How do crown ethers surround metal ions?

Crown ethers are cyclic molecules that function as size-selective ionophores by using their ether oxygen atoms to form a ring, or crown, around a metal cation.1

The mechanism of interaction depends on the relationship between the size of the cation and the size of the ring's central cavity:

• Direct Encapsulation: A cation of the appropriate size fits into the hole at the center of the crown ether molecule, forming a stable 1:1 complex. For example, the potassium cation ($K^{+}$) fits precisely into the center of an 18-crown-6 ring.1
• Sandwich Complexes: If a cation is too large to fit within the ring, it may be complexed on one side or encapsulated between two crown ether molecules in a "sandwich" fashion, such as the 1:2 complex formed by cesium ($C{s}^{+}$) and 18-crown-6.1

The selectivity of these ethers for specific cations is primarily determined by the ring size, allowing chemists to target specific ions like $L{i}^{+}$, $N{a}^{+}$, or $K^{+}$ by using appropriately sized molecules.1