NONAQUEOUS ANION-EXCHANGE CHROMATOGRAPHY .2. CHANGING ANION-EXCHANGE SELECTIVITY BY RESIN SURFACE COMPLEX-FORMATION OF CROWN-ETHERS

Ion-exchange selectivity can be varied with the extent of desolvation, the charge distribution of ion-exchange sites, and the hydrogen bond formation between the ion-exchange site and a counter ion. These are c hanged by varying the structure of ion-exchange sites. Thus, we can mo dify the ion-exchange selectivity by changing the chemical structures of ion-exchange sites as can be seen in a variety of references. When an ion-exchange site comprises a primary ammonium ion, the distributio n of the charge of the ammonium ion can be modified by complex formati on of a crown ether with this anion-exchange site. Though it is diffic ult to quantitatively describe the electrostatic potential on a molecu lar basis, a number of data accumulated so far apparently indicate tha t ion-exchange sites with high charge density force the desolvation of solute ions and form tight ion pairs with small anions while ion-exch ange sites with low charge density interact with solvated ions and fav orably bind large anions. Comparison of data implies that the surface complex formation of a crown ether apparently lowers hydrogen bond for mation ability of the anion-exchange site and the affinity of small an ions, and changes anion-exchange selectivity.