Prediction of the Standard Gibbs Energy of Transfer of Organic Ions Across the Interface between Two Immiscible Liquids

The non-Bomian solvation model was applied for evaluation of the standard Gibbs energy (Delta G(tr)(o,W -> O)) of transfer of organic ions from water (W) to organic solvent (O = nitrobenzene). The solvation energy of an ion in either W or O is basically formulated as the energy required for the formation of a nanosized ion-solvent interface around the ion; however, many organic ions with strongly charged groups (e.g., -SO3-, -CO2-, NH3+) are preferentially hydrated in O. Here we divided the surface of an ion into hydrated and non-hydrated surfaces and then carried out regression analyses with experimental values of Delta G(tr)(o,W -> O). In the analyses, the local electric field on the surface of an organic ion was evaluated through density functional theory calculation. Good regression results were then obtained with the mean absolute error of 1.9 and 2.4 kJ mol(-1) for 34 anions and 63 cations, respectively. These errors correspond to the error of similar to 20 mV in the standard ion-transfer potential (Delta(W)(O)phi degrees), being only two times larger than the typical experimental error (similar to 10 mV) in the voltammetric measurement. This non-Bomian model is promising for theoretical prediction of Delta G(tr)(o,W -> O)' (or Delta(W)(O)phi degrees) for organic ions and possibly of the biomembrane permeability for ionic drugs.