Estimating the Organic Carbon Partition Coefficient and Its Variability for Hydrophobic Chemicals

Abstract

Numerous correlations have been developed between the organic carbon/water partition coefficient K OC and various molecular properties and descriptors, but most notably the octanol/water partition coefficient K OW and water solubility. From an analysis of the theory underlying in this partitioning and an examination of the existing database, it is suggested that the preferred approach is to correlate the quantity log( K OC / K OW ), which is essentially the ratio of activity coefficients in octanol and organic carbon, with a molecular property or with log K OW rather than log K OC with log K OW . Such an approach is presented for estimating K OC for hydrophobic chemicals, including an expression of uncertainty limits, based on a correlation derived between log( K OC / K OW ) and log K OW . In its simplest form the correlation is that K OC = 0.35 K OW subject to variation by a factor of 2.5 in either direction. It is suggested that new experimental data be assessed for consistency and achievement of true equilibrium by comparison with this correlation.