Coarse Dispersions: Suspensions and Emulsions Explained

coarse dispersion

A heterogeneous system where the dispersed phase has particles larger than 1 µm, visible under a microscope.

two main types of coarse dispersions

Suspensions and emulsions.

pharmaceutical suspension

A coarse dispersion in which insoluble solid particles are dispersed in a liquid phase.

pharmaceutical emulsion

A coarse dispersion where two immiscible liquids are mixed, with one dispersed as droplets in the other.

dispersed phase in suspensions

The insoluble solid particles.

continuous phase in suspensions

The liquid vehicle in which the solid is dispersed.

two phases in emulsions

Dispersed (internal) phase and continuous (external) phase.

oil-in-water (O/W) emulsion

An emulsion where oil droplets are dispersed in water.

water-in-oil (W/O) emulsion

An emulsion where water droplets are dispersed in oil.

advantages of suspensions

Allows delivery of insoluble drugs, masks taste, flexible dosing, improved stability over solutions.

disadvantages of suspensions

Physical instability, requires shaking, potential for caking, bulky packaging.

advantages of emulsions

Improved bioavailability, masks taste, suitable for external and internal use, customizable release.

disadvantages of emulsions

Thermodynamically unstable, prone to creaming and breaking, require emulsifiers.

sedimentation in suspensions

Settling of solid particles under gravity over time.

caking in suspensions

Hard, compact sediment that is difficult to redisperse after settling.

creaming in emulsions

Separation where the dispersed droplets rise or sink, forming a cream layer; reversible.

cracking in emulsions

Irreversible separation where the dispersed droplets coalesce into a separate layer.

phase inversion in emulsions

A switch in emulsion type (O/W to W/O or vice versa) due to temperature or composition changes.

flocculated suspension

Particles loosely aggregated, settle rapidly, easy to redisperse.

deflocculated suspension

Particles settle slowly, form hard cake, difficult to redisperse.

ideal properties of pharmaceutical suspensions

Uniform dispersion, easy to redisperse, physical stability, palatability, pourability.

ideal properties of pharmaceutical emulsions

Fine droplet size, stable against coalescence and creaming, pleasant texture and appearance.

emulsifying agent

A substance that stabilizes an emulsion by reducing interfacial tension between immiscible liquids.

examples of emulsifying agents

Tween 80 (polysorbate), Span 80 (sorbitan monooleate).

HLB

Hydrophilic-Lipophilic Balance; a measure of surfactant affinity for water vs. oil.

HLB value favors oil-in-water emulsions

HLB 8-18.

HLB value favors water-in-oil emulsions

HLB 3-6.

role of wetting agents in suspensions

They improve dispersion of hydrophobic particles by lowering surface tension.

examples of wetting agents

Tween 80, glycerin.

Stokes' Law

A formula that predicts the sedimentation rate of particles in a suspension.

equation for Stokes' Law

v = (2r²(ρ₁-ρ₂)g) / (9η), where v = velocity, r = radius, ρ = density, η = viscosity, g = gravity.

ways to improve suspension stability

Reduce particle size, increase viscosity of the vehicle, add flocculating or suspending agents.

examples of suspending agents

Methylcellulose, xanthan gum, sodium carboxymethylcellulose.

Ostwald ripening

Growth of larger particles at the expense of smaller ones, causing instability in suspensions and emulsions.

tests to differentiate O/W and W/O emulsions

Dilution test, conductivity test, dye solubility test.

dilution test for emulsions

Emulsion is mixed with water—if miscible, it's O/W; if immiscible, it's W/O.

conductivity test for emulsions

O/W emulsions conduct electricity (due to water phase); W/O emulsions do not.

dye solubility test for emulsions

A water-soluble dye colors the continuous phase; O/W turns uniformly colored, W/O shows globules.

coalescence in emulsions

Fusion of small droplets into larger ones, leading to separation and instability.

common uses of suspensions

Antibiotic suspensions, antacid preparations, pediatric or geriatric formulations.

common uses of emulsions

Oral formulations, topical creams/lotions, IV fat emulsions.

zeta potential in suspensions

A measure of particle surface charge that affects repulsion and stability.

surfactant

A compound that reduces surface and interfacial tension between phases (e.g., in emulsions).

nonionic surfactants

Surfactants without charge; they are stable over a wide pH range and less irritating.