The reverse micellar phase L2 of the triblock
copolymer poly(ethylene
oxide)-b-poly(propylene
oxide)-b-poly(ethylene oxide)
EO13PO30EO13 (commercial
name Pluronic L64), where EO is ethylene oxide
and PO is propylene oxide, was studied in the ternary mixture
L64/water/o-xylene and in the binary
mixture L64/water, using ESR spectroscopy with nitroxide spin probes.
The spin probes differed in size,
structure, and polarity and belong to two main types: (a) cationic
probes
4-(N,N-dimethyl-N-alkyl)ammonio)-2,2‘,6,6‘-tetramethylpiperidine-1-oxyl iodide (CATn)
with n, the number of carbon atoms in
the alkyl substituent, equal to 1, 4, 8, 11, and 16; (b) amphiphilic
probes based on x-doxylstearic acid
(xDSA) with x, the carbon atom to which the doxyl
group is attached, equal to 5, 7, 10, and 16. X-band
ESR spectra reflect the intercalation of the probes in the
self-assembled L64 system, but different probes
choose different locations in the aggregates and report on the
local polarity, hydration, and degree of
order. The hydration gradient in the poly(ethylene oxide)
(PEO) core of the reverse micelles was estimated
on a scale of ≤48 Å from the analysis of a
N,
the isotropic hyperfine splitting of 14N, in the
CATn series by
comparison with a “calibration curve” based on CAT4 in aqueous
solutions of PEO. The lower homologs
in the series (n = 1 and 4) are located near the center of
the hydrated core, while the higher homologs (n
= 11 and 16) are close to the interface between the hydrated core and
the o-xylene-swollen poly(propylene
oxide) (PPO) blocks. The xDSA probes have their head
group near the interface between the core and
the PPO regions. The nitroxide group in 5DSA is at the core/PPO
interface, but the nitroxide groups in
the other doxyl probes are at different depths in the PPO regions,
depending on the value of x. The
dynamics of the CATn probes was elucidated by comparing the
experimental B and C parameters in
the
expression for the line width in motionally averaged ESR spectra of
nitroxides ΔH(m
I) = A
+ Bm
I +
Cm
I
2, with the corresponding values
calculated as a function of the direction of the axis of rotation
and
N, the degree of anisotropy of the rotational reorientation.
The comparison suggests that the dominant
motional mechanism for all probes is rotation around the N−O bond
(x axis), but the anisotropy of the
lower spin probe homologs is significantly higher (N = 10)
compared to that of the higher homologs (N
= 2−3). These results are consistent with the existence of a
hydration gradient in the core of the reverse
micelles.
