Bonded Phases - The Different Classes of Bonded Phases
The Different Classes of Bonded Phase
There are basically three classes of bonded phase and they are "the brush phase", "the oligomeric phase" and "the bulk phase". The different classes are produced from the three different types of silane reagents, that is, the mono-substituted, di-substituted and tri-substituted silanes. The mono-chlorsilanes, dichlorsilanes and trichlorsilanes are examples of such reagents. The production of these classes of bonded phase and the effect of each type of reagent will be discussed using, as examples, the reagents octyldimethylchlorsilane, octylmethyldichlorsilane and for cross linked phases, octyltrichlorsilane. Silane reagents are readily available and a list of those commonly used for bonded phase synthesis are given in table 1.
Table 1. Some Commonly Used Silane Reagents Used in Bonded Phase Synthesis
"Brush" Type Bonded Phases
The brush type phase was the first class of bonded phase to be prepared and the basic methodology for its production, employing a mono-chlorsilane reagent, has been discussed. Details of the synthesis will be discussed later and at this time it can be said that silica is reacted at elevated temperatures in contact with a solution of the reagent in a high boiling solvent. In all the bonding procedures, excluding the fluidized bed process, the bonding is usually achieved by dispersing the silica gel in a suitable solvent and then by adding the reagent. The product is made of a surface that is covered with dimethyloctyl chains, attached like the bristles of a brush, hence the term 'brush' phase. A 'brush' phase is depicted in figure 1.
A 'Brush' Type Bonded Phase Structure (A)
A 'Brush' Type Bonded Phase Structure (B)
Figure 1. Brush Type Bonded Phases
However, the actual organization of the dimethyloctylsilyl groups on the surface as depicted in figure 1 (A) is not generally accepted. It is believed by many that the two methyl groups on the silicon atom of each attached group will sterically hinder the possibility of the reagent reacting with any adjacent hydroxyl group. Actually, many workers in the field believe that each attached organic group is separated by a silicon atom with a hydroxyl group attached as depicted in figure 1 (B).