Biology Reference
In-Depth Information
Capsule Permeability:
The most widely characterized polyelec-
trolytes in the formation of hollow capsules have been alternating
layers of polystyrenesulphonate (PSS, negative) and polyallylamine
hydrochloride (PAH, positive), and therefore the following perme-
abilitydiscussionwillbebasedonthatsystem.(PAH/PSS)
n
willrefer
to a film of
n
bilayers.
The relatively loose, layered structure of the PEs renders
them porous to low-molecular-weight compounds. (When those
compounds are charged it has been suggested that their movement
throughtheshellisby“hopping”fromoppositelychargedsites[93].)
The shell porosity has been examined by entrapping fluorescein
microparticles at low pH and then measuring the fluorescence in
bulk after the microparticles are dissolved through a pH increase
[94]. It was found that the permeability to small molecules
decreased with increasing film thickness. For more than 8 layers,
the decrease was roughly linear with the film thickness increase
and corresponded to a diffusion coe
cient of fluorescein through
the shell wall in the order of 10
−
12
cm
2
s
−
1
. For less than 8 layers,
the shell permeability decreased more quickly than described by
a linear relation, which is consistent with the finding that the first
eight layers have a more dense conformation than the subsequent
coatings[95].
Effect of ionic strength.
As noted earlier, the initial structure of the
PE layers is affected by ionic strength. After hollow capsules are
formed from the PEs, they are also affected by the ionic strength
[96-98]. The exact reasons for the effect of ionic strength are
complex (see Ref. 91 for a detailed discussion), but in general
permeability increases nonlinearly with salt concentration. Human
serumalbumin(HSA)hasbeenincorporatedintoPSS/PAHcapsules
by increasing the bulkNaCl concentration to 5 mM [99].
Effect of pH.
IfionicstrengthchangescannotrenderPAH/PSScap-
sules permeable to larger species (e.g., macromolecules, enzymes,
nanoparticles), then manipulation of pH or solvent polarity can
be used. The point about the (PAH/PSS)
n
system is that PSS is a
strong polyelectrolyte and remains fully ionized, whereas PAH is a
weak polyelectrolyte and so its dissociation is dependent on pH.
