Surface and interfacial phenomenon: Complex films and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam

Surface and interfacial phenomenon: Complex films and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam

SURFACE FILMS: Surface-active agents tend to concentrate at interfaces and are adsorbed at oil–water interfaces as monomolecular films. These monomolecular films formed at the interface depend on the nature, characteristics, concentration and combination of the surfactant.

Insoluble Monomolecular Films: Insoluble amphiphilic compounds such as surfactants with very long hydrocarbon chains form films one molecule in thickness on water surfaces when the surfactant is dissolved in a volatile solvent and carefully injected onto the surface. The molecules are orientated at the surface in the same way as typical surfactants, i.e. with the hydrophobic group protruding into the air and the polar group acting as an anchor in the surface. The thickness of such films can be determined if the area of the film and the volume of the spreading liquid are known. The properties of the film can be studied using the film balance.

The surfactant is dissolved in a volatile solvent and is spread as a film on the surface of water (substrate). The volatile solvent evaporates, leaving behind the surfactant film over water. With the help of stationary and movable barrier, the area of the film is determined and the movable barrier is then forced to move towards the stationary barrier, thus compressing the film gradually. At each position, the area of the film (A) and the film pressure (S) are measured. The results are presented as plots of area of film against the film pressure (S = Jo – Jm, where Jo is the surface tension of water surface and Jm the surface tension of the film-covered surface), called S–A curves.

Gaseous films: In gaseous films, the adsorbed surfactant molecules do not adhere to each other laterally, and move freely around the interface. The charged groups repel one another in the aqueous solution as the droplet covered with the film moves closer to one another. In such films, there is only a gradual change in the surface pressure as the film is compressed. One example of a gaseous film is that formed by the anionic surfactant, sodium dodecyl sulphate.

Expanded films: Films formed by oleic acid are expanded. The hydrocarbon chains in oleic acid are less cohesive and less orderly packed due to the higher polarity and affinity for water. The presence of branched and bent-shaped hydrocarbon chains, bulky head groups and multiple polar groups causes lateral cohesion to be reduced and films to expand. The S–A curves are quite steeply curved but extrapolation to a limiting surface area yields a value that is usually several times greater than the cross-sectional area from molecular models.

Condensed films: If the concentration of the surfactant is high, it forms a rigid film between the immiscible phases and acts as a mechanical barrier to both adhesion and coalescence of the liquid droplets. The molecules of the long straight-chain fatty acids, such as palmitic acids, are more tightly packed due to the cohesive contact of hydrocarbon chains. As the chains interlock, the molecules do not freely move in the interface, leading to a stable emulsion. In these films, the film pressure remains very low at high film areas and increases abruptly when the molecules become tightly packed on compression.

Interfacial Complex Condensed Films: To improve stability, the combinations of surfactants are often used rather than a single surfactant. Combination of a water-soluble surfactant that produces a gaseous film and an oil-soluble auxiliary surfactant produces a stable interfacial complex condensed film. This film is flexible, highly viscous, coherent, elastic and resistant to rupture since the molecules are efficiently packed between each other. Thus, a tightly packed surfactant film explains the well-known fact that mixed surfactants are often more effective than single surfactants. The ability of the mixture of surfactants to pack more tightly contributes to the strength of the film, and hence, to the enhanced stability.

Lamellar Liquid Crystalline Films: Stable emulsions are believed to comprise liquid crystalline layers on the interface of emulsified droplets with the continuous phase. Mixed emulsifiers can interact with water to form three- dimensional association structures. Emulsions should be viewed as three-component systems comprising oil, water and lamellar liquid crystals, the latter consisting of consecutive layers of water–emulsifier–oil–water.

 

Multiple choice questions (MCQs)

1.Surface-active agents tend to concentrate at interfaces and are adsorbed at oil–water interfaces as monomolecular films. These monomolecular films formed at the interface depend on which of the following?

a)Nature

b)Characteristics

c)Concentration and combination of the surfactant

d)All of these

2.The properties of the film can be studied using 

a)Film balance

b)Viscometer

c)Stalagmometer

d)All of these

3.Which of the films are expanded films?

a)Films formed by oleic acid

b)Film formed by dissolving surfactant in a volatile solvent

c)Film formed if the concentration of the surfactant is high

d)All of these

4.Which of the following is condensed film?

a)Films formed by oleic acid

b)Film formed by dissolving surfactant in a volatile solvent

c)Film formed if the concentration of the surfactant is high

d)All of these

5.Which of the following is gaseous film?

a)Films formed by oleic acid

b)Film formed by dissolving surfactant in a volatile solvent

c)Film formed if the concentration of the surfactant is high

d)If adsorbed surfactant molecules do not adhere to each other laterally

6.Why Interfacial Complex Condensed Films are formed?

a)To improve stability

b)To increase solubility

c)To increase viscosity

d)All of these

7.The ability of the mixture of surfactants to pack more tightly contributes to the 

a)Strength of the film

b)Enhanced stability

c)Both of these

d)None of these

8.What is the main result of adding surfactants into a liquid composed of two immiscible phases such as oil and water?

a)Reduction in the interfacial tension between the phases

b)Increase in the interfacial tension between the phases

c)Catalysation of a chemical reaction between the phases

d)Nothing happens

9.Insoluble amphiphilic compounds such as surfactants with very long hydrocarbon chains form films one molecule in thickness on water surfaces when the surfactant is dissolved in a volatile solvent and carefully injected onto the surface. This type of layer formed is called 

a)Insoluble Monomolecular Films

b)Gaseous film

c)Expanded films

d)Condensed films

10.S–A curves are plotted in which type of films?

a)Insoluble Monomolecular Films

b)Gaseous film

c)Expanded films

d)Condensed films

11.Which of the following is not an adsorbent?

a) Carbon

b) Polymers and resins

c) Clay

d)  Dry sponge

12.What do you mean by the term “Sorption”?

a)Attachment

b)Detachment

c)Diffusion

d)Thermal Expansion

13.One example of a gaseous film is

a)Film that is formed by the anionic surfactant, sodium dodecyl sulphate

b)Water–emulsifier–oil–water film

c) Both of these

d)None of these

14.Which of the following statements regarding the physical adsorption of a gas on surface of solid is not correct?

a)On increasing temperature, adsorption increases continuously

b)Enthalpy changes are negative

c)Adsorption is specific

d)It is reversible in nature

15.Which of the following is not characteristic of chemisorption?

a)It is irreversible

b)It is specific

c)It is multilayer phenomenon

d)Heat of adsorption is about 400kj

Solutions:

  1. d)All of these
  2. a)Film balance
  3. a)Films formed by oleic acid
  4. c)Film formed if the concentration of the surfactant is high
  5. d)If adsorbed surfactant molecules do not adhere to each other laterally
  6. a)To improve stability
  7. c)Both of these
  8. a) Reduction in the interfacial tension between the phases
  9. a)Insoluble Monomolecular Films
  10. a)Insoluble Monomolecular Films
  11. d) Dry sponge
  12. a) Attachment
  13. a)Film that is formed by the anionic surfactant, sodium dodecyl sulphate
  14. a) On increasing temperature, adsorption increases continuously
  15. c) It is multilayer phenomenon

References:

1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 127-128.

2. Martins Physical Pharmacy, 6th edition 2011, page no. 681-698.

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