Description of adsorption equilibria in liquid chromatography systems withbinary mobile phases

Adsorption of some simple compounds from pure and mixed solvents and of som e solvents from mixed binary solvent mixtures on columns used in normal-pha se, aqueous-organic and non-aqueous liquid chromatography was investigated. The distribution of the compounds between the liquid and the stationary ph ase is affected by the composition of the solvent mixture. Although prefere ntial sorption of stronger solvents can often be described by the Langmuir isotherm, significant deviations are observed in some systems. A model was suggested accounting for the deviations from Langmuir isotherm by associati on on already adsorbed molecules. In most systems studied in this work, a s imple competitive Langmuir isotherm did not provide a good fit to the exper imental distribution data from mixed solvents when competition between the solute and the strong solvent for the adsorption sites was considered. A co mpetitive isotherm taking into account possible association of solute on it s own already adsorbed molecules and on the adsorbed molecules of strong so lvent improves the fit to the experimental distribution data and enables de scription of the distribution data in dependence on the concentration of th e strong solvent in mixed solvents in normal-phase, aqueous-organic and non -aqueous reversed-phase systems. Even though we need more data to prove the validity of the present model, we observe a good qualitative agreement of the experimental isotherm coefficients with the relative strength of the in teractions expected in various chromatographic systems employing mixed solv ents. (C) 2001 Elsevier Science B.V. All rights reserved.