W1.3_Absorption and Adsorption

Question 1

What are the similarities between absorption and adsorption? What are their purposes respectively?

Answer 1

• Both absorption and adsorption are sorption process (sorbate is captured by sorbent in condensed state (s/l)
• Absorption: essential part of pharmacokinetics (LADME)
• Adsorption: observed in formulation of medicines

Question 2

Compare and contrast absorption and adsorption. (6)

Answer 2

Absorption:
- Accumulation of particles throughout another substance
- Bulk phenomenon
- Endothermic
- Not affected by temperature
- Uniform rate
- Concentration eventually becomes the same throughout the material (limited by partitioning)
Adsorption:
- Accumulation of particles onto a surface/interface
- Surface phenomenon
- Exothermic
- Favoured by lowering the temperature
- Rate steadily increases until reaching equilibrium
- Surface concentration differs from internal concentration

Question 3

Explain the two types of adsorption.

Answer 3

Physisorption (physical adsorption)
- Adsorbate held on adsorbate via weak intermolecular forces
- Reversible, low enthalpy change of adsorption
- Forms multilayers
Chemisorption (chemical adsorption)
- Highly specific (chemical bonding between adsorbent and adsorbate)
- Irreversible, high enthalpy change of adsorption
- Forms monolayer

Question 4

Give out examples of adsorption in S/L interface (2) and S/V interface (2). State the factors that affect adsorption rate (4). Define leachables and extractables.

Answer 4

S/L interface:
- Solid particles dispersed in liquid: basis of stabilisation by surfactants
- Treatment of overdose (ex. activated charcoal contains pores to adsorb paracetamol in GIT)
S/V interface:
- Adsorption of drugs/excipients on wall container during manufacturing/storage can affect potency/stability of drug
- Analysis methods (ex. Thin Layer Chromatography, High Pressure Liquid Chromatography)
- pH/solubility/temperature/surface area affects adsorption rate (eqm process)
- Leachables (compounds that have direct contact with formulation and can leach into the drug product formulation in container closure system) and extractables (compounds that can be extracted from container closure system in presence of solvent)