Membranes 5 Flashcards
(21 cards)
Define driving force
Potential difference ac. membrane/ membrane thickness
For a given membrane there is a proportionality relationship between flux and driving force. What is this relationship
Flux (J) = Proportionality factor (A) * Driving Force (F)
Ficks law) (J = -D*(dC/dx)
What is flux defined as
Flux per unit pressure difference between two sides of a membrane
What can be done if the thickness of the membrane is known
The permeability coefficient (P) can be found by normalising the permeance by the thickness
In asymmetric membranes, which layer mainly determines the resistant to transport?
top layer (more dense)
What is the difference (in relation to pores) between the poiseulle equation and the Kozeny-Carman equation
In the poisseuille equation pores are considered cylinders. In Kozeny-Carman the pores are voids between closed packed spheres of equal diameter
When does knudsen diffusion occur, what is the diffusion rate and separation like?
Occurs when the pore size in the membrane is smaller than the mean free path.
The diffusion rate is then related to the inverse square root of molecule molar mass
The separation is low
What is molecular sieving
When molecules are separated based on their kinetic diameters
What does solution-diffusion involve
Gases are separated by their solubility within the membrane and their diffusions through the dense membrane matrix
What are the 6 mechanisms of transport
Bulk diffusion Surface diffusion Viscous flow Capillary condensation Knudsen diffusion Activated transport
When does poisseulle flow occur
Mention pore sizes
High concentration where molecule - molecule collisions are dominant
Larger pores is dominated by viscous flow
When does knudsen flow occur?
in much smaller pores where collisions between molecules are far less than the pore wall
describe solubility and how is it used in the permeability equation
A thermodynamic parameter and is measure of the amount of molecules sorbed by a membrane
P = S*D
When can Henry’s law apply for solubility
When solubility of gases in polymers is low
If the solubility is high it is considered, not ideal
What does diffusivity indicate
How fast a molecule is transported through the membrane
When does a system show linear trends between pressure and solubility
With gases in elastomers
With glassy polymers what does the graph look like
A curved graph with an initial high gradient which reduces in size
What is a non ideal system caused by
Strong interaction between vapours and polymer
looks like exponential
What is the significance of the interactions in rubbery membranes
permeant - permeant and permeant polymer interactions are taken to be relatively weak which means ideal solution behaviour and Henry’s law applies
What is the concentration gradient also equal to
The pressure gradient x solubility
Imagine how Ficks law would be converted to an equation involving permeability
J = D(C1-C2)z (C1-C2) = S(P1-P2) DS = P J = P(P1-P2)/z
P = JZ/(P1-P2)
