Surfactants and Suspending Agents Flashcards
Why can insects walk on water?
Surface tension
How is surface tension created>
-Water molecules are attracted to each other and like to stay together, especially on the surface where there is only air above
- hydrogen bonds pull down from the air = tension on surface
surface tension problematic in pharamceutics
- Wetting process hindered if surface tension too high
- Add surfactant molecules to replace some of the water molecules in the surface
- Forces of attraction between surfactant and water molecules are less than those between two water molecules, hence the contraction force is reduced
Surfactant
Surface
Active
Agent
What is a surfactant?
- An amphiphilic/amphipathic molecule that consists of a distinct hydrophobic and hydrophilic region
- A molecule that accumulates at interfaces
- A molecule that aggregates or self-assembles
Everyday applications of surfactants:
- Paint (pigment dispersions)
- ice cream (smooth texture - resistance to rapid meltdown)
- Detol/washing up liquid (trap and remove dirt)
Pharmaceutical applications of surfactants:
Surfactants are present in suspensions and emulsions
Aggregated surfactants can be used as drug delivery vehicles: e.g. micelles and vesicles to solubilise insoluble drugs
- cetyl
- myristyl alcohol (aveeno)
- polyethylene glycol (pfizer injection)
Phospholypid:
hydrophobic tail
hydrophilic head
in water
face hydrophilic head
in oil
face hydrophobic tail
What lowers interfacial tension?
Adsorption at the oil-water
Types of surfactants
- ionic surfactants (anionic - dissociate at high pH/cationic - dissociate at low pH)
- Non-ionic surfactants (less toxic and irritant than ionic - widely used)
- Mixtures of surfactants (produce more stable emulsions)
Classifications of surfactants - ANIONIC
-negatively charged surfactant, +ive couterion
-e.g sodium dodecyl sulfate
-soluble in water
-forms self-emulsifying bases with fatty alcohols
-detergent in medicated shampoos
-skin cleanser in topical applications
Classification of surfactant - NOn-IONIC
-uncharged
-e.g ‘Brij’ - alkyl chain linked to polyoxyethylyne gycol
-spans = sorbitan acid esters
-tweens = polyoxethylene sorbitan fatty acid esters
Classification of surfactants - MIXTURE
-Zwitterionic
-two charges, overall neutral
Surfactants can be…
- Drugs
- Naturally occurring: Bile salts, Lecithin,Synovial fluid, Lungs surfactant
Co-surfactants
- To achieve very low interfacial tension, a second amphiphile (co-surfactant) is incorporated
-Usually short chain alcohols or amines ranging from C4 to C10
- Helps in the formation and stabilisation of micelles/microemulsions
- Increases the flexibility and fluidity of the interface
Co-surfactant mixture role =
covering the whole droplet of oil
Surfactants/Co-surfacants > Classification> self-assesmby>
Micelles
What is Micelles?
Molecular dynamics simulation of a micelle formed by non-ionic surfactants dissolved in water, with a hydrophobic core
Micelles size and shape:
- Colloidal spherical or near spherical nanoparticles
- Loose aggregates of 50 – 100 surfactant molecules into a single entity
- Size of micelles ~ 5 – 10 nm
Micelles are dynamic structures:
- Individual monomers may enter or leave the micelles = rapid process
- Micelles are constantly breaking down and reforming = slow process
Surfactant self-assembly…
-60 randomly positioned molecules of polysorbate 80 first aggregate into small clusters, which then come together to form a single micelle
-The micelle then undergoes restructuring to yield its final equilibrium arrangement
Micelle formation in water;
- Extensively hydrogen bonded
- “Flickering clusters”
- Highly ordered water molecules form “cages” around the hydrophobic alkyl chains
