Applications of Colloids
Colloids play a very important role in nature, in our daily life and in industry. Some of the important applications of colloids are discussed below.
1. Food stuffs and medicines: Many of our food stuffs are colloidal in nature. Milk, butter, whipped cream, fruit jellies, ice cream, bread etc. are all colloidal in nature. For example, milk is an emulsion of butter fat in water, stabilized by milk protein (casein). Ice cream is a dispersion of colloidal ice particles in cream. Similarly, bread consists of air dispersed in baked dough.
Colloidal medicines are more effective and are easily absorbed by the body system. Therefore a large number of pharmaceutical preparations are emulsions. Halibut-liver oil, cod-liver oil, skin ointments etc. are emulsions. Antibiotics such as penicillin, streptomycin etc. are usually injected in the body in colloidal form. Several metal sols are also used as medicines.
2. Purification of water: In water works, water is usually purified by the addition of certain electrolytes such as potash alum, aluminium sulphate etc. This involves the phenomenon of coagulation. The impure water usually contains dispersed colloidal particles which cannot be removed by filtration. When potash alum is added to impure water, the negatively charged colloidal particles of impurities get coagulated by the action of Al3+ ions furnished by the alum and can be removed by filtration or decantation.
3. Sewage disposal: Sewage water contains particles of dirt, mud etc. which are colloidal in nature and carry some electrical charge. These particles may be removed by using the phenomenon of electrophoresis. The sewage water is passed through a tunnel fitted with metallic electrodes and maintained at a high potential difference. The colloidal particles present in the sewage water migrate to the oppositely charged electrodes and get coagulated. This solves the problem of sewage disposal. Moreover, the rubbish matter obtained on account of the coagulation of colloidal dirt particles may be used as manure.
Cottrell smoke precipitator
4. Smoke precipitation: Smoke is colloidal system and consists of electrically charged colloidal particles of carbon dispersed in air. As smoke is a big source of pollution. It is always desirable to precipitate it, i.e., to remove colloidal carbon particles present in it. The removal of colloidal carbon particles from smoke can be effected by using the phenomenon of electrophoresis. This is achieved in an apparatus called Cottrell precipitator as shown in figure. Smoke is allowed to pass through a chamber having a number of metal plates attached to a metal wire connected to a source of high potential (20,000 to 70,000V). The electrically charged colloidal particles of carbon and dust get discharged when come in contact with the oppositely charged plates and fall down to the bottom. The clean hot air leaves the precipitator from an exit near the top.
5. Artificial rain: Clouds are colloidal systems and consist of water vapour mixed with dust particles. The water molecules present in a cloud develop some electrical charge. Therefore, clouds can be made to rain by neutralizing the charge present on colloidal particles (water molecules). This type of rain is called artificial rain and may be carried out by spraying oppositely charged colloidal dust or sand particles over a cloud. This neutralizes the charge on water molecules and compels them to get coagulated, i.e. to rain.
6. Rubber industry: Latex obtained from rubber trees is an emulsion consisting of a dispersion of negatively charged rubber particles in water. In order to obtain rubber from latex, the latter is boiled when rubber particles get coagulated. The coagulated mass is vulcanized and sold as natural rubber.
Rubber-plated articles can be prepared directly from latex by electrically depositing the negatively charged rubber particles over the article to be rubber-plated by making the article an anode in the rubber-plating bath.
7. Leather tanning: During the tanning of leather, the positively charged colloidal particles of raw skin and hides are coagulated by the negatively charged tanning materials which include tannin and compounds of aluminium and chromium. On soaking hides in the solutions of tanning materials, the coagulation of negatively charged tanning materials takes place in the pores of hides and the latter get hardened.
8. Cleansing action of soaps: Soaps solution is colloidal in nature and removes dirt and oil from clothes by forming water soluble emulsion as explained earlier.
9. Smoke screen: In chemical warfare, smoke screens are generally used to hide the movement of troops. Smoke screens are the colloidal dispersions of very fine particles of titanium oxide in air.
10. Formation of delta: When a river falls into the sea, extensive deposits of sand and clay are formed at the mouth of the river in the sea. Such deposits are called delta. The river water contains colloidal particles of sand and clay which carry negative charge. The sea water contains a number of positive ions such as Na+, Mg2+, Ca2+ ion etc. When the river water comes in contact with sea water, the negative charge present on colloidal sand and clay particles get neutralized by the positively charged ions (present in sea water) and they get coagulated. The coagulated sand and clay particles settle down and take the shape of delta in due course of time.
Formation of delta
11. Blue colour of the sky: The sky is the empty space around earth and as such has no colour. It appears blue due to the scattering of light by the colloidal dust particles present in air (Tyndall effect).
12. Preparation of Nano-materials: Nano-materials which are used as catalyst are prepared by using reverse micelles.
13. Building roads: Now a day, roads are built asphalt emulsified in water. This technique does not require the necessity of melting the asphalt.
14. Metallurgical operations: Several metal ores are concentrated by froth floatation process. This process involves the treatment of the pulverized ore in emulsion of pine oil.