Types of bonds
A very broad variation of functionality of bonded species could be attached to the silica surface. The most popular is an organofunctional groups, R. R may carry substituents of various functionality such as alcoholic or phenolic hydroxyl, amine, phenyl, carbonyl, nitril etc.
The functional group R can be linked to the surface silicon atoms in the following ways:
- Si-R
-
R is directly bonded to the surface silicon atoms. The elimination of the original hydroxyl groups can be achieved by chlorination of the surface with subsequent treatment of the chlorinated surface with organometallic compounds. This type of surface modification is usually very laborious and does not give a stable reproducible surface modification.
- Si-O-R
-
This bond is known as an ester bond and is easily formed by the reaction between an alcohol and surface hydroxyl groups.
- Si-O-Si-R
-
This structure is obtained by treatment of a hydroxylated silica surface with organosilanes of RnSiX4-n where X is the reactive groups such as halogen, ethoxy and methoxy. This type of bonded ligands is the most widely used in HPLC.
Polymeric and monomeric bonding
Reaction of monochlorosilane with hydroxilated silica surface leads to the attachment of only one alkylsilane ligand to one silanol group. The type of bonded layer which involves the formation of a monomolecular organic layer on the silica surface is called "monomeric bonding".
Polymeric phases can be prepared by reacting silica with di- or trichlorosilane in the presence of trace amount of water, which leads to the formation of tree-like structure on one reacted silanol group of silica surface.
Schematic of monomeric and polymeric bonded layers
