Ice-like Water Structure in Carbon Nanotube (8,8) Induces Cationic Hydration Enhancement

It is well recognized that ice-like water can be formed in carbon nanotubes (CNTs). Here, we perform molecular dynamics simulations of the hydration of Na+, K+ and Cl- in armchair CNT(n,n) (n = 6, 7, 8, 9 and 10) at 300 K to elucidate the effect of such water structures on ionic hydration. It is found that the interaction of Na+ and K+ with the water molecules is enhanced in CNT(8,8), but is similar or weaker than in bulk in the other CNTs. In bulk, water molecules orient in specific directions around ions due to the electrostatic interaction between them. Under the confinement of CNTs, the hydrogen bonds formed in the first hydration shell of Na+ and K+ disturb this orientation greatly. An exception is in CNT(8,8), where the dipole orientation is even more favorable for cations than in bulk due to the formation of a unique ice-like water structure that aligns the water molecules in specific directions. In contrast, the coordination number is more important than hydration shell orientation in determining the Cl- -water interaction. Additionally, the preference for ions to adopt specific radial positions in the CNTs also affects ionic hydration.