  We study the selective adsorption of various cations into a layered slit
system using grand canonical Monte Carlo simulations. The slit system is formed
by a series of negatively charged membranes. The electrolyte contains two kinds
of cations with different sizes and valences modelled by charged hard spheres
immersed in a continuum dielectric solvent. We present the results for various
cases depending on the combinations of the properties of the competing cations.
We concentrate on the case when the divalent cations are larger than the
monovalent cations. In this case, size and charge have counterbalancing
effects, which results in interesting selectivity phenomena.
