When choosing the optimum acidity for the reaction of cationic dye with CPAT,
the dependence of the deviation of absorbance from additivity on solution pH was
plotted for each of the polymers. On all the curves, one can see the range of pH values
at which ΔA is approximately constant and close to the maximum. For all polymers
(with the exception of a polymer with a charge density of 5%), the optimum pH value
is approximately in the range from 5 to 7 (Figure 10b). The form of this dependence
does not depend on the charge density, but the equilibrium shifts much more toward
the ionic associate for polymers with high charge density values.
Interaction of Bromphenol Blue with polyelectrolyte CPAT. In a solution of
BPB at pH <3, the single-ionized form of the dye with λ
max
437 nm dominates, and at
pH> 4.6 the doubly ionized form λ
max
592 nm. When CPAT is added to the solution of
BPB in the pH range from 2.5 to 4.0, the intensity of the band of doubly ionized form
increases, while the single-charge band decreases (Fig. 11).
Figure 11. Spectra of IAs formed between Bromphenol Blue and CPAT.
C
CPAT
is changed from 0.12 to 7.2 mg L
-1
, C
Dye
= 0.04 mmol L
-1
, pH = 3.6, l = 5 cm
In the investigated system, under certain conditions, the formation of IA is
accompanied by the appearance of a new band, hypsochromically shifted with respect
to the band of dianionic form. It appears in the spectra as a band with a maximum of
about 568 nm or in the form of a shoulder.
The second maximum, in our opinion, is due to the processes of aggregation of
the dye. Aggregation occurs because of the approaching of dye molecules in the
composition of the polymer, which contributes to the dispersive π-π interaction of
conjugated aromatic system of the dye. According to exciton theory, dye molecules
can be oriented like a "tail to head" (one after another), forming J-aggregates or parallel
400
500
600
700
800
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Abs
orb
anc
e
λ, nm
C
CPA
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