A phase transfer catalyst is a type of chemical compound. It is a quaternary ammonium salt, which facilitates the migration of a particular chemical component in one phase into another phase in a heterogeneous system. The chemical component is soluble in one phase but insoluble in the other unless the phase transfer catalyst is present. A phase transfer catalysts works by encapsulating the ion. The PTC-ion system has a hydrophilic interior containing the ion and a hydrophobic exterior. Crown ethers are also efficient phase transfer catalysts.