Classification of Colloids

Dhristi JEE 2022-24

Colloids

Colloids is 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 solutions 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”.


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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 sulphur sol consists of particles containing 1000s of S8 sulphur 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 a very large molecular mass. 

Example: Starch, proteins, cellulose, enzymes, and 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 colloidal solutions with examples to get a better understanding.


Name of Colloid

Dispersed Phase

Dispersed Medium

Example

Sol

Solid

Liquid

Paints, Soap solution

Solid Sol

Solid

Solid

Gemstone

Aerosol

  1. Solid

  2. Liquid

  1. Gas

  2. Gas

  1. Smoke

  2. Fog, mist, cloud

Emulsion

Liquid

Liquid

Milk, Butter

Foam

Gas

Liquid

Shaving cream

Solid Foam

Gas

Solid

Foam rubber, sponge 

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 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


Stabilisation 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 Stabilisation

  2. Steric Stabilisation

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 are another colloid used as medicine.


Colloid Elimination

The best solution is to perform a first step particle coagulation with a coagulant agent to remove colloids from water. To promote the meeting and their future agglomeration during the flocculation step is the only objective of destabilizing the colloid electrostatic charge of the above step.


What are the Methods to Purify Colloids?

Peptization: It's a process with the addition of a small amount of an electrolyte to stabilize a colloidal dispersion.


Coagulation: This is also known as coagulation because of clotting or thickening of colloid particles coagulation: It’s a process of thickening (clotting) of colloids.


Bredig's Arc Method: For the preparation of colloids, it is the method that we used.


Dialysis: When we separate through the membrane such as with paper parchment or cellulose etc. crystalloid, which are true particles of solution from colloidal solution by diffusion, we termed it as so.


Examples of Colloids Chemistry

  1. The high surface area of the dispersed phase and the chemistry of these interfaces are linked closely to the properties of colloidal dispersions. This natural blend of colloids and surface chemistry stands in for crucial research space and based on these basic properties, we get to see a variety of categories of colloids.

Examples: smog, fog, and sprays

  1. These are generally called liquid aerosols. As the above example has a dispersed phase of liquid and a medium of dispersion of gas.

Examples: dust and smoke present in the air

  1. These are known as solid aerosols as the above-mentioned dispersed phase is solid and the medium of dispersion is gas.

Examples: mayonnaise and milk

  1. This is mentioned as an emulsion as the examples above have a dispersed phase of liquid and the medium for dispersion is liquid as well.

Examples: plastics that are pigmented

  1. Such a blend is termed suspension.

Example: Au sol, silver iodide sol, and toothpaste


It is acknowledged as the colloidal solution. The dispersed phase for the above-mentioned examples is solid and the dispersion medium is liquid.


The natural fact that particulate matter has a high surface area to mass proportion follows from the huge difference in surface area of colloids and surface. This is one of the leading properties as a factor for colloidal solutions of the surface.


For example, by the method of adsorption onto particulate-activated charcoal, the molecules of organic pollutants and dye can be removed effectively from water. It is because of the high surface area of the coal.


These are widely used properties and processes for all kinds of oral treatments and water purification.


With many nearest neighbors than those at the surface, molecules in the bulk of liquid can interact via attractive forces. They are partially freed from bonding with neighboring molecules that must have higher energy molecules than those in bulk. To create a new surface, work must be done to take fully interacting molecules from the bulk of the liquid. This gives rise to the tension of a liquid or the surface energy; hence, the greater the work done, the stronger the molecular force between liquid molecules.


What are the Characteristics of Colloid Solutions?

Below are some characteristics of colloid solutions:

  • Mostly the colloid solutions have these characteristics.

  • The mobility is helped by the thermal kinetic energy

  • The absence of fluid affects the inertia.

  • There is either no or negligible effect of gravitation.

  • Due to electromagnetic radiation, the type of interaction is closer.

The best example which is considered to be colloid at home is the shampoo that we use for hair, milk, the metal polisher liquid, and hand wash liquid that is usually used at home.


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FAQs (Frequently Asked Questions)

1. What are associated colloids?

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 example synthetic detergents, soap, organic dyes, tanning agents, and alkaloids.

2. How are multimolecular and macromolecular colloids different from each other? Provide examples of each.

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 have 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 macro because of their large molecular mass. Since their molecules are flexible, they can take on various shapes. Examples of multimolecular colloids: Gold sol, sulphur sol. Examples of macromolecular colloids: Cellulose, starch.

3. What do you mean by the stability of colloids?

The stability usually depends on the interaction forces between the particles and is also defined by the particles remaining suspended in solution. If the case is this, then the substance will remain a suspension and the colloidal particles will repel or only weakly attract each other. The overall free energy of the system contributes to both the electrostatic interactions and Van der Waals forces. If the case is such that a colloid is stable in the interaction energy due to force of attraction between the colloidal particles is less than kT, where k is denoted as Boltzmann constant and the absolute temperature is T, then the particles of colloid will repel or weakly attract each other and the suspension will remain of the substance.

4. How are colloids used to purify water?

For the turbidity or the color of the surface of the water, colloids of very low diameter particles are responsible. The best way to eliminate them is the coagulation-flocculation processes because of the very low sedimentation speed that they hold. To promote colloidal meetings, the aim of the coagulation is to destabilize the electrostatic charge. Suspended and soluble particles in the form of collisions are contained in river lakes and canal water. The pure water is decanted because the coagulated \[Al^{3+}\] ions into ammonium hydroxide and settle down, which are negatively charged colloidal particles of impurities.

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