The Mechanism of Chromatographic Retention - Solute Stationary Phase Interactions > Mobile Phase Component Dispersive or Weakly Polar > Page 51
Assuming the total area covered by the first layer of ethyl acetate will be very similar to the area covered by the second layer, only about one third of the layer is complete at an ethyl acetate concentration of about 4%w/v. This is in striking contrast to the formation of the first layer which is virtually complete at an ethyl acetate concentration of 1%w/v.
The interacting surface, with respect to interacting solutes, is now very complex indeed. In contrast to the weaker held solvents, retention will be modified dramatically as the concentration of the polar solvent increases from 0 to 1%w/v. Above 1%w/v, however, the retention of a solute will be reduced more subtly, allowing a more controlled adjustment of the chromatographic conditions.
It should be pointed out that multi-layer adsorption is quite feasible. The second layer of ethyl acetate might have another absorbed layer of ethyl acetate on it. However, any multi solvent layers that may be formed must be progressively more weakly held and sparse in nature. Under such circumstances their, presence, if in fact real, will have little impact on solute retention
Solute Stationary Phase Interactions
Mobile Phase Component Dispersive or Weakly Polar
There are two types of interaction that can take place between a solute and an adsorbent 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. This type of interaction is called displacement interaction and occurs when the interactive forces between the solute molecules and the silica surface are much stronger than those between the solvent molecules and the silica surface. This type of interaction 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. An example of Sorption Interaction is depicted in figure 22.