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204
F
A
E
A
(kJ mol
-1
)
< 0.0003 158
0.002
152
0.47
130
0.62
134
The figure shows in fact several important things:
a) The data fit very well to the Arrhenius equations in all cases
b) Chitosans with high F
A
have lower activation energies than for low F
A
c) Chitosans with high F
A
are hydrolysed much faster than low F
A
chitosans.
If we draw a vertical straight line at 1/T = 3.0
⋅
10
-3
K
-1
(T = 333 K = 60
°
C,
dotted line in figure), we can observe an intercept with the upper curve at ln k
= ca. -11.2. The difference in ln k is (-11.2 – (-18)) = 6.8. In other words, the
high F
A
chitosans are hydrolysed e
6.8
≈
900 times faster than almost fully de-
N-acetylated chitosans.
[This is easy to explain: The GlcNAc residues are hydrolysed relatively
easily, whereas the GlcN (glucosamine) residues are difficult to
hydrolyse because the –NH
2
group becomes protonated in acid: -NH
3
+
,
and thus repel protons (H
+
) which must bind to the adjacent glycosidic
oxygen to catalyze the hydrolysis. See also part I of this document.]
Differences in activation energies have an interesting consequence: The
Arrhenius lines are not parallel, and therefore they will at one point (some
temperature) cross each other. Above that temperature the reaction that was
the slower at 60
°
C will in fact be the faster one. Oppositely, using low
temperatures the hydrolysis of the highly N-acetylated chitosans are even
more favoured. The figure below shows this more clearly:
ln
k
1/T (K
-1
)