Liquid Chromatography - Types of Bonded Phase (Oligomeric Phases)
The di-substituted silanes such as methyloctyldichlorosilanes can be used to synthesis the oligomeric bonded phases which is a far more complicated procedure (24). The silica is first reacted with the methyloctyldichlorosilane to link methyloctylchlorosilyl groups to the surface. The product is then treated with water, which hydrolyses the methyloctylchlorosilyl groups to methyloctylhydroxysilyl groups with the elimination of hydrochloric acid. The "hydroxy" product is then reacted with more methyloctyldichlorosilane, attaching another methyloctylchlorosilyl group to the previous groups.
This process of alternately treating the product with the silane reagent and then water can be repeated until, in the original synthesis, eight or ten oligomers are linked to each other and attached to each stearically available hydroxyl group on the surface. However, it should be noted that the hydroxylation of the bonded moiety is not the only possible reaction that can take place with the water. There will be situations where it will be stearically possible for a water molecule to react with two adjacent chains and thus produce some cross-linking. However, a small amount of cross-linking would indeed strengthen the bonded system and could be advantageous. The product is finally treated with trimethylchlorosilane or some other capping reagent to eliminate the last hydroxyl groups formed at the end of the oligomer. The oligomers are layered over the surface making the product extremely stable exhibiting almost no polar characteristics whatsoever. However, due to the complexity of the synthesis [which needs to be carried out in a fluidized bed for efficient reaction] oligomeric phases are expensive to manufacture and, consequently, are not often used and, at this time, are not commercially available. Nevertheless, this type of procedure, if used to synthesize a reversed phase, would produce material that would probably be completely dispersive in character with no measurable polarity.