Most products that stand a chance of getting damaged due to coming in contact with moisture are protected by using an adsorbent material. This material removes the humidity or moisture from its surroundings. This process is called desiccation, and the products that make it possible are known as desiccants. Silica Gel is one such product. Apart from its standard form where it is used in a sachet, it can also be used as colloidal silica and nano-silica. Read on to find out more about this desiccant known as Silica Gel.
What Exactly Is Silica Gel?
Chemically, silica gel is nothing but silicon dioxide and is amorphous. These appear as granules that look like small beads. They are instrumental as a desiccant because of their molecular structure. They have porous cavities that give them a large surface area, making them ideal for high adsorption levels.
The water molecules stick to the gel easily because its vapour pressure is lower than that of the surrounding air. As the pressure equalises, the adsorption capacity reduces. High humidity in the air allows greater adsorption, but overly high humidity can damage the gel.
Benefits of Using Silica Gel as Desiccant
Silica gel has specific chemical properties that make it widely accepted as a desiccant. Some of these are:
- It Is Non-Flammable
This makes it very safe to use with materials of all kinds.
Heating the gel can release the molecules that it has adsorbed. It regenerates the gel and can be used again.
- High Adsorption Capacity
It can adsorb up to 40% of its weight.
- No Expiry Date
If stored in airtight conditions, it can last indefinitely.
It will not react with materials to damage them in any way. It is non-toxic and non-corrosive.
It can be packed easily and is available in varied sizes that make it useful for different applications.
Other Desiccant Forms of Silica
When present as hard granules, it is known as silica gel. In a suspension, they take the form of colloidal silica, and the nano-silica suppliers provide the same in a fluffy powder form. Their applications are very different from each other.
Uses of Silica Gel
Silica Gel is used in many ways, as detailed below:
- It is often used wrapped up in permeable paper sachets to absorb moisture from commercial goods and dry food packets. It helps prevent any damage to the goods by way of reacting with the moisture.
- It protects chemicals from moisture preventing their decomposition.
- It protects electronics from damage due to moisture.
- It is used for drying the air in compressed air systems (air dryers).
- It is used to control relative humidity in setups where the systems can be adversely affected, such as transmission systems, museums, libraries, etc.
- It is also used as cat litter, either on its own or in combination with clays. Its property of being odorless is put to fair use here.
- It is added to food as an anti-caking agent, stabilizer, absorbent, defoaming agent, and many other roles.
- It is used to filter water as its surface allows adsorption of some unwanted minerals dissolved in water.
- Adding a color-changing indicator to the gel makes it act like a humidity indicator, an indicator for the presence of moisture. The gel changes color when it absorbs the moisture. Typical indicators are cobalt chloride and methyl violet.
Dangers of Using Silica Gel
- While silica gel is non-reactive under most circumstances, it does react with strong acids, strong bases, and oxidisers.
- The gel is not suitable for the respiratory tract and can irritate the digestive tract. Hence, it is used with a warning to prohibit consumption.
Industrial Use as Air Dryer
A desiccant dryer removes the moisture present in the air with a desiccant that can successfully adsorb it. This kind of dryer finds application in various industrial processes. An air dryer typically has two cylinders filled with a desiccant like silica gel and connected by a valve. The first cylinder, when saturate, passes the air to the second cylinder for adsorption. Since the desiccant is regenerative, it can be used again.
The fact that silica gel is non-toxic, non-corrosive, and odourless allows the dryers to function economically with premium desiccants to be used in large volumes. Silica gel is thermally and chemically stable and hence suitable for this purpose.
Comparison with Other Desiccants
Silica gel is one of the most used desiccants; the other most used one is the molecular sieve. While they are both equally adsorbent, the molecular sieve needs higher temperatures for regeneration. This makes it more suitable than silica gel for use when products are stored under conditions with high temperature and high humidity.