Crown ethers demonstrate specificity for shape as well as size and charge.
Rebek's self-replicating molecules may model natural synthesis and replication. A crown ether is a cyclic, organic molecule containing ether groups. Simple crown ethers contain 9-18 carbon atoms. The lone pairs on the oxygen atoms are available to non-covalently bond metal ions. Crown ethers bond specifically, and may distinguish between a potassium and Sodium or Silver ions .