One of the more important chemical differences between PAHs is their solubility in water, which can be expressed in terms of hydrophobicity. In general, PAHs are more hydrophobic as molecular weight and alkylation increase. The most common way to express hydrophobicity is with the octanol-water partition coefficient (Kow), which can be obtained from many sources. The Kow is the dominant physical parameter that explains a substantial amount of the partitioning behavior exhibited by PAHs in the environment, which is a crucial feature for understanding toxicity from exposure to PAHs found in water or sediment. The Kow values for all PAHs vary by approximately 4 orders of magnitude («10 000-fold).
Another useful partition coefficient is the ratio between PAH concentrations in aquatic sediment or soil organic carbon and water (Koc), which is determined by the concentration of PAH per gram of organic carbon in sediment or soil divided by the concentration of PAH in either overlying or interstitial water extracted from the sediment or soil. For example, this coefficient is useful for predicting the amount of waterborne PAH for a given sediment concentration under equilibrium conditions. The Koc for a PAH may be predicted with the Kow because organic carbon behaves similarly to octanol as a partition phase. This partitioning behavior is also similar for the lipid found in organisms. Organic carbon and lipid are hydrophobic phases that PAHs prefer and will accumulate according to these physical-chemical properties (Kow, water solubility, and other factors). Based on these partitioning properties, predictions for the distribution of PAHs between water, sediment, and tissue can be generated, which is very useful for predicting bioaccumulation factors and toxicity values.
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