Crown ethers:

The crown ethers contain a central varying size hydrophilic cavity which is ringed with electronegative binding polyether oxygen atoms. These oxygen items are surrounded by a collar of -CH₂ groups imparting it a hydrophobicity. They complex with the cation converting it into a lipophilic species which extracts in the organic solvent. The complexes are composed of two ionic species held together by electrostatic attraction.