Surface Chemistry: Coagulation

Coagulation is the transformation of blood from a liquid to gel, resulting in the formation of blood clots. It could lead to hemostasis or the cessation of blood loss from a damaged vessel and then repair. The addition of electrolytes usually causes coagulation to take place. The coagulating ion, also known as a flocculating ion, is the ion that is responsible for neutralising the charge on colloidal particles.

What is Coagulation?

Coagulation is a process that involves colloidal particles coming together to form larger particles that eventually settle as residue or float on the surface. The coagulating ion, also known as a flocculating ion, is the ion that is responsible for neutralising the charge on colloidal particles. The neutral particles then begin to clump together to form larger particles that settle down.

What are Colloidal Solutions?

Heterogeneous systems with dispersed particle sizes ranging from 1 nm to 1000 nm (i.e. 10-9-10-6 m) are colloidal solutions.

Hardy Schulze Law

The valency of an ion with an opposite charge to the colloidal particles determines the amount of electrolyte required to coagulate a specific amount of colloidal solution. The electrolyte ions, that are most effective in causing coagulation, are those which have the opposite charge as the colloidal particles. Coagulating ions are the ions that cause the blood to clot. We can say that coagulation is faster if the valency of the oppositely charged ion is greater than the added electrolyte. 

Causes of Coagulation

Coagulation of Lyophobic Sol can be caused by the following:

1. By Prolonged Dialysis: The presence of a small quantity of electrolyte in a colloidal sol gives it stability. The electrolyte is entirely removed during prolonged dialysis. As a result, the colloidal sol coagulates and becomes unstable.
2. By heating and cooling: Sometimes, heating the sol causes it to thicken, such as butter coagulation. Similarly, in some situations, cooling the sol causes coagulation, such as coagulation of milk, in which fats begin to float on the surface as the milk cools down.
3. Electrophoresis: The dispersed phase particles move towards oppositely charged electrodes in electrophoresis and are neutralised. If the process is allowed to run long enough, the neutral particles will unite, grow in size, and settle down. 
4. Mutual Precipitation: A process in which oppositely charged sols are mixed in proper proportions to neutralise each other’s charges is known as mutual precipitation. It causes both sols to coagulate. When positively charged arsenious sulphide sols are mixed, both sols coagulate. 
5. The stability of lyophilic sols is determined by two different factors: charge and association of colloidal particles. A lyophilic sol could be coagulated after these two factors are removed. This is accomplished by adding electrolytes and a suitable solvent.

What is Sol?

It is a type of colloid in which the dispersed phase is solid, but the dispersion medium is fluid. 

How are Lyophilic and Lyophobic Sol different?

1. Lyophilic Sol: In Lyophilic Sol, the dispersed phase and dispersion medium attract each other. 
2. Lyophobic Sol: In Lyophobic Sol, the dispersed phase and dispersion medium repel each other.

Mechanism of Coagulation

When we add electrolyte to the sol, the colloidal particle absorbs ions having opposite charges from the electrolyte. As a result, this charge gets neutralised, and the uncharged particles become coagulated. 

Coagulation in Water Treatment

Coagulation in water treatment involves the addition of substances that promote the clumping of small flocs into bigger flocs to be separated from the water more efficiently. Coagulation can be used as a pre or post-treatment step in water or wastewater treatment processes such as filtration and sedimentation. 

Coagulation treatment is typically performed before sedimentation and filtration. A coagulant is added to the water during the process, and its positive charge balances out the negative charge of the suspended contaminants. Suspended particles bind together due to neutralisation. These particles collect at the bottom of the treatment tank in clumps known as flocs, making it easier to filter out of the water.

The coagulant is quickly added to the water and mixed during this process, allowing it to disperse throughout the entire sample of water. To remove the settled particles, the water can be filtered through an ultrafiltration or microfiltration membrane, or a medium filter, after it has coagulated. 

Conclusion 

Coagulation is a process that involves colloidal particles coming together to form larger particles that eventually settle as a precipitate or float on the surface. The addition of electrolytes usually causes coagulation to take place. Coagulation could lead to hemostasis, cessation of blood loss from a damaged vessel, and then repair. The electrolyte ions that are most effective in causing coagulation are those that have the opposite charge as the colloidal particles.