The Mechanism of Chromatographic Retention - Solute Stationary Phase Interactions > Mobile Phase Component Polar > Page 53


Figure 23 depicts displacement interaction, when the silica surface is covered with a layer of molecules from one or more different solvents. Such layers can impede the interaction of the solute directly with the stationary phase as in sorption. If the solute interacts strongly with the stationary phase, however, the solute molecule will replace a solvent molecule on the surface which will be accompanied by the release of a solvent molecule into the mobile phase. In the case of silica gel, if the solute is strongly polar, it may be capable of displacing either, or both, solvents from the surface. If the solute has an intermediate polarity, the solute molecules may displace the weaker solvent component but interact directly with the layer of the stronger solvent by sorption. In this way, both interactive processes would take place at the same time, sorption and displacement.

Mobile Phase Component Polar

Mobile phases consisting of mixtures of polar and dispersive solvents frequently produce surface bi-layers when used with silica gel as a stationaryphase. Thispossibilityhasalreadybeendiscussedand the formation of a surface bi-layer on silica gel is depicted in figure 24.

Figure 24. Bi-layer Adsorption of Solvent