Crown ethers are ring-like structures composed of several units of ethylene glycol. Because the radius of their pores are similar to that of alkali metal ions, crown ethers are well known for their ability to bind metallic ions.  For example, 12-crown-4, 15-crown-5, 18-crown-6, 21-crown-7, and 24-crown-8 interact with potassium, sodium, ammonium, and calcium ions, respectively. Among the aforementioned crown ethers, all the ions have the strongest binding affinity with 18-crown-6 because the size of 18-crown-6 most closely matches that of the ions. Besides ionic species, crown ethers can also bind to neutral molecules, e.g., 1, 2, 3- triazole. Crown ethers can also be threaded with long, linear molecules and/or polymers, giving rise to supramolecular structures called rotaxanes. Given that the crown ethers are not bound to the chains, they can move up and down the threading molecule.