The differential Gibbs free energy of sorption of an ionizable organic compound: eliminating the contribution of solute–bulk solvent interactions

Sorption of ionizable organic compounds (IOCs) from solutions is of great significance in multiple processes but its understanding is complicated by IOC speciation. Experimental separation of contributions of differently ionized species to the overall IOC sorption is not always feasible, due to the possibility of sorbent changes/decomposition upon varying pH. Even when such a separation is successful, accounting of ionized solute–bulk solvent interactions is not obvious. Therefore, actual interactions in a sorbed phase are “masked” by solute–bulk solvent interactions, thus complicating relating IOC sorption interactions to a compound structure. A new thermodynamic concept is proposed which reformulates the differential Gibbs free energy of IOC sorption in such a way that the initial thermodynamic state of the sorption process is the solution reference state associated with a non-ionized form of IOC. Hence, IOC ion–bulk solvent interactions are excluded from consideration. These redefined differential Gibbs free energies of sorption do not require separating the whole IOC sorption into the contributions of different species; however, if such a separation is possible, it converts the overall sorption process into the two quantified steps: (1) neutral IOC sorption and (2) turning on the sorbed IOC ion–sorbent interactions. The suggested definition of the sorption process makes also possible the further elimination (or minimizing) of IOC–bulk solvent interactions, by converting the molecular IOC solution reference state to another molecular reference state in an inert medium (i.e., the gas phase or inert solvents). Finally, the differential Gibbs free energies of sorption “cleaned” of IOC–bulk solvent interactions can be examined in terms of organic sorbate structure effects, with the focus on the interactions in a sorbed state. The concept is illustrated by the experimental data on soil sorption of IOCs from aqueous solutions.