Classification of Colloids
Colloidal solutions can be classified in different ways:
- on the basis of interaction between the phases
- on the basis of molecular size
Classification Based Upon Interaction
Depending upon the interaction between dispersed phase and the dispersion medium colloidal solutions have been classified into two categories.
Lyophilic Colloids
The word Lyophilic means solvent lover. Lyophilic colloidal solutions are those in which the dispersed phase have a great affinity (or love) for the dispersion medium. Substances like gum, gelatine, starch, etc. when mixed with suitable dispersion medium, directly pass into colloidal state and form colloidal solution. Therefore, such solutions are easily formed simply by bringing dispersed phase and dispersion medium in direct contact with each other.
However, these colloidal solutions have an important property i.e. they are reversible in nature. This means that once lyophilic colloidal solution has been formed then dispersed phase and dispersion medium can be separated easily. Once separated these can again be formed by remixing the two phases. These sols are quite stable. If water is used as dispersion medium then it is termed as hydrophilic colloid.
Lyophobic Colloids
The word Lyophobic means solvent hating. Lyophobic colloidal solutions are those in which the dispersed phase has no affinity for the dispersion medium. Metals like Au, Ag and their hydroxides or sulphides, etc., when simply mixed with dispersion medium do not pass directly into colloidal state. These sols have to be prepared by special methods.
These sols can be readily precipitated and once precipitated they have little tendency to go back into the colloidal state. Thus, these sols are irreversible in nature. Also they are not very stable and require a stabilizing agent to remain in the colloidal form. In case water is used as dispersion medium it is called as hydrophobic sol.
Classification Based on Molecular Size
Depending upon the molecular size the colloids have been classified as
Macromolecular colloids: In this type of colloids, the size of the particles of the dispersed phase are big enough to fall in the colloidal dimension. Examples of naturally occurring macromolecular colloids are starch, cellulose, proteins, etc.
Multi-molecular colloids: Here, individually the atoms are not of colloidal size but they aggregate to join together forming a molecule of colloidal dimension. For example, sulphur sol contains aggregates of S8 molecules which fall in colloidal dimension.
Associated colloids: These are substances which behave as normal electrolyte at low concentration but get associated at higher concentration to form miscelle and behave as colloidal solution. Soap is an example. Soap is sodium salt of long chain fatty acid R COONa. When put in water, soap forms RCOO— and Na+.