L48 - liquid dosage forms: suspensions and emulsions 1

Question 1

what are characteristics of suspensions?

Answer 1

• solubility of drug in vehicle is low
• diameter of disperse phase: 0.5 to 100mcm
• particle size <0.5 mcm - colloidal

Question 2

what is a suspension?

Answer 2

dispersion in which the drug is dispersed in the external phase (vehicle)

Question 3

what do suspensions being unstable lead to?

Answer 3

• sedimentation
• particle-particle interactions
• caking (compaction)

Question 4

what can help understand suspensions’ physical stability?

Answer 4

• electrical properties of dispersed particles
• effect of distance of separation between particles on their subsequent interaction

Question 5

why may particles acquire a charge due to?

Answer 5

• ionisation of functional groups
• adsorption of ions to surface of the particle

Question 6

what is the electrical double layer?

Answer 6

phenomenon referred to following adsorption of ion on to the surface

Question 7

what is the zeta potential?

Answer 7

potential that the boundary of second layer possesses

Question 8

what does zeta potential measure?

Answer 8

degree of electric charge on particles relative to bulk medium in which they are suspended

Question 9

what can be used to stabilise pharmaceutical suspensions regarding electrical properties of dispersed particles?

Answer 9

compression of the electrical double layer by inc conc of electrolyte

Question 10

what are the three states on interaction possible?

Answer 10

• no interaction
• coagulation (agglomeration)
• loose aggregation (termed floccules)

Question 11

what is the no interaction state like?

Answer 11

• particles sufficiently distant from one another
• thermodynamically stable (absence of sedimentation)

Question 12

what is the coagulation state like?

Answer 12

• particles form intimate contact with each other
• pharmaceutically unacceptable formulation - inability to redisperse the particles upon shaking

Question 13

what is the loose aggregation state like?

Answer 13

• loose reversible interaction between particles
• enable particles to be redispersed upon shaking

Question 14

what are the opposing forces in particle suspensions like?

Answer 14

• thermodynamic instability tends to aggregation = attractive forces (vdw potential eng, Va)
• electrical charge (zeta potential) tends to separation = repulsive forces (electrostatic potential eng, Vr)

Question 15

what is the DLVO theory?

Answer 15

when dispersed in liquid medium, particles will experience (electrical), repulsive forces and attractive (london/vdw) forces

Question 16

what is the eqn for the overall energy of interaction between the particles?

Answer 16

Vt = Va + Vr

Question 17

what does the potential energy curve for particles show?

Answer 17

• 2 minima
• 1 maximum

Question 18

what happens as you increase interparticles distance?

Answer 18

• 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)

Question 19

what is key to determining the stability of the system?

Answer 19

depth of secondary minimum

Question 20

what happens in sedimentation?

Answer 20

• large particles reach bottom initially
• smaller particles occupy space between larger particles

Question 21

when does sedimentation occur?

Answer 21

under the influence of gravity

Question 22

what is caking?

Answer 22

• 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

Question 24

What is the equation to calculate the velocity of sedimentation?

Answer 24

Vsed = 2a^2g(pp-pm) / 9n

a - particle radius
pp - density of particles
pm - density of suspension medium
n - viscosity of suspension medium
g - gravitational constant

Question 25

How can you control particle sedimentation + rate?

Answer 25

- controlling may enhance physical stability of pharmaceutical suspensions - rate may be dec by red avg particle diameter and in viscosity of vehicle

Question 26

When is manipulation of magnitude of secondary minimum required? (Controlled flocculation)?

Answer 26

For patciles in which zeta potential is high

Question 27

How is flocculation controlled?

Answer 27

- addition of electrolyte - charged surfactants that reduce potential to give satisfactory 2ndary min (where flocs can be formed)