Violation of the local electroneutrality condition in an inhomogeneous macroions solution

A simple mathematical model for a multivalent macroions solution, next to a charged wall of planar geometry, is solved through a well-established integral equation theory. The macroions structure and the charge induced into the fluid are obtained, as a function to the distance to the electrode. The macroions adsorption to the surface and the induced charge density, both show an atypical structure, not consistent with the predictions of the classical theory of Poisson–Boltzmann. In particular, the induced charge density exhibits an enormous charge overcompensation, localized just next to the electrode, implying a violation of the local electroneutrality condition. A breakdown of the charge neutrality, in confined, charged fluids, has been theoretically predicted in the past, by means of integral equations, density functional approaches, and computer simulations, and recently experimentally reported. However, the results presented here show a charge neutrality breakdown, in unconfined, inhomogeneous fluids. Our results are in qualitative agreement with experimental data for Langmuir films of amphiphilic molecules, in contact with a macroions solution.