Disperse systems 8 Flashcards
Coagulation
- primary minimum phenomenon
- settle very quickly
- particles are closely aggregated
- much of the vehicle is trapped, reducing the amount of free vehicle and making the formulation very viscous and difficult to pour
- a single large aggregate is formed on standing: shaking may be enough to deform the aggregate but it is very difficult or impossible to redisperse
Deflocculation
• primary maximum phenomenon
• particles remain separate
• particles are small so settle slowly (Stoke’s Law)
• V R allow particles to slide past each other as
they sediment
• these characteristics prevent liquid being trapped in sediment
• compact sediments of low sedimentation volume
are produced
• sediment does not easily redisperse on shaking.
Flocculation
• secondary minimum phenomenon
• formation of loose aggregates (groups) of particles which constantly break up and reform
• aggregates are relatively large so sedimentation
is rapid
• liquid is trapped within and between aggregates
• sediment has a high sedimentation volume
• shaking can re-disperse the sediment and a
homogenous system may be obtained.
Flocculation vs Deflocculation
• Ideally we’d like the slow sedimentation
characteristics of a deflocculated suspension,
but that comes with the risk of caking and
irreversible sedimentation
• So we usually compromise with a flocculated
suspension (easy to re-disperse) with increased
viscosity to minimize sedimentation, but which
remains pourable
Altering ζ with electrolytes
• The greater the charge on two like particles, the greater their repulsion for each other
• If an electrolyte with the opposite charge is added, it will associate with the dispersed particle, decreasing their VR and zeta potential, making them repel each other less and allowing flocculation to occur.
• The amount of electrolyte added controls the degree of flocculation
– small amount > flocculation
– large amounts > coagulation
– very large amounts > charge reversal > deflocculation
• The ability of an electrolyte to flocculate particles
depends on its valency (Schultz-Hardy rule):
trivalent > divalent > monovalent
Bridging agents
• non-ionic SAAs adsorb onto more than one particle to form a bridge and ∴ loose flocculated structures
• can act with electrolytes in bridging particles
• hydrophilic polymers are long chain branched molecules
• part of the chain adsorbs onto a particle and the rest
bridges to other particles, forming a gel-like network
and flocculating the system
Steric Stabilisation
- non-ionic polymers can adsorb onto the particle surface
* the polymer chains keep the particles apart due to a repulsive steric interaction
Assessing Suspension Stability
• rate of sedimentation
• final volume or height of sediment
• ease of redispersion
• particle size distribution
• during measurement a low rate of shear is
usually used to mimic the change in structure
upon storage, as a high rate may damage the
suspension
Accelerated stability testing
• shortens the time of some experiments,
• it is not always possible to accurately predict normal
behaviour from the results
• Centrifugation
– increases sedimentation rate by increasing g (Stoke’s Law).
– may destroy the structure of a flocculated system by
forming a tightly packed sediment, which may not have
occurred under normal conditions.
• Temperature cycling
– extremes of temperature (e.g. 40 to 0ºC) increases rate of degradation, especially crystal growth.
Sedimentation Volume (F)
• Plots of F vs time allows comparisons between
formulations. For a graph of F / time, when F is constant, sedimentation is complete.
• If F = 1 the volume of sediment is equal to the original
volume of suspension, no clear supernatant is seen on
standing and the product is in “flocculation equilibrium” -
this is a desirable state for a pharmaceutical.
• F is normally < 1, but can be > 1 if the flocs in a system
are particularly loose and fluffy.
Degree of Flocculation
This is a more useful measure than F.
It relates the volume of the flocculated sediment to that in a deflocculated system.
