Polyparameter linear free energy relationships for estimating the equilibrium partition of organic compounds between water and the natural organic matter in soils and sediments

Values of the organic-carbon-based partition coefficient (K_oc) have often been estimated using one-parameter linear free energy relationships (op-LFERs), which include both correlations between log K_oc and log K_ow, where K_ow is the octanol-water partition coefficient, and op-LFERs that are based on first-order molecular connectivity indices. For chemicals with tendencies toward strong hydrogen-bonding or other specific interactions with the organic phase, however, these methods are not sufficiently accurate. Polyparameter LFERs (pp-LFERs) address these shortcomings by explicitly considering contributions toward free energy change from multiple kinds of molecular interactions with both water and bulk organic phases. This paper reviews pp-LFER theory and presents the development of a new pp-LFER for organic chemical partitioning with soil/sediment organic matter (SOM) using a data set of 356 carefully selected experimental values of log K_oc for 75 chemicals, including apolar, monopolar, and bipolar compounds. The paradigm of representing SOM by a single pp-LFER is qualitatively supported by our results, and the set of coefficients for the regression log K_oc= (1.10 ± 0.10)E - (0.72 ± 0.14)S + (0.15 ± 0.15)A - (1.98 ± 0.14)B + (2.28 ± 0.14)V + (0.14 ± 0.10) represents a proposed set of water - SOM-specific properties for estimating log K _oc. The developed correlation outperformed other currently recommended approaches with the given K_oc data set and also compared favorably against the use of new multiple class-specific op-LFER regressions. Overall, the pp-LFER approach is recommended over other current methods for the purpose of K_oc estimation and especially for polar chemicals.