Chemistry Reference
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Fig. 13
Time-resolved
Stokes shift,
C(t)
,ofpatman
in PS-PVP-PEO micelles at
pH values of 2, 3, and 4, as
indicated on the
corresponding curves.
Inset
:
Time-dependent halfwidth,
δ
1.00
4500
2.0
3.0
4000
0.75
4.0
3500
0.50
(
t
), of the time-resolved
emission spectra of patman in
PS-PVP-PEO micelles at
different pH values, as
indicated
3.0
3000
0
10
20
4.0
t
/ns
0.25
2.0
0.00
0
10
20
t
/ns
The pH dependence of the solvent relaxation rate offers an explanation of the
observed instability of micellar solutions at low pH and formation of a fraction of
micellar clusters. The stability of micellar solutions assumes a proper solvation of
PEO chains. However, PEO solvation promotes the ice-like structure and reduces
entropy, which may cause solubility problems in systems with crowded PEO chains
such as micellar shells formed by long PEO blocks. The addition of HCl (or other
small ions) breaks the water structure, increasing the fraction of free and mobile
water molecules, and reduces the fraction of PEO solvation-capable structured wa-
ter molecules. Regarding the enthalpy-to-entropy interplay, the effect of increasing
acidity resembles the effect of increasing temperature in the lower critical solu-
tion temperature (LCST) region. In both cases, the mobile solvent molecules would
have to “condense” at the chain to assure its solubility and sacrifice considerable
translational entropy. Regarding the LCST, the unfavorable entropy contribution in-
creases with increasing temperature and at LCST, the phase separation occurs. In
the studied case, the entropy contribution increases with the concentration of HCl:
the mobile water molecules “liberated” due to the breakdown of the ice-like struc-
ture after the addition of small ions would have to form the structured solvation shell
of PEO monomer units to provide sufficient thermodynamic stability of micelles in
the solution. The complex entropy-to-enthalpy balance shifts towards free water
molecules with decreasing pH, which promotes the formation of micellar clusters
and minimizes the fraction of water molecules engaged in solvation shells.
The relaxation behavior at pH 1 is very interesting, but we could not analyze it be-
cause the time-resolved emission bands were bimodal. We studied the possibility of
analyzing complex time-resolved spectra and in our recent paper [
135
] we describe
a successful method of decomposition and treatment of bimodal time-resolved spec-
tra. We used two probes with the same fluorescent headgroup differing in the length
of the aliphatic tail. They have different affinity to micelles that allows study of
their partitioning between micelles and bulk solvent. A detailed description and ex-
tended discussion exceeds, unfortunately, the scope of this paper, but we not only
succeeded in treating the two time-resolved contributions (from free and micelle-
solubilized probes) separately, but we also identified a slow contribution due to the
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