Solvent/Solute Interactions with the Silica Gel Surface
In all chromatography systems both the solvent and the solutes interact with the stationary phase. It follows that, when the silica surface is in contact with a solvent, the surface is covered with a layer of the solvent molecules. If the mobile phase consists of a mixture of solvents the surface is partly covered by one solvent and partly with the other (32). Thus, any solute interacting with the stationary phase may well be presented with two, quite different types of surface with which to interact. The probability that a solute molecule will interact with one particular type of surface will be statistically controlled by the proportion of the total surface area that is covered by that particular solvent.
Dispersive solvents appear to be adsorbed from a solvent mixture on the surface of silica gel according to the Langmuir adsorption isotherm (33). Examples of mono-layer adsorption isotherms obtained for benzene, chloroform and butyl chloride are shown in figure 40.
Figure 40. Langmuir Adsorption Isotherms for Benzene, Butyl Chloride and Chloroform
