Vicinal silanol functionalities

There are vicinal silanol functionalities characterized by hydroxyl groups attached to adjacent silicon atoms. Here steric effects become important. For instance, vicinal silanols represent a rather weak adsorptive site but in the presence of water can be rendered active (51) ... 

Once the silanol number as a function of temperature is known, further distinction between isolated, vicinal and geminal hydroxyls should be made. Since all silanol types exhibit different reactivities, an exact knowledge of the ratio isolated/vicinal silanols is crucial for a thorough study of a modification reaction. 

The active sites (centers) of silica are mainly the free and bonded silanol groups as shown in Figure 1. Two vicinal silanol groups connected by hydrogen bonds form the bonded hydroxyl groups. In the case of normal-phase liquid chromatography (NPLC), these sites interact with the polar analyte functional group/s. Thus, silica is considered as a polar stationary phase. The properties of a specific silica depend on its surface area and the number of active sites. 

As noted earlier, three types of functional silanol groups exist on the silica surface isolated hydroxyl groups, geminal groups (two -OH groups on one Si atom), and vicinal groups (see Figure 9.6). The silanization reaction is illustrated in Eq. (9.5). 

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