Stability Of Colloids

Stability Of Colloids

The stability of colloids may be owing to one or more of the following factors :

(I) Electric charge

The dispersed particles of lyophobic colloidal systems have the same kind of electric charge. Particles with like charge repel each other and their mutual repulsion prevents them from joining together and settling out. The accumulation of similar kind of charges on the suspended particles holds them part and thus stabilizes the system.

The charge is generally acquired by adsorbing +ve or −ve ions from the dispersion medium. The dispersion medium (or internal phase) has the charge opposite to that on the dispersed particles and the system as a whole is neutral. Most of colloidal metal sulphides and acid dyestuffs acquire a negative charge. Most of oxides and hydroxides of the metal and basic dyestuffs become positively charged. Protein appears to gain either +ve or a −ve charge with equal ease.

The stability of lyophobic colloid is intimately related with the electrical charge on the particle. Thus in the formation of an arsenic (III) sulphide sols by precipitation with hydrogen sulphide in acid solution, sulphide ions are primarily adsorbed (since every precipitate has a tendency to adsorb its own ions), and some hydrogen ions are secondarily adsorbed. The hydrogen ions or other ions, which are secondarily adsorbed, have been termed as ‘counter−ions’. Thus the so called electrical double layer is set up between the particles and the solution. An arsenic (III) sulphide particle is represented diagrammatically in figure. The colloidal particles of arsenic (III) sulphide has the a negatively charged surface, with positively charged counter ions which impart a positive charge to the liquid immediately surrounding it. If electric current is passed through the solution, the negative particles will move towards the anode; the speed is comparable with that of electrolytic ions. The electrical conductivity of a sol is, however, quite low because the number of current−carrying particles are small compared with that in a solution of an electrolyte at an appreciable to compensate for their smaller number.

If the electrical double layer is destroyed the colloid will no longer stable and when BaCl2 is added, barium ions are preferentially adsorbed by the particles; the charge distribution on the surface is disturbed and the particles flocculate. After flocculation, it is found that the dispersion medium is acid owing to the liberation of the hydrogen counter−ions. It appears that ions of opposite charge to those primarily adsorbed on the surface are necessary for coagulation. The minimum amount of electrolyte necessary to cause flocculation of the colloid is called the flocculation or coagulation value.

(II) Coating of protective substances

In some systems (like emulsion) the stability is due to the presence of some protective agent. The protective agent is adsorbed on the surface of the dispersed particles coating them and thus preventing actual contact and their subsequent coagulation.

(III) Solvation

The stability of Lyophilic sols is basically due to solvation. The dispersed phase becomes solvated in the presence of liquid dispersion medium. In other words the dispersed particles adsorb molecules of the dispersing liquid and thus surrounded by a protective envelope of dispersion medium. Such sols are very stable. The disperse phase cannot be separated from the envelope of the medium. The precipitate (or coagulum) is formed only by removing first the solvent envelope by dehydration, followed by the addition of a huge amount of electrolyte.