Determining octanol-water partition coefficients for extremely hydrophobic chemicals by combining 'slow stirring' and solid phase micro extraction

Octanol-water partition coefficients (Kow ) are widely used in fate and effects modelling of chemicals. Still, high quality experimental Kow data are scarce, in particular for very hydrophobic chemicals. This hampers reliable assessments of several fate and effect parameters and the development and validation of new models. One reason for the limited availability of experimental values may relate to the challenging nature of Kow measurements. In the present study, Kow values for 13 polycyclic aromatic hydrocarbons (PAHs) were determined with the gold standard 'slow stirring' method (logKow 4.6-7.2). These values were then used as reference data for the development of an alternative method for measuring Kow . This approach combined slow stirring and equilibrium sampling of the extremely low aqueous concentrations with polydimethylsiloxane-coated solid phase micro extraction (SPME) fibers, applying experimentally-determined fiber-water partition coefficients. It resulted in Kow values matching the slow stirring data very well. Therefore, the method was subsequently applied to a series of 17 moderately to extremely hydrophobic petrochemical compounds. The obtained Kow values spanned almost 6 orders of magnitude, with the highest value measuring 10(10.6) . The present study thereby demonstrates that the hydrophobicity domain within which experimental Kow measurements are possible can be extended with the help of SPME; and that experimentally-determined Kow values can exceed the proposed upper limit of 10(9) . This article is protected by copyright. All rights reserved.