Liquid Chromatography - Types of Bonded Phase (Bulk Phases)
If the silica surface is saturated with water and octyltrichlorosilane is used as the reagent, reaction occurs with both the hydroxyls of the silica surface and the adsorbed water causing a cross-linking reaction, and an octylsilyl polymer can be built up on the surface. The same procedure can be used as that in the synthesis of oligomeric phases and the material can be alternatively treated with water and the trichlorosilane reagent. Layers of bonded phase are built up on the surface but, in this case, due to the tricoloro function of the reagent, extensive cross-linking occurs. As a result of the polymerization process, the stationary phase has a chemically cross-linked, multi-layer character and, consequently, is termed a "bulk" phase. The "bulk" phases are almost as popular as the "brush" phases as they tend to have a higher carbon content (more organic material bonded to the surface) and thus provide a little greater retention and selectivity. "Bulk" phases have about the same stability to aqueous solvents and pH as the "brush" phases.
Using appropriate organic chlorosilanes, polar or polarizable groups such as nitriles or aromatic rings can be bonded to the silica to provide stationary phases covering a wide range of polarities. Bonded ion exchange materials have also been synthesized, although they are not as stable to salt solutions and extremes of pH as the ion exchange resins.