Liquid Chromatography - The Preparation of Irregular Silica Gel
The size of the primary particles depend, among other factors, on the conditions of synthesis, (e.g., reaction temperature, the pH of the mixture at the time of gelling and even the subsequent treatment of the gel, including the conditions of washing). The product at this stage is called a 'hydrogel'.
If a neutralized silicate solution is allowed to age with gentle stirring to prevent gel formation, Ostwald Ripening occurs, causing the larger primary particles to grow in size at the expense of the formation of smaller particles. In general, the aging and ripening of silica solutions will increase the size of the larger particles which, in turn, will decrease the surface area of the silica and increase its porosity.
It is apparent that the formation of the primary particles of condensed silica, and their subsequent fusion by gelling, that confers on silica gel its high porosity and high surface area, which are so important in its use as a stationary phase in LC. Furthermore, as has already been stated, it is the condensation of the surface silanol groups of the primary particles, that causes their adhesion and the onset of gel formation and, consequently, the mechanical strength of the gel.
After the gel has first set, it is a very soft and is usually transferred to vats or trays where it is then allowed to stand for a number of days. During this period, condensation between the primary particles continues to take place and the gel shrinks and exudes salinated water. This shrinking process accompanied by saline elimination is called sinerisis and eventually a firm, almost rigid gel is produced which is called the hydrogel. The compact hydrogel is then well washed under controlled conditions to eliminate the last of the sodium chloride and then heated for a few hours at 120oC. The resulting product is a hard amorphous mass called the xerogel. The xerogel, ground and graded is the material that is used for packing LC columns and manufacturing bonded phases. The product, prepared in this way, is called irregular silica gel, to differentiate it from spherical silica gel which is prepared by employing an entirely different synthetic procedure. Irregular silica gel is also the basic material from which bonded phases can be prepared.