Adsorption and Absorption Flashcards
What is absorption?
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
What is adsorption?
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)
What are the features of absorption?
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
What are the features of adsorption?
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
What are examples of products functioning by adsorption?
enterosgel intestinal adsorbent 225g silica gel activated characoal activated aluminates zeolites
What is an adsorbent and adsorbate?
adsorbent - surface where adhesion occurs
adsorbate - molecules of solute
How does activated charcoal work?
used to remove harmful substances from the body
What are the requirements for an effective adsorbent?
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
What are the three different classes of adsorbents?
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
What are the advantages of adsorption?
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
What are the disadvantages of adsorption?
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
How does adsorption occur?
occurs via weak physical forces rather than chemical alteration
- hydrogen bonding
- hydrophobic forces
- Van der Waals forces
What is Langmuir-Hinshelwood mechanism?
reaction occurs with 2 species
- between 2 adsorbed reactants
- both are adsorbed onto the surface of the catalyst and a product is formed
How do you work out the rate of formation in Langmuir-Hinshelwood mechanism?
to find out rate, the extent of surface coverage is needed
- depends on surface covered
expect rate law to be in second order
What is the Eley-Rideal mechanism?
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
