Theoretical study of hydration effects on the selectivity of 18-crown-6 between K⁺ and Na⁺

A combination of molecular dynamics (MD) and density functional theory (DFT) calculations were used to study the hydration structures of K⁺ and Na⁺ ions under the confinement of 18-crown-6 in order to identify the role of water in the selectivity of 18-crown-6 towards K⁺. The radial distribution functions, coordination numbers, orientation structures and interaction energies were analyzed to investigate the hydration of K⁺ and Na⁺ in 18-crown-6/cation complexes. All calculations of K ⁺ and Na⁺ in bulk water were also conducted for comparison. The simulation results show that the orientation distributions of the water molecules in the first coordination shell of K⁺ are more sensitive to the confinement of 18-crown-6 than those of Na⁺. It is more favorable to confine a K⁺ in 18-crown-6 than a Na⁺ in terms of interaction energy. Good agreement is obtained between MD results and DFT results.