Bonded Phases - Bulk Type Bonded Phases
"Bulk" Type Bonded Phases
The bulk type phase consists of a somewhat open polymeric film of stationary phase covering the silica surface that is formed by the use of trichlorsilyl reagents. The polymer is cross-linked by water that is added to the silica prior to reaction. When the silica surface is saturated with water and is treated with octyltrichlorsilane a reaction occurs with both the hydroxyls of the silica surface and the adsorbed water, causing a cross-linking reaction and the octylsilanyl polymer is built up on the surface. As a result of the polymerization process, the stationary phase assumes a multi-layer character and that is why it is termed a 'bulk' phase. Initially, the silica gel is first exposed to atmosphere at 100% humidity for and hour; if necessary, additional water can be added to the silica if so desired. The wet silica is suspended in a suitable solvent and excess octyltrichlorsilane is added with vigorous stirring and the mixture is then heated. The first stage of the reaction results in the formation of an 'octyldichlor' chain being attached to the silica with the evolution of hydrochloric acid.
In the second stage it is presumed that the 'octyldichloro' chain reacts with some of the excess water to from a 'octyldihydroxy' chain.
The 'octyldihydroxy chain can, in turn react, with two more octyltrichlorsilane molecules and start building the polymer on the surface.
The pendant chlorsilane groups can now have sufficient freedom of movement so that it is sterically possible to react with another surface silanol group. In this way, a cross-linked polymer layer is built up, attached to the silica surface at different points, providing a fairly rigid structure for the solvent and solutes to interact. By making alternate treatments with octyltrichlorsilane and water a layer of polymerized bulk phase can be sequentially deposited on the surface to any desired thickness.