JEE Main & Advanced Chemistry Surface & Nuclear Chemistry / भूतल और नाभिकीय रसायन Phases Of Colloids And Their Classification    

(1) Phases of colloids : We know that a colloidal solution is of heterogeneous nature. It consists of two phases which are as follows

(i) Internal phase or Dispersed phase (Discontinuous phase) : It is the component present in small proportion and is just like a solute in a solution. For example in the colloidal solution of silver in water (silver acts as a dispersed phase)

(ii) External phase or Dispersion medium (continuous phase) : It is generally component present in excess and is just like a solvent in a solution. For example, in the colloidal solution of silver in water. Water act as a dispersion medium.      

(2) Classification of colloids : The colloids are classified on the basis of the following criteria

(i) Classification based on the physical state of the dispersed phase and dispersion medium : Depending upon the physical state of dispersed phase and dispersion medium whether these are solids, liquids or gases, eight types of colloidal systems are possible.

Different types of colloidal systems

(ii) Classification based on Nature of interaction between dispersed phase and dispersion medium: Depending upon the nature of interactions between dispersed phase and the dispersion medium, the colloidal solutions can be classified into two types as (a) Lyophilic and (b) Lyophobic sols.

(a) Lyophilic colloids (water loving) : “The colloidal solutions in which the particles of the dispersed phase have a great affinity for the dispersion medium, are called lyophilic collodis.”

(b) Lyophobic colloids (water hateing) : “The colloidal solutions in which there is no affinity between particles of the dispersed phase and the dispersion medium are called lyophobic colloids.”

Distinction between lyophilic and lyophobic sols

(iii) Classification based on types of particle of dispersed phase : Depending upon the type of the particles of the dispersed phase, the colloids are classified as follows.

(a) Multimolecular colloids

• When on dissolution, atoms or smaller molecules of substances (having diameter less than 1nm) aggregate together to form particles of colloidal dimensions, the particles thus formed are called multimolecular colloids.
• In these sols the dispersed phase consists of aggregates of atoms or molecules with molecular size less than 1
• For example, sols of gold atoms and sulphur \[({{S}_{8}})\]molecules. In these colloids, the particles are held together by Vander Waal's forces. They have usually lyophilic character.

(b) Macromolecular colloids

• These are the substances having big size molecules (called macromolecules) which on dissolution form size in the colloidal range. Such substances are called macromolecular colloids.
• These macromolecules forming the dispersed phase are generally polymers having very high molecular masses.
• Naturally occurring macromolecules are starch, cellulose, proteins, enzymes, gelatin etc. Artificial macromolecules are synthetic polymers such as nylon, polythene, plastics, polystyrene etc.
• They have usually lyophobic character.

(c) Associated colloids

• These are the substances which on dissolved in a medium behave as normal electrolytes at low concentration but behave, as colloidal particles at higher concentration due to the formation of aggregated particles. The aggregates particles thus formed are called
• Their molecules contain both lyophilic and lyophobic

Micelles

• Micelles are the cluster or aggregated particles formed by association of colloid in solution.
• The common examples of micelles are soaps and detergents.
• The formation of micelles takes place above a particular temperature called Kraft temperature \[({{T}_{k}})\]and above a particular concentration called critical micellization concentration (CMC).
• They are capable of forming ions.
• Micelles may contain as many as 100 molecules or more.
• For example sodium stearate \[({{C}_{17}}{{H}_{35}}COONa)\]is a typical example of such type of molecules.
• When sodium stearate is dissolved in water, it gives \[N{{a}^{+}}\]and \[{{C}_{17}}{{H}_{35}}CO{{O}^{-}}\] ions.

    \[\underset{\text{Sodium stearate}}{\mathop{{{C}_{17}}{{H}_{35}}COONa}}\,\]\[\underset{\text{Stearate  ion}}{\mathop{{{C}_{17}}{{H}_{35}}CO{{O}^{-}}}}\,+N{{a}^{+}}\]

     The stearate ions associate to form ionic micelles of colloidal size.

• It has long hydrocarbon part of \[{{C}_{17}}{{H}_{35}}\]radical. Which is lyophobic and \[CO{{O}^{-}}\]part which is lyophilic.

In the figure, the chain corresponds to stearate ion, \[({{C}_{17}}{{H}_{35}}CO{{O}^{-}})\]. When the concentration of the solution is below from its CMC \[({{10}^{-3}}\,mol\,\,\,{{L}^{-1}})\], it behaves as normal electrolyte. But above this concentration it is aggregated to behave as micelles.

• The main function of a soap is to reduce oily and greasy dirt to colloidal particles (an emulsion). Soap therefore, are known as emulsifying agents.
• Some other examples of micelles are sodium palmitate \[({{C}_{15}}{{H}_{31}}COONa)\], Sodium lauryl sulphate \[[C{{H}_{3}}{{(C{{H}_{2}})}_{11}}S{{O}_{3}}{{O}^{-}}N{{a}^{+}}]\], Cetyl trimethyl ammonium bromide \[C{{H}_{3}}{{(C{{H}_{2}})}_{15}}{{(C{{H}_{2}})}_{3}}{{N}^{+}}B{{r}^{-}}\] etc.