CLASSIFICATION OF COLLOIDS
The colloidal dispersion are thermodynamically unstable and tend to coagulate and precipitate on standing unless suitable precautions are taken. Since both the disperse phase and dispersion medium may be solid, liquid or gaseous, there can be several different types of colloidal system. No example of colloidal dispersion of one gas in another gas is known. There are two main basis for the classification of colloids.
A) BASED ON THE NATURE OF DISPERSED PHASE AND DISPERSION MEDIUM
B) BASED ON THE INTERACTION OF PHASES
The colloidal system in which a solid is dispersed in a liquid is given the general name ‘Sol’. When the dispersion medium is water, the soles are called hydrosols or sometimes “aquasols”. If the dispersion medium is alcohol, the soles are called alcosols. When the dispersion medium is benzene, the soles are called benzosols. Based on interaction a phases, soles are divided into two categories lyophilic sols and lyophobic sols.
LYOPHILIC SOLS (SOLVENT LOVING)
These are the colloidal solution in which the dispersed phase shows a definite affinity for the dispersion medium or the solvent, when lyophilic soles are precipitated they can easily be converted back into soles by addition of the dispersed medium. Hence are also called reversible soles and behave like true solutions to sonic extent. They are more stable than the lyophobic sols e.g. gums, starches and proteins.
LYOPHOBIC SOLS (SOLVENT HATING)
These are the colloidal solution in which the dispersed phase shows very little affinity for the dispersed medium and disperse phase. On heating or cooling the lyophobic systems, solids are obtained which cannot be reconverted into sols either by adding solvent or by warming. Hence are also called irreversible sols. Lyophobic sols are not very stable. Typical examples of this category of colloids are sols of metals, sulphur and silver halides.
The lyophilic sols are called hydrophilic when water is used as the medium.
The lyophobic sols are called hydrophobic when water is used as the dispersion medium.