conditions, formation of an ionic associate can be observed accompanied with
spectrum change.
In fact, for phenol red and other sulfonphtaleine dyes, a pH interval was found in
which changes in the spectra, interpreted by us as a manifestation of the ion-associative
interaction, were observed. With increasing polyelectrolyte concentration (or
increasing the charge density, which is equivalent to an increase in the concentration
of positively charged groups in solution), in all cases an increase in the intensity of the
band corresponding to one form of the dye with a parallel decrease in the intensity of
the other form was observed. In absorption spectra with an increase in the fraction of
the ion associate, an isobestic point is observed. One form of the dye passes into
another, as it always does when the acid-base equilibrium is shifted. Neither the
absorption maxima nor the shape of the absorption bands change in this case. Both
maximums in the difference spectra are at wavelengths, which coincide with the
maximum of the corresponding protonated forms of the dye.
Figure 10. Interaction of polyelectrolyte CPAT and Phenol Red: a) Absorption
spectra of Phenol Red in absence (1) and in the presence of polyelectrolytes CPAT
with different charge density (2-6).
1 – without CPAT; 2 – CPAT 5%; 3 – CPAT 30%; 4 – CPAT 55%; 5 – CPAT 70%; 6 – CPAT
95%. C
PE
= 32 mg
L-
1, pH 5.5, C
Dye
= 10 μmol L
-1
, l = 2 cm. b) Dependence of ΔA on solution pH in
the presence of polyelectrolytes CPAT with different charge density. ΔA – difference between
absorbances of solutions with and without polyelectrolyte. 1 – CPAT 5%; 2 – CPAT 30%; 3 – CPAT
55%; 4 – CPAT 70%; 5 – CPAT 95%. C
PE
= 32 mg
L-
1, C
Dye
= 10 μmol L
-1
, λ = 560 nm, l = 2 cm
In the structure of the CPAT, there are no voluminous and extended hydrophobic
regions where hydrophobic interaction and the associated with it solvatochromic effect
would be significant. A shift of the dye bands was noted for the interaction of
sulfonphtaleine dyes with other cationic PEs [35]. The dianionic bands of
Chromazurole and Eriochromcyanine were shifted to the long-wave region by 9-55
nm.
a
b
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