Liquid Chromatography - Solute Stationary Phase Interactions 2
These two types of interaction are shown in figure 42. Displacement would occur if the solute was strongly polar such as an alcohol, which would interact more strongly with the polar silanol group than the dispersive chloroform layer. Sorption is depicted as a solute molecule on interacting with each solvent layer and can not interact strongly enough with the silica gel surface to displace the solvent..
Mobile phases consisting of mixtures of polar and dispersive solvents frequently produce surface bi-layers when used with silica gel as a stationary phase and therefore a far more complicated set of interactive possibilities exist. These possibilities are depicted in figure 43.
Figure 43. Different Types of Solute Interaction that can occur on Silica Surfaces Covered with a Solvent Bi-layer
The surface offers the opportunity of a number of sorption and displacement processes that can take place between the solute and the stationary phase surface. There are three different surfaces on which a molecule can interact by sorption and three different surfaces from which molecules of solvent can be displaced and allow the solute molecule to penetrate. In any separation all the alternatives are possible but it is more likely that for one particular solute, one type of interaction will dominate. the various types of interaction are depicted in figure 43. Where there are multi-layers of solvent, the solvent that interacts directly with the silica surface is the most polar, 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).