Properties of Colloids
Heterogeneous Character
Colloidal particles remain within their own boundary surfaces which separate them from the dispersion medium. So a colloidal system is a heterogeneous mixture of two phases. The two phases are dispersed phase and dispersion medium.
Brownian movement
It is also termed as Brownian motion and is named after its discoverer Robert Brown (a Botanist). Brownian Motion is the zig-zag movement of colloidal particles in continuous random manner.
Brownian motion arises because of the impact of the molecules of the dispersion medium on the particles of dispersed phase. The forces are unequal in different directions. Hence, it causes the particles to move in a zig-zag way.
Tyndall Effect
Tyndall in 1869, observed that if a strong beam of light is passed through a colloidal solution then the path of light is illuminated. This phenomenon is called Tyndall Effect. This phenomenon is due to scattering of light by colloidal particles. The same effect is noticed when a beam of light enters a dark room through a slit and becomes visible. This happens due to the scattering of light by particles of dust in the air.
Electrical Properties
The particles of a colloidal solution are electrically charged and carry the same type of charge, either negative or positive. The dispersion medium has an equal and opposite charge. The colloidal particles therefore repel each other and do not cluster together to settle down. For example, arsenious sulphide sol, gold sol, silver sol, etc. contain negatively charged colloidal particles whereas ferric hydroxide, aluminium hydroxide, etc. contain positively charged colloidal particles.
Origin of charge on colloidal particles is due to:
- (a) Preferential adsorption of cations or anions by colloidal particles.
- (b) Miscelles carry a charge on them.
- (c) During the formation of colloids especially by Bredig arc method, colloidal particles capture electrons and get charged. The existence of charge on a colloidal particle is shown by a process called electrophoresis.
Electrophoresisis a process which involves the movement of colloidal particles either towards cathode or anode under the influence of electrical field.