Bonded Phases - The Adsorption of Ion Exchange Reagents on the Surface of Reverse Phases
Secondly, it has also been suggested that, at higher solvent concentrations, the PIC reagents work in an entirely different manner. As already discussed, mobile phase components can act like solvents or solutes. As the solvent concentration increases, the PIC reagents will interact more strongly with the mobile phase and will be less strongly adsorbed on the reverse phase surface. Consequently, there will be less ion exchange material on the stationary phase surface. This is clearly demonstrated by the adsorption isotherm of octane sulphonate shown in figure 21. In pure water, (or at very low concentrations of solvent) octane sulphonate will cover the outer reverse phase surface when there is only about 0.5 %w/w of octane sulphonate in the mobile phase.
Figure 21. The Adsorption Isotherm for Octane Sulphonate on a Reverse Phase
It is seen, from figure 21, that a very different situation holds at higher solvent concentrations. From the adsorption isotherm it is seen that the sulphonate must be present in the mobile phase at about 10 to 15 %w/w in order to cover most of the reverse phase surface. Such levels of PIC reagents would be prohibitive in chromatography for many reasons. It is clear that if low a concentration of the PIC reagent is added to mobile phases that contains significant amounts of solvent then the reagent will remain dominantly in the mobile phase. However, under such circumstances, experimentally it has been found that the retention of ionic materials is still significantly changed by the presence of the PIC reagents. An explanation given for this significant increase in retention is as follows. The PIC reagent interacts with the complementary ionic substances in the sample, forming a salt with a very poor dissociation constant. The ionic component of the sample is now attached to a substance with a relatively long dispersive 'tail' that (as has already been shown) can interact strongly with reverse phase and thus, be retained. However, for this situation to occur the pH of the mobile phase is adjusted so that the 'ion-pair' are strongly associated so that they can exist in the unionized form long enough to allow the normal chromatographic distribution processes to take place. The exact mechanism of PIC reagents remains, at least in the author's mind, still a little uncertain particularly at high solvent concentrations. Nevertheless, PIC reagents can be used to a great practical advantage under certain circumstances to help resolve difficult mixtures of ionic materials.