Ion distributions within a microdroplet without surface charge: Fluctuation-correlation effects

The mean-field approach like the Poisson-Boltzmann theory predicts a trivial result that ions from a symmetric electrolyte are uniformly distributed within an aqueous microdroplet without surface charge. However, the phenomenon of an increase in surface tension due to salt addition indicates ion depletion near the air-water interface. It was believed that electrical polarization associated with dielectric differences is responsible for salt depletion. Using Monte Carlo simulations with the restrictive primitive model and Poisson equation, a depletion zone near the water-oil interface is observed. Nevertheless, without any polarization, ion depletion can still be attained near a hard interface and was theoretically predicted by the Debye-Hückel theory [Phys. Rev. E 60, 3174 (1999)]. The nonuniform ion distribution is caused by the fluctuation-correlation effect with broken symmetry, which is furnished by the boundary.