Predicting the Partitioning Behavior of Various Highly Fluorinated Compounds

Abstract

Due to their high degree of fluorination, highly fluorinated compounds (HFCs) have unique substance properties that differ from many other organic contaminants. To predict the environmental behavior of HFCs, models that predict both absorptive and adsorptive partitioning are needed; however, the accuracy of existing models has not heretofore been thoroughly investigated for these compounds. This report has two parts: first we show that a well-established polyparameter linear free energy relationship used to predict experimental adsorption constants underestimates values for HFCs by several orders of magnitude. We found a mechanistic explanation for the model's inaccuracy and adjusted it accordingly. In the second part of this report, we evaluate various models that predict saturated subcooled liquid vapor pressure (pL*), air-water partition constant (Kaw), and the octanol-water partition constant (Kow) based on molecular structure. These parameters are typically required for general environmental fate and transport models. Here, we found that SPARC and COSMOtherm make predictions usually within 1 order of magnitude of the experimental value, while the commonly used EPI SUITE and ClogP perform more inaccurately. The least accurate predictions occurred with ClogP for the fluorotelomer alcohols, where the estimated values were off by 2 to almost 5 orders of magnitude.

Experimental hydrophobicity parameters of perfluorinated alkylated substances from reversed-phase high-performance liquid chromatography

“…The contribution of each CF 2 group to the increase in hydrophobicity is therefore equal to 0.66, a value that is close to the value of 0.61 derived from voltammetric measurements, [5] between values of 0.50 and 0.80 estimated by earlier model calculations (COSMOtherm and SPARC models respectively) [13] and again close to the average value (0.62) calculated recently with COSMOtherm. [14] One should bear in mind though, that such models can probably only properly estimate the partitioning properties of neutral, monomeric species.…”

“…[27] Hydrophobicity Most log K ow values reported for PFASs in the literature have been calculated using partitioning models. Apart from the experimental log P 0 0 values determined using voltammetry by Jing et al, [5] Kelly et al provided an overview table with log K ow values using original data calculated by Arp et al [13] and correcting these because of 'apparent overestimation' (see table S3 of the supporting information of Kelly et al [29] ). Wang et al [14] presented new results obtained with the COSMO-therm software.…”

“…[5] To predict the partitioning behaviour of PFASs, mathematical models have been presented in the literature. [13] These relate distribution coefficients of these pollutants, such as e.g. air-to-water and octanol-to-water partitioning, to physicochemical or molecular properties of PFASs.…”

“…The partition coefficients, in turn, are typically required for general environmental fate and transport models. [2] However, experimental partition coefficients for PFASs are largely missing for the reasons outlined above and therefore mathematical models like EPI Suite, ClogP, SPARC and COSMOtherm [13,14] applied to estimate or calculate these coefficients may suffer from poor calibration and lack of validation. The possibly resulting inaccurate partitioning data could lead to misinterpretations, erroneous risk assessments and incorrect policies to regulate this new generation of pollutants.…”

Using COSMOtherm to predict physicochemical properties of poly- and perfluorinated alkyl substances (PFASs)

“…[20] As a supplement to laboratory determination, studies using computational methods have been conducted to estimate some physicochemical properties for a few PFASs. [2,21] COSMOtherm is a quantum chemistry-based method that requires no specific calibration, and thus is expected to be adaptable to estimating properties for PFASs, for which only few calibration data are available. In support of this view, properties estimated with COSMOtherm in a recent study showed good agreement with the experimental data for a limited number of PFASs.…”