Silica gel, chemical formula: mSiO2 • nH2O, appears as slightly transparent white solid substance. It belongs to genus amorphous. Silica is a highly active, porous material with the internal surface area being large. The specific surface area per g silica can reach above 450 m2. Therefore, it has strong absorption capability on the liquid or gas. Silica gel can’t be dissolve in water, not soluble in other solvents except hydrofluoric acid and strong alkali.

The commercially available solid gel silica has spherical and irregular, two kinds of shapes. According to the pore size, it can be divided into the fine-pore silica gel (with average pore size of less than 1.5 ~ 2.0nm) and coarse pore gel silica (with average pore size of greater than 4.0 ~ 5.0nm). There is another kind of discoloration gel silica containing cobalt chloride with anhydrous cobalt chloride exhibiting blue color. However, after absorbing water, the hydrated cobalt chloride will exhibit pink color with returning back to blue after drying. According the change of the color during dehydration and hydration of cobalt chloride, we can know moisture absorption condition of the silica gel. The pink silica gel has no absorbent capacity, but after thermal dehydration, it will have its color return back to blue and get its absorbent capacity restored.

The silica gel can be manufactured through the reaction between water glass and sulfuric acid, and further gelling, washing, desalting, amidation, drying and screening. The reaction is as follows:

nNa2O • mSiO2 + nH2SO4 → mSiO2 • nH2O + nNa2SO4. General industrial silica gel has a mechanical strength of 85 to 95%, water content being below 2% and the moisture absorption rate of 25 to 30%.

Gel silica is mainly used for gas absorption or drying, liquid dehydration and liquid chromatography, also used as catalysts or catalyst carrier. The textile industry uses it as sizing agent. Pore silica gel is mainly used for the drying and purification of gas, the dehydration and purification of organic product, also used as a supported carrier. Coarse-pore gel silica is mainly used to process the macromolecular protein contained in the beer as well as the drying of gas and liquid and defogging as well as dehumidification of gas upon high humidity, as the catalyst and catalyst carrier in the petrochemical industry. Discoloration gel is mainly used to indicate the relative humidity of airtight packaging, precision instruments and the inner space of instrument as well as being used for the moistureproof maintenance of precision instrumentation.

Chromatography-purpose silica gel
Chromatography-purpose silica gel is a kind of porous material, being able to be represented by the formula SiO2 • xH2O. The molecule contains the cross-link structure of siloxane (-Si-O-Si), while there are a lot of silanol groups on the surface of the particle. The absorption strength of the gel silica is dependent on the content of the silanol. Silanol group can absorb the water molecule through the formation of hydrogen bond so the absorption force of silica gel will decrease with the increase of the absorbed moisture. If the water absorption capacity exceeds 17%, the absorption force is too weak to be used as an adsorbent, but can be used as the supporting agent in the partition chromatography. For the activation of the silica gel, when silica gel is heated to 100 ~ 110 ℃, the hydrogen bond-absorbed water molecule on the silica gel surface can be removed. When the temperature rises to 500 ℃, the silanols on the silica surface can also undergo dehydration and condensation to convert into a siloxane bond, thus losing the activity of absorbing water through the hydrogen bonds, thereby no longer there having the nature of the adsorbent. Even water treatment can’t restore its adsorption activity. Therefore, the activation of the silica gel should not proceed at higher temperatures (generally at above 170 ℃, there will be a small amount of bound-water lost).

Silica gel is an acidic adsorbent, being suitable for the chromatography of neutral or acidic components. Meanwhile, the silica gel is also a kind of weak acid cation exchanger with the silanol group on the surface being able to release weakly acidic hydrogen ions. When meeting strong alkali compound, it is capable of absorbing alkaline compound due to ion exchange reaction.

Classification of the silica gel
Based on the application, silica gel can be divided into desiccant silica gel, absorbent silica gel, analysis-purpose silica gel, packaged drying-purpose silica gel, catalyst carrier-purpose silica gel and special dedicated silica gel.
Silica gel is characterized by being chemically inert, high surface area, high internal porosity, and strong adsorption capacity, widely used as dehumidifying desiccant, dehydrating agent, adsorbents, fillers and catalyst carrier. It is easy to adsorb polar substances, difficult to adsorb non-polar organic substances.

The moisture contained in the absorbed gas of silica gel can reach 50% of its mass while in the air stream of 60% humidity; the water amount adsorbed on micropore silica gel can also reach 24% of the mass of the silica gel. Therefore, it is commonly used in the drying of high-humidity gas. We will introduce the major categories of commercial silica gel products and their application below.
(1) Spherical silica gel. Spherical silica gel is the spherical particle of particle size of 2.0 ~ 8.0mm, There are certain differences in the surface area, pore volume, bulk density and some other indicators of the spherical silica gel produced in different places, which are mainly used for gas drying, gas absorption, humidity control of air, oil product refinement and catalyst carrier and other aspects.
(2) Granular silica gel. Irregular particle of the size of 6 ~ 20 mm with the specific surface area of 700 ~ 800 m2/g. It is mainly used for the drying of natural gas, nitrogen and recycled hydrogen gas as well as the hydrocarbon separation of the natural gas.
(3) Waterproof silica gel. It is mostly spherical with the specific surface area of 300 ~ 400m2 / g. It is characterized by not burst upon meeting water. It is mainly used for gas drying, dehydration of liquid saturated hydrocarbon as well as catalyst carrier.
(4) Micro-powder silica gel. Its average particle size is about 2 ~ 10μm. It is amorphous and has a oil absorption value of 60 ~ 300 g/100g. It is mainly used as the additives of food, medicine and cosmetics. It is non-toxic with the addition amount of food, medicines not exceeding 2%.
(5) Selective adsorbed silica gel. It is a silica gel manufactured from finishing, also known as "adsorption silica gel of chromatography". The column chromatography silica gel on the domestic market falls into this category. It appearance contains bulk-shape and microsphere-shape. It has the granularity of 60 to 300 meshes and its specific surface area of 300 ~ 750m2 /g. It is mainly used for the absorption, separation and purification of various substances.
(6) Blue-indicating silica gel. It is a kind of silica gel which can exhibit different colors with the change of the relative humidity of the environment. Its appearance can be bulky and spherical shape. It is mainly used as the indicator of relative humidity for the moisture proof packaging. The moisture absorption degree of the indicating drying agent can be used to judge of relative humidity of the environment. For example, the "Ocean" brand blue gel indicator, respectively exhibit light-blue color, purple and red under the relative humidity of 20%, 40% and 60%.
(7) Package-purpose silica gel desiccant. According to different situations of applications and packaging, the packaging-purpose silica gel desiccant contains many types and varieties, such as bagged category, unique shape type and dedicated type.
Bagged category contains paper bags, cotton bags, non-woven bags, nylon bags, polyethylene bags, and is the largest category in the package type of the silica gel. The heteromorpha type includes plate shape, cylindrical, bar form, plugged-shape silica gel desiccant. Dedicated type is specific silica gel desiccant specifically designed and manufactured based on the requirements of the user and design

Reference:
Zhu-Hongfa, ZHU-Yuxia editor “fine chemical products manufacturing technology” Beijing: 2002 Golden Shield Press, page 168.
Lin Chongde, Lu Jiang, Wang Desheng editor; Wang Desheng sub-volume editor; Chinese Adult Education encyclopedia • Chemical • Chemical Industry.