Stability problems in suspensions and corrective methods MCQs With Answer

Suspension stability problems and corrective methods are essential for B.Pharm students to ensure safe, effective liquid formulations. This topic covers common instability phenomena—flocculation, deflocculation, caking, creaming, Ostwald ripening—and their physicochemical causes such as particle size, zeta potential, interfacial tension, and ionic strength. You will learn corrective methods including wetting agents, flocculating agents, suspending agents (e.g., xanthan, bentonite), viscosity modifiers, pH and electrolyte control, steric and electrostatic stabilization, and milling techniques. Understanding Stoke’s law, DLVO theory, sedimentation volume, redispersibility tests, and structured vehicles helps formulate robust suspensions with prolonged shelf life. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is ‘caking’ in pharmaceutical suspensions?

• Formation of a loose, easily redispersible floc
• Irreversible aggregation forming a hard compact sediment
• Upward migration of particles forming a cream layer
• Gradual increase in particle solubility over time

Correct Answer: Irreversible aggregation forming a hard compact sediment

Q2. Which phenomenon describes the upward movement of dispersed particles in a suspension?

• Caking
• Flocculation
• Creaming
• Coagulation

Correct Answer: Creaming

Q3. Flocculation in suspensions is best characterized by which statement?

• Permanent chemical fusion of particles forming a cake
• Loose reversible aggregation that is easily redispersed
• Complete dissolution of particles into the medium
• Separate phases forming immiscible layers

Correct Answer: Loose reversible aggregation that is easily redispersed

Q4. A deflocculated suspension typically shows which behavior?

• Rapid sedimentation and easy redispersion
• Slow sedimentation but formation of hard cake
• No sedimentation due to perfect stabilization
• Immediate flocculation on standing

Correct Answer: Slow sedimentation but formation of hard cake

Q5. According to Stoke’s law, sedimentation velocity is most strongly dependent on which factor?

• Particle density linearly
• Particle diameter squared
• Viscosity squared
• Surface charge magnitude

Correct Answer: Particle diameter squared

Q6. Ostwald ripening in suspensions results from which mechanism?

• Electrostatic repulsion between particles
• Smaller particles dissolving and redepositing on larger particles
• Mechanical breakdown of aggregates by stirring
• Formation of air bubbles that lift particles

Correct Answer: Smaller particles dissolving and redepositing on larger particles

Q7. High absolute zeta potential in a suspension generally indicates what?

• Poor wettability of particles
• Enhanced steric stabilization
• Strong electrostatic repulsion and good dispersion stability
• High tendency for Ostwald ripening

Correct Answer: Strong electrostatic repulsion and good dispersion stability

Q8. Which additive is primarily used to improve wetting of hydrophobic powders for suspension?

• Suspending agent (e.g., xanthan gum)
• Wetting agent (surfactant)
• Preservative
• Density adjusting agent

Correct Answer: Wetting agent (surfactant)

Q9. Which of the following is commonly used as a suspending agent to increase viscosity?

• Sodium chloride
• Xanthan gum
• Toluene
• Polysorbate 80 as sole stabilizer

Correct Answer: Xanthan gum

Q10. Which class of substances commonly promotes flocculation by compressing the electrical double layer?

• Non-ionic polymers
• Electrolytes (salts)
• Antioxidants
• Non-polar solvents

Correct Answer: Electrolytes (salts)

Q11. Sedimentation volume (F) is used to evaluate which property of suspensions?

• Particle solubility
• Flocculation and sedimentation behavior
• Microbial contamination level
• Viscosity at high shear

Correct Answer: Flocculation and sedimentation behavior

Q12. The degree of flocculation (β) is calculated by which formula?

• β = Vo / Vf
• β = Vf / Vo
• β = sediment density × particle size
• β = zeta potential / ionic strength

Correct Answer: β = Vf / Vo

Q13. DLVO theory explains stability of suspensions by considering the balance between which forces?

• Hydrophobic and hydrophilic forces
• Van der Waals attraction and electrostatic repulsion
• Gravitational and buoyant forces
• Covalent bonding and hydrogen bonding

Correct Answer: Van der Waals attraction and electrostatic repulsion

Q14. Which manufacturing technique is commonly used to reduce particle size for improved suspension stability?

• Lyophilization
• Jet milling / wet milling
• Spray painting
• Crystallization without milling

Correct Answer: Jet milling / wet milling

Q15. To prevent irreversible caking of a sediment, formulators commonly do which of the following?

• Avoid any viscosity modifiers
• Promote controlled flocculation and increase viscosity
• Use only low-molecular-weight electrolytes
• Maximize particle aggregation

Correct Answer: Promote controlled flocculation and increase viscosity

Q16. The HLB value of a surfactant indicates what property?

• Hydrophilic-lipophilic balance
• Heat of lipid binding
• Hydrogen-bonding level
• High-lipid bioavailability

Correct Answer: Hydrophilic-lipophilic balance

Q17. Which simple test evaluates ease of redispersion of a suspension?

• Freeze–thaw cycling
• Shake test / redispersibility test
• pH titration
• Thermogravimetric analysis

Correct Answer: Shake test / redispersibility test

Q18. Steric stabilization of particles is typically provided by which type of additive?

• Small inorganic electrolytes
• Adsorbed polymers (e.g., PVP, PEG)
• Strong acids
• Volatile organic solvents

Correct Answer: Adsorbed polymers (e.g., PVP, PEG)

Q19. Which approach most directly reduces creaming according to Stoke’s law?

• Increase particle size
• Reduce particle size
• Add volatile solvents
• Increase temperature

Correct Answer: Reduce particle size

Q20. Brownian motion in suspensions helps to:

• Increase aggregation of large particles
• Counteract sedimentation for very small particles
• Cause irreversible chemical reactions
• Enhance Ostwald ripening exclusively

Correct Answer: Counteract sedimentation for very small particles

Q21. Which term describes irreversible aggregation due to strong attractive forces between particles?

• Flocculation
• Coagulation
• Dispersion
• Emulsification

Correct Answer: Coagulation

Q22. Increasing temperature generally affects suspension stability by:

• Increasing viscosity and reducing sedimentation
• Decreasing viscosity and increasing sedimentation
• Eliminating all electrostatic charges
• Converting suspensions into true solutions

Correct Answer: Decreasing viscosity and increasing sedimentation

Q23. Preservatives are added to suspensions primarily to:

• Prevent particle aggregation
• Improve wetting of powders
• Inhibit microbial growth and spoilage
• Adjust density of the dispersed phase

Correct Answer: Inhibit microbial growth and spoilage

Q24. Which surfactant is frequently used as a wetting and emulsifying agent in pharmaceutical suspensions?

• Calcium carbonate
• Polysorbate 80 (Tween 80)
• Benzalkonium chloride as sole suspending polymer
• Magnesium stearate as dispersant

Correct Answer: Polysorbate 80 (Tween 80)

Q25. A consequence of Ostwald ripening on particle size distribution over time is:

• Decrease in mean particle size
• Increase in mean particle size
• No change if temperature changes
• Immediate flocculation only

Correct Answer: Increase in mean particle size

Q26. Zeta potential of a suspension is typically measured by which technique?

• Mass spectrometry
• Electrophoretic light scattering (zeta potential analyzer)
• UV-visible spectroscopy
• Gas chromatography

Correct Answer: Electrophoretic light scattering (zeta potential analyzer)

Q27. A ‘structured vehicle’ in suspension formulation refers to:

• A presaturated solvent system with no rheology
• A gel-like network that immobilizes particles
• An oil phase that replaces water completely
• A solid matrix used for tablets only

Correct Answer: A gel-like network that immobilizes particles

Q28. Which rheological behavior is most desirable for easy pouring and good redispersibility of suspensions?

• Newtonian viscosity only
• Pseudoplastic (shear-thinning) with thixotropy
• Dilatant (shear-thickening)
• Perfectly elastic solid-like behavior

Correct Answer: Pseudoplastic (shear-thinning) with thixotropy

Q29. To improve wettability of a hydrophobic drug powder, formulators often add:

• Non-ionic wetting agent or surfactant (e.g., sodium lauryl sulfate)
• Only inorganic fillers
• Anhydrous solvents exclusively
• High concentrations of simple sugars

Correct Answer: Non-ionic wetting agent or surfactant (e.g., sodium lauryl sulfate)

Q30. What is the ideal sedimentation volume (F) for a well-flocculated suspension?

• Close to 0 (0%)
• Close to 0.5 (50%)
• Close to 1 (100%)
• Negative value

Correct Answer: Close to 1 (100%)

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