Measurement of surface and interfacial tensions MCQs With Answer

Measurement of surface and interfacial tensions MCQs With Answer

This concise introduction explains measurement of surface and interfacial tensions for B.Pharm students, covering key concepts like surface tension, interfacial tension, tensiometry methods (Du Noüy ring, Wilhelmy plate, pendant drop), contact angle, Gibbs adsorption and critical micelle concentration (CMC). Understanding these parameters is essential in pharmaceutical formulation—emulsions, suspensions, wetting, coating and pulmonary delivery—because surfactants and interfacial phenomena control stability and bioavailability. Practical considerations include dynamic versus equilibrium tension, temperature effects, measurement errors and instrument selection. Emphasizing methods and pharmaceutical applications helps prepare you for exams and lab work. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the SI unit of surface and interfacial tension?

• mN/m
• N/m
• dyn/cm
• J/m

Correct Answer: N/m

Q2. Which technique measures surface tension by pulling a ring and recording the maximum force?

• Pendant drop method
• Du Noüy ring tensiometer
• Wilhelmy plate method
• Maximum bubble pressure method

Correct Answer: Du Noüy ring tensiometer

Q3. In the Wilhelmy plate method, the measured force F for a fully wetted plate is related to surface tension γ by which relation (P is perimeter)?

• F = γ·P·cosθ
• F = 2γ/R
• F = γ·V
• F = γ/P

Correct Answer: F = γ·P·cosθ

Q4. Which value is closest to the surface tension of pure water at 20°C?

• 7.28 mN/m
• 72.8 mN/m
• 720 mN/m
• 0.0728 N/m

Correct Answer: 72.8 mN/m

Q5. The pendant drop technique determines surface tension by analyzing what feature of a droplet?

• Contact angle on a solid substrate
• Shape of the axisymmetric hanging drop
• Oscillation frequency of the drop
• Evaporation rate of the drop

Correct Answer: Shape of the axisymmetric hanging drop

Q6. Which equation relates pressure difference across a curved interface to surface tension and curvature?

• Young-Laplace equation
• Gibbs adsorption isotherm
• Arrhenius equation
• Stokes equation

Correct Answer: Young-Laplace equation

Q7. For a spherical droplet of radius R, the Laplace pressure difference ΔP is:

• ΔP = γ·R
• ΔP = 2γ/R
• ΔP = γ/(2R)
• ΔP = 4πR²γ

Correct Answer: ΔP = 2γ/R

Q8. The Gibbs adsorption equation links change in surface tension with which quantity?

• Surface excess concentration (Γ)
• Viscosity of the bulk
• Contact angle
• Capillary length

Correct Answer: Surface excess concentration (Γ)

Q9. Critical micelle concentration (CMC) is characterized by what behavior of surface tension versus surfactant concentration?

• Continuous increase of surface tension
• Sharp increase at CMC
• Plateau or leveling-off of surface tension above CMC
• Oscillatory changes with concentration

Correct Answer: Plateau or leveling-off of surface tension above CMC

Q10. Which method is most suitable for measuring very low interfacial tensions (near zero)?

• Wilhelmy plate method
• Du Noüy ring method
• Spinning/drop rotation (spinning drop tensiometer)
• Capillary rise

Correct Answer: Spinning/drop rotation (spinning drop tensiometer)

Q11. The capillary rise height h in a tube is inversely proportional to which property?

• Surface tension
• Tube radius
• Liquid vapor pressure
• Drop volume

Correct Answer: Tube radius

Q12. Spreading coefficient S for a liquid on a solid is defined as S = γ_sv – (γ_sl + γ_lv). If S > 0, what occurs?

• Partial wetting
• Non-wetting
• Complete spreading (spontaneous spreading)
• Contact angle equals 90°

Correct Answer: Complete spreading (spontaneous spreading)

Q13. Which parameter is directly measured in the maximum bubble pressure method?

• Equilibrium surface tension only
• Dynamic (age-dependent) surface tension
• Contact angle hysteresis
• Interfacial rheology modulus

Correct Answer: Dynamic (age-dependent) surface tension

Q14. Young’s equation relates contact angle θ to which interfacial tensions?

• γ_lv and density
• γ_sv, γ_sl and γ_lv
• γ_sl and viscosity
• γ_sv and temperature only

Correct Answer: γ_sv, γ_sl and γ_lv

Q15. Which factor commonly decreases surface tension of water?

• Addition of ionic or nonionic surfactant
• Increasing pressure at constant temperature
• Adding nonvolatile salts that increase ionic strength only
• Decreasing temperature drastically

Correct Answer: Addition of ionic or nonionic surfactant

Q16. Surface excess concentration Γ can be calculated from Gibbs equation. Which of these is a correct form for a nonionic surfactant?

• dγ = -RT Γ d(ln c)
• dγ = RT Γ dc
• dγ = -Γ dT
• dγ = γ·dV

Correct Answer: dγ = -RT Γ d(ln c)

Q17. Which phenomenon arises from gradients of surface tension along an interface and can affect spreading and mixing?

• Marangoni effect
• Brownian motion
• Capillary condensation
• Ostwald ripening

Correct Answer: Marangoni effect

Q18. Which tensiometry method requires knowledge of a geometric correction factor (commonly called the Z-factor)?

• Pendant drop
• Wilhelmy plate
• Du Noüy ring
• Spinning drop

Correct Answer: Du Noüy ring

Q19. In pharmaceutical emulsions, lower interfacial tension between oil and water primarily improves what?

• Viscosity
• Color stability
• Emulsification and droplet breakup (ease of emulsification)
• Crystallinity

Correct Answer: Emulsification and droplet breakup (ease of emulsification)

Q20. Which statement about dynamic and equilibrium surface tension is correct?

• Dynamic surface tension equals equilibrium surface tension at infinite surfactant adsorption time
• Dynamic surface tension is independent of surfactant adsorption kinetics
• Equilibrium surface tension changes rapidly with time
• Dynamic surface tension is always lower than equilibrium tension

Correct Answer: Dynamic surface tension equals equilibrium surface tension at infinite surfactant adsorption time

Q21. Which technique measures interfacial tension by fitting the droplet profile to the Young-Laplace equation (ADSA)?

• Du Noüy ring
• Wilhelmy plate
• Pendant drop axisymmetric drop shape analysis (ADSA)
• Maximum bubble pressure

Correct Answer: Pendant drop axisymmetric drop shape analysis (ADSA)

Q22. What is the effect of increasing temperature on surface tension for most liquids?

• Surface tension increases with temperature
• Surface tension decreases with temperature
• Surface tension remains constant
• Surface tension oscillates with temperature

Correct Answer: Surface tension decreases with temperature

Q23. The Eötvös rule relates surface tension to which molecular property?

• Molar volume (or molecular volume) and temperature
• Diffusion coefficient
• Dielectric constant
• Viscosity

Correct Answer: Molar volume (or molecular volume) and temperature

Q24. For a clean Wilhelmy plate fully wetted by liquid, what is the contact angle θ assumed to be?

• 180°
• 90°
• 0°
• 45°

Correct Answer: 0°

Q25. Which process describes adsorption of surfactant molecules at an interface lowering the interfacial free energy?

• Desorption
• Surface activation
• Surface adsorption
• Bulk crystallization

Correct Answer: Surface adsorption

Q26. When measuring surface tension, one common experimental artifact is contamination. Which contaminant effect is typical?

• Random noise but no systematic change
• Apparent increase in measured surface tension over time
• Apparent decrease in measured surface tension due to surface-active impurities
• Elimination of interfacial tension

Correct Answer: Apparent decrease in measured surface tension due to surface-active impurities

Q27. Which of the following is a dimensionless number comparing gravitational and capillary forces (via capillary length)?

• Reynolds number
• Weber number
• Bond number (Bo)
• Peclet number

Correct Answer: Bond number (Bo)

Q28. Capillary length (lc) is defined as sqrt(γ/(ρg)). What does it represent?

• Characteristic length where viscous forces dominate
• Characteristic length where capillary forces balance gravity
• Mean free path of molecules
• Distance for diffusion across interface

Correct Answer: Characteristic length where capillary forces balance gravity

Q29. Which tensiometer method is best for measuring dynamic surface tension at very short surface ages (milliseconds)?

• Pendant drop ADSA
• Maximum bubble pressure method
• Wilhelmy plate slowly immersed
• Capillary rise method

Correct Answer: Maximum bubble pressure method

Q30. In emulsification, surfactant packing at the oil–water interface affects stability by modifying which property?

• Melting point
• Interfacial tension and interfacial film elasticity
• Bulk ionic conductivity only
• Color of the emulsion

Correct Answer: Interfacial tension and interfacial film elasticity

Q31. When using the Du Noüy ring, why is a correction factor required?

• Because the ring dissolves partially in the liquid
• Because the ring shape and wetting create a non-ideal geometry for force-to-tension conversion
• Because surface tension changes during measurement
• Because temperature is not accounted for

Correct Answer: Because the ring shape and wetting create a non-ideal geometry for force-to-tension conversion

Q32. Which of the following best describes interfacial rheology?

• Viscosity of the bulk phase only
• Elastic and viscous response of the interfacial layer to deformation
• Electrical conductivity of the interface
• Surface tension at equilibrium only

Correct Answer: Elastic and viscous response of the interfacial layer to deformation

Q33. Which surfactant property is most relevant to lowering surface tension at very low concentrations?

• Hydrophilic-lipophilic balance (HLB) and adsorption efficiency
• Molecular weight only
• Color
• pKa only

Correct Answer: Hydrophilic-lipophilic balance (HLB) and adsorption efficiency

Q34. In the context of pulmonary surfactants, lowering alveolar surface tension primarily prevents what?

• Protein denaturation
• Alveolar collapse (atelectasis)
• Oxidative stress
• Bacterial growth

Correct Answer: Alveolar collapse (atelectasis)

Q35. What does a contact angle θ < 90° typically indicate about a liquid on a solid?

• Non-wetting
• Partial wetting or good wettability
• Complete non-adhesion
• Hydrophobicity always

Correct Answer: Partial wetting or good wettability

Q36. Which factor is NOT typically important when preparing samples for tensiometry in pharmaceutical labs?

• Avoiding dust and contamination at the interface
• Degassing to remove bubbles
• Using impure solvents to mimic real formulations
• Controlling temperature precisely

Correct Answer: Using impure solvents to mimic real formulations

Q37. What happens to surface tension as surfactant concentration approaches the CMC from below?

• Surface tension increases sharply
• Surface tension decreases until it plateaus near the CMC
• Surface tension becomes infinite
• Surface tension remains unchanged

Correct Answer: Surface tension decreases until it plateaus near the CMC

Q38. Which measurement is most suited to characterize time-dependent adsorption kinetics of surfactants to a newly created interface?

• Equilibrium Wilhelmy plate static measurement
• Dynamic tensiometry (maximum bubble pressure or rapidly generated pendant drops)
• Simple density measurement
• pH titration

Correct Answer: Dynamic tensiometry (maximum bubble pressure or rapidly generated pendant drops)

Q39. Which interfacial property is critical for the formation of stable liposomes and drug delivery vesicles?

• Surface roughness of container
• Membrane bending rigidity and interfacial tension
• Boiling point of solvent
• Color of phospholipids

Correct Answer: Membrane bending rigidity and interfacial tension

Q40. Which of the following is a direct consequence of a positive spreading coefficient for a liquid on another immiscible liquid?

• Dewetting occurs
• Droplet forms a spherical cap with high contact angle
• Complete spreading of the liquid as a thin film over the other liquid
• Immediate phase separation by density

Correct Answer: Complete spreading of the liquid as a thin film over the other liquid

Q41. The Wilhelmy plate method provides surface tension by measuring force per unit what?

• Mass
• Perimeter (or wetted length) of the plate
• Area of the plate
• Volume displaced

Correct Answer: Perimeter (or wetted length) of the plate

Q42. Why are rotating/spinning drop methods preferred for measuring very low interfacial tensions between two liquids?

• They are very fast but inaccurate
• They generate large centrifugal forces to elongate a drop where interfacial tensions are too low for other methods
• They do not require image analysis
• They measure contact angle directly

Correct Answer: They generate large centrifugal forces to elongate a drop where interfacial tensions are too low for other methods

Q43. Which of these statements about surfactant adsorption is true?

• Adsorption always increases surface tension
• Adsorption decreases surface tension until surface saturation
• Adsorption has no effect on interfacial properties
• Adsorption always causes precipitation

Correct Answer: Adsorption decreases surface tension until surface saturation

Q44. What is the main limitation of capillary rise method for quantifying surface tension in complex pharmaceutical formulations?

• It requires perfectly wetting liquids only
• It is highly sensitive to contact angle and pore geometry, making it unreliable for complex mixtures and surfactants
• It measures interfacial rheology instead of tension
• It can only be used at 0°C

Correct Answer: It is highly sensitive to contact angle and pore geometry, making it unreliable for complex mixtures and surfactants

Q45. Which analytical approach helps extract concentration-dependent surface excess from surface tension vs concentration data?

• Langmuir-Blodgett plotting
• Gibbs adsorption isotherm application
• Beer-Lambert law
• HPLC calibration curve

Correct Answer: Gibbs adsorption isotherm application

Q46. In drop shape analysis, what physical constants are needed to calculate γ from the droplet profile?

• Temperature and pH only
• Density difference between phases and gravitational acceleration
• Viscosity and dielectric constant
• Surface roughness and conductivity

Correct Answer: Density difference between phases and gravitational acceleration

Q47. Which pharmaceutical application directly depends on contact angle and surface wetting?

• Tablet disintegration and coating coverage
• Sterility testing only
• Genetic sequencing
• Color stability of dyes

Correct Answer: Tablet disintegration and coating coverage

Q48. What does a plateau in surface tension vs log(surfactant concentration) commonly indicate?

• Onset of precipitation
• Formation of a monolayer only
• Micelle formation at the CMC and surface saturation
• Evaporation limit reached

Correct Answer: Micelle formation at the CMC and surface saturation

Q49. Which parameter is most likely monitored to assess dynamic interfacial adsorption during spray drying or aerosol formation?

• Static surface tension measured after hours
• Dynamic surface tension at millisecond to second timescales
• Optical rotation of the solution
• Thermal conductivity

Correct Answer: Dynamic surface tension at millisecond to second timescales

Q50. Which precaution improves accuracy when measuring surface tension in lab for formulation studies?

• Allow instrument to run with dirty probes to simulate real life
• Maintain strict cleanliness, control temperature, avoid evaporation and degas samples
• Use arbitrary sample volumes without calibration
• Measure only at very high surfactant concentrations

Correct Answer: Maintain strict cleanliness, control temperature, avoid evaporation and degas samples

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