Suspensions

Fifty question-and-answer flashcards covering key principles of pharmaceutical suspensions, including definitions, stability theories, flocculation control, wetting, and formulation considerations.

1. What is a pharmaceutical suspension?

A coarse dispersion in which insoluble particles, generally larger than 1 µm, are dispersed within a liquid (usually aqueous) medium.

2. Why are suspensions used for water-insoluble drugs?

They allow administration of drugs that cannot be dissolved in acceptable volumes of solvent.

3. How does a suspension differ from a solution?

In a solution the drug is molecularly dispersed; in a suspension the drug remains as solid particles dispersed in the liquid.

4. How does a suspension differ from a colloid?

Colloids contain much smaller particles (

5. List four characteristics of an ideal suspension.

Homogeneous during dosing, easy to re-suspend, proper/acceptable viscosity, and particles that are small and uniform in size.

6. Name four common routes for administering pharmaceutical suspensions.

Oral suspensions, ear/eye drops, intramuscular injections (not intravenous), and topical suspensions.

7. Why must the dispersed drug remain essentially insoluble in the vehicle?

To avoid Ostwald ripening, recrystallisation, and particle growth that destabilise the formulation.

8. What is Ostwald ripening?

The process in which smaller particles dissolve and redeposit onto larger particles, leading to crystal growth and instability.

9. Why is sedimentation a greater problem in suspensions than in colloids?

Because the larger particle size in suspensions makes gravitational settling dominate over Brownian motion.

10. Which physical law governs the sedimentation rate of particles in suspensions?

Stokes’ law.

11. According to Stokes’ law, how does particle radius affect sedimentation velocity?

Sedimentation velocity is proportional to the square of the particle radius (v ∝ a²).

12. At what approximate particle size does Stokes’ law apply for sedimentation?

For particles larger than about 0.5 µm.

13. What is meant by ‘creaming’ in a suspension?

Upward movement of particles (or droplets) when their apparent density is less than that of the continuous phase (Ap < 0).

14. What two opposing forces are considered in DLVO theory?

Electrostatic repulsion (VR) and van der Waals attraction (VA).

15. Which minimum in the DLVO potential energy curve is most important for pharmaceutical suspensions?

The secondary minimum, because it produces weak, reversible flocculation that can stabilise a suspension.

16. What happens when attractive forces greatly exceed repulsive forces in a suspension?

Particles can aggregate irreversibly, forming a dense cake that cannot be re-dispersed.

17. Define flocculation in the context of suspensions.

The reversible aggregation of particles into loosely bound clusters (flocs) that settle rapidly but are easily re-suspended.

18. Define deflocculation.

A state in which particles remain as discrete units; they sediment slowly but can form a hard, compact cake.

19. What is the major disadvantage of an over-flocculated suspension?

It may be irreversible, highly viscous, and visually unattractive, with very rapid sedimentation.

20. What is the main drawback of an under-flocculated suspension?

It forms a compact sediment that is difficult to re-suspend.

21. List two visual/physical differences between flocculated and deflocculated suspensions.

Flocculated: fast sedimentation, fluffy sediment with large volume; Deflocculated: slow sedimentation, compact sediment with small volume.

22. What is sedimentation volume ratio (F) and its formula?

F = Vu / Vo, where Vu is the final settled volume and Vo is the original volume of suspension.

23. What does an F value greater than 1 indicate?

The sediment occupies more volume than the original suspension, often due to strong floc formation (high sediment expansion).

24. How can addition of surfactants sometimes lead to deflocculation?

Surfactants can increase electrostatic repulsion (zeta potential), preventing particles from forming flocs.

25. Name three main approaches to achieving controlled flocculation.

Adjust particle size, add electrolytes to control zeta potential, and incorporate flocculating agents such as surfactants or polymers.

26. State the Schulze-Hardy rule in one sentence.

The higher the valency of the counter-ion, the lower the concentration needed to cause particle aggregation.

27. Give an example illustrating the Schulze-Hardy rule (NaCl vs AlCl3).

NaCl requires the highest concentration to flocculate, CaCl₂ a lower amount, and AlCl₃ the lowest due to its trivalent counter-ion.

28. Describe the effect of low electrolyte concentration on DLVO energy profile.

Large diffuse double layer → high primary maximum → no secondary minimum, so particles remain dispersed.

29. How does intermediate electrolyte concentration influence particle interaction?

It compresses the diffuse layer, lowers the primary maximum, and introduces a secondary minimum suitable for controlled flocculation.

30. What happens to DLVO interactions at very high electrolyte concentration?

The diffuse layer collapses, eliminating the primary maximum and leading to irreversible aggregation (instability).

31. Name two ways polymers can act as flocculating agents.

They can bridge between particles and increase medium viscosity, both promoting stable flocs.

32. Explain steric repulsion.

A stabilising force arising when adsorbed macromolecules prevent particles from approaching closely, producing positive enthalpy and osmotic effects that oppose aggregation.

33. List the three energetic contributions to steric repulsion.

Positive enthalpy of interaction, negative entropy due to polymer conformational restriction, and an osmotic pressure effect upon dilution.

34. What is the main function of wetting agents in suspensions?

To facilitate dispersion of hydrophobic drug particles in an aqueous vehicle by lowering surface tension and contact angle.

35. How is wettability of a powder quantified?

By the contact angle; a smaller contact angle indicates better wetting.

36. What does a large contact angle signify about a powder’s wettability?

Poor wettability and difficulty in dispersing the powder into the liquid.

37. Why are surfactants often chosen as wetting agents?

They lower surface tension and can adsorb at solid–liquid interfaces, improving particle wetting and preventing adhesion to container walls.

38. Which injectable route is suitable for suspensions and which is not?

Intramuscular injection is suitable; intravenous injection is not, due to the risk of embolism from large particles.

39. What term describes suspensions containing particles larger than colloidal size?

Coarse suspensions.

40. Why must suspensions be homogeneous at the moment of dosing?

To ensure dose uniformity and therapeutic efficacy for each administered portion.

41. How do lyophobic systems interact with the dispersion medium?

They have poor affinity for the solvent, contributing to physical instability and sedimentation.

42. What role does Brownian motion play in stabilising very small particles?

It counteracts gravitational settling for colloidal-size particles, but is insufficient for coarse suspension particles.

43. Define caking in suspensions.

The formation of a dense, compact sediment caused by close packing of particles, often irreversible and difficult to redisperse.

44. When does creaming occur instead of caking?

When the particle density is lower than the medium, causing upward movement and separation rather than downward compaction.

45. Which particle property can be altered to manipulate the depth of the secondary minimum and floc stability?

Particle size and surface charge (controlled by electrolytes or surfactants).

46. State three key reasons why small and uniform particle size is desirable in suspensions.

Reduces sedimentation rate, improves homogeneity, and promotes easier re-suspension.

47. According to Stokes’ law, list the three main variables (besides radius) that influence sedimentation velocity.

Density difference between particle and medium, gravitational acceleration, and viscosity of the continuous phase.

48. What effect does increasing continuous-phase viscosity have on sedimentation?

It decreases sedimentation velocity, helping maintain particles in suspension longer.

49. How can addition of electrolytes promote controlled flocculation?

By compressing the electrical double layer, reducing zeta potential, and allowing particles to enter the secondary minimum for loose floc formation.

50. Why is easy re-suspension critical for patient compliance and dose accuracy?

Because patients and caregivers must be able to shake the bottle briefly to obtain a uniform dose without excessive effort or specialized equipment.