Thermodynamics of organic compound transfer from the gas phase to environmentally important sorbents

Linear Free Energy Relationships (LFERs) based on distribution coefficients between the aqueous phase and the sorbent are often unsuccessful for interpreting sorption mechanisms in environmentally important sorbents because they comprise contributions not only from solute interactions with the sorbent, but also from hydration of the solute in bulk water. To avoid this problem, we suggest an approach for studying interactions between organic sorbates and environmentally important sorbents using a thermodynamic cycle to eliminate interactions between the solute and bulk water. In this approach, aqueous distribution coefficients are converted to gas phase distribution coefficients that are related to the free energy of transfer from the gas phase to the hydrated sorbent, and include contributions from all interactions occurring during the transfer. Gas phase distribution coefficients are compared for different compounds with similar molar refraction values to give insights into the nature of specific and nonspecific interactions of organic molecules with environmental sorbents. This methodology is illustrated for sorption of organic molecules in soil organic carbon and at mineral surfaces, and examples of insights obtainable from this methodology are given.