L48 - liquid dosage forms: suspensions and emulsions 1 Flashcards
what are characteristics of suspensions?
- solubility of drug in vehicle is low
- diameter of disperse phase: 0.5 to 100mcm
- particle size <0.5 mcm - colloidal
what is a suspension?
dispersion in which the drug is dispersed in the external phase (vehicle)
what do suspensions being unstable lead to?
- sedimentation
- particle-particle interactions
- caking (compaction)
what can help understand suspensions’ physical stability?
- electrical properties of dispersed particles
- effect of distance of separation between particles on their subsequent interaction
why may particles acquire a charge due to?
- ionisation of functional groups
- adsorption of ions to surface of the particle
what is the electrical double layer?
phenomenon referred to following adsorption of ion on to the surface
what is the zeta potential?
potential that the boundary of second layer possesses
what does zeta potential measure?
degree of electric charge on particles relative to bulk medium in which they are suspended
what can be used to stabilise pharmaceutical suspensions regarding electrical properties of dispersed particles?
compression of the electrical double layer by inc conc of electrolyte
what are the three states on interaction possible?
- no interaction
- coagulation (agglomeration)
- loose aggregation (termed floccules)
what is the no interaction state like?
- particles sufficiently distant from one another
- thermodynamically stable (absence of sedimentation)
what is the coagulation state like?
- particles form intimate contact with each other
- pharmaceutically unacceptable formulation - inability to redisperse the particles upon shaking
what is the loose aggregation state like?
- loose reversible interaction between particles
- enable particles to be redispersed upon shaking
what are the opposing forces in particle suspensions like?
- thermodynamic instability tends to aggregation = attractive forces (vdw potential eng, Va)
- electrical charge (zeta potential) tends to separation = repulsive forces (electrostatic potential eng, Vr)
what is the DLVO theory?
when dispersed in liquid medium, particles will experience (electrical), repulsive forces and attractive (london/vdw) forces
what is the eqn for the overall energy of interaction between the particles?
Vt = Va + Vr
what does the potential energy curve for particles show?
- 2 minima
- 1 maximum
what happens as you increase interparticles distance?
- short - attractive forces predominate, particles tend to agglomerate (first minimum)
- as it increases - repulsive forces predominate, particles remain in suspension (max)
- increased further - repulsive force decreases, particles weakly attracted (second minimum)
what is key to determining the stability of the system?
depth of secondary minimum
what happens in sedimentation?
- large particles reach bottom initially
- smaller particles occupy space between larger particles
when does sedimentation occur?
under the influence of gravity
what is caking?
- particles at the bottom gradually compressed by weight of those above
- sufficient energy available to overcome primary max (repulsive force)
- particles become sufficiently close to form irreversible interaction at primary min