Basic HTML Version
53
its maximum charge). Thus, it is common to dissolve chitosans in acidic
buffers such as dilute acetic acid.
Chitins (F
A
> 0.8) are generally insoluble in water
17
, but can be dissolved in
special solvents (DMAc/LiCl) or cold alkali.
A special case is found for high molecular weight chitosans with intermediate
degree of acetylation (0.4 < F
A
< 0.6), which turn out to be soluble in water
even at high pH (no charges). This is ascribed to the random distribution of N-
acetyl groups.
The ability to precipitate polymers from solution is thought to be linked to
association of homogeneous regions, which dominate at low F
A
(homogeneous deacetylated regions) or high F
A
(homogeneous acetylated
regions):
At intermediate F
A
we have maximum disorder (given random distribution of
N-acetyl groups), preventing association of homogeneous regions.
1.3.9. Chitosans:
Free amine
form and
salts
Chitosans are manufactured either as ‘free amines’ or in various ‘salts forms’,
or example as chitosan chloride or chitosan acetate. Chitosan salts are often
termed ‘water soluble chitosans’ because they dissolve directly in pure water.
In contrast, most chitosans in the free amine form do not dissolve directly in
pure water. However, they dissolve when a little acid is added, bringing pH
down to about 6 or below. Why is this so?
In fact, this is all a matter of the acid-base properties and the polyelectrolyte
properties mentioned above. To dissolve in water, the chitosan must be in the
acidic form: -NH
3
+
. The type of counter-ion (Cl
-
, CH
3
COO
-
etc.) is less
important
18
. It is mostly a matter of bringing pH closer to (or below) pK
a
.
Remember from general acid-base theory:
17 Otherwise shrimp and insect could only live in extremely dry places
18 Not entirely true. Acids such as H2SO4 or H3PO3 give insoluble sulfate or phosphate salts, respectively. Stick to HCl or acetic acid.
F
A
> 0.8
F
A
< 0.4