Surface active agents MCQs With Answer

Surface active agents MCQs With Answer are essential for B.Pharm students preparing for pharmaceutics and pharmaceutical analysis exams. This concise, keyword-rich introduction covers surfactant classification, micelle formation, CMC, HLB, adsorption, solubilization, Krafft and cloud points, and common examples like SDS, Tween 80 and Span 20. Understanding these concepts helps in formulation design—emulsions, suspensions, creams, and solubilized drug systems—and in predicting effects of electrolytes, pH and temperature on stability. These MCQs focus on both fundamental theory and practical pharmaceutical applications to boost exam readiness and formulation skills. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary characteristic of a surface active agent (surfactant)?

• High molecular weight polymer with no polar group
• Molecule with both hydrophobic and hydrophilic regions
• Neutral small organic solvent
• Inert inorganic salt that increases viscosity

Correct Answer: Molecule with both hydrophobic and hydrophilic regions

Q2. Which of the following is an anionic surfactant commonly used in pharmaceutical formulations?

• Polysorbate 80 (Tween 80)
• Sodium dodecyl sulfate (SDS)
• Cetyltrimethylammonium bromide (CTAB)
• Lauryl betaine

Correct Answer: Sodium dodecyl sulfate (SDS)

Q3. The Critical Micelle Concentration (CMC) is defined as:

• The concentration above which surfactant molecules begin to precipitate
• The concentration at which surfactant molecules start to form micelles
• The minimal concentration required for complete wetting of solids
• The concentration where surface tension is maximum

Correct Answer: The concentration at which surfactant molecules start to form micelles

Q4. Which method is commonly used to experimentally determine CMC by measuring surface tension?

• Wilhelmy plate or Du Nouy ring tensiometry
• HPLC with UV detection
• Infrared spectroscopy
• Gel permeation chromatography

Correct Answer: Wilhelmy plate or Du Nouy ring tensiometry

Q5. A surfactant that carries no net charge is called:

• Anionic
• Cationic
• Nonionic
• Zwitterionic

Correct Answer: Nonionic

Q6. Which parameter estimates the balance between hydrophilic and lipophilic portions of a surfactant?

• Gibbs free energy of micellization
• Hydrophile-Lipophile Balance (HLB)
• Diffusion coefficient
• Partition coefficient (log P)

Correct Answer: Hydrophile-Lipophile Balance (HLB)

Q7. Which surfactant is typically chosen to stabilize oil-in-water (o/w) emulsions due to a high HLB value?

• Span 80 (HLB ~4.3)
• Span 20 (HLB ~8.6)
• Polysorbate 80 (Tween 80, HLB ~15)
• Span 85 (HLB ~1.8)

Correct Answer: Polysorbate 80 (Tween 80, HLB ~15)

Q8. Which property decreases as surfactant concentration increases until the CMC is reached?

• Electrical conductivity for ionic surfactants
• Surface tension of the solution
• Viscosity drastically
• pH consistently

Correct Answer: Surface tension of the solution

Q9. The Gibbs adsorption equation relates surface excess concentration to:

• Temperature only
• Change in surface tension with concentration
• Viscosity changes
• pKa of the surfactant

Correct Answer: Change in surface tension with concentration

Q10. Which surfactant type is useful as antimicrobial and interacts with bacterial membranes due to positive charge?

• Anionic surfactants
• Nonionic surfactants
• Cationic surfactants
• Zwitterionic surfactants

Correct Answer: Cationic surfactants

Q11. Cloud point is a critical property for which surfactant class?

• Anionic surfactants
• Nonionic surfactants
• Cationic surfactants
• Inorganic salts

Correct Answer: Nonionic surfactants

Q12. Krafft temperature describes the temperature above which:

• Nonionic surfactants become ionic
• An ionic surfactant’s solubility increases sharply and micelles form
• Surfactants lose hydrophobic chains
• Surface tension becomes zero

Correct Answer: An ionic surfactant’s solubility increases sharply and micelles form

Q13. Which of the following is a zwitterionic surfactant often used for mildness in formulations?

• Lauryl sulfate
• Cocamidopropyl betaine
• CTAB
• Span 60

Correct Answer: Cocamidopropyl betaine

Q14. Micelles typically solubilize hydrophobic drugs in which region?

• Outer hydration shell
• Micelle core (hydrophobic core)
• At the air–water interface only
• Inside the counterion cloud exclusively

Correct Answer: Micelle core (hydrophobic core)

Q15. Increasing the hydrophobic chain length of a homologous series of surfactants generally causes the CMC to:

• Increase exponentially
• Remain unchanged
• Decrease
• Vary randomly without pattern

Correct Answer: Decrease

Q16. Which surfactant measurement technique uses fluorescence probes (e.g., pyrene) to detect micellization?

• Tensiometry
• Conductivity
• Fluorescence spectroscopy
• Mass spectrometry

Correct Answer: Fluorescence spectroscopy

Q17. For ionic surfactants, addition of electrolytes typically causes the CMC to:

• Increase due to electrostatic screening
• Decrease due to counterion screening of head groups
• Remain unaffected
• Become undefined

Correct Answer: Decrease due to counterion screening of head groups

Q18. In emulsions, a surfactant that reduces interfacial tension between oil and water most directly improves:

• Suspension sedimentation only
• Wetting of solid powders exclusively
• Emulsion formation and initial droplet breakup
• pH stability of active drug

Correct Answer: Emulsion formation and initial droplet breakup

Q19. Which surfactant is commonly used as a solubilizer for poorly water-soluble drugs in oral and parenteral formulations?

• Sodium chloride
• Polysorbate 80 (Tween 80)
• Glucose
• Magnesium stearate

Correct Answer: Polysorbate 80 (Tween 80)

Q20. Which factor does NOT directly influence micelle size and shape?

• Surfactant molecular structure
• Temperature
• Concentration relative to CMC
• Color of the formulation container

Correct Answer: Color of the formulation container

Q21. The area per molecule at the air–water interface can be estimated from surface excess using which relation?

• Graham’s law
• Area per molecule = 1 / (N_A * surface excess)
• Henry’s law
• van’t Hoff equation

Correct Answer: Area per molecule = 1 / (N_A * surface excess)

Q22. Which surfactant class tends to produce stable foams often used in topical and cleansing products?

• Nonionic surfactants only
• Anionic surfactants
• Neutral salts
• Hydrophobic oils

Correct Answer: Anionic surfactants

Q23. The Davies method is used to calculate HLB values based on:

• Molecular weight and number of ethylene oxide units only
• Contribution numbers of hydrophilic and lipophilic groups
• Surface tension at CMC
• Partition coefficient between octanol and water

Correct Answer: Contribution numbers of hydrophilic and lipophilic groups

Q24. In pharmaceutical suspensions, surfactants may act as wetting agents to:

• Increase interfacial tension between powder and vehicle
• Improve spreading of liquid over powder particles
• Cause rapid flocculation always
• Eliminate need for preservatives

Correct Answer: Improve spreading of liquid over powder particles

Q25. Which of the following is a cationic surfactant used as a preservative and antiseptic?

• Benzalkonium chloride
• Sodium lauryl sulfate
• Polysorbate 20
• Sorbitan monooleate

Correct Answer: Benzalkonium chloride

Q26. Which statement about HLB values is correct?

• Lower HLB favors oil-in-water emulsions
• Higher HLB favors water-in-oil emulsions
• Higher HLB favors oil-in-water emulsions
• HLB is irrelevant to emulsion type selection

Correct Answer: Higher HLB favors oil-in-water emulsions

Q27. Which surfactant property is most directly measured by a Du Nouy ring tensiometer?

• Viscosity
• Surface or interfacial tension
• Dielectric constant
• Micelle aggregation number

Correct Answer: Surface or interfacial tension

Q28. Aggregation number refers to:

• Number of surfactant molecules forming a single micelle
• Number of ions neutralized per surfactant
• Molecular weight divided by CMC
• The area per molecule at interface

Correct Answer: Number of surfactant molecules forming a single micelle

Q29. Which factor increases the tendency for micelle formation (lowering free energy of micellization)?

• Shortening the hydrophobic tail
• Increasing temperature above Krafft point for ionic surfactants
• Adding large amounts of organic solvents that solubilize tails freely
• Adding strong detergents to break micelles

Correct Answer: Increasing temperature above Krafft point for ionic surfactants

Q30. Surfactant adsorption at interfaces typically follows which thermodynamic tendency?

• System increases free energy to adsorb surfactant
• Adsorption reduces interfacial free energy by lowering surface tension
• Adsorption raises surface tension
• Adsorption only occurs for nonionic surfactants

Correct Answer: Adsorption reduces interfacial free energy by lowering surface tension

Q31. Which method can determine CMC by monitoring changes in electrical conductivity?

• Surface tension measurement
• Conductivity vs concentration plot showing breakpoint
• Light scattering only
• pH titration curve

Correct Answer: Conductivity vs concentration plot showing breakpoint

Q32. Which surfactant is known for its strong anionic character and protein-denaturing properties, widely used in laboratory detergents?

• Sodium dodecyl sulfate (SDS)
• Polysorbate 60
• Cetyl alcohol
• Glyceryl monostearate

Correct Answer: Sodium dodecyl sulfate (SDS)

Q33. Which change is expected when temperature approaches the cloud point for nonionic surfactants?

• Solution becomes more transparent due to increased solubility
• Solution becomes turbid as micelles aggregate and separate
• CMC becomes zero
• Surfactant ionizes fully

Correct Answer: Solution becomes turbid as micelles aggregate and separate

Q34. Which surfactant property influences drug permeability and absorption from formulations?

• Only the odor
• Interaction with biological membranes and effect on membrane permeability
• Color of the surfactant
• Nonpolar solvent miscibility only

Correct Answer: Interaction with biological membranes and effect on membrane permeability

Q35. A mixed surfactant system can show synergistic effects such as:

• Higher CMC than either component alone
• Lower interfacial tension and lower mixed CMC
• Complete immiscibility at all concentrations
• No change from individual surfactants

Correct Answer: Lower interfacial tension and lower mixed CMC

Q36. Which statement best describes counterion binding in ionic surfactant micelles?

• Counterions always remain fully dissociated in bulk
• Counterions may bind to micelle surface reducing effective charge and electrostatic repulsion
• Counterion binding increases CMC dramatically
• Counterions convert surfactants to nonionic molecules

Correct Answer: Counterions may bind to micelle surface reducing effective charge and electrostatic repulsion

Q37. Which surfactant is preferred for parenteral formulations due to low toxicity and good solubilizing ability?

• Polysorbate 80
• Sodium lauryl sulfate
• CTAB
• Phenol

Correct Answer: Polysorbate 80

Q38. In reverse micelles formed in nonpolar solvents, the polar heads are located:

• On the outer surface facing solvent
• In the micelle core forming a polar interior
• Distributed uniformly in the solvent
• Always bound to solid surfaces only

Correct Answer: In the micelle core forming a polar interior

Q39. Emulsifying efficiency of a surfactant is influenced by:

• Surfactant HLB, concentration, and oil phase properties
• Only color of the surfactant
• Only the ambient light during mixing
• None of the formulation variables

Correct Answer: Surfactant HLB, concentration, and oil phase properties

Q40. Which parameter describes the tendency of surfactant molecules to adsorb at an interface per unit area?

• Surface excess concentration (Γ)
• Dielectric constant
• Bulk viscosity
• Boiling point

Correct Answer: Surface excess concentration (Γ)

Q41. Which surfactant is commonly used as an emulsifier in topical creams and ointments: Span or Tween?

• Span (sorbitan esters) generally for W/O emulsions
• Tween exclusively for solid dosage forms only
• Span never used in creams
• Tween is ionic while Span is neutral

Correct Answer: Span (sorbitan esters) generally for W/O emulsions

Q42. The primary driving force for micelle formation in aqueous solutions is:

• Increase in entropy of water due to release of structured water molecules around hydrophobic tails
• Formation of covalent bonds between surfactant molecules
• Decrease in temperature below freezing point
• Increase in pH only

Correct Answer: Increase in entropy of water due to release of structured water molecules around hydrophobic tails

Q43. The aggregation number can be experimentally estimated using which technique?

• Static light scattering or cryo-TEM and fluorescence quenching
• Simple pH paper test
• UV absorption at 280 nm for all surfactants
• Melting point determination

Correct Answer: Static light scattering or cryo-TEM and fluorescence quenching

Q44. Which surfactant property is most relevant when designing transdermal formulations to enhance penetration?

• Ability to form solid crystals at skin surface
• Interaction with skin lipids and reversible disruption of stratum corneum
• Only its color
• Its boiling point only

Correct Answer: Interaction with skin lipids and reversible disruption of stratum corneum

Q45. Which of the following statements about biodegradable surfactants is true?

• They are always nonionic
• They are designed to break down into less harmful products in the environment
• They cannot be used in pharmaceuticals
• They are always synthetic polymers

Correct Answer: They are designed to break down into less harmful products in the environment

Q46. Which measurement provides information about size distribution of micelles or droplets in colloidal systems?

• Dynamic light scattering (DLS)
• Tensiometry only
• Melting point analysis
• Conductivity at a single concentration

Correct Answer: Dynamic light scattering (DLS)

Q47. Which surfactant class tends to be least sensitive to added salt in terms of CMC change?

• Anionic surfactants
• Cationic surfactants
• Nonionic surfactants
• Zwitterionic surfactants

Correct Answer: Nonionic surfactants

Q48. The term “solubilization” in the context of surfactants refers to:

• Conversion of surfactant into solid crystals
• Incorporation of hydrophobic molecules into micellar structures increasing apparent solubility
• Complete degradation of drugs
• Removal of water from formulation

Correct Answer: Incorporation of hydrophobic molecules into micellar structures increasing apparent solubility

Q49. Which of the following is an amphiphilic block copolymer surfactant used for advanced drug delivery systems?

• Pluronic (Poloxamer) series
• Sodium chloride
• Polyvinyl alcohol only
• Stearic acid alone

Correct Answer: Pluronic (Poloxamer) series

Q50. When optimizing a pharmaceutical emulsion, why might a formulator choose a blend of surfactants rather than a single surfactant?

• Blends always increase CMC making formulation easier
• Blends can provide improved interfacial packing, lower interfacial tension, and better stability across conditions
• Single surfactants cannot reduce surface tension
• Regulatory guidelines mandate blends only

Correct Answer: Blends can provide improved interfacial packing, lower interfacial tension, and better stability across conditions

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com