  The Debye-H\" uckel formula for ionic activity coefficients is extended for
concentrated solutions by solving a simple model of many-body Coulomb
correlations and adding the Born solvation energy. Given the bulk permittivity,
our formula is able to fit activity data for diverse electrolytes with only one
parameter to adjust the correlation length, which interpolates between the
Bjerrum length and the mean ion spacing. The results show that ionic activity
in most electrolytes is dominated by three types of electrostatic forces: (i)
mean-field charge screening, (ii) solvation, and (iii) Coulomb correlations,
both "over-screening" (charge oscillations) and "under-screening" (extending
beyond the Debye screening length).

Chemical Physics;Statistical Mechanics