DrX %20(1).png)
DrX Whiz Niraj
Biphasic Liquids: Suspensions
Comprehensive exam notes on Suspensions. Master the concepts of Flocculated vs Deflocculated systems, Stokes' Law, and Stability problems like Caking.
Definition: A Pharmaceutical Suspension is a biphasic liquid dosage form containing finely divided insoluble solid particles (Dispersed phase) ranging from 0.5 to 5.0 microns, uniformly distributed in a liquid vehicle (Dispersion medium).
Advantages
- Ideal for drugs that are insoluble in water or standard solvents.
- Provides higher chemical stability than aqueous solutions (e.g., Penicillin G is formulated as a suspension).
- Can successfully mask the bitter taste of drugs (e.g., Chloramphenicol palmitate).
- Can be formulated to provide sustained/prolonged drug release.
Disadvantages
- Physical instability: Particles tend to settle down over time.
- Must be shaken vigorously before each use to ensure an accurate dose.
- Risk of Caking (formation of a hard, un-redispersible solid mass at the bottom).
- Bulky to carry and prone to breakage (glass bottles).
Based on the electrokinetic nature of solid particles, suspensions are classified into two critical types. This is the most frequently asked question in university exams.
| Parameters | Flocculated Suspension | Deflocculated Suspension |
|---|---|---|
| Particle Arrangement | Particles form a loose network (flocs). | Particles exist as separate, individual entities. |
| Rate of Sedimentation | High / Fast (Because flocs are heavy and settle quickly). | Low / Slow (Because individual particles are small and light). |
| Supernatant Liquid | Appears Clear after settling. | Appears Cloudy / Turbid even after settling. |
| Sediment Nature (Caking) | Sediment is loosely packed. No Hard Cake is formed. | Sediment is very closely packed. Hard Cake is formed. |
| Redispersion | Easy. A gentle shake redistributes the particles uniformly. | Very Difficult. Hard cake cannot be broken easily by shaking. |
| Bioavailability | Comparatively less. | Relatively higher. |
Formulating a stable suspension involves particle size reduction and the addition of specific excipients to maintain stability.
Key Excipients Used:
- Wetting Agents: Insoluble drugs (like Sulphur) repel water and float on top. Wetting agents (Surfactants, Glycerin, Alcohol) reduce the interfacial tension, allowing water to penetrate and wet the particles.
- Suspending / Thickening Agents: They increase the viscosity of the dispersion medium, thereby slowing down the sedimentation rate (e.g., Methylcellulose, Tragacanth, Sodium Alginate).
- Flocculating Agents: Electrolytes (like NaCl) or polymers added to artificially induce flocculation, preventing hard cake formation.
1. Sedimentation & Caking
Particles settling down and fusing together into an unbreakable rock-hard mass (Cake) is the biggest failure of a suspension.
Remedy: Deliberately formulate a Flocculated suspension by adding electrolytes. Flocs are bulky and porous, preventing particles from packing tightly.
2. Crystal Growth (Ostwald Ripening)
Temperature fluctuations cause small particles to dissolve and re-precipitate onto larger particles. Over time, particles grow so large they cannot be suspended.
Remedy: Add protective colloids (polymers) that coat the particles, and avoid extreme temperature storage conditions.
Formula Alert: Stokes' Law
The velocity of sedimentation (settling rate) of particles is mathematically expressed by Stokes' Equation:
Where: V = Sedimentation rate, r = Particle radius, d₁ & d₂ = Densities, η = Viscosity of medium.
Takeaway: To minimize settling (make V smaller), we must decrease the particle size (r) or increase the viscosity (η) of the dispersion medium.