A Non-Bornian Analysis of the Gibbs Energy of Ion Hydration

A non-Bornian model for understanding the Gibbs energy of ion hydration (Delta G(hyd)degrees) has been proposed, in which only short-range interactions between an ion and solvent molecules adjacent to the ion are considered, while the long-range electrostatic interaction between the ion and the outer bulk solvent is not explicitly included in the model. As the short-range interactions, Coulomb, polarization, and charge-transfer interactions are taken into account. It has thus been shown that the Delta G(hyd)degrees scaled by the solvent-accessible surface (SAS) area of an ion can be given by a quadratic function of the electric field strength E at the SAS. Regression analyses with the derived analytical equation have been performed for the literature values of Delta G(hyd)degrees for spherical ions, including 85 cations (with the charge number, z = 1 to 4) and 53 anions (with z = -1 and -2); good fitting results have been obtained for either the cations or the anions. An additional regression analysis has been performed for 10 representative cations, using the data set employed by Koehl et al. (J. Phys. Chem. B 2009, 113, 5694). Our non-Bornian model has been found to be one of the highest performance models for the prediction of Delta(hyd)degrees.