Classification of Colloids: Milk looks like a homogenous mixture, but it is a heterogeneous mixture of fat particles dispersed in water. Similarly, in smoke, solid particles are dispersed in gas. These are some examples of the class of substance called colloids. In this article, you will explore different methods of classification of colloids based on the physical state of the dispersed phase and dispersion medium and the interaction between them.

What are Colloids?

Colloids are heterogeneous systems having a particle size ranging from \(1\,{\rm{nm}}\) to \(100\,{\rm{nm}}{\rm{.}}\) It consists of two phases, i.e., dispersed phase and a dispersion medium.

Dispersed phase: The substance present in the form of colloidal particles in the colloidal system is called the dispersed phase.

Dispersion medium: The medium in which the colloidal particles are dispersed is called dispersion medium.

Example: In a colloidal solution of ferric hydroxide in water, ferric hydroxide is the dispersed phase, and water is the dispersion medium.

What is the Classification of Colloids?

Colloids are classified in three different ways as follows:

1. Based on the physical state of the dispersed phase and the dispersion medium
2. Based on the nature of the interaction between the dispersed phase and dispersion medium
3. Based on the types of particles of the dispersed phase.

Classification Based on Physical State of Dispersed Phase and Dispersion Medium

Depending upon the physical state, i.e., solid, liquid, and gas of the dispersed phase and dispersed medium, colloids are classified into the following eight types.

S. No.	Dispersed Phase	Dispersion medium	Name	Examples
\(1\)	Solid	Solid	Solid Sol	coloured glasses, gemstones, ruby, pigmented plastics
\(2\)	Solid	Liquid	Sol	Paints, cell fluids, muddy water, gold sol, sulphur sol
\(3\)	Solid	Gas	Aerosol	Smoke, dust
\(4\)	Liquid	Solid	Gel	Cheese, butter, jellies, boot foolish
\(5\)	Liquid	Liquid	Emulsion	Milk, Hair cream, emulsified oils
\(6\)	Liquid	Gas	Aerosol	Fog, mist, cloud, insecticide sprays
\(7\)	Gas	Solid	Solid foam	Pumice stone, foam rubber, styrene foam
\(8\)	Gas	Liquid	Foam	Froth, whipped cream, soap lather

The colloidal system of a gas dispersed in the gaseous dispersion medium is not possible because a gas with another is a homogeneous mixture. However, the colloid system is a heterogeneous mixture.

A colloidal system in which solid dispersed in a liquid medium is called sol. Based on the dispersed medium, sols are named hydrosol, alcosols, benzosols. In hydrosol dispersion medium is water. In alcosols dispersion medium is alcohol and in benzosols dispersion medium is benzene.

Classification Based on the Nature of Interaction Between the Dispersed Phase and Dispersion Medium

Depending upon the affinity of the dispersed phase for the dispersion medium, colloidal systems can be classified into two types, i.e., lyophilic and lyophobic colloids. On the other hand, if water is the dispersion medium, colloids are called hydrophilic and hydrophobic sols.

Lyophilic colloid: The word lyophilic means liquid-loving. The colloidal solution in which the dispersed phase particles have a great affinity for the dispersion medium is called lyophilic colloids. These are also called intrinsic colloids. These are reversible, stable in nature. Therefore, it cannot be easily precipitated.

Examples of classification of colloids lyophilic are gelatin, albumin, starch, and gum.

Lyophobic colloids: The word lyophobic means liquid-hating. The colloidal solutions in which the dispersed phase particles have no affinity for the dispersion medium are called lyophobic colloids. They are irreversible in nature. These are also called extrinsic colloids. It easily gets precipitated or coagulated by adding a small amount of electrolyte by heating or agitation. Hence these are unstable.

Example: Gold sol, and platinum sol.

Difference between Lyophilic and Lyophobic Sol

Lyophilic Sols	Lyophobic Sols
1. Prepared by directly mixing with dispersed particles and the liquid dispersion medium.	1. Prepared by special methods.
2. They are stable and are not easily precipitated or coagulated.	2. The sols are easily precipitated by the addition of a small amount of suitable electrolyte.
3. They are highly hydrated.	3. They are not very hydrated.
4. They are reversible.	4. They are irreversible.
5. These sols are usually formed by organic substances like starch, gum, proteins, etc.	5. These sols are usually formed by inorganic materials like metals, and their sulphides.
6. Their viscosity is much higher than that of the medium.	6. The viscosity is almost the same as that of the medium.
7. Their surface tension is usually lower than that of the dispersion medium.	7. The surface tension is nearly the same as that of the dispersion medium.

Classification Based on the Types of Particles of the Dispersed Phase

Colloidal particles have a size in the range of \(1\,{\rm{nm}} – 100\,{\rm{nm}}\) in the colloidal system. Depending on how different substances forming colloidal solutions acquired the particle range of \(1\,{\rm{nm}} – 100\,{\rm{nm,}}\) colloidal solutions are classified into three types, i.e., multimolecular colloids, macromolecular colloids, and associated colloids.

Multimolecular Colloids (micromolecular colloids)

When a large number of atoms or small molecules having a diameter of less than 1 nm of a substance combine in a dispersion medium to form aggregates having the size in the colloidal range, the colloidal solution thus formed is called multimolecular colloids or micromolecular colloids. The atoms or molecules constituting the dispersible particles in multimolecular colloids held together by Van der Waals’ forces.

Example: Gold sol, sulphur sol, etc. A gold sol may contain particles of various sizes composed of several atoms of gold. For example, sulphur sol consists of particles containing thousands of \({{\rm{S}}_{\rm{8}}}\) molecules.

Macromolecular Colloids

In this type of colloids, the dispersed phase particles are sufficiently large to be a colloidal dimension. Such molecules have a very high molecular mass and are termed macromolecules. When such substances are dispersed in a suitable dispersion medium, colloidal solutions formed are known as macromolecular colloids. Most of the lyophilic sols are macromolecular colloids.

Starch, proteins, gelatin, cellulose, nucleic acids, etc., are examples of natural macromolecules. On the other hand, synthetic polymers like polyethylene, synthetic rubber, etc., are examples of manufactured macromolecular colloids.

Associate Colloid

Associate colloids are substances that behave as normal electrolytes at a low concentration but behave as colloidal particles at higher concentration. These associated particles are also called micelles.

The formation of micelles occurs only above a particular temperature called Kraft temperature \(\left( {{{\rm{T}}_{\rm{k}}}} \right)\) and above a particular concentration called Critical Micelle Concentration (CMC).

Example: Surface active agents like soaps and synthetic detergents.

In an aqueous solution, soap (sodium stearate) ionises as:

\({{\text{C}}_{{\text{17}}}}{{\text{H}}_{{\text{35}}}}{\text{COONa}} \rightleftharpoons {{\text{C}}_{{\text{17}}}}{{\text{H}}_{{\text{35}}}}{\text{CO}}{{\text{O}}^{\text{-}}}{\text{ + N}}{{\text{a}}^{\text{ + }}}\)

In concentrated solutions, these ions get associated, forming an aggregate of colloidal size.

Comparison of Multimolecular, Macromolecular and Associated Colloids

Multimolecular Colloids	Macromolecular Colloids	Associated colloids
1. They are formed by aggregating a large number of atoms or molecules that generally have diameters less than \(1\,{\rm{nm}}.\)	1. They are molecules of large size.	1. They are formed by an aggregation of a large number of ions in a concentrated solution.
2. Their molecular masses are not very high.	2. They have high molecular masses.	2. They have high molecular masses.
3. Their atoms and molecules are held together by weak van der Waals forces.	3. Due to the long chain, the van der Waals forces holding them are comparatively stronger.	3. Higher the concentration, the greater the van der Waals forces.

Classification of Colloids Based on Appearance

Based on appearance, colloids are classified into two types, i.e., sols and gels

Sols: Colloidal solution with fluid appearance is called sols. In the naming of sols, the dispersion medium is named before the sol.

Example: In hydrosol dispersion medium is water. In alcosols dispersion medium is alcohol and in benzosols dispersion medium is benzene.

Gels: Colloids with a solid-like appearance is called gel.

Example: Butter, jelly, cheese, curd, etc.

Classification of Colloids Based on Dispersion Medium

Based on dispersion medium, colloids are classified into hydrophilic colloids and hydrophobic colloids.

Hydrophilic colloids: The water-loving colloids are called hydrophilic colloids.

Example: Agar, gelatin, pectin, etc.

Hydrophobic colloids: The colloids that repelled by water are called hydrophobic colloids.

Example: Gold sol, clay particles, etc.

Summary

With this article, we studied the classification of colloids system- \(8\) types of colloids based on the type of dispersed phase and the dispersion medium. We looked at the meaning and examples of lyophilic colloid, lyophobic colloid, macromolecular colloid, multimolecular colloid and associated colloids.

FAQs on Classification of Colloids

Q.1. What are the \(3\) types of colloidal systems?
Ans: Based on the types of dispersed phase particles, colloidal systems are of three types. They are multimolecular colloids, macromolecular colloids, and associated colloids.

Q.2. What are the \(8\) types of colloids?
Ans: Colloids are classified into the following eight types depending upon the physical state of the dispersed phase and the dispersion medium, i.e., solid sol, sol, aerosol, gel, emulsion, solid foam and foam

Q.3. What are the applications of colloids?
Ans: The applications of colloids are:
a. Colloids are used for creating artificial rain by throwing oppositely charged sand or common salt to bring about coagulation of water particles.
b. Blue colour of the sky is due to the scattering of light by colloidal dust particles.
c. Purification of water by coagulation of mud particles on adding alum.
d. The formation of the delta is due to the coagulation of sand or clay particles by \({\rm{NaCl}}\) present in seawater.

Q.4. What is a colloidal solution? How are they classified? Give examples?
Ans: A substance dispersed in a dispersion medium is called a colloidal solution.
Based on the interaction between the dispersed phase and dispersed medium, colloids are classified into lyophilic and lyophobic colloids.
Example: Starch in water, and gold sol.

5. What are Multimolecular colloids? Give examples.
Ans: When a large number of atoms or small molecules, having a diameter of less than 1 nm, of a substance combine in a dispersion medium to form aggregates having a size in the colloidal range, the colloidal solutions thus formed are called multimolecular colloids or micromolecular colloids.
Example: Gold sol, and sulphur sol.

Q.6. How are the colloidal solutions classified?
Ans: Colloids are classified in three different ways as follows:
a) Based on the physical state of the dispersed phase and dispersion medium, colloids are classified into eight types, i.e., solid sol, sol, aerosol, gel, emulsion, solid foam and foam.
b) Based on the interaction between the dispersed phase and dispersed medium, colloids are classified into lyophilic and lyophobic colloids.
c) Based on the types of dispersed phase particles, colloids are classified into multimolecular colloids, macromolecular colloids, and associated colloids.

Study Preparation of Colloids