Beyond steric selectivity of ions using angstrom-scale capillaries

TL;DR

This study demonstrates that angstrom-scale artificial channels can distinguish between same-charge ions with similar hydrated sizes by exploiting differences in ion positioning within nanoconfined water, surpassing traditional steric selectivity mechanisms.

Contribution

The paper introduces a new mechanism for ion selectivity in artificial channels based on ion positioning, not just size or charge, enabling separation of similar ions.

Findings

Angstrom-scale channels can differentiate ions with similar hydrated diameters.

Ion selectivity arises from different positions within nanoconfined water layers.

Mechanism extends beyond traditional steric sieving principles.

Abstract

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.