MCQs on Surface Active Agents, HLB, Adsorption & Interfacial Phenomena. GPAT / NIPER / AIIMS / HSSB / SSB / DSSSB/RRB / DSSSB/Pharmacist Recruitment.

1. Which one defines a surface-active agent?

A. Substances to increase surface tension
B. Substances to accumulate at interfaces and lower surface tension
C. Substances to increase viscosity only
D. Substances to prevent solubility

Answer: B. Substances to accumulate at interfaces and lower surface tension

Explanation:

Surface-active agents (surfactants) are substances that preferentially adsorb or accumulate at interfaces such as:

• liquid–air interface

• liquid–liquid interface

• solid–liquid interface

They reduce the surface or interfacial tension by decreasing the free energy at the interface. This property improves wetting, emulsification, detergency, spreading, and solubilization.

Examples:

• Sodium lauryl sulfate

• Tween 80

• Span 60

Why Other Options Are Incorrect

A. Substances to increase surface tension: Most surfactants decrease surface tension rather than increase it. By accumulating at the surface, they weaken cohesive forces between liquid molecules.

Example:
Water has high surface tension due to hydrogen bonding. The addition of surfactants lowers it.

C. Substances to increase viscosity only: Some surfactants may indirectly affect viscosity, but that is not their defining property. Their primary characteristic is adsorption at interfaces and reduction of surface/interfacial tension.

D. Substances to prevent solubility: Surfactants generally enhance the solubility of poorly soluble drugs through micelle formation. They do not prevent solubility.

Example:
Tween 80 improves the solubility of lipophilic drugs.

GPAT / NIPER Tip

Remember the keyword:

“Adsorption at interface + lowering surface tension = Surface-active agent”

Frequently asked linked concepts:

• HLB value

• CMC (Critical Micelle Concentration)

• Wetting agents

• Emulsifying agents

• Detergency

• Micelle formation

Quick Concept Trick: If a substance:

• accumulates at a boundary/interface, and

• lowers surface tension,

Then it is acting as a surfactant.

GPAT Point:

Surfactants possess:

• Hydrophilic part → water-loving

• Lipophilic part → oil-loving

This amphiphilic nature is responsible for their surface activity.
Explanation: Surface-active agents (surfactants) adsorb at interfaces and reduce surface/interfacial tension.

2. A high HLB-value surfactant is

A. Water-insoluble
B. Hydrophobic
C. Lipophilic
D. Hydrophilic

Answer: D. Hydrophilic

Explanation:

HLB stands for Hydrophilic–Lipophilic Balance. It indicates the relative proportion of the hydrophilic (water-loving) and lipophilic (oil-loving) portions of a surfactant.

• High HLB value (≈ 8–18): more hydrophilic

• Low HLB value (≈ 1–6): more lipophilic

Therefore, a surfactant with a high HLB value has a greater affinity for water and is generally used for:

• Oil-in-water (O/W) emulsions

• Solubilization

• Detergency

• Wetting

Examples:

• Tween 80: high HLB: hydrophilic

• Span 80: low HLB: lipophilic

Why Other Options Are Incorrect

A. Water-insoluble: High HLB surfactants are usually water-soluble, not water-insoluble.

B. Hydrophobic: Hydrophobic means “water-repelling.” High HLB surfactants are the opposite — they are more water-attracting.

C. Lipophilic: Lipophilic substances have an affinity for oils/fats and generally possess low HLB values.

GPAT / NIPER Tip. Easy Memory Trick:

High HLB = High Love for Water

• High HLB: Hydrophilic: O/W emulsion

• Low HLB: Lipophilic: W/O emulsion

Important HLB Range (Frequently Asked)

• HLB Range Function

• 1–3 Antifoaming agents

• 3–6 W/O Emulsifying agents

• 7–9 Wetting agents

• 8–18 O/W Emulsifying agents

• 13–18 Detergents/Solubilizers

Exam Points

Surfactants with:

• High HLB → better in aqueous systems

• Low HLB → better in oily systems

This concept is repeatedly asked in

3. The surfactants having an HLB value between 8 and 18 are generally suitable for

A. Multiple emulsion
B. Microemulsion only
C. W/O emulsion
D. O/W emulsion

Answer: D. O/W emulsion

Explanation:

Surfactants with an HLB value between 8 and 18 are predominantly hydrophilic and are generally used as oil-in-water (O/W) emulsifying agents.

In an O/W emulsion:

• Oil droplets are dispersed in water.

• The emulsifier must have a greater affinity for water.

• Hence, surfactants with higher HLB values are preferred.

Examples:

• Tween 20

• Tween 80

These are commonly used to stabilize O/W emulsions in pharmaceutical formulations.

Why Other Options Are Incorrect

A. Multiple emulsion: Multiple emulsions (e.g., W/O/W) require specialized combinations of surfactants and are not defined solely by the HLB range of 8–18.

B. Microemulsion only: Microemulsions require a specific balance of surfactant, co-surfactant, oil, and water. HLB alone does not define microemulsion formation.

C. W/O emulsion: Water-in-oil (W/O) emulsions require lipophilic surfactants with low HLB values (3–6).

Example:

• Span 80: low HLB: W/O emulsifier

GPAT / NIPER Tip

HLB Rule:

• HLB Range Application

• 3–6 W/O emulsifier

• 8–18 O/W emulsifier

Quick Memory Trick:

“High HLB: High water affinity: O/W emulsion”

• O/W emulsion needs a hydrophilic surfactant

• W/O emulsion needs a lipophilic surfactant

Frequently Asked Concept

Difference Between O/W and W/O Emulsion

• Feature O/W W/O

• External phase Water Oil

• Surfactant type Hydrophilic Lipophilic

• HLB range 8–18 3–6

The following are the most repeated topics in the exam:

Questions on:

• HLB ranges

• O/W vs W/O emulsions

• Tween vs Span surfactants

4. Span surfactants are

A. Cationic
B. Amphoteric detergents
C. Lipophilic nonionic surfactants
D. Highly hydrophilic

Answer: C Lipophilic nonionic surfactants
Explanation: Spans (sorbitan esters) possess low HLB values and are lipophilic.

Span surfactants are a group of nonionic surfactants derived from sorbitan esters. They are generally:

• Lipophilic (oil-loving)

• Have low HLB values

• Commonly used as W/O emulsifying agents

Examples:

• Span 20

• Span 60

• Span 80

Because they are nonionic, they do not ionize in solution and are less affected by changes in pH or electrolytes.

Why Other Options Are Incorrect

A. Cationic: Cationic surfactants carry a positive charge.

Examples:

• Cetrimide

• Benzalkonium chloride

Span surfactants are nonionic, not cationic.

B. Amphoteric detergents: Amphoteric surfactants can act as either acidic or basic, depending on the pH.

Examples:

• Lecithin

• Betaines

Span surfactants do not possess both positive and negative charges.

D. Highly hydrophilic: Span surfactants are mainly lipophilic, with low HLB values (around 1.8–8.6). Therefore, they are not highly hydrophilic.

Hydrophilic surfactants are typically Tween surfactants.

GPAT / NIPER Easy Memory Trick:

“SPAN = Soluble in oil”

• Span: lipophilic: low HLB: W/O emulsion

• Tween: hydrophilic: high HLB: O/W emulsion

Comparison

• Property Span Tween

• Nature Nonionic Nonionic

• Affinity Lipophilic Hydrophilic

• HLB Low High

• Emulsion type W/O O/W

Frequently Asked Exam

Examples:

• Span 80: W/O emulsifier

• Tween 80: O/W emulsifier

This comparison is extremely important.

5. Which one is most suitable for Tween 80?

A. Suspending agent
B. Antioxidant
C. Preservative
D. O/W emulsifying agent

Answer: D. O/W emulsifying agent
Explanation: Tween 80 has a high HLB and is widely used for O/W emulsions.

Tween 80 (Polysorbate 80) is a hydrophilic nonionic surfactant with a high HLB value (~15). Because of its strong affinity for water, it is mainly used as an:

• Oil-in-water (O/W) emulsifying agent

• Solubilizing agent

• Wetting agent

It helps disperse oil droplets uniformly in an aqueous phase.

Common pharmaceutical applications:

• Emulsions

• Suspensions

• Injectable formulations

• Solubilization of poorly soluble drugs

Why Other Options Are Incorrect

A. Suspending agent: Suspending agents mainly increase viscosity and keep particles uniformly dispersed.

Examples:

• Tragacanth

• Sodium CMC

• Methylcellulose

Tween 80 primarily acts as a surfactant/emulsifier, not as a suspending agent.

B. Antioxidant: Antioxidants prevent oxidation of drugs.

Examples:

• BHT

• Sodium metabisulfite

• Ascorbic acid

Tween 80 does not function as an antioxidant.

C. Preservative: Preservatives inhibit microbial growth.

Examples:

• Parabens

• Benzalkonium chloride

• Chlorobutanol

Tween 80 has no significant preservative action.

GPAT / NIPER exams memory trick:

"A tween loves water."

Therefore:

• High HLB

• Hydrophilic

• Suitable for O/W emulsions

Frequently Asked Exam Point

Tween surfactants are:

• Nonionic

• Less toxic

• Stable over a wide pH range

• Compatible with many drugs

They are widely used in pharmaceutical formulations and are commonly asked in exams.

6. The detergents' cleaning action depends upon:

A. Neutralization
B. Hydrolysis
C. Oxidation
D. Wetting and micelle formation

Answer: D Wetting and micelle formation
Explanation: Detergents lower surface tension and form micelles that solubilize grease

The cleansing or detergency action of detergents mainly depends on

1. Wetting action

   • Detergents lower surface tension.

   • This allows water to spread easily over dirty surfaces and penetrate grease.

2. Micelle formation

   • Detergent molecules arrange themselves into micelles.

   • The hydrophobic tails trap oily dirt/grease inside the micelle.

   • The hydrophilic heads remain in water, allowing dirt to be washed away.

Thus, detergents clean by emulsifying and solubilizing oily substances through micelle formation.

Why Other Options Are Incorrect

A. Neutralization: Neutralization is an acid–base reaction and is not responsible for the cleaning mechanism of detergents.

B. Hydrolysis: Hydrolysis involves chemical breakdown by water. Detergency is primarily a physical surface phenomenon, not a hydrolysis process.

C. Oxidation: Oxidation may occur with bleaching agents, but ordinary detergents clean mainly through wetting and micelle formation.

GPAT / NIPER Exam Tip:

A detergent molecule has:

• Hydrophobic tail: dissolves grease/oil

• Hydrophilic head: interacts with water

This dual nature is called

Amphiphilic character

Micelle Concept (Very Important)

When detergent concentration exceeds the

Critical Micelle Concentration (CMC) molecules aggregate to form

Micelles

These micelles trap oily dirt and remove it during washing.

Quick Memory Trick:

“Wetting loosens dirt; micelles remove dirt.”

Frequently Asked Exam Points

Questions commonly asked are from:

• CMC

• Micelle formation

• Wetting action

• Surface tension lowering

• Hydrophilic/lipophilic groups

They are extremely important for exams.

7. The critical micelle concentration (CMC): what happens

A. Adsorption stops completely
B. Zero surface tension
C. Formation of micelles begins
D. Surfactant becomes insoluble

Answer: C. Micelles begin to form
Explanation: Above CMC, surfactant molecules aggregate into micelles.

The Critical Micelle Concentration (CMC) is the concentration of a surfactant above which surfactant molecules start aggregating to form micelles in solution.

Before CMC:

• Surfactant molecules mainly accumulate at the interface.

• Surface tension decreases progressively.

At and above CMC:

• The interface becomes saturated.

• Additional surfactant molecules aggregate in the bulk solution to form micelles.

Micelles help in:

• Solubilization

• Detergency

• Emulsification

Why Other Options Are Incorrect

A. Adsorption stops completely: Adsorption does not stop completely. The interface becomes nearly saturated, but equilibrium still exists between adsorbed molecules and those in solution.

B. Surface tension becomes zero: Surface tension decreases significantly up to the CMC, but it never becomes zero.

After CMC, surface tension changes very little.

D. Surfactant becomes insoluble: At CMC, surfactants actually become organized into soluble aggregates (micelles), not insoluble substances.

GPAT / NIPER Tip

Definition to Remember:

CMC = Minimum concentration at which micelles start forming

This is one of the most repeated definitions in pharmaceutical physical chemistry.

Important Behavior Around CMC

• Below CMC At/Above CMC

• Surfactants adsorb at the interface. Micelles form

• Surface tension decreases rapidly Surface tension becomes nearly constant

• Few aggregates Large number of micelles

Quick Memory Trick:

“CMC: Clustering into Micelles Commences."

Exam Point

Properties changing at CMC:

• Surface tension

• Conductivity

• Osmotic pressure

• Solubilization

Graph-based questions on CMC are very common.

8. Which surfactant has the lowest CMC?

A. Inorganic electrolytes
B. Amphoteric buffers
C. Long-chain surfactants
D. Short-chain surfactants

Answer: C. Long-chain surfactants
Explanation: Longer hydrocarbon chains promote micelle formation at lower concentrations.

The Critical Micelle Concentration (CMC) decreases as the hydrophobic chain length of the surfactant increases.

Therefore:

• Long-chain surfactants form micelles more easily.

• They require lower concentration to start micelle formation.

• Hence, they have lower CMC values.

Reason:
Long hydrophobic chains increase hydrophobic interactions, promoting aggregation into micelles.

Why Other Options Are Incorrect

A. Inorganic electrolytes: Inorganic electrolytes are not surfactants and therefore do not possess a characteristic CMC like surfactants.

B. Amphoteric buffers: Buffers are used to maintain pH and are not defined by micelle formation behavior.

D. Short-chain surfactants: Short hydrophobic chains have weaker hydrophobic interactions and require higher concentrations to form micelles.

Therefore:

Short-chain surfactants: higher CMC

GPAT / NIPER Tip, Quick Memory Trick:

Longer hydrocarbon chain → Lower CMC

This is a very important trend question.

“Long tail, less concentration needed for micelles.”

Important Factors Affecting CMC

• Factor Effect on CMC

• Increase chain length Decreases CMC

• Increase hydrophobicity Decreases CMC

• Increase temperature (sometimes) Variable effect

• Addition of electrolytes (ionic surfactants) Usually decreases CMC

Frequently Asked in Exam:

Surfactants with:

• low CMC

• high aggregation tendency

• longer alkyl chains

are more efficient micelle-forming agents.

This concept is commonly asked

9. Detergent action is maximum in:

A. High Contact angle
B. Poor Wetting
C. Zero Contact angle
D. Increased Surface tension

Answer: C. Zero Contact angle
Explanation: A smaller contact angle indicates better wetting and detergency.

Detergency action is closely related to the wetting ability of a liquid. Wetting is measured by the contact angle formed between a liquid and a solid surface.

• Zero contact angle → complete wetting

• The liquid spreads fully over the surface.

• Better penetration into dirt and grease occurs.

• Hence, detergency action becomes maximum.

Surfactants reduce surface tension and contact angle, thereby improving cleaning efficiency.

Why Other Options Are Incorrect

A. High contact angle: A high contact angle indicates poor wetting.
The liquid remains as droplets and cannot spread effectively on the surface.

Result:

• Poor penetration

• Reduced cleaning action

B. Poor Wetting: Detergency depends heavily on efficient wetting. Poor wetting reduces contact between the detergent solution and the dirty surface.

D. Increased Surface tension: High surface tension resists the spreading of liquid.
Detergents work by lowering surface tension, not increasing it.

GPAT / NIPER Tip

Important Relationship:

Lower contact angle: Better wetting: Better detergency

Contact Angle Interpretation

• Contact Angle Wetting

• 0° Complete wetting

• < 90° Good wetting

• > 90° Poor wetting

Quick Memory Trick:

“More spreading = More cleaning”

Thus:

• Low surface tension

• Low contact angle

• Good wetting

• Maximum detergency

all are interconnected concepts.

Exam Point

Questions are frequently asked on

• Contact angle

• Wetting phenomenon

• Surface tension

• Detergency

• Spreading coefficient

10. The angle between a liquid surface and a solid surface is called the:

A. Capillary angle
B. Contact angle
C. Wetting angle
D. Phase angle

Answer: B. Contact angle
Explanation: Contact angle is a measure of wetting behavior.

The contact angle is the angle formed between:

• the tangent to the liquid surface, and

• the solid surface

at the point where the liquid, solid, and air meet.

It is an important measure of

• Wetting

• Spreading

• Adhesion

Interpretation:

• Low contact angle: good wetting

• High contact angle: poor wetting

Example:
Water spreads on clean glass with a very small contact angle, indicating excellent wetting.

Why Other Options Are Incorrect

A. Capillary angle: “Capillary angle” is not the standard term used for this phenomenon in surface chemistry.

C. Wetting angle: Although sometimes informally related to wetting, the correct scientific term is contact angle.

D. Phase angle: Phase angle is used in physics/electronics and does not describe wetting phenomena.

GPAT / NIPER Tip,

Important Relationship:

Contact angle ∝ Poor wetting

• Smaller angle → better spreading

• Larger angle → poorer spreading

Quick Memory Trick:

“Contact angle tells how well liquid contacts the surface.”

Values

• Contact Angle Wetting Behavior

• 0° Complete wetting

• < 90° Good wetting

• > 90° Poor wetting

Frequently Asked Questions in the Exam

Contact angle is important in:

• Suspension formulation

• Tablet granulation

• Coating

• Detergency

• Spreading of liquids

11. Complete wetting occurs when the contact angle is:

A. 0°
B. 45°
C. 90°
D.180°

Answer: A
Explanation: Zero contact angle indicates perfect spreading.

12. Activated charcoal removes coloring matter by which property?

A. Adsorption
B. Absorption
C. Neutralization
D. Sedimentation

Answer: A
Explanation: Adsorption is a surface phenomenon involving accumulation on the surface.

Activated charcoal removes coloring matter mainly by the process of adsorption.

In adsorption:

• Molecules of coloring impurities accumulate on the surface of activated charcoal.

• Activated charcoal has:

  • very large surface area

  • porous structure

• This makes it an excellent adsorbent.

It is widely used for:

• Decolorization

• Purification

• Poison treatment

• Water purification

Why Other Options Are Incorrect

B. Absorption

Absorption involves penetration of a substance into the bulk of another material.

Example:
Water is absorbed by the sponge.

Activated charcoal works mainly through surface phenomena, i.e., adsorption.

C. Neutralization: Neutralization is an acid-base reaction and is unrelated to the removal of coloring matter by charcoal.

D. Sedimentation: Sedimentation involves the settling of particles under gravity. Activated charcoal removes impurities by surface binding, not settling.

GPAT / NIPER Tip

Most Important Difference:

• Adsorption Absorption

• Surface phenomenon Bulk phenomenon

• Molecules accumulate on surface Molecules penetrate inside

• Example: Example:

• Charcoal decolorization Sponge soaking water

Quick Memory Trick:

“Adsorption = molecules ADhere to surface”

Exam Point

Activated charcoal is a classic example of:

• Physical adsorption

• Surface phenomenon

• Large surface area adsorbent

Frequently asked questions about applications:

• Gas masks

• Antidote for poisoning

• Decolorizing agent

• Water purification

13. The Freundlich adsorption isotherm is represented by:

A. PV = nRT
B. x/m = kC^(1/n)
C. F = ma
D. η = F/A

Answer: B
Explanation: The Freundlich isotherm describes adsorption on heterogeneous surfaces.

14. Langmuir adsorption isotherm assumption is

A. A Multilayer adsorption
B. A Heterogeneous surface
C. A Monolayer adsorption on a homogeneous surface
D. A Infinite adsorption

Answer: C Monolayer adsorption on a homogeneous surface
Explanation: Langmuir theory assumes uniform adsorption sites and monolayer formation.

The Langmuir adsorption isotherm is based on the assumption that adsorption occurs as a single molecular layer (monolayer) on a homogeneous surface.

Main Assumptions of Langmuir Isotherm:

1. Surface contains a fixed number of adsorption sites.

2. All sites are identical (homogeneous surface).

3. Each site adsorbs only one molecule.

4. Adsorption forms a monolayer.

5. No interaction occurs between adsorbed molecules.

The Langmuir equation is:

x​/m = abP / 1 + b

where:

• (x/m) = amount adsorbed per unit mass

• (P) = pressure (for gases)

• (a, b) = constants

Why Other Options Are Incorrect

A. Multilayer adsorption: Multilayer adsorption is explained by the BET adsorption theory, not Langmuir theory.

B. Heterogeneous surface: Langmuir theory assumes a homogeneous surface with identical adsorption sites.

D. Infinite adsorption: Langmuir theory assumes a finite number of adsorption sites. Once all sites are occupied, adsorption reaches saturation.

GPAT / NIPER Tip, Remember:

“Langmuir = Monolayer + Homogeneous surface”

This is one of the most frequently asked theoretical concepts.

Quick Comparison

• Langmuir Isotherm BET Theory

• Monolayer adsorption Multilayer adsorption

• Homogeneous surface May involve multilayers

• Finite adsorption sites Continued layer formation

Quick Memory Trick:

“L for Langmuir = L for Layer (single layer)."

Exam Point

Commonly asked concepts:

• Adsorption isotherms

• Physical vs chemical adsorption

• Freundlich isotherm

• Langmuir assumptions

• Activated charcoal adsorption

15. Which one of the adsorption processes is reversible?

A. Covalent adsorption
B. Chemisorption
C. Ionic adsorption only
D. Physical adsorption

Answer: D. Physical adsorption
Explanation: Physical adsorption involves weak van der Waals forces and is reversible.

Physical adsorption (physisorption) is generally reversible because the adsorbate molecules are held on the surface by weak intermolecular forces such as:

• Van der Waals forces

• Dipole interactions

Since these forces are weak, the adsorbed molecules can be easily removed by:

• increasing temperature,

• reducing pressure, or

• changing concentration.

Why Other Options Are Incorrect

A. Covalent adsorption: Covalent adsorption involves strong chemical bond formation and is usually not easily reversible.

B. Chemisorption: Chemisorption involves the formation of strong chemical bonds between the adsorbate and adsorbent.

Characteristics:

• Strong bonding

• Usually irreversible

• High heat of adsorption

C. Ionic adsorption only: Ionic adsorption may involve stronger electrostatic interactions and is not the classic reversible adsorption referred to in surface chemistry questions.

The standard reversible adsorption process is physical adsorption.

GPAT / NIPER Tip

Important Difference:

• Physical Adsorption Chemisorption

• Weak Van der Waals forces Strong chemical bonds

• Reversible Usually irreversible

• Low heat of adsorption High heat of adsorption

• Multilayer possible Usually monolayer

• Favored at low temperature Favored at higher temperature

Quick Memory Trick:

“Physical adsorption is physically weak, so it is reversible.”

Exam Point

Physical adsorption increases with:

• Increase in surface area

• Decrease in temperature

• Increase in pressure (for gases)

16. Chemisorption differs from physical adsorption because it involves:

A. Weak intermolecular force
B. Chemical bond formation
C. No heat evolution
D. Multilayer formation only

Answer: B
Explanation: Chemisorption involves strong chemical bonding.

17. Surface excess concentration is explained by:

A. Arrhenius equation
B. Nernst equation
C. Henderson equation
D. Gibbs adsorption equation

Answer: D. Gibbs adsorption equation
Explanation: Gibbs equation relates surface tension with adsorption.

The Gibbs adsorption equation explains the relationship between:

• surface excess concentration, and

• change in surface tension with concentration.

It describes how surfactant molecules accumulate at an interface.

The Gibbs adsorption equation is:

Γ = −1/RT (dγ/d ln C)

where:

• Γ = surface excess concentration

• γ (gamma) = surface tension

• C = concentration

• R = gas constant

• T = absolute temperature

Interpretation:

• If surface tension decreases with increasing concentration,

• positive adsorption occurs at the interface.

This is especially applicable to surfactants.

Why Other Options Are Incorrect

A. Arrhenius equation: The Arrhenius equation relates to

• rate constant and temperature.

• It is used in chemical kinetics, not adsorption.

B. Nernst equation: The Nernst equation is related to:

• electrode potential

• electrochemistry.

C. Henderson equation: The Henderson–Hasselbalch equation is used for:

• buffer pH calculations.

• It has no relation to surface excess concentration.

GPAT / NIPER Tip, Concept:

The Gibbs adsorption equation connects adsorption with surface tension changes.

Quick Memory Trick:

“Gibbs = Gathering at Interface”

Meaning:
Surfactant molecules gather at the surface/interface.

Exam Point

According to Gibbs equation:

• Surface-active agents accumulate at interfaces.

• Surface tension decreases as adsorption increases.

This concept is highly important for:

• Surface chemistry

• Surfactants

• Interfacial phenomena

• Detergency

18. Select a monolayer formed after very high attractive forces between molecules.

A. Micellar layer
B. Gaseous monolayer
C. Condensed monolayer
D. Expanded monolayer

Answer: C. Condensed monolayer
Explanation: Strong intermolecular attraction results in closely packed condensed films.

A condensed monolayer is formed when the attractive forces between molecules are very high.

Characteristics:

• Molecules are tightly packed.

• Strong intermolecular attraction exists.

• The film behaves like a condensed liquid or solid layer.

• Molecular mobility is low.

Such monolayers are commonly observed with long-chain fatty substances at interfaces.

Why Other Options Are Incorrect

A. Micellar layer: Micelles are aggregates formed in bulk solution above the CMC and are not monolayers at an interface.

B. Gaseous monolayer: In gaseous monolayers:

• molecules are far apart,

• Intermolecular attraction is very weak.

This is the opposite of the condition given in the question.

D. Expanded monolayer: Expanded monolayers have

• moderate intermolecular attraction,

• loosely packed molecules,

• greater molecular mobility compared to condensed films.

They are less tightly packed than condensed monolayers.

GPAT / NIPER Tip

Types of Monolayers Based on Molecular Packing

• Type Molecular Attraction Packing

• Gaseous monolayer Very low Widely separated

• Expanded monolayer Moderate Loosely packed

• Condensed monolayer Very high Tightly packed

Quick Memory Trick:

“Condensed = Compressed molecules."

Hence:

• high attractive force

• close packing

• rigid film

Exam Point Takeaway

Monolayer questions are frequently asked about:

• Surface chemistry

• Langmuir films

• Surface-active agents

• Interfacial phenomena

Important associated concepts:

• Surface pressure

• Molecular packing

• Film expansion/compression

19. In expanded monolayers, molecules are:

A. Present as precipitate
B. Closely packed
C. Covalently linked
D. Randomly dispersed with a larger intermolecular space

Answer: D. Randomly dispersed with a larger intermolecular space
Explanation: Expanded films have loosely packed molecules.

In an expanded monolayer, the molecules are

• loosely arranged,

• relatively far apart,

• and possess greater mobility.

The intermolecular attractive forces are moderate, so the molecules are not tightly packed as in condensed monolayers.

Characteristics of expanded monolayers:

• Larger intermolecular distance

• Flexible film

• Greater molecular motion

• Lower surface pressure compared to condensed films

Why Other Options Are Incorrect

A. Present as precipitate: Expanded monolayers exist as surface films at interfaces, not as precipitates.

B. Closely packed: Closely packed molecules are characteristic of condensed monolayers, not expanded monolayers.

C. Covalently linked: Monolayer molecules are generally held together by intermolecular forces, not covalent bonds.

GPAT / NIPER Tip

Comparison of Monolayers

• Property Expanded Monolayer Condensed Monolayer

• Molecular packing Loose Tight

• Intermolecular space Large Small

• Attractive force Moderate Strong

• Mobility High Low

Quick Memory Trick:

“Expanded = molecules spread apart.”

Exam Point

Questions commonly focus on:

• Gaseous monolayer

• Expanded monolayer

• Condensed monolayer

• Molecular packing

• Surface pressure

20. Which one is an anionic surfactant?

A. Benzalkonium chloride
B. Sodium lauryl sulfate
C. Tween 20
D. Cetrimide

Answer: B. Sodium lauryl sulfate
Explanation: Sodium lauryl sulfate (SLS) carries a negative charge.

Sodium lauryl sulfate is a classic anionic surfactant because it carries a negative charge on its hydrophilic part in an aqueous solution.

Characteristics of anionic surfactants:

• Excellent detergency

• Good foaming property

• Widely used in shampoos, toothpaste, and pharmaceutical formulations

The negatively charged group in sodium lauryl sulfate is the sulfate ion.

Why Other Options Are Incorrect

A. Benzalkonium chloride: Benzalkonium chloride is a cationic surfactant because it carries a positive charge.

It is mainly used as

• preservative

• disinfectant

• antiseptic

C. Tween 20: Tween 20 is a nonionic surfactant.

Characteristics:

• No charge

• Hydrophilic

• Used as an O/W emulsifying agent

D. Cetrimide: Cetrimide is a cationic surfactant with antiseptic and disinfectant properties.

GPAT / NIPER Tip

Classification of Surfactants (This classification is repeatedly asked.)

• Type Charge Example

• Anionic Negative Sodium lauryl sulfate

• Cationic Positive Cetrimide, Benzalkonium chloride

• Nonionic No charge Tween 20, Span 80

• Amphoteric Both + and − Lecithin

Exam Point

Important Uses of Different Surfactants:

• Anionic: detergents, foaming agents

• Cationic: antiseptics, preservatives

• Nonionic: emulsifiers

• Amphoteric: mild surfactants

21. Name the class of surfactants to which cetrimide belongs

A. Cationic
B. Anionic
C. Nonionic
D. Amphoteric

Answer: A. Cationic
Explanation: Cetrimide is a quaternary ammonium cationic surfactant.

Cetrimide belongs to the class of cationic surfactants because it carries a positive charge on the hydrophilic portion of the molecule.

Cetrimide is a quaternary ammonium compound and is widely used as:

• antiseptic

• disinfectant

• preservative

Cationic surfactants possess:

• antimicrobial activity

• surface-active properties

Why Other Options Are Incorrect

B. Anionic: Anionic surfactants carry a negative charge.

Example:

• Sodium lauryl sulfate

Cetrimide is positively charged, not negatively charged.

C. Nonionic: Nonionic surfactants do not possess any ionic charge.

Examples:

• Tween 80

• Span 80

Cetrimide contains a charged ammonium group.

D. Amphoteric: Amphoteric surfactants can behave as both positive and negative ions depending on pH.

Example:

• Lecithin

Cetrimide remains positively charged and is not amphoteric.

Quick Memory Trick:

“Cetrimide = CATionic”

The “CET/CAT” sound helps remember that it is cationic.

Exam Point Takeaway

Properties of Cationic Surfactants:

• Good antimicrobial action

• Used as preservatives and disinfectants

• Incompatible with anionic surfactants

This incompatibility is a very frequently asked GPAT/NIPER concept.

22. Which surfactant type generally possesses maximum antimicrobial activity?

A. Nonionic
B. Cationic
C. Anionic
D. Amphoteric

Answer: B
Explanation: Cationic surfactants disrupt microbial cell membranes.

23. Zeta potential definition is the potential difference between the

A. Two immiscible liquids
B. Stern layer and bulk liquid
C. Solid surface and vacuum
D. Electrode and electrolyte

Answer: B. Stern layer and bulk liquid
Explanation: Zeta potential exists between the slipping plane and the bulk liquid

The zeta potential is the electrical potential difference between

• the stationary layer of liquid attached to the particle surface (Stern layer/slipping plane), and

• The bulk liquid surrounding the particle.

It is an important parameter for determining:

• stability of colloidal dispersions,

• emulsions,

• suspensions.

Significance:

• High zeta potential → strong repulsion between particles → stable system

• Low zeta potential → particle aggregation/flocculation

Why Other Options Are Incorrect

A. Two immiscible liquids: Potential difference between immiscible liquids is related to interfacial phenomena, not specifically zeta potential.

C. Solid surface and vacuum: This does not describe the electrokinetic potential involved in colloidal systems.

D. Electrode and electrolyte: This refers to electrochemical electrode potential, not zeta potential.

GPAT / NIPER Tip

Important Concept:

A colloidal particle in solution develops:

1. Surface charge

2. Electrical double layer

3. Zeta potential

Quick Memory Trick:

“Zeta potential measures particle stability.”

Higher magnitude (positive or negative):

• better repulsion

• better stability

Exam Point

Applications of Zeta Potential:

• Stability of suspensions

• Emulsion stability

• Flocculation studies

• Colloidal characterization

24. High zeta potential means

A. Poor stability
B. Zero electrostatic repulsion
C. Greater particle aggregation
D. Better dispersion stability

Answer: D
Explanation: A high zeta potential produces electrostatic repulsion, preventing aggregation.

25. Electrolyte addition to a colloidal dispersion causes:

A. Remove adsorbed ions
B. Compress the electrical double layer
C. Increase in zeta potential indefinitely
D. Prevent coagulation completely

Answer: B. Compress the electrical double layer
Explanation: Electrolytes compress the diffuse double layer and reduce repulsion.

When an electrolyte is added to a colloidal dispersion, the ions from the electrolyte accumulate around the charged colloidal particles and compress the electrical double layer.

As a The

• zeta potential decreases,

• electrostatic repulsion between particles decreases,

• particles come closer together,

• Coagulation/flocculation may occur.

This is the basic mechanism behind electrolyte-induced coagulation of colloids.

Why Other Options Are Incorrect

A. Remove adsorbed ions: Electrolytes do not primarily remove adsorbed ions; instead, they neutralize or shield surface charges and compress the double layer.

C. Increase in zeta potential indefinitely: Electrolyte addition generally reduces zeta potential by compressing the diffuse layer.

Lower zeta potential promotes instability.

D. Prevent coagulation completely: Electrolytes usually promote coagulation rather than prevent it, especially at higher concentrations.

GPAT / NIPER Tip

Important Sequence:

Electrolyte addition → Double layer compression → Reduced zeta potential → Coagulation

Electrical Double Layer Concept

• Layer Description

• Stern layer Tightly bound ions

• Diffuse layer Loosely distributed ions

• Compression of diffuse layer Reduces repulsion

Quick Memory Trick:

“More electrolyte → less repulsion → more coagulation.”

Exam Point Takeaway

Stable colloids require

• high zeta potential,

• strong electrostatic repulsion.

Coagulation occurs when:

• zeta potential decreases,

• particles aggregate.

26. The Nernst potential arises due to:

A. Micelle formation
B. Temperature change only
C. Surface tension gradient
D. Difference in ionic concentration across the membrane

Answer: D. Difference in ionic concentration across the membrane
Explanation: Nernst potential develops because of unequal ion distribution.

The Nernst potential (or equilibrium potential) develops due to a difference in ionic concentration across a membrane or interface.

When ions are unevenly distributed across two sides of an interface:

• ions tend to diffuse from higher to lower concentration,

• electrical potential develops,

• Equilibrium is reached when electrical and chemical forces balance.

The Nernst equation expresses this relationship:

E = RT/nF ​ln C₁/C₂

where:

• (E) = Nernst potential

• (R) = gas constant

• (T) = temperature

• (n) = valency of ion

• (F) = Faraday constant

• (C1, C2) = ionic concentrations

Why Other Options Are Incorrect

A. Micelle formation: Micelle formation is related to surfactants and CMC, not electrode or membrane potentials.

B. Temperature change only: Temperature affects the magnitude of Nernst potential, but the primary cause is concentration difference of ions.

C. Surface tension gradient: Surface tension gradients are associated with Marangoni effects and interfacial phenomena, not Nernst potential.

GPAT / NIPER Tip

Nernst potential = concentration gradient potential

Quick Memory Trick:

“Different ion concentration creates electrical potential.”

Exam Point

The Nernst equation is important in

• Electrochemistry

• Membrane transport

• pH measurement

• Biological potentials

• Electrodes

27. The electrical double layer consists of:

A. Two solid phases
B. Stern layer and diffuse layer
C. Adsorbed gases only
D. Two immiscible liquids

Answer: B
Explanation: The electrical double layer includes a fixed Stern layer and mobile diffuse layer.

28. Name a factor that decreases the gases' adsorption on solids.

A. Increase in pressure
B. Increase in surface area
C. Decrease in vapour pressure
D. Increase in temperature

Answer: D
Explanation: Gas adsorption is generally exothermic; a higher temperature decreases adsorption.

29. Wetting agents act by:

A. Increasing viscosity
B. Reducing interfacial tension
C. Increasing contact angle
D. Producing electrical charges

Answer: B
Explanation: Wetting agents improve spreading by lowering interfacial tension.

30. Which of the following is the most suitable wetting agent in suspensions?

A. Talc
B. Sodium lauryl sulfate
C. Acacia
D. Starch

Answer: B. Sodium lauryl sulfate
Explanation: SLS lowers interfacial tension and improves wetting of hydrophobic powders.

Sodium lauryl sulfate is widely used as a wetting agent in pharmaceutical suspensions.

Wetting agents:

• reduce surface tension,

• decrease contact angle,

• displace air from powder surfaces,

• improve dispersion of hydrophobic powders in liquid.

Since sodium lauryl sulfate (SLS) is an anionic surfactant with excellent wetting properties, it is highly suitable for suspension formulations.

Why Other Options Are Incorrect

A. Talc: Talc is mainly used as:

• glidant,

• antiadherent,

• dusting powder.

• Adsorbent.

It does not act as a wetting agent.

C. Acacia: Acacia is primarily used as:

• suspending agent,

• emulsifying agent,

• binder.

Although it may aid dispersion, it is not the most suitable wetting agent compared with surfactants like SLS.

D. Starch: Starch is mainly used as:

• binder,

• disintegrant,

• thickening agent.

It is not primarily used for wetting hydrophobic particles.

GPAT / NIPER Tip

Role of Wetting Agents in Suspensions:

Hydrophobic powders tend to float because air remains adsorbed on their surface.

Wetting agents:

• lower interfacial tension,

• replace air,

• allow easy penetration of liquid.

Common Wetting Agents:

• Sodium lauryl sulfate

• Polysorbates (Tweens)

Common Suspending Agents:

• Acacia

• Tragacanth

• Sodium CMC

Questions comparing:

• wetting agents,

• suspending agents,

• emulsifiers

31. Hydrophile-Lipophile Balance (HLB) system was proposed by:

A. Ostwald
B. Freundlich
C. Griffin
D. Langmuir

Answer: C
Explanation: Griffin introduced the HLB system for surfactants.

32. Which surfactant is most suitable for W/O emulsion formation?

A. Tween 20
B. Tween 80
C. Span 80
D. Sodium lauryl sulfate

Answer: C
Explanation: Span 80 has low HLB and favors W/O emulsions.

33. Which statement about adsorption is TRUE?

A. Adsorption never reaches equilibrium
B. Adsorption decreases with an increase in surface area
C. Adsorption is a surface phenomenon
D. Adsorption is a bulk phenomenon

Answer: C
Explanation: Adsorption occurs at interfaces or surfaces.

34. The effectiveness of activated charcoal as an adsorbent is due to

A. High viscosity
B. High density
C. Chemical instability
D. Large surface area

Answer: B
Explanation: A finely porous structure gives an enormous surface area.

35. Which electrolyte is most efficient at causing coagulation in negatively charged sols?

A. Na⁺
B. K⁺
C. Ca²⁺
D. Al³⁺

Answer: D. Al³⁺
Explanation:

According to the Hardy–Schulze rule, the coagulating power of an electrolyte depends mainly on the valency of the ion carrying the opposite charge to the colloidal particles.

For a negatively charged sol:

• Positively charged ions (cations) cause coagulation.

• The higher the valency of the cation, the greater is the coagulating power.

Thus:
Al³⁺>Ca²⁺>Na⁺≈K⁺

Therefore, Al³⁺ is the most effective coagulating ion.

Why Other Options Are Incorrect

A. Na⁺: Sodium ion is monovalent (+1), so its coagulating power is relatively weak.

B. K⁺: Potassium ions are also monovalent and have low coagulating efficiency.

C. Ca²⁺: Calcium ions have higher coagulating power than Na⁺ and K⁺ because it is divalent, but it is still less effective than Al³⁺.

GPAT / NIPER Tip

Hardy–Schulze Rule:

“The higher the valency of an oppositely charged ion, the greater the coagulating power.”

Quick Memory Trick:

3+ beats, 2+ beats, 1+

For negatively charged sols:
Al³⁺>Ca²⁺>Na⁺

Exam Point

For Positively Charged Sols:

Negative ions cause coagulation, and their effectiveness increases as
PO43−​>SO42−​>Cl−

Frequently Asked Topics

• Zeta potential

• Electrical double layer

• Coagulation of colloids

• Gold number

• Hardy–Schulze rule

GPAT / NIPER Revision Tips

• High HLB → O/W emulsion

• Low HLB → W/O emulsion

• Adsorption = surface phenomenon

• Absorption = bulk phenomenon

• High zeta potential = better stability

• Contact angle ↓ → wetting ↑

• Activated charcoal works by adsorption

• Cationic surfactants = antimicrobial activity

Dr. Alok Singh