Silanol geminal

Isolated silanol Vicinal silanols Geminal silanols... 

Figure 2.3. Structure of a silica gel showing siloxane bond, single free silanol, geminal silanols, and vicinal silanols.

Only two kinds of functional groups can be found on silica surfaces sUoxane bridges and hydroxyls (silanols). However, among the hydroxyls, different types can be identified, namely single free silanols, geminal hydroxyls. 

Fig. 1. Silanol groups of amorphous silica surface, where 0= Si Q — O and = H (a) isolated, (b) vicinal, and (c) geminal.

A review of the properties of silica as applied to reversed phase separations summarizes a number of issues that have been debated for many years.71 The review categorizes unmodified silanols as free, geminal, vicinal, and internal. The pK, values of silanols average about 7.1, but some silanols may have pK, values as low as 3. As reported in Chapter 4, heavy metal... 

Gemifloxacin, 21 224 Geminal dihydroperoxides, 13 455-456 Geminal silanol groups, 22 380, 381 silica surface chemistry and, 22 373 Geminate recombination, 14 620 Gemopatrilat, 5 159... 

Isolated silanol group Vicinal silanol groups Geminal silanol groups... 

The siliceous surface provides various reactive sites that have been assigned to distinct silanol groups (isolated, perturbed isolated, vicinal, geminal) and strained... 

Figure 14.1 Silsesquioxanes used as models for Isolated, vicinal and geminal surface silanols.

Figure 9. Schematic representation of active groups on silica surfaces (A) isolated silanols (B) siloxane bonds (C) geminal silanols (D) hydrogen-bonded silanols ( ) hydrogen-bonded water. (Reproduced with permission from L. Johnston in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 359.)...

Silicagel is also called silica or bare silica. Its adsorptive properties depend on the hydroxyl groups attached to surface silicon atoms. Silicagel has a maximum silanol density of 8.0 yumolcs/ m2. Many of these silanols are buried deep in the porous structure and are available only to the smallest analytes. Silanols are either isolated, geminal, or vicinal they can be distinguished by means of Si solid-state nuclear magnetic resonance (NMR). The surface also contains siloxane bonds (Si-O-Si), which are considered hydrophobic. 

Thus, heating the silica gel to temperatures above 500°C followed by rehydration changes the proportion and distribution of isolated geminal silanol sites. At higher temperatures only isolated silanol groups remain. 

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