A gel is a colloidal system in which dispersed phase is liquid and dispersion medium is solid
Examples: When warm sol of gelatin is cooled, it sets to a semi-solid mass which is a gel. Jellies, jams, curd, butter, shoe polish etc. are gels.
Types of gels:
Elastic gels are gels which when heated carefully, form a dry mass and if this dried mass, when placed in contact with liquid, absorb liquid, swell up and regain their original form. They are called as elastic gels or reversible gels
e.g. agar agar, gelatin, fruit jams etc.
Elastic gels are lyophilic and reversible.
Characteristics of Elastic Gels:
On heating (dehydration) they give elastic solid.
The original gel can be obtained by addition of water or liquid to elastic solid.
They are reversible
They are lyophilic
They show imbibition
Gels, made up of organic substances are elastic.
Non Elastic Gels:
Non-elastic gels are gels which when on heating loose liquid and gets converted into dry mass but cannot absorb liquid and regain their original form when they are placed in contact with the liquid. They are called as elastic gels or irreversible gels.
e.g. Silica gel, solid alcohol, hydroxides of Fe, Al, Cr etc. are non-elastic gels.
Non-elastic gels are lyophobic and irreversible.
Non Elastic Gels:
On heating (dehydration) they give a powder.
The original gel cannot be obtained by addition of water or liquid to powder.
They are irreversible.
They are lyophobic
They do not show imbibition
Gels, made up of inorganic substances are non-elasitc.
Properties of Gels:
The tendency of a gel to take up a large quantity of water or liquid and go on increasing in volume is called as swelling of gel or smiling of gel or imbibition of gel.
Only elastic gel shows this property.
The decrease in the volume of a gel due to the loss of liquid on standing is called as syneresis or weeping of gel.
Many inorganic gels on standing, undergo shrinkage which is accompanied by exudation of liquid.
This process is the reverse of imbibition.
Some gels turn into a sol on shaking and reset to the gel on standing. This reversible gel-sol transformation is called thixotropy.
Iron oxide and silver oxide gels exhibit this property.
Gels do not show Tyndall effect, Brownian movement, Electrophoresis.
Uses of Gels:
Alcohol jellied with calcium acetate is used as solid fuel for military field services.
Silica gel is most valuable adsorbing and drying (desiccating) agent which is used in industry and laboratory.
Many articles of common use are gels. e.g. curd, fruit jams, butter, cheese, jellies, shoe polish, various foodstuffs etc.
A colloidal system in which both the dispersed phase as well as dispersion medium are immiscible liquids is called an emulsion.
e.g. Milk, cod liver oil, oil paints, vanishing cream, cold creams, etc.
Generally, one of the two liquids is water and the other which is immiscible with water is designated as oil.
The liquid in excess is dispersion medium and the liquid which forms droplets or globules is the dispersed phase.
Emulsifying Agent or Emulsifier:
To obtain a stable emulsion a small quantity of third substance is added along with two immiscible liquids. This third substance is called Emulsifying agent or Emulsifier.
Emulsifier forms a protective layer around disperse phase droplets and prevent coagulation.
e.g. Soaps, Detergents, Lyophilic substances like gelatin, gum etc.
Types of Emulsions and Their Preparation:
Oil in Water Type (O/W):
In this type of emulsion, oil is the dispersed phase and water is the dispersion medium.
e.g. Milk is (fats) oil in water type of emulsion, vanishing creams.
Preparation: The emulsifier is dissolved in water and oil is added to it drop by drop with continuous agitation.
Characteristics of Oil in Water Emulsion:
Oil is the dispersed phase and water is dispersion medium.
If water is added it is miscible with the emulsion.
If oil is added it is not miscible with the emulsion.
Addition of small amount of electrolyte makes emulsion conducting.
Water is continuous phase.
Basic metal sulphates, water-soluble alkali metal soaps are used as emulsifiers.
Water in Oil Type (W/O):
In this type of emulsion, water is the dispersed phase and oil is the dispersion medium.
e.g. Cod liver oil in which particles of water are dispersed in oil, Cold creams.
Preparation: The emulsifier is dissolved in oil and water is added to it drop by drop with continuous agitation.
Characteristics of Water in Oil Emulsion:
Water is the dispersed phase and oil is dispersion medium.
If oil is added it is miscible with the emulsion.
If water is added it is not miscible with the emulsion.
Addition of small amount of electrolyte does not make emulsion conducting.
Oil is continuous phase.
Water-insoluble soaps such as those of Zn, Al, Fe, alkaline earth metals are used as emulsifiers.
Properties of Emulsions:
In many emulsions, the size of the dispersed droplets is larger than the particles found in sols.
These droplets can be precipitated by adding suitable electrolytes.
Separation of two liquid phases (two distinct layers) from emulsion due to the removal of emulsifier by any means is called as de-emulsification or demulsification.
It can be done by heating, freezing, centrifuging or by addition of electrolytes.
Emulsions can be diluted with any amount of dispersion medium. But it is to be noted that any increase in the concentration of dispersed phase results in demulsification.
Emulsions show similar properties as that of sols. When the droplets are very small they show Tyndall effect, Brownian movement and electrophoresis.
Characteristics of Emulsions:
It is the colloidal system in which the dispersed phase and the dispersion medium both are liquid.
It is a mobile liquid.
Droplets of one liquid dispersed in another liquid.
No tendency to absorb a liquid or to swell.
The emulsifying agent is needed.
It is classified as oil in water and water in oil.
They show Tyndall effect, Brownian movement and electrophoresis.
Characteristics of Gels:
It is the colloidal system in which the dispersed phase is liquid and the dispersion medium is solid.
It is an immobile semi-solid.
It has honeycomb like structure.
Many gels have a tendency to absorb liquid and swell.
No such agent is required.
It is classified as elastic gel and non-elastic gel.
They do not show Tyndall effect, Brownian movement and electrophoresis.
Uses of Emulsions:
Phenyl, when poured in water, gives oil in water emulsion. It is used as a disinfectant.
Cleansing action of soap or detergent is due to emulsion formation of oil with water.
Emulsions are used in medicines such as emulsions of cod liver oil, malt and yeast.
They are used in oil paints, plastic emulsion paints.
In the concentration of sulphide ore by forth floatation method, oil in water type of emulsion is formed.
Milk is an important household emulsion.
Asphalt emulsified in water is used for the construction of roads.
Method of Identification of Type of Emulsion:
Water is added to the emulsion. If water added is readily miscible with the emulsion, then water is the dispersion medium and oil is a dispersed phase. Thus the emulsion is of type oil in water.
Oil is added to the emulsion. If oil added is readily miscible with the emulsion, then oil is the dispersion medium and water is the dispersed phase. Thus the emulsion is of type water in oil.
A small amount of electrolyte is added to the emulsion. If the emulsion becomes conducting then water is the dispersion medium and oil is the dispersed phase. Thus the emulsion is of type oil in water. Conversely, if emulsion shows negligible conductivity then oil is the dispersion medium and water is the dispersed phase. Thus the emulsion is of type water in oil.
Method of Breaking of Emulsion:
The two liquids in the emulsion can be separated by heating, freezing or centrifuging.
The addition of a large quantity of electrolyte causes coagulation of dispersed phase.
The chemical destruction of the emulsifier causes the separation of the two liquids.