Bonded Phases - The Retention Properties of Oligomeric Phases
It is seen that the relationship shown is quite the opposite to that shown in figure 7 for the brush phases. In addition, and within experimental error, the corrected retention times of the solutes appear to be independent of the carbon load on the stationary phase. It should be pointed out that the scatter is to be expected, because the retention volume will be proportional to with the quantity of packing in the column, that, of course, will differ somewhat from column to column. Nevertheless, it is seen that the slopes of the curves are very close to zero indicating that as each oligomer is formed, no increase in interactive surface is produced. Actually, as each chain is added to the bonded phase, a new layer, or covering 'scale', is laid down on the outer surface and, as a consequence the effective chromatographic surface area remains the same.
This data again supports the concept of a oligomeic phase being a pellicular type of packing that attains its surface area because long hydrocarbon chains are attached to the surface. An oligomeric phase may, in fact, be viewed as a multi-layer system and as a result, after each synthesis stage the chromatographically interactive surface is moved further away from the silica gel surface and (more important) away from any unreacted silanol groups that, as already explained, accounts much for the stability of the phase under low pH conditions Akapo (28) also carried out some broad thermodynamic investigations into oligomeric phases with the intent of identifying the type of interactions that were occurring between solutes and the interactive moieties on the bonded phase. As a result of the heavy layering of the hydrocarbon chains over the silica surface and the apparent formation of a pellicular form of reverse phase, it would be expected that the interactions involved between the oligomeric phase and any solute or solvent would be exclusively dispersive in nature. If this were so, one would expect the relationship between the standard enthalpy of distribution and the molecular weight of a homologous series of solutes would be linear. Akapo carried out an extensive series of retention volume measurements over a range of temperatures for a homologous series of esters and aromatic hydrocarbons. The standard enthalpies were calculated and the curves relating standard enthalpy and molecular weight are shown in figure 14.
It is clear that the expected linear relationship was indeed revealed, thus, demonstrating that retention by an oligomeric phase is almost exclusively controlled by dispersive interactions. As a consequence, and in contrast to other types of bonded phases, no residual silanol groups appear to be present or, to say the least are not readily available. Consequently, provided there are a sufficient number of chains present in the oligomeric phase, residual silanol groups will not effect solute retention to any significant extent.
In summary, if our comments are confined to reverse phases only, during synthesis the oligomers become layered over the surface rendering the product extremely stable and as a result exhibit almost virtually no polar characteristics whatsoever. Nevertheless, as already has been discussed, as a result of the complexity of the synthesis, oligomeric phases are, using the system described, expensive to manufacture and, consequently, are still not readily available.
Figure 14. Graph of Standard Enthalpy from Data obtained from Column 10 against Solute Molecular Weight
The bulk reverse phases appear to be the most stable and predictable when used with mobile phases of high water content and also appear to exhibit greater retention for a given carbon content, perhaps due to the bulk phase being more open and the interacting chains more available. It would appear that the synthetic procedure for brush phases can provide a product with more reproducible chromatographic properties and, consequently, has become the more popular type of reverse phase.