Structure of primitive electrolytes in semi-permeable shells

Abstract

We have theoretically studied the competing adsorption of the two- and three-component electrolytes with different sizes and ionic valences near a semi-permeable shell. The present theory provides interesting insights about the steric charge separation occurring at the shell, which cannot be explained by the linearized and nonlinear Poisson-Boltzmann theories. The smaller and divalent ions interact more strongly with the shell than the larger and monovalent ions because the smaller ions can approach more closely to the shell surface and the divalent ions interact more strongly with the charged shell than the monovalent ions. The large steric charge separation occurs at a highly asymmetric electrolyte and a high bulk concentration. The competition between the cations and anions gives rise to the charge reversal and charge inversion even for an uncharged shell. A positively charged surface enhances the charge surface amplification because the shell is impermeable for the anion, whereas a negatively charged surface enhances the charge reversal. The excess osmotic pressure, which is based on a nonlinear Poisson-Boltzmann theory, depends only on the permeated ion density at the shell, and increases with increasing the shell size and surface charge density.