Volatility and Nonspecific van der Waals Interaction Properties of Per- and Polyfluoroalkyl Substances (PFAS): Evaluation Using Hexadecane/Air Partition Coefficients

Abstract

Per-and polyfluoroalkyl substances (PFAS) form weak van der Waals (vdW) interactions, which render this class of chemicals more volatile than nonfluorinated analogues. Here, the hexadecane/air partition coefficient (K Hxd/air ) values at 25 °C, as an index of vdW interaction strength and volatility, were determined for 64 neutral PFAS using the variable phase ratio headspace and gas chromatographic retention methods. Log K Hxd/air values increased linearly with increasing number of CF 2 units, and the increase in log K Hxd/air value per CF 2 was smaller than that per CH 2 . Comparison of PFAS sharing the same perfluoroalkyl chain length but with different functional groups demonstrated that K Hxd/air was highest for the N-alkyl perfluoroalkanesulfonamidethanols and lowest for the perfluoroalkanes and that the size of the nonfluorinated structure determines the difference in K Hxd/air between PFAS groups. Two models, the quantum chemistry-based COSMOtherm model and an iterative fragment selection quantitative structure− property relationship (IFS-QSPR) model, accurately predicted the log K Hxd/air values of the PFAS with root-mean-square errors of 0.55 and 0.35, respectively. COSMOtherm showed minor systematic errors for all PFAS, whereas IFS-QSPR exhibited large errors for a few PFAS groups that were outside the model applicability domain. The present data set will be useful as a benchmark of the volatilities of the various PFAS and for predicting other partition coefficient values of PFAS.

Experimental Determination of Air/Water Partition Coefficients for 21 Per- and Polyfluoroalkyl Substances Reveals Variable Performance of Property Prediction Models

“…K Hxd/w can be measured more reliably than K aw because gas phase quantification is not needed and hexadecane has a larger solubilization capacity than water, which minimizes the problem of third-phase sorption. Experimental K Hxd/air values for 61 PFAS have already been reported in our previous study . A similar approach could be taken with the octanol/air/water cycle.…”

Evaluating Neutral PFAS for Potential Dermal Absorption from the Gas Phase

“…Estimate the dermal-to-inhalation ( D / I ) exposure ratio D / I = k normalp normal_ normalg · C air · BSA Q normalb · C air = k normalp normal_ normalg · BSA Q normalb where BSA is the body surface area [m 2 ] and Q b is the breathing rate (m 3 /h), which (for adults) have a nominal ratio of 4 [h/m] in the Weschler and Nazaroff protocol. Note that 12 assumes that the airborne chemical is overwhelmingly in the vapor phase, which is true for neutral PFAS, , as opposed to sorbed on particulate. This assumption generally holds for compounds with log K oa below 10.…”

“…In the Weschler and Nazaroff protocol, B is estimated using the method of Cleek and Bunge here k p_cw has units of [cm/h]. Estimate the overall air-to-skin permeability coefficient ( k p_b ) from k p_w and K aw Estimate the transdermal permeability coefficient [m/h] from air to skin ( k p_g )­ where v d is an air-to-skin deposition rate and is assumed to be 6 m/h in the Weschler and Nazaroff protocol. Estimate the dermal-to-inhalation ( D / I ) exposure ratio where BSA is the body surface area [m 2 ] and Q b is the breathing rate (m 3 /h), which (for adults) have a nominal ratio of 4 [h/m] in the Weschler and Nazaroff protocol. Note that 12 assumes that the airborne chemical is overwhelmingly in the vapor phase, which is true for neutral PFAS, , as opposed to sorbed on particulate. This assumption generally holds for compounds with log K oa below 10.…”

“…Estimate the dermal-to-inhalation ( D / I ) exposure ratio where BSA is the body surface area [m 2 ] and Q b is the breathing rate (m 3 /h), which (for adults) have a nominal ratio of 4 [h/m] in the Weschler and Nazaroff protocol. Note that 12 assumes that the airborne chemical is overwhelmingly in the vapor phase, which is true for neutral PFAS, , as opposed to sorbed on particulate. This assumption generally holds for compounds with log K oa below 10.…”

“…Prior dermal absorption experiments indicate limited dermal absorption of ionic PFAS, such as perfluoroalkyl carboxylates, − from aqueous solution, while dermal absorption of neutral PFAS from liquid vehicles or from the gas phase has not been addressed. Neutral PFAS, often referred to as volatile or semivolatile PFAS due to their volatility, − are also listed in a U.S. Environmental Protection Agency (EPA) list of PFAS, yet to the best of our knowledge; there are no data on the occurrence of many of these neutral PFAS in consumer products, clothing, or indoor air. In the absence of data, it is important to use modeling approaches to identify specific PFAS with properties that make them potentially amenable to dermal uptake.…”