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What is a Colloid?
Colloids are regarded as false solutions
– the solutes are not really dissolved in the solvent but appear so to the optical eyes (due to the very tiny size of the solutes particles). The solute particles are actually in suspension, but are not large enough to settle to the bottom of the containing vessel, hence, colloids are also not true suspensions.
Therefore, based on particle size, colloids are in-between true solutions and suspensions. The generally agreed limits of the colloids particle size are:
Lower limit - 1 and 5 nm
Upper limit – 100 and 500 nm
Terms Used to Describe Colloids:
- Dispersed phase – this is the particle of colloidal size (it is like the solute of a true solution)
- Dispersion medium – this is the medium containing the dispersed phase (it is like the solvent of a true solution). For example, in a colloidal solution of silver chloride: solid silver chloride is the dispersed phase, while water is the dispersion medium.
- If the dispersed phase (i.e. the colloidal particle) has strong affinity for the dispersion medium, the colloid will tend towards being a true solution. Such colloids are called emulsoids or lyophilic colloids (hydrophilic, if water is the dispersion medium). Examples are aqueous solutions of gelatin, proteins, gum, and starch.
- If the dispersed phase has little affinity for the dispersion medium, such colloids will tend towards being true suspension – these colloids are called suspensoid or Iyophobic colloids (hydrophobic, if water is the dispersion medium). Examples include: alcoholic solutions of gelatin and sodium chloride ; and a suspension of gold in water.
- Sol – this describes a colloidal system when it is fluid in physical state.
- Gel - when a colloid is a paste or something more rigid, it is described as a gel.
Note: all lyophobic colloids are sols, while
lyophilic colloids can either be sol or gel
depending on the existing condition, such
as: temperature; concentration and pH.
For example, keeping a fluid solution of
jelly in a refrigerator (to reduce its
temperature) for sometime transforms it to a thick mass. A starch sol at its boiling point turns into a gel when it cools.
Types and Examples of Colloidal Systems
The table below gives the types and examples
of colloidal systems:
|
Dispersed
Phase |
Dispersion
Medium |
Examples |
|
Solid |
Liquid |
Milk of magnesia
(Mg(OH)2), silver chloride (AgCl), and
barium sulphate (BaSO4) in water; gold in water;
sulphur in water or benzene. |
|
Liquid |
Liquid |
Water in oils or
oils in water (emulsion). Milk (animal fat in water).
|
|
Gas |
Liquid |
Soap foams,
whipped egg or cream and the froth on palm wine or beer consist
of air in liquid.
|
|
Solid |
Solid |
Ruby glass
(consist of gold of colloidal size in glass); black diamond (consist of
colloidal carbon in
crystalline carbon).
|
|
Liquid |
Solid |
Pearls (consist
of water particles of colloidal size in solid calcium carbonate, CaCO3).
|
|
Gas |
Solid |
Pumice, cakes,
pastry and certain other confectionery.
|
|
Solid
|
Gas |
Smoke (consist
of small carbon particles in air), dust clouds of harmatan (fine dust
particles in air).
|
|
Liquid |
Gas |
Fog, mist,
clouds - consist of small liquid particles of colloidal size dispersed
in air. Other examples
are aerosols, e.g., insect sprays, perfume. |
Properties of Colloids
Tyndal Effect
When strong light is passed through a colloid, the light comes out scattered – this is know as Tyndal effect.
The reason for this effect is that the particle size of the dispersed phase is of the same magnitude as the wavelength of the incident light. Therefore, the rays are obstructed from freely passing through by the colloidal particles, hence, the light comes out scattered.
Note: for Iyophilic colloids, the scattering is not much pronounced compared with Iyophobic colloids.
Brownian Movement
Colloidal particles are observed to move randomly and continuously in straight lines. This type of movement is known as Brownian movement (recall that Brownian movement was discussed earlier in the lecture on the kinetic theory of matter).
Stability
By adding an electrolyte, the stability of colloids are reduced or destroyed – colloidal particles coagulate and are precipitated.
Note: the properties discussed above are not shown by either true solutions or suspensions.
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