Size Selectivity of Narrow Pores

The selectivity of micropores and ion channels is examined for simple pore topologies within the framework of density functional theory of highly confined fluids. In an infinite cylindrical pore purely steric (excluded volume) effects are shown to lead to strong, nontrivial size selectivity, which is highly sensitive to the pore radius. A crude modeling of electrostatic effects does not alter the relative absorbance of ${\mathrm{Na}}^{+}$ and ${\mathrm{K}}^{+}$ ions in a significant way.