The competitiveness of solvent adsorption on polar-embedded stationary phases.

Solvation processes on polar embedded stationary phases were investigated by determination of excess isotherms in binary acetonitrile/water mobile phase. The obtained results indicate the competitiveness of acetonitrile and water adsorption on heterogeneous surfaces of chemically bonded stationary phases. The calculation of excess isotherm as a volume of excessively accumulated solvent confirms the competitiveness of solvents' adsorption in qualitative and quantitative meaning. Corresponding excess isotherms of water and acetonitrile are symmetrical with respect to the x-axis. It confirms that the displaced volume of acetonitrile is replaced by an equal volume of water and vice versa. Additionally, fitting the two-site adsorption model to experimental data provides quantitative values with low experimental errors. The obtained results confirm the advantage of volumetric assumption of excess adsorption over direct adsorption layers of solvents, which is commonly postulated.