From milk and fog to injections and emulsions, colloids quietly dominate everyday science and pharmaceutical technology. These systems consist of tiny particles dispersed in another medium, creating mixtures that appear uniform but behave uniquely. In modern pharmacy, colloidal dispersions are crucial for drug delivery, stability, and bioavailability.
UNIT 1 explores how these microscopic systems work, how they are classified, and why their physical properties matter in medicine and industry.
Classification of Dispersed Systems
What Is a Dispersed System?
A dispersed system contains two components:
- Dispersed phase (particles)
- Dispersion medium (continuous phase)
Depending on particle size, dispersed systems are categorized into different types.
Types Based on Particle Size
1. True Solutions
Particles less than 1 nm.
Example: salt in water.
2. Colloidal Dispersions
Particles between 1–1000 nm.
Example: milk, gels, suspensions.
3. Coarse Dispersions
Particles greater than 1000 nm.
Example: sand in water.
Colloids lie between solutions and suspensions, giving them special properties.
Size and Shapes of Colloidal Particles
Particle size determines behavior such as stability and diffusion.
Common Shapes
- Spherical
- Rod-like
- Disc-shaped
- Fibrous
Shape influences surface area and interaction with the dispersion medium, which is especially important in pharmaceutical suspensions and emulsions.
Classification of Colloids
Colloids can be classified based on different criteria.
Based on Physical State
- Sol (solid in liquid)
- Gel (liquid in solid)
- Emulsion (liquid in liquid)
- Foam (gas in liquid)
Based on Affinity for Medium
Lyophilic Colloids
- Strong attraction
- Stable
- Reversible
Example: starch solution
Lyophobic Colloids
- Low attraction
- Less stable
- Irreversible
Example: gold sol
Lyophilic colloids are commonly preferred in pharmaceuticals due to better stability.
Comparative General Properties of Colloids
Colloids show characteristics intermediate between solutions and suspensions.
Key Properties
- Do not settle easily
- Pass through filter paper
- Show light scattering
- Exhibit Brownian motion
These features help identify colloidal systems.
Optical Properties
Tyndall Effect
Colloids scatter light, making the beam visible when passed through the system. This phenomenon confirms the presence of dispersed particles.
Importance
Used to distinguish colloids from true solutions.
Kinetic Properties
Brownian Motion
Colloidal particles show continuous zig-zag motion due to collision with solvent molecules.
Significance
- Prevents sedimentation
- Maintains stability
- Indicates particle size
Smaller particles show more rapid movement.
Electrical Properties
Charge on Colloidal Particles
Colloidal particles usually carry electric charges due to:
- Ion adsorption
- Ionization
- Surface interactions
This charge creates repulsion between particles, preventing aggregation.
Zeta Potential
Higher zeta potential means better stability of the colloidal system.
Electrical properties are important in designing injectable suspensions and emulsions.
Effect of Electrolytes
Adding electrolytes may neutralize particle charges, causing coagulation.
Effects Include
- Reduced repulsion
- Particle aggregation
- Precipitation
This process is used in water purification and formulation adjustments.
Coacervation
Coacervation refers to phase separation of a colloidal solution into two liquid layers.
Applications
- Microencapsulation
- Controlled drug delivery
- Taste masking
It is widely used in pharmaceutical technology.
Peptization
Peptization converts precipitates into colloidal particles by adding suitable electrolytes.
Importance
- Restores colloidal state
- Improves dispersion
- Enhances stability
This process helps prepare stable pharmaceutical suspensions.
Protective Action
Certain lyophilic colloids protect lyophobic colloids from coagulation.
Examples
Gelatin protecting gold sol.
Benefits
- Increased stability
- Prevention of precipitation
- Improved shelf life
Protective colloids are widely used in injections and emulsions.
Why Colloidal Chemistry Matters in Pharmacy
Colloids are vital in:
- Creams and ointments
- Suspensions
- Emulsions
- Aerosols
- Controlled drug delivery systems
Understanding colloidal behavior ensures better drug performance and patient safety.