Control of retention by molecular interactions in reversed-phase chromatography

Abstract

The procedures used in the synthesis of the different types of reversed phases are described, together with the physical and chemical properties of the products. The nature of the surface of the different reversed phases is discussed and the changes that take place when in contact with water explained on the basis of the dispersive character of the bonded phase and the high polarity of the water. The innate stability of the oligomeric phases is considered and the adsorption of solvent from aqueous solvent mixtures onto the surface of a reversed phase described, including the form of the adsorption isotherm. The different ways a solute molecule can interact with the reversed-phase surface is discussed and the interactions that take place in the mobile phase examined using simple liquid—liquid distribution systems. It is shown that the probability of solute interaction with a given solvent in a solvent mixture is proportional to the volume fraction of the solvent. Conditions are then considered where association between the components of the solvent mixture occurs using, as an example, aqueous solvent mixtures of methanol. The resulting ternary mixture is seen to add to the complexity of the retention mechanism. Finally, a general equation is given that describes the retention volume of a solute in terms of the distribution coefficient of the solute between each pair of stationary phase and mobile phase components and the fraction of each phase component available for solute interaction.