Band splitting in overloaded isocratic elution chromatography III. Modeling of adsorbate-adsorbate interactions by a two-component extension of a BET kinetic isotherm model

A new two-component competitive adsorption model was derived to account for the competitive adsorption data of mixtures of ethylbenzoate and 4-tert-butylphenol, on a C-18-Kromasil column under RPLC conditions (mobile phase, methanol/water, 62/38, v/v). The derivation is based on kinetic arguments and is an extension to multicomponent systems of the single-component BET isotherm. The model assumes that the molecules of the first compound (A) can adsorb on layers made of molecules of either A or B, while molecules of B can only adsorb on layers made of molecules of A. This makes the competitive isotherm consistent with the single-component isotherms of ethylbenzoate and 4-tert-butylphenol, the multilayer BET and the monolayer Langmuir isotherm models, respectively. The competitive adsorption data were acquired by frontal analysis (FA) with equimolar mixtures of eight different concentrations. For the seven lowest concentrations, these data were derived from the retention times of the shocks of the two compounds and the concentration of the intermediate plateau of the less retained compound. At the highest concentration (25 g/l), the individual band profiles were measured by collecting and analyzing twenty fractions. The low concentration data (C less than or equal to 10 g/l) are well accounted for by the two competitive isotherm models derived previously but these models fail to describe the experimental data of 4-tert-butylphenol at high concentrations. By contrast, the new model predicts very well the experimental adsorption data for mixtures of ethylbenzoate and 4-tert-butylphenol in the whole range of concentration studied. Our results suggest that the adsorption constant of 4-tert-butylphenol onto layers made of ethylbenzoate (b(B,A) = 0.0120 l/g) is intermediate between those of ethylbenzoate on layers made of 4-tert-butylphenol (b(A,B) = 0.0105 l/g) and of ethylbenzoate on itself (b(A,A) = 0.01451/g). This new model should give an improved description of the band splitting observed for 4-tert-butylphenol in the presence of ethylbenzoate. (C) 2004 Elsevier B.V. All rights reserved.