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The following points highlight the seven important properties of colloidal solution.
Colloidal Solution Property # 1. Filterability:
The colloidal particles are unable to pass through parchment membrane or collodion.
Colloidal Solution Property # 2. Adsorption and Increased Surface Area:
The colloidal particles have a tendency to attract and retain at their surfaces other particles with which they come in contact. This is called as adsorption. The adsorption is increased if the surface area of the same mass of an adsorbent is also increased.
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In a colloidal solution the little mass of dispersed phase is present in the form of a large number of tiny particles thus increasing its total surface areas. The adsorption and the large surface area offered by the colloidal particles help to carry on many complex biochemical reactions in the protoplasm.
Colloidal Solution Property # 3. Tyndall Effect:
If a strong beam of light is passed through a colloidal solution and viewed from the side, the path of the beam is illuminated by a bluish light cone. (Fig. 2.4). This phenomenon is called as Faraday-Tyndall Phenomenon or Tyndall Effect and results due to the scattering or diffraction of light by the colloidal particles. The intensity of this effect depends on the difference in the index of refraction between the two phases of the colloidal solution. The Tyndall Effect is less striking with hydrophilic sols than with the hydrophobic sols.
Under the ultra-microscope, a colloidal solution shows bright spots against a dark field (Fig. 2.4). In fact these bright spots are not colloidal particles but only the halos of scattered light around them.
Colloidal Solution Property # 4. Brownian Movement:
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In a colloidal solution the suspended particles (dispersed phase) are in continuous and rapid zig-zag motion called as Brownian Movement (Fig. 2.5 A) after the name of its discoverer, a Scottish botanist Robert Brown (1828) who first observed the similar movement while examining the pollen grains suspended in water under the microscope.
Brownian movement in a colloidal solution results due to an unequal bombardment of the particles of dispersed phase by the molecules of the dispersion medium (Fig. 2.5 B). With an increase in temperature the kinetic energy and activity of the molecules of the dispersion medium increases which results in increased Brownian movement.
But the amplitude of Brownian movement becomes lesser with an increase in the size of the sol. particle because the chances of its unequal bombardment gradually diminish (Fig. 2.5 C). In more viscous colloidal solution also, the Brownian movement becomes more sluggish. Usually the phenomenon of Brownian movement is more clearly exhibited by the micelles of hydrophobic sol. than by those of hydrophilic sols.
Colloidal Solution Property # 5. Electric Properties:
The colloidal particles constituting the dispersed phase carry an electric charge probably due to the preferential adsorption of ions present in the dispersion medium. All these colloidal particles in a particular colloidal system carry electric charge of the same sign. As a result, they repel each other and remain dispersed in the dispersion medium, and if the colloidal solution is placed under an electric field all these particles move towards the oppositely charged pole (Fig. 2.6).
This phenomenon is called as cataphoresis or electrophoresis. Gum, starch, eosin etc., are some of the examples of negative sols while basic dyes such as methylene blue, metallic hydroxides such as Al (OH)3 are examples of positive sols. However, a colloidal solution is electrically neutral as a whole because the particles of dispersion medium have equal electric charge of opposite sign.
Colloidal Solution Property # 6. Coagulation or Flocculation:
The precipitation of the colloidal particles constituting the dispersed phase of the colloidal solution by the addition of an electrolyte is called as coagulation or flocculation. It is because the electric charge carried by the particles of dispersed phase is neutralized by the electrically charged ions resulting from the dissociation of the electrolyte in colloidal solution. Now these colloidal particles can no longer repel each other. They come close to each other due to Brownian movements and soon settle down due to the force of gravity.
While coagulation of the hydrophobic sol. takes place by the simple addition of an electrolyte, the coagulation of hydrophilic sol will take place only if the electrolyte is added along with a dehydrating substance (Fig. 2.7). Otherwise, the film of water around the hydrophilic sol particles will keep them away from each other and coagulation will not occur.
Mutual coagulation of a negatively charged sol and a positively charged sol takes place if the two are mixed together.
Colloidal Solution Property # 7. Osmotic Pressure:
The osmotic pressure of the colloidal solution is usually very small.
