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DrX Whiz Niraj
Unit 1: Colloidal Dispersions
Physical Pharmaceutics II | B.Pharm 4th Sem
1. Introduction to Dispersed Systems
A Dispersed System is a mixture where one substance (the Dispersed Phase) is distributed as particles throughout another substance (the Dispersion Medium).
Analogy to Understand:
Think of sugar dissolved in water. Sugar is the dispersed phase, and water is the dispersion medium. However, based on the size of the particles, these systems behave very differently.Colloids are unique because their particles are larger than molecules (True solutions) but smaller than visible coarse particles (Suspensions). Because of this size (1nm - 1000nm), they do not settle down by gravity, making them highly stable.
2. Lyophilic vs Lyophobic Colloids
Colloids are classified based on how much the dispersed particles "love" or "hate" the dispersion medium.
| Feature | Lyophilic (Solvent Loving) 💖 | Lyophobic (Solvent Hating) 💔 |
|---|---|---|
| Affinity | High affinity for solvent. | No affinity for solvent. |
| Preparation | Prepared easily by direct mixing. | Requires special chemical methods. |
| Reversibility | Reversible. If dried, can be remixed with solvent to form sol again. | Irreversible. Once precipitated, cannot be remixed easily. |
| Stability | Highly stable. Does not coagulate easily. | Unstable. Coagulates easily on adding electrolytes. |
| Examples | Gelatin, Acacia, Starch in water. | Gold sol, Silver sol, Sulfur sol. |
3. Optical Properties (Tyndall Effect)
When a strong beam of light is passed through a true solution, the path is invisible. But when passed through a Colloidal Dispersion, the path of the light becomes bright and visible. This is called the Tyndall Effect.
Why does this happen?
Colloidal particles (1-1000 nm) are large enough to scatter the light rays falling on them. True solution particles (<1 nm) are too small to scatter light.4. Kinetic Properties (Brownian Motion)
If you observe a colloidal solution under an ultramicroscope, the particles are in a continuous, random, zig-zag motion. This is called Brownian Movement.
Cause of Brownian Motion:
It happens due to the unequal bombardment (collisions) of the dispersion medium molecules against the dispersed colloidal particles from different sides. This constant knocking prevents the particles from settling down, giving stability to the colloid.5. Stability, Coagulation & Peptization
Why are Colloids Stable?
Colloidal particles carry an electric charge (either all positive or all negative). Because like charges repel, the particles push each other away and do not aggregate (combine) to settle down. This forms an Electrical Double Layer.
Coagulation (Flocculation)
If we add an Electrolyte (like NaCl or AlCl3), the oppositely charged ions neutralize the charge on the colloidal particles. Without their protective charge, particles collide, aggregate, and settle down as a precipitate. This is Coagulation.
Hardy-Schulze Rule:
"The higher the valency (charge) of the added ion, the greater is its power to cause coagulation."Example to precipitate a Negative Sol: Al³⁺ > Ba²⁺ > Na⁺ (Al³⁺ is the most powerful).
Peptization (Reverse of Coagulation)
Peptization is the process of converting a freshly prepared precipitate back into a colloidal sol by adding a small amount of electrolyte (Peptizing agent).
Example: Adding a few drops of FeCl3 to a fresh precipitate of Ferric Hydroxide converts it back into a Red Colloidal Sol.
6. Protective Colloids & Gold Number
Lyophobic sols (like gold sol) are highly unstable and coagulate easily with a pinch of salt. To save them, we add a Lyophilic sol (like Gelatin). The lyophilic particles form a protective layer around the lyophobic particles, saving them from the salt. This is called Protective Action.
🌟 Definition of Gold Number:
It is the minimum amount of protective colloid (in milligrams) required to prevent the color change (coagulation) of 10 ml of standard Gold Sol when 1 ml of 10% NaCl solution is added to it.Important Rule: Lower the Gold Number, Higher is the protective power.
(Gelatin has a gold number of 0.005, making it an excellent protector. Starch has a gold number of 25, making it a very weak protector).