Liquid Chromatography - Solvent/Solute Interactions with the Reversed Phase Surface 2
The weak nature of the methanol adsorption, relative to the other alcohols is clear and it is seen that the surface of the reverse phase is being modified over one third of the methanol concentration range. The reverse phase surface can be modified in a controlled manner, over the range of 0 to about 40 % methanol, but between methanol concentrations of 40% and 100 % the nature of the reverse phase surface remains sensibly constant and it is the solute interactions in the mobile phase that are progressively modified. Acetonitrile and tetrahydrofuran behave in a similar manner but there adsorption isotherms are closer in magnitude to those of ethanol than of methanol.
The types of interactions that can take place between the solute and the reverse phase are similar to those that can take place between the solute and the silica gel surface. Solutes can interact by the sorption process, the displacement process or a combination of both. The same rules apply; if the solvent interacts more strongly with the surface than the solute then the solute interacts with the adsorbed layer of solvent by sorption.
Figure 45. The Adsorption Isotherms of a Homologous Series of Aliphatic Alcohols
If, on the other hand, the solute interacts more strongly with the reverse phase than the layer of solvent molecules then the solute will displace the solvent and interact directly with the surface by displacement. In, general, those solutes that elute early in the chromatogram will interact by sorption, those that elute late in the chromatogram will interact by displacement and at some intermediate point in the elution scale, solute stationary phase interactions will probably involve both sorption and displacement. Bi-layer adsorption is also possible with reverse phases but, at this time, experimental evidence of this does not appear to be available in the literature.