Liquid Chromatography - Types of Bonded Phase (Brush Phases)
Types of Bonded Phase
There are three basic types of bonded phases, the brush phases, the bulk phases and the oligomeric phases. The different phases are produced by the use of the mono, di- and tri- substituted silanes respectively in the bonding process, e.g. the monochloro, dichloro and tricoloro silanes. The monochlorosilanes, for instance octyldimethyl-chlorosilane, react with the hydroxyl groups on the silica surface to produce dimethyloctylsilyl chains attached to the silica.
The alkyl chains are thought to stand out from the surface like bristles of a brush, hence the term brush phase. The extent to which the silanyl groups are reacted is a subject of debate at this time. It is thought that the two methyl groups next to the silicon atom hinder the reaction of adjacent hydroxyl groups with the reagent and thus there will be a considerable amount of unreacted hydroxyl groups remaining even after capping. In the extreme, it has been suggested that there is a hydroxyl group situated between each bonded chain. There is certainly evidence of some polar interactions with reverse phases which if completely covered with hydrocarbon chains should only exhibit dispersive interactions. However, reverse phases are predominantly dispersive in character and it would appear that if there are any hydroxyl groups still present on the surface it is likely that they would be relatively few in number compared with the bonded moieties.