Predicting the Temperature Dependence of the Octanol–Air Partition Ratio: A New Model for Estimating $$\Delta {U^{ \circ}_{\text{OA}}}$$

Abstract

The octanol–air partition ratio (KOA) describes the partitioning of a chemical between air and octanol and is often used to approximate other partitioning phenomena in environmental chemistry (e.g., blood–air, atmospheric particulate matter–air, polyurethane foam-air). Such partitioning processes often occur at environmental temperatures other than 25 °C. Enthalpies $$\Delta {H^{ \circ}_{\text{OA}}}$$ Δ H OA ∘ or internal energies $$\Delta {U^{ \circ}_{\text{OA}}}$$ Δ U OA ∘ of phase transfer are used to express the temperature dependence of the KOA. Existing poly-parameter linear free energy relationships (ppLFERs) for predicting $$\Delta {H^{ \circ}_{\text{OA}}}$$ Δ H OA ∘ were developed using a relatively small dataset. In this work we utilize a recently developed comprehensive KOA database to create and curate a $$\Delta {U^{ \circ}_{\text{OA}}}$$ Δ U OA ∘ dataset containing 195 chemicals and use this dataset in the development of new predictive equations. Using the QSAR development platform QSARINS we evaluate the use of Abraham descriptors, other molecular descriptors, and the log10KOA at 25 °C as variables in different multilinear regression equations for $$\Delta {U^{ \circ}_{\text{OA}}}$$ Δ U OA ∘ . The $$\Delta {U^{ \circ}_{\text{OA}}}$$ Δ U OA ∘ of neutral organic chemicals can be reliably predicted using only the log10KOA (RMSEEXT = 6.86 kJ·mol−1, $${\text{R}^{2} _{\text{adj}}}$$ R adj 2 = 0.94), only the solute’s hydrogen acidity A and the logarithm of the hexadecane–air partition ratio L (RMSEEXT = 7.23 kJ·mol−1, $${\text{R}^{2} _{\text{adj}}}$$ R adj 2 = 0.93), or A and log10KOA (RMSEEXT = 6.76 kJ·mol−1, $${\text{R}^{2} _{\text{adj}}}$$ R adj 2 = 0.95).

Cross-Species Evaluation of Bioaccumulation Thresholds for Air-Breathing Animals

“…A similar observation was noted for Chukar partridge for thresholds derived from G R values measured at low (K OA@25 = 10 4.72 ) and high (K OA@25 = 10 5. 19 ) elevations. Additionally, thresholds calculated for flying birds were higher on average (log K OA@25 = 6.17 ± 0.25) than those calculated for the same species at rest (log K OA@25 = 5.28 ± 0.35).…”

“…In general, reptiles had lower reported G R values compared to those of mammals and birds (Figure S2 and Appendix S2). Because K OA is dependent on temperature, , we calculated K OA@25 from individual G R values reported across a range of different temperatures, which often included several measurements for the same species [ n = 86 observations (Appendix S4)]. Most of the reptile G R measurements [64% ( n = 50)] were reported at T B > 25 °C.…”

“…While the temperature dependence of K OW is generally minor and can be neglected, 18 a temperature adjustment needs to be applied to K OA . Here we applied a temperature correction according to Baskaran et al 19 (Text S1).…”

Expanding the Applicability Range of the Generator Column Technique to Measure the Octanol–Air Partition Ratio of Volatile Compounds

“…For example, the reported K OA values for fluorotelomer alcohols by Goss et al 12 and Thuens et al 13 are inconsistent with each other, and regressions on the reported log K OA values against reciprocal temperature show poor linearity. 15 These measurements were presumably made with the plateau method and involved large volumes of air passing through the generator column. While it is assumed that the octanol concentrations at the end of the column during these experiments remain constant, 13,14 the octanol concentration was not measured after column preparation, and it is possible that the octanol concentrations within that column may have declined significantly during the measurements.…”

“…27,28 The ΔU°O A value was estimated from the estimated K OA at 25 °C using a simple correlation equation. 15…”