Surface modification by calcium ions—Effect on chromatographic properties of silica

Summary

The modification of silica surfaces by calcium ions has been studied by high-performance liquid chromatography. After treatment at pH 7.0 and 9.0 with calcium hydroxide, the modified silicas were characterised by physical and chemical analysis. From measurements by atomic absorption, the amount of calcium adsorbed was 650 ppm on silica modified at pH 7.0, and 4800 ppm on silica modified at pH 9.0. The amount of calcium on the modified silica was 0.574 groups nm⁻² at pH 9.0 (original silica: mean pore diameter 176 Å, specific surface 279 m² g⁻¹, pore volume 1.23 ml g⁻¹, mean particle size 9.8 μm). On the other hand, the amount of calcium in the original silica was 58 ppm. The separation factor, α for N-methylaniline and N,N-dimethylaniline was 5.2 on silica modified at pH 9.0, but could not be measured on silica modified at pH 7.0 and the original silica, using iso-propyl alcohol—n-hexane (0.5∶99.5) eluent.