Adsorption of mandelic acid enantiomers on a chiral stationary phase with a grafted antibiotic eremomycin

Abstract

Adsorption of mandelic acid enantiomers on the Nautilus-E chiral stationary phase with eremomycin as a grafted antibiotic, as influenced upon by composition of the water–ethanol mobile phase, was studied. The surface of the adsorbent was evinced to be heterogeneous and to have two types of adsorption sites: the enantioselective and nonselective ones. Nonselective interactions contribute to adsorption of mandelic acid mainly. With the ethanol fraction in the eluent increased from 40 vol% to 60 vol%, contribution of nonselective interactions to adsorption was increase. The adsorption isotherms of S- and R-mandelic acid were plotted by the Glueckauf method and described by the bi-Langmuir and Henry–Langmuir isotherm models. The Henry–Langmuir model was found out to adequately describe the experimental equilibrium findings in the concentration range under study. The temperature effect on the adsorption equilibrium was discussed. The thermodynamic characteristics of enantioselective adsorption sites were characterized by their adsorption constants, saturation capacities as influenced upon by temperature. A significant increase in the true enantioselectivity at the high-energy adsorption sites with an increase in temperature was revealed. Retention and recognition of mandelic acid enantiomers on the enantioselective adsorption sites were governed by the entropy term.