Sedimentation, Surface Tension & Surfactants

Sedimentation

Settling of suspended particles under gravity

Stokes’ law

Sedimentation velocity v=2r2g(s-l)/9

Particle radius (r)

Half particle diameter; smaller → much slower settling

Viscosity ()

Fluid resistance to flow; increased viscosity slows sedimentation (e.g., xanthan gum)

Density difference (s-l)

Difference between solid and liquid density; smaller difference → slower settling

Caking

Dense, irreversible sediment; particles cannot be redispersed easily

Surface tension ()

Net inward force at liquid surface; minimises surface area

Surface free energy

Work to expand surface; numerically equal to surface tension

Contact angle ()

Angle between liquid drop and solid; small angle = good wetting

Young’s equation

SA=SL+LA; relates contact angle to interfacial tensions

Wetting

Liquid spreading over solid; low SL and small contact angle needed

Surfactant

Amphiphilic molecule that lowers interfacial tension; improves wetting and dispersion

Amphiphile

Molecule with hydrophilic and hydrophobic regions

Anionic surfactant

Negatively charged head; strong surface activity; can be irritant

Cationic surfactant

Positively charged head; antimicrobial; e.g., cetrimide

Non-ionic surfactant

Uncharged; low irritation; pH-insensitive; safe for parenteral use

Amoxicillin sodium

Highly soluble salt; dry powder injection; reconstituted just in time to prevent hydrolysis

Amoxicillin trihydrate

Slightly soluble; oral suspension; most drug undissolved → chemically stable

Chloramphenicol palmitate

Prodrug; practically insoluble; no bitter taste; needs surfactant to form suspension