Look~~ This is my supplementary bottle of Calcium which my grandma bought me 2 months ago. Can you see those little packets of substances? I found them inside the bottle. I think you can certainly guess what it is called as we can often see them in a variety of products and it s name is always written on the package , can't you?
Here's the clearer pictures :
It is SILICA GEL which is a type of desiccant!
It gave me an idea for what to write in this the posting you are reading right now :p
What is a DesiCcAnT?
A desiccant has synonym term of drying agent. It is a substance with great hygroscopic properties, meaning great tendency to attract water molecules from surrounding. Due to this property, desiccant is most commonly used to in preservation of many products to remove excess humidity to sustain a dryness state so that all those products which are sensitive to moisture will neither be degraded nor destroyed! This process is generally termed desiccation. There are a number of substances classified as desiccant, a few of main among them include :
Hygroscopic desiccant has the abillity to attract water molecules frome the surrounding environment through ABSORPTION or ADSORPTION, or both.
Puzzled?
Lets explore about absorption and adsorption of water, they are NOT the same...
Absorption
Is the chemical phenomenon of atoms, ions or molecules enter phase in bulk of gas, liquid and solid. In simpler, it is a process of a substance permeates another or being dissolved by another substance. Absorption is taken up by volume, not surface.
While....
Adsorption
Is the accumulation of molecules of a gas to form a thin film of adsorbate on the surface of the adsorbent. Silica gel functions mostly on the adsorption basis!
Molecules accumulate on the surface!
Silica gel was known since 1640 but its desiccant properties were found only during the World War I as it's adsorbent ability was realised to be useful in gas masks canisters.
Silica gel is an oxide of the Silicon element as well as a partially hydrated structure of silica, which is - the sand. It belongs to a type of highly activated porous adsorbent and amorphous substance appears in granular form of solid.
HOW is it being manufactured in the industrial process today?
Generally, Silica gel, the high capicity adsorbent of silicon dioxide is made synthetically from an acidification process of the solution of Sodium Silicate, which is "water glass" sold in drug store and a Strong acid such as Sulphuric acid and Hydrochloric acid. This chemical reaction come out with a product known as Silicic acid, which is a kind of hydrated silica with the chemical formula xSiO2 . yH2O due to the polymerization process. This resulting acid appears in the precipitate form of a transparent gel or colloid. Some soluble substances on the gel is to be then removed by washing with water.
Lets use hydrochloric acid as an example,
Na2SiO3 (aq) + 2 HCl (aq) ------------> H2SiO3 (s) + 2 NaCl (aq)
Sodium silicate(aq) + Hydrochloric acid(aq) -----------> Silicic acid(s) + Sodium chloride(s)
Sodium Silicate, Na2SiO3
Silic Acid, H2SiO3 
In the primary state of silical gel, each particle consists of a core of a silicon atom join with oxygen atoms by silicon-oxygen-silicon bond known as Siloxane bonds. On its surface, there are still uncondensed hydroxyl groups attaching to the particle for its polar properties. The hydroxyl group are then ti be eliminated from the polymeric silicic acid. There are three types of hydroxyl group with as you could seen from the picture.
When the Silicic acid is obtained, water molecules have to be first eliminated from the acid through heating process, by drying it up or dyhydrating it we can then obtain the solid granular desiccant, sillion dioxide.
Silicic acid(aq) ----------------> Silicon dioxide (s) + water (l)
H2SiO3 (aq) -----------------> SiO2(s) + H2O (l)
General Structural formula of silicon dioxide :
Silicon dioxide is actually a Giant covalent structure... There are three different crystal forms of silicon dioxide.Let me just show you the simplest one :
Remember this is just a tiny part of the whole covalent structure extending in 3 dimensional!
~Its Charateristics~
Silica gel comes with a wide range of particles and packaging sizes in semi-permeable or breathable bulk bags or plastics to suit various kind of applications in industries, science fields, medical, as well as in our daily lifes. Besides high permfomance of adsorbing moisture, silica gel is widely used because of its tasteless, odourless, non-corrosive and non- harzardous nature. Additionally, silica gel is also non-flamable, has good termal stabilty and high mechanical strength. Silica gel can be produced in different range in sense of their funtional charateristics by controlling the conditions during the production processes.
How does it work?
Being a desiccant, silica gel is chemically inert and stable that it does not dissolve in any solvents with alkali and hyfdrofluoric acid an exception. The highly porous characteristic has contributed dominantly to the adsorption performance of silica gel. Different size of pore diameters and dimessions will lead to different adsorption performance!
The porous structure of the silica gel!
The internal surface of silica gel is consisted of a network of interconnected pores with extremetly large surface area, that is around 800 m/g allows it to adsorb water molecules, alcohols, hydrocarbons and other types of chemical readily and holds them to itself. Silica gel adsorb water physically by the phenomena of physisorption basis, meaning due to capillary condesation of water molecules by van der waals interations at high humidity in air or liquids. Therefore, during adsorption process, it does not undergo any chemical reaction and does not have any equation as well as products.
Silica gel will be appearing dry and no changing in size or shape even when it is saturated with water! Like a sponge, Silica gel is capable to adsorb 40% of it's own weight in water vapours at a humidity of 100%. With this capability, silica gel is said to have the highest capability of water adsorption among many other commercial desiccants!
Have a look at the following table :)
Desiccant Needed to Adsorb 6 Grams of Water Vapour |
Desiccant | Mass (grams) |
Silica Gel | 15 |
Indicating Silica Gel | 75 |
Montmorillonite Clay | 24 |
Calcium Oxide (quicklime) | 21.5 |
Calcium Sulfate (gypsum, drierite) | 60 |
Wood 43 | 43 |
Silica gel requires in smallest amount to adsorp 6g of water vapour.
There are two types of silica gel which are the non-indicating and self-indicating types. White or colourless gel is the non-indicating type while coloured gel is the self-indicating type as it will undergo colour changing to indicate the moisture level of the gel due to the presence of colour indicator. The most common type is the blue indicating silica gel which will turn to purple then to pink when being hydrated due to presence of Cobalt Chloride indicator. In contrast to indicating type, non-indicating one will be much cheaper as no clour indicator is present. It is effective too after all.
Can silica gel be REUSED?
Yes, used Silica gel can be recycled or regenerated by thermally-induced drying or heating by solar energy or oven at about 120 degree Celsius to dyhydrate it.
Silica gel has multiple functions, they are being widely used in chemistry field for drying of solvents, drying of analytical samples, thicken liquid, combine with alkali metals to form a M-SG reducing agent and chromatograpy which is the separation of mixtures by laboratory techniques. Besides, silica gel is used in supplementary bottles, electronic appliances, pepperoni, leather products, optical instrument, drying and storage of flower and seed and many more.
If you do raise a cat, do you know your cat litter contains silica gel too to adsorb moisture from your cat's faeces and urine to reduce foul odors from ammonia?
Cat : "Yeah~ My sweet home will not be smelly with Silica gel!"
Labels: Desiccant
