P7: Emulsions & Suspensions Flashcards
what are colloids
emulsions, microemulsions, creams
what are some examples of colloids
oral suspensions, topical dosage forms, injections, aerosols
what is the dispersion phase of an emulsion
phase that is sub-divided
what is the continuous phase of an emulsion
phase in which disperse phase is distributed
what are the steps of colloidal stability
high surface area -> high surface energy -> aggregation -> thermodynamically unstable
what are emulsions and suspensions
thermodynamically unstable two-phase systems
when do emulsions and suspensions attain equilibrium
when internal droplets coalesce to form one ‘macro-phase’
surface area is minimised
how do suspension particles achieve lower surface area
flocculating or aggregating (no coalescence)
how do fine particles dispersed in liquid encounter one another
brownian movement, creaming or sedimentation, convection
what is the effect on sedimentation rate when particle diameter increases
increased sedimentation rate
what is the effect on sedimentation rate when particle density increases
increased sedimentation rate
what happens to sedimentation rate when viscosity increases
decreased sedimentation rate
what happens to particles at short inter-particle distances
attractive forces predominate and particles tend to agglomerate
what happens to particles as inter-particle distance increases
sufficient energy is added to separate the particles, repulsive forces dominate and particles remain in suspension
what is flocculation
prevents rigid cohesion by forming loose aggregates
held together with comparatively weak interparticulate forces
what type of structure occurs with flocculation
lattice type structure; resists complete settling, reduces caking and aids re-dispersion
what happens in flocculated systems
repulsive barriers have been reduced and particles form loosely bonded structures in secondary minimum
particles settle as flocculates not individual particles
sediment is not closely packed and caking does not occur
suspension formulation aims for partial or controlled flocculation
what is caking
particles not associated in deflocculated systems; pressure on individual particles - their close packing
what happens to particles in low cetrimide concentration
head-groups attach to kaolin
hydrophobic tails point out into aqueous phase
hydrophobic groups interact to form flocculates
what happens to particles in high cetrimide concentration
cetrimide bilayer with head groups pointing out
interfacial tension reduced and deflocculation
how is flocculation controlled
with non-ionic polymers to increase aqueous phase viscosity
what are the characteristics of an ideal suspending agent for controlled flocculation
easily incorporated into formulation, readily dissolves/disperses in water without the need for special techniques, ensures formation of loosely-packed system that does not cake, does not affect dissolution rate or absorption rate of drug, is inert, non-toxic and free from incompatibilities
how can colloids be stabalised
by repulsive forces due to adsorption of macromolecules or surfactants to their surfaces
in aqueous dispersions, absorbed molecules are hydrated
why are non-ionic surfactants used in emulsions
to absorb onto oil droplets and increase stability by creating hydrated layer on hydrophobic particles
what is HLB
measure of balance between hydrophobic and hydrophilic components of a surfactant
what does HLB stand for
hydrophile-lipophile balance system
what is the range of the HLB scale
1 to 20
what does a low HLB value indicate
oil-soluble surfactants
what are microemulsions
homogenous, transparent, low-viscocity colloidal solutions
