NEET Chemistry Notes Surface Chemistry Preparation of Lyophobic Solution
Preparation of Lyophobic Solution
Preparation of Lyophobic Solution
Lyophilic colloids may be prepared by simply warming the solid with the liquid dispersion medium. On the other hand, lyophobic colloids have to be prepared by special methods. Substances are converted into colloidal solutions by the following two methods.
- Dispersion Methods
These methods involve the breaking of bigger particles to the size of colloidal particles. The various dispersion methods are :
- Electrodisintegration methods (Bredig’s arc method), by this method colloidal solutions of metals like gold, silver etc., are obtained.
- Peptisation, e.g.
- Condensation or Chemical Methods
These methods involve the growing of size of the dispersed phase or the size of colloidal particles.
In periodic precipitation, precipitation reaction is carried out in gel medium. As a result of this, rings or layers of precipitates are formed at definite intervals.
Purification of Colloidal Solutions
The following methods are commonly used to purify the colloids.
- Dialysis
It is the process of removing small molecules or ions from a soil by diffusion through a semipermeable membrane. In this process, impure colloidal solution is placed in a bag of semipermeable membrane, dipping in water, the ions diffuse through membrane. Ferric hydroxide sol can be purified by this method. - Electrodialysis
If dialysis is carried out under the influence of electric field, it is called electrodialysis. This speeds up the migration of ions to the opposite electrodes. - Ultrafiltration
Separation of sol particles from the liquid medium and electrolytes by filtration through an ultrafilter is called ultrafiltration. - Ultra-Centrifugation
In ultra-centrifuge, the colloidal particles settle down at the bottom and impurities remain in the solution.
Properties of Colloidal Solutions
The important properties of the colloidal solutions are given below:
- Brownian Movement
Colloidal particles are always in a state of rapid random motion, which is termed as Brownian movement. - Tyndall Effect
When a strong and converging beam of light is passed through a colloidal solution, its path becomes visible due to scattering of light by particles. It is called Tyndall effect. - Electrophoresis
The phenomenon, involving the migration of colloidal particles under the influence of electric field towards the oppositely charged electrode, is called electrophoresis.
Sedimentation potential or Dom effect is the reverse of electrophoresis. It is set up when a particle is forced to move in a resting liquid. - Coagulation or Flocculation
The precipitation of particles of the dispersed phase in a sol is known as coagulation. .
The minimum amount of an electrolyte required to cause precipitation of one litre of a colloidal solution is called coagulation value or flocculation value. The reciprocal of coagulation value is regarded coagulating power.
Hardy-Schulze Rule
Higher the valency of the active ion, the greater will be its power to precipitate the sol.
e.g. order of coagulating power is
Protective Action of Lyophilic Colloids
As lyophobic sols are unstable (e.g. Au, Ag) and get easily precipitated, the addition of lyophilic colloids like gums, soaps etc., makes it difficult. The process is known as protection and the lyophilic colloids are termed as protective colloids.
Gold Number
It is the minimum weight (in mg) which must be added to 10 mL of standard red gold sol so that no coagulation of it takes place when 1 mL of 10% NaCl solution is rapidly added to it.
Emulsions
These are the colloidal solutions of two immiscible liquids in which the liquids act as the dispersed phase as well as the dispersion medium.
Types of Emulsions
There are two types of emulsions :
- Oil in water type, e.g. milk in which tiny droplets of liquid fat are dispersed in water.
- Water in oil type, e.g. stiff greases, in which water being dispersed in lubricating oil.
Emulsifying Agents
During the preparation of emulsion, a small amount of some substances such as soap, gum, agar and protein etc., are added to stabilise the emulsion. These substances are known as emulsifying agents.
Breaking of Emulsions
Emulsions can be broken into constituent liquids by heating, freezing, centrifuging or chemical destruction of emulsifying agent.