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Chemistry
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PROPERTIES OF SILICA

 

Silica is the material that is used most frequently to prepare bonded phase packings. A number of dfferent procedures are used to manufacture it. Different procedures give silica gels with different properties with regards to shape, particle diameter, pore radius and specific surface area. Typical values are:

 

Pore diameter:                6 - 10 nm

Specific surface area:    200 - 400 m2/g

Particle diameter:            3 - 10 mm

 

The chromatographic properties are determined by the physical properties as well as the chemical composition of the silica surface. There are four different functional groups present on a silica surface, see also the silica figure:

-siloxane groups

- free silanols groups

-geminal silanols groups

- associated silanols groups

 

The total concentration of silanol groups on commercially made silica is usually in the range 5 - 7 mmole/m2.

 

During the manufacturing of the silica gel, impurities such as Fe and Al in the raw material tend to concentrate at the surface. There are strong indications that these impurities influence the chromatographic properties of the final product.

 

 

PROPERTIES OF REVERSED PHASE SURFACES

 

The chemical reaction that is used to synthesise silica based chemically bonded phases can be written as folloows:

 

surface  -  SiOH + ClSiRR´R´´  = Si - O - Si - RR`R``+HCl

 

Here SiOH is a silanol group that is situated on the surface of the silica particle. This group is reacted with a chlorosilane to form a siloxane ( Si-O-Si ) bond. Since the chemical structure of the three R groups can be chosen in many ways, a large number of surface properties can be obtained by this procedure. See also the RP-surface figure.

 

The most common chemically bonded phase is made from dimethyl-octadecyl- chlorosilane which is used to make the monomeric RP-C18 phase. It is called monomeric because each C18 chain is bound to one surface silica group.

 

When the reaction is completed about 50% of the silanol groups originally present on the silica surface still remain. These can be further reacted with trimethylchlorosilane. The purpose is to give the retentive layer more homogenous physical properties. This procedure is called end-capping.

 

Another type of RP-C18 phase  is the polymeric. In this case octadecyl -trichlorosilane is in the first reaction step reacted with silanol groups on the silica surface. In a later reaction step a controlled amount of water is present. Since there are several leaving groups ( i.e. Cl bound to Si ) the silane will hydrolyse and then react with other leaving groups to form a polymeric network.

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