Comparison between heterogeneous multi-Langmuir and homogeneous electrostatically modified Langmuir models in accounting for the adsorption of small organic ions in reversed-phase liquid chromatography

Adsorption isotherm data were measured by frontal analysis (FA) for sodium nicotinate (anionic species, pHWS 9.14) and atenololium chloride (cationic species, pHWS 2.60) on a column packed with hybrid porous particles (BEH-C18), eluted with an aqueous solution of methanol at two different ionic strengths. These data were well accounted for by both a heterogeneous bi-Langmuir adsorption isotherm model and a homogeneous electrostatically modified Langmuir isotherm. In the case of the former model, two different types of adsorption sites are available for the ions. The saturation capacity and the binding constant of the stronger type of adsorption sites are nearly independent of the ionic strength of the eluent. These sites could be consistent with residual, accessible silanol groups, the surface concentration of which was estimated at 0.08 μmol/m2. In contrast, the equilibrium constant of the weaker type of adsorption sites markedly increases with increasing elution strength of the eluent, a result consistent with a salting-out process. These weak adsorption sites are likely located at the interface between the C18-bonded layer and the bulk eluent. In the case of the latter isotherm model, there is only one type of adsorption sites, its equilibrium constant increases with decreasing eluent ionic strength, consistent with classical electrostatics, but its saturation capacity strongly decreases when the ionic strength drops. This last result suggests that the homogeneous electrostatic adsorption model is physically inconsistent.