Many membrane-based ion separation technologies require monovalent cations selectivity. However, available membranes usually show limited specific ion selectivity. In order to provide basic information on the membrane ion selectivity, the confinement effect on narrow pores disrupting the ion hydration is investigated in this work, with the aim to provide a new alternative to classical Cation Exchange Membranes (CEM). Starting from solvated cations structures used as templates and obtained by a quantum approach, Single-Wall Carbon Nanotubes (SWCNT) with ad-hoc designed diameters were chosen to perform in-silico experiments of single-cation permeation through CNT by means of Molecular Dynamics simulations, for which partial charges at nanotube inlets were parametrized through ab initio calculations. Cations' trajectories and energy decomposition analysis suggest that 100 % perm-selectivity towards Na+ with respect to Ca2+ and Mg2+ is virtually attainable with CNT of 1.33 nm diameter. Interestingly, the origin of this behaviour lays on thermodynamics rather than on size exclusion mechanisms. Opposite to polymeric homogeneous CEM, showing higher affinity for multivalent cations, an inverted trend was found for CNT in terms of hydration free energies, where Na+ shows more affinity up to 36 kJ/mol. Finally, total rejection of Cl− was observed in the range of CNT diameters investigated. Display omitted •Hydrated cations DFT-COSMO optimized structures as templates for CNT diameters.•Selectivity of monovalent versus divalent cations: a proof of concept•Single cation 200 ns MD trajectories through 4 nm-long CNT models in water•Na+ 36 kJ/mol affinity vs. Mg2+ and Ca2+ total rejection with CNT 1.33 nm diameter•Total rejection of Cl− anions through zig-zag CNTs 1.33–1.96 nm diameter