Exploring organic compound interactions with organic matter: The thermodynamic cycle approach

Specifically we demonstrate that: (i) specifically interacting compounds have an essentially greater ability to undergo interactions with hydrated soil organic matter as compared with non-specifically interacting compounds of the same molar refraction; (ii) in many cases, greater interaction ability is correlated with compound ability to undergo hydrogen bond formation with the hydrated soil organic matter phase; (iii) interactions of solute molecules with the bulk aqueous phase compensate for specific interactions between sorbate molecules and the soil organic matter phase, resulting in 'apparent' hydrophobic partitioning behavior of specifically-interacting solutes that masks compound specific interactions with soil organic matter; (iv) the overall trend in increasing strength of compound interactions with OM is as follows: aromatic and Cl-substituted aromatic hydrocarbons ≤ aliphatic hydrocarbons and Cl-substituted aliphatic hydrocarbons ≈ anisole ≈ o-Cl-nitrobenzene < nitrobenzene < acetophenone < 2,4-di-Cl-phenol ≈ atrazine ≈ benzyl alcohol < pyridine < phenol < m-nitrophenol; (v) only those compounds that interact most strongly with OM exhibit significant sorption isotherm nonlinearity; (vi) other commonly used standard states such as pure compound (liquid) state and solution in n-octanol are considerably less useful in deciphering sorption mechanisms; (vii) strength of compound interaction with hydrated OM is correlated with the effect of OM hydration on compound sorption. Thus, using a single inert reference state (either gas phase or inert solvent), is shown to be valuable for delineating the differences in organic compound interactions with OM.