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What are Colloids?

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  • Last Updated : 09 Nov, 2021

Solutions are systems that are homogeneous. When sand is mixed with water, it forms a suspension that settles over time. Colloidal dispersions, or simply colloids, are a large group of systems that exist between the two extremes of suspensions and solutions.

Colloids are a combination of two substances in which one component is split into minute particles referred to as colloidal particles (diameters range from 1 to 1000 nm) and dispersed or suspended over another material in chemistry. Filtering or centrifuging will not separate these stubborn particles. Sol, Emulsion, Foam, and Aerosol are examples of colloids. Let’s look at what colloids are, what they’re used for, and how they’re made.

What are Colloids?

A colloid is a heterogeneous system in which one substance is dispersed as very fine particles in another substance known as the dispersion medium.

The particle size difference is what distinguishes a solution from a colloid. While the constituent particles of a solution are ions or small molecules, the dispersed phase of a colloid may consist of particles of a single macromolecule (such as protein or synthetic polymer) or an aggregate of many atoms, ions, or molecules. Colloidal particles are larger than simple molecules but small enough to float. Their diameters range between 1 and 1000 nm.

Because of their small size, colloidal particles have a massive surface area per unit mass. Consider a cube with a side length of 1 cm. It has a 6cm2 total surface area. If divided into 1012 equal cubes, the cubes would be the size of large colloidal particles with a total surface area of 6m2.

Example of colloids

  • Blood: A water-soluble respiration pigment-containing albumin protein. Albumin serves as the dispersed phase in the pigmented part, and water serves as the dispersion medium. It is a kind of hydrosol.
  • Cloud: It has air as the dispersion medium and water droplets as the dispersed phase. These are aerosol cans.
  • Gold sol: It’s a metallic sol with gold particles dispersed in water.

Classification of Colloids

Colloids are classified on the basis of the following criteria:

  1. The physical state of the dispersed phase and the dispersion medium
  2. Nature of interaction between the dispersed phase and the dispersion medium
  3. Type of particles of the dispersed phase.

Classification Based on Physical State of Dispersed Phase and Dispersion Medium

There are eight different types of colloidal systems depending on whether the dispersed phase and the dispersion medium are solids, liquids, or gases. When one gas is mixed with another, the result is a homogeneous mixture, which is not a colloidal system. The following are examples of various types of colloids, along with their common names.

Dispersed Phase

Dispersion Medium

Type of Colloid


SolidSolidSolid solGem Stones
SolidGasAerosolSmoke, dust
LiquidSolidGelCheese, butter
LiquidLiquidEmulsionMilk, hair cream
LiquidGasAerosolFog, mist, cloud, insecticide spray
GasSolidSolid solFoam Rubber
GasLiquidFoamWhipped cream

Colloids are found in a wide range of commercial and natural products. Whipped cream, for example, is a foam, which is a gas dispersed in a liquid. Colloidal systems include firefighting foams, which are used in emergency aeroplane landings. The majority of biological fluids are aqueous sols (solids dispersed in water). Within a typical cell, proteins and nucleic acids are colloidal-sized particles dispersed in an aqueous solution of ions and small molecules. Sols (solids in liquids), gels (liquids in solids), and emulsions are the most common (liquids in liquids). If the dispersion medium is water, the sol is referred to as aqua sol or hydrosol; if the dispersion medium is alcohol, the sol is referred to as alcosol.

Classification Based on Nature of Interaction between Dispersed Phase and Dispersion Medium

Colloidal sols are classified into two types based on the nature of the interaction between the dispersed phase and the dispersion medium: lyophilic (solvent attracting) and lyophobic (solvent repelling) (solvent repelling). When water is used as the dispersion medium, the terms hydrophilic and hydrophobic are used.

  1. Lyophilic colloids: The term lyophilic refers to a person who enjoys liquids. Lyophilic sols are colloidal sols formed directly by combining substances such as gum, gelatin, starch, rubber, and others with a suitable liquid (the dispersion medium). One important feature of these sols is that if the dispersion medium is separated from the dispersed phase (for example, by evaporation), the sol can be reconstituted by simply mixing with the dispersion medium. As a result, these sols are also known as reversible sols.
  2. Lyophobic colloids: The term lyophobic refers to a dislike of liquids. Substances such as metals, their sulphides, and so on do not form colloidal sols when simply mixed with the dispersion medium. Only through the use of specialized techniques can they produce colloidal sols. These sols are known as lyophobic sols. These sols are easily precipitated (or coagulated) when small amounts of electrolytes are added, heated, or shaken, and thus are not stable. Furthermore, they do not return colloidal sol by simply adding the dispersion medium once precipitated. As a result, these sols are also known as irreversible sols. Lyophobic sols require stabilizing agents to be preserved.

Classification Based on Type of Particles of the Dispersed Phase, Multimolecular, Macromolecular and Associated Colloids

Colloids are classified into three types based on the type of particles in the dispersed phase: multi molecular, macromolecular, and associated colloids.

  1. Multimolecular colloids: When a substance dissolves, a large number of atoms or smaller molecules aggregate together to form colloidal species. The resulting species are known as multimolecular colloids. A gold sol, for example, may contain particles of varying sizes and many atoms. Sulphur sol is made up of particles that contain a thousand or more S8 sulphur molecules.
  2. Macromolecular colloids: Macromolecules in suitable solvents form colloidal solutions with macromolecules. These are known as macromolecular colloids. These colloids are quite stable and, in many ways, resemble true solutions. Starch, cellulose, proteins, and enzymes are examples of naturally occurring macromolecules, while polythene, nylon, polystyrene, synthetic rubber, and other man-made macromolecules are examples of man-made macromolecules.
  3. Associated colloids (Micelles): Some substances behave as normal strong electrolytes at low concentrations but exhibit colloidal behaviour at higher concentrations due to aggregate formation. Micelles are the aggregated particles that form as a result of this process. These are also referred to as associated colloids. Micelle formation occurs only above a certain temperature known as the Kraft temperature and a certain concentration known as the critical micelle concentration.

Application of Colloids

  1. Colloids are widely used in industries, medicine, and everyday life.
  2. Syrup, Halwa, and Soup are examples of colloidal systems in food.
  3. Medicine: Colloidal silver, also known as Argyrols, acts as an antiseptic for eye infections.
  4. In Cottrell precipitator for air purification: Coagulation of solution particles is involved in this process. When dust or smoke enters an electrified chamber with a central electrical plate charged with the opposite charge of a dent or a smoke particle, the particles coagulate and pure air exits through another outlet.
  5. Tanning of leather: Animal skins are very soft; when immersed in a tannin solution with the opposite charge of the animal skin, particles coagulate and the skin hardens; this is known as tanning of leather.
  6. Delta formation is caused by the coagulation of river clay particles with a seawater electrolyte.

Sample Questions

Question 1: What is the primary distinction between lyophilic and lyophobic sols?


In lyophilic sols, there is a strong interaction between the dispersed phase and the dispersion medium, which is highly stable and resistant to coagulation. Lyophobic sols are irreversible and ready-to-coagulate Van Der Waals forces of attraction between dispersed phase and dispersion medium.

Question 2: What are gels? Give an example.


Gels are a type of sol in which a solid serves as the dispersion medium and a liquid serves as the dispersed phase; they are stable at low temperatures.

Question 3: Give examples of macromolecular colloids.


These are typically biomolecular particles, such as enzymes or proteins, that aggregate to form sols when immersed in a suitable dispersion medium.

Question 4: Which phenomenon of colloids involves the formation of a delta?


The formation of the delta is aided by coagulation or flocculation. The river contains clay particles that coagulate when combined with seawater and electrolyte due to the presence of opposite ions.

Question 5: Name the substance that is used during ultrafiltration to reduce the pore size of filter paper.


Because of the large pore size, normal filter paper cannot be used in ultrafiltration. The pores are reduced in size by using collodion solution, which is a 4 percent cellulose nitrate solution in an alcohol ether mixture.

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