Role of anions in the complex formation of crown ethers with ammonium ions chemically bonded on silica gel

Abstract

The complex formation of crown ethers with ammonium ions chemically bonded on silica gel has been chromatographically investigated. Although the selectivity in the complex formation of crown ethers is basically the same as in solution, a counter anion existing in the diffuse double layer of the silica gel also plays an important role, even when a polar solvent with hydrogen-bond formation ability such as methanol, is used as a medium. The overall complex formation at the silica gel surface can be divided into three processes: (i) the dissociation of a counter anion from its ion-pair with the ammonium ion; (ii) complex formation of a crown ether with the ammonium ion; (iii) formation of an ion-pair between the anion and the ammonium ion complexed by the crown ether. This mechanism is evaluated from thermodynamic viewpoints including the solvation of anions, the ion-pair formation of anions with ammonium, etc.