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. Activity coefficients for some ionic liquids were taken from articles by Sadeghi and Shekaari . 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. The spherical shape is adopted due to the isotropic nature of bulk electrolyte solutions.