Bonded Phases - The Retention Properties of Oligomeric Phases

The Retention Properties of Oligomeric Phases

The carbon content of the 10 oligomeric phases prepared by Akapo et al (28) was obtained by micro-analysis and the results obtained are given as curves relating carbon content to reaction step in figure 12.

Figure 12. Graph of %w/w Carbon on the Oligomeric Phase against Synthesis Step

It is seen that the linear relationship between carbon content and reaction step (which might have been anticipated) is not, in fact, realized. As the first chains are laid down on the surface there is an initial, rapid rise in carbon content. This might indicate that throughout the first five synthetic steps previously unreacted silanol groups are being reacted, in addition to the newly formed hydroxyl groups arising from the previously bonded oligomer. However, after the fifth step, it would appear that silanization of the surface silanol groups that were not sterically hindered was complete, and only those hydroxyl groups generated by the hydrolysis of the previous oligomer were now available for reaction. It is seen that subsequent to the addition of the fifth oligomer, the carbon content does linearly increase with each step, which from a linear curve fit to the data for steps 6-10, the carbon content appears to increase at a rate of about 0.79%w/w per synthesis step. Now, from equation (1) an increase of 0.79%w/w in carbon content per oligomer addition is only about 0.29mmol.m-2 increase in the surface area.

This would appear to imply that either the reaction was very inefficient or that the surface area of the bonded phase had been significantly reduced. In fact, as the efficiency of the fluidized bed method of synthesis has already been well established in the preparation of brush phases, it would seem that the surface area of the bonded phase must have been reduced. The surface area increase appears to be reduced to a constant value subsequent to about 5 chains having been added to the oligomeric surface. This might indicate that while the synthesis proceeded, the pores became filled and blocked and the bonded material assumed similar properties to that of a pellicular packing. After 5 oligomers have been added, the surface area increase arises largely from the hydrocarbon chains on the surface of the particles (the packing particle not the silica primary particle). Unfortunately, information on the characteristics of oligomeric phases is very limited and whether this 'pellicular' characteristic would still occur if the oligomers were made with chains of shorter length is unknown. In any event, this pellicular concept is also supported by curves relating the retention volume of pyrene and pentyl benzoate to the carbon content of the bonded phases as shown in figure 13.

Figure 13. Graph of (V r) for Pyrene and Pentyl Benzoate against Carbon % w/w