Emulsions, Gels and Application of Colloids

Emulsions

Emulsions are the colloidal solutions in which both the dispersed phase and the dispersion medium are liquids.

Emulsions are liquid-liquid colloidal systems i.e. the dispersion of finely divided droplets in another liquid. Any two immiscible liquids form an emulsion.

For example: milk is a naturally occurring emulsion in which the particles (or globules) of liquid fats are dispersed in water.

 

Emulsions are generally prepared by vigorously shaking two liquids or by passing the mixture through a colloid mill, called the homogenizer. Since the two do not mix well, the emulsion is generally unstable and the two liquids may separate out on standing for some time.

Therefore, to form stable emulsions, small quantities of certain other substances are added during their preparation.

The substances which are added to stabilize the emulsions are called emulsifier or emulsifying agent.

The substances that are commonly used as emulsifying agents are gum, soap of different forms, gelatin, albumin, etc.

Role of Emulsifier

The emulsions are generally unstable and are stabilised by the addition of suitable emulsifier. The stabilisation of an emulsion by means of emulsifier is called emulsification.

The main function of the emulsifier is to reduce the interfacial tension between the two liquids forming the emulsion. Emulsifiers are generally long chain molecules having polar groups.

a) The soaps are sodium or potassium salts of higher fatty acids such as sodium palmitate (C₁₅H₃₅COONa), sodium stearate (C₁₇H₃₅COONa).

b) A molecule of soap consists of two parts : hydrocarbon non-polar part (e.g., C₁₅H₃₅ , C₁₇H₃₅ etc.) which is soluble in oil and the polar group (-COO‾Na⁺) which is soluble in water.

c) When a drop of oil is surrounded by soap solution, R-part of the soap remains in oil and the  -COO‾Na⁺ part remains in water.

d) As a result, soap molecules get concentrated over the surface of the drop of oil and therefore form protective film around each oil drop.

e) As a result, the interfacial tension between oil and water decreases and therefore they are intermixed to form the emulsion.

Casein (a protein), a lyophobic colloid present in milk acts as an emulsifying agent as it forms a protective layer around fat molecules dispersed in water. Therefore, milk is a fairly stable emulsion.

 

Types of Emulsions

These are of two types :

1) Oil-in-water (o/w type) emulsions. Oil acts as the dispersed phase (small amount) and water as the dispersion medium (excess).

For example: milk is an emulsion of soluble fats in water and here casein acts as an emulsifier. Vanishing cream is another example of this class. Such emulsions are called aqueous emulsions.

(ii) Water-in-oil (w/o type) emulsions: Water acts as the dispersed phase while the oil behaves as the dispersion medium. Such types of emulsions are called oily emulsions.

For example: butter, cod liver oil, cold cream, etc.

The two types of emulsions can be interconverted by simply changing the ratio of the dispersed phase and dispersion medium.

For example: an oil-in-water emulsion can be converted to water-in-oil emulsion by simply adding excess of oil in the first case.

Identification of Emulsions

The oil in water or water in oil type emulsions can be identified by the following tests:

1)  Dilution Test

If the emulsion can be diluted with water, this means that water acts as the dispersion medium and it is an example of oil-in-water emulsion. In case, it is not diluted. then oil acts as dispersion medium and it is an example of water-in-oil emulsion.

(ii) Dye test: An oil soluble suitable dye is shaken with the emulsion. If colour is noticed on looking at a drop of the emulsion, it is oil-in-water type emulsion. In case the entire background is coloured, it is an example of water in-oil type.

 

Applications of Emulsions

(1) Concentration of ores in metallurgy: The concentration of the sulphide ore of a metal by froth floatation process involves the use of some oil such as pine oil. The oil forms emulsion with ore particles. When air is bubble through the emulsion, it rises to the surface as foam and is skimmed off.

(2) In medicine: The various pharmaceuticals and cosmetics available in liquid form such as cod-liver oil, B-complex, ointments etc. are emulsions of water-in-oil type. These are readily adsorbed in the intestines.

(3) Cleansing action of soaps: The cleansing action of soap is based upon the formation of oil-in-water type emulsion.

(4) Milk: Milk which is an important constituent of our diet is an emulsion of fat in water.

Gels

A gel is a colloidal system in which a liquid is dispersed in a solid. The lyophilic sols may be coagulated to give a semi solid jelly like mass which encloses all the liquid present in the sol.

The process of gel formation is called gelation and the colloidal system formed is called gel.

The Common examples of gel are : gum arabic, gelatin, processed cheese, silicic acid, ferric hydroxide, etc.

Gels may be classified into two types :

(1) Elastic gels

(2) Non-elastic gels

Elastic gels

(i) Elastic Gels

These are the gels which possess the property of elasticity.They readily change their shape on applying force and return to original shape when the applied force is removed. They change to solid mass on dehydration which can again be converted into gel by addition of water followed by heating and cooling. When these gels are placed in contact with water, they absorb water and swell. This property is called Imbibition.

Examples are gelatin, agar, starch etc.

 

(ii) Non-elastic Gels

These are the gels which are rigid and do not have the property of elasticity. They change into solid mass on dehydration which becomes rigid and cannot be converted into original form by heating with water.

They do not show the phenomenon of imbibition.

For example : silica gel.

Application of Colloids

1) Smoke precipitation

Smoke is a colloidal solution of solid particles such as carbon, arsenic compounds, dust, etc. in air. Smoke is a colloidal system in which the carbon particles are suspended in air. The carbon particles are charged in nature and they do not get coagulated or precipitated.

The charge on the carbon particles is neutralised by bringing them in contact with the oppositely charged metal plate. They, thus, get precipitated and the smoke coming out of the chimney is free from dust. This precipitation of smoke 

particles is carried out by Cottrell smoke precipitator shown in Fig. 5.34. In this method, the smoke is allowed to pass through a chamber having a series of plates charged to very high potential (20,000 to 70,000 V). Charged particles of smoke get attracted by charged plates, get precipitated and the gases coming out of chimney become free of charged particles.

2) Sewage Disposal

The sewage disposal contains particles of dirt, rubbish etc. suspended in water. They are, therefore, colloidal in nature and are also charged. These particles do not settle down easily. The particles can be removed by discharging them at electrodes. The dirty water is either placed in big tanks or passed through a tunnel fitted with metallic electrodes which are maintained at high potential difference. The colloidal particles migrate towards the oppositely. charged electrodes where their charge is neutralised and they get coagulated.

The coagulated mass can be used as a manure and the water left behind is used for irrigation.

 

3) Purification of Drinking Water

The water coming from natural sources often contains suspended impurities. The drinking water can be purified by precipitation of suspended colloidal particles. For this purpose, a small amount of alum [K₂SO₄Al₂(SO₄)₃.24H₂O] is added. The Al³⁺ ions neutralise the charge on the particles and they get coagulated.

4) Colloidal Medicines

The colloidal medicines are quite effective on account of their easy assimilation and adsorption. A few important medicines are colloidal gold, manganese, sulphur, antimony, etc.

For example

(i) Argyrol is a silver sol used as an eye lotion.

(ii) Colloidal antimony is used as an eye lotion.

(iii) Milk of magnesia, an emulsion is used for stomach disorders.

(iv) Colloidal gold is used for intramuscular injection. Colloidal medicines are more effective because these have large surface area and therefore, are easily assimilated.

(v) Ferric chloride is quite effective to stop bleeding from a minor cut. It coagulates the blood due to its charged nature.

5) In disinfectants

The disinfectants such as dettol and lysol give emulsions of the oil-in-water when mixed with water.

6) Photographic Plates

These are thin glass plates or celluloid films coated with gelatin containing a fine suspension of silver bromide. The particles of silver bromide are colloidal in nature.

7) Chemical Warfare

Smoke or mist screens are formed by the dispersion of harmful substances by explosion of bombs, etc. Gas masks are essential to filter out the toxic smokes. These masks contain in them colloidal animal charcoal to adsorb the poisonous gases.

8) Artificial Rain

Artificial rain can be caused by spraying oppositely charged colloidal dust or sand particles over a cloud from an aeroplane. The colloidal water particles present in the cloud will get neutralised and will coagulate to form bigger water drops causing artificial rain.

9) Rubber Industry

Latex is a colloidal solution of negatively charged colloidal rubber particles. Rubber can be obtained from latex by coagulation. These rubber particles can be deposited over articles (wares or handles of different tools) to be rubber plated by electrophoresis. The article to be rubber plated is made the anode in the rubber plating bath.

10) In metallurgical Operations

The metal ores are concentrated by froth-floatation process which involves the treatment of the pulverised ore in emulsion of pine oil.

 

11) Blue Colour of the Sky and Sea

Blue colour of the sky is due to the scattering of light by colloidal dust particles present in the air (Tyndall effect). These dust particles along with water suspended in air scatter blue light which reaches our eyes and therefore, sky looks blue to us. Similarly, sea water looks green due to scattering of light by the colloidal impurities present in sea water looks green due to scattering of light by the colloidal impurities present in sea water.

12) Formation of Delta

River water is muddy and contains charged colloidal particles of clay, sand and many other materials. Sea water contains in it a number of dissolved electrolytes. When the river water comes in contact with the sea water, the electrolytes present in sea water coagulate the suspended colloidal particles which ultimately settle down at the point of contact. As a result, the level of the river bed rises. Thus, there is the formation of the delta at the point where the river enters the sea.

13) Tail of comets

Tail behind a comet is seen as Tyndall cone due to scattering of light by the tiny solid particles of colloidal range left by the comet in its path.

14) Blood

It is a colloidal solution of an albuminoid substance. Bleeding stops when alum and ferric chloride solution is applied to a bleeding part. The styptic action of alum or ferric chloride is due to coagulation of blood by an electrolyte forming a blood clot which stops further bleeding.

15) Fog, Mist and Rain

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

Clouds are aerosols having small droplets of water being suspended in air.Because of condensation in the upper atmosphere, the colloidal droplets of water become bigger and bigger in size till they fall down in the form of rain.

16) Food Articles

Many food articles such as milk, butter, ice-creams fruit juices, fruit jellies, whiped cream, etc. are colloidal in nature.

17) Soils

Fertile soils are also colloidal in nature. In the soil, the humus acts as a protective colloid. Because of colloidal nature soils adsorb moisture and other nourishing substances.

18) Dyeing

In the dyeing of fabrics, the fabric is first treated with a colloidal suspension of a substance called mordant (e.g., aluminium hydroxide). The mordant should be such that it gets strongly adsorbed on the fabric and then the dye gets strongly adsorbed on it. As a result, the dye sticks fast to the fabric.

19) Varnishes, paints, enamels, resins, cellulose gums, glues, soaps, detergents, etc. are all colloidal in nature.

20) Asphalt emulsified on water is used for building roads without the necessity of melting the asphalt is colloid.