The mean activity coefficient expresses the excess chemical potential of the electrolyte in units of RT . Activity coefficients, γ ± , as function of molality, m B , are reported for a large number of salts in two books by Lobo. To describe γ ± based on a microscopic picture, our idea is to split the total electrolyte system into a spherical "observation" droplet (d) with radius, R d , and the remaining environment (e). The excess chemical potential is hence a function of the interaction free energy, U de , between the two (see Figure ). We choose the observation volume to contain exactly one electrolyte unit (ν + cations and ν -anions) and the stoichiometric amount of water (n w = x w /x B ) where x B and x w are the electrolyte and water mol fractions, respectively. This definition is uniquely determined by the molality of the solution and its density. The spherical shape is adopted due to the isotropic nature of bulk electrolyte solutions. It is important to note that our observation droplet is a purely statistical entity, defined to be charge neutral and containing on average the smallest possible, stoichiometric number of water and ions. Hence, its size, charge and average composition is totally unrelated to the microscopic ion configurations that can be observed e.g. from simulations or diffraction experiments. The heterogeneity in the distribution of such configurations contributes, in our approach, to the instantaneous charge distribution inside the observation droplet, due to both, ions and water. It can be complex depending on the electrolyte composition and concentration, for example for solutions of strongly interacting ions where long-range ion-ion correlations and ion-clustering Figure : Schematics of the coarse grained approach used to determine the interaction energy in aqueous electrolyte solutions: The solution is separated into a uniquely defined central "observation volume" with radius, R d (black line) and its environment. All distances R = n R R h are measured in units of the hydrated electrolyte radius, R h (dashed circle at the center) so that n d = R d /R h identifies the droplet radius in this coordinate system. In a hypothetical solution without ionion interactions, the charge distribution inside and outside the droplet is uniform. When switching on ion-ion interactions, the solution averaged charge distribution inside the central droplet produces an electrostatic potential outside the droplet which attracts charges of the opposite sign. At high electrolyte concentration R d decreases, ion-ion interactions become more complex and the interaction energy increases. become important.