FIG. 1. Dehydration of ions going through multilayer graphene pores.

(a) A nanopore through trilayer graphene. As K⁺ (red) translocates through the pore, it retains only part of its hydration. In this case, the pore radius is r_p = 0.34 nm and the first hydration layer is essentially complete. The second hydration layer, though, is significantly diminished due to the carbon of the graphene (gray) preventing the water molecules (cyan and white) from fluctuating about 0.5 nm away from the ion, except along the pore axis. (b) Water density quantified by its oxygen location around K⁺ and Cl⁻ ions fixed in bulk and in the center of mono-, bi-, and tri-layer graphene (shown as grey bars) pores with radius r_p = 0.34 nm. The white dotted circles demarcate the first and the second hydration layers. The first hydration layers remain but acquire some additional structure. The second hydration layer is greatly reduced (see Fig. 2, and Fig. S2 and Table S2 in the SI). For this pore size, the free energy barrier due to the second layer dehydration significantly contributes to the ion currents and selectivity. The bi- and tri-layer graphene are AB and ABA stacked, respectively, but similar results occur for perfectly aligned multilayer graphene.