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

The non-Bornian solvation
model was applied for evaluation of the
standard Gibbs energy (ΔGtr°,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 ΔGtr°,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 ∼20 mV in the standard ion-transfer
potential (ΔOWϕ°), being only two times larger than the typical
experimental error (∼10 mV) in the voltammetric measurement.
This non-Bornian model is promising for theoretical prediction of
ΔGtr°,W→O (or ΔOWϕ°) for organic ions
and possibly of the biomembrane permeability for ionic drugs.