Adsorption and Absorption

Question 1

What is absorption?

Answer 1

a fluid permeates or is dissolved by a liquid or solid

process where a substrate originally present in one phase or compartment is diffused into another phase/compartment

Question 2

What is adsorption?

Answer 2

adhesion of molecules of gas, liquid or dissolved solid to a solid surface

describes the tendency of materials to locate at a solid surface in a concentration different than that found in the surrounding medium (bulk)

Question 3

What are the features of absorption?

Answer 3

irreversible - drug cannot be retrieved when absorbed

bulk phenomenon - whole substance is affected and taken in

endothermic - energy of absorbent increases/ energy taken in

uniform rate

temperature has no effect

Question 4

What are the features of adsorption?

Answer 4

reversible

• can be reversed by reducing pressure, reducing temperature, changing pH
• interaction is only between particles at the interface
• interaction is based on Van der Waals forces which are weak

surface phenomenon = takes place at surface/ only those at the surface are affected
exothermic - energy of surface decreases
non-uniform rate - increases steadily until it reaches equilibrium
temperature dependent - favours low temperatures

Question 5

What are examples of products functioning by adsorption?

Answer 5

enterosgel intestinal adsorbent 225g
silica gel
activated characoal
activated aluminates
zeolites

Question 6

What is an adsorbent and adsorbate?

Answer 6

adsorbent - surface where adhesion occurs

adsorbate - molecules of solute

Question 7

How does activated charcoal work?

Answer 7

used to remove harmful substances from the body

Question 8

What are the requirements for an effective adsorbent?

Answer 8

1 - small size = usually spherical pellets or rods with diameters between 0.5-10mm
2 - high surface area = small pore diameters and high thermal stability increase area exposed for adsorption
3 - active sites or hydrophobic sites = must have distinct reactive functional groups or hydrophobic groups

Question 9

What are the three different classes of adsorbents?

Answer 9

oxygen containing compounds
- typically hydrophilic and polar
carbon based compounds 
- typically hydrophobic and non-polar
polymer based compounds 
- polar or non-polar functional groups in a porous polymer matrix

Question 10

What are the advantages of adsorption?

Answer 10

can be used in emergency treatment of drug overdose

• administration of a water powder slurry (suspension) of activated carbon into the GIT by either oral or nasogastric tube
• many drugs have a high binding affinity for it

Question 11

What are the disadvantages of adsorption?

Answer 11

loss of drug potency

• drug can accumulate at the surface of the container through unspecified hydrophobic forces = instead of interacting in solution
• loss of proteins from the bulk solution
• unfolding of proteins can occur = non-functional

Question 12

How does adsorption occur?

Answer 12

occurs via weak physical forces rather than chemical alteration

• hydrogen bonding
• hydrophobic forces
• Van der Waals forces

Question 13

What is Langmuir-Hinshelwood mechanism?

Answer 13

reaction occurs with 2 species

• between 2 adsorbed reactants
• both are adsorbed onto the surface of the catalyst and a product is formed

Question 14

How do you work out the rate of formation in Langmuir-Hinshelwood mechanism?

Answer 14

to find out rate, the extent of surface coverage is needed
- depends on surface covered

expect rate law to be in second order

Question 15

What is the Eley-Rideal mechanism?

Answer 15

reaction occurs between 2 species

• between an adsorbed reactant and a non-adsorbed reactant
• gas molecules collides with another molecule already adsorbed onto the catalyst

the rate determining step is the collision between the them

Question 16

How do you work out the rate of formation in Eley-Rideal mechanism?

Answer 16

to find out the rate, partial pressure of the rolling gas and surface coverage by the adsorbed species is needed

• allows prediction of how much of a drug can be adsorbed onto a specific surface

Question 17

What are the assumptions made by the Langmuir-Hinshelwood mechanism?

Answer 17

solid surface contains a constant number of identical adsorption sites

each adsorption sites takes one molecule forming a monolayer

no adsorbate-adsorbate interactions

whole process is at constant temperature