Again using molecular dynamics, Liu et al demonstrate that a hexagonal array of noncharged CNTs, each with an effective diameter of 0.44 nm, is able to discriminately select potassium ions over sodium in the presence of an applied pressure of 5 MPa. They suggest that the key to this ion selectivity is the higher binding energy of water molecules in the first hydration shell of sodium, which renders it more stable, and therefore sodium has a higher desolvation energy, making it more difficult to permeate through a narrow NT. The flexibility of the hydration shell of a potassium ion enables it to maintain better coordination within the NT. Similarly, Song and Corry showed that narrow CNTs with no functionalization have intrinsic ion selectivity.