote in the paperSubstantial claim - Findingown data
Unsubstituted PFASAs are water-soluble (log Kaw < −3), even if their ionization in water is disregarded.
“In contrast, unsubstituted PFASAs are water-soluble (log Kaw < −3), even if their ionization in water as discussed above is disregarded.”
Quote in the paperNot a substantial claim - Findingreasoning
Larger differences were found for 4:4, 6:2, 8:2 FTOHs, and 4:2 FTMAC.
“However, larger difference was found for 4:4, 6:2, 8:2 FTOHs, and 4:2 FTMAC”
Quote in the paperSubstantial claim - Findingown data
Fluorotelomer substances (FTIs, FTACs, FTMACs) favor air over water (log Kaw > 0), except for FTOHs.
“Fluorotelomer substances (i.e., FTIs, FTACs, FTMACs) favor air over water (log Kaw > 0), except for FTOHs.”
Quote in the paperNot a substantial claim - Findingreasoning
Further N-hydroxyethyl substitution (e.g., from MeFBSA to MeFBSE) decreases Kaw back by 1.5 log units, because of the introduction of the polar −OH group.
“Further N-hydroxyethyl substitution (e.g., from MeFBSA to MeFBSE) decreases Kaw back by 1.5 log units, because of the introduction of the polar −OH group.”
Quote in the paperNot a substantial claim - Findingown data
The group contribution model of HenryWin was not any better (RMSE 2.1 log units; n = 13).
“The group contribution model of HenryWin was not any better (RMSE 2.1 log units; n = 13; see Table S5).”
Quote in the paperNot a substantial claim - Findingown data
Log Kaw of the four groups (two X:1 FTOHs, three X:2 FTOHs, three PFASAs, and two N-methyl FASAs) increases consistently by 0.43 ± 0.02 log units per CF2.
“Log Kaw of these four groups increases consistently by 0.43 ± 0.02 log units per CF2.”
Quote in the paperSubstantial claim - Findingown data
The CF2 and CH2 increments have similar influences on log KHxd/w (0.74 ± 0.02 and 0.64 ± 0.04, respectively).
“Interestingly, the CF2 and CH2 increments have similar influences on log KHxd/w (0.74 ± 0.02 and 0.64 ± 0.04, respectively; Figure 1A), as opposed to the differing influences on log Kaw.”
Quote in the paperSubstantial claim - Findingown data
log Kaw of 6:1 FTI-7H is 1.6 log units lower than that of 6:1 FTI.
“and that log Kaw of the former is 1.6 log units lower than that of the latter.”
Quote in the paperSubstantial claim - Findingown data
All OPERA predictions were outside the global AD defined by the leverage calculation of OPERA.
“As shown in Table S6, all predictions were outside the global AD defined by the leverage calculation of OPERA.”
Quote in the paperSubstantial claim - Findingown data
Even with a polar functional group such as acrylate or methacrylate, FTIs, FTACs, and FTMACs readily partition from water to air.
“Even with a polar functional group such as acrylate or methacrylate, these chemicals readily partition from water to air.”
Quote in the paperNot a substantial claim - Findingreasoning
Log Kaw increases only by 0.14 ± 0.04 log units per CH2.
“Noteworthy is that log Kaw increases only by 0.14 ± 0.04 log units per CH2.”
Quote in the paperNot a substantial claim - Findingown data
OPERA resulted in a high RMSE (1.8 log units).
“OPERA also resulted in a high RMSE (1.8 log units).”
Quote in the paperSubstantial claim - Findingown data
LSER-IFSQSAR consistently underestimated sulfonamides and their derivatives (errors, −2.7 to −1.0).
“LSER-IFSQSAR consistently underestimated sulfonamides and their derivatives (errors, −2.7 to −1.0)”
Quote in the paperNot a substantial claim - Findingreasoning
If the COSMOtherm-predicted log Kaw values are corrected by adding 0.34, the RMSE would become 0.26 log units.
“Note that, if we correct the COSMOtherm-predicted log Kaw values by adding 0.34, the RMSE would become 0.26 log units.”
Quote in the paperNot a substantial claim - Findingown data
COSMOtherm was by far the most accurate of the four models tested (RMSE, 0.4 log units).
“COSMOtherm was by far the most accurate of the four models tested (RMSE, 0.4 log units).”
Quote in the paperSubstantial claim - Findingown data
The ME of log KHxd/air predictions for the current 21 compounds is +0.48.
“The ME of log KHxd/air predictions for the current 21 compounds is +0.48 (Figure S4).”
Quote in the paperNot a substantial claim - Findingown data
The ULs provided by EAS-E Suite did not have a relationship with the observed absolute errors (R2, 0.01).
“The ULs provided by EAS-E Suite did not have a relationship with the observed absolute errors (R2, 0.01)”
Quote in the paperNot a substantial claim - Findingown data
log KHxd/w predictions by COSMOtherm have little bias (ME, +0.14).
“In contrast, log KHxd/w predictions by COSMOtherm have little bias (ME, +0.14; Figure S4)”
Quote in the paperSubstantial claim - Findingown data
The estimated errors from EAS-E Suite did have a positive correlation with the observed absolute errors (R2, 0.38).
“but the estimated errors from EAS-E Suite did have a positive correlation with the observed absolute errors (R2, 0.38) (Table S6).”
Quote in the paperNot a substantial claim - Findingown data
There was slight overall underprediction with a mean error (ME) of −0.34 log units for COSMOtherm.
“There was slight overall underprediction with a mean error (ME) of −0.34 log units.”
Quote in the paperNot a substantial claim - Findingown data
LSER-IFSQSAR tended to overestimate the others including FTOHs (errors, −0.2 to +1.5), except 5:2s FTOH (error, −1.6).
“and tended to overestimate the others including FTOHs (errors, −0.2 to +1.5), except 5:2s FTOH (error, −1.6).”
Quote in the paperSubstantial claim - Interpretationreasoning
The H atom at the end of the perfluoroalkyl chain can serve as a relatively strong H-bond donor site because of the strong electron-withdrawing property of the F atoms, substantially strengthening the polar interactions with water molecules.
“The H atom at the end of the perfluoroalkyl chain can serve as a relatively strong H-bond donor site because of the strong electron-withdrawing property of the F atoms, substantially strengthening the polar interactions with water molecules.”
Quote in the paperSubstantial claim - Premisecitation
Reported Kaw values for a single compound differ between studies by up to a factor of 130.
“Some of the reported values, however, differ between the studies by up to a factor of 130 for a single compound.”
Quote in the paperSubstantial claim0 of 3 sources checked - Premisecitation
An experimental pKa value of 6.33 was reported for trifluoromethane sulfonamide.
“While measured pKa values of these compounds are unavailable, an experimental pKa value of 6.33 was reported for trifluoromethane sulfonamide (CF3-SO2NH2).25”
Quote in the paperSubstantial claim0 of 1 sources checked- cites [25] source
- Interpretationreasoning
The batch partition and shared-headspace methods are largely complementary.
“Thus, the batch partition and shared-headspace methods are largely complementary”
Quote in the paperSubstantial claim - Interpretationcitation
The data from the new paper are inconsistent with the available knowledge for Kaw of PFAS.
“These data, however, are inconsistent with the available knowledge for Kaw of PFAS (see SI-5 and Figures S5, S6 for more details)”
Quote in the paperSubstantial claim0 of 1 sources checked- cites [37] source
- Study design and conductown data
In the shared-headspace method, analysis of the hexadecane phase confirmed 85-100% mass conservation.
“The hexadecane phase was also analyzed, which confirmed 85–100% mass conservation (Table S3).”
Quote in the paperNot a substantial claim - Interpretationown data
The high consistency of the observed CF2 dependence over four classes of PFASs supports the accuracy of the authors' data.
“The high consistency of the observed CF2 dependence over four classes of PFASs supports accuracy of our data.”
Quote in the paperSubstantial claim - Premisecitation
Only a few scientific articles have reported direct measurements of Kaw for PFAS with a perfluorocarbon chain length of 4 or more.
“There have been only a few scientific articles that reported direct measurements of Kaw for PFAS with a perfluorocarbon chain length of 4 or more.”
Quote in the paperSubstantial claim0 of 5 sources checked - Premisereasoning
No experimental data means no model calibration and validation.
“However, no experimental data means no model calibration and validation; indeed, Arp et al.20 reported prediction errors of several orders of magnitude in Kaw by some models.”
Quote in the paperSubstantial claim - Study design and conductcitation
The VPR-HS measurement for Kaw could result in substantial errors if the mass balance assumption does not hold true, as was experimentally demonstrated before.
“This measurement was conducted for a comparison purpose because the VPR-HS measurement for Kaw could result in substantial errors if the mass balance assumption does not hold true, as was experimentally demonstrated before.8”
Quote in the paperSubstantial claim0 of 1 sources checked- cites [8] source
- Premisenone given
The only structural difference between 6:1 FTI-7H and 6:1 FTI is the F substitution pattern of the terminal methyl group (−CF2H vs −CF3, respectively).
“The only structural difference between 6:1 FTI-7H and 6:1 FTI is the F substitution pattern of the terminal methyl group (−CF2H vs −CF3, respectively).”
Quote in the paperSubstantial claim - Premisecitation
Arp et al. reported prediction errors of several orders of magnitude in Kaw by some models.
“However, no experimental data means no model calibration and validation; indeed, Arp et al.20 reported prediction errors of several orders of magnitude in Kaw by some models.”
Quote in the paperSubstantial claim0 of 1 sources checked- cites [20] source
- Interpretationown data
There appears to be no major improvement in predicting log Kaw for PFAS.
“There appears to be no major improvement in predicting log Kaw for PFAS.”
Quote in the paperSubstantial claim - Interpretationown data
The use of the modified VPR-HS method may be limited to weakly sorptive compounds.
“Thus, although the method is by far the simplest, the use of this method may be limited to weakly sorptive compounds.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The modified VPR-HS method assumes that the relationship between PA and VHxd expected by eq 2 holds.
- Premisecitation
Experimental KHxd/air values for 61 PFAS have already been reported in the authors' previous study.
“Experimental KHxd/air values for 61 PFAS have already been reported in our previous study.21”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites [21] source
- Premisecitation
Despite the proliferation of literature on PFAS, experimental data for basic physicochemical properties, including Kaw, are scarce.
“Despite the proliferation of literature on PFAS, experimental data for basic physicochemical properties, including air/water partition coefficients (Kaw), are scarce.”
Quote in the paperSubstantial claim0 of 1 sources checked- cites [13] source
- Interpretationown data
The results indicate the advantage of a theoretical model over empirical models for a data-poor class like PFAS.
“The results indicate the advantage of a theoretical model over empirical models for a data-poor class like PFAS and the importance of experimentally filling data gaps in the chemical domain of environmental interest.”
Quote in the paperSubstantial claim - Premisecitation
Two studies measured Kaw of the neutral species of PFOA and provided values that are an order of magnitude different.
“Two other studies took on the challenge of measuring Kaw of the neutral species of the strong acid PFOA and provided values that are an order of magnitude different.10,11”
Quote in the paperSubstantial claim0 of 2 sources checked - Interpretationreasoning
OPERA clearly indicated that its log Kaw predictions for the 21 PFAS were not trustworthy.
“Thus, OPERA clearly indicated that its log Kaw predictions for the 21 PFAS were not trustworthy.”
Quote in the paperSubstantial claim - Interpretationreasoning
The presence of 0.006 vol % hexadecane in water suggests that the current batch partition method is applicable to measure log KHxd/w values up to 3.6.
“The presence of 0.006 vol % hexadecane in water suggests that the current batch partition method is applicable to measure log KHxd/w values up to 3.6.”
Quote in the paperSubstantial claim- The authors state this as an assumption: In the batch partition method, KHxd/w can be calculated from the measured water phase concentration under the mass conservation assumption.
- The authors state this as an assumption: In the calculation reproducing the batch method apparent log KHxd/w values, the water phase contained 0.006 vol % hexadecane droplets and the shared-headspace log KHxd/w is accurate.
- Interpretationreasoning
The small change in log Kaw with the addition of CH2 can be explained by the increased van der Waals interaction energy with water molecules being nearly offset by the increased cavity formation energy cost.
“This small change in log Kaw with the addition of CH2 can be explained by the fact that the increased van der Waals interaction energy with water molecules is nearly offset by the increased cavity formation energy cost.”
Quote in the paperNot a substantial claim - Premisecitation
Even including indirectly measured Kaw data, a recent review listed experimental data for only seven PFAS.
“Even including such indirectly measured Kaw data, the authors of a recent review13 listed experimental data for only seven PFAS.”
Quote in the paperSubstantial claim0 of 1 sources checked- cites [13] source
- Premisecitation
In 2006, Arp et al. demonstrated similar overprediction for four FTOHs by a former version of HenryWin.
“In 2006, Arp et al.20 demonstrated similar overprediction for four FTOHs by a former version of HenryWin.”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Arp et al. [20] source
- Interpretationreasoning
For log KHxd/w higher than 3.6, an error of >0.1 log unit would occur in the current batch partition method.
“For higher KHxd/w, an error of >0.1 log unit would occur.”
Quote in the paperSubstantial claim- The authors state this as an assumption: In the batch partition method, KHxd/w can be calculated from the measured water phase concentration under the mass conservation assumption.
- The authors state this as an assumption: In the calculation reproducing the batch method apparent log KHxd/w values, the water phase contained 0.006 vol % hexadecane droplets and the shared-headspace log KHxd/w is accurate.
- Interpretationreasoning
6:1 FTI-7H has substantially higher water affinity than 6:1 FTI.
“Both indicate that 6:1 FTI-7H has substantially higher water affinity than 6:1 FTI does.”
Quote in the paperSubstantial claim - Interpretationreasoning
The shared-headspace method is useful for compounds with sufficient volatility and hydrophobicity, as these properties ensure efficient transfer from hexadecane to water via air and negligible concentration depletion in hexadecane.
“The shared-headspace method is useful for compounds with sufficient volatility (relatively high Kaw, low KHxd/air) and hydrophobicity (relatively high KHxd/w), as these properties ensure an efficient transfer from hexadecane to water via air and negligible concentration depletion in hexadecane.”
Quote in the paperSubstantial claim - Premisecitation
The accuracy of log Kaw prediction for neutral PFAS is comparable to that for nonfluorinated chemicals (∼0.3 log units).
“The accuracy of log Kaw prediction for neutral PFAS is comparable to that for nonfluorinated chemicals (∼0.3 log units).43”
Quote in the paperSubstantial claim0 of 1 sources checked- cites [43] source
- Interpretationown data
Liquid/liquid partitioning can be predicted more accurately.
“suggesting the liquid/liquid partitioning can be predicted more accurately.”
Quote in the paperSubstantial claim - Interpretationown data
Fully empirical models (e.g., HenryWin, OPERA) do not seem to be useful for predicting Kaw of PFAS, at least for the moment.
“Overall, the evaluation of four models presented above suggests that fully empirical models (e.g., HenryWin, OPERA) do not seem to be useful for predicting Kaw of PFAS, at least for the moment.”
Quote in the paperSubstantial claim - Interpretationnone given
HenryWin lacks fragment values and correction factors required to describe the influence of F substitutions on log Kaw.
“Apparently, HenryWin lacks fragment values and correction factors required to describe the influence of F substitutions on log Kaw.”
Quote in the paperSubstantial claim - Interpretationown data
The high prediction performance of COSMOtherm indicates that this theoretically based model is suitable for predicting Kaw of PFAS.
“The high prediction performance of COSMOtherm indicates that this theoretically based model is suitable for predicting Kaw of PFAS.”
Quote in the paperSubstantial claim - Interpretationreasoning
COSMOtherm accurately modeled the influence of the perfluoroalkyl structure on log Kaw discussed in the previous section.
“Thus, COSMOtherm accurately modeled the influence of the perfluoroalkyl structure on log Kaw discussed in the previous section.”
Quote in the paperSubstantial claim - Interpretationreasoning
COSMOtherm appears to slightly overestimate the partitioning of PFAS from air to liquid phases.
“COSMOtherm appears to slightly overestimate the partitioning of PFAS from air to liquid phases.”
Quote in the paperNot a substantial claim
- It may state two separate claims as one. 28
- The quote only partly states this claim. 1
- Probably not a substantial claim. 12
It may state two separate claims as one.
PremisecitationKaw prediction models have been evaluated against experimental data in the literature but only for four FTOHs and four additional PFAS so far because of the data limitation.
“Kaw prediction models have been evaluated against experimental data in the literature but only for four FTOHs20,24 and four additional PFAS13 so far because of the data limitation.”
Quote in the paperSubstantial claim1 of 3 sources checked- cites [20] source
- cites [24] source
In ‘Limitations of property estimations’ (page unavailable), the paper says it compared estimates with available experimental data for several PFASs other than FTOHs, and that the data covered only some properties and parts of each homologous series. It does not establish that KAW models were evaluated for exactly four additional PFASs. The four-FTOH portion of the citing sentence is attached to other citations, not this one. (LLM, conf=0.84)
Experimental — accuracy not yet measured - cites [13] source
It may state two separate claims as one.
FindingreasoningNine PFAS meeting the shared-headspace requirements were measured by this method, while all eight sulfonamides and four short-chain FTOHs were not.
“Therefore, nine PFAS that meet this requirement were measured by this method, while all eight sulfonamides and four short-chain FTOHs were not.”
Quote in the paperSubstantial claimIt may state two separate claims as one.
Findingown dataThe local AD indices generated by OPERA were low (<0.4) for all but 3:1 FTOH, meaning that the five nearest neighbors were not similar to the predicted compound.
“Moreover, the local AD indices generated by OPERA were low (<0.4) for all but 3:1 FTOH, meaning that the five nearest neighbors were not similar to the predicted compound.”
Quote in the paperSubstantial claimIt may state two separate claims as one.
Findingown dataA few calibration chemicals (e.g., 6:2, 8:2 FTOHs) were frequently identified as neighboring compounds, which resulted in similar predictions for many of the tested PFAS.
“Looking closer at the output file, we found that a few calibration chemicals (e.g., 6:2, 8:2 FTOHs) were frequently identified as neighboring compounds, which resulted in similar predictions for many of the tested PFAS.”
Quote in the paperSubstantial claimIt may state two separate claims as one.
Findingown dataThe batch method-measured log KHxd/w values for the five compounds were all approximately 4 with wide 95% CIs.
“The batch method-measured values for the five compounds were all ∼4 with wide 95% CIs.”
Quote in the paperSubstantial claim- The authors state this as an assumption: In the batch partition method, KHxd/w can be calculated from the measured water phase concentration under the mass conservation assumption.
It may state two separate claims as one.
Findingown dataThe OPERA prediction did not follow the increasing trend of log Kaw with chain length, but instead calculated a narrow range of log Kaw from −2.9 to −1.5 for 19 out of 21 compounds.
“The OPERA prediction did not even follow the increasing trend of log Kaw with the chain length, but instead, it calculated a narrow range of log Kaw from −2.9 to −1.5 for 19 out of 21 compounds.”
Quote in the paperSubstantial claimIt may state two separate claims as one.
Findingown dataCOSMOtherm predicted pKa values of 6.4 and 6.3 for PFBSA and PFHxSA, respectively, and 7.3 and 7.2 for MeFBSA and MeFHxSA, respectively.
“COSMOtherm predicted pKa values of 6.4 and 6.3 for PFBSA and PFHxSA, respectively, and 7.3 and 7.2 for MeFBSA and MeFHxSA, respectively.”
Quote in the paperSubstantial claimIt may state two separate claims as one. The quote only partly states this claim.
Findingown dataPredictions for FTOHs by HenryWin were better but still errors up to +2.1 were found.
“Predictions for FTOHs were better but still errors up to +2.1 were found.”
Quote in the paperNot a substantial claimIt may state two separate claims as one.
Findingown dataKaw values were obtained by dividing KHxd/w by KHxd/air and span 7 orders of magnitude (10-4.9 to 102.3).
“Hexadecane/water partition coefficients (KHxd/w) were measured with batch partition, shared-headspace, and/or modified variable phase ratio headspace methods and were divided by hexadecane/air partition coefficients (KHxd/air) to obtain Kaw values over 7 orders of magnitude (10–4.9 to 102.3).”
Quote in the paperSubstantial claimIt may state two separate claims as one.
Findingown dataN-methyl substitution of PFASAs increases Kaw by 2 log units, making them substantially less water favoring.
“N-methyl substitution of PFASAs increases Kaw by 2 log units, making them substantially less water favoring.”
Quote in the paperNot a substantial claimProbably not a substantial claim. The paper’s conclusions may not depend on it.
Findingown dataFive FTOHs were measured with the modified VPR-HS method.
“Five FTOHs were measured with this method, which were measurable by headspace GC/MS and expected to have KHxd/w that is low enough for this method.”
Quote in the paperNot a substantial claim- The authors state this as an assumption: In the modified VPR-HS method, GC/MS response linearity and no significant mass of the compound in the headspace or any other interface can be assumed.
It may state two separate claims as one.
Findingown dataOPERA calculated ∼ −1.6 for 10 and ∼ −2.3 for 3 compounds.
“More specifically, OPERA calculated ∼ −1.6 for 10 and ∼ −2.3 for 3 compounds.”
Quote in the paperNot a substantial claimIt may state two separate claims as one.
Findingown dataThis approach did not result in major improvement in RMSE (1.7 log units) even though all 21 compounds were considered within AD of both Kow and Koa prediction models.
“This approach, however, did not result in major improvement in RMSE (1.7 log units, Table S5) even though all 21 compounds were considered within AD of both Kow and Koa prediction models.”
Quote in the paperSubstantial claimIt may state two separate claims as one.
Findingown dataLSER-IFSQSAR provided a better RMSE (1.3 log units) than the former two methods, and it captured the chain-length dependence of log Kaw.
“LSER-IFSQSAR provided a better RMSE (1.3 log units) than the former two methods, and it captured the chain-length dependence of log Kaw.”
Quote in the paperSubstantial claimIt may state two separate claims as one.
Findingown dataThe predicted log Kaw values using COSMOtherm for 222 PFAS were in the range of −10.0 to 9.7, and 200 (90%) of the 222 values were from −5 to 5, indicating a large variation of the partition properties.
“The predicted log Kaw values were in the range of −10.0 to 9.7, and 200 (90%) of the 222 values were from −5 to 5, indicating a large variation of the partition properties.”
Quote in the paperNot a substantial claimIt may state two separate claims as one.
Interpretationown dataThe authors' values from eq 1 are internally consistent and should represent the most reliable set of experimental values available for PFAS.
“As shown below, our values from eq 1 are internally consistent and should represent the most reliable set of experimental values available for PFAS.”
Quote in the paperSubstantial claimProbably not a substantial claim. The paper’s conclusions may not depend on it.
PremisecitationA new paper reported Kaw values for 15 PFAS measured by the standard VPR-HS method.
“In the course of this work, a new paper appeared that reported Kaw values for 15 PFAS measured by the standard VPR-HS method.37”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites [37] source
It may state two separate claims as one.
PremisecitationThere exist many neutral or largely neutral PFAS, some of which are known precursors of PFOS and PFOA.
“Nevertheless, there exist many neutral or largely neutral PFAS, some of which are known precursors of the PFAS of high concern (i.e., PFOS, PFOA).14−16”
Quote in the paperSubstantial claim0 of 3 sources checkedProbably not a substantial claim. The paper’s conclusions may not depend on it.
Study design and conductown dataIn the batch partition method, analysis of the hexadecane phase confirmed 93-112% of the expected concentrations present in hexadecane.
“The hexadecane phase was also analyzed with LC/MS or GC/MS, which confirmed 93–112% of the expected concentrations present in hexadecane.”
Quote in the paperNot a substantial claimProbably not a substantial claim.
PremisecitationIt has been reported in the literature that H atoms of halogenated aliphatic compounds can serve as H-bond donor sites (e.g., chloroform, hexachlorocyclohexanes, chlorinated paraffins) and affect the partition properties of the compounds.
“It has been reported in the literature that H atoms of halogenated aliphatic compounds can serve as H-bond donor sites (e.g., chloroform,38 hexachlorocyclohexanes,39 and chlorinated paraffins)40 and affect the partition properties of the compounds.”
Quote in the paperSubstantial claim0 of 3 sources checkedIt may state two separate claims as one.
Interpretationown dataIt is likely that the water phase equilibrated in the batch method contained invisible microdroplets of hexadecane, which increased the apparent concentration of the water phase and decreased the measured apparent KHxd/w.
“It is likely that the water phase equilibrated in the batch method contained invisible microdroplets of hexadecane, which increased the apparent concentration of the water phase and decreased the measured (apparent) KHxd/w.”
Quote in the paperSubstantial claim- The authors state this as an assumption: In the batch partition method, KHxd/w can be calculated from the measured water phase concentration under the mass conservation assumption.
- The authors state this as an assumption: In the calculation reproducing the batch method apparent log KHxd/w values, the water phase contained 0.006 vol % hexadecane droplets and the shared-headspace log KHxd/w is accurate.
It may state two separate claims as one.
Study design and conductreasoningKHxd/w can be measured more reliably than Kaw because gas phase quantification is not needed and hexadecane has a larger solubilization capacity than water, which minimizes the problem of third-phase sorption.
“KHxd/w can be measured more reliably than Kaw because gas phase quantification is not needed and hexadecane has a larger solubilization capacity than water, which minimizes the problem of third-phase sorption.”
Quote in the paperSubstantial claimProbably not a substantial claim.
PremisecitationWater-saturated octanol has been shown to have slightly different properties than pure octanol.
“Additionally, there is some mutual solubility between octanol and water, and water-saturated octanol has been shown to have slightly different properties than pure octanol.22”
Quote in the paperSubstantial claim0 of 1 sources checked- cites [22] source
It may state two separate claims as one.
PremisecitationHexadecane has very little mutual solubility with water and is an ideal solvent to be used for the thermodynamic cycle approach.
“In contrast, hexadecane has very little mutual solubility with water and is an ideal solvent to be used for the thermodynamic cycle approach.23”
Quote in the paperSubstantial claim0 of 1 sources checked- cites [23] source
It may state two separate claims as one.
InterpretationreasoningIn the case of CF2, the increase in cavity formation energy exceeds the increase in interaction energy, and thus CF2 has a substantially larger influence on log Kaw than CH2.
“In the case of CF2, the increase in cavity formation energy exceeds the increase in interaction energy, and thus, CF2 has a substantially larger influence on log Kaw than CH2.”
Quote in the paperNot a substantial claimIt may state two separate claims as one.
InterpretationreasoningThese results suggest that PFASAs and their N-alkyl compounds can be deprotonated in unbuffered water but remain largely neutral in 1 mM HCl.
“These results suggest that PFASAs and their N-alkyl compounds can be deprotonated in unbuffered water but remain largely neutral in 1 mM HCl.”
Quote in the paperSubstantial claimProbably not a substantial claim. The paper’s conclusions may not depend on it.
PremisecitationVarious researchers derived Kaw from the saturated vapor pressure relative to the aqueous solubility (VP/S) to fill the data gap.
“To fill the huge data gap, various researchers derived Kaw from the saturated vapor pressure relative to the aqueous solubility (VP/S).8,9,12,13”
Quote in the paperNot a substantial claim0 of 4 sources checkedIt may state two separate claims as one.
Interpretationown dataSuch a wide range of log Kaw would not be achievable by direct measurement methods alone, demonstrating an advantage of the thermodynamic cycle approach used here.
“Such a wide range would not be achievable by direct measurement methods alone, demonstrating an advantage of the thermodynamic cycle approach used here.”
Quote in the paperSubstantial claimIt may state two separate claims as one.
Interpretationown data4:4, 6:2, 8:2 FTOHs, and 4:2 FTMAC are less water-soluble and less volatile than 3:1, 3:3, and 4:2 FTOHs and are likely more susceptible to experimental artifacts.
“which are less water-soluble and less volatile than the former three compounds and are likely more susceptible to experimental artifacts.”
Quote in the paperSubstantial claimProbably not a substantial claim. It may be too vague to check against evidence.
InterpretationreasoningThe results indicate the importance of experimentally filling data gaps in the chemical domain of environmental interest.
“The results indicate the advantage of a theoretical model over empirical models for a data-poor class like PFAS and the importance of experimentally filling data gaps in the chemical domain of environmental interest.”
Quote in the paperNot a substantial claimIt may state two separate claims as one.
Study design and conductreasoningThe shared-headspace approach does not require direct contact between water and hexadecane, avoiding the possible formation of emulsions or microdroplets in water.
“The shared-headspace approach does not require a direct contact between water and hexadecane, avoiding the possible formation of emulsions or microdroplets in water.”
Quote in the paperNot a substantial claimIt may state two separate claims as one.
Interpretationnone givenThe shared-headspace method appears applicable for compounds with log KHxd/w values higher than 4, provided the test compound is sufficiently volatile and hydrophobic.
“The shared-headspace method appears applicable for compounds with log KHxd/w values higher than 4, although the test compound should be sufficiently volatile and hydrophobic (see the Methods section).”
Quote in the paperSubstantial claimIt may state two separate claims as one.
PremisecitationAs Kaw data are largely missing, environmental transport models inevitably use model-based predictions.
“As Kaw data are largely missing, environmental transport models inevitably use model-based predictions.18,19”
Quote in the paperSubstantial claim0 of 2 sources checkedProbably not a substantial claim. It may state two separate claims as one.
InterpretationreasoningLSER-IFSQSAR has been built on the mechanically based LSER equation and calibrated not only on log Kaw but on many different partition data available, which could explain the better predictions.
“LSER-IFSQSAR has been built on the mechanically based LSER equation and calibrated not only on log Kaw but on many different partition data available, which could explain the better predictions.”
Quote in the paperSubstantial claimProbably not a substantial claim. It may state two separate claims as one.
PremisecitationLampic and Parnis proposed to compute Kow and Koa with OPERA and apply a thermodynamic cycle to calculate Kaw.
“Lampic and Parnis13 proposed to compute Kow and Koa with OPERA and apply a thermodynamic cycle to calculate Kaw.”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Lampic and Parnis13
Couldn’t be matched to a paper in Crossref, so it can’t be checked.
Probably not a substantial claim. The paper’s conclusions may not depend on it.
PremisecitationThis minor but consistent bias is similar to the prediction for log KHxd/air, reported in the authors' previous paper.
“This minor but consistent bias is similar to the prediction for log KHxd/air, reported in our previous paper.21”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites [21] source
Probably not a substantial claim.
InterpretationreasoningThe low performance of empirical models might be no surprise, as there have not been sufficient calibration data for PFAS.
“The low performance might be no surprise, as there have not been sufficient calibration data for PFAS.”
Quote in the paperSubstantial claim- Rests on an assumption: There have not been sufficient calibration data for PFAS to support empirical models.
Probably not a substantial claim. It may be too vague to check against evidence.
InterpretationreasoningOPERA needs far more Kaw data for PFAS that could serve as useful nearest neighbors.
“OPERA needs far more Kaw data for PFAS that could serve as useful nearest neighbors.”
Quote in the paperNot a substantial claim
The authors state this as an assumption.
In the batch partition method, KHxd/w can be calculated from the measured water phase concentration under the mass conservation assumption.
“KHxd/w was calculated using the measured water phase concentration under the mass conservation assumption.”
The authors state this as an assumption.
In the modified VPR-HS method, GC/MS response linearity and no significant mass of the compound in the headspace or any other interface can be assumed.
“Here, response linearity as well as no significant mass of the compound in the headspace and any other interface (e.g., vial wall, hexadecane/water interface, septum) was assumed.”
The authors state this as an assumption.
In the calculation reproducing the batch method apparent log KHxd/w values, the water phase contained 0.006 vol % hexadecane droplets and the shared-headspace log KHxd/w is accurate.
“Indeed, if we assume that the water phase contained 0.006 vol % hexadecane droplets and that log KHxd/w determined by the shared-headspace method is accurate, we can reproduce the “apparent log KHxd/w” values measured by the batch method (see SI-4 for the calculation).”
The authors state this as an assumption.
The modified VPR-HS method assumes that the relationship between PA and VHxd expected by eq 2 holds.
“which violates the assumption of eq 2”
The paper relies on this without support.
There have not been sufficient calibration data for PFAS to support empirical models.
“The low performance might be no surprise, as there have not been sufficient calibration data for PFAS.”
The paper relies on this without support.
PFAS are an extremely data-poor class of chemicals.
“Arguably, for an extremely data-poor class of chemicals such as PFAS”
Claims other papers make about this one
Each is checked against this paper’s own text: does it say what the citing paper says it does?
The electron-withdrawing F atoms make the H atom at the terminal methyl group of 6:1 FTI-7H significantly H-bond donating.
“The electron-withdrawing F atoms were thought to make the H atom at the terminal methyl group of 6:1 FTI-7H significantly H-bond donating.5”
From Intermolecular Interactions, Solute Descriptors, and Partition Properties of Neutral Per- and Polyfluoroalkyl Substances (PFAS) (2023) · cites this paper as [5]
Reliable air/water partition coefficients (Kaw) were available for only a few neutral PFAS with (CF2)≥4 until our group recently reported experimental values for 21 PFAS.
“reliable air/water partition coefficients (Kaw) were available for only a few neutral PFAS with (CF2)≥4 until our group recently reported experimental values for 21 PFAS.”
From Intermolecular Interactions, Solute Descriptors, and Partition Properties of Neutral Per- and Polyfluoroalkyl Substances (PFAS) (2023) · cites this paper as [5]
The current version of COSMOtherm likely slightly overestimates the partitioning of PFAS from air to liquid phases.
“It is likely that the current version of COSMOtherm slightly overestimates the partitioning of PFAS from air to liquid phases, as reported in previous articles.5,18”
From Intermolecular Interactions, Solute Descriptors, and Partition Properties of Neutral Per- and Polyfluoroalkyl Substances (PFAS) (2023) · cites this paper as [5]
Kaw for 21 PFAS in ref (5) was not considered here to avoid duplicate data use, as these data were derived from KHxd/w and KHxd/air through the thermodynamic cycle calculation.
“Note that Kaw for 21 PFAS in ref (5) was not considered here to avoid duplicate data use, as these data were derived from KHxd/w and KHxd/air through the thermodynamic cycle calculation.”
From Intermolecular Interactions, Solute Descriptors, and Partition Properties of Neutral Per- and Polyfluoroalkyl Substances (PFAS) (2023) · cites this paper as [5]
In a previous article, it was shown that 6:1 FTI-7H has a relatively high affinity for water compared to 6:1 FTI.
“In a previous article,5 it was shown that 1H,1H,7H-perfluoroheptyl iodide (6:1 FTI-7H) has a relatively high affinity for water compared to 6:1 FTI.”
From Intermolecular Interactions, Solute Descriptors, and Partition Properties of Neutral Per- and Polyfluoroalkyl Substances (PFAS) (2023) · cites this paper as [5]
PFASAs and FASAs are weak acids and may ionize in water and partition more into water.
“Note that PFASAs and FASAs are weak acids and may ionize in water and partition more into water,5 which is not accounted for in the plot.”
From Intermolecular Interactions, Solute Descriptors, and Partition Properties of Neutral Per- and Polyfluoroalkyl Substances (PFAS) (2023) · cites this paper as [5]
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Metadata
- Authors
- Endo, S., Hammer, J., & Matsuzawa, S.
- Affiliations
- National Institute for Environmental Studies (JP)
- Corresponding author
- Endo, Satoshi
- Venue
- Environmental Science & Technology
- Publisher
- American Chemical Society (ACS)
- Published
- 2023-05-26
- Volume
- 57
- Issue
- 22
- Pages
- 8406-8413
- ISSN
- 0013-936X (ISSN-L)
- 1520-5851
- PMC
- PMC10249623
- OpenAlex
- W4378347347
- Semantic Scholar
- 1d576666d333c3788b63db61494ff164d4241132
- Cited by
- 54
- References
- 41
- Citations by year
- 2026: 19 · 2025: 14 · 2024: 18 · 2023: 3
- FWCI
- 4.10
- Type
- journal article
- Language
- en
- Open access
- hybrid
- doi.org
- Locations
- Environmental Science & Technology (publishedVersion, cc-by)
- PubMed Central (publishedVersion, cc-by)
- PubMed (publishedVersion)
- Funders
- Environmental Restoration and Conservation Agency (JPMEERF20213002)
- Topics
- Physical Sciences › Environmental Science › Environmental Chemistry › Per- and polyfluoroalkyl substances research
- Physical Sciences › Environmental Science › Health, Toxicology and Mutagenesis › Toxic Organic Pollutants Impact
- Physical Sciences › Chemistry › Organic Chemistry › Fluorine in Organic Chemistry
- Keywords
- per- and polyfluoroalkyl substances, PFAS, air-water partition coefficient, linear solvation energy relationships, property prediction models, data gaps
- MeSH terms
- Air, Alkanes, Fluorocarbons, Water
About these checks
Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.
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