Interfacial tension MCQs With Answer

Interfacial tension MCQs With Answer provides B. Pharm students a focused, exam-oriented review of interfacial phenomena essential for formulation science. This concise introduction and question set cover definitions, measurement techniques (du Noüy ring, Wilhelmy plate, pendant drop), thermodynamics (Gibbs adsorption), dynamic vs equilibrium tension, surfactant behavior, contact angle, spreading, and pharmaceutical applications like emulsification, stability, and drug delivery. Each MCQ emphasizes practical implications for formulation, characterization, and quality control while reinforcing theoretical concepts. Keywords: interfacial tension, surface tension, surfactants, emulsions, Gibbs adsorption, pendant drop, du Noüy, Wilhelmy, B. Pharm. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the definition of interfacial tension between two immiscible liquids?

• The force per unit length acting along the line in the interface between two immiscible phases
• The force per unit area acting normal to the surface of a liquid
• The pressure difference across a curved surface
• The energy required to create a unit volume of new interface

Correct Answer: The force per unit length acting along the line in the interface between two immiscible phases

Q2. Which unit is commonly used to express interfacial tension in pharmaceutics?

• milliNewton per meter (mN/m)
• Pascal-second (Pa·s)
• mole per liter (mol/L)
• Newton per square meter (N/m²)

Correct Answer: milliNewton per meter (mN/m)

Q3. The du Noüy ring method measures interfacial tension by:

• measuring the maximum force required to detach a ring from the interface
• measuring the weight of a pendant drop
• measuring the capillary rise of liquid in a tube
• measuring surface pressure with a Langmuir trough

Correct Answer: measuring the maximum force required to detach a ring from the interface

Q4. The Wilhelmy plate method estimates surface or interfacial tension by:

• measuring the force on a vertically suspended plate wetted by the liquid
• measuring the oscillation frequency of a bubble
• directly counting molecules at the interface
• measuring the evaporation rate of a droplet

Correct Answer: measuring the force on a vertically suspended plate wetted by the liquid

Q5. The principle behind the pendant drop method is based on:

• analyzing the equilibrium shape of a drop under gravity and interfacial tension using the Young-Laplace equation
• measuring the contact angle of a drop on a solid surface
• creating a monolayer and measuring surface pressure
• counting surfactant molecules adsorbed per unit area

Correct Answer: analyzing the equilibrium shape of a drop under gravity and interfacial tension using the Young-Laplace equation

Q6. Dynamic interfacial tension differs from equilibrium interfacial tension because it:

• depends on the time required for surfactant adsorption to the interface
• is measured only at very high temperatures
• is independent of surfactant concentration
• is always higher than surface viscosity

Correct Answer: depends on the time required for surfactant adsorption to the interface

Q7. The Gibbs adsorption isotherm relates surface excess (Γ) to:

• the change in surface/interfacial tension with the logarithm of bulk concentration
• the viscosity of the bulk phase
• the electrical conductivity of the solution
• the contact angle hysteresis

Correct Answer: the change in surface/interfacial tension with the logarithm of bulk concentration

Q8. Addition of surfactant to an oil–water system generally causes:

• a decrease in interfacial tension between oil and water
• an increase in interfacial tension between oil and water
• no change in interfacial tension regardless of concentration
• complete elimination of interfacial tension at any concentration

Correct Answer: a decrease in interfacial tension between oil and water

Q9. What happens to interfacial tension when surfactant concentration reaches the critical micelle concentration (CMC)?

• Interfacial tension levels off and shows little further decrease
• Interfacial tension increases sharply
• Interfacial tension becomes infinite
• Interfacial tension becomes zero immediately

Correct Answer: Interfacial tension levels off and shows little further decrease

Q10. A high HLB value surfactant (hydrophilic-lipophilic balance) is most suitable for which emulsion type?

• Oil-in-water (O/W) emulsions
• Water-in-oil (W/O) emulsions
• Solid dispersion systems
• Gas-in-liquid foams

Correct Answer: Oil-in-water (O/W) emulsions

Q11. The spreading coefficient S determines whether a liquid will spread on a solid and is defined by which relation?

• S = γ_SV – (γ_SL + γ_LV)
• S = γ_LV + γ_SL – γ_SV
• S = γ_SL / (γ_SV + γ_LV)
• S = γ_SV × γ_LV / γ_SL

Correct Answer: S = γ_SV – (γ_SL + γ_LV)

Q12. Young’s equation relating contact angle θ is written as:

• γ_SV = γ_SL + γ_LV cosθ
• γ_SV = γ_SL – γ_LV cosθ
• γ_LV = γ_SV + γ_SL cosθ
• γ_SL = γ_SV cosθ + γ_LV

Correct Answer: γ_SV = γ_SL + γ_LV cosθ

Q13. A contact angle less than 90° indicates:

• good wetting of the solid by the liquid
• complete non-wetting
• an unstable interface
• that interfacial tension is zero

Correct Answer: good wetting of the solid by the liquid

Q14. The Marangoni effect arises from:

• surface tension gradients that induce flow along an interface
• electrostatic attraction between droplets
• thermal decomposition of surfactants
• gravitational settling of particles

Correct Answer: surface tension gradients that induce flow along an interface

Q15. Which statement correctly describes nonionic surfactants?

• They have no net electrical charge on their hydrophilic head
• They always precipitate in the presence of salt
• They only form micelles at extremely high temperatures
• They are composed of only hydrocarbons with no polar groups

Correct Answer: They have no net electrical charge on their hydrophilic head

Q16. Adding electrolyte (salt) to a solution of ionic surfactant typically:

• lowers the CMC and can reduce interfacial tension at lower concentrations
• raises the CMC dramatically
• destroys the surfactant molecules
• has no effect on micellization or interfacial tension

Correct Answer: lowers the CMC and can reduce interfacial tension at lower concentrations

Q17. Interfacial rheology refers to:

• viscoelastic properties of the interface affecting film and droplet stability
• bulk viscosity of the continuous phase only
• the electrical resistance of a liquid film
• the chemical reactivity of surfactant head groups

Correct Answer: viscoelastic properties of the interface affecting film and droplet stability

Q18. The Laplace pressure across a spherical droplet of radius r is given by:

• ΔP = 2γ / r
• ΔP = γ × r²
• ΔP = γ / (2r)
• ΔP = γ × r

Correct Answer: ΔP = 2γ / r

Q19. Ostwald ripening in emulsions is driven primarily by:

• difference in solubility and Laplace pressure between small and large droplets
• electrostatic repulsion between droplets
• surfactant monolayer collapse only
• gravitational settling of identical-sized droplets

Correct Answer: difference in solubility and Laplace pressure between small and large droplets

Q20. Coalescence in emulsions refers to:

• merging of two droplets into a larger droplet
• formation of micelles from surfactants
• precipitation of active pharmaceutical ingredient
• adsorption of proteins at the interface without droplet fusion

Correct Answer: merging of two droplets into a larger droplet

Q21. DLVO theory explains colloidal stability in terms of which two primary forces?

• van der Waals attraction and electrostatic double-layer repulsion
• steric hindrance and gravitational force
• hydrophobic interaction and Marangoni flow
• ionic bonding and covalent bonding

Correct Answer: van der Waals attraction and electrostatic double-layer repulsion

Q22. In the du Noüy ring measurement, why is a correction factor often required?

• to account for ring geometry and buoyant forces affecting the measured force
• to correct for the color of the liquid
• to eliminate the need for temperature control
• because the ring changes the chemical composition of the interface

Correct Answer: to account for ring geometry and buoyant forces affecting the measured force

Q23. The Young-Laplace equation in general form relates pressure difference to curvature as:

• ΔP = γ(1/R1 + 1/R2)
• ΔP = γ × R1 × R2
• ΔP = γ / (R1 – R2)
• ΔP = γ × (R1 – R2)

Correct Answer: ΔP = γ(1/R1 + 1/R2)

Q24. Surface tension of pure water at 25°C is approximately:

• 72 mN/m
• 7.2 mN/m
• 720 mN/m
• 0.72 mN/m

Correct Answer: 72 mN/m

Q25. Which method is most suitable for measuring very low interfacial tensions between two liquids (e.g., microemulsions)?

• Pendant drop method with image analysis
• capillary rise in a thin tube
• direct weighing of an evaporated film
• colorimetry

Correct Answer: Pendant drop method with image analysis

Q26. The surface excess concentration Γ has units of:

• mol per square meter (mol/m²)
• mol per liter (mol/L)
• Newton per meter (N/m)
• dimensionless

Correct Answer: mol per square meter (mol/m²)

Q27. Which of the following best describes a Langmuir monolayer?

• A single molecular layer spread on a water surface with measurable surface pressure
• A micelle formed in bulk solution above CMC
• A bilayer vesicle containing encapsulated drug
• A precipitated crystalline film at the bottom of a container

Correct Answer: A single molecular layer spread on a water surface with measurable surface pressure

Q28. According to Gibbs adsorption, a negative slope of surface tension vs ln(concentration) indicates:

• adsorption of surfactant at the interface (positive surface excess)
• desorption of surfactant from the interface
• no adsorption and a clean interface
• a chemical decomposition of surfactant

Correct Answer: adsorption of surfactant at the interface (positive surface excess)

Q29. The critical packing parameter (CPP) of a surfactant influences:

• the preferred self-assembled aggregate shape (micelle, cylinder, bilayer)
• the molecular weight of the surfactant only
• the color of the surfactant solution
• the electrical conductivity of water

Correct Answer: the preferred self-assembled aggregate shape (micelle, cylinder, bilayer)

Q30. Lowering interfacial tension during emulsification generally leads to:

• smaller droplet sizes and easier dispersion formation
• larger droplets and rapid phase separation
• increased crystallinity of the active ingredient
• higher glass transition temperature of polymers

Correct Answer: smaller droplet sizes and easier dispersion formation

Q31. Which of the following can be used to measure dynamic interfacial tension at very short timescales?

• maximum bubble pressure method
• static Wilhelmy plate immediately at equilibrium
• Langmuir trough with slow compression
• visual observation of droplet color

Correct Answer: maximum bubble pressure method

Q32. Surface-active proteins at interfaces often provide stabilization by which mechanism?

• forming viscoelastic interfacial films that resist coalescence
• increasing bulk viscosity without interfacial adsorption
• raising the interfacial tension to very high values
• induces immediate precipitation of oil

Correct Answer: forming viscoelastic interfacial films that resist coalescence

Q33. Contact angle hysteresis is caused mainly by:

• surface roughness and chemical heterogeneity of the solid
• temperature variations in bulk solution only
• presence of volatile organic solvents exclusively
• light-induced changes in surface tension

Correct Answer: surface roughness and chemical heterogeneity of the solid

Q34. Which factor generally decreases interfacial tension between oil and water?

• adding an appropriate surfactant below or near CMC
• raising the temperature slightly always increases tension
• removing all dissolved gases from the phases
• stirring without surfactant indefinitely

Correct Answer: adding an appropriate surfactant below or near CMC

Q35. Which surfactant type provides steric stabilization to emulsions?

• nonionic surfactants with large polymeric chains (e.g., PEGylated)
• small ionic surfactants only
• metal salts with high ionic strength
• volatile oils

Correct Answer: nonionic surfactants with large polymeric chains (e.g., PEGylated)

Q36. In the Wilhelmy plate method it is commonly assumed that the plate is:

• completely wetted (contact angle ≈ 0°) for force calculation simplification
• non-wettable so contact angle ≈ 180°
• chemically reacting with the liquid
• electrically charged to measure surface potential

Correct Answer: completely wetted (contact angle ≈ 0°) for force calculation simplification

Q37. Which of the following best describes why temperature generally decreases interfacial tension?

• higher temperature increases molecular motion and reduces cohesive forces at the interface
• temperature increases always polymerize surfactants at the interface
• temperature adds surfactant molecules to the interface from the bulk
• temperature converts liquids into solids increasing interfacial area

Correct Answer: higher temperature increases molecular motion and reduces cohesive forces at the interface

Q38. In an oil-water system, hydrophobic tails of an amphiphile typically orient:

• toward the oil phase, with hydrophilic heads toward water
• completely buried in the aqueous phase only
• randomly with no preferred orientation
• only at the solid-support interface

Correct Answer: toward the oil phase, with hydrophilic heads toward water

Q39. Which phenomenon is most directly reduced by lowering interfacial tension in emulsions?

• the energy barrier for droplet breakup during homogenization
• the pH of the aqueous phase
• the molecular weight of the oil
• the ionic strength of water independently

Correct Answer: the energy barrier for droplet breakup during homogenization

Q40. A positive spreading coefficient (S > 0) indicates:

• the liquid will completely spread over the substrate (spontaneous spreading)
• the liquid will form a stable spherical droplet only
• the interface is electrically charged
• the contact angle must be greater than 90°

Correct Answer: the liquid will completely spread over the substrate (spontaneous spreading)

Q41. In pendant drop analysis, the controlling forces are:

• gravity and interfacial tension
• electrostatic attraction and viscosity only
• surface charge and pH exclusively
• magnetic forces and capillary waves

Correct Answer: gravity and interfacial tension

Q42. Steric stabilization of emulsions is typically provided by:

• adsorbed polymers or bulky nonionic surfactant layers preventing close approach of droplets
• adding high concentrations of electrolytes
• increasing temperature to dissolve surfactants
• mechanical agitation only

Correct Answer: adsorbed polymers or bulky nonionic surfactant layers preventing close approach of droplets

Q43. The numerical equivalence between dyn/cm and mN/m is:

• 1 dyn/cm = 1 mN/m
• 1 dyn/cm = 10 mN/m
• 1 dyn/cm = 0.1 mN/m
• 1 dyn/cm = 100 mN/m

Correct Answer: 1 dyn/cm = 1 mN/m

Q44. Which of the following will most effectively lower oil–water interfacial tension for formulation purposes?

• adding an appropriate amphiphilic surfactant up to near its CMC
• adding non-surface-active salt only
• filtration through a fine membrane without surfactant
• shaking vigorously without any additive

Correct Answer: adding an appropriate amphiphilic surfactant up to near its CMC

Q45. Which parameter of a solid surface most strongly affects the equilibrium contact angle of a liquid?

• surface free energy (surface tension) of the solid
• the bulk density of the liquid
• the molecular weight of atmospheric gases
• the absolute amount of light on the surface

Correct Answer: surface free energy (surface tension) of the solid

Q46. Which method is commonly used to form a Langmuir monolayer in laboratories?

• spreading a solution of amphiphile on a water surface and compressing with movable barriers
• heating a bulk surfactant until it vaporizes
• adding surfactant directly into oil with no interface
• shaking a solution vigorously to create foam

Correct Answer: spreading a solution of amphiphile on a water surface and compressing with movable barriers

Q47. In pharmaceutical emulsions, what is the role of interfacial tension in drug release?

• Lower interfacial tension can lead to smaller droplets and potentially faster drug release from dispersed phase
• Higher interfacial tension always increases drug solubility
• Interfacial tension has no effect on droplet size or release
• Interfacial tension only affects the color of the emulsion

Correct Answer: Lower interfacial tension can lead to smaller droplets and potentially faster drug release from dispersed phase

Q48. Which phenomenon describes convective flow from low to high surface tension regions?

• Marangoni flow
• Brownian motion
• Stokes settling
• Langmuir compression

Correct Answer: Marangoni flow

Q49. Which of the following is NOT a typical effect of surfactant adsorption at an oil–water interface?

• increase interfacial tension dramatically
• reduce interfacial tension
• change interfacial rheological properties
• modify wettability of solids

Correct Answer: increase interfacial tension dramatically

Q50. For accurate interfacial tension measurements, which practice is most important?

• careful control of cleanliness, temperature, and elimination of contamination at the interface
• using colored dyes to visually calibrate surface area
• adding a large excess of volatile solvents just before measurement
• measuring only in outdoor uncontrolled environments

Correct Answer: careful control of cleanliness, temperature, and elimination of contamination at the interface

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