Stability problems in emulsions and remedies MCQs With Answer

Introduction

Emulsions are thermodynamically unstable dispersed systems widely used in pharmaceutics for oral, topical, and parenteral delivery. Stability problems include creaming, flocculation, coalescence, cracking, Ostwald ripening, and phase inversion. Key determinants are droplet size, interfacial tension, zeta potential, required HLB matching, emulsifier type (ionic/nonionic), viscosity, density difference, temperature, oxidation, and microbial contamination. Remedies span high-pressure homogenization, mixed emulsifier systems, steric stabilizers, rheology modifiers, antioxidants, chelators, and appropriate preservatives, alongside accelerated stability testing (centrifugation, thermal cycling, freeze–thaw) and diagnostic tests (conductivity, dye, drop dilution). Understanding Stokes’ law, PIT behavior, and interfacial film mechanics enables rational design of robust emulsions. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which emulsion instability is generally reversible and can often be corrected by gentle shaking?

• Creaming
• Coalescence
• Cracking
• Phase inversion

Correct Answer: Creaming

Q2. According to Stokes’ law, which change(s) will directly decrease the creaming rate of an o/w emulsion?

• Decreasing droplet radius
• Increasing continuous-phase viscosity
• Reducing the density difference between phases
• All of the above

Correct Answer: All of the above

Q3. Ostwald ripening in emulsions primarily occurs due to:

• Aggregation of droplets without fusion
• Diffusion of dispersed-phase molecules from smaller to larger droplets via the continuous phase
• Breaking of the interfacial film under shear
• Water migration into oil phase due to osmosis

Correct Answer: Diffusion of dispersed-phase molecules from smaller to larger droplets via the continuous phase

Q4. The most effective strategy to suppress Ostwald ripening in o/w emulsions is to:

• Increase the processing temperature during homogenization
• Add a small amount of a high molecular weight, low-solubility oil (e.g., squalane) as a ripening inhibitor
• Use a strongly ionic emulsifier
• Reduce packaging headspace

Correct Answer: Add a small amount of a high molecular weight, low-solubility oil (e.g., squalane) as a ripening inhibitor

Q5. Which action most directly reduces coalescence in emulsions?

• Using highly volatile oils
• Ensuring complete interfacial coverage with an optimized mixed-emulsifier system
• Lowering product viscosity
• Adding excess electrolytes

Correct Answer: Ensuring complete interfacial coverage with an optimized mixed-emulsifier system

Q6. Which action is most likely to increase flocculation in an electrostatically stabilized o/w emulsion?

• Adding electrolyte to increase ionic strength
• Adjusting pH away from the emulsifier’s isoelectric point
• Incorporating a nonionic polymer for steric stabilization
• Reducing temperature below the cloud point

Correct Answer: Adding electrolyte to increase ionic strength

Q7. Which test best indicates that an emulsion is oil-in-water (o/w)?

• Low electrical conductivity
• High electrical conductivity
• Floats on water and sinks in oil
• Forms a water-insoluble film on dilution

Correct Answer: High electrical conductivity

Q8. For an oil phase with a required HLB of 12 (o/w), a suitable emulsifier system should have an HLB value:

• Much lower than 12 (HLB 3–6)
• Around 12
• Much higher than 12 (HLB 16–18)
• Irrelevant, HLB does not apply

Correct Answer: Around 12

Q9. What percentage of Tween 80 (HLB 15) mixed with Span 80 (HLB 4.3) gives an HLB ≈ 10?

• 53% Tween 80
• 47% Tween 80
• 32% Tween 80
• 68% Tween 80

Correct Answer: 53% Tween 80

Q10. The primary objective of freeze–thaw cycling during accelerated stability testing of emulsions is to:

• Measure osmotic pressure
• Promote micelle formation
• Reveal susceptibility to coalescence/cracking due to ice-induced stress
• Determine required HLB

Correct Answer: Reveal susceptibility to coalescence/cracking due to ice-induced stress

Q11. Centrifugation testing primarily accelerates which instability mechanism?

• Ostwald ripening
• Creaming or sedimentation
• Oxidation
• Microbial contamination

Correct Answer: Creaming or sedimentation

Q12. Which of the following is NOT a typical emulsion instability?

• Creaming
• Coalescence
• Ostwald ripening
• Caking

Correct Answer: Caking

Q13. When choosing preservatives for an o/w emulsion, the formulator should primarily:

• Select oil-soluble preservatives only
• Use water-soluble preservatives effective at product pH and consider partitioning into oil
• Avoid preservative mixtures
• Ignore pH because it does not affect activity

Correct Answer: Use water-soluble preservatives effective at product pH and consider partitioning into oil

Q14. Microbial growth in emulsions predominantly occurs in the:

• Oil phase
• Aqueous phase
• Interfacial film only
• Headspace

Correct Answer: Aqueous phase

Q15. Which antioxidant is most appropriate for protecting the oil phase from autoxidation?

• Sodium metabisulfite
• Butylated hydroxytoluene (BHT)
• EDTA disodium
• Sorbic acid

Correct Answer: Butylated hydroxytoluene (BHT)

Q16. The primary role of chelating agents (e.g., EDTA) in emulsion stability is to:

• Act as primary antioxidants
• Bind trace metals that catalyze oxidation, enhancing antioxidant efficacy
• Increase viscosity of the continuous phase
• Reduce droplet size during homogenization

Correct Answer: Bind trace metals that catalyze oxidation, enhancing antioxidant efficacy

Q17. For o/w emulsions stabilized by ethoxylated nonionic surfactants, raising temperature toward the phase inversion temperature (PIT) typically:

• Improves hydration and increases o/w stability
• Dehydrates the headgroups, lowering effective HLB and risking inversion/coalescence
• Has no effect on interfacial properties
• Prevents Ostwald ripening

Correct Answer: Dehydrates the headgroups, lowering effective HLB and risking inversion/coalescence

Q18. Which ingredient class most effectively reduces creaming without altering interfacial chemistry?

• High–molecular weight hydrophilic polymers (e.g., xanthan gum)
• Low-boiling solvents
• Electrolytes (e.g., NaCl)
• Short-chain alcohols

Correct Answer: High–molecular weight hydrophilic polymers (e.g., xanthan gum)

Q19. The creaming index of an emulsion is commonly calculated as:

• (Total height / Cream layer height) × 100
• (Cream layer height / Total height) × 100
• (Sediment volume / Total volume) × 100
• (Droplet size / Viscosity) × 100

Correct Answer: (Cream layer height / Total height) × 100

Q20. In the drop dilution test, an o/w emulsion will typically:

• Readily dilute with water without separation
• Readily dilute with oil without separation
• Not dilute with either water or oil
• Invert immediately upon dilution with water

Correct Answer: Readily dilute with water without separation

Q21. Which processing step most directly decreases both creaming and coalescence?

• High-pressure homogenization to reduce and narrow droplet size distribution
• Prolonged storage at elevated temperature
• Reducing surfactant concentration
• Open-vessel mixing to increase aeration

Correct Answer: High-pressure homogenization to reduce and narrow droplet size distribution

Q22. Which zeta potential magnitude suggests good resistance to flocculation due to electrostatic repulsion?

• +5 mV
• −10 mV
• ±15 mV
• −40 mV

Correct Answer: −40 mV

Q23. Adding CaCl₂ to an o/w emulsion stabilized by sodium stearate is most likely to cause:

• Improved stability via increased zeta potential
• Soap precipitation and emulsion breaking
• No change in stability
• Reduced oxidation rate

Correct Answer: Soap precipitation and emulsion breaking

Q24. For parenteral lipid emulsions, a typical target mean droplet diameter to minimize embolic risk is:

• Greater than 5 μm
• Approximately 0.2–0.5 μm
• 1–2 μm
• 10–20 μm

Correct Answer: Approximately 0.2–0.5 μm

Q25. Which instability is driven by Laplace pressure differences between droplets?

• Flocculation
• Ostwald ripening
• Creaming
• Foaming

Correct Answer: Ostwald ripening

Q26. Which GMP practice best minimizes microbial contamination during emulsion manufacture?

• Using potable water and open transfers
• Using purified water, sanitized equipment, and closed transfers
• Skipping in-process bioburden testing
• Eliminating preservatives entirely

Correct Answer: Using purified water, sanitized equipment, and closed transfers

Q27. Why are mixed emulsifier systems often superior to single emulsifiers?

• They always lower viscosity dramatically
• They form more elastic, closely packed interfacial films with synergistic stabilization (electrostatic + steric)
• They eliminate the need for preservatives
• They prevent oxidation of the oil phase

Correct Answer: They form more elastic, closely packed interfacial films with synergistic stabilization (electrostatic + steric)

Q28. Which statement best differentiates microemulsions from conventional emulsions?

• Microemulsions are thermodynamically stable and can form spontaneously
• Microemulsions have larger droplet sizes than emulsions
• Microemulsions cannot solubilize lipophilic drugs
• Emulsions are clear while microemulsions are always opaque

Correct Answer: Microemulsions are thermodynamically stable and can form spontaneously

Q29. Adding a small amount of ethanol to the aqueous phase of an o/w emulsion will most likely:

• Increase continuous-phase viscosity and reduce creaming
• Decrease continuous-phase viscosity and increase creaming tendency
• Eliminate the need for surfactants
• Prevent phase inversion at all temperatures

Correct Answer: Decrease continuous-phase viscosity and increase creaming tendency

Q30. For w/o emulsions, which emulsifier would be most appropriate?

• Polysorbate 80 (HLB ~15)
• Sodium lauryl sulfate (HLB ~40)
• Sorbitan monooleate, Span 80 (HLB ~4.3)
• Poloxamer 188 (HLB ~29)

Correct Answer: Sorbitan monooleate, Span 80 (HLB ~4.3)

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