Colloids 2 Flashcards
What is a suspension?
are dispersions of an insoluble drug (or other substance) in an aqueous or non-aqueous continuous phase
How can surfactants be used as wetting agents?
surfactants are compounds of amphiphilic nature
- reduce the surface tension of water/solid-liquid interface
= surfactants adsorb at the interface with their hydrophobic region escaping the aqueous environment and the hydrophilic region dissolving in the water
= the adsorbed surfactants displace the air at the interface and take the place of the water molecules
separates each particle from the adjacent particle
What are examples of surfactants commonly used?
oral
- polysorbates = Tweens
- sorbitan esters = Spans
external use
- sodium lauryl sulphate
parenteral
- polysorbates
- lecithin
hydrophilic colloids
- acacia
- tragacanth
- alginates
- cellulose derivatives
What are the factors affecting suspension stability?
particle size
- reducing particle size increases stability
= smaller particles remain suspended for longer
viscosity
- increasing viscosity increases stability
= particles will take longer to sediment
density difference between particles
- reducing density difference increases stability
= reduces sedimentation rate
What is a flocculate?
a cluster of particles held together in a loose open structure
What are the difference between flocculated and deflocculated systems?
flocculated system = beneficial
- high sedimentation
- high volume
- contains many voids
- can be re-dispersed
- no caking
deflocculated
- low sedimentation
- low volume
- contains few voids
- is difficult to re-disperse
What does flocculation depend on?
relative strength of
- electrostatic forces of repulsion
- electrostatic forces of attraction
What is the DVLO theory?
assumes that the electrostatic repulsion and Van der Waals attraction forces are the only forces involved in the interactions between colloidal particles
electrostatic repulsion - Vr
Van der Waals attraction forces - Va
How does distance affect electrostatic repulsion? How does the thickness of the double layer affect electrostatic repulsion?
electrostatic repulsion forces decay as an exponential function of the distance between the two particles
- they decrease as distance increases
electrostatic repulsion forces have a range of the order of the thickness of the double layer
How does distance affect the Van der Waals attraction forces?
Van der Waals attraction forces vary inversely with interparticulate distances
- at short and large distances, they dominate
What is the total potential energy of interaction between particles?
total potential energy of interaction is the sum of the electrostatic repulsion forces and Van der Waals attraction forces
How do the electrostatic repulsion forces and Van der Waals attraction forces vary with distance?
both electrostatic repulsion forces and Van der Waals attraction forces decrease as distance between particles increases
at short and large distances
- Van der Waals attraction forces predominate
at intermediate distances
- electrostatic repulsion forces predominate
How can the curve of the total potential energy of interaction between particles be described?
at short and large distances
- Van der Waals attraction forces predominate
= are known as the primary and secondary minimum
at intermediate distances
- electrostatic repulsion forces predominate
= is known as the primary maximum
What is the effect of the electrostatic repulsion forces and Van der Waals attraction forces on the colloidal system?
at short distances
- Van der Waals attraction forces predominate
= particles are more likely to coagulate, system is unstable
at intermediate distances
- electrostatic repulsion forces predominate
= particles are more likely to form a suspension
at large distances
- Van der Waals attraction forces predominate
= particles are more likely to form flocculates, as they are more loosely bound (can be easily re-suspended/re-dispersed)
When does flocculation occur? How can particles escape flocculation?
flocculation occurs when at a certain distance, an approaching particle becomes trapped in the secondary minimum energy region
to escape flocculation
- energy must be supplied
= shows it is easier to re-suspend a flocculated system than to have a deflocculated system
the deeper the secondary minimum, the more stable the floc
- energy of the system is less than that of the floc
