A colloidal solution is a heterogeneous system which is made up of two phases; dispersed phase (as solute) and a dispersion medium (as solvent).
The substance distributed as the colloidal particles is called the dispersed phase and the second phase in which the colloidal particles are scattered is called the dispersion medium. Size of colloidal particles is in between 1 nm to 1000 nm.
Milk, face creams, sponge, rubber, pumice, blood, gems, etc are the examples of colloids. When one constituent particle of a solution is scattered around the another, then it is called dispersion.
Classification of Colloids
- On the basis of physical state of dispersed phase and dispersion medium, colloids are classified into following types
Dispersed Phase Dispersion Medium Type Examples Solid Solid Solid Sol Coloured Gemstone, Milky glass Solid Liquid Sol Milk of Magnesia, Mud, paints, cell fluids Solid Gas Aerosol Smoke, automobile exhaust Liquid Solid Gel Jelly, cheese, butter Liquid Liquid Emulsion Milk, face cream, Hair cream Liquid Gas Aerosol Fog, Clouds, Mist, insecticide sprays Gas Solid Solid sol Foam, rubber, sponge, pumice stone Gas Liquid Foam Shaving cream, froth, whipped cream
- On the basis of nature of interaction between dispersed phase and dispersion medium, colloidal sols are divided into two categories
Lyophilic Colloids Lyophobic Colloids These are solvent loving colloids These are solvent hating colloids These are directly formed by mixing substances like gum, gelatin, starch, rubber, etc with a suitable solvent. These sols can be prepared only by special methods These are quite stable sols These are not stable. These are also called reversible sol These sols are also called irreversible sols. e.g., Sol of starch e.g., Gold Sol
- On the basis of the type of particles of the dispersed phase, colloids are classified into three categories.
- Multimolecular Colloids. In this type of colloids, colloidal particles are aggregates of a large number of atoms or smaller molecules. e.g., gold sol, sulfur sol, etc.
- Macromolecular Colloids. Macromolecules in suitable solvents form solutions in which the size of macromolecules may be in colloidal range. These colloids are quite stable and resemble true solutions in many respects, e.g., naturally occurring macromolecules starch, cellulose, proteins and enzymes; and those of man-made macromolecules polythene, nylon, polystyrene, synthetic rubber, etc.
- Associated Colloids (Micelles) The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called Critical Micelle Concentration (CMC).
These substances behave as normal strong electrolytes at low concentration e.g., soap solution in water at particular temperature and at particular concentration
- Cleansing action of soap and detergents is due to the emulsification and micelle formation
Properties of Colloidal Solutions
- It is a heterogeneous permanent system and can not be filtered by ordinary filter papers. The dispersed particles can not be seen through naked eye but can be distinctly seen through ultra microscope.
- When light passes through a sol, its path becomes visible due to scattering of light by colloidal particles. It is called Tyndall effect
- The continuous zig–zag motion of colloidal particles is called Brownian Movement (first observed by British botanist, Robert Brown). This is independent of the nature of colloid but depends on the size of the particles and viscosity of the solution. Smaller the size, lesser is the viscosity and faster is the motion. Brownian movement is a stirring effect, so it is responsible for the stability of soil. Suspension and true solution do not show Brownian movement.
- Colloidal particles always carry an electric charge. e.g., Haemoglobin (blood) is positively charged sol while sols of starch, gelatin, charcoal are negatively charged sols.
- Colloidal solutions impart color due to scattering of light. The color of the colloidal solution depends on the wavelength of light scattered by dispersed particles.
- The process of precipitation of colloid on adding a small quantity of electrolyte is called coagulation. During this process, the particles of electrolyte carrying a charge opposite to that present on the colloidal particles, neutralize them, so they get precipitated. Alum or ferric chloride is applied on minor acts, they stop bleeding by coagulating the blood.