Surface Chemistry Part - 4 | Class 12th

THE COLLOIDAL STATE

Colloid is an intermediate state between true solution and suspension. In a true solution, the size of the particles is < 1nm. The particles do not settle down under the influence of gravity or by any method and they cannot be filtered by a filter paper.

ü  Colloids are heterogeneous systems containing two phases – dispersed phase and dispersion medium.

Ø  Dispersed Phase: The phase which is dispersed in the other (medium) is called Dispersed Phase or internal phase, or discontinuous phase.

Ø  Dispersion Medium: The phase or medium in which the dispersion is made is called dispersion medium or external phase or continuous phase.

ü  Classification colloids

Ø  Based on the physical state of dispersed phase and the dispersion medium:

Depending upon physical state of the dispersed phase and the dispersion medium, there are eight types of colloidal systems.

Dispersed Phase	Dispersion medium	Type of colloid	Examples
Solid	Solid	Solid sol	Some coloured glasses, gems stones
Solid	Liquid	Sol	Paints, muddy water, cell fluids
Solid	Gas	Aerosol	Smoke, dust
Liquid	Solid	Gel	Cheese, butter, jellies
Liquid	Liquid	Emulsion	Milk, hair cream
Liquid	Gas	Aerosol	Fog, mist, cloud, insecticide sprays
Gas	Solid	Solid foam	Pumice stone, foam rubber
Gas	Liquid	Foam	Froth, whipped cream, soap lather

Ø  Based on the nature of the dispersion medium:

Dispersion medium	Name of colloid (Sol)
Air	Aerosol
Water	Hydrosol
Alcohol	Alcosol
Benzene	Benzosol

Ø  Based on the attraction between the dispersed phase and the dispersion medium:

On the basis of nature of interaction of dispersed phase and dispersion medium colloids are of two types: lyophilic (solvent loving) and lyophobic (solvent hating). If the force of attraction between dispersed phase and dispersion medium is strong, it is called lyophilic sol e.g. gum, gelatin, starch, rubber etc in suitable dispersion medium.

If the force of attraction between dispersed phase and dispersion medium is weak, it is called lyophobic sol. e.g. Arsenic sulphide (As₂S₃) sol, sulpher sol and metal sols like gold sol, silver sol etc.

Difference Between Lyophilic and Lyophobic Sols:

Properties	Lyophilic sol	Lyophobic sol
Force of attraction	Strong	Weak
Preparation	Can be easily prepared by mixing the dispersed phase with the dispersion medium	Some special methods are used for the
Reversibility	Reversible (i.e. they can be easily separated and remixed.	Irreversible
Stability	Self-Stabilized	Less stable and requires some stabilizing agent
Coagulation	large amount of electrolyte is required for coagulation	Only small amount of electrolyte is required.

Ø  Based on Nature of Particles:

On this basis, the colloids are classified into three types namely Multimolecular colloids, Macromolecular colloids and Associated colloids.

·       Multimolecular colloids. They are formed by the aggregation of a large number of atoms or molecules which generally have diameter less than 1nm, e.g., sols of gold, sulphur etc. Their atoms or molecules are held together by weak van der waals forces and their molecular masses are not high.

·       Macromolecular colloids. They are molecules of large size, e.g., polymers like rubber, nylon, polythene, starch, cellulose, proteins, enzymes, etc. These substances when dissolved in a suitable liquid, directly form the colloidal solution. They have high molecular masses and have lyophobic character.

·       Associated colloids. The substances which when dissolved in a medium at low concentration behave as normal strong electrolytes but at higher concentration exhibit colloidal state properties due to the formation of aggregated particles are called associated colloids. The aggregated particles thus formed are called micelles.

Micelle formation takes place above a particular temperature called Kraft temperature (T_k) and above a particular concentration called Critical Micelle Concentration (CMC). These molecules contain both lyophilic and lyophobic groups.