BET isotherm, Factors Affecting Adsorption, Adsorption Phenomenon(Part 3) and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam

BET isotherm: Langmuir and Freundlich isotherms are based on the formation of a saturated monolayer of the adsorbate on the surface of the adsorbent. The BET isotherm assumes that a multimolecular layer of adsorbate molecules covers the surface of the adsorbent and that each layer behaves as the Langmuir isotherm. The BET isotherm is written as follows:

where [A]s is the saturated concentration of the adsorbate, q_o is the number of moles of the adsorbate adsorbed per unit weight of adsorbent in a monolayer and b is a constant related to the energy of interaction with the surface.

Plotting the left hand-side term of the equation versus [A]/[As] gives a slope of (b – 1)/bq_o and an intercept of 1/bq0. For a simple monomolecular layer, the BET equation reduces to the Langmuir equation.

Factors Affecting Adsorption:

The adsorption of solute molecules from its solution may be influenced by the following factors:

1. Nature of adsorbent: The physicochemical nature of the adsorbent can have decisive impacts on the rate and capacity for adsorption. Every solid material can be used as an adsorbent, but activated carbon and clays such as kaolin and bentonite have been used as particular adsorbents in pharmaceutical applications.
2. Nature of adsorbate: The solubility of the adsorbate in the solvent from which adsorption takes place has an inverse relationship with the extent of adsorption (Lundelius’ rule). The forces between the adsorbate and solvent need to be broken for adsorption to occur. Thus, higher the solubility of the adsorbate in a solvent, the greater the forces and the smaller the extent of adsorption.
3. Adsorbent–solute interaction: Adsorption of a solute from a dilute solution involves the breaking of bonds between the solute and the solvent molecules as well as the formation of bonds between the solute and adsorbent molecules. As an example, the higher molecular weight solutes are usually more readily adsorbed than low molecular weight solutes. This is due to van der Waals forces of attraction, which increases with the size of molecules.
4. Adsorbate concentration: The amount of adsorption increases with the increase in the concentration of solute at equilibrium until it reaches a limiting value. However, the relative amount of solute removed from the solution is greater in dilute solutions.
5. Surface area of adsorbent: Adsorption is a surface phenomenon and the amount of solute adsorbed depends on the surface area available. Thus, reducing the particle size of the adsorbent will increase the adsorption.
6. Temperature: Physical adsorption is an exothermic process and thus a decrease in temperature will increase the extent of adsorption.
7. Removal of adsorbed impurities: Removal of adsorbed impurities such as gases or moisture from the surface of solid adsorbent activates the active adsorption sites and increases the efficiency of adsorbents. This can be achieved by heating the adsorbent at high temperature(at 110°C for 1 h).
8. pH of the medium: pH of a solution influences the extent of adsorption since pH affects both the degree of ionization and the solubility of the adsorbate drug molecule. More ionized (i.e.polar) and soluble adsorbates adsorb much less than their unionized forms (i.e. lipophilic).Amphoteric adsorbates such as proteins are usually best adsorbed at the isoelectric point where the net charge of the adsorbate becomes zero, and at the lowest solubility.

Multiple choice questions (MCQs)

1.Which of the following factors effect adsorption?

a)Nature of adsorbent

b)pH of medium

c)Temperature

d)All of the above

2.Which of the following are used as particular adsorbents in pharmaceutical applications?

a)Activated carbon

b)Clays

c)Kaolin and Bentonite

d)All of the above

3.The solubility of the adsorbate in the solvent from which adsorption takes place has a/an _____ relationship with the extent of adsorption

a)Direct

b)Inverse

c)No relation

d)Stable

4.The higher molecular weight solutes are usually more readily adsorbed than low molecular weight solutes.

a)True

b)False

5.The amount of adsorption _____ with the increase in the concentration of solute at equilibrium until it reaches a limiting value. 

a)Increase

b)Decreases

c)No change

d)Both a nd b

6.The solubility of the adsorbate in the solvent from which adsorption takes place has an inverse relationship with the extent of adsorption. This rule is known as

a)Schulze hardy rule

b)Lundelius’ rule

c)Hofmeister rule

d)Avgadro’s rule

7.Adsorption is a

a)Bulk phenomena

b)Surface phenomena

c)Absorption phenomena

d)All of these

8.Reducing the particle size of the adsorbent will ____ the adsorption.

a)Increases

b)Is infinite

c)Remains unchanged

d)May decrease or increase depending upon size

9.Physical adsorption is an

a)Endothermic process

b)Exothermic process

c)Both of these

d)None of these

10.Decrease in temperature will ____ the extent of adsorption.

a)Increase

b)Decreases

c)No change

d)Both a nd b

11.Removal of adsorbed impurities such as gases or moisture from the surface of solid adsorbent activates the active adsorption sites and increases the efficiency of adsorbents. This can be achieved by heating the adsorbent at

a)110°C for 1 h

b)110°C for 2 hrs

c)110°C for 30 minutes

d)110°C for 20 minutes

12.pH of a solution influences the extent of adsorption since pH affects 

a)Degree of ionization

b)Solubility of the adsorbate drug molecule

c) Both of these

d)None of these

13.Which of the following adsorbs lesser?

a)Ionized adsorbates

b)Soluble adsorbates

c)Both of these

d)None of these

14.Amphoteric adsorbates such as proteins are usually best adsorbed at

a)Kraft point

b)CMC

c)Isoelectric point

d)Cloud point

15.At the isoelectric point the net charge of the adsorbate becomes

a)1

b)0

c)2

d)<1

Solutions:

1. d)All of the above
2. d)All of the above
3. b)Inverse
4. a)True
5. a)Increase
6. b)Lundelius’ rule
7. b)Surface phenomena
8. a)Increases
9. b)Exothermic process
10. a)Increase
11. a)110°C for 1 h
12. c) Both of these
13. c)Both of these
14. c)Isoelectric point
15. b)0

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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