Ion-Partitioning between Charged Capillaries and Bulk Electrolyte Solution:  An Example of Negative “Rejection”

The equilibrium partitioning of a primitive model +3:−3 electrolyte between a charged cylindrical capillary and the bulk solution was studied using the grand canonical Monte Carlo method. The ions were considered to be charged hard spheres embedded in a dielectric continuum with the fixed charge smeared on the hard inner walls of the capillary. The electrolyte in a micropore was assumed to be in equilibrium with the external bulk electrolyte of the same chemical composition. Simulation results are presented for several charge densities of the fixed charge and for different concentrations of the external electrolyte. Under certain conditions, that is, for moderate to high charge densities on the inner walls and at low enough concentration of the external electrolyte, we obtained an increase of the electrolyte concentration within the microcapillary, and not a decrease as expected on the basis of the traditional electrical double-layer theory. This “negative rejection” seems to be a consequence of the strong correlation between multivalent ions.