Brownian Movement in Colloids

A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance known as the dispersion medium. The primary difference between solution and colloid is the particle size. In solution, the constituent particles are ions or small molecules. In colloidal solutions, the dispersed phase may consist of particles of a single macromolecule, such as a protein or synthetic polymer, or aggregates of many atoms, ions, or molecules. Colloidal particles are larger than single molecules but small enough to stay in suspension.  

Their diameters range from 1 to 1000 nanometres. Colloidal particles have a huge surface area per unit mass due to their small size.

Brownian movement of colloids

When the colloidal solution is observed under a powerful microscopic microscope, the colloidal particles appear to be in continuous zigzag motion throughout the critical field. This motion was first observed by the British botanist Robert Brown and is known as the Brownian motion.

The nature of colloids does not influence Brownian motion, but the viscosity of the medium and the size of the particle play an active role in it. The smaller the size and viscosity, the faster the movement. Brownian motion has been explained by the unbalanced bombardment of particles by molecules of the dispersion medium. The agitating Browning motion does not allow the particles to settle and is therefore responsible for the stability of the sol.

Properties of colloids

Colligative Properties

Colloidal particles are in larger aggregates. The number of particles in the colloidal solution is relatively low compared to a real solution. Therefore, the values of the matching properties, including osmotic pressure, vapour pressure, freezing point reduction, and boiling point elevation, are orders of magnitude smaller than those reported by real solutions at the same concentration.

Tyndall Effect

If a homogeneous solution is placed in the dark, it is observed in the direction of light, but if observed from a direction perpendicular to the direction of the light beam, it is completely dark. Colloids viewed similarly may also appear reasonably transparent or translucent by transmitted light, but they exhibit a mild to intense opacity. When viewed perpendicular to the transmitted light stream, the beam’s path is illuminated by a bluish light. Faraday first observed this effect, which was later studied in detail and is known as the Tyndall effect.

Tyndall effect is observed in the following situations: 

• The diameter of the dispersed particles is not much smaller than the wavelength of the light used
• The refractive indices of the dispersed phase and the dispersion medium differ significantly in magnitude.

The Tyndall effect distinguishes between a colloidal and a true solution. Ultramicroscopy does not provide any information about the size and shape of colloidal particles.

Colour

The colour of the colloidal solution depends on the wavelength of the light scattered by the dispersed particles. The wavelength of light also depends on the size and nature of the particles. The colour of the colloidal solution also changes according to how the observer receives the light. For example, a mixture of milk and water is blue when viewed with reflected light and red with transmitted light. The best yellow soil is red; as the particle size increases, it appears purple, blue, and finally yellow.

Charge on colloidal particles

Colloidal particles always carry a charge. The nature of this charge is the same across all particles of a given colloid and can be positive or negative.

Conclusion

Colloidal systems exhibit interesting optical, mechanical, and electrical properties. Converting the colloidal particles of the sol into an insoluble precipitated by the addition of certain suitable electrolytes is called coagulation. An emulsion is a colloidal system in which the dispersed phase and dispersion medium are liquid. They can be (i) oil in water and (ii) water in oil. The process of creating emulsions is called emulsification. Colloids are widely used in our daily life. Colloidal drugs are more effective due to their large surface area, as they are easily absorbed. Latex is a colloidal solution of negatively charged rubber particles. Paints, inks, synthetic resins, rubber, graphite lubricants, cement, etc., are colloidal solutions. Therefore, we must know their characteristics to use them daily.