colloids and suspensions Flashcards
what is colloidal dispersions
dispersions in which the size of the dispersed particles in the continuous phase is in the range of 10^-9 - 10^-6 m
what is a colloid
particles of drug
aggregates consisting of many drug molecules
why do we use suspensions
poorly soluble drugs cannot always be made into solutions
taste making
drug may be more stable (if not stable, esters can hydrolyse and the drug becomes degraded)
what is a property of a good suspension
suspension must be homogeneous so the particles are evenly distributed to ensure the same dose= avoids overdosing and underdosing
what is the tyndall effect
colloidal systems assessed based on how beam of light behaves when it comes in contact with the system
what causes the sedimentation velocity to go up
increase radius of particles
increase the density of solid
decrease the density of liquid
decrease the viscosity of the liquid
what causes the sedimentation velocity to go down
increase density of liquid
increase the viscosity of liquid
take the suspension to the moon= less gravity
how does caking occur
the gravity pulls the drug particles to drop to the bottom of the container
what is a wetting agent with examples
breaks the interfacial tension= decreases surface tension
ensures the solid particles disperse easily through the liquid
e.g surfactants, hydrophilic colloids
what makes a good suspension
low interfacial tension= liquid spreads around the particle
difference between flocculated system and deflocculated
flocculated=
loose aggregates of particles
large volume of final sediment
rapid sedimentation rate
suspension clears quickly
easy to disperse
deflocculated=
discrete units of particles
small volume of final sediment
slow sedimentation rate
cloudy
difficult to redisperse
coagulation
particles are closely aggregated and difficult to redisperse
permanent
viscosity enhancing agents
polysaccharides
celluloses
hydrated silicates
flocculating agents
electrolytes
surfactants
polymers
electrical properties of colloids
negative charge at the particle surface will attract positive ions
attracts negative ions
electrical double layer
How do electrolytes cause flocculation
Reducing the charge on particles
Reduces repulsive forces between particles
Why does caking not occur in a flocculated system
Large amounts of continuous phase is entrapped in the sediment
Volume of sediment is larger
Suspension is easier to resuspend
What is the zeta potential
Tells you how strong the repulsive forces are between two particles
Boundary between inner layer and surface layer
What does Vr depend on (repulsive force)?
Surface charge
Thickness of the double layer
What is Vt (total force)
Va +Vr
= attractive force + repulsive force
What is the stern layer
Counter and co ions are in direct contact with the surface
What is the shear plane
Separates liquid adhering to the solid particle surface from the bulk liquid in the continuous phase
What will an increase in electrolytes do
Decrease the thickness of the double layer
Decrease the zeta potential
Increase the depth of the secondary minimum
Reduce the repulsive forces
Leads to flocculation
What does secondary mimimum lead to
Flocculation
How does surfactants reduce repulsion’s
Neutralise the surface charge of a particle
How does polymers affect flocculation
Chemical groups in polymer interact with the surface of the particles
Free end of polymer attaches to another particle
Inter particle bridging leads to flocculation
If there are no other particles to interact with, the free end of the polymer coats the particle and leads to re stabilisation and deflocculation
What is a restabilised particle
Both ends of polymer end up getting stuck to same particle
Flocculation
Loosly bound particles
Dvlo theory
Repulsive interactions arise from electrostatic forces
At what potential of zp does flocs and caking appear
Between -30 and +30 mV = flocs
Less than -30 or greater than +30mV= dispersed system which means particles sink slowly and leads to caking
What is clumping and clinging
Clumping= particles cluster together
Clinging= to the side of container
