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Colloids: Classification - Type Of Particles

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Colloids: Classification - Type Of Particles - Lesson Summary

Colloidal solutions are classified into three categories.

They are:

  • Multi-molecular colloids
  • Macromolecular colloids
  • Associated colloids

Multi-molecular colloids:

Multi-molecular colloids are formed when a large number of atoms or smaller molecules of the dispersed substance aggregate together to form a specie whose size lies in the colloidal range.

EX: a gold sol consists of particles of various sizes that are a cluster of several gold atoms.

In the aggregate are held together by Van der Waal's forces.

Macromolecular colloids:

Certain substances, like starch, proteins and cellulose, have molecules of big size, which lie in the colloidal range. The solutions of these substances in suitable solvents are called macromolecular colloids.

Synthetic macromolecules, such as polyethylene, nylon and polystyrene, also form colloids when dispersed in suitable solvents.

Associated colloids:

There are some substances that behave as normal strong electrolytes at low concentration, but behave as colloidal solutions at a higher concentration.

The colloidal behaviour is due to the formation of aggregates of small particles. Such aggregated particles are called micelles, and the colloid thus formed is called an aggregated or associated colloid.

EX: soaps and detergents.

Micelles are generally formed by specific types of molecules that have both lyophilic as well as lyophobic ends. Soaps consist of sodium or potassium salts of higher fatty acids, and are represented as RCOO -Na+ in soaps

Mechanism of micelle formation:

In sodium stearate, the long hydrocarbon part of stearate radical, i.e. C 17 H 35, is the lyophobic end COO-is the lyophilic end.

Sodium stearate behaves as a normal electrolyte and ionises to give sodium and stearate ions. These stearate ions remain on the surface of water and orient themselves in such a way that the lyophilic end of the COO- dips in water, while the lyophobic part, the C17H35 part stays away from it.

At the critical micelle concentration, the polar COO- part is pulled into the bulk of the solution.

Thus, a cluster is formed with the hydrocarbon chains pointing towards the centre of the sphere, and the COO- part oriented outwards on the surface of the sphere.

The aggregate thus formed has the dimensions of a colloidal particle, and is known as an ionic micelle.

Detergents also undergo micelle formation in a similar manner.

The stearate ions of soap arrange themselves around an oil droplet in such a way that the hydrophobic part of the stearate ions is directed towards the oil and the hydrophilic part projects outside.

The hydrophilic part, being polar interacts with the water molecules and the oil droplet is pulled away from the cloth into the water to form an ionic micelle.

The stearate ions of soap molecules help in making a stable emulsion of oil with water, which is later washed away with excess of water.

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