Bonded Phases - The Retention Properties of Bulk and Brush Phases
As explained in the synthesis of bonded phases, this reduces reagent exclusion during bonding and promotes a higher bonding efficiency. It should also be noted, that to produce a reverse phase having low retentive capacity, (ODS), Whatman reduced the extent of bonding and in the process left a significant proportion of the silica gel surface unreacted. As a result, much of the surface will be available for erosion in aqueous solvents and, in addition, retention will involve mixed interactions, dispersive interactions with the bonded phase and polar interactions with the free silanol groups. The retentive properties of the five reverse phases are shown in figure 7 where the corrected retention volume (V'r) of 2-ethyl anthraquinone is plotted against carbon content of the reverse phase. It is seen, perhaps with somewhat of a surprise, that the relationship between retention volume and carbon content is linear for the brush phases (R2, R8, R18). This linear relationship can only be expected to occur if all the stationary phase is available to the solute and the packing procedure is very reproducible so that each column contains the same amount of packing.
Figure 7. Graph of Retention Volume against Carbon Content for a Series of Brush and Bulk Type Phases
It is important to stress that all three reverse phases were produced from base silica s of very different surface areas but, in spite of this, the linear relationship between carbon content and corrected retention volume still remained. It would appear that when all the bonded material is made available to the solute, the retention volume for a given type of reverse phase depends on the amount of hydrocarbon chain that is accessible and is independent of whether the chain is 2 carbon atoms long or eighteen carbon atoms long. This also might imply that interaction between the solutes and the stationary phase perhaps may only involve one or two methylene groups and not all or a significant part of the chain length. Again, it should be emphasized, that if the relationship in figure 7 is to hold, the total bonded phase must be chromatographically available and not part of it contained in packed pores or screened by layers of bonded material as with oligomeric phases. If during the bonding process, pores are packed with reverse phase, then the reverse phase contained in the pores is not available to interact with solutes and the simple linear relationship between retention and carbon content will no longer be valid. However, the carbon content of the unavailable reverse phase that cannot contribute to solute retention will still be included in the carbon content given by micro-analysis.
The data for the bulk phase indicates that the availability of the hydrocarbon chains for interaction with a solute differs significantly from that of a brush phase. Indeed, it might appear from the greater retention values obtained from ODS2, that bulk phases could be chromatographically more open than a brush phase but one cannot to draw too many conclusions from one single experiment.
Nevertheless, the retention data for the ODS2 does not fall on the same linear curve for the brush phases. It could be inferred that the data for the ODS phases fall on a separate linear curve (the dotted line in figure 7) but a straight line drawn through two points through two points has little or no significance. The data could indicate that the hydrocarbon chains of the brush phases are so tightly crowded, that their chromatographic availability is greatly restricted and, consequently, the total length of the individual chains are not available to the solute. This conclusion, if indeed true, must occur for chains of all lengths if the relationship shown in figure 7 was to prevail.