  A Poisson-Fermi model is proposed for calculating activity coefficients of
single ions in strong electrolyte solutions based on the experimental Born
radii and hydration shells of ions in aqueous solutions. The steric effect of
water molecules and interstitial voids in the first and second hydration shells
play an important role in our model. The screening and polarization effects of
water are also included in the model that can thus describe spatial variations
of dielectric permittivity, water density, void volume, and ionic
concentration. The activity coefficients obtained by the Poisson-Fermi model
with only one adjustable parameter are shown to agree with experimental data,
which vary nonmonotonically with salt concentrations.
