Adsorption at liquid interfaces MCQs With Answer

Adsorption at liquid interfaces MCQs With Answer is a concise, SEO-focused revision resource tailored for B.Pharm students studying interfacial phenomena in pharmaceutical sciences. This set covers surface adsorption fundamentals, Gibbs adsorption, adsorption isotherms (Langmuir, Freundlich, Temkin), dynamic adsorption, surfactant adsorption and CMC, adsorption kinetics, and relevance to drug formulation, emulsions, and stability. Questions include conceptual, calculation-based, and application scenarios commonly encountered in dosage form design and characterization. Designed by educators and aligned with the B.Pharm syllabus, questions range from fundamental theory to calculation-based problems and experimental interpretation. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is adsorption at a liquid interface?

• The accumulation of molecules at the boundary between two phases
• The formation of a chemical bond in the bulk liquid
• The increase of bulk concentration due to heating
• The conversion of liquid to gas at the interface

Correct Answer: The accumulation of molecules at the boundary between two phases

Q2. Which equation relates surface excess concentration to change in surface tension with concentration?

• The Arrhenius equation
• The Gibbs adsorption equation
• The Nernst equation
• The Van’t Hoff equation

Correct Answer: The Gibbs adsorption equation

Q3. In the Gibbs adsorption equation for a nonionic solute, surface excess (Γ) is proportional to which derivative?

• dγ/dT (change of surface tension with temperature)
• dγ/d ln c (change of surface tension with logarithm of concentration)
• dμ/dP (change of chemical potential with pressure)

Correct Answer: dγ/d ln c (change of surface tension with logarithm of concentration)

Q4. Which isotherm assumes monolayer adsorption on a homogeneous surface with finite identical sites?

• Freundlich isotherm
• Langmuir isotherm
• Temkin isotherm
• Henry’s law

Correct Answer: Langmuir isotherm

Q5. The Freundlich isotherm is best described as:

• An empirical multilayer adsorption model on heterogeneous surfaces
• A theoretical monolayer model for homogeneous surfaces
• A model based on ideal gas behavior
• A kinetic rate equation for adsorption

Correct Answer: An empirical multilayer adsorption model on heterogeneous surfaces

Q6. Which parameter indicates the concentration at which surfactants begin forming micelles?

• Surface excess (Γ)
• Critical micelle concentration (CMC)
• Partition coefficient (Kp)
• Langmuir constant (b)

Correct Answer: Critical micelle concentration (CMC)

Q7. Which experimental method measures static surface tension commonly used to study adsorption?

• Gel permeation chromatography
• Wilhelmy plate or Du Noüy ring tensiometry
• UV-visible spectrophotometry
• Dynamic light scattering

Correct Answer: Wilhelmy plate or Du Noüy ring tensiometry

Q8. Surface excess Γ has units of:

• mol·m^-2 or mol/cm^2
• mol·L^-1
• moles
• J·mol^-1

Correct Answer: mol·m^-2 or mol/cm^2

Q9. Which process primarily controls early-time dynamic adsorption of surfactants to a fresh interface?

• Bulk convection only
• Diffusion from bulk to interface
• Chemical reaction at the interface
• Evaporation of solvent

Correct Answer: Diffusion from bulk to interface

Q10. The Ward–Tordai equation is used to describe:

• Equilibrium adsorption isotherms
• Dynamic diffusion-controlled adsorption kinetics
• Thermodynamics of partitioning between phases
• Solubility of gases in liquids

Correct Answer: Dynamic diffusion-controlled adsorption kinetics

Q11. Which assumption is NOT part of the Langmuir isotherm?

• Adsorption occurs at specific homogeneous sites
• Each site holds at most one molecule (monolayer)
• Adsorbate molecules interact strongly with each other on the surface
• Equilibrium between adsorption and desorption exists

Correct Answer: Adsorbate molecules interact strongly with each other on the surface

Q12. A negative value of surface excess from Gibbs equation indicates:

• Adsorption of solute at the interface
• Desorption or depletion of solute from the interface
• No change in surface concentration
• Formation of a multilayer at the interface

Correct Answer: Desorption or depletion of solute from the interface

Q13. How does increasing electrolyte (salt) concentration generally affect adsorption of ionic surfactants at the air–water interface?

• Decreases adsorption by increasing repulsion
• Increases adsorption by screening electrostatic repulsion
• No effect on adsorption
• Causes immediate desorption

Correct Answer: Increases adsorption by screening electrostatic repulsion

Q14. The phenomenon where proteins sequentially displace each other at a surface due to differing affinities is called:

• Langmuir exchange
• Vroman effect
• Gibbs switching
• Henry displacement

Correct Answer: Vroman effect

Q15. Temkin isotherm accounts for:

• Constant adsorption energy regardless of coverage
• Linear decrease of adsorption energy with coverage due to adsorbate interaction
• Exponential adsorption kinetics only
• Multilayer formation at high pressure

Correct Answer: Linear decrease of adsorption energy with coverage due to adsorbate interaction

Q16. Which factor most directly reduces surface tension when added to water?

• Nonpolar gas
• Surface-active surfactant molecules
• Inert salts at low concentration
• Increased temperature only

Correct Answer: Surface-active surfactant molecules

Q17. In adsorption at liquid–liquid interfaces, partitioning differs from adsorption because:

• Partitioning involves bulk-to-bulk distribution across phases, not only accumulation at a surface
• Partitioning only occurs for gases
• Adsorption only occurs in solids
• They are identical processes

Correct Answer: Partitioning involves bulk-to-bulk distribution across phases, not only accumulation at a surface

Q18. Which quantity is minimized when a system reaches equilibrium at a fixed temperature and pressure?

• Gibbs free energy
• Surface excess
• Rate of adsorption
• Surface area

Correct Answer: Gibbs free energy

Q19. Contact angle measurement provides information about:

• Surface roughness only
• Wettability and interfacial free energies
• Bulk viscosity
• Adsorption isotherm parameters directly

Correct Answer: Wettability and interfacial free energies

Q20. Which model would you choose for heterogeneous surface adsorption fitting experimental data?

• Langmuir model
• Freundlich model
• Ideal gas model
• Henry’s law only

Correct Answer: Freundlich model

Q21. The slope of a Langmuir linear plot of 1/θ vs 1/C gives information about:

• Adsorption enthalpy directly
• Langmuir constant and maximum adsorption capacity
• Diffusion coefficient in bulk
• Surface tension at infinite dilution

Correct Answer: Langmuir constant and maximum adsorption capacity

Q22. Zero-order adsorption kinetics implies:

• Rate independent of concentration at the interface
• Rate proportional to concentration
• Rate proportional to square of concentration
• No adsorption occurs

Correct Answer: Rate independent of concentration at the interface

Q23. Which technique gives a direct measurement of interfacial tension for small droplets?

• Pendulum viscometry
• Pendant drop tensiometry
• HPLC
• Atomic force microscopy

Correct Answer: Pendant drop tensiometry

Q24. Adsorption of amphiphilic drugs at interfaces can influence:

• Drug color only
• Emulsion stability, bioavailability, and release kinetics
• Only solubility in organic solvents
• Only melting point

Correct Answer: Emulsion stability, bioavailability, and release kinetics

Q25. In the context of adsorption, the Gibbs dividing surface is:

• A physical film at the interface
• An imaginary plane used to define surface excess
• The actual location of adsorbed molecules
• A measurement instrument

Correct Answer: An imaginary plane used to define surface excess

Q26. Which statement about protein adsorption at liquid interfaces is true?

• Proteins never change conformation on adsorption
• Proteins often unfold and irreversibly adsorb, affecting function
• Protein adsorption is always reversible like surfactants
• Proteins reduce surface tension more effectively than surfactants at equal molar concentration always

Correct Answer: Proteins often unfold and irreversibly adsorb, affecting function

Q27. The term “surface pressure” (Π) in monolayer studies is defined as:

• Π = γ0 + γ (sum of tensions)
• Π = γ0 – γ (decrease in surface tension due to monolayer)
• Π = γ × γ0 (product of tensions)
• Π = γ/γ0 (ratio of tensions)

Correct Answer: Π = γ0 – γ (decrease in surface tension due to monolayer)

Q28. Adsorption isotherm fitting yields which useful pharmaceutical parameter?

• Maximum surface coverage and affinity constants
• Drug melting point
• Particle size distribution
• pH of maximum solubility

Correct Answer: Maximum surface coverage and affinity constants

Q29. Which phenomenon explains slower adsorption at high surface coverage due to lateral interactions?

• Ideal adsorption
• Steric hindrance and lateral repulsion between adsorbed molecules
• Complete desorption
• Bulk precipitation

Correct Answer: Steric hindrance and lateral repulsion between adsorbed molecules

Q30. Which parameter from the Langmuir isotherm represents maximum monolayer adsorption capacity?

• Kd (dissociation constant)
• Qmax or Γmax
• n from Freundlich
• Rt (resistance time)

Correct Answer: Qmax or Γmax

Q31. Which effect would increasing temperature generally have on adsorption of a nonionic surfactant?

• Always increases adsorption due to higher diffusion
• May decrease or increase adsorption depending on enthalpy of adsorption and solubility changes
• Has no effect on adsorption equilibria
• Causes immediate desorption always

Correct Answer: May decrease or increase adsorption depending on enthalpy of adsorption and solubility changes

Q32. Which concept links surface tension reduction to surface concentration for ionic surfactants including counterion effects?

• Gibbs adsorption with activity and ionic stoichiometry considerations
• Langmuir purely
• Ideal gas law
• Arrhenius kinetics

Correct Answer: Gibbs adsorption with activity and ionic stoichiometry considerations

Q33. Which is NOT a typical method to study adsorption kinetics at liquid interfaces?

• Dynamic tensiometry
• Pendant drop relaxation
• Stopped-flow interfacial tensiometry
• Infrared spectroscopy of bulk crystal lattice

Correct Answer: Infrared spectroscopy of bulk crystal lattice

Q34. Competitive adsorption in mixed surfactant systems can lead to:

• Synergistic lowering of surface tension
• Preferential adsorption of the surfactant with higher affinity
• Time-dependent composition changes at the interface
• All of the above

Correct Answer: All of the above

Q35. Which interfacial parameter is crucial for predicting emulsion stability?

• Bulk pH only
• Interfacial tension and interfacial rheology
• Color of the dispersed phase
• Molecular weight of solvent

Correct Answer: Interfacial tension and interfacial rheology

Q36. In adsorption thermodynamics, a negative adsorption free energy indicates:

• Spontaneous adsorption
• Non-spontaneous adsorption
• Equilibrium at infinite dilution
• No adsorption possible

Correct Answer: Spontaneous adsorption

Q37. Which property of surfactant head groups affects adsorption at charged interfaces?

• Head group polarity and charge
• Only tail length matters
• Color of the head group
• Atomic number of hydrogen

Correct Answer: Head group polarity and charge

Q38. The Gibbs adsorption isotherm for ionic surfactants requires accounting for:

• Only temperature changes
• Ionization degree and counterion association
• Surface roughness only
• Adsorbent porosity

Correct Answer: Ionization degree and counterion association

Q39. Adsorption-desorption rate constants are used to calculate which equilibrium parameter?

• Partition coefficient only
• Langmuir affinity constant (b) or equilibrium constant
• Solubility product constant
• pKa directly

Correct Answer: Langmuir affinity constant (b) or equilibrium constant

Q40. Which is a consequence of strong irreversible adsorption of drug molecules to excipient surfaces?

• Improved solubility always
• Loss of available drug and altered bioavailability
• Guaranteed enhanced stability
• Elimination of need for preservatives

Correct Answer: Loss of available drug and altered bioavailability

Q41. The initial slope of surface tension vs log concentration plot gives:

• Information irrelevant to adsorption
• Quantitative indication of surface activity and via Gibbs equation the surface excess
• Only temperature dependence
• Bulk viscosity changes

Correct Answer: Quantitative indication of surface activity and via Gibbs equation the surface excess

Q42. Which interfacial phenomenon is important in pulmonary drug delivery formulations?

• Surface adsorption of pulmonary surfactants affecting spreading and stability
• Bulk crystallization only
• Magnetic alignment of droplets
• Gas phase combustion

Correct Answer: Surface adsorption of pulmonary surfactants affecting spreading and stability

Q43. The term “adsorption kinetics limited by convection” implies:

• Adsorption solely driven by chemical reaction at surface
• Bulk convection dominates transport to interface rather than diffusion
• Adsorption is instant
• No transport limitations exist

Correct Answer: Bulk convection dominates transport to interface rather than diffusion

Q44. Which physical property of an adsorbed layer is measured by interfacial rheology?

• Electrical conductivity of bulk
• Viscoelasticity and mechanical strength of the interfacial film
• Color intensity of the interface
• Boiling point elevation

Correct Answer: Viscoelasticity and mechanical strength of the interfacial film

Q45. How does pH influence adsorption of weakly ionizable drugs at interfaces?

• pH affects ionization state and therefore affinity for the interface
• pH has no effect on ionizable drugs
• Only temperature matters for ionizable drugs
• pH only affects color not adsorption

Correct Answer: pH affects ionization state and therefore affinity for the interface

Q46. Desorption energy barriers are important because they determine:

• How colorless a solution is
• Whether adsorption is reversible or effectively irreversible
• Bulk solubility only
• Shape of the molecules

Correct Answer: Whether adsorption is reversible or effectively irreversible

Q47. The Vroman effect is most relevant to which application area?

• Pharmaceutical surface interactions such as blood-contacting devices and formulations
• Gas chromatography columns only
• Paint drying times
• Metal alloy formation

Correct Answer: Pharmaceutical surface interactions such as blood-contacting devices and formulations

Q48. Which parameter describes the tendency of an adsorbate to displace another at an interface?

• Relative surface affinity or competitive adsorption constant
• Bulk density only
• Absolute refractive index
• Compressibility alone

Correct Answer: Relative surface affinity or competitive adsorption constant

Q49. For adsorption controlled by diffusion, the amount adsorbed typically varies with time as:

• Linear with time (t)
• Proportional to square root of time (t^0.5)
• Exponential growth with time
• Independent of time

Correct Answer: Proportional to square root of time (t^0.5)

Q50. Which strategy can reduce unwanted adsorption of drugs to container interfaces?

• Use of appropriate surfactants or blocking agents to occupy interfacial sites
• Increase container surface area indefinitely
• Store at extremely high temperatures always
• Use pure gas headspace only

Correct Answer: Use of appropriate surfactants or blocking agents to occupy interfacial sites

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