Local inversion of the mean electrostatic potential, maximum charge reversal, and capacitive compactness of concentrated 1:1 salts: The crucial role of the ionic excluded volume and ion correlations

Abstract

• A local inversion of the electrostatic potential in one and three ionic radii from the macroions’s surface is observed. • If the zeta potential is in the above region, an inversion of the macroion’s mobility could be driven by 1:1 electrolytes. • The capacitive compactness around a macroion immersed in very concentrated indiferent 1:1 salts is studied. Recent restricted primitive model Monte Carlo simulations (Takamichi Terao, Mol. Phys. 119 , e1831634, 2020) have suggested the possibility of observing the phenomenon of charge inversion in highly concentrated monovalent salts, in which ionic specific adsorption is absent. By using the non-linear Poisson-Boltzmann equation supplemented by a hard-spheres contribution, integral equations theory, and Monte Carlo simulations, the associated mean electrostatic potential, maximum charge reversal and capacitive compactness are studied here. The main finding of this study is the observation of a local inversion of the mean electrostatic potential in a region bounded by one and three ionic radii measured from the macroions’s surface. If the zeta potential is located in this region, the above result suggests the possibility of observing an inversion of the macroion’s electrophoretic mobility, driven by 1:1 aqueous electrolytes in the absence of ionic specific adsorption . On the other hand, the maximum charge reversal and the capacitive compactness increases and decreases montonically, respectively, as a function of the ionic volume fraction when the salt concentration and/or the ionic size of the aqueous electrolytes increase.