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 th e grand canonical Monte Carlo method. The ions were considered to be charge d hard spheres embedded in a dielectric continuum with the fixed charge sme ared on the hard inner walls of the capillary. The electrolyte in a micropo re was assumed to be in equilibrium with the external bulk electrolyte of t he same chemical composition. Simulation results are presented for several charge densities of the fixed charge and for different concentrations of th e 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 conce ntration within the microcapillary, and not a decrease as expected on the b asis of the traditional electrical double-layer theory. This "negative reje ction" seems to be a consequence of the strong correlation between multival ent ions.