Properties of Colloidal Solutions

When a colloidal solution is viewed under an ultra microscope, the colloidal particles are seen continuously moving in a zigzag path. Brownian movement is due to the unequal bombardments of the moving molecules of the dispersion medium on colloidal particles. The moving molecules of the dispersion medium continuously attack on colloidal particles from all sides and impart momentum to them.

It is more rapid when the size of the particles is small and the solution is less viscous. Since the chances of their collisions are unequal, the net driving force on a colloidal particle forces it to move a particular direction. As the particle moves in that direction, other molecules of the medium again collide with it and the particle changes its direction. The process continues. This results in a random zigzag movement of the colloidal particle.

Diffusion is the property of particles to move from higher concentration to lower concentration through a membrane until equilibrium is reached. Diffusion is a direct result of Brownian movements — because due to Brownian motion, particles gain the forces that allow them to move.

The diffusion follows Fick's first law. It can be written as:

dq = - D S (dc/dx) dt

• D = diffusion coefficient
• dc/dx = concentration gradient
• dt = time fraction
• dq = quantity diffused in a given time
• S = surface area for particle diffusion

When an intense converging beam of light is passed through a colloidal solution kept in the dark, the path of the beam gets illuminated with a bluish light. This phenomenon is called the Tyndall effect and the illuminated path is known as the Tyndall cone.

The Tyndall effect is due to the scattering of light by colloidal particles. Since the dimensions of colloidal particles are comparable to the wavelength of ultraviolet and visible radiations, they scatter these radiations and become illuminated. Individual colloidal particles appear as bright stars against a dark background when observed with a microscope at right angles to the beam.

Tyndall effect is not exhibited by true solutions because particles (ions or molecules) in true solutions are too small to scatter light. It therefore helps distinguish a colloidal solution from a true solution and is used in instruments such as the ultramicroscope.

The Tyndall effect is observed only when the following two conditions are satisfied:

• The diameter of 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 greatly in magnitude.

A colloidal solution as a whole is electrically neutral; the dispersion medium carries an equal and opposite charge to that of the particles of the dispersed phase. When the movement of the dispersed phase is prevented, the dispersion medium can be made to move under an applied electric field or potential. This phenomenon is referred to as electro‑osmosis.

Thus electro‑osmosis may be defined as the movement of the dispersion medium under the influence of an applied electric field. If the dispersed phase particles have positive charge they will move towards the cathode, and if they have negative charge they move towards the anode.