Classification of Colloids

Types Of Colloids

Colloids are a mixture of two substances where microscopically dispersed particles get suspended over another. The size of particles ranges from 0 to 1000 nanometres. This range is usually larger than the particles found in the solution. The mixture is classified as a colloid only when the particles of the mixture do not settle down after leaving them undisturbed. Colloidal solution exhibit the property of the Tyndall effect where a beam of light on colloids is scattered due to interaction between the light and colloidal particles. 

The IUPAC definition of colloid is as follows:

“The colloidal state is the state of subdivision in which molecules or polymolecular particles having at least one dimension in the range of 1 nanometre and 1 micrometre, are dispersed in some medium”

[Image will be uploaded soon]


Classification of Colloid

A colloid is termed as a mixture where one substance that has fine particles gets mixed with another. The substances which are dispersed in the solution are called dispersed phase and the solution in which it is to be dispersed is called dispersion medium. 

Based on the types of colloids, their classification is done. These are classified as follows:

  1. Multimolecular colloids

  2. Macromolecular colloids

  3. Associated colloids

Multimolecular Colloids

A large number of smaller molecules of a substance add on together on mixing and form species that are in the colloidal range. 

Example: A sulfur sol consists of particles containing 1000s of S8 sulfur molecules.

Macromolecular Colloids

In this colloid, the macromolecule forms a solution with a solvent. The size of particles remains in the range of colloidal particle size. Here the colloidal particles are macromolecules having very large molecular mass and  

Example: Starch, proteins, cellulose, enzymes, polystyrene 

Associated Colloids

Few substances react as strong electrolytes when they are in low concentration, but act as colloidal sols when they are in high concentration. In high concentration, particles aggregate and show colloidal behaviour and these particles are known as the micelles. They are also known as associated colloids. The formation of micelles occurs above a certain temperature and specific concentration. These colloids can be reverted by diluting it.

Example: Soap, synthetic detergents.

Below is the tabular explanation of different types of the colloidal solution with examples to get a better understanding:

Name of Colloid

Dispersed Phase

Dispersed Medium

Example

1. Sol

Solid

Liquid

Paints, Soap solution

2. Solid Sol

Solid

Solid

Gemstone

3. Aerosol

  1. Solid

  2. Liquid

  1. Gas

  2. Gas

  1. Smoke

  2. Fog, mist, cloud

4. Emulsion

Liquid

Liquid

Milk, Butter

5. Foam

Gas

Liquid

Shaving cream

6. Solid Foam

Gas

Solid

Foam rubber, sponge 

7. Gel

Solid

Liquid

Gelatin

Having known all the different types of colloids based on their dispersed phase and medium with an example, it becomes quite understandable of the topic. 

Colloids can also be classified based on the nature of the interaction between the Dispersed phase and medium:

  1. Hydrophilic Colloid: These are water-loving or are attracted to the water. They are also known as reversible sols.

Example: Agar, Gelatin, and pectin

  1. Hydrophobic Colloid: These are the opposite in nature and are repelled by water. These are also called irreversible sols.

Example: Gold sols and clay particles.


Methods of Preparation

Colloids are formed by two principal ways namely:

  1. Dispersion - It is formed by the dispersion of large particles or droplets to colloidal dimension or by application of shear ( e.g, shaking or mixing )

  2. Condensation - Condensation plays its role by condensing of small dissolved molecules by precipitation and condensation.

Methods by which lyophobic colloids can be prepared:

  1. Dispersion Method

  2. Aggregation Method

It also mentions the method of purification of colloidal solution

  1. Dialysis

  2. Electrodialysis

  3. Ultrafiltration

  4. Electro Decantation


Stabilization of Colloids

The colloid solution is said to be stable when the suspended particles in the mixture do not settle down. Stability is hindered by aggregation and sedimentation phenomena. 

There are two traditional methods for colloidal stability.

  1. Electrostatic Stabilization

  2. Steric Stabilization

However, stability improvement has rarely been considered. 


Application of Colloids

Colloids are used widely and they have varied applications. Some of its applications are:

  1. Medicine: Medicines in the colloidal form are absorbed by the body tissues and therefore are used widely and effectively.

  2. The cleansing action of soap explained as a soap solution is colloidal, and it removes dirt by emulsifying the greasy matter.

  3. Purification of water: The precipitation of colloidal impurities can be done by adding certain electrolytes like alum. The negatively charged colloidal particles of impurities get neutralized by the effect of alum.

  4. A colloid is used as a thickening agent in industrial products such as lubricants and lotion.

  5. Colloids are useful in the manufacture of paints and ink. In ballpoint pens, the ink used is a liquid-solid colloid.

  6. Colloidal gold is injected into the human body. Silver sol is used as an eye lotion. Dextran and Hetastarch is another colloid used as medicine.

FAQs (Frequently Asked Questions)

Q1. What are Associated Colloids?

Ans: Associated colloids are micro heterogeneous in which the micelles are formed by a substance dissolved in the dispersion medium. In low concentration, they generally behave as a normal strong electrolyte but in higher concentration, they exhibit colloidal properties due to the formation of aggregated particles.

Two terms are used with associated colloids:

  1. Kraft Temperature:  The formation of micelles occurs above a certain temperature known as Kraft temperature.

  2. Specific Concentration: The formation of micelles also occurs above a specific concentration known as critical micelle concentration. 

For eg: synthetic detergents, soap, organic dyes, tanning agents, and alkaloids.

Q2. How multimolecular and macromolecular colloids are different from each other? Consider examples of each.

Ans: Multimolecular colloid particles are aggregates of a large number of atoms and molecules having a diameter of less than 1nm. They also possess weak van der Waals between particles whereas macromolecular colloids having large molecular mass. They possess strong chemical bonds between macromolecular particles. Associated colloids are different from multimolecular and macromolecular as they show colloidal properties at high concentrations due to the formation of aggregated particles. They behave like a macro because of their large molecular mass. Since their molecules are flexible, they can take on various shapes.

Example of Multimolecular Colloid: Gold sol, sulfur sol

Example of Macromolecular Colloid: Cellulose, starch.