Formation of Trilayer Ices in Graphene Nanocapillaries under High Lateral Pressure

Using molecular dynamics simulation, we investigate the phase behavior of water confined in graphene nanocapillaries at room temperature (300 K). Here, the lateral pressure P_zz is used as the primary controlling variable, and its effect on the behavior of trilayer water is systematically studied. Three (meta)­stable trilayer (TL) crystalline/amorphous ice phases, namely, TL-ABAI, TL-ABA, and TL-AAAI, are observed in our simulations with the lateral pressure in the range of 1.0 GPa ≤ P_zz ≤ 6.0 GPa. TL-ABAI exhibits a square lattice in every layer, and the three layers exhibit the ABA stacking pattern; i.e., the oxygen atoms in the two outer layers are in registry. This new trilayer ice structure can also be viewed as a bilayer clathrate hydrate with water molecules in the middle layer serving as the guest molecules. With increasing lateral pressure, typically, the solid-to-liquid-to-solid phase transition occurs, during which the structural transformation from triangular to square-like in the ice layer is accompanied by a sudden jump in P_⊥ (normal pressure) and in potential energy (per molecule). The oxygen density profiles of the three trilayer structures show a common feature; that is, the peak of the middle layer is markedly lower than that of the two outer layers. The computed diffusivity suggests that water in the middle layer exhibits behavior different from that in the two outer layers in contact with the graphene. For TL-AAAI, the diffusion of water molecules in the layer next to the graphene is faster than that in the middle layer.