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O
CH
3
(I)
CH
3
Cl
−
N
+
N
H
3
C
CH
3
CH
3
O
CH
3
CH
3
(II)
N
CH
3
O
CH
3
(III)
CH
3
N
CH
3
Fig. 10
Structures of (I) patman, (II) prodan, and (III) laurdan
Even though the aggregation of evenly charged micelles is a slightly surprising
phenomenon, the effect of pH is an understandable result of electrostatic inter-
actions. The addition of HCl supports the protonization of PVP blocks, but the
H
3
O
+
and Cl
−
ions efficiently screen the electrostatic forces, which promotes the
aggregation.
We performed the solvent relaxation study of acidic PS-PVP-PEO solutions with
the aim of (a) proving the assumption that the formation of micellar clusters is a
result of an insufficient solvation of PEO units, and (b) understandinghow the sol-
vation is affected by pH and ionic strength.
First, we studied the solvent relaxation in solutions of diblock copolymer
structure I), frequently used in phospolipid bilayer studies [
123
], was added to
aqueous solutions of PS-PEO micelles. The probe binds strongly to micelles be-
cause its hydrophobic aliphatic chain has a strong affinity to the nonpolar PS core.
The positively charged fluorescent headgroup is supposed to be located in the PEO
shell close to the core-shell interface. The assumed localization has been supported
by time-resolved anisotropy measurements.
The time-resolved emission spectra (TRES),
F
TR
(
ν
,
t
)
, were reconstructed us-
ing the steady-state emission spectrum,
F
(
ν
), and the fluorescence decays,
I
(
ν
,
t
)
,
measured at different wave numbers
ν
I
(
ν
,
t
)
F
(
ν
)
F
TR
(
ν
,
t
)=
.
(14)
∞
I
(
ν
,
t
)
d
t
0
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