Study of solvent adsorption on chemically bonded stationary phases by microcalorimetry and liquid chromatography.

A detailed, molecular-level description of the sorption mechanism in reversed-phase liquid chromatography is of great interest to analytical chemists. For this purpose, solvent adsorption in the octadecyl stationary bonded phase was investigated. Preferential adsorption of solvents from an acetonitrile-water and methanol-water mobile phase was measured on a series of non-end-capped octadecyl bonded phases with different coverage densities of bonded ligands using the minor disturbance method. For a comparison, a microcalorimetric study of organic solvent adsorption on the stationary phase was executed. The results from the excess isotherm measurement agree well with the experimental measurement of the heat of immersion of the bonded stationary phases by the test solvents. The microcalorimetric measurement is another method for determination of solvation processes of the stationary phases. Changes of the heat of immersion provide information about the surface accessibility for interaction with solvent molecules. The increase of the stationary phase coverage density reduces the free space between bonded chains and penetration of solvent between organic chains.