Surface free energy MCQs With Answer

Introduction: Surface free energy is a key property of pharmaceutical solids that influences wetting, adhesion, powder cohesion, tablet coating, dissolution rate and stability. This concise, student-friendly guide on Surface free energy MCQs With Answer helps B.Pharm learners master concepts such as contact angle, surface tension, Young’s equation, work of adhesion, polar and dispersive surface energy components, measurement techniques (sessile drop, Wilhelmy plate, pendant drop), and practical formulation applications like granulation and coating. Questions range from fundamental definitions to applied problems relevant to drug formulation and surface modification. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the definition of surface free energy for a solid?

• The energy required to remove a unit mass from the bulk
• The excess energy per unit area at the surface compared to the bulk
• The kinetic energy of surface molecules
• The thermal energy at the solid–liquid interface

Correct Answer: The excess energy per unit area at the surface compared to the bulk

Q2. Which units are commonly used to express surface free energy?

• Joules per mole (J/mol)
• Newton (N)
• Millinewton per meter (mN/m) or mJ/m²
• Pascal-second (Pa·s)

Correct Answer: Millinewton per meter (mN/m) or mJ/m²

Q3. Surface tension is most directly measured for which phase boundary?

• Solid–solid
• Liquid–vapor
• Solid–liquid
• Liquid–liquid

Correct Answer: Liquid–vapor

Q4. Young’s equation relates contact angle with which three interfacial tensions?

• Solid–gas, liquid–gas, and solid–liquid
• Liquid–liquid, solid–vapor, and liquid–vapor
• Solid–solid, liquid–liquid, and gas–gas
• Adhesion, cohesion, and friction forces

Correct Answer: Solid–gas, liquid–gas, and solid–liquid

Q5. In Young’s equation, a low contact angle indicates:

• High hydrophobicity
• Poor wetting
• Good wetting and high surface free energy of the solid
• High surface roughness only

Correct Answer: Good wetting and high surface free energy of the solid

Q6. Which technique is commonly used to measure solid surface energy by observing a liquid droplet?

• Gel permeation chromatography
• Sessile drop contact angle measurement
• Mass spectrometry
• Differential scanning calorimetry

Correct Answer: Sessile drop contact angle measurement

Q7. The work of adhesion (Wadh) between a liquid and a solid is given by which relation (Dupré equation)?

• Wadh = γLV + γSL − γSV
• Wadh = γSV − γSL − γLV
• Wadh = γLV (1 + cos θ)
• Wadh = γSL × γSV

Correct Answer: Wadh = γLV (1 + cos θ)

Q8. Which parameter describes difference between advancing and receding contact angles?

• Surface tension
• Contact angle hysteresis
• Spreading coefficient
• Zeta potential

Correct Answer: Contact angle hysteresis

Q9. Which model describes wetting on rough homogeneous surfaces?

• Cassie-Baxter model
• Langmuir model
• Wenzel model
• Hildebrand model

Correct Answer: Wenzel model

Q10. The Cassie-Baxter model is used when:

• The liquid completely penetrates surface roughness
• The surface is chemically heterogeneous and air pockets are trapped under the droplet
• The interface is strictly atomically smooth
• There is no contact angle hysteresis

Correct Answer: The surface is chemically heterogeneous and air pockets are trapped under the droplet

Q11. Which method measures surface tension by recording the force on a plate immersed in liquid?

• Pendulum method
• Wilhelmy plate method
• Sessile drop method
• Ellipsometry

Correct Answer: Wilhelmy plate method

Q12. Critical surface tension (γc) introduced by Zisman is used to:

• Predict spreading of liquids on solids by extrapolating contact angles of probe liquids
• Measure the electrical conductivity of a surface
• Calculate bulk tensile strength of powders
• Determine glass transition temperature

Correct Answer: Predict spreading of liquids on solids by extrapolating contact angles of probe liquids

Q13. Which surface energy component accounts for London dispersion forces?

• Polar component
• Acid-base component
• Dispersive component
• Hydrogen-bonding only

Correct Answer: Dispersive component

Q14. The Owens-Wendt method determines solid surface energy by using:

• Two or more probe liquids with known polar and dispersive contributions
• Only water as a probe liquid
• Mass spectrometric analysis of surface atoms
• Thermogravimetric data

Correct Answer: Two or more probe liquids with known polar and dispersive contributions

Q15. Van Oss acid-base approach separates surface energy into which components?

• Polar and nonpolar only
• Electron donor (basic), electron acceptor (acidic), and Lifshitz–van der Waals components
• Hydrophobic and hydrophilic only
• Mechanical and thermal components

Correct Answer: Electron donor (basic), electron acceptor (acidic), and Lifshitz–van der Waals components

Q16. Gibbs adsorption equation relates changes in surface tension to:

• Temperature only
• Surface excess concentration of an adsorbed species
• Bulk viscosity
• Crystal lattice energy

Correct Answer: Surface excess concentration of an adsorbed species

Q17. Contact angle greater than 90° typically indicates:

• Complete wetting
• Hydrophilicity
• Hydrophobic surface
• Zero surface free energy

Correct Answer: Hydrophobic surface

Q18. Which instrument is primarily used to capture and analyze droplet shape for contact angle?

• UV-Vis spectrophotometer
• Contact angle goniometer / optical tensiometer
• Atomic absorption spectrometer
• Rheometer

Correct Answer: Contact angle goniometer / optical tensiometer

Q19. Surface free energy influences which formulation property most directly?

• Tablet color only
• Lubricant miscibility only
• Wetting and adhesion of liquids to solid excipients
• Melting point of drug

Correct Answer: Wetting and adhesion of liquids to solid excipients

Q20. Increasing surface free energy of a powder surface generally will:

• Increase powder hydrophobicity
• Decrease wettability by polar liquids
• Improve wettability and often improve granulation with aqueous binders
• Reduce tablet hardness

Correct Answer: Improve wettability and often improve granulation with aqueous binders

Q21. Surfactants reduce surface tension primarily by:

• Increasing bulk viscosity
• Adsorbing at interfaces and lowering interfacial free energy
• Raising the temperature of the system
• Forming covalent bonds with the surface

Correct Answer: Adsorbing at interfaces and lowering interfacial free energy

Q22. Which phenomenon can lead to contact angle hysteresis?

• Chemical heterogeneity and surface roughness
• Purely elastic deformation of the droplet
• Constant temperature conditions
• Absence of air at the interface

Correct Answer: Chemical heterogeneity and surface roughness

Q23. In emulsion formulation, compatibility of oil and water phases is often quantified by:

• Surface free energy and interfacial tension
• pKa of the drug only
• Melting point depression
• Colorimetric titration

Correct Answer: Surface free energy and interfacial tension

Q24. AFM (atomic force microscopy) can provide surface energy-related information by:

• Measuring surface topography and local adhesion forces between tip and sample
• Measuring bulk drug solubility
• Recording infrared spectra
• Determining crystalline polymorphs exclusively

Correct Answer: Measuring surface topography and local adhesion forces between tip and sample

Q25. Which of the following reduces powder cohesion by modifying surface energy?

• Adding hydrophobic glidants like colloidal silica to lower surface energy
• Heating the powder to decomposition
• Compressing the powder at very high force only
• Mixing the powder with water without drying

Correct Answer: Adding hydrophobic glidants like colloidal silica to lower surface energy

Q26. Sessile drop, pendant drop and captive bubble are methods to determine:

• Surface free energy of solids directly
• Liquid–vapor surface tension or contact angles used for solid surface energy estimation
• Crystalline melting points
• Viscosity of polymers only

Correct Answer: Liquid–vapor surface tension or contact angles used for solid surface energy estimation

Q27. The spreading coefficient S of a liquid on a solid is positive when:

• Spreading is spontaneous and the liquid will wet the surface
• Spreading is impossible
• The contact angle is exactly 180°
• The liquid evaporates instantly

Correct Answer: Spreading is spontaneous and the liquid will wet the surface

Q28. Which equation expresses Young’s relation in terms of contact angle θ?

• γSV = γSL + γLV cos θ
• γLV = γSV + γSL cos θ
• γSL = γLV + γSV cos θ
• γSV = γLV + γSL cos θ

Correct Answer: γSV = γSL + γLV cos θ

Q29. Contact angle measurement on powders is commonly performed by which adapted method?

• Rolling ball method
• Capillary rise with packed powder bed or sessile drop on compressed tablet
• Direct tensiometry of powder
• Flotation analysis only

Correct Answer: Capillary rise with packed powder bed or sessile drop on compressed tablet

Q30. A decrease in liquid surface tension generally does what to contact angle on a given solid?

• Always increases contact angle
• Has no effect
• Usually decreases contact angle and improves wetting
• Makes the contact angle equal to 90°

Correct Answer: Usually decreases contact angle and improves wetting

Q31. Which surface modification technique increases surface free energy by introducing polar groups?

• Plasma treatment
• Coating with paraffin wax
• Annealing under inert gas only
• Mechanical polishing without chemistry

Correct Answer: Plasma treatment

Q32. In tablet coating, poor wetting of the coating solution results in:

• Uniform smooth coating
• Poor adhesion, patchy coverage and coating defects
• Improved dissolution only
• Instant disintegration

Correct Answer: Poor adhesion, patchy coverage and coating defects

Q33. Which statement about surface free energy and dissolution is true?

• Higher solid surface energy can enhance initial wetting and dissolution rate
• Surface energy has no impact on dissolution
• Lower surface energy always increases dissolution rate
• Dissolution depends only on pH and not on surface properties

Correct Answer: Higher solid surface energy can enhance initial wetting and dissolution rate

Q34. Zeta potential is mainly a measure of:

• Surface free energy directly
• Electrical potential at the slipping plane, related to colloidal stability
• Contact angle hysteresis
• Bulk viscosity of a suspension

Correct Answer: Electrical potential at the slipping plane, related to colloidal stability

Q35. Which factor does NOT typically influence measured contact angle?

• Surface roughness
• Surface chemical heterogeneity
• Ambient temperature
• Atomic number of the bulk material only

Correct Answer: Atomic number of the bulk material only

Q36. Spreading coefficient S for liquid on solid is given by which relation?

• S = γLV + γSL + γSV
• S = γSV − (γSL + γLV)
• S = γSL − γSV − γLV
• S = γLV − γSV − γSL

Correct Answer: S = γSV − (γSL + γLV)

Q37. Which probe liquids are commonly used for Owens-Wendt analysis?

• Water and diiodomethane (or formamide)
• Hexane only
• Glycerin only
• Any two colored liquids

Correct Answer: Water and diiodomethane (or formamide)

Q38. Surface free energy decreases when the surface is made more:

• Polar and hydrogen-bonding
• Rough and chemically heterogeneous
• Hydrophobic by low-energy coatings
• Oxidized with polar groups

Correct Answer: Hydrophobic by low-energy coatings

Q39. What is contact angle hysteresis caused by?

• Uniform, atomically flat surfaces only
• Surface defects, roughness and chemical heterogeneity
• Pure evaporation alone
• Identical advancing and receding angles

Correct Answer: Surface defects, roughness and chemical heterogeneity

Q40. Which surface analytical technique provides chemical composition of the outermost surface layer?

• X-ray photoelectron spectroscopy (XPS)
• Thermogravimetric analysis
• Gel electrophoresis
• Optical microscopy only

Correct Answer: X-ray photoelectron spectroscopy (XPS)

Q41. Increasing temperature generally does what to liquid surface tension?

• Increases it
• Decreases it
• Has no effect
• Makes it infinite

Correct Answer: Decreases it

Q42. In wetting terminology, a surface is termed lyophobic when:

• It has a strong affinity for the liquid
• It repels the liquid and contact angle is high
• It spontaneously dissolves
• It becomes charged in water

Correct Answer: It repels the liquid and contact angle is high

Q43. How does plasma treatment improve adhesion of coatings?

• By depositing a thick hydrophobic film
• By cleaning the surface and introducing polar functional groups, increasing surface energy
• By making the surface chemically inert
• By reducing surface temperature only

Correct Answer: By cleaning the surface and introducing polar functional groups, increasing surface energy

Q44. Which of the following describes the relation between work of cohesion (Wcoh) for a liquid?

• Wcoh = 2 × γLV
• Wcoh = γSV + γSL
• Wcoh = γLV / 2
• Wcoh = γSL − γSV

Correct Answer: Wcoh = 2 × γLV

Q45. For a pharmaceutical powder, high surface energy can lead to which unwanted effect?

• Decreased tendency to agglomerate
• Increased powder cohesion and poor flowability
• Instantaneous dissolution always
• Loss of crystallinity only

Correct Answer: Increased powder cohesion and poor flowability

Q46. Which is the best description of the role of surfactants in wetting of drug particles?

• Surfactants increase interfacial tension and prevent wetting
• Surfactants adsorb at interfaces, lower interfacial tension, and improve wetting and dispersion
• Surfactants only change taste
• Surfactants react chemically and dissolve the particles

Correct Answer: Surfactants adsorb at interfaces, lower interfacial tension, and improve wetting and dispersion

Q47. Contact angle of water on a solid decreased from 90° to 30° after treatment. This indicates:

• Surface became more hydrophobic
• Surface energy decreased dramatically
• Surface became more hydrophilic and surface energy increased
• No change in wetting behavior

Correct Answer: Surface became more hydrophilic and surface energy increased

Q48. Which is a limitation when measuring contact angles on heterogeneous powder surfaces?

• Powder surfaces are ideal and give reproducible results
• Surface roughness, porosity and heterogeneity can cause variability and pinning of contact line
• Contact angle is independent of liquid choice
• Contact angle methods measure bulk instead of surface properties

Correct Answer: Surface roughness, porosity and heterogeneity can cause variability and pinning of contact line

Q49. Which parameter would you manipulate to improve coating adhesion on a hydrophobic tablet core?

• Increase tablet temperature only
• Lower coating formulation surface tension, add wetting agents, or increase core surface energy by treatment
• Reduce coating solids content only
• Make coating more hydrophobic

Correct Answer: Lower coating formulation surface tension, add wetting agents, or increase core surface energy by treatment

Q50. Which statement correctly links interfacial tension to emulsion stability?

• Higher interfacial tension always stabilizes emulsions
• Lower interfacial tension facilitated by surfactants aids droplet breakup and can improve kinetic stability when surfactant layers are present
• Interfacial tension has no relevance to emulsions
• Only viscosity determines emulsion stability

Correct Answer: Lower interfacial tension facilitated by surfactants aids droplet breakup and can improve kinetic stability when surfactant layers are present

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