Biology Reference
In-Depth Information
Therefore, the effect of pH on the capsule can be understood by
considering the effect of pH on the PAH layers. Those layers are
protonated according to the equilibrium
R
−
NH
2
+
H
3
O
+
R
−
NH
3
+
+
OH
−
The protonation has two effects: (1) Mutual repulsion from
neighboring
−
NH
3
+
sites causes a “stretching out” of the molecule.
Simulations on commercial software suggest the PAH length
increasesby7%fromuncharged(pH10.0)tofullycharged(pH3.0)
[91]. (2) The formation of
−
NH
3
+
requires charge compensation
by counterions. Each counterion is surrounded by a shell of H
2
O
molecules and their entry into the film causes osmotic pressure
between the PEs. These two factors combine to result in an opening
upofthefilmstructure.Theopeninghasbeenobservedbyscanning
forcemicroscopy[93].Capsulesexposedtoacidicsolutionexhibited
pores of up to 100 nm diameter, while capsules at pH 10.0 showed
no such effect. The same thing has also been observed for the
PAH/PSS system deposited on a planar surface [100]. Importantly,
whencapsulesfromanacidicsolutionweretransferredtoasolution
at pH 10.0 the pores could not be observed [93]. Hence, the capsule
openingisreversible,andsopHmaybemanipulatedtoentraplarge
molecules within the capsules. Such entrapment has been studied
by confocal microscopy using fluorescent-labeled dextran, and it
was demonstrated that in acidic conditions dextrans entered the
capsules [91]. Polyions and proteins have also been entrapped by
this method [101].
Effect of solvent polarity.
Solvent polarity affects capsule perme-
ability by changing the solubility of the capsule walls. In the case
ofPAH/PSSpairs,theyareinsolubleinwaterandsolubleinethanol.
Hence,varyingthewater-to-ethanolratioofthesuspensionmedium
canleadtoalooseningofthefilmstructure.At20%ethanolcontent
asignificantincreaseintheshellpermeabilitywasnotedforarange
of high-molecular-weight materials including dextrans and proteins
[102]. As with pH-induced changes, the opening was found to be
reversible and could therefore be used forencapsulation.
Maximum loading.
If the loading of the PE capsules is driven
solely by the concentration gradient across the capsule walls,
