Is vapor pressure or the octanol–air partition coefficient a better descriptor of the partitioning between gas phase and organic matter?

Both the sub-cooled liquid vapor pressure (PL) and the octanol–air partition coefficient (KOA) are used to describe the partitioning of non-polar organic compounds between the gas phase and a variety of natural organic substrates in soil, atmospheric particles and foliage. Whether the former is preferable over the latter depends on whether the interaction of the organic compound with the organic matter (OM) resembles more those in the pure liquid than those in liquid octanol.

The activity coefficient in octanol (γOct) is a quantitative measure of the difference between these two interactions. An analysis of PL and KOA values for several sets of non-polar and semi-volatile organic compounds (chlorobenzenes, PCBs, PCNs, PCDD/Fs, PBDEs), and of the γOct values derived from these, reveals that γOct tends to range from 1 to 10 suggesting that PL and KOA are very highly correlated.

Furthermore, the estimated standard deviation of γOct tends to be so large that PL and KOA are virtually indistinguishable within the measurement uncertainty. Whether γOct within a group of related compounds increases, decreases or stays the same with increasing molecular mass depends on the specific KOA and PL data set used in the calculation of γOct.

This implies that with the current precision of KOA, PL and partition coefficients involving OM it is impossible to judge one parameter better than the other.