Disperse Systems Flashcards
What is happening to water molecules for surface tension?
Are attracted to each other, with hydrogen bonds. They are also pulling away from the air.
What are the problems involving surface tension in pharmaceutics?
Displace water molecules
Wetting process hindered if surface tension too high
Need to add surfactant molecules to replace some of the water molecules in the surface.
What is a surfactant?
Surface active agent
What type of molecule is a surfactant?
Amphiphilic and amphipathic molecule that consists of a distinct hydrophobic and hydrophilic region.
What are the two types of surfactants?
Ionic surfactants
-an ionic dissociate at a high pH
-cationic dissociate at low pH
Non ionic surfactants
What are co-surfactants used for?
To achieve a very low interfacial tension, by incorporating a second amphiphile.
What is the structure of micelles?
Oil inside, water outside.
Micelles formation in water is…
Dispersion of lipids in water
Water surrounds the tail
Cluster of lipid molecules
Micelles cluster
What side does a reaction favour in thermodynamics?
Lowest free energy state
What are suspending agents used for?
Reduce the rate of settling of suspended active ingredients
Increases viscosity of the suspension
what are non-ionic surfactants used for?
widely used, less toxic and irritant than ionic surfactants
what do mixtures of surfactants produce?
more sable emulsions
how does lowering the interfacial tension help the adsorption of the oil-water interface?
aids the dispersal of the oil into droplets of a small size
maintains the particles in a dispersed state
what are anionic surfactants?
negatively charged surfactants, positive counterion
what are cationic urfactants?
positively charged surfactants, negative counterions
what are zwitterions?
two charges, overall neutral
what are some naturally occuring surfactants?
bile salts
lecithin
synovial fluid
lugns surfactant
what are the characteristics of co-surfactants?
usually short chain alcohols or amines ranging from C4 to C10
helps in the formation and satabilisation of micelles/ microemulsions
increases feklxi bility and fluidity at the interface
what is the rapid process involving micelles?
individual monomers may enter or leave the micelles
what is the slow process involving micelles?
micelles are constantly breaking down and reforming
what is the self- assembly of surfactants?
60 randomly positioned molecules of polysorbate 80 first aggregate into small clusters, which the come together to form a single micelle
the micelle then undergoes restructuring to yield its final euilibrium
what happens when the head groups of micelles come too close together?
start to repel each other
which stops the micelles from growing
what type of repulsion do ionic micelles have?
strong repulsion which becomes a smaller micelle
what type of repulsion do non-ionic micelles have ?
less strong repulsion which becomes a larger micelle
what is the critical micelle concentration (CMC)?
the surfactant concentration at and above which micelles are formed
detrmined by measuring thensurface tension at different [surfatants]
what is the surface tension like above the CMC?
constant
what does the osmotic pressure in water depend on after CMC?
number of molecules present in water
what are the sites of possible solubilisation?
a) hydocarbon core
b) core/mantle region
c) head group (mantle) region
d) surface adsorption
what are suspending agents used for?
to reduce the rate of settling of suspended active ingredients
increases the viscosity of the suspension
drugs can be readily and uniformly resuspended before use
what are the characteristics of an ideal suspending agent?
Readily and uniformly incorporated into formulation
* Readily dispersed in water without special techniques
* Ensure the formation of a loosely packed system which does not cake
* Does not influence the dissolution rate or absorption rate of the drug
* Be inert, non-toxic and free from incompatibilities
