On the enantioselectivity of the mass transfer kinetics and the adsorption equilibrium of Naproxen on the chiral stationary phase (R,RR,R)-Whelk-O1 under reversed-phase conditions

The adsorption of the Naproxen enantiomers on a Pirkle-type chiral stationary phase (CSP) (R,RR,R)-Whelk-O1 from 0.01 M acetic acid solution in a methanol–water (85/15, v/v) mixture was studied using techniques of frontal analysis and elution chromatography. Adsorption was found to follow a model that assumes retention of a solute on two types of surface sites, enantioselective and nonselective. Some minor deviations from the classical bi-Langmuir model were found, but they were well accounted for by assuming the coexistence of two groups of enantioselective sites: sparse sites of strong affinity toward (RR)-Naproxen and a more numerous type of low-energy sites exhibiting a somewhat different affinity toward the two enantiomers. Special consideration is paid to the adsorption of the methanol–water mixture on the surface of the CSP. The bonded organic layer exhibits hydrophobic properties, so the adsorbed layer is enriched in methanol throughout the whole range of mobile phase compositions. The study of the mass transfer kinetics was carried out by analyzing the dependence of the HETP on the flow velocity. It revealed the enantioselective character of the intraparticle transport processes. Other aspects of the column dynamics, such as axial dispersion, the external and internal mass transfer were also discussed.