Surface and Interfacial Phenomenon One Shot Notes and MCQs

Surface and Interfacial Phenomenon One Shot Notes

1. Liquid Interface

Interface: Boundary between two phases (e.g., liquid-liquid, liquid-gas, or liquid-solid).
Liquid interface: Forms when a liquid contacts either a solid, gas, or another immiscible liquid. Examples: water-air interface or oil-water interface.
Important in drug formulation, especially emulsions, suspensions, and foams.

2. Surface and Interfacial Tensions

Surface tension: Force per unit length that acts along the surface of a liquid, minimizing surface area.
Interfacial tension: Force per unit length at the interface between two immiscible liquids or a liquid and solid.
Units: Dyne/cm or N/m.
Example: Water exhibits high surface tension compared to organic solvents like ethanol.

Factors affecting surface tension:

Temperature (increases temperature → decreases surface tension).
Presence of surfactants (lowers surface tension).

3. Surface Free Energy

Energy required to increase the surface area of a liquid by a unit amount.
Higher surface energy indicates stronger molecular interactions at the surface.
Application: Helps in understanding wettability and spreading behavior, critical for tablet coating and emulsification.

4. Measurement of Surface and Interfacial Tensions

Capillary rise method: Based on the height to which a liquid rises in a thin tube.
Drop weight method: Measures the weight of drops falling from a nozzle.
Wilhelmy plate method: Measures force exerted by a liquid meniscus on a plate.
Du Noüy ring method: Measures the force required to detach a ring from the liquid surface.
Pendant drop method: Analyzes the shape of a droplet to determine interfacial tension.

5. Spreading Coefficient

Describes the tendency of a liquid to spread over another liquid or solid surface.
Formula:
S=γs−(γl+γsl)S = \gamma_s – (\gamma_l + \gamma_{sl})S=γs−(γl+γsl)
Where:
- SSS = Spreading coefficient
- γs\gamma_sγs = Surface tension of substrate
- γl\gamma_lγl = Surface tension of liquid
- γsl\gamma_{sl}γsl = Interfacial tension between liquid and substrate
Application: Critical in emulsification and wetting behavior.

6. Adsorption at Liquid Interfaces

Adsorption: Process where molecules accumulate at the surface or interface.
Surface-active agents (surfactants) reduce surface tension by adsorbing at the liquid interface.
Types of surfactants:
- Anionic: Sodium lauryl sulfate
- Cationic: Cetyltrimethylammonium bromide
- Nonionic: Polysorbates (Tween)
- Amphoteric: Lecithin
Application: Surfactants are used in emulsions, suspensions, and detergents.

7. Hydrophilic-Lipophilic Balance (HLB) Classification

HLB value: Measures the balance between hydrophilic and lipophilic groups in a surfactant.
HLB scale:
- < 9: Lipophilic (used in water-in-oil emulsions)
- 9: Hydrophilic (used in oil-in-water emulsions)
Applications:
- Emulsions: Tweens and Spans are common emulsifiers.
- Solubilization of poorly water-soluble drugs.

8. Solubilization and Detergency

Solubilization: Process where water-insoluble substances are incorporated into micelles formed by surfactants, improving drug solubility.
Detergency: Ability of a surfactant to remove dirt or oil from surfaces by forming micelles.

9. Adsorption at Solid Interfaces

Solid-Gas Interface: Gas molecules adsorbed on solid surfaces (e.g., adsorption of oxygen on activated charcoal).
Solid-Liquid Interface: Liquid molecules adsorbed on solid surfaces, crucial in chromatography and tablet formulations.

Adsorption types:

Physical adsorption: Weak van der Waals forces.
Chemical adsorption: Involves chemical bonding, stronger than physical adsorption.

10. Complex Films at Interfaces

Complex films: Monolayers or multilayers of molecules at the liquid or solid interfaces.
Application: Formation of protective layers in emulsions or barriers on solid surfaces to prevent degradation.
Example: Use of phospholipids in liposomes.

11. Electrical Properties of Interfaces

Zeta potential: Electrical potential at the slipping plane of a particle in a liquid.
Significance: High zeta potential indicates stability of colloidal dispersions.
Electrical double layer: Formed at the interface between a charged solid and surrounding liquid.
- Stern layer: Tightly bound layer of ions on the particle surface.
- Diffuse layer: Loosely associated ions surrounding the Stern layer.
Applications: Zeta potential is used to assess the stability of suspensions and emulsions.

MCQs on Surface and Interfacial Phenomenon

What is surface tension?
a) Force acting on the volume of a liquid
b) Force per unit length acting along the surface of a liquid
c) Energy absorbed by a liquid
d) Pressure applied at the surface
Answer: b
Which method is commonly used to measure interfacial tension?
a) Sedimentation method
b) Du Noüy ring method
c) Centrifugation
d) Rheometry
Answer: b
Which factor decreases the surface tension of a liquid?
a) Cooling the liquid
b) Adding surfactants
c) Increasing viscosity
d) Applying pressure
Answer: b
What does the HLB value indicate?
a) Surface charge
b) Balance between hydrophilic and lipophilic properties of a surfactant
c) Weight of the surfactant in the formulation
d) Molecular weight
Answer: b
What type of surfactants typically have HLB values above 9?
a) Lipophilic surfactants
b) Hydrophilic surfactants
c) Amphoteric surfactants
d) Zwitterionic surfactants
Answer: b
Which of the following methods can be used to measure surface tension?
a) X-ray diffraction
b) Wilhelmy plate method
c) Chromatography
d) Viscosity measurement
Answer: b
What is the primary function of surfactants in emulsions?
a) Increase viscosity
b) Reduce surface tension between oil and water phases
c) Increase solubility of all components
d) Decrease temperature
Answer: b
What is solubilization?
a) Removal of solid particles from a solution
b) Incorporation of water-insoluble substances into micelles
c) Separation of immiscible phases
d) Increase in surface energy
Answer: b
Which surfactant is commonly used in water-in-oil (W/O) emulsions?
a) Tween 80
b) Span 60
c) Sodium dodecyl sulfate
d) Polysorbate 20
Answer: b
Which type of adsorption occurs through weak van der Waals forces?
a) Chemisorption
b) Physical adsorption
c) Electrostatic adsorption
d) Hydrophobic bonding
Answer: b
What is zeta potential?
a) The mass of charged particles in a liquid
b) The potential difference between the bulk liquid and the surface of a solid
c) The electrical potential at the slipping plane of a particle
d) The viscosity of a suspension
Answer: c
What happens to the zeta potential when a suspension becomes unstable?
a) Increases
b) Decreases
c) Remains constant
d) Becomes zero
Answer: b
What is detergency?
a) Reduction of particle size
b) Removal of dirt or oil from a surface using surfactants
c) Adsorption of gases on solid surfaces
d) Formation of emulsions
Answer: b
Which of the following is an example of a solid-gas interface?
a) Oxygen adsorbed on activated charcoal
b) Water on glass
c) Oil on the surface of water
d) Protein binding in plasma
Answer: a
Which property is measured by the spreading coefficient?
a) Viscosity
b) Tendency of a liquid to spread over another surface
c) Electrical conductivity
d) Adsorption potential
Answer: b
What is the significance of the electrical double layer in colloidal systems?
a) Reduces viscosity
b) Increases solubility
c) Stabilizes colloidal particles by preventing aggregation
d) Increases surface tension
Answer: c
Which technique is used to measure surface tension using a thin tube?
a) Capillary rise method
b) Sedimentation method
c) Drop-weight method
d) Ultrafiltration
Answer: a
What type of adsorption involves chemical bonding at the interface?
a) Physical adsorption
b) Chemisorption
c) Electrostatic adsorption
d) Hydrophilic adsorption
Answer: b
Which surfactant is amphoteric?
a) Sodium lauryl sulfate
b) Cetyltrimethylammonium bromide
c) Lecithin
d) Polysorbate 80
Answer: c
What is the purpose of complex films at the interface?
a) To increase surface area
b) To act as a barrier to degradation
c) To increase solubility of drugs
d) To reduce viscosity
Answer: b
Which factor increases surface tension?
a) Addition of surfactants
b) Increasing temperature
c) Decreasing temperature
d) Stirring the solution
Answer: c
What does a low HLB value indicate?
a) The surfactant is highly hydrophilic
b) The surfactant is highly lipophilic
c) The surfactant has no effect on the interface
d) The surfactant is amphoteric
Answer: b
What does a higher zeta potential indicate in a colloidal system?
a) Lower viscosity
b) Higher stability
c) Increased particle aggregation
d) Decreased electrical conductivity
Answer: b
Which method involves measuring the shape of a droplet to determine surface tension?
a) Capillary rise method
b) Wilhelmy plate method
c) Pendant drop method
d) Ultrasonic method
Answer: c
Which film type forms at a liquid-liquid interface?
a) Micellar film
b) Complex film
c) Monolayer film
d) Polymer film
Answer: c
Which parameter is essential for assessing the flow properties of powders?
a) Zeta potential
b) Bulk density
c) Interfacial tension
d) Spreading coefficient
Answer: b
Which surfactant is commonly used in the pharmaceutical industry to increase solubility?
a) Polysorbate 80
b) Span 20
c) Sodium benzoate
d) Magnesium stearate
Answer: a
What does a negative spreading coefficient indicate?
a) Spreading will occur
b) Spreading will not occur
c) The liquid is miscible
d) Surface energy is zero
Answer: b
What is the primary role of surface-active agents in detergency?
a) Increasing surface tension
b) Forming micelles to remove oils and dirt
c) Increasing viscosity
d) Reducing bulk density
Answer: b
In the Wilhelmy plate method, the force measured is due to:
a) Friction
b) Surface tension acting on the plate
c) Bulk density
d) Temperature changes
Answer: b