Classical density functional analysis of ion selectivity in nanopores: The coupling between hard-sphere and electrostatic interactions

The selective adsorption of ions in nanopores is an important subject in various fields. Many interesting phenomena have been observed in experiments but which have not yet been explained by theory. We propose a classical density functional theory (CDFT) to examine selective adsorption. This theory provides a rationale for the extremely high ion selectivity that has been observed in experiments and shows that the coupling mechanism between hard spheres and electrostatic interactions is the dominating factor in many cases. Based on this mechanism, the dependence of selectivity on the ion diameter and dielectric constant are nonmonotonic, the functional relation between selectivity and voltage has a superexponential form, the uptake of the larger ion decreases with voltage in the high-voltage region, and the selectivity increases with the bulk ion density. Moreover, we found that the favorable pore size for ion separation is always 1.7 times that of the smaller ion size despite any other conditions due to the formation of a face-centered cubic (FCC) structure.

Automated summary

Selective adsorption in nanopores is a significant research topic. Recent experiments have observed interesting phenomena, which can be explained by classical density functional theory. The coupling mechanism between hard spheres and electrostatic interactions plays a dominant role in this process.