The specific retention behavior of solutes in adsorption chromatography at given eluent composition is governed mainly by the chemical nature of the silica surface. The synthesis and stability of surface bonded layers are also dependent on the silica surface chemistry, types of the surface silanols, and their reactivity. Here, we briefly consider the silica surface chemistry.
There are five different types of adsorption sites on the surface of the fully hydroxilated silica:
1. free silanol
2. silanol groups with physically adsorbed water
3. siloxane bonds
4. geminal silanol groups
5. hydrogen-bonded silanol groups.
The silanol groups are weakly acidic. Maximum concentration of
silanol groups on the surface of porous silica is 8 to 9 Ámol/m2 or around 5
-OH groups per 100 ┼2 . Most of the chromatographic properties of the silica
surface are related to the interactions with silanol groups, although siloxane and
hydrogen bonded silanols may also contribute to the surface activity.