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Bull. Chem. Soc. Jpn. Vol. 83, No. 6 (2010)
Role of the Succinate Skeleton
but also stabilization of a specific torsion angle for the 1¤C-1C-
2C-3O skeleton.16 The IR spectral data of aqueous solutions for
SDES, SDPS, SDHS, and AOT, in this present study, provide
clear evidence that this steric effect brings about the restricted
state of the succinate skeleton in aqueous solutions.
Thus, the conformational change between type A and type B
conformations in the succinate skeleton, in addition to the
variation of fractional populations (PI, PII, and PIII) about
the 1¤CH2-1CH single bond, may play a critical role in the
disorder-order transition.
In our previous paper,25 we found that, for the reversed
micellar system of the AOT-C6D6-D2O (2:2:1), the rotational
Structural Model of the SDHS-Lamellar and the IR
Data. We may now discuss a model for the lamellar structure
in the SDHS aqueous solutions, in which type A is preferen-
tially stabilized. The type A conformation, with the two hexyl
chains fanning out each other (Figure 1a), is apparently not
suited to formation of the stacking structure. However, when
the hydrocarbon chains of the SDHS molecules, constituting a
lamellar, take up a position in which they overlap each other (as
in a finger-joint), a lamellar formation of the type A con-
formation may be possible. In this model, we must assume that
the thickness of the lamellar should be smaller than twice the
molecular length of SDHS. The thickness of the lamellar
structure for the present samples, obtained by the X-ray
diffraction method, is approximately 30 ¡. This value is clearly
smaller than that (35.50 ¡) of the bilayer calculated using
Tanford’s equation37 (Lmax = 1.5 + 1.265nc, where Lmax is the
maximum length of hydrocarbon chain and nc is the number of
carbon atoms) and the molecular model of adopted by Sheu
et al.38 This evidence supports our suggestion of a finger-joint
model for the lamellar structure in the SDHS aqueous
solutions.
isomer III (Scheme 2) is preferentially stabilized (PI:PII:PIII
=
0.21:0.04:0.75; PI, PII, and PIII: fractional populations of the
rotational isomers I, II, and III, respectively, Scheme 2). This
result showed that the 1¤CH2-1CH group of the succinate
segment is extremely restricted in the reversed micelles.
We have no evidence for the values of PI, PII, and PIII for
aqueous micellar samples of AOT and its homologs. However,
we may expect that such populations will change more in the
normal micelles, compared with those in the reversed micelles.
The reason for this variation is that the internal rotation of the
succinate segment in the aqueous micelles will be restricted,
due to an increase in the hydrophobic interactions among the
chains which will be much more extensive than those in the
reversed micells.36
Furthermore, in this present study, for the SDMS-D2O and
SDES-D2O systems, which are unable to form micelles in
aqueous solution, we have calculated the fractional populations
of the three rotational isomers (PI, PII, and PIII) using 1H NMR
coupling constants, according to the method described in
Ref. 26. The calculated fractional populations are PI:PII:PIII
=
The thickness (17.9 ¡) of a lamellar structure in the AOT-
water binary system has been determined by Mori et al.35 using
the small angle X-ray scattering method. It is clear that this
value is smaller than twice (ca. 24 ¡) the AOT molecular
length. The authors presented a model with bending hydro-
carbon chains or a tilted bilayer model, in addition to a finger-
joint model, to explain this thickness.
0.2:0.2:0.6 for the D2O solutions (concentration: 20 wt %) of
SDMS and SDES, implying that the rotational isomers I (20%)
and II (20%) coexist although isomer III (60%) is stabilized.
This result may indicate that isomer III is relatively stable even
in an unaggregated state, compared with the other two isomers.
However, we may assume that the presence of isomers I and
II with such fractional populations (PI and PII: 0.2) causes
deviation of the torsion angles of types A and B in the 1¤C-1C-
2C-3O skeleton, resulting in broadening of the 1350 cm¹1 band
of the aqueous samples upon dilution.
The present IR data provide ample evidence that both types
A and B are stabilized in the lamellar of the AOT-water
system, indicating that the extent of ordering of molecules in
the AOT lamellar is as high as seen in the SDHS lamellar.
Moreover, the C13 spin-lattice relaxation data24 provide the
following analogy. That is, the segmental mobility of two
tertiary CH groups, both their side-CH2 groups, and two ethyl
groups in the 2-ethylhexyl chains, may be extremely restricted
in the lamellar, compared with those in the normal and reversed
micelles.24 This great rigidity probably allows the ethyl groups
as a branching segment to prevent stacking of the ethylhexyl
chains, furnishing the grooves among the AOT molecules.
Thereby, the bending portions of 2-ethylhexyl chains may fill
the grooves to increase the spatial proximity of inter- and
intramolecular 2-ethylhexyl chains, possibly providing the
thickness, 17.9 ¡, of the AOT lamellar. Thus, we present a
mixed model of the AOT lamellar, with contributions from a
finger-joint model and a bending-hydrocarbon model.
For the SDES aqueous solutions (20 and 40 wt %), we find
¹1
that the intensity of the 1397 cm band becomes greater
with an increase in concentration (Figures 3a and 3b). This
observation may be interpreted by further stabilization of the
isomer III, as follows. In the normal mode analysis of the
SDES anion,17 the isomers I, II, and III provided calculated
values of 1387, 1392, and 1399 cm¹1, respectively, for the
1CH-deformation mode (or its mode coupled with the scissor-
ing mode of the 1¤CH2-group). Accordingly, intensification of
the 1397 cm¹1 band may be explained by invoking an increased
fractional population of isomer III, providing a calculated
value of 1399 cm¹1. Furthermore, in the IR spectra of the
two SDES aqueous samples, it is evident that an increase
in concentration brings about the increased width of the
¹1
1376 cm band, probably caused by the variation of the
Conclusion
fractional populations (PI, PII, and PIII).
We may assume that dilution of the samples results in
variation of such fractional populations for the cases of AOT
and its homologs. This variation may be reflected in the very
broad and strong IR bands at 1350 cm¹1 for the diluted samples
and in the very broad spectral feature at 1330-1370 cm for
the concentrated samples.
Two conformations, type A and type B, originating from the
difference in the torsion angles of the succinate skeleton (1¤C-
1C-2C-3O), in sodium dialkylsulfosuccinate (SDAS) aggre-
gates, can be detected by the IR spectral method in both
aqueous solution and in the crystalline state. We may assume
that the conformational change between types A and B in the
¹1