Estimating n-octanol-water partition coefficients for neutral highly hydrophobic chemicals using measured n-butanol-water partition coefficients.

Abstract Direct measurement of the n -octanol partition coefficients (K OW) for highly hydrophobic organic chemicals is extremely difficult because of the extremely low concentrations present in the water phase. n -Butanol/water partition coefficients (K BW) are generally much lower than K OW due to the increased solubility of solute in the alcohol saturated aqueous phase, and therefore become easier to measure. We measured the K BW for 25 neutral organic chemicals having measured log K OW s ranging from 2 to 9 and 4 additional highly hydrophobic chemicals, with unmeasured K OW s, having estimated log K OW s ranging from 6 to 18. The measured log K BW and log K OW values were linearly related, r2 = 0.978, and using the regression developed from the data, K OW s were predicted for the 4 highly hydrophobic chemicals with unmeasured K OW s. The resulting predictions were orders of magnitude lower than those predicted by a variety of computational models and suggests the estimates of K OW in the literature for highly hydrophobic chemicals (i.e., log K OW greater than 10) are likely incorrect by several orders of magnitude. Highlights • n -Butanol/water partition coefficients are measurable for hydrophobic chemicals. • Plot of log K OW vs log K BW is linear, r2 = 0.98, for K OW s extending into 109 range. • K OW s were estimated using K OW -K BW relationship and measured K BW s. • K OW s estimated for highly hydrophobic chemicals are lower than their QSAR estimates. [ABSTRACT FROM AUTHOR]