Biphase Liquid Dosage Form (w4,5 + 6 ) Flashcards
List the factors that can cause emulsion instability to occur …
-Phase inversion
-Creaming
-Flocculation
-Coalescence
-Ostwald ripening
What are properties of a stable emulsion ?
The droplets retain their initial character and remain uniformly distributed throughout the continuous phase.
What is phase inversion ?
-Oil-in-water emulsion stabilised by ionic surfactant/co-surfactant
(If charge on emulsion droplet is reduced (with the addition of ions through buffer or drug), emulsion droplets will come together)
(Once droplets are in contact, interfacial surfactant film re-aligns forming water-in-oil droplets and phase inversion occurs)
What is flocculation and what causes it ?
-Flocculation is when two or more emulsion droplets aggregate without losing their individual identity
Larger droplets (> 2 µm) flocculate fastest and flocculation is promoted by creaming
-Addition of salt (Na3PO4) causes flocculation
What us coalescence and when does it occur and how is it caused ?
-Coalescence occurs when two or more droplets collide and form one larger droplet and is irreversible
-It is caused by various factors, including surfactant type and concentration, pH, temperature etc
What is Ostwald ripening ?
-With polydispersed droplets, collision between two droplets may cause one bigger droplet and one smaller droplet
-Upon repeated collisions, the small droplets become very small and become solubilised in the continuous medium. They eventually diffuse and re-deposit on larger droplets making them even larger in size
What is creaming ?
-Due to density difference between oil and water, the oil droplets tend to concentrate at the top of the emulsion
-To avoid this, increase the oil density or viscosity of the emulsion
-Fat globules (coloured with dye) tend to accumulate as a cream layer on top of the milk
-Due to the differences in densities between fat globules and the plasma phase of milk
What is the secondary minimum ?
When at large distances of separation the particles experience a minimum attraction.
- forced of attraction are weak, flocculations not coalescence (can redisperse upon shaking)
What is the primary maximum ?
- As particles come closer together, they start to experience some repulsion which will peak at the primary maximum
- The height of this repulsive force (Vmax) determines the stability of the system
(The height varies with different surfactants and electrolyte concentration)
(A high value will ensure coagulation is so slow that the system displays long term stability)
-The energy barrier that leads to irreversible particle aggregation
What happens to the primary maximum on addition of an electrolyte ?
-Neutralisation or reduction of charge on droplets
-Decrease in Vmax
-Destabilisation of the emulsion
What are entropic (steric) effects ?
- When two particles come into close contact, the polymer chains start to overlap
- This leads to a loss in the freedom of motion of the polymer chains, i.e. a loss of entropy
- This situation is thermodynamically unfavourable and forces the droplets apart again
What are osmotic (solvation) forces ?
- When two particles come into close contact the polymer chains start to overlap, effectively leading to a concentrated polymer solution
- This induces an osmotic gradient in the solution: a concentrated polymer solution in the overlap region and a dilute solution in the bulk solution
- Water enters the concentrated region in an attempt to dilute it and in doing so forces the polymer chains (and droplets) apart
What are the 4 main types of forces ?
1) Van der Waals (attractive)
2) Electrostatic (repulsive)
3) Steric forces (repulsive)
4) Solvation forces (repulsive)
For charged colloids (emulsions stabilised by ionic surfactants), van der Waals and electrostatic forces are the most important
For uncharged colloids (emulsions stabilised by non-ionic surfactants) van der Waals and steric and solvation forces are the most important
Define the term stability …
The capacity of a product to remain within specifications to ensure potency, quality or purity.
What is chemical degradation ?
-Decomposition of chemical moiety
-Due to effects of moisture, oxygen, light & heat
-Results in loss of active drug
