  We revisit the role of the local solvent structure on the activity
coefficient of electrolytes with a general non-local dielectric function
approach. We treat the concentrated electrolyte as a dielectric medium and
suggest an interpolated formula for the dielectric response. The pure water
limit is calibrated based on MD simulations and experimental data. Solving our
model around a central ion, we find strong over-screening and oscillations in
the potential, which are absent in the standard "primitive model" predictions.
We obtain mathematically tractable closed-form expressions for the activity
coefficients and show reasonable agreement with experimental data.
