Silica gel is another form of silica. Now for silica, as the name suggests it is basically an alternative abbreviation for silicon dioxide that contains one molecule of silica and two molecules of oxygen. Silica is found in abundance in the earth crust. Sand is basically silicon dioxide and quartz is the crystalline form of silica. Glass is also made of silica.
Now, silica gels are derived from silica and are synthesized by a process that involves sodium silicate and an acid. In this process, the solution of sodium silicate is acidified with an acid. The resulting silicic acid either forms a solid mass or a gelatinous solution. The insoluble precipitates are then removed from the solution and then washed thoroughly with water. The water is then finally removed by heating the precipitate leaving a clear granule of silica gel. The chemical reaction is mentioned below.
nNa2O + nH2SO4 → mSiO2• nH2O + nNa2SO4
Silica gel is basically a highly porous, non crystalline, slightly transparent form of silicon dioxide commonly known as silica. Silica gels are majorly used to remove moisture from solid and gases, for liquid thickening, for polishing a surface to paint and glass making. The silica gel chemical formula is mSiO2.nH2O. Silica gel structure is-
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By analysing silica gel structure and silica gel formula together, it is evident that silica gel has a large internal surface area that makes it a highly active and porous substance. Thus, it belongs to the genus amorphous group. The silica gel structure shows that it has an irregular three-dimensional framework where one silica and oxygen atoms alternatingly spread throughout the molecular framework. This framework has voids and pores of nanometer-scale and these voids are either filled with water or liquid. Alternatively, if the voids are filled with gas or vacuum the substance is called silica xerogel.
Silica Gel Properties
As silica gel is highly porous in nature, therefore, it can hold about 40-50% more weight of the moisture absorbed compared to its actual weight. As per the commercial availability of silica gel, it has been broadly classified into two shapes, spherical and irregular. There are a few important physical and chemical properties of silica gel mentioned in the table:
Types and Classification of Silica Gel
On the basis of pore size, it can be divided into three categories, namely-
Type A: In this type, the pore size ranges between 2.5 - 2.8 nm in diameter and clear palettes. It is moisture-proof and dry in nature. Thus, it is used as a catalyst carrier, absorbents, variable pressure absorbents and separators.
Type B: In this type, the pore size ranges between 4.5 - 7.0 nm in diameter and translucent white palettes. Thus, it is used as car litters, driers, catalyst carriers and perfume carriers.
Type C: In this type, the silica gel is translucent and has a micro-pore structure to it. It is used for manufacturing car litters. When it is further dried it forms micropore silica gel which makes good absorbents, drier as well as catalyst career.
Based on the application silica gel has been categorized as following into many parts. A few of them are mentioned below.
Silica Alumina Gel: They are light yellow in colour and are chemically unstable in nature. They are flame-resistant and possess thermal stability. As they also have a superficial polarity, therefore, they are of greater performance than fine-pored silica gel. They are generally insoluble in water but show partial solubility in alkali and hydrofluoric acid.
Stabilizing Silica Gel: They are non-crystalline, non toxic, flame resistance microporous silica gel. Therefore they are used in microbreweries of beer to improve its taste, clearance, foam, colour and to remove the impurities present in the beer.
Spherical Silica Gel: Spherical silica gel has spherical microparticles whose sizes vary from 2.0 - 8.0 nm. Based on their availability in various places, their physical indicators like bulk density, pore volume, surface area and other properties vary. They are mainly used for gas drying, gas absorption, humidity control of air, oil product refinement and catalyst career.
Granular Silica Gel: The particles of granular silica gel vary from 6nm - 200nm and its specific surface area range from 700-800 m2/g. It has irregular particle size and is used for drying natural gas, nitrogen as well as hydrogen gas and helps in hydrocarbon separation from natural gas.
Waterproof Silica Gel: The particles of silica gel are mostly spherical in nature and its surface area varies from 300 -400 m2/g. It has a unique property where it does not burst when it comes in contact with water. Therefore, it is mostly used in career catalysts, drying of gas, dehydration of liquid saturated hydrocarbons.
Micro Powder Silica Gel: The size of the particles of this category varies from 2-10 μm. Thus, it is amorphous and has an oil absorption power that ranges from 60-300 g/100g. It is non-toxic in nature and therefore it is used as an additive for food, medicines and cosmetics. In medicine, the quantity of micro powder silica gel must not exceed above 2%.
Selective Absorbed Silica Gel: This category of silica gel has bulk-shaped and microsphere-shaped particles whose granularity varies from 60-300 meshes and its specific surface area ranges from 300-750 m2/g. Thus it is mainly used for absorption, separation and purification of any substance. Thus, it is commonly known as “adsorption silica gel of chromatography”.
Silica Gel in Water
Though there is no silica gel reaction with water chemically, silica gel or silicon dioxide are desiccant which means it can adsorbed water on its surface due to its high porosity and wide range of surface area of adsorption. So when a silica gel comes in contact with the water surface due to its widespread molecular framework, it adsorbs the water molecules on its surface and lets them sit on the core of its molecule or have any chemical interaction with each other. The process of adsorption of water by silica gel is a reversible process, as the water molecules can be separated from the host by heating the silica gel crystals. A silica gel crystal can hold up to 40% of water content on its surface compared to its own weight.
Uses of Silica Gel
Some of the uses of silica gel are:
Due to high humidity that is the moisture content in the air, there is a high possibility of growth of moulds and fungus. These may accelerate the decomposition of food items being accidentally exposed to moisture for a long time and even chemicals like those present in vitamin pills. Condensation of the excess moisture inside electronic gadgets causes their damage. This placing packets of silica gel inside helps the absorption of extra moisture into the silica gel keeping the objects, chemicals as well as food dry and preventing decomposition action.
The waveguide that is built inside the radio and the satellite are very sensitive to moisture. If accidentally a lot of moisture builds up inside the waveguide of the radio and satellite transmitting signals, it will cause an arcing inside the waveguide itself. Also, the power amplifier that is feeding the power to the waveguide will be damaged. Moreover, the water particles inside the waveguide deteriorate the signal frequency by changing the characteristic impedance. Thus silica gel helps in keeping the moisture inside the waveguide as low as possible.
Silica gel is also used to dry the air of the industrial compressed air system. In this process, the air is passed through the bed of silica gel that absorbs the excess moisture from the air so that it does not damage the electrical gadgets present at the end point of use of compressed air. The same system is applied for the compressed air system for locomotives and in railways.
In chemistry, silica gel is used in column chromatography and is termed as stationary phase. The stationary phase in the column chromatography is often made of silica gel particles 40-63 μm. Different chromatography uses different silica particles as the silica particles are related to the surface area. In this process, due to the non polarity of the silica gel, the non polar component in the solution eludes before the polar components. Thus it is known as stationary phase chromatography. However, when a hydrophilic group is attached to the silica gel the reverse happens. This process is known as reverse phase chromatography. Also for thin layer chromatography, silica gel is applied on the surface of aluminium, glass or plastic sheets.
Due to the adsorption property of silica gel, it is used as a domestic water filter. As the water gets adsorbed on the surface of the silica gel, some minerals along with water get trapped onto the surface of the silica gel that is commonly known as ion exchange.