Ion-water interaction potentials in the semimicroscopic model

摘要

The semimicroscopic model separates the electronic and reorientational components of the solvent dielectric response. The former is accounted for by embedding the computational system in an electrical background that describes the high-frequency (optical) contributions to the dielectric constant (epsilon approx 2). The latter is treated by means of the force field. We use this decomposition to describe the solvation of simple monovalent ions in water. The free energy of solvation is composed of two terms: one Born-like, describing the transfer of the ion from vacuum to the electrical background and the other describing the shielding free energy due to interaction between the ion and the surfounding solvent. We employ a highly simplified model: ions are hard spheres with crystal radii and water is a hard sphere with an embedded (gas-phase) multipole. Absolute ionic hydration energies cannot be directly measured. They hinge on the value accepted for the absolute potential of the standard hydrogen electrode. Using the most recent determination [M. D. Tissandier et al., J. Phys. Chem. A 102, 7787 (1998)] our description reproduces experimental hydration free energies using intuitively reasonable geometric parameters to describe water and the ions.