Citation:

Hanson, K., D. Hoff, T. Lahren, Dave Mount, A. Squillace, AND L. Burkhard. Estimating n-octanol-water partition coefficients for neutral highly hydrophobic chemicals using measured n-butanol-water partition coefficients. CHEMOSPHERE. Elsevier Science Ltd, New York, NY, 218:616-623, (2019). https://doi.org/10.1016/j.chemosphere.2018.11.141

Impact/Purpose:

Measurement of the n-octanol-water partition coefficients (KOW) for methodologically challenging chemicals (MCCs) that are highly hydrophobic, i.e., log KOW greater than 10, is analytically extremely difficult and resulting in limited/no measured values for these MCCs. We have demonstrated that n-butanol-water partition coefficients (KBW) can be easily measured for MCCs. Collander (1951) and others have demonstrated that KOW and KBW are linearly correlated, and we have developed a Collander relationship to estimate KOWs of MCCs. KOWs estimated using measured KBWs are lower than estimates from QSAR calculators for KOW, and measured KOWs for highly hydrophobic MCCs are much lower than predicted using current QSAR calculators. Lowering uncertainties in KOWs for MCCs will result in improved assessments of MCCs environmental behavior and bioaccumulation in aquatic food webs.

Description:

Direct measurement of the n-octanol partition coefficients (KOW) of many highly hydrophobic organic chemicals is extremely difficult because of the vanishingly small concentrations present in the water phase. n-Butanol/water partition coefficients (KBW) are generally much lower than KOW due to the increased solubility of solute in the alcohol saturated aqueous phase, and therefore become easier to measure. We measured the KBW for 25 nonionic organic chemicals having measured log KOWs ranging from 2 to 9 and 4 additional highly hydrophobic chemicals having estimated log KOWs ranging from 6 to 18. When relating KBW-KOW using a Collander relationship, measured log KBW and log KOW values were tightly correlated with an r2= 0.978. The regression developed from data within this study was used to extrapolate KOW estimations for the 4 highly hydrophobic chemicals; the resulting predictions were orders of magnitude lower than those predicted by a variety of computational models. The extended log KBW-log KOW relationship enables an accurate estimation of KOWs for highly hydrophobic chemicals where direct measurement is nearly impossible.

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