Block 4 -- Diffusion & Dissolution Flashcards
Define diffusion:
mass transfer of individual particles brought about by random molecular motion
What factors affect the diffusion coefficient?
1) T
2) Pressure
3) Solvent properties
4) Solute properties
Give examples of diffusion coefficients for big and small particles.
Big – 10^-6
Small – 10^-7
Define flux:
the amount of material (M) flowing through a cross-section (S) of a barrier in a specified amount of time (t)
What does Fick’s First Law show?
The amount of material (M) flowing through a unit barrier in a unit time
What does the negative sign in Fick’s first law signify?
Diffusion occurs from higher to lower concentration
What does Fick’s Second Law show?
How concentration changes with time at a particular location
Describe the diffusion cell:
1) donor compartment
2) receptor compartment
3) Membrane
4) Inflow of solvent to receptor
Why would C_1 be greater than C_D?
If the drug is lipophilic and sequesters in the membrane
Describe conditions and kinetics at steady state in the diffusion cell?
Zero-order release
Sink conditions
How can you find the Permeability coefficient from a graph?
M = PSC_dt Slope = PSC_d
Why is a lag time associated with non-steady-state conditions?
Lag time is present to establish a uniform concentration gradient within membrane
Define dissolution rate:
the amount of API in a solid dosage form dissolved per unit time under a set of standardized conditions of liquid/solid interface, T, media composition
Why is the dissolution rate tested?
1) To measure quality control in formulation & manufacturing.
2) To ensure bioequivalency when F correlates with dissolution.
3) To establish the intrinsic dissolution rate of drug.
4) To identify “F” problems in preformulation
5) To quantify stability/shelf-life.
Describe the in-vivo and in-vitro sink conditions.
In-vitro 1) large volume of dissolution medium 2) replenishing medium at specific rate [solute] always < 10-15% of max solubility in-vivo 1) GI tract
