A new equation for the retention of solutes in liquid–solid adsorption chromatography with mixed mobile phases

Abstract

A new equation for the dependence of the capacity ratio k′ of solutes on the composition of a binary mobile phase in liquid–solid adsorption chromatography has been derived. Starting from the general definition of the chromatographic retention in terms of surface excess quantities, and then introducing a monolayer model for adsorption from three-component mixtures, a three-parameter relation for k′ in terms of the compositions of the surface layer and the mobile phase is obtained.

This equation is tested against experimental data for the adsorption isotherms of two binary solvent systems (cyclohexane–benzene and cyclohexane–1,2,-dichloroethane) on silica gel (µ-Porasil) and new measurements of the capacity ratio of non-polar and polar solutes (aromatic hydrocarbons and dinitrobenzenes) on the same chromatographic systems. The surface excess isotherms of the binary solvent systems are analysed on the basis of the parallel-layer model to determine the composition of the adsorbed layer. Using that information, the retention data for the solutes are then analysed in terms of the new equation. The resulting parameters (molecular size ratio, r_s, interaction parameter, S₂₁, and surface phase fraction, p) and their physical significance are discussed.