[Explained] Preparation of Colloidal Solutions and Colloids - Dispersion Methods, Condensation

Preparation of Lyophilic (hydrophilic) Sols

Lyophilic colloids have a strong affinity for the dispersion medium and readily form a sol by bringing them into contact or by warming them with medium. 

The preparation of of Lyophilic sol is not a tough job to do almost anyone can do it at their home. So here is a small experiment you can try yourselves at home. If you want to brush up your knowledge on different kinds of Colloidal systems check this post.

Types of Colloidal System and Examples

For example, starch, gelatin, Arabic gum and many more hydrophilic sols can be prepared by dissolving them in finely divided state in a suitable solvent such as water at high temperature and then later cooling them at room temperature. 

You should also learn about Properties of Colloidal Solutions and Preparation of Colloidal Solutions to understand the preparation of colloidal solutions better.

The dispersed can be achieved again by just heating the solution which evaporates the dispersion medium in this case we took water. So if we want to convert a normal solution or a suspension we just have to make the particle size smaller by physical or chemical means. 

Similarly, a colloidal solution of cellulose nitrate can be prepared by dissolving it in an organic solvent such as ethyl alcohol. The product obtained is commercially known as collodion. So now you know what a collodion is.

Preparation of Lyophobic (Hydrophobic) sols

 

Since lyophobic sols have close to no affinity for the dispersion medium, they do not readily mix into the medium to form a colloidal solution. Hence, special methods are required for the preparation of lyophobic sols. The methods used for the preparation of lyophobic sols can be broadly divided into two following categories:

1. Dispersion methods
2. Condensation or aggregation methods

Dispersion Methods of Preparing Colloidal solutions

The bigger particles of a substance are broken down to form smaller particles of colloidal dimensions in the presence of a dispersion medium. The colloidal dispersions thus obtained are stabilized by the addition of certain stabilizing agents.

Mechanical dispersion method for Preparing colloidal solution

 

A Colloidal Mill

In this method, the dispersion of the coarse material (whose colloidal solution is to be prepared) is carried out in a machine called colloid mill. It consists of two heavy steel discs separated by a little gap. 

The gap may be adjusted according to the particle size desired. The two discs rotate at very high speed (about 80000 revolutions per minute in the opposite direction. A suspension of the substance in water is added to the mill. 

The coarse particles present in the suspension are grounded so that the particles are reduced in size suitable for the formation of the sol. And by suitable size we mean the size of colloidal particles. The particles then get dissolved in water to form a sol. 

Finer dispersion can be obtained by adding an inert diluent which prevents the colloidal particles to grow in size. A very good example of an inert diluent which prevents the colloidal particles to grow in size would be glucose which is used in the preparation of Sulphur sol.

Electrical dispersion method (Bredig's Arc method) for Preparing Colloidal solutions

Electrical dispersion method using Bredig's arc method

 

This method is used for preparing sols of metals such as gold, silver, platinum etc. In this method, an electric arc  is truck between the two electrodes of the metal (a colloidal solution will be prepared with the metal) immersed in the dispersion medium like, water. 

The dispersion medium is cooled by surrounding it with a freezing mixture. High temperature of the arc vaporizes some of the metal. The vapor condenses to the particles of colloidal size on cooling. The colloidal particles thus formed get dispersed in the medium to form a sol of the metal.

Peptization

In this method, a freshly prepared precipitate of the substances is made to pass into the colloidal state by the addition of a suitable electrolyte. 

The process of dispersing a freshly prepared precipitate into colloidal form by using a suitable electrolyte is called peptization. The electrolyte added is called peptizing agent.

Let us look at some examples of peptization.

• When a small amount of ferric chloride solution is added to the freshly precipitated ferric hydroxide, a reddish brown colored colloidal solution of ferric hydroxide is obtained. this occurs due to the adsorption of Ferric ions over ferric hydroxide particles which causes them to disperse into the solution due to the electrostatic repulsion between the similarly charged particles.
• When a freshly prepared precipitate of silver iodide is shaken with a dilute solution of silver nitrate, a colloidal solution of silver iodide is obtained. 
  

Condensation Methods (Aggregation Methods) for Preparing Colloidal solutions

In condensation methods, the smaller particles of the dispersed phase are aggregated, to form larger particles of colloidal dimensions. Some important condensation methods are described below. 

 Chemical Methods for Preparing Colloidal solutions

Some chemical reactions may be used to aggregate smaller particles of the atomic or ionic sizes to form larger particles of colloidal dimensions. These reactions actually involve the formation of the dispersed phase as insoluble reaction products. Some important reactions leading to the formation of hydrophobic sols are as follows.

1. Oxidation

Colloidal solution of Sulphur can be prepared by oxidizing an aqueous solution of

H₂S with a suitable oxidizing agent such as bromine water, nitric acid or SO₂.

H₂S  +  Br₂  → S  +  2HBr

 

2H₂S   +    SO₂   → 3S  + 2H₂O

 

2. Reduction

Sols of gold, silver, platinum etc. can be obtained by the reduction of dilute solutions of their salts with a suitable reducing agent. For example, gold sol can be obtained by reducing a dilute aqueous solution of gold chloride with stannous chloride.

2AuCl₃   + 3SnCl₂  → 2Au + 3Sncl₄

 

3. Hydrolysis

Sols of ferric hydroxide and aluminum hydroxide can be prepared by boiling the aqueous solutions of the corresponding chlorides. For example,

 

FeCl₃  +  3H₂O → Fe(OH)₃    +  3HCl

 

4. Double decomposition

 The sols of inorganic insoluble salts such as arsenious sulphide, silver halides, etc. may be prepared by using double decomposition reactions. For example, arsenious sulphide sol can be prepared by passing H₂S  gas through a dilute solution of arsenious oxide.

As₂O₃   + 3H₂S    → As₂S₃  + 3H₂O

 

Physical Methods for Preparing Colloidal solutions

 

1. Exchange of solvent

This method involves the pouring of the true solutions of a substance to another solvent in which the solute is insoluble but the solvent is completely miscible. An exchange of solvent gives the colloidal solution of the solute. 

The method may be used for the preparation of the sols of Sulphur and phosphorus. For example, Sulphur is soluble in alcohol but less soluble in water. When an alcoholic solution of Sulphur is poured into water, a colloidal solution of Sulphur is obtained. 

 

2. Excessive cooling

A colloidal solution of ice in a non polar organic solvent like ether can be obtained by freezing a mixture of water and the solvent.