PROPERTIES OF COLLOIDS
The properties of colloids are discussed under three types
i. Kinetic property
ii. Optical property
iii. Electrical property
(i) Kinetic property
When sol is examined with an ultramicrosope, the suspended particles are seen as shining specks of light. By following an individual particle, it is observed that the particle is undergoing a constant rapid motion. It moves in a series short straight line paths in the medium, changing direction abruptly.
The continuous rapid zig-zag chaotic random and ceaseless movement executed by a colloidal particle in the dispersion medium is called brownian movement. This is due to the unbalanced bombardment of the particles by the molecules of the dispersion medium.
(ii) Optical property
When a strong beam of light is passed through a sol and viewed at right angles, the path of light shows up as a hazy beam. This is due to the fact that sol particles absorb light energy and then emit it in all directions. This scattering of light illuminates the path of the beam. The phenomenon of the scattering of light by the sol particles is called Tyndall effect.
(iii). Electrical Properties
(i) Charge on Colloidal particles
The important property of colloidal dispersions is that all the suspended particles possess either a positive or negative charge. The mutual forces of repulsion between similarly charged particles prevent them from aggregating and settling under the action of gravity. This gives stability to the sol.
The surface of colloidal particle acquires a positive charges by selective adsorption of a layer of positive ions around it. This layer attracts counterions from the medium which form a second layer of negative charges. The combination of the two layers of charges around the sol particle is called Helmholtz double layer.
If electric potential is applied across two platinum electrodes immersed in a hydrophilic sol, the dispersed particles move toward one or the other electrode. The movement of sol particles under an applied electric potential is called electrophoresis or cataphoresis. If the sol particles here negatively charged, they migrate toward the positive electrode. On the other hand, if they have positively charged they move toward the negative electrode. From the direction of movement of the sol particles, we can determine the charge of the sol particles.
The phenomenon of electrophoresis can be demonstrated by placing a layer of As2S3 sol under two limbs of a U-tube. When a potential difference of about 100 volts is applied across the two platinum electrodes dipping in deionised water, it is observed that the level of the sol drops on the negative electrode side and rises on the positive electrode side (Fig.) This shows that As2S3 sol has migrated to the positive electrode, indicating that the particles are negatively charged.
(iii) Electro osmosis
In a sol, the dispersion medium carries an equal but opposite charge to that of the dispersed particles. Thus, the medium will move in opposite direction to the dispersed phase under the influence of applied electric potential. The movement of the dispersion medium under the influence of applied potential is known as electro-osmosis.
The phenomenon of electro-osmosis can be demonstrated by using a U-tube in which a plug of wet clay (a negative colloid) is fixed. The two limbs of the tube are filled with water to the same level. The platinum electrodes are immersed in water and potential applied across them. It will be observed that water level rises on the cathode side and falls on anode side. This movement of the medium towards the negative electrode, shows that the charge on the medium is positive. Similarly, for a positively charged colloid electro-osmosis will take place in the reverse direction.