# Ionic selectivity and filtration from fragmented dehydration in multilayer graphene nanopores

## TL;DR

This study uses molecular dynamics simulations to show how multilayer graphene nanopores can be engineered for ion selectivity based on dehydration effects, offering insights for designing selective filtration membranes.

## Contribution

It reveals how pore size and number of graphene layers influence ion selectivity through dehydration, providing a new approach to membrane design.

## Key findings

- Ion selectivity varies with pore radius and layer number.
- Multilayer graphene shows dehydration-induced selectivity at larger pore sizes.
- Energy barriers for ion passage are significant, enabling selective filtration.

## Abstract

Selective ion transport is a hallmark of biological ion channel behavior but is a major challenge to engineer into artificial membranes. Here, we demonstrate, with all-atom molecular dynamics simulations, that bare graphene nanopores yield measurable ion selectivity that varies over one to two orders of magnitude simply by changing the pore radius and number of graphene layers. Monolayer graphene does not display dehydration-induced selectivity until the pore radius is small enough to exclude the first hydration layer from inside the pore. Bi- and tri-layer graphene, though, display such selectivity already for a pore size that barely encroaches on the first hydration layer, which is due to the more significant water loss from the second hydration layer. Measurement of selectivity and activation barriers from both first and second hydration layer barriers will help elucidate the behavior of biological ion channels. Moreover, the energy barriers responsible for selectivity -- while small on the scale of hydration energies -- are already relatively large, i.e., many $k_B$. For separation of ions from water, therefore, one can exchange longer, larger radius pores for shorter, smaller radius pores, giving a practical method for maintaining exclusion efficiency while enhancing other properties (e.g., water throughput).