Monday, October 29, 2012

Colloids

Selmi (1843) was first to study colloids systematically. He prepared colloidal solutions of sulphur, prussian blue, casein etc. According to him, these colloids were not true solutions but suspensions of extremely small particles in water. Later on Thomas Graham in 1861 studied the process of diffusion of dissolved substances through a Parchment membrane. He observed that some substances like glue, geletin starch etc in solution form diffused very slowly and rates of diffusion were found to be very small in comparison to those other substances like sugar, salt and urea in water. 
In view of his observation, Thomas Graham divided substances into two distinct classes as under:-
            (i).        Crystalloids   
            (ii).       Colloids
(I)        Crystalloids:- Substances which diffuse readily in solution and can easily pass through a parchment membrane or animal membrane are known as Crystalloids. For instance, inorganic acids, bases, salts and organic compounds like sugar, urea etc.
(II)       Colloids:- Substances which diffuse very slowly in solution and cannot pass through parchment paper or animal membrane are called Colloids. For instance, starch, glue, geletin, albumin etc.
In recent years, Graham’s classification of substances into Crystalloids and Colloids has undergone major change because it has been observed that every substance irrespective of its nature can be a colloidal under suitable conditions. For instance, NaCl a typical crystalloid in water behaves like a colloid in benzene. Soap, a typical colloid in water, behaves crystalloid in alcohol. Similarly insoluble metal like copper, silver and gold can be converted into colloidal form by suitable methods.
COLLOIDAL STATE OF MATTER
                        A colloidal is not a substance but it is a particular state of substance which depends on the particle size. Any substance can be brought into colloidal state by suitable methods. There are three distinct types of solutions of substances based on the size of particle. These are:–
1.         True Solution :-
                        It is a homogeneous solution which contains small solute particles like molecules or ions dispersed throughout the solvent. The solute particle in a true solution are invisible even under microscope and these particles pass through a filter paper or an animal membrane. The particle size is 0.1 nm – 1 nm.
2.         Suspension :-
                        It is a heterogeneous mixture which contains large insoluble particles. The particles of a suspension can be seen by naked eye and these particles can neither be passed through filter paper nor through animal membrane. The size of particles is greater than 100 nm.
3.         Colloidal solution :-
                        It is a heterogeneous solution which contains solute particles of intermediate size. The particles of colloidal solution can pass through ordinary filter paper but not through animal membrane. These particles cannot be seen with a naked eye but can be seen under ultramicroscope. The particle size is 1nm – 100 nm.
DISTINGUISHING CHARACTERISTICS
OF
TRUE SOLUTION, COLLOID AND SUSPENSION

S.No.
PROPERTY
TRUE SOLUTION
COLLOIDAL SOLUTION
SUSPENSION
1.
Nature
Homogenous
Heterogeneous
Heterogeneous
2.
Size of Particle
0.1 nm – 1nm.
1nm – 100nm.
> 100 nm
3.
Filterability
Passes through ordinary filter paper as well as animal membrane.
Passes through filter paper but not through animal membrane.
Does not passes through filter paper or animal membrane.
4.
Separation
It can not be separated by ordinary filter paper nor by ultra filtration.
It can not be separated by ordinary filter paper but can be separated by ultra filtration.
It can be separated by both.
5.
Appearance
Transparent
Generally transparent
Opaque.
6.
Setting
Does not settle.
Settles in centrifuge.
Settles under the action of gravity.
7.
Ionisation
May be ionised.
Not ionised.
Not ionised.
8.
Diffusion
Diffuses readily
Diffuses slowly
Does not diffuse.
9.
Tyndal Effect
Does not show Tyndal effect.
Shows Tyndal effect.
Does not show Tyndal effect.
10.
Brownian Movement
Does not show Brownian movement.
Shows Brownian movement.
May show.
11.
Electrical Charge
Uncharged
Charged either positively or negatively
Uncharged.

PHASES OF COLLOIDAL SOLUTION
                        A colloidal solution is heterogeneous in nature. It consists of two phases, dispersed phase and dispersion medium.
1.         DISPERSED PHASE
                          In a colloidal solution, the component present in smaller proportion (solute) is known as Dispersed Phase. Example: In colloidal solution of silver in water, the component silver acts as dispersed phase.
2.         DISPERSION MEDIUM
                        In a colloidal solution, the component present in larger proportion (solvent) is called Dispersion Medium. Example: In colloidal solution gold in water, the component water acts as a dispersion medium.

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