Bonded Phases - Solute Interaction with the Reverse Phase Surface

Solute Interaction with the Reverse Phase Surface

Those interactions that can take place between the solute and the reverse phase are similar in principle 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 basic 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. 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. As a general rule, those solutes that elute early in the chromatogram will have interacted by sorption, those that elute late in the chromatogram will have interacted by displacement. At some intermediate point in the elution scale, solute stationary phase interactions are likely to involve both sorption and displacement.

Scott and Kucera (41) carried out some experiments to experimentally identify the type of interaction that took place between acetophenone and a RP18 brush type reverse phase using a gravimetric procedure. Due to the poor wettability of the RP18 reverse phase a concentration of over 40%w/v of solvent was necessary to achieve complete wetting. Consequently, any displacement of the solvent from the surface would need to be determined as a change in solvent concentration at the 40%w/v level then stringent demands were made on the accuracy and precision of the sampling and analytical procedure. The experiment was carried out by adding acetophenone, progressively, to a thermostatted dispersion of RP18 reverse phase in an aqueous solvent mixture containing 40.4%w/v of acetonitrile. The supernatent liquid was sampled and analyzed after equilibrium had been established after each addition of solute and the concentration of acetophenone and solvent in the mobile phase determined. The results that were obtained are shown in figure 22.

It is clear from figure 22 that as the acetophenone is added the concentration rises (for the most part linearly) to a level of about 22 mg/g of stationary phase during which the concentration of acetonitrile in the mobile phase appears to remain constant. The position of the 4s bars (four standard deviations) are shown as dotted lines on the graph. In fact, about 5 g of the reverse phase was used and 50 ml of solvent mixture, it can be calculated that if the acetophenone had displaced acetonitrile from the surface, then the concentration of acetonitrile in the solvent mixture would have increased to about 41.8%w/v. It is seen from the curve in figure 22 that such an increase in acetonitrile concentration would have been well within the accuracy and precision of the analytical procedure. It, therefore must be concluded that in the case of acetophenone interacting with RP18 the interacting process was by sorption and not displacement.

Figure 22. The Adsorption Isotherm of Acetophenone between a Reverse Phase and an Aqueous Solvent Mixture Containing 40.4 %w/v of Acetonitrile.

Nonetheless, as with silica gel, displacement interactions will occur with solutes eluted at higher (k') values and both processes are integral parts of all chromatographic distribution systems. It is important for the chromatographer to be aware that, when using either silica gel or some type of bonded phase, if the retention and separation ratios of the solutes require to be adjusted to achieve separation, then either or both the phases can be changed. It is equally important to understand that changing the composition of the mobile phase does not merely change the interactions in the mobile phase, but will also change the nature of the stationary phase surface and, thus, the interactions between the solute and the stationary phase.