Walk into any pharmacy and you’ll see bottles labeled “shake well before use,” thick creams, and milky lotions. These everyday medicines are examples of coarse dispersions—systems where particles or droplets are dispersed in another medium but are large enough to settle or separate over time.
Unlike true solutions, coarse dispersions need careful formulation to stay stable and effective. UNIT 3 dives into the science of suspensions and emulsions, explaining how pharmaceutical scientists design these products to ensure uniform dosing, long shelf life, and patient convenience.
Coarse Dispersions: An Overview
What Are Coarse Dispersions?
Coarse dispersions contain particles larger than 0.5–1 µm dispersed in a continuous medium.
They include:
- Suspensions (solid in liquid)
- Emulsions (liquid in liquid)
Because of their size, particles tend to settle or separate, requiring stabilization techniques.
Suspensions
Definition
Suspensions are heterogeneous systems where insoluble solid particles are dispersed in a liquid medium.
Examples
- Antacid suspensions
- Pediatric antibiotic syrups
- Insoluble drug mixtures
They are useful when drugs are poorly soluble in water.
Interfacial Properties of Suspended Particles
Particles possess surface energy and charge, which influence stability.
Important Factors
- Surface tension
- Wetting properties
- Electrical charge
- Particle size
Proper wetting agents help disperse particles uniformly.
Settling in Suspensions
Particles settle due to gravity. The rate of sedimentation follows Stokes’ law.
Factors Affecting Settling
- Particle size
- Density difference
- Viscosity of medium
- Temperature
Reducing particle size and increasing viscosity slows sedimentation.
Flocculated and Deflocculated Suspensions
Flocculated Systems
- Loose aggregates form
- Faster settling
- Easy redispersion
Deflocculated Systems
- Separate particles
- Slow settling
- Hard cake formation
Flocculated suspensions are generally preferred to avoid caking.
Emulsions
What Are Emulsions?
Emulsions consist of two immiscible liquids where one is dispersed as droplets in the other.
Common types:
- Oil-in-water (O/W)
- Water-in-oil (W/O)
They are widely used in creams, lotions, and oral emulsions.
Theories of Emulsification
Surface Tension Theory
Emulsifiers reduce interfacial tension, allowing droplet formation.
Oriented Wedge Theory
Emulsifiers orient themselves between oil and water phases.
Interfacial Film Theory
A protective film surrounds droplets to prevent coalescence.
These theories explain how stable emulsions are formed.
Microemulsions and Multiple Emulsions
Microemulsions
Clear, thermodynamically stable systems with very small droplet size.
Benefits
- Enhanced drug solubility
- Improved absorption
- Long-term stability
Multiple Emulsions
Systems like W/O/W or O/W/O.
Used for:
- Controlled drug release
- Taste masking
- Protection of sensitive drugs
These advanced systems are widely used in modern pharmaceutics.
Stability of Emulsions
Emulsions may destabilize through:
- Creaming
- Coalescence
- Cracking
- Phase separation
Stability Enhancement Methods
- Emulsifiers
- Viscosity enhancers
- Proper storage
- Antioxidants and preservatives
Stability ensures consistent therapeutic effect.
Preservation of Emulsions
Since emulsions contain water, microbial growth is possible.
Preservatives Used
- Parabens
- Benzalkonium chloride
- Sorbic acid
These agents maintain product safety and shelf life.
Rheological Properties of Emulsions
Viscosity plays a major role in:
- Spreadability
- Pourability
- Stability
Most emulsions exhibit non-Newtonian, pseudoplastic behavior, making them easy to apply but stable at rest.
Emulsion Formulation by HLB Method
What Is HLB?
Hydrophilic–Lipophilic Balance (HLB) measures the affinity of emulsifiers for water or oil.
Rules
- Low HLB → W/O emulsion
- High HLB → O/W emulsion
By selecting proper HLB values, formulators achieve stable emulsions.
Why Coarse Dispersions Matter in Pharmacy
Suspensions and emulsions:
- Improve drug delivery
- Enhance stability of insoluble drugs
- Increase patient compliance
- Allow flexible dosing
These systems are essential in pediatric, topical, and cosmetic formulations.