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
How do you work out the rate of formation in Eley-Rideal mechanism?
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
What are the assumptions made by the Langmuir-Hinshelwood mechanism?
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
