Mechanical Dispersion Method: in This Method, the Large Particles of the Material Whose Sol Is to Be Prepared Are Broken Down in a Machine Called Colloid Mill

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Learning objectives: Types of colloidal solution. Types of colloidal solution

Lyophilic Solution Lyophobic solution Lyophilic Solutions: ➢ Solvent loving ➢ Easy to prepare ➢ Quite stable ( Not easily coagulated) ➢ Reversible in nature

Eg. Protein + H2O

Starch + H2O Lyophobic Solutions: ➢ Solvent hating ➢ Difficult to prepare ➢ Unstable (easily coagulated) ➢ Irreversible in nature

Eg. Sulphur sol + H2O

Fe(OH)3 solution + H2O Methods to prepare colloidal solutions:

For Lyophilic: It can be prepared easily by just adding hot/cold water Eg. Protein powder + cold water

For Lyophobic solutions:

It is not easy to prepare. There are some methods to prepare lyophobic colloids. Mechanical dispersion Mechanical dispersion method: In this method, the large particles of the material whose sol is to be prepared are broken down in a machine called colloid mill. Colloid mill consists of two heavy steel discs separated by a little gap. The can be adjusted according to the particle size desired. The two discs rotate at high speed (about 8000 revolutions per minute) in the opposite direction.

Notes for reference A suspension of the substance in water is added into the mill. The particles present in the suspension are grinded to form the particles of colloidal size and als get dispersed in water to form a sol. Finer dispersion can be obtained by adding an inert diluents which prevents the colloidal particles to grow in size. For example, glucose is used as diluents in the preparation of sulphur sol.

Notes for reference Bredig’s Arc method Bredig’s Arc Method: This method is used for the preparation of sols metals such as gold, silver, platinum etc. In this method, two electrodes of the metals whose sol is to be prepared are immersed into the dispersion medium and an electric arc is produced bas passing high voltage current. The dispersion medium is cooled by surrounding it with a freezing mixture. Due the heat produced from electric arc , the metal starts vaporizing and the vapour condense into the cold dispersion medium to form to the particles of colloidal size on cooling.

Notes for reference Peptization

Breaking of precipitate by adding an electrolyte How peptization happens?

Electrolyte (A+B-) ● Ions get adsorbed on surface of precipitate part. ● Every surface has a unique tendency to attract either cation or anion. Let us say it is adsorbing +ve part: Examples of peptization:

1. CdS + S2-

2. AgI + I-

3. AgI + Ag+

3+ 4. Fe(OH)3 + Fe Chemical methods for the preparation of lyophobic sols.

Charge on colloidal particle: For a stable colloid, it must have a charge on it. If neutral, they will coagulate.

+ve (generally oxides, hydroxides)

-ve (generally halides, sulphides)

Neutral (clay, blood)

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