Chemical Mechanism of the Solid-Phase Extraction of Metal Complexes: Barium Complexes with Crown Ethers

The adsorption of barium complexes of dibenzo-18-crown-6 (DB18C6) and dicyclohexano-18-crown-6 (DCH18C6) ethers on alkylated silica gel in the presence of sodium dodecylsulfonate was studied. The adsorbent capacity for the barium complex of DCH18C6 was higher than that for the complex of DB18C6 by a factor of about 4. Depending on the proton-acceptor ability of the adsorbate, the surface layer of alkyl radicals and residual silanol groups behaves as a solid surface or a liquid phase. In the latter case, a supramolecular cavity complementary to the adsorbate is formed in the surface phase. Thus, alkylated silica gel undergoes self-organization with regard to the properties of the adsorbate. This hypothesis explains the unusual adsorption of barium complexes.