The Mechanism of Chromatographic Retention - Solute Stationary Phase Interactions > Mobile Phase Component Polar > Page 54


It is seen that a wide range of sorption and displacement processes can occur between the solute and the stationary phase surface. There are three different surfaces available for interaction by sorption and three corresponding different surfaces available for interaction by displacement. All the alternatives are possible but it is more likely that for any particular solute, one type of interaction will dominate. The various types of interaction are depicted in figure 25. In multi-layer adsorption the most polar solvent is the one that interacts directly with the silica surface, and consequently constitutes the first layer. Depending on the concentration of the polar solvent the next layer may be a second layer of the same polar solvent as in the case of ethyl acetate. If, however, the quantity of polar solvent is limited, then the second layer might consist of a less polar component of the solvent mixture. If a ternary mixture of solvents is used, the nature of the surface, and the solute interactions with the surface can become very complex indeed. In general the stronger the polarity of the solute the more likely it is to interact with the surface by displacement even to the extent of displacing both layers of solvent (one of the alternative processes that is not depicted in figure 25). Less polar solutes a more likely to interact with the surface by sorption.


Figure 25. Different Types of Solute Interaction that can occur on Silica Surfaces Covered with a Solvent Bi-layer