Bonded Phases - OligomericType Bonded Phases 1
"Oligomeric" Type Bonded Phases
In the synthesis of oligomeric phases, a di-substituted silane, (i.e. dichlorsilanes) must be employed as the silanizing reagent as an alternative to the mono-chlorsilanes that are employed in the production of "brush" phases. The reagent to be discussed will be the octylmethyldichlorsilane. As one chlorine atom of the dichlor reagent reacts with a surface hydroxyl group, a mono-chlor silane is bonded to the silica accompanied by the evolution of hydrochloric acid. When using dichlor or trichlorsilanes it is important to understand that any water present is likely to cause cross-linking and, as will be seen later, a polymeric type of stationary phase is developed on the surface. In the production of 'bulk' phases this is the desirable target, but when synthesizing oligomeric phases, polymerization must be avoided. Consequently, as in the case of Grignard reactions, where the presence of water must also be avoided, the silica gel should be heated to at least 300oC for an hour prior to use to ensure that any strongly bound or hydrogen bonded water that may be present is removed.
After bonding the methyloctylchlor group to the silica surface any excess reagent must be washed away and the mono-chlor derivative is then reacted with water. The water hydrolyses the bonded mono-chlor derivative to the mono-hydroxy derivative with more evolution of hydrochloric acid.
The mono-hydroxy silyl derivative should then be dried, after which it can again be reacted again with more dichlorsilane. The second reaction with the dichlorsilane is accompanied by further evolution of hydrochloric acid, and will join another mono-chlorsilane to the previous methyloctylsilyl residue.