Silica gel

The silica gel is a silicic polymer of acid If (OH) prepared starting from sodium silicate.

Structure and properties

The interior of each silica grain is composed of silicon atoms connected to each other by atoms of oxygen (a silicate). On the surface, silanol groups (If-OH) remain and are responsible for very strong the Polarité of silica gel. In the presence of water, this surface is hydrated what causes a reduction in the Polarité.

The silica gel grains are porous and the size of the grains and the pores very strongly depends on the method of preparation used. This structure is responsible for very large the specific surface, typically 500-600 m 2 /g.

Use in chemistry

The silica gel is mainly used like stationary Phase for the Chromatographie. It can also be used as dessicant, reagent or support for reagents.

Use in chromatography

The silica gel is used as support in thin layer chromatography (CCM), chromatography on column (liquid chromatography, flash, MPLC, HPLC…) etc

In the case of the thin layer chromatography, the silica gel is mixed with a binder, like plaster, to which one often adds a fluorescent pigment which allows a detection of made up by exposure to an ultraviolet light. The mixture is deposited on an inert support (plate of glass, plastic, aluminum), let dry then activated hot. In liquid chromatography on column, the silica gel is used such as it is.

Silica can be returned Hydrophobe by grafting hydrophobic groups on the functions silanol present at surface. For example, silica known as " C18" is a silica gel on which one grafted groups octadécyle (18 carbon atoms). In this case, one speaks about Chromatographie in phase reverses because the most polar products are élués in first whereas the most hydrophobic products remain fixed longer on silica.

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