Physical and chemical adsorption, Adsorption Isotherm, Adsorption phenomenon(Part1) and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam
ADSORPTION PHENOMENA: Adsorption is the process in which materials of one phase (adsorbate) accumulate or concentrate at the interfacial surface of the other phase (adsorbent). It is a spontaneous phenomenon driven by a reduction of the surface free energy. Adsorption occurs at the interfaces of two phases such as liquid/liquid, gas/liquid, liquid/solid or gas/solid.
Two different types of adsorption processes exist: physical adsorption (physisorption) and chemical adsorption (chemisorption).
In physical adsorption, adsorbates adsorb on the surface of a solid by van der Waals forces, which are relatively weak and nonspecific forces. The adsorbate is not fixed to the surface of the solid and can move freely within the interfacial surface. Physical adsorption is fast, reversible and results in multilayer adsorption.
In chemical adsorption, the substance is held on the surface of a solid by specific covalent forces between the adsorbate and the adsorbent. The chemically adsorbed materials are not free to move on the surface. Chemical adsorption is slow, not readily reversible and results in monolayer adsorption.
Table 1 – Physical and chemical adsorption
| Parameter | Physical adsorption | Chemical adsorption |
| Force | Weak van der Waals forces | Strong forces |
| Specificity | Non-Specificity | Specificity |
| Reversibility | Reversible | Often irreversible |
| Effect of temperature | Exothermic, adsorption decreases as temperature increases | Surface reaction proceeds above certain temperature |
| Adsorbed layers | Multilayer formation | Monolayer formation |
Adsorption is a dynamic phenomenon that is opposed by desorption, i.e. the transfer of a surfactant to a bulk phase. The adsorption and desorption steps are often very rapid; consequently, adsorption–desorption equilibrium is reached after some time, which depends on the surfactant concentration in the bulk phase. Since the surfactant has a lower free energy when it is adsorbed at the interface than in the solvent bulk phase, the equilibrium is displaced towards the adsorbed state. In fact, the interface is rapidly covered by a monolayer of surfactant and everything happens as if the interface is coated with a thin layer of a new material.
Adsorption Isotherm: When a substance moves away from a solution and adsorbs at the surface of a solid, the concentration of the solute remaining in solution is in dynamic equilibrium with the adsorbed concentration at the surface. This distribution ratio of the solute in solution and at the surface is a measure of the adsorption equilibrium. Adsorption studies using gases generally involve the determination of the amount of gas adsorbed, x, by a given mass, m, of the adsorbent at constant temperature. Determinations are carried out at different equilibrium pressures p(the pressure attained after adsorption) to yield an adsorption isotherm (isotherm refers to a plot at constant temperature). Based on the IUPAC classification, the isotherms obtained can generally be classified into six types as follows:
Table 2 – Adsorption Isotherm
| Type I isotherm: Shows a rapid rise in the adsorption with increasing pressure followed by levelling off.
1. Characteristic of chemical adsorption by microporous adsorbents. 2. Langmuir type isotherms, adsorption being restricted to monolayer. 3. Rapid rise is due to micropore łlling at relatively low pressures. 4. Levelling off is because chemical groups available for chemisorption get saturated very rapidly (monolayer).
|
Example: adsorption of N2 on carbon at 77°K, adsorption of ammonia on charcoal at 273°K. |
| Type II isotherm: Sigmoidal in shape.
1. Characteristic of multilayer physical adsorption onto nonporous solids. 2. First inŃection point is due to the formation of a monolayer. 3. As the pressure is increased further, multilayer formation occurs. 4. Described by BET equation.
|
Example: activated carbons with mixed micro and mesoporosity. |
| Type III isotherm: Convex curve to the relative pressure axis.
1. Characteristic of weak adsorbate–adsorbent interactions and is most associated with nonporous and microporous adsorbents. 2. Low adsorption at low pressures due to weak interactions between the adsorbate and the adsorbent. 3. Accelerated uptakes at higher relative pressure due to strong adsorbate–adsorbate interaction after adsorption at primary sites.
|
Example: adsorption of N2 water on carbon, the primary adsorption sites are oxygen based. |
| Type IV isotherm:
1. Characteristic of adsorption of gases on porous solids. 2. First inŃection point represents the amount of gas required to form a monolayer on the surface of the solid. 3. Further adsorption is due to multilayer formation and capillary condensation within the pores of the solid. 4. Limiting value is due to saturation vapour pressure.
|
Example: condensation of gases on porous solids. |
| Type V isotherm:
1. Similar to Type III isotherm, shows convex curve characteristic of weak adsorbate–adsorbent interactions. 2. Further adsorption is due to capillary condensation as seen for Type IV isotherm. 3. Adsorption reaches a limiting value before the saturation vapour is reached. |
|
| Type VI isotherm: Stepwise isotherm
1. Characteristic of extremely homogeneous, nonporous surfaces where the monolayer capacity corresponds to step height. 2. Formation of complete monolayer before each subsequent multilayer commences. 3. Shows distinct steps corresponding to the complete formation of each monolayer.
|
Example: adsorption of krypton on carbon black at 90K. |
Multiple choice questions (MCQs)
1.The process in which materials of one phase (adsorbate) accumulate or concentrate at the interfacial surface of the other phase (adsorbent) is called
a)Adsorption
b)absorption
c)Both of these
d)None of these
2.Adsorption is a spontaneous phenomenon driven by a reduction of the
a)Surface free energy
b)Bonds between ions
c)Bonds between molecules
d)Density
3.Adsorption occurs at the interfaces of two phases such as
a)liquid/liquid
b)gas/liquid
c)liquid/solid
d)All of the above
4.Which of the following are types of adsorption process?
a)Physical adsorption
b)Physisorption
c)Chemical adsorption
d)All of the above
5.Chemical adsorption is also known as
a)Physisorption
b)Chemisorption
c)Physical adsorption
d)Chemical absorption
6.In physical adsorption, adsorbates adsorb on the surface of a solid by
a)Ionic bonds
b) Van der Waals forces
c)Covalent bonds
d)All of these
7.Physical adsorption is
a)Fast
b)Reversible
c)Multilayer adsorption
d)All of the above
8.In chemical adsorption, the substance is held on the surface of a solid by
a)Ionic bonds
b) Van der Waals forces
c)Covalent bonds
d)All of these
9.Chemical adsorption is
a)Slow
b)Not readily reversible
c)Monolayer adsorption
d)All of the above
10.As per Langmuir adsorption isotherm, the rate of adsorption is proportional to
a)Occupied spots
b)Unoccupied spots
c)Pressure
d)Both a and b
11.Adsorption is a dynamic phenomenon that is opposed by
a)Chemisorption
b)Physisorption
c)Absorption
d)Desorption
12.The term ‘Sorption’ stand for
a)Absorption
b)Adsorption
c)Desorption
d)Both a and b
13.Isotherm refers to a plot at
a)Varying temperature
b)Varying pressure
c)Constant pressure
d)Constant temperature
14.Based on the IUPAC classification, the isotherms obtained can generally be classified into how many types?
a)2
b)4
c)6
d)8
15.Which of the following is example of type II isotherm?
a)Adsorption of N2 on carbon at 77°K, adsorption of ammonia on charcoal at 273°K
b)Activated carbons with mixed micro and mesoporosity
c)Adsorption of krypton on carbon black at 90K
d)Condensation of gases on porous solids
Solutions:
- c)Both of these
- a)Surface free energy
- d)All of the above
- d)All of the above
- b)Chemisorption
- b) Van der Waals forces
- d)All of the above
- c)Covalent bonds
- d)All of the above
- d) Both a and b
- d)Desorption
- d)Both a and b
- d)Constant temperature
- c)6
- b)Activated carbons with mixed micro and mesoporosity
References:
1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 128-136.
2. Martins Physical Pharmacy, 6th edition 2011, page no. 669-696.
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