As mentioned earlier, 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 the medium. For example, sols of starch, gelatin, gum Arabic etc. can be prepared just by warming them with water. 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 called collodion.
The hydrocarbon residue R of RCOO– ion dissolves in the greasy dirt on the cloth (b) Soap micelle containing greasy dirt (Grease micelle)
Since lyophobic colloids practically have no affinity for the dispersion medium, they do not readily pass 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 broadly be divided into the following two categories.
(i) Dispersion methods
(ii) Condensation or aggregation methods
Dispersion Methods
In these methods, the bigger particles of a substances are broken down to form smaller particles of colloidal dimensions thus obtained are stabilized by the addition of certain stabilizing agents. Some important dispersion methods are as follows:
(i) Mechanical dispersion method: 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 high speed (about 8000 revolutions per minute) in the opposite direction. A suspension of the substance in water is introduced into the mill. The coarse particles present in the suspension are grinded to the particles of colloidal dimensions and 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.
|