  Ion-selective channels play a key role in physiological processes and are
used in many technologies. While biological channels can efficiently separate
same-charge ions with similar hydration shells, it remains a challenge to mimic
such exquisite selectivity using artificial solid-state channels. Although,
there are several nanoporous membranes that show high selectivity with respect
to certain ions, the underlying mechanisms are based on the hydrated ion size
and/or charge. There is a need to rationalize the design of artificial channels
to make them capable of selecting between similar-size same-charge ions, which
in turn requires understanding of why and how such selectivity can occur. To
address this issue, we study angstrom-scale artificial channels made by van der
Waals assembly, which are comparable in size with typical ions and carry little
residual charge on channel walls. This allows us to exclude the first-order
effects of steric and Coulomb-based exclusion. We show that the studied
two-dimensional angstrom-scale capillaries can distinguish between same-charge
ions with similar hydrated diameters. The selectivity is attributed to
different positions occupied by ions within the layered structure of
nanoconfined water, which depend on the ion-core size and differ for anions and
cations. The revealed mechanism points at possibilities of ion separation
beyond the simple steric sieving.
