Adsorption characterization of octyl bonded phases for high performance liquid chromatography

Adsorption-desorption isotherms were measured for selected chromatographic silica gels with chemically bonded octyl groups. The standard sorption characterization of these packings provided the BET specific surface area, the total pore volume and the pore size distribution. Advanced numerical methods based on the density functional theory and the regularization method were used to calculate the pore volume and the adsorption energy distributions. The surface and structural properties of the silicas with chemically bonded octyl phases were compared to those of the original silica gels. The surface coverage of octyl groups occurred to be similar for all samples studied. After the octyl groups were bonded, the porous structure of the silica gels was not altered significantly, resulting in similar adsorption behavior of both unmodified and modified silica gels at pressures close to the saturation pressure. However, the adsorption properties in low pressure region were changed significantly. Namely, all modified silicas exhibited very low adsorption and similar shape of adsorption isotherms at low pressures. Therefore, their adsorption energy distributions resembled each other and did not feature the higher energy sites present on the surface of unmodified silicas. The low pressure adsorption behavior indicates that essentially all higher energy sites on the silica surface reacted in the course of the chemical modification.