Liquid Chromatography - Solute Stationary Phase Interactions 1
If it is assumed that the total area covered by the first layer of ethyl acetate will be very similar to the area covered by the second layer, then only about one third of the second layer is complete at an ethyl acetate concentration of about 4%w/v. In contrast, the first layer is virtually complete at an ethyl acetate concentration of 1%w/v. It should be pointed out that solvent layer formation on the surface of the silica is not necessarily restricted to binary systems and multi-layers are quite feasible.
Solute Stationary Phase Interactions
There are basically two types of interaction that can take place between a solute and the silica gel surface. Firstly, the solute molecule can interact with the adsorbed solvent layer and rest on the top of it. This type of interaction is called sorption interaction and occurs when the molecular forces between the solute and the silica are relatively weak compared with the forces between the solvent molecules and the silica. The second type of interaction is where the solute molecules displace the solvent molecules from the surface and interact directly with the silica gel itself, for example, the silanol groups.
Figure 42. Diagram Depicting Sorption and Displacement Occurring on the Silica Surface.