Molecular Theory of Liquid Adsorption Chromatography

Abstract

A statistical-mechanical theory, based on a lattice model, has been developed to address the molecular mechanism of retention and selectivity in both normal-phase and reversedphase liquid adsorption chromatography. The model is a natural “competitive-equilibrium” one, where possible contributions from solvent-solvent and solute-solvent interactions, and, hence, from solution nonideality, are not neglected. Homogeneous and heterogeneous adsorbent surfaces, single-solvent and binary mixed-solvent mobile phases, and solute molecules of different size and shape are treated. Practical applications of the theory are presented to demonstrate its utility and significance. For homogeneous adsorbents and neat solvents, the molecular energetics of retention and selectivity are examined, with special emphasis on the effects of solute size and shape, and, relatedly, the modes of solute adsorption. Separations of geometrical isomers and homologous series in real and simulated chromatographic processes are investigated, confirming predictions of the theory and the important role of solvent-solvent and solute-solvent interactions in reversed-phase systems. The implications of a more general retention equation for microscopically heterogeneous adsorbents are discussed. The dependence of capacity ratio on mobile-phase composition for binary solvents is analyzed in some detail. An often important contribution arising from solution nonideality is predicted theoretically. This is shown to be consistent with experimental results on normal-phase and reversed-phase systems.