How can I ensure Discrete locations for ions in a nanoscale ion-transport system ?

I'm using the Transport of dilute species module, and have coupled this with the Poission-Boltzmann equation (with Steric accomodations) to model transport of Sodium ions and Cholride ions in a nanotube with a charged wall. I'm getting a rise in concentration of oppositely charged ions at the wall surface, and a corresponding dip for likely charged ions, but this doesn't go to 0 mol/m^3 . Considering the dimensions of the system, I believe I should expect the Likely charged ions to not be present at the wall's surface, as it's physically impossible for both kinds of ions to be present at the same location. How can I ensure this ? Or is my assumption inherently wrong ? I have attached an image that has concentration plots for the ionic species. Please note that I have used an Axisymmetric 2D geometric model.