Summarizing the results above, we have demonstrated that ln γ ± can be represented by a multipole expansion of the interaction between the central observation droplet and the surrounding solution (see Equ. 3). Each expansion exponent, λ l , reflects the combination of the thermally averaged interaction between the multipole inside the observation droplet and the charge distribution in its environment. The interaction spatially decays as 1/n 3λ l +1 R . The dipole-dipole interaction term (l = 1 in Equ. 3) compares well with the simplest form of the Debye-Hückel law (ln γ ± ∝ m 1/2 B ) for dilute solutions where x ∝ m B when choosing λ Dipole = 0.5 (see Fig. ).