Pharmaceutics · Short notes

Disperse Systems: Short Notes & Cheat Sheet

Suspensions, emulsions and colloids: stability, Stokes' law, HLB, flocculation and emulsion instability.

Short notes

Read these before you attempt the topic's MCQs.

Particle size decides how a dispersion behaves: true solutions never settle, colloids scatter light, and coarse dispersions settle under gravity.

Molecular dispersions (true solutions) have particles under 1 nm; colloids are 1 nm to 0.5 µm; coarse dispersions (suspensions, emulsions) are larger than about 0.5 µm.
Colloids are lyophilic (solvent-loving, e.g. gelatin, acacia) or lyophobic (e.g. gold sols). Lyophobic colloids are easily destabilised by electrolytes.
Colloids show the Tyndall effect (light scattering) and Brownian motion.

Stokes' law: settling velocity = d²(ρ₁ − ρ₂)g / 18η. Settling slows if particle size falls, density difference falls or viscosity rises.
Particle size has the biggest effect because it is squared.
Flocculated suspensions settle quickly into a loose sediment that is easy to redisperse. Deflocculated suspensions settle slowly but form a hard cake.
Controlled flocculation (using electrolytes, surfactants or polymers) is usually preferred.
Wetting agents (e.g. polysorbates) help hydrophobic powders disperse. Suspending agents (e.g. tragacanth, CMC, xanthan gum) raise viscosity.
Sedimentation volume F = Vu/Vo; a higher F means a better suspension.

Oil-in-water (o/w) emulsions are washable and suit oral use; water-in-oil (w/o) emulsions are greasy and suit dry skin.
Emulsifier type is predicted by HLB: low HLB (3–6) forms w/o emulsions, high HLB (8–18) forms o/w emulsions.
Emulsion type is identified by dilution, dye solubility and conductivity tests. An o/w emulsion conducts electricity and mixes with water.
Instability: creaming (reversible), flocculation (reversible), coalescence and cracking (irreversible), and phase inversion.
Gum acacia is the classic emulsifier for extemporaneous oral emulsions (dry gum method, 4:2:1 oil:water:gum for fixed oils).

High-yield facts in one place. Cover the right-hand column and test yourself.

Property	Flocculated · Deflocculated
Settling rate	Fast · Slow
Sediment	Loose, easy to redisperse · Hard cake
Supernatant	Clear · Cloudy
Preferred?	Yes (controlled) · No

Low HLB Loves oil: Low-HLB (lipophilic) emulsifiers make w/o emulsions; high-HLB make o/w.
4 : 2 : 1: Dry gum method for fixed oils: 4 parts oil, 2 parts water, 1 part acacia.

Creaming is reversible; cracking is not.
In Stokes' law particle diameter is squared, so halving particle size cuts settling rate to a quarter.
Deflocculated suspensions look better at first (slow settling) but form a hard cake.
Span surfactants are lipophilic (low HLB); Tween surfactants are hydrophilic (high HLB).

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