Coagulation: Lyophobic and Lyophilic Sols

Coagulation, also known as precipitation, is the process of settling colloidal particles. It is the phenomenon of aggregation or accumulation of colloidal particles to settle down as a precipitate.

Due to the existence of charge on colloidal particles, lyophobic sols are stable. The same type of charge on colloidal particles repels one another and keeps them in a suspended state. The particles will move closer to one another to form aggregates (or coagulate) and settle down under the influence of gravity if the charge is somehow eliminated. This process is called coagulation.

Process of Coagulation of lyophobic sols

Some of the methods of coagulation of lyophobic sols are:

1. Electrophoresis: Moving towards electrodes with opposing charges, the colloidal particles discharge, and then get precipitated.
2. Mixing two oppositely charged sols: When oppositely charged sols are mixed in almost equal amounts, they neutralize their charges and get precipitated. For example,  Arsenic sulfide (-ve sol) and hydrated ferric oxide (+ve sol) are combined to generate precipitated forms. 
3. Addition of electrolyte: The colloidal particles precipitate when an excess of an electrolyte is introduced. The interaction of colloids with ions having charges opposite to their own causes neutralization, and hence lead to their coagulation. 
4. Boiling: Boiling a sol causes more collisions with the molecules of the dispersion medium, which disturbs the adsorbed layer. This lowers the charge on the particles, which eventually causes them to settle as a precipitate.
5. Dialysis: Continuous dialysis is one of the methods to coagulate lyophobic sols. Persistent dialysis causes the sol to lose its stability and eventually coagulate as parts of the electrolytes are fully removed.

Hardy Schulze Rule of coagulation

Hardy Schulze’s rule states that the greater the valency of the flocculating ion added, the greater will be its power to cause precipitation. In other words, the coagulation power depends on the valency of the coagulation ion.

The flocculating/coagulating force in the coagulation of a negative sol is as follows:

Al³⁺>Ba²⁺>Na⁺

Similarly, the flocculating/coagulating force in the coagulation of a positively charged sol is as follows:

[Fe(CN)₆]^4– > PO₄^3– > SO₄^2– > Cl^–

Coagulation of lyophilic sols

The stability of lyophilic sols is primarily attributed to two factors;  colloidal particles’ charge and solvation.  This is accomplished by adding an electrolyte and a suitable solvent, respectively. The dispersion phase of hydrophilic sols dehydrates when solvents like alcohol and acetone are introduced. A minimal amount of electrolyte can cause coagulation in this condition.

Protection of Colloids

Compared to lyophobic sols, lyophilic sols are more stable. This is because lyophilic colloids are heavily solvated, or the liquid in which they are dispersed surrounds the colloidal particles like a sheath. The ability to shield lyophobic colloids is a special quality of lyophilic colloids. In the presence of a lyophilic sol, lyophobic particles are shielded from electrolytes by a layer of lyophilic particles that surrounds them. Protective colloids are lyophilic particles that are employed for this purpose.

Coagulation Video

FAQs

References

• Arun Bahl, B. S. Bahl & G. D. Tuli, Essentials of Physical Chemistry, S. Chand and Company Ltd., New Delhi, 2012.
• Principles of colloid and surface chemistry, Paul C. Hiemenz and Raj Rajagopalan
• Introduction to Applied Colloid and Surface Chemistry; Georgios M. Kontogeorgis, Søren Kiil.