Porous silica is the most widely used packing material in HPLC. Its chemical composition could be expressed as SiO2 x H2O, which means that water is chemically bound in non-stoichiometric amount.

Formation of the porous silica consists of several steps:

Formation of silica sols. Sol is basically a type of emulsion of colloidal silica particles of size 1 - 100 nm formed by polycondensation of dissolved silicates at pH~8. These particles are spherical in shape, nonporous and amorphous.

Formation of hydrogels. Dispersed silica particles in sol tend to aggregate to form a three-dimensional structure of sole particles linked together. This process leads to the formation of gelatinous mass, called silica hydrogel.

Formation of xerogels. After washing the silica hydrogel, water is usually removed by heat treatment to form a xerogel. Xerogel is a rigid and porous silica. Its internal surface is not smooth because it is formed by crosslinked small nonporous silica particles. Special hydrothermal treatments needed for further cementation of silica particles by dissolution-deposition process.