Surfactant

Surfactants

(Surface active agents/amphiphiles) substances that can spontaneously collect at interfaces of solids, liquids or gases, thus lowering surface tensions or interfacial tensions.

Anionic surfactant characteristics - pH

- not stable at pH < 10 as they are weak acids and unionized fatty acid is formed at this pH - weakly antibacterial - laxative and unpleasant soapy taste - incompatible with high concentration of electrolytes (salting out effect)

Pharmaceutical applications of surfactants

1. Promote wetting by reducing the interfacial tension between liquid/solid (zinc oxide) 2. Antibacterial (benzalkonium chloride) 3. Stabilize colloid and foams 4. Stabilize emulsions

Particle size: Molecular dispersion Colloidal dispersion Coarse dispersion

<1.0 nm 1-500 nm >500 nm

Critical Micelle Concentration (CMC)

Cut off concentration when the concentration of a surfactant is greater than the concentration that can be accommodated by the surface or interface, micelles are formed.

What solutions are not always possible to formulate?

Drugs that are insoluble or solution is not practical (e.g., drug not stable in aqueous solutions, hydrolysis

Solutions not required

Ointment or lotion or solution prolonged effect targeting effect taste effect

Wetting of solid by liquid

Prevent floating The degree of wetting is determined by the contact angle (θ) which is a measure of the solvent's ability to wet a solid. For example, for contact lenses, you want maximum wetting. complete wetting: θ = 0° incomplete wetting: 0° < θ < 90° poor wetting: 90° < θ < 180°

Soft soap/Hard soap

Soft H2O soluble Anionic ( COO-, Na+, K+ NH4+) Hard H2O insoluble Anionic (COO-, Ca2+, Mg2+, Al3+)

Dispersed systems

When one component (e.g., drug) is distributed more or less uniformly throughout the second component (solvent or medium), the "mixture" is called a dispersion system in which surfactants are often added.

Surface tension

a special type of interfacial tension, referring to the tension between gas/solid or gas/liquid

Surfactants structure

amphipathic

Cationic surfactant characteristics - pH range

antibacterial preservatives mostly Could be used in emulsion skin preparations due to antiinfective properties. The suitable pH range is 4 - 6

Hydrophilic Lipophilic Balance (HLB)

determines how a surfactant is used higher the value, more hydrophilic lower the value, more lipophilic

Aerosol

dispersion system with solid or liquid dispersed in air

Can anion and cation surfactants be mixed?

no

cohesive > adhesive

no mixing;

Interfacial tension

owing to phase separation, there is a tension between any two separated phases (defined as the force acting at the right angles to a line 1 m in length along the interface).

cohesive force

refers to interactions between like molecules

Adhesive force

refers to interactions between unlike molecules.

Nonionic surfactant

stable low toxicity compatible with most H20 insoluble (hydrophobic) - e.g. Spans (fatty acid esters of Sorbitan). Sorbitan monolaureate (Span 20), Sorbitan trioleate (Span 80). H20 soluble (hydrophilic) - e.g. Tweens (derived from spans by adding polar polyoxyethylene glycol chains to non-esterified hydroxyls. Polyoxyethylene sorbitan monooleate (Tween 80), also known as polysorbate 80 (USP).

Anionic surfactant

the negatively charged polar head usually contains a carboxylate (e.g., soaps) and sulfonate. Aerosol OT (bis (2-ethylhexyl) sodium sulfosuccinate)

Zwitterionic surfactant

the polar head contains both positive and negative charges such as carboxylate (-), phosphate (-), quaternary ammonium (+). Many natural products such as protein, lecithin, gelatin, and phosphatidylcholine are zwitterionic surfactants

Cationic surfactant

the positively charged polar head usually contains quaternary ammonium

highest surface tension

water

adhesive > cohesive

→ mixing