Colloids Vs Crystallloids | A Brief Story

What are Colloids?

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A colloid is a homogeneous mixture of two substances in which one substance (small proportion) is dispersed in another substance (large proportion).

In a colloid, the substance present in a larger amount is called dispersing medium and the substance present in a less amount is called the dispersed phase.

Any substance can be converted into a colloid by reducing its particle size to 1-200nm.

Discovery of Colloids

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Origin of study of colloid starts with Thomas Graham who observed diffusion of that a solution of sugar, urea or sodium chloride (NaCl) through a membrane but no glue, gelatin or gum. He called the former substances as crystalloids and the latter colloids ( In Greek, kola as gum, eidos-like).

Classification of Colloids

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Probably the most important colloidal systems have dispersed phase as solid and the dispersion medium as a liquid.

If the dispersion medium considered is water, then the colloids are referred to as hydrosols or aquasols.

If the dispersion medium is alcohol, then the colloid is termed as alcosol, and if benzene is the dispersion medium, it is called benzosol.

One more type of classification is based on the forces acting between the dispersal phase and dispersion medium, they are named as

• Lyophilic Colloid 
• Lyophobic Colloid

In lyophilic colloids, definite attractive force or affinity exists between the dispersion medium and dispersed phase. Examples: sols of protein and starch.

In a lyophobic colloid, no attractive force exists between the dispersed phase and dispersion medium. They are less stable and precipitated readily, but can not be produced again by just adding the dispersion medium. They are also called irreversible sols. Examples: sols of gold, silver, platinum and copper.

Preparation of Colloids

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Many lyophilic substances are made in their colloidal form by warming with water. Rubber forms a colloidal solution with benzene. Soap spontaneously forms a colloidal solution by just mixing with water.

In general, colloidal is prepared by the following methods.

1. Dispersion methods: In this method, larger particles are broken to colloidal dimension.
2. Condensation method: In this method, smaller atoms or molecules are converted into larger colloidal sized particles.

Properties of Colloids

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Colour: 

The colour of a sol is not always the same as the colour of the substance in the bulk. For example, bluish tinge is given by diluted milk in reflected light and reddish tinge in transmitted light.

Colour of the sol, generally depends on the following factors.

• Method of preparation
• The wavelength of the source of light
• Size and shape of colloidal particle
• whether the observer views the reflected light or transmitted light

Size:

The size of colloidal particles ranges from 1 to 1000 nm in diameter.

Filterability:

As the size of the pores in the ordinary filter paper is large the colloidal particles easily pass through the ordinary filter papers.

Non-Setting nature:

Colloidal solutions are quite stable i.e. they are not affected by gravity.

Concentration and Density:

When the colloidal solution is dilute, it is stable. When the volume of the medium is decreased coagulation occurs. Generally, the density of sol decreases with increase in the concentration.

Diffusability:

Unlike a true solution, colloids diffuse less rapidly through membranes.

Colligative Properties:

The colloidal solutions show colligative properties i.e. elevation of boiling point, depression in freezing point and osmotic pressure. Measurements of osmotic pressure are used to find the molecular weight of the colloidal particle.

The shape of Colloidal particle:

It is very interesting to know the various shapes of colloidal particles. For example, Arsenic trisulfide is spherical in shape and ferric oxyhydroxide is in the shape of a Disc or plate.

Kinetic Property:

Robert Brown observed that when the pollen grains suspended in water was viewed through an ultramicroscope, they showed a random, zigzag ceaseless. This is called the Brownian motion of colloidal particles. 

This can be explained as follows, 

The colloidal sol particles are continuously bombarded with the molecules of the dispersion medium and hence they follow a zigzag, random, continuous movement.

Brownian Movement

Optical Property:

Colloids have optical properties. When a homogeneous solution is seen in the direction of light, it appears clear but it appears dark, in a perpendicular direction.

Tyndall Effect

But when light passes through the colloidal solution, it is scattered in all directions. This effect was first observed by Faraday, but investigations are made by Tyndall in detail, hence called as Tyndall effect.

Coagulation or Precipitation:

The flocculation and settling down of the sol particles is called coagulation.

Various method of coagulation are given below:

• Addition of electrolytes
• Electrophoresis
• Mixing oppositely charged sols
• Boiling

Protective Action:

Generally, lyophobic sols are precipitated readily even with a small number of electrolytes. But they are stabilised by the addition of a small amount of lyophilic colloid.

A small amount of gelatine sol is added to gold sol to protect the gold sol.

Conclusion:

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I hope you had some interesting knowledge today. Ok, let us see what we've gone through from top to bottom...

• What are Colloids?
• Discovery of Colloids
• Preparation of Colloids
• Properties of Colloids

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