Questions
1-2 questions in semester exams
Difficulty
Easy
Importance
Core foundational concept for Pharmaceutics
Overview
Colloidal systems are heterogeneous mixtures where particle sizes range between 1nm and 1000nm, acting as a bridge between true solutions and coarse suspensions. Understanding this topic is critical for pharmacy and biotechnology students, as it governs the stability, shelf-life, and bioavailability of drug delivery systems. Success requires mastering the phase distribution between the dispersed phase and dispersion medium.
Classification of Colloids
Colloids are classified primarily based on the physical state of the dispersed phase and the dispersion medium. A secondary classification involves the interaction between these two components, defining them as lyophilic (solvent-loving) or lyophobic (solvent-fearing).
- Aerosol: Liquid/Solid dispersed in gas
- Foam: Gas dispersed in liquid
- Emulsion: Liquid dispersed in liquid
- Lyophilic: Reversible, stable, and highly hydrated
- Lyophobic: Irreversible, unstable, requires stabilizers
Suspensions Basics
Suspensions are coarse dispersions where insoluble solid particles are dispersed in a liquid medium. They are kinetically unstable and often require the addition of suspending agents to increase the viscosity of the continuous phase and retard sedimentation.
- Particle size usually greater than 1000nm
- Visible to the naked eye or light microscope
- Separated via filtration or centrifugation
- Follows Stokes' Law for sedimentation rate
- Requires flocculating agents for uniform dispersion
Emulsion Fundamentals
Emulsions consist of two immiscible liquids where one is dispersed as droplets within the other, stabilized by an emulsifying agent. The type of emulsion (O/W or W/O) is determined by the phase ratio and the nature of the emulsifier used.
- O/W (Oil in Water): Water is the continuous phase
- W/O (Water in Oil): Oil is the continuous phase
- HLB system (Hydrophilic-Lipophilic Balance) for surfactant selection
- Microemulsions are thermodynamically stable
- Cracking and coalescence are signs of instability
Formula Sheet
v = [d^2(ρ1 - ρ2)g] / 18η (Stokes' Law for sedimentation)
HLB = 20 * (1 - M_o / M)
n = (Total surface area) / (Area of one particle)
Exam Tip
Always draw the table comparing True Solution, Colloid, and Suspension; it is the highest-scoring part of this unit in descriptive exams.
Common Mistakes
- Confusing particle size ranges for true solutions, colloids, and suspensions.
- Failing to distinguish between physical classification (phase states) and interaction-based classification (lyophilic/phobic).
- Applying Stokes' law without accounting for the density difference or viscosity correctly.
More Revision Notes
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