Two Calix-Crown Based Stationary Phases. Synthesis, Chromatographic Performance and X-ray Photoelectron Spectroscopy Investigation

Two chromatographic stationary phases, obtained by covalently linking two suitably derivatized calix[4]crown-6 and calix[4]crown-5 receptors onto silica gel, are described here. A study of their chromatographic behaviour as well as their surface composition is reported. The packing materials exhibit high selectivity toward alkali metal ions. Complete separations of Cs⁺ from K⁺ and Na⁺ were obtained when using a water/methanol mixture (80:20) as the mobile phase. In these conditions, a selectivity factor Cs⁺/Na⁺=4.10 was achieved with the silica gel-bound calix[4]crown-6. The degree of coverage of the activated silica-gel surface following the covalent attachment of the macrocycle was estimated by mean of X-ray photoelectron spectroscopy (XPS). This technique allows the first few monolayers of the material to be analysed and thus is to be preferred to conventional Elemental Analysis for similar studies. Three methods for an adequate correction of the XP signals for the presence of organic contaminants are suggested and critically compared.