chp3 diffusion Flashcards
what is diffusion?
is the process of mass transfer of
individual molecules of a substance brought about by
random molecular motion and associated with a
driving force such concentration gradient.
give examples on diffusion in pharmaceutical sciences
1, drug release from tablets, capsules, ointments and suppository bases
2.drug absorption and elimination
- permeation and distribution of drug molecules
- osmosis and dialysis
what is osmosis?
involves the passage of a solvent only through a semipermeable membrane.
what is dialysis?
involves the passage of solvent and solute through a membrane
give an example on the osmotic drug release system
release systems that use osmotic pressure to control the delivery of drugs aka “osmotic pumps”.
example: Oros technology used in Pfizer
Procardia XL
what is the difference between procardia and procardia XL?
About a four-fold lower fluctuation
index (ratio of peak to trough) was
observed with once daily
PROCARDIA XL® than conventional
immediate-release PROCARDIA ®
capsule at t.i.d. dosing
- minimizing adverse effects
- improved patient compliance
The label indication for PROCARDIA
XL® was expanded to include
treatment of both angina and
hypertension.
both have the same active ingredient: Nifedipine
the passage of matter through a solid barrier (polymeric membrane) can occur by?
• Simple molecular diffusion or permeation through
nonporous media: it depends on the solubility of
the permeating molecules in the bulk membrane
• Movement through solvent-filled pores and
channels: it is influenced by the relative size of the
penetrating molecules and the diameter and shape
of the pores
both require dissolution first
what causes diffusion? is it slow? does the slow rate matter?
Diffusion is caused by random
molecular motion therefore, it is a slow process.
• The diffusion rate of a molecule is estimated to be:
– In gases = 10 cm/minute
– in liquids = 0.05 cm/minute
– In solids = 0.0001 cm/minute
• Is that slow process significant? The answer is yes.
Because the thickness of cell membrane is 5 nm
(0.0000005 cm) it will take a molecule a fraction of a
second to cross that thickness.
give an example on a drug that we need the slow diffusion rate for
levonorgestrel-releasing implants
(Norplant®): a contraceptive approved
for 5 years of continuous use in
human. It provides 40 – 50 g/day in
the first year that decreases to 25 – 30
g/day in the fifth year.
how can the mixing of the drug in solution with intestinal contact effects the diffusion
– At first glance, mixing appears to be a simple
process; however, several molecular- and
macroscopic-level processes must occur in
parallel for efficient mixing to occur.
– It is important to remember that diffusion
depends on random molecular motions that
take place over small molecular distances
“microscopic phenomenon”
give some examples on these macroscopic processes
convection,dispersion, and stirring.
remember that diffusion is kinetic and needs the equilibrium to not be balanced in order to occur
ok
what are the different diffusion driving forces
1.Concentration:
– Passive diffusion in absorption
– Drug dissolution
- Pressure:
– Osmotic drug release - Temperature:
– Lyophilization - Electrical potential:
– Iontophoretic dermal drug delivery
– Electrophoresis
what is fick’s first law of diffusion
J= (d * M) \ A . dt
it then becomes
J= -D* (dC\dX)
Where
D: the diffusion coefficient of a penetrant (also called the diffusant) in cm2/sec
C: concentration in g/cm3
X: the distance in cm of movement perpendicular to the surface of the barrier
the negative sign is for the direction. the flux is always positive
J is g\sec
if dC\dX is 0 the diffusion stops
what is D the diffusion coefficient affected by?
– Concentration
– Temperature
– Pressure
– Solvent properties
– Chemical nature of the diffusant
