Competitive Adsorption of Solute and the Modifier in a Mixed Partition/displacement Mechanism in Reversed-phase Liquid Chromat

Competitive adsorption of the solute and the polar component of the mobile phase (the modifier) in reversed-phase liquid chromatography is discussed. Retention properties on surfaces differing in alkyl chain length (RP-18 compared with RP-8e) have been investigated. Typical solvent mixtures were used as mobile phases–methanol–water and acetonitrile–water. Three different types of compound were selected as solutes–two homologous cycloketones, cyclopentanone and cyclohexanone, and phenol. For all these solutes the adsorption mechanism was heterogeneous – the compounds had affinity for the polar uncovered surface and for the alkyl chains. Adsorption of organic solvents on the RP-8e (endcapped) adsorbent was measured and compared with that on RP-18e and RP-18 (non-endcapped) measured in previous studies. The adsorption equilibrium of solvents was strongly affected by the structure of the adsorbent surface and the polarity of the mobile phase. Variation of separation selectivity as a function of surface properties and mobile-phase composition was also investigated, and is discussed. The model of competitive adsorption proposed in our previous study has been used for analysis of the retention behaviour of the solutes and for prediction of non-linear chromatographic band profiles.