appearance of the 1259 cm~1 band may also imply the pres-
the C2O stretch band reÑects the extent of hydration of the
ester portions.
ence of another SO ~É É ÉNa` type interaction arising from a
3
di†erence in the hydration modes.
The concentration dependence of the band frequency of the
C2O stretch mode for aqueous SDHS solutions above the
c.m.c. (0.8 wt.%, 18.8 mM) was also examined. It has been
found that micellization brings about the high frequency shift
of the l(C2O) value. Since the concentration of the boundary
between the micellar solution phase and the lamellar phase is
close to 32 wt.% (30 ¡C) (to be the subject of a separate
publication), we may assume that formation of the lamellar
structure results in a further high frequency shift. Such a high
frequency shift of the C2O stretch band may be caused by
exclusion of the hydrated water molecules around the ester
portions upon formation of micelles or a lamellar structure.
Similar observations were made for the aqueous SDBS
samples.
For the samples in the A series, the Raman and IR spectral
features of the la(SO ~) and ls(SO ~) modes are very similar
3
3
to those of SDES(1H O) or SDES(3H O) (Table 2). More-
2
2
over, the band frequency of the la(SO ~) modes and the *la
3
value of the la(SO ~) modes are much closer to those of
3
SDES than to those of SDMS, showing that formation of the
Na`É É É~OÈSO É É ÉH O complex may be possible in crystals
2
2
of the A series and that there exists an SO ~É É ÉNa` inter-
3
action mode similar to that identiÐed in the SDES mono- or
tri-hydrate crystals.
For the B series, as seen in Tables 1 and 2, the Na`É É ÉSO ~
3
interaction modes are obviously di†erent from those of
SDMS(1H O) and SDES(3H O), since the Raman (IR) bands
2
2
of the la(SO ~) mode were observed at 1224È1226 (1226) and
As has already been pointed out by Maitra et al.19 and
Yoshino et al.,23 isomer III is preferentially stabilized in the
AOT-reversed micelles. Therefore, we may assume that stabili-
3
1242 (1242) cm~1, reÑecting the manner of the SO ~É É ÉNa`
coordination in the crystal structure of the B series (Fig. 3).
3
For the frozen SDBSÈH O system, the spectral features of
zation of a speciÐc rotational isomer about the CH ÈCH
single bond varies the hydration environment of the ester por-
2
2
the la(SO ~) and ls(SO ~) modes were examined at various
3
3
SDBS concentrations (spectra not shown). In addition to the
shoulder bands at 1210È1220 cm~1, Raman bands were
observed at 1234 and 1260 cm~1, which closely correspond to
the bands at 1238 and 1259 cm~1, respectively, for the frozen
SDES(5H O and 23H O) samples, showing that the
tions, leading to the high frequency shift of the l(C2O) value.
For concentrated samples (80 and 90 wt.%) of SDBS,
SDHS and SDOS in the lyotropic liquid crystalline state,
splitting patterns of the C2O stretch modes (very weak shoul-
der bands at around 1720 and 1740 cm~1 and predominant
bands at 1732È1736 cm~1) were observed in the Raman
spectra (spectra not shown).
2
2
SO ~É É ÉNa` interaction in the SDBS sample is similar to
3
that in these frozen SDES samples. However, for more con-
centrated SDBS (di- and penta-hydrate) samples in the frozen
state, in addition to these three bands, a new shoulder band at
1224 cm~1 and Raman band at 1245 cm~1 appeared, which
correspond well to the 1221 and 1250 cm~1 bands for the
crystalline SDES(3H O) and frozen SDES(4H O) samples.
The Raman spectra of the SDBS samples in the coagel state
were examined (spectra not shown) and it was found that the
spectral features of coagel SDBS samples depend upon their
concentration in the original aqueous solution. When the
SDBS concentration in the coagel was relatively low, shoulder
bands at 1720 and 1740 cm~1 appeared, in addition to the
predominant 1732È1734 cm~1 bands, while, as the concentra-
tion increased, the band at 1720 cm~1 disappeared and only
the last two bands were seen. Similar observations were made
for SDHS and SDOS. We therefore conclude that for the
coagel samples of the SDAS-series, the C2O stretch modes
reÑect the microstructures of the polar region of aggregates in
the original aqueous solutions.
2
2
These observations reveal that as the extent of hydration for
the SO ~ group decreases, the SO ~É É ÉNa` interaction
3
3
modes approach that in the SDES trihydrate. Similar obser-
vations were made for the aqueous SDHS and SDOS samples
in the frozen state.
For the IR spectra of the C series, it was found that the
spectral patterns of the la(SO ~) bands (1226 and 1242È1244
3
cm~1) obviously di†er from those (1219È1222 and 1255È1257
cm~1) for the A series and are very similar to those for the B
When we take into account the distance (R
) between
COÕÕÕO
series (Table 1), showing that in the C series there exists
the C2O group oxygen atom and the oxygen atom of the
nearest neighboring water molecule, obtained by X-ray dif-
fraction analysis of SDMS and SDES18 (Table 3), then the
extent of hydration of the b C2O group is larger than that of
the a C2O group. Moreover, in our previous 13C NMR spinÈ
lattice relaxation study7 of the carbonyl carbonÈwater inter-
action in Aerosol-OT reversed micelles, we have already
presented evidence that the hydration shell of the b C2O
group may be larger than that of the a C2O group. Therefore,
for the coagel samples of SDHS, the bands at 1732È1736
cm~1 arise from the hydrated b C2O groups and the 1740
cm~1 band from the unhydrated (free) a C2O groups, while
the bands at ca. 1720 cm~1 arise from the b C2O groups,
which interact weakly with the Na` ion and are considerably
hydrated.
another type of SO ~É É ÉNa` interaction mode, which is dif-
3
ferent from that in the SDMS(1H O) and SDES(3H O) crys-
2
2
tals but similar to that seen in the B series. Two bands at
1053È1055 and 1063È1064 cm~1, coming from the ls(SO ~)
3
stretch mode, are observed in common for the B and C series,
implying that splitting of the la(SO ~) mode may occur
3
because of the di†erent SO ~É É ÉNa` interaction modes.
3
Discussion
C2O environment in the SDAS–H O systems and hydration
2
e†ect
The splitting of the C2O stretch modes, which was found in
the crystalline SDMS and SDES samples, disappeared for the
isotropic aqueous samples and only the Raman bands at
1732È1735 cm~1 were observed (spectra not shown). These
Raman bands may be regarded as characteristic of fully
hydrated C2O groups.
In the isotropic aqueous solutions of the SDAS-series,
Raman (IR) bands of the C2O stretch modes arising from a-
and b-carbonyl groups were also observed as one broad band
in the 1720È1735 cm~1 region, and the frequencies of the C2O
stretch bands for the a C2O and b C2O groups were indistin-
guishable. It was found that the band frequency depends upon
the concentration, indicating that the frequency [l(C2O)] of
For the Raman spectra of these coagel samples, the depen-
dence of the C2O stretch modes on the SDAS-concentration
in the original aqueous solutions may be interpreted as
follows. Below the c.m.c. the surfactant molecules are in the
monomeric state. Therefore, when the monomeric solutions
form a coagel phase at low temperatures, the SDAS molecules
may be in a fully hydrated state. For surfactant molecules in
the micellar state, the extent of hydration should be less than
that for the monomers, since micellization brings about partial
exclusion of hydrated water molecules around the C2O
groups. Furthermore, even when the micellar solutions
become coagels, this hydration environment in the micellar
state should remain. When the liquid crystalline samples form
4404
Phys. Chem. Chem. Phys., 1999, 1, 4395È4407