Chemistry Reference
In-Depth Information
H
2
O
!
3
þ
Ce
4
þ
þ
H
þ
ðÞ
þ
Ce OH
(2.27)
As known, ceric ion in CAN exists as the species of Ce
4+
, Ce(OH)
3+
, and
(Ce-O-Ce)
6+
in its aqueous solution. It was reported that the efficiency of Ce
4+
and Ce(OH)
3+
species to form radicalic sites on cellulose backbone is higher than
that of (Ce-O-Ce)
6+
since the size of the former is smaller than that of the latter
[
124
] and the former is more mobile than the latter. Goya et al. [
124
] also reported
that at high acid concentrations, Ce
4+
and Ce(OH)
3+
species affects the grafting
adversely, namely, the termination reaction dominates over the propagation. A
possible explanation for this adverse effect of high acid concentration on the
grafting may be the difficulty in hydrogen abstraction from graft substrate (opinion
of G¨rda˘). The concentration of these species depends on the amount of acid
present in the medium. At high nitric acid concentrations, the equilibrium reaction
(
2.27
) shifts to the left and ceric ions in CAN occur in the form of Ce
4+
which are
responsible for the creation of active radicals preferably on the cellulose than
monomer. In the case of low acid concentrations, the equilibrium shifts to the
right, and the formation of high amount of Ce(OH)
3+
led to the formation of
considerable amount of (Ce-O-Ce)
6+
which is not active for the creation of
radicalic sites.
3
þ
!
6
þ
2Ce OH
ðÞ
ð
Ce
O
Ce
Þ
þ
H
2
O
(2.28)
For that reason, CAN or another ceric salt should be used together with an acid.
Dhiman et al. [
96
] found that the increase in the concentration of HNO
3
from 0.3 M
to 0.5 M led to ca. 20 % increase in grafting percentage of 4-VP onto cellulose by
CAN initiator, and further increase in HNO
3
concentration resulted in ca. 10 %
decrease in grafting percentage. The decrease in grafting with the increase in acid
concentration beyond the optimum value was attributed to the effect of excess H
+
ions as free-radical terminator [
96
]. Ibrahim et al. [
49
,
50
] investigated the effects
of reaction parameters on the water absorbency of graft products in the grafting of
NVP onto CMCNa/HEC in homogeneous medium using ceric ammonium sulfate
(CAS) as initiator. They determined that the increase in Ce
4+
concentration from
0.02 % to 0.2 % reduced the water absorption capacity of graft product from 209 g/g
to 131 g/g. They explained this finding so that higher initiator concentrations lead to
the increase in grafting frequency, namely, a graft product with high amount of
short graft chains occurs. As known, the increase in polymer chain ends does not
contribute to the water absorption capacity [
49
,
50
]. Therefore, the water absorption
capacity of NVP-grafted CMCNa/HEC decreased with the initiator CAS. Misra
et al. [
89
] investigated the effect of complexing agent such as KF, ascorbic acid,
and EDTA on the grafting of ethyl acrylate (EA) onto cellulose by Fenton reagent
(Fe
2+
-H
2
O
2
). In order to avoid the negative effect of Fe
3+
ions on the grafting,
namely, the wastage of
•
OH radicals by reaction with Fe
3+
ions, the grafting was
carried out in the presence of some complexing agents with Fe
3+
ions such as
ascorbic acid, potassium fluoride (KF), and ethylenediaminetetraacetic acid
(EDTA) [
85
,
89
]. At low concentration (81
10
4
M), KF gave highest amount
