Stability of Colloids Chemistry Notes

Stability of Colloids Chemistry Notes

Stability of Colloids :

A colloidal solution is quite stable due to the following reasons:

→ Existence of electrical charge on colloidal particles : All the colloidal particles in sols carry similar charge. The repulsive interation created by these similar charges do not allow the particles to come closer to each other and hold them apart. It prevents the aggregation of sol particles thus stabilizing the system.

→ Brownian movement. The continuous, random, zig-zag motion of sol particles is also responsible for the stabilty of the sol. This zig-zag motion is due to continuous bombardment of the particles. Due to this Brownian movement, particles do not settle under the influence of gravitational force. Thus it helps in stabilizing the colloidal system.

Stability of Colloids Chemistry Notes

→ Presence of small amount of electrolyte : A trace of electrolyte in lyophobic sol is responsible for the origin of charge on collodial particles, thus stabilizing the colloidal solution. On the other hand, a traces of electrolyte in lyophilic sols diminishes the zeta potential of colloidal particles and thus prevents the aggregation of particles and helps in stabilizing the colloidal system.

→ Solvation of sol particles : As we known that lyophilic sols are more stable than lyophobic sols. It is due to their greater affinity for dispersion medium. In lyophilic sols, the sol particles are surrounded by a sheath or layer of the dispersion medium which prevents their actual contacts, so the lyophilic sols are stable.

Emulsions :

→ The colloidal system, in which the dispersed phase and dispersion medium both are liquids, is known as emulsion, In emulsion, the finely divided droplets are dispersed in another liquid. It can be obtained by shaking a mixture of two immiscible of partially miscible liquids. Generally one of the two liquids is water.

Emulsions are of two types :

  • Oil dispersed in water (O/W type)
  • Water dispersed in oil (W/O type)

→ Oil dispersed in water (O/W type): In this type of emulsion, oil acts as dispersed phase while water acts as dispersion medium. Examples: milk, vanishing cream, lysol etc.

Stability of Colloids Chemistry Notes 1

→ Water dispersed in oil (W/O type): In this type of emulsion, water acts as dispersed phase while oil acts as dispersion medium. Example : Cold cream, butter, cod-liver oil etc.

Stability of Colloids Chemistry Notes 2

Emulsifying Agent :

Emuslion of oil and water is unstable and some time they are distributed into two layers. So to make the system stable, a third component is added to it. It is known as emulsifying agent or emulsifier. Actually an emulsifier is an agent which stabilises the emulsion by lowering the interfacial tension between the constituents of the emulsion. It forms a film around the droplets of dispersed phase and prevents them from coming together and thus prevents coagulation.
Examples:
For oil/water (O/W) type emulsion, main emulsifier are protein, gum and syntheic surface active agents such as soaps, detergrents etc. Sometimes lyophilic sols also act as emulsifier.
For water/oil (W/O) type emulsion, main emulsifier are salts of heavy metals of higher fatty acids, alcohols of long chain, lamp black etc.

Stability of Colloids Chemistry Notes

Properties of Emulsions :

The main characteristics of emulsions are as follow:

  • The size of colloidal particles are comparatively larger than sols.
  • Emulsion exibits all the properties of colloidal solutions such as Tyndall effect, Brownian movement, electrophoresis etc.
    Emulsions are coagulated by the addition of electrolytes containing polyvalent metal ions, indicating the negative charge on the globules, i.e., the droplets in emulsions are negatively charged.
  • Emulsions are diluted by adding any amount of dispersion medium. However, addition of dispersed phase results in the formation of separate layer.
  • Emulsion can be separated into two layers of liquids.

De mulsification :

The process of breaking down of emulsion into two separate liquids is called demulsification.

Demulsification can be done by :

  • By boiling or freezing or ceritrifuging the emulsion. Cream is separated from milk by centrifugation.
  • By adding a substance which destroys the emulsifier i.e., either by electrostatic precipitation or chemical methods.
  • W/O type emulsion can be demulsified by the addition of dehydrating agent.

Methods for the Detection of Dmulsions :

To determine the type of emulsion, following methods are used:

→ Indicator Method: In this method, an oil soluble dye is shaken with emulsion. If coloured droplets are seen on looking at it under microsocope it is of O/W type emulsion. However, if the whole background is coloured, the emuslion is W/O type emulsion. It also known as dye test.

→ Dilution Method : Dilution of any emulsion can be done by adding dispersion medium in any amount. Whereas the addition of dispersed phase causes the formation of separate layer. It means if emulsion is diluted by oil then it is W/O type emulsion or if it is diluted by water then it is O/W type emulsion.

Stability of Colloids Chemistry Notes

→ Conductivity method : Electrical conductivity of oil in water type emulsion is much greater than water in oil type emulsion. If an electrolyte like NaCl is added to oil in water type emulsion its conductivity increases rapidly. Other methods can also be used to measure conductivity.

→ Viscosity Method : Viscosity of water in oil type emulsion is larger than oil in water type emulsion.

→ Spreading Method : Water in oil type emulsion easily spreads on the surface of of an oil but not on the surface of water. In this way, type of emulsion can be detected.

Applications of Emulsions :

Some important applications of emulsions are given below:

→ In froth floation process : Froth floation process is used to concentrate sulphide ores. In this process, the finely powdered ore is treated with an oil emulsion. On passing air through this solution then ore particles come with froth to the surface, where it is collected.

  • In medicines : A number of tonics, cough syrups, cod-liver oils, digestive juices are emulsions. As Cosmetics : Various pharmaceuticals, lotions, creams, ointments are emulsions.
  • In digestion of fats and lipids : In intestine, the digestion of fats is done by emulsification.
  • As disinfectants : The disinfectants like dettol, lysol etc. form emulsions in water.
  • Some important products : Milk, toothpaste, B-complxes, butter, jam etc. are important products present in the form of emulsions.
  • In Building Roads : An emulsion of asphalt and water is used for building roads without the necessity of melting asphalt.

→ Cleansing action of soaps : The cleansing action of soap is based upon the formation of W/O type emulsion. When dirty clothes are dipped in soap or detergent solution then hydrophobic end of soap or detergent dissolves in oil or grease. While the hydrophilic or polar end of soap or detergent projected in water.

→ Due to the presence of negatively charged groups, each oil droplet is surrounded by an ionic atmosphere. Due to repuslion, oil or grease particles change into very small drops. Thus a stable emulsion of the oil in water type is obtained. This emulsion can be washed away by stirring or rubbing the clothes and finally clothes become clean. In this way, soap and detergent help in cleaning the dust or dirt from the cloth by emulsifying the fat and grease containing the dirt.

Applications of Colloids :

Some important applications of colloids are as follow:

→ Cottrell Smoke Precipitator: Smoke can be made free of colloidal particles of carbon by passing it through cottrell precipitator installed in the chimney of an industrial plants. Cottrell precipitator consists of two metal discs charged to a high potential. The charge present on dust and carbon particles present in smoke is neutralised and it in precipitated. While waste gas comes out through chimney.

Stability of Colloids Chemistry Notes 3

→ Purification of drinking water : Water obtained from natural sources has suspended impurities in it. To coagulate these impurities, alum is added to it. In this way, water can be made pure for drinking

→ Medicines : Various medicines are colloidal in nature. Colloidal medicines are highly effective because they have large surface area and are therefore easily assimilated.

For examples:

  • Any eye lotion argyrol, is a colloidal solution of silver.
  • Colloidal antimony is used in curing Kala-azar.
  • Colloidal sol is used for intramuscular injection.
  • Milk of Magnesia, an emulsion, is used for stomach disorders.

→ Cleansing action of Soap and Detergent: It is also an colloidal process.
→ Industrial Products : Paints, inks, synthetic plastics, rubber, graphite, lubricants, cement etc. are all colloidal solutions.
Photographic plates and Films : The photographic plates and films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin over glass plates as celluloid films.

→ Tanning : Animals hides are colloidal in nature. When a hide, which has positively charged colloidal particles, is soaked in tannin, which has negatively charged particles, the mutual coagulation takes place. In this way, hardening of leather takes place. This process is known as tanning. In place of tannin chromium salts can also be used.

Stability of Colloids Chemistry Notes

→ As Disinfectants : Disinfectants like dettol, lysol, savlon etc. are colloidal solutions.

→ In sewage disposal : Sewage water has particles of dirt, rubbish etc, which are charged colloidal particles and do not settle down easily. For removing these particles, the process of electrophoresis is used. This sewage water is passed through metallic electrode having high potential difference. Now colloidal particles present in sewage migrate towards oppositely charged electrode where they are coagulated. The water obtained after purification can be used for irrigation.

→ Rubber Industry: Rubber is emulsion of negatively charged rubber particles present in water. It is called rubber milk. The protective layer of protein, which covers the particles naturally is broken on boiling rubber milk.

→ So, the particles of rubber are precipitated in the reaction of salts present in dispersion medium. The precipitated rubber on treatment with sulphur forms vulcanized rubber. If a layer of rubber is covered on an object then the layer of rubber is covered by making that object as anode by electrophoresis. The negatively charged particles of rubber are electrically neutral at anode.

Stability of Colloids Chemistry Notes 4

→ Formation of Delta : River water is colloidal solution of clay particles which carry negative charge. Sea water contains a number of electrolytes like NaCI, MgCl, etc. When river water meets the sea water, the electrolytes present in sea water coagulates the colloidal solution of clay which gets deposited with the formation of delta at the mouth of the river.

→ Blue Colour of Sky : Colloidal dust particles along with water suspended in air scatter blue light which reaches our eyes and thus sky appears blue in colour.

Stability of Colloids Chemistry Notes

→ Fog and Mist : When a large mass of air containing dust particles is cooled below its dew point, then the moisture present in air condenses on the surface of these particles forming fine droplets. These droplets are colloidal in nature and hence float in air in the form of mist or fog.

→ Natural Rain : Clouds are aerosols having small droplets of water suspended in air. These clouds sondense in the upper atmosphere and thus bigger in size, till they come down in the form of natural rain.

→ Artificial Rain : Sometimes artificial rainfall can be done by meeting two oppositely charged clouds. So artificial rain can be done by throwing electrified sand or by spraying a sol carrying charge opposite to the one on clouds from an aeroplane.

→ Blood : Blood is also an colloidal solution of an albuminoid substance. The styptic action of alum and ferric chloride solutions is due to the congulation of blood forming a clot which stops the further bleeding.

→ Soils : Fertile soils are colloidal in nature in which moisture or humus acts as a protective colloids. Due to colloidal nature, it adsorbs moisture and nourishing materials.

Chemistry Notes

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