The interfacial properties of suspended particles determine colloidal stability, drug release, and formulation performance in pharmaceutical suspensions and emulsions. This introduction covers surface and interfacial tension, adsorption of surfactants and polymers, zeta potential, electrical double layer, DLVO theory, steric and electrostatic stabilization, wetting, coalescence, and measurement techniques relevant to B. Pharm students. Understanding these concepts helps predict aggregation, sedimentation, and emulsion stability, and guides excipient selection and process control in drug formulations. Keywords: interfacial properties, suspended particles, zeta potential, surface tension, adsorption, colloidal stability, emulsions, flocculation. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the primary cause of instability in colloidal suspensions of pharmaceutical particles?
- High viscosity of the medium
- Aggregation due to attractive interparticle forces
- Presence of preservatives
- High molecular weight polymers
Correct Answer: Aggregation due to attractive interparticle forces
Q2. Surface tension is defined as which of the following?
- Force per unit length acting at a surface
- Mass per unit area of a particle
- Viscosity per unit volume
- Electrical charge per unit area
Correct Answer: Force per unit length acting at a surface
Q3. Which unit is commonly used to report surface or interfacial tension in pharmaceutical literature?
- Pascal-second (Pa·s)
- Millinewton per meter (mN/m)
- Mol per liter (M)
- Newton-meter (N·m)
Correct Answer: Millinewton per meter (mN/m)
Q4. Zeta potential is most closely associated with which region around a particle?
- The Stern (slipping) plane separating bound and mobile ions
- The particle core crystalline lattice
- The bulk solvent far from the particle
- The primary hydration shell only
Correct Answer: The Stern (slipping) plane separating bound and mobile ions
Q5. According to DLVO theory, the total interaction energy between particles is the sum of which forces?
- Hydrophobic attraction and Brownian motion
- Electrostatic repulsion and van der Waals attraction
- Viscous drag and gravitational force
- Steric hindrance and hydration forces only
Correct Answer: Electrostatic repulsion and van der Waals attraction
Q6. Increasing ionic strength of the dispersion medium typically causes which effect on the electrical double layer?
- Expansion of the double layer thickness
- Compression (reduction) of the double layer thickness
- Conversion to a permanent dipole
- No change to the double layer
Correct Answer: Compression (reduction) of the double layer thickness
Q7. In electrophoretic mobility measurements, the Smoluchowski equation relates mobility to zeta potential. Which property is directly proportional to zeta potential in this relation?
- Viscosity of the medium
- Dielectric constant of the medium
- Particle density
- Surface tension
Correct Answer: Dielectric constant of the medium
Q8. Which stabilization mechanism involves bulky polymer chains adsorbed on particle surfaces preventing close approach?
- Steric stabilization
- Electrostatic stabilization
- Pflücking stabilization
- Ostwald stabilization
Correct Answer: Steric stabilization
Q9. The Gibbs adsorption equation links change in surface tension to what thermodynamic quantity at the interface?
- Surface excess concentration (Γ)
- Bulk viscosity
- Particle zeta potential
- Interfacial curvature
Correct Answer: Surface excess concentration (Γ)
Q10. Which of the following describes the isoelectric point of particles in suspension?
- pH at which particle solubility is maximum
- pH at which the net surface charge and zeta potential are zero
- pH at which surface tension is minimum
- pH at which ionic strength is highest
Correct Answer: pH at which the net surface charge and zeta potential are zero
Q11. Which measurement technique is commonly used to measure interfacial tension of emulsions by analyzing a droplet shape?
- Pendant drop tensiometry
- UV-visible spectrophotometry
- Dynamic light scattering
- Gas chromatography
Correct Answer: Pendant drop tensiometry
Q12. Pickering emulsions are stabilized primarily by which of the following?
- Low molecular weight surfactants forming micelles
- Adsorption of solid particles at the oil–water interface
- High temperature emulsification
- Strong ionic bonding between phases
Correct Answer: Adsorption of solid particles at the oil–water interface
Q13. Ostwald ripening in emulsions is best described as:
- Coalescence due to Brownian collisions
- Growth of larger droplets at the expense of smaller ones via molecular diffusion
- Decrease in ionic strength over time
- Polymer bridging between droplets
Correct Answer: Growth of larger droplets at the expense of smaller ones via molecular diffusion
Q14. Contact angle measurements provide quantitative information about which property?
- Particle size distribution
- Wetting and surface hydrophobicity/hydrophilicity
- Zeta potential magnitude
- Viscosity of the continuous phase
Correct Answer: Wetting and surface hydrophobicity/hydrophilicity
Q15. Which surfactant parameter helps in selecting surfactants for oil-in-water versus water-in-oil emulsions?
- Critical micelle concentration (CMC)
- Hydrophilic–lipophilic balance (HLB)
- Melting point
- Viscosity
Correct Answer: Hydrophilic–lipophilic balance (HLB)
Q16. Which phenomenon explains increased surface elasticity upon uneven surfactant distribution leading to flow from low to high surface tension?
- Marangoni effect
- Ostwald ripening
- Brownian motion
- Steric repulsion
Correct Answer: Marangoni effect
Q17. Which of the following reduces interfacial tension most effectively at concentrations above CMC?
- Further addition of ionic salt
- Adsorption of surfactant monomers at the interface until CMC is reached, then micelle formation reduces free monomers
- Heating the system to boiling
- Increasing particle size
Correct Answer: Adsorption of surfactant monomers at the interface until CMC is reached, then micelle formation reduces free monomers
Q18. Which equation describes the settling velocity of a small spherical particle under laminar flow conditions?
- Arrhenius equation
- Stokes’ law
- Henderson–Hasselbalch equation
- Gibbs–Duhem equation
Correct Answer: Stokes’ law
Q19. A high magnitude (positive or negative) zeta potential usually indicates:
- High tendency for flocculation
- Enhanced colloidal stability due to electrostatic repulsion
- Immediate coalescence of droplets
- Low particle mobility
Correct Answer: Enhanced colloidal stability due to electrostatic repulsion
Q20. Steric stabilization effectiveness increases with which of the following polymer characteristics?
- Low molecular weight and low chain density
- High molecular weight and appropriate graft density or thickness of adsorbed layer
- Complete desorption from particle surfaces
- Short hydrophobic tails only
Correct Answer: High molecular weight and appropriate graft density or thickness of adsorbed layer
Q21. Which interfacial phenomenon is directly responsible for the formation of a monolayer of surfactant at an air–water or oil–water interface?
- Electrostatic screening
- Adsorption driven by amphiphilicity of surfactant molecules
- Crystallization of solvent molecules
- Mechanical agitation only
Correct Answer: Adsorption driven by amphiphilicity of surfactant molecules
Q22. In the context of protein-stabilized emulsions, which property of proteins mainly contributes to interfacial stabilization?
- Their enzymatic activity
- Ability to unfold and form viscoelastic interfacial films
- Their color
- High crystalline melting point
Correct Answer: Ability to unfold and form viscoelastic interfacial films
Q23. Which factor does NOT lower the energy barrier to coalescence of emulsion droplets?
- Thinning of the interfacial film
- Electrostatic repulsion between droplets
- Surfactant desorption from the interface
- Reduced interfacial viscosity
Correct Answer: Electrostatic repulsion between droplets
Q24. The term ‘surface excess’ refers to:
- The excess number of molecules in the bulk compared to the surface
- The amount of a component adsorbed per unit area at an interface
- The difference in particle size at the surface vs. core
- The excess viscosity in the interfacial layer
Correct Answer: The amount of a component adsorbed per unit area at an interface
Q25. Which of the following best describes coagulation in suspensions?
- Reversible aggregation controlled by weak repulsive forces
- Irreversible aggregation leading to large aggregates that often settle
- Increase in solubility of particles
- Formation of micelles
Correct Answer: Irreversible aggregation leading to large aggregates that often settle
Q26. Which change would you expect to reduce the zeta potential magnitude of colloidal particles with negatively charged surfaces?
- Lowering ionic strength of the medium
- Adding counter-ions (cations) to the medium
- Raising pH to make surface more negative
- Increasing temperature slightly
Correct Answer: Adding counter-ions (cations) to the medium
Q27. Which of the following is an effect of surfactant adsorption kinetics on emulsion preparation?
- Faster adsorption always leads to larger droplet size
- Slow adsorption may cause insufficient coverage during droplet formation and increased coalescence
- Adsorption kinetics do not affect emulsion stability
- Only the bulk surfactant concentration matters, not kinetics
Correct Answer: Slow adsorption may cause insufficient coverage during droplet formation and increased coalescence
Q28. Which parameter directly quantifies the electrical screening length in an electrolyte solution?
- Zeta potential magnitude
- Debye length (κ−1)
- Surface tension
- Hydrodynamic diameter
Correct Answer: Debye length (κ−1)
Q29. In DLVO theory, increasing temperature generally affects stability by:
- Directly increasing van der Waals attraction only
- Altering thermal energy (kT) and possibly changing balance between attractive and repulsive interactions
- Eliminating the double layer completely
- Converting colloids to crystals
Correct Answer: Altering thermal energy (kT) and possibly changing balance between attractive and repulsive interactions
Q30. Which technique is commonly used to estimate particle size distribution in suspensions relevant to interfacial studies?
- Dynamic light scattering (DLS)
- Infrared spectroscopy
- Atomic absorption spectroscopy
- pH titration
Correct Answer: Dynamic light scattering (DLS)
Q31. Which condition favors steric stabilization over electrostatic stabilization?
- High ionic strength that screens electrostatic repulsion
- Low polymer concentration
- Near-zero zeta potential only
- Low molecular weight surfactants exclusively
Correct Answer: High ionic strength that screens electrostatic repulsion
Q32. Which of the following best explains contact angle hysteresis on a solid surface?
- Evaporation of the continuous phase
- Surface heterogeneity and pinning of the contact line
- Constant surface tension with temperature
- Brownian motion of particles
Correct Answer: Surface heterogeneity and pinning of the contact line
Q33. A biodegradable polymer adsorbs strongly to particle surfaces and provides a hydrated brush layer. What primary stabilization does this confer?
- Electrostatic stabilization
- Steric stabilization
- Reduction of ionic strength
- Magnetic stabilization
Correct Answer: Steric stabilization
Q34. Which property of surfactants determines whether they carry a charge at the interface?
- Hydrophobic tail length only
- Head group ionicity (anionic, cationic, nonionic, zwitterionic)
- Color of the compound
- Molecular weight only
Correct Answer: Head group ionicity (anionic, cationic, nonionic, zwitterionic)
Q35. Which factor most directly promotes coalescence between emulsion droplets?
- Thick adsorbed polymer layer at the interface
- Rapid drainage and rupture of the thin liquid film between droplets
- High interfacial viscoelasticity
- Strong electrostatic repulsion
Correct Answer: Rapid drainage and rupture of the thin liquid film between droplets
Q36. Which of the following is TRUE about the relationship between particle size and Brownian motion?
- Brownian motion increases with increasing particle size
- Brownian motion is independent of particle size
- Brownian motion decreases with increasing particle size
- Brownian motion only occurs for charged particles
Correct Answer: Brownian motion decreases with increasing particle size
Q37. Which interfacial parameter can be reduced by adding an appropriate surfactant to an aqueous phase to facilitate wetting of a solid particle by oil?
- Zeta potential of the particle
- Interfacial tension between oil and water
- Particle density
- Viscosity of the oil
Correct Answer: Interfacial tension between oil and water
Q38. In an emulsion, coalescence differs from flocculation in that coalescence results in:
- Aggregation without film rupture
- Merging of droplets and formation of a larger single droplet
- Only reversible cluster formation
- Formation of stable Pickering layers
Correct Answer: Merging of droplets and formation of a larger single droplet
Q39. Which surfactant type is least likely to impart a high zeta potential to particle surfaces?
- Anionic surfactants
- Cationic surfactants
- Nonionic surfactants
- Zwitterionic surfactants (depending on pH)
Correct Answer: Nonionic surfactants
Q40. Which calculation or relation would you use to estimate electrophoretic mobility from zeta potential in a typical aqueous medium (thin double layer)?
- Arrhenius relation
- Smoluchowski equation
- Henderson–Hasselbalch equation
- Henry’s law
Correct Answer: Smoluchowski equation
Q41. When surfactant adsorption at an interface reaches saturation, further increase in bulk concentration mainly leads to:
- Decrease in micelle formation
- Formation of micelles in the bulk phase above CMC
- Immediate precipitation of surfactant
- Increase in interfacial tension
Correct Answer: Formation of micelles in the bulk phase above CMC
Q42. Which of the following best explains why nanoparticles can stabilize emulsions more effectively than larger particles in Pickering systems?
- Nanoparticles always dissolve in the continuous phase
- Higher specific surface area and stronger adsorption energy per particle at the interface
- They reduce Brownian motion drastically
- They increase the ionic strength of the medium
Correct Answer: Higher specific surface area and stronger adsorption energy per particle at the interface
Q43. Which interfacial property is directly measured by a spinning drop tensiometer?
- Electrical conductivity
- Low interfacial tension between immiscible liquids
- Zeta potential
- Surface charge density
Correct Answer: Low interfacial tension between immiscible liquids
Q44. The primary reason proteins can stabilize emulsions at low concentrations is their ability to:
- Form covalent bonds between droplets
- Unfold and adsorb to form viscoelastic interfacial films
- Increase the bulk ionic strength dramatically
- React chemically with oil
Correct Answer: Unfold and adsorb to form viscoelastic interfacial films
Q45. Which process is most important when controlling shelf-life to prevent creaming in an oil-in-water suspension?
- Reducing the interfacial area
- Controlling droplet size and density difference plus viscosity of the continuous phase
- Increasing temperature to reduce viscosity
- Removing surfactants entirely
Correct Answer: Controlling droplet size and density difference plus viscosity of the continuous phase
Q46. Which statement about contact angle and wettability is correct?
- Large contact angle indicates better wetting
- Contact angle less than 90° indicates the liquid wets the solid surface
- Contact angle is unrelated to adhesion
- Contact angle increases as hydrophilicity increases
Correct Answer: Contact angle less than 90° indicates the liquid wets the solid surface
Q47. Which approach is commonly used to increase colloidal stability in pharmaceutical suspensions by modifying surface charge?
- Add nonpolar solvents
- Adjust pH or add charged surfactants/polyelectrolytes to increase zeta potential magnitude
- Reduce particle surface area
- Freeze the suspension
Correct Answer: Adjust pH or add charged surfactants/polyelectrolytes to increase zeta potential magnitude
Q48. Which phenomenon describes the time-dependent change in interfacial tension due to adsorption of surfactant molecules?
- Interfacial rheology
- Dynamic (or time-dependent) interfacial tension
- Static light scattering
- Thermal expansion
Correct Answer: Dynamic (or time-dependent) interfacial tension
Q49. Which is a typical effect of polymer bridging between particles in suspension?
- Steric stabilization and prevention of aggregation
- Formation of reversible flocs via adsorbed polymer chains connecting particles
- Instant dissolution of particles
- Reduction of ionic strength
Correct Answer: Formation of reversible flocs via adsorbed polymer chains connecting particles
Q50. For ionic surfactants, why does the Gibbs adsorption equation often require modification compared to nonionic surfactants?
- Ionic surfactants do not adsorb at interfaces
- Electrostatic dissociation results in different numbers of species and activity coefficients affecting the relation between surface tension and bulk concentration
- Ionic surfactants increase viscosity only
- Nonionic surfactants are always insoluble
Correct Answer: Electrostatic dissociation results in different numbers of species and activity coefficients affecting the relation between surface tension and bulk concentration
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.
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