Description of adsorption equilibria in liquid chromatography systems with binary mobile phases

Adsorption of some simple compounds from pure and mixed solvents and of some solvents from mixed binary solvent mixtures on columns used in normal-phase, aqueous–organic and non-aqueous liquid chromatography was investigated. The distribution of the compounds between the liquid and the stationary phase is affected by the composition of the solvent mixture. Although preferential 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 association on already adsorbed molecules. In most systems studied in this work, a simple competitive Langmuir isotherm did not provide a good fit to the experimental distribution data from mixed solvents when competition between the solute and the strong solvent for the adsorption sites was considered. A competitive isotherm taking into account possible association of solute on its own already adsorbed molecules and on the adsorbed molecules of strong solvent improves the fit to the experimental distribution data and enables description of the distribution data in dependence on the concentration of the 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 interactions expected in various chromatographic systems employing mixed solvents.