Structural characterization of solid trivalent metal dodecyl sulfates: From aqueous solution to lamellar superstructures

Article abstract of DOI:10.1039/c2ra21906a

Metal dodecyl sulfates of trivalent aluminium, chromium, lanthanum and gadolinium were prepared by addition of the corresponding salts to aqueous solutions of sodium dodecyl sulfate at the natural pH (ca. 6). Using X-ray diffraction, FT-IR and NMR spectroscopy, DSC, thermogravimetry and polarizing light thermomicroscopy it is shown that metal dodecyl sulfates are formed with lamellar structures. These have different degrees of hydration, which depend upon the metal ion. In some cases there is evidence for coexistence of different lamellar phases. The metal is strongly bound electrostatically to the sulfate group, and although the alkyl chain is in an extended conformation, there are suggestions of local disordering of the methylene groups adjacent to the anionic head group. SEM with an EDS probe on the trivalent metal complexes shows a 1:3 ratio at the surface identical to the bulk structure. Studies by SEM and AFM provide evidence of periodicity, which is likely to be induced by the lamellar, layered structures. Differences are observed in the thermal behavior, which appear to reflect both the coordination behavior of the metal ion and the degree of hydration.

Full text of DOI:10.1039/c2ra21906a

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Structural characterization of solid trivalent metal
dodecyl sulfates: from aqueous solution to lamellar
superstructures3
Cite this: RSC Advances, 2013, 3,
1420
a
a
a
´
Rui F. P. Pereira,* Artur J. M. Valente, Hugh D. Burrows, Veronica de Zea
Bermudez,b Rui A. Carvalhoc and Ricardo A. E. Castrod
Metal dodecyl sulfates of trivalent aluminium, chromium, lanthanum and gadolinium were prepared by
addition of the corresponding salts to aqueous solutions of sodium dodecyl sulfate at the natural pH (ca.
6). Using X-ray diffraction, FT-IR and NMR spectroscopy, DSC, thermogravimetry and polarizing light
thermomicroscopy it is shown that metal dodecyl sulfates are formed with lamellar structures. These have
different degrees of hydration, which depend upon the metal ion. In some cases there is evidence for
coexistence of different lamellar phases. The metal is strongly bound electrostatically to the sulfate group,
and although the alkyl chain is in an extended conformation, there are suggestions of local disordering of
the methylene groups adjacent to the anionic head group. SEM with an EDS probe on the trivalent metal
complexes shows a 1 : 3 ratio at the surface identical to the bulk structure. Studies by SEM and AFM
provide evidence of periodicity, which is likely to be induced by the lamellar, layered structures. Differences
are observed in the thermal behavior, which appear to reflect both the coordination behavior of the metal
ion and the degree of hydration.
Received 22nd August 2012,
Accepted 15th November 2012
DOI: 10.1039/c2ra21906a
www.rsc.org/advances
relevant for the development of layered hydroxide composite
systems for a variety of materials applications.18,19
1. Introduction
Colloidal systems involving long chain anionic amphiphiles
and trivalent metal ions have applications in detergency,1
wastewater treatment,2,3 gel formation and development of
thickeners and dispersants,4,5 catalysis,6,7 analytical chemis-
try,8 froth flotation for metal ion recovery,9 and contrast agents
for magnetic resonance imaging.10 They form ordered nanos-
tructures through self-assembly which also have considerable
potential in materials science, including tunable UV/visible
light emitters,11 templated synthesis of mesoporous materi-
als,12,13 production of lanthanide-based glasses for photonic
applications,14,15 preparation of nanoparticles,16 and forma-
tion of metal–organic frameworks (MOFs).17 They are also
The addition of trivalent metal salts to sodium dodecyl
sulfate (SDS) in aqueous solutions almost invariably leads to
precipitation, due to formation of the trivalent metal dodecyl
sulfates. However, as has been discussed elsewhere,20–25 the
metal–dodecyl sulfate interaction depends on the nature and
concentration of the metal ion and also on the precipitation
conditions (pH, temperature, etc.).
The synthesis of metal dodecyl sulfates, such as Cu(DS)2
and Zn(DS)2, was first reported by Matuura et al. who used the
addition of SDS to a solution of the metal nitrate or
chloride.26–28 Firouzabadi et al. synthesized Al(DS)3?H2O and
investigated its successful application in the Michael addition
of indoles and pyrrole to a,b-unsaturated electron-deficient
compounds in aqueous media at room temperature. In this
study, they found that the catalytic activity of Al(DS)3?3H2O is
greater than that of Sc(DS)3.29 Al(DS)3?3H2O was also employed
efficiently as a catalyst for the conversion of epoxides into
thiiranes with thiourea in water at room temperature in
excellent yields.30
aDepartment of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal.
E-mail: rpereira@qui.uc.pt; Fax: (+351) 239 350480; Tel: (+351) 239 350253
b
´
Department of Chemistry and CQ-VR, University of Tras-os-Montes e Alto Douro,
5001-801 Vila Real, Portugal
cDepartment of Life Sciences and Center for Neurosciences, University of Coimbra,
3004-295 Coimbra, Portugal
dFaculty of Pharmacy, University of Coimbra, 3004-295 Coimbra, Portugal
These compounds are frequently referred in literature as
Lewis acid-surfactant-combined catalysts.30–32 The develop-
ment and use of new environmentally-friendly Lewis acid
catalysts is increasing due to the introduction of increasingly
restrictive environmental regulations applied to industrial
processes. When conventional Lewis acid catalysts are used,
Electronic supplementary information (ESI) available: XRD patterns of the
3
metal dodecyl sulfate complexes, EDS spectrum of all M(DS)3 complexes, TGA
and DTG curves of the metal dodecyl sulfate complexes M(DS)3 in the high
temperature region, FT-IR spectra of the metal dodecyl sulfate complexes
M(DS)n: in the nOH, nC–C and sulfate regions, and discussion of interpretation
of FT-IR spectra for the sulfate group vibrations. See DOI: 10.1039/c2ra21906a
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1
an additional step is often required to remove the acid–base
adduct formed between products and catalyst.33 This process
usually leads to catalyst degradation and generation of
chemical wastes.33,34 In contrast, ‘‘green’’ Lewis acid catalysts
are not corrosive, do not require neutralization steps for
separation, are more selective and can be reused.33
scopic analysis (FT-IR, H and 13C NMR) and X-ray diffraction
(XRD). In addition, morphological studies have been per-
formed using scanning electron microscopy (SEM) and atomic
force microscopy (AFM). The SEM studies of the trivalent
metal complexes have been complemented by energy-disper-
sive X-ray spectroscopy (EDS).
Metal centers combined with surfactant molecules have
been proposed as powerful catalysts for several other
reactions: aldol condensation, regioselective ring opening,
Michael addition, Diels–Alder, and esterification;30,32,34–36
furthermore, these materials are capable of performing
organic reactions in water.
2. Experimental
2.1 Materials
Aluminum(III) nitrate nonahydrate (Al(NO3)3?9H2O, 98.0%),
chromium(III) nitrate nonahydrate (Cr(NO3)3?9H2O, 99.0%),
lanthanum(III) nitrate hexahydrate (La(NO3)3?6H2O, 99.999%),
gadolinium(III) perchlorate (Gd(ClO4)3, 40 wt % solution in
water), and sodium dodecyl sulfate (CH3(CH2)11OSO3Na, SDS,
¢98%) were purchased from Sigma-Aldrich. These reagents
were used as received, and all solutions were prepared using
Millipore-Q water. All experiments were carried out at 25 uC
unless otherwise stated.
In addition, metal-surfactant compounds can be used as so-
called organic-inorganic hybrid nanocomposites. The devel-
opment of such new types of porous hybrid materials allows
the design of composites with different guests.37 Mesoporous
materials can also be synthesized in the presence of
surfactants; here the surfactant acts as a template for the
formation of the mesoporous network, with the length of the
alkyl chain determining the size of the pores.14,17 These
materials show suitable morphologies for applications in
optical, electronic, and sensing devices, as well as for
separation, due, in particular, to their versatility as coatings
on various substrates and commercial supports such as optical
fibers.14,15,17,37 In general, such trivalent metal hybrid materi-
als containing, for example, lanthanides, have improved
mechanical properties and better processability than the pure
molecular complexes, in addition to showing improvements in
thermal stability and luminescence output.14,15
A detailed understanding of structural aspects of metal
surfactant systems is important for many of these applica-
tions. We report the structural, thermal and spectroscopic
characterization of dodecyl sulfates of trivalent metals. Four
model metal ions have been chosen for their different
characteristics: Al3+, Cr3+, La3+ and Gd3+. The first two have
environmental importance, in addition to showing different
coordination behavior. Although Al3+ and Cr3+ are normally
both hexacoordinated, differences are seen in substitution of
their water molecules, with aluminum(III) being kinetically
labile, while chromium(III) is inert, only undergoing very slow
water substitution reactions.38 With lanthanum(III) and
gadolinium(III), both show a wide range of useful physical
properties, including luminescent and magnetic characteris-
tics.39 In addition, because of the fairly uniform complexation
behavior of this group of elements, once the characterization
of one lanthanide surfactant system has been made, its
behavior can readily be extended to other trivalent lantha-
nides. However, subtle differences exist along the series of the
lanthanide ions, sometimes leading to a decrease in coordina-
tion number,40 and it is anticipated that the behavior of La3+
and Gd3+ may be different, and may reflect the behavior of
lanthanide ions at the beginning and end of the series.
The structural and physical-chemical characterization of
the metal dodecyl sulfates has been carried out by thermo-
gravimetric analysis (TGA), differential scanning calorimetry
(DSC), polarized light thermal microscopy (PLTM), spectro-
2.2 Preparation of metal dodecyl sulfates
Recently we reported that DS2 ions interact at the natural pH
in water with trivalent metal ions (M3+ = Al3+, Cr3+, Gd3+ and
La3+) to form solids in a 3 : 1 stoichiometry,21,22 corresponding
to charge neutralization. However, in contrast to what is
expected for metal tri(dodecyl sulfate)s, (M(DS)3), it has been
found that the maximum interaction stoichiometry occurs at
around 4 : 1, suggesting a change in the solid structure which
allows the best packing of DS2 ions around the metal ions.21
Based on these observations, the following experimental
procedure was adopted: a certain volume of 0.1 M SDS
solution was added, under continuous stirring, to a 1 mM
trivalent metal salt solution until the concentration ratio r
(where r = [SDS]/[M3+]) of 4 was attained, and then left stirring
for at least 6 h. The resulting precipitate was collected,
centrifuged, washed with water and then lyophilized for about
24 h. All metal(III) solids were prepared in the same manner
from their respective salts.
2.3 Experimental methods
1H and 13C NMR spectra were acquired in deuterated
methanol (99.80%) on a Varian 600 MHz spectrometer using
a 5 mm broadband probe. Typical acquisition parameters for
1H NMR spectra included a 45u radiofrequency observation
pulse and 32 000 points covering a spectral width of 8 kHz. For
13C NMR spectra 132 000 points were acquired for resolving
the 35 kHz spectral widths; a sequence with a 45u radio-
frequency pulse and an interpulse duration of 3 s were used to
allow full relaxation of aliphatic carbons.
XRD patterns of the dried (lyophilized) samples were
recorded at room temperature using
a Philips PW-170
diffractometer. The samples were exposed to monochromatic
Cu-Ka radiation (l = 1.5418 Å, 50 kV, 40 mA) over the 2h range
between 20 and 70u in steps of 0.02u.
High Resolution Scanning Electronic Microscopy (HR-SEM)
micrographs were obtained using a FEI Quanta 400FEG/Edax
Genesis X4M scanning electron microscope. Elemental analy-
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sis on microscopic sections of each M(DS)3 sample was
performed by Energy Dispersive Spectroscopy (EDS). SEM
images were obtained at high or low vacuum, depending on
sample charging effects, and in the secondary electrons mode
or backscattered electrons mode.
Real Time Video Measurement System software by Linkam was
used for image analysis. The images were obtained by
combined use of polarized light and wave compensators,
using a 2006 magnification.
The FT-IR spectra were acquired at room temperature in
transmission mode using a Nicolet 6700 FT-IR spectrometer
from Thermo Scientific. The spectra were collected over the
4000–400 cm21 range by averaging 64 scans at a spectral
resolution of 2 cm21. Solid dried samples were finely ground,
mixed with dried potassium bromide (Merck, spectroscopic
grade) and pressed into pellets.
AFM images were recorded with
a Veeco Metrology
Multimode/Nanoscope IVA equipment (CEMUP-Porto contract
REEQ/1062/CTM/2005), in tapping mode using a super sharp
silicon tip, curvature radius 10 nm, and frequency resonance
equal to #300 kHz. Flattening and elimination of line noise
tools and a Lowpass filter provided by the WSXM software41
were used to improve the quality of the images.
For the DSC measurements a dried (lyophilized) sample
with a mass of approximately 8 mg was placed in a 50 ml
perforated aluminium pan and the thermogram was recorded
using a Perkin Elmer Pyris1 Differential Scanning Calorimeter.
An empty pan was employed as reference. The sample was
heated at 10 uC min21 from 0 to 190 uC using dried high purity
nitrogen as purge gas supplied at a constant flow rate of 20 mL
min21. All samples for each material were collected from the
same precipitate.
For the TGA measurements dried samples were transferred
to open platinum crucibles and analyzed from room tempera-
ture to 900 uC using a TA Q50 Thermogravimetric Analyser at a
heating rate of 10 uC min21 using dried high purity nitrogen as
purge gas supplied at a constant flow rate (40 mL min21 and
60 mL min21 in the balance and sample chamber, respec-
tively).
3. Results and discussion
Initial characterization of the M(DS)3 complexes was carried
out in solution using NMR spectroscopy. 1H and 13C NMR
spectra of SDS dissolved in CD3OD were obtained and
compared with those acquired for each of the complexes of
the trivalent metal ions. The SDS 1H NMR spectrum (Fig. 1(a))
showed chemical shifts at d = 4.02 (i), 1.70 (ii), 1.44 (iii), 1.33
(iv) and 0.94 (v) ppm. 1H peak assignments are given in the
scheme in Fig. 1, and are similar to those reported for other
dodecyl compounds.6,29,31,42 The peak at 3.35 ppm is from
vestigial CHD2OD. The 13C NMR spectrum of SDS (Fig. 1(b))
presents peaks at d = 69.43, 33.34, 31.06, 31.03, 30.99, 30.97,
30.74, 30.67, 27.18, 24.00 and 14.72 ppm. The resonances a, b
and c (Fig. 1(b)) indicate the methylene groups immediately
following to the sulfate in the dodecyl sulfate aliphatic chain.
The intense peak at 49.30 ppm is from CD3OD. 1H and 13C
NMR spectra of each of the complexes are also shown in the
The dried solids were characterized by PLTM using a hot
stage Linkam system (model DSC600) equipped with a Leica
DMRB microscope and a Sony CCD-IRIS/RGB video camera.
Fig. 1 1H (a) and 13C (b) NMR spectra of metal dodecyl sulfate complexes M(DS)n in CD3OD.
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figure and are similar to spectra reported for other metal
dodecyl complexes.6,31
methylenic group in the Gd(DS)3 complex, consistent with
the large paramagnetic character of this metal ion.
The complexation with the M3+ ions induced changes in
both chemical shifts and linewidths. A complete analysis of
those changes was made and revealed that the noteworthy
alterations were those associated with the methylenic groups
in close proximity to the sulfate group. As expected, the
paramagnetic Cr3+ and Gd3+ ions induced significant broad-
ening in both 1H and 13C resonances. All metals cause a small
downfield shift of the proton resonance at y4.0 ppm,
assigned to the methylene group adjacent to the sulfate group
(i), which is caused by the trivalent metal ions binding the
surfactant head group. The largest shift is seen for La(DS)3 (Dd
of 0.09 ppm) and the largest broadening for the same
Changes in 13C chemical shifts are observed for all
complexes, but are less pronounced for the Al(DS)3 complex.
In terms of relaxation, significant increases in relaxation rates
(as derived from 13C resonance linewidths) can be seen for the
methylene group attached to the sulfate (a from Fig. 1(b)) with
Dd = 0.9, 0.2, 0.3 and 2.7 ppm for La(DS)3, Al(DS)3, Cr(DS)3 and
Gd(DS)3 complexes, respectively. Although less pronounced, it
was also possible to observe some changes in both chemical
shifts and linewidths in the signals of the two methylene
groups at 30 and y27 ppm (b and c from Fig. 1(b)), suggesting
that these are the two CH2 groups immediately following the
a-methylene in the dodecyl sulfate aliphatic chain. It is worth
noting that only one set of DS2 resonances is observed both
for 1H and 13C NMR spectra, showing that all coordinated DS2
ions have the same chemical and magnetic environments in
solution. In addition, the fact that relatively sharp resonances
are seen for the terminal methyl and associated methylene
groups in the DS complex with the paramagnetic Gd3+ ion
indicates that the alkyl chains of the DS2 ligands are extended,
outside the volume of paramagnetic influence of this
lanthanide. This NMR analysis clearly demonstrates that the
chemical and magnetic environments of the proton and
carbon nuclei in close proximity to the sulfate group are the
most affected and such effects differ depending on the M3+
ion.
The XRD patterns of the solid M(DS)3 compounds are
shown in Fig. 2. The solids show a reasonable degree of
crystallinity, as indicated by the relatively sharp peaks
observed in the low q region (q = 4psin h/l where 2h is the
scattering angle and l is the wavelength) where at least three
(001) reflections are distinctly seen. At q , 8 nm21 the XRD
patterns of SDS, Al(DS)3 and Cr(DS)3 (red, blue and green
vertical continuous lines in Fig. 2(a), respectively) and SDS,
La(DS)3, and Gd(DS)3 (red, blue and green vertical continuous
lines in Fig. 2(b), respectively) are compared. All the patterns
exhibit a series of peaks which correspond to the kth order
reflections of a lamellar structure with an interlamellar
spacing d1 (where d1 = k2p/qk). The d1 values, listed in
Table 1, calculated for all the complexes using the position of
Table 1 Characteristic distances (in nm) of the metal dodecyl sulfate complexes
and ionic radii (in nm) of the corresponding cations
M(DS)n
n + 40
n = 1 n = 3 d1
d2
d3
QS
d
dW
rM
Na+
3.818
0.909 4.200 0.382 0.0990a
0.1020b
0.196
15.454 12.425 0.0535
0.238
Al3+
Cr3+ 3.818
3.818
3.029 3.273 2.649
3.056 3.273 2.649
12.840 9.784 0.0615
0.333 10.618 7.082 0.0938
0.588 6.202 2.555 0.1032
Gd3+ 3.536
La3+ 3.647
—
—
Fig. 2 XRD patterns of the metal dodecyl sulfate complexes M(DS)n in the low-q
a
b
Tetrahedral coordination. Octahedral coordination.
region.
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hydration of the cation, as we will show below, this
explanation can be ruled out from TGA data, which shows
the smallest number of bound water molecules for the Na+-
based complex. Instead, we believe that in the M(DS)n systems
there are significant differences in the conformational
arrangement of the alkyl chains, essentially dictated by
packing restrictions. As with other lipid systems,50 in metal
complexes of long chain amphiphiles the alkyl chains are
normally thought to be fully extended and to adopt all-trans
conformations.51 However, single crystal XRD studies on
copper(II) decanoate52 show that, while one of the chains has
this all-trans conformation, the other is distorted by rotation of
bonds connecting the methylene groups closest to the anionic
head group to ensure maximum packing. We suggest that a
similar conformational disordering occurs to M(DS)3 systems,
but not with SDS, explaining why the sodium complex does
not behave as the other compounds (Fig. 3). XRD and
molecular modelling studies on anhydrous sodium dodecyl
sulfate suggest a fully extended alkyl chain.46 The observed
decrease in the lamellar bilayer thickness of SDS on increasing
hydration is suggested to result from increased chain tilting.
The shifts in the signals of the terminal methylene peaks
Fig. 3 Ionic radii dependence of the lamellar spacing of the main bilayer
structure (d1, filled symbols) and of the thickness of the water layer (dW, open
symbols) of the metal dodecyl sulfate complexes M(DS)n . The lines drawn are
just guides for the eyes.
the most intense diffraction peak, are characteristic of
lamellar arrangements.6,43 These data are in agreement with
those obtained for other M(DS)3 compounds incorporating
lanthanide ions (M = Nd, Yb, Gd and Ce).6,44 The d1 values
correspond to the successive (001) reflections through planes
of atoms of a lamellar bilayer structure, whose separations are
proportional to twice the length of the DS2 group (ca. 3.5 nm
based on XRD and molecular modeling)45,46 plus twice the
ionic radius of the corresponding metal ion (rMn+) (Table 1).47
This trend was observed for all the trivalent metal dodecyl
sulfates, including the reflections of Al(DS)3 at lowest q,
supporting the idea of a lamellar structure. Lee and co-
workers18 have suggested from the interlamellar spacings that
with Gd(DS)3 prepared at pH , 7 that there is tilting of the
lamellar bilayer relative to the metal plane. Our results are
consistent with this idea. However, as we will see later, there is
evidence for slight conformational disordering in the alkyl
chains, which will also reduce interlamellar spacings.
1
noted in the H and 13C NMR spectra of the trivalent dodecyl
sulfates give support to the idea of conformational disorder-
ing. Although, caution needs to be exercised with this
argument, since the NMR spectra are observed in solution
where aggregation effects may be important,53 literature data
on 2H NMR spectra of a heterogeneous system containing
sodium dodecyl sulfate adsorbed at the alumina/water inter-
face show conformational disordering of the a-methylene
group.54
Interestingly, while the La(DS)3, Gd(DS)3 and SDS com-
plexes produce a single lamellar structure (Fig. 2(b) and
Table 1), in the case of Al(DS)3 and Cr(DS)3, the main lamellar
phase coexists with two other lamellar structures (Fig. 2(a), Fig.
S1 and Table 1), indicating the presence of extra ordering. A
3
common structure with interlamellar spacing less (d3 = 2.649
nm, dotted blue vertical lines in Fig. S1 ) than those of the
3
main lamellar phases of Al(DS)3 (d1 = 3.029 nm, continuous
blue vertical lines in Fig. S1 ) and Cr(DS) (d1 = 3.056 nm,
3
3
We believe that the metal ions are present in a two-
continuous green vertical lines in Fig. S1 ) is detected. A
3
dimensional (2D) plane with
a relatively homogeneous
superstructure with higher interlamellar spacing (d2 = 3.273
distribution, and are coordinated by sulfate (and in some
cases water). Support for this comes from a recent study of
closely related lanthanide surfactant lamellar assemblies,11
where transmission electron microscopy (TEM) and SEM-EDS
measurements showed the trivalent metal ions are present in
an organized 2D structure. This is also consistent with results
of Langmuir–Blodgett films formed from trivalent lanthanide
ions and surfactants.48 Based on this and the structural
parameters involved in the lamellar spacings of this type of
material,49 there should be a linear relationship between these
spacing and the ionic radii. However, from a plot of the d1
nm, dotted green vertical lines in Fig. S1 ) also emerges
3
concomitantly in both compounds. Previous reports55,56 have
attributed the formation of extra ordering in structurally
analogous systems to the entrapment of water molecules. We
will return to this particular issue shortly. Finally, it is worth
mentioning that the XRD patterns of Al(DS)3 and Cr(DS)3
provide evidence that the three lamellar structures referred
above coexist with the lamellar order of the SDS phase.
The SEM images reproduced in Fig. 4 show the morphology
of three representative samples of the metal dodecyl sulfate
complexes: Al(DS)3, Cr(DS)3 and La(DS)3. These compounds
were produced as dense aggregates (Fig. 4(a), 4(e) and 4(k),
respectively) which, in the case of the La3+-based compound,
are homogeneously dispersed (Fig. 4(i) and 4(j)). The aggre-
spacing as a function of rM (Fig. 3) the monovalent Na+ ion
n+
shows a larger interlamellar spacing than expected from its
ionic radius. Although it may be thought that this is due to
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Fig. 4 SEM images of the Al(DS)3 (a)–(d), Cr(DS)3 (e)–(h) and La(DS)3 (i)–(l) complexes.
gates share in common a rather irregular shape, but differ
considerably in terms of size (average diameters of ca. 200–100
mm for Al(DS)3 (Fig. 4(a)) and Cr(DS)3 (Fig. 4(e)) and ca. 10 mm
4 nm, according to the profile plot of Fig. 6(c). It is quite
interesting that the lamellar structuring observed with the
XRD (ca. 3.647 nm, Table 1) and AFM (ca. 4 nm) studies
coincides beautifully.
The thermal behaviour of metal dodecyl sulfate compounds
is expected to depend upon the presence of water, the
for La(DS)3 (Fig. 4(k)). Higher magnification shows that in the
Al(DS)3 and Cr(DS)3 samples these micro-objects are composed
of randomly organized microplatelets assembled along an
edge-to-face fashion, giving rise to an ‘‘origami’’-like morphol-
ogy (Fig. 4(b) and 4(f), respectively). In the case of Cr(DS)3, the
surface of the micro-objects exhibits a waxy appearance
(Fig. 4(g)). The right bottom area of Fig. 4(l) reveals that in
the case of La(DS)3 these platelets are made of lamellae of
nanometer thickness. In La(DS)3 thin films with submicrom-
eter thickness stack along a sort of tile-to-tile piling (Fig. 4(l)).
On the basis of elemental analysis performed on micro-
scopic sections of Al(DS)3, Cr(DS)3, La(DS)3 and Gd(DS)3 by
EDS (Fig. S2 ) the atomic percentage of Al, Cr, La and Gd, and
3
of S was calculated to be 22.40 and 77.60%, 21.35 and 78.65%,
26.02 and 73.98%, and 16.17 and 83.83%, respectively,
suggesting the 1 : 3 stoichiometry. X-ray mapping of a zoom
area of Fig. 5(a), 5(d) ,5(g) and 5(j) demonstrated that in these
samples, as expected, the S atoms are found all over the
surface of the aggregates (Fig. 5(b), 5(e) , 5(h) and 5(k),
respectively). Fig. 5(c), 5(f) , 5(i) and 5(l) demonstrate that the
Al3+, Cr3+, La3+ and Gd3+ ions are also homogeneously
distributed, respectively. These results show that the metal-
surfactant ratio at the surface is identical to that in the bulk
material. This may be relevant to the superstructuring which
we will discuss later.
The texture of La(DS)3, which yields a single lamellar
structure (see above), was further analyzed by AFM upon
scanning the surface of this complex in tapping mode (Fig. 6).
The 3D perspective of the complex, depicted in Fig. 6(a),
confirms its lamellar arrangement. The average periodicity
observed in the amplified 2D image (Fig. 6(b)) is approximately
Fig. 5 SEM images of the Al(DS)3 (a) (zoom of Fig. 4(a)), Cr(DS)3 (d) (zoom of
Fig. 4(e)), La(DS)3 (g) (zoom of Fig. 4(j)) and Gd(DS)3 (j) complexes and
corresponding EDS mapping for S ((b), (e), (h) and (k), respectively) and La ((c),
(f), (i) and (l), respectively).
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Fig. 7 TGA curves of the metal dodecyl sulfate complexes M(DS)n.
is extremely valuable, as it provides information regarding the
release of water molecules and the formation of the anhydrous
complexes.58 Although all the samples undergo a mass loss
within this temperature interval, Fig. 8 and Table 2 provide
evidence that there is a trend in the compounds for the water
content to increase in the order: SDS , La(DS)3 , Gd(DS)3 ,
Cr(DS)3 , Al(DS)3. Clearly, Al(DS)3 and Cr(DS)3 are the
compounds which display the most complex TGA curves
(Fig. 8). In both cases the process of water loss begins already
at 40 uC and ends up at about 112 uC (Table 2). These results
strongly suggest not only the existence of both occluded and
bound water molecules, but also the occurrence of a large
amount of water in these two compounds. Based on the weight
variation corresponding to the water loss measured over this
temperature range (Table 2), it was possible to estimate the
number of water molecules (x) present in the M(DS)n?xH2O
complexes: x = 0.1, 4.1, 3.2, 0.7 and 2.0 for M = Na+, Al3+, Cr3+,
La3+ and Gd3+, respectively. The number of water molecules
Fig. 6 AFM Phase images of the La(DS)3 complex: 3D (a), 2D (b) and profile plot
(c) corresponding to the line drawn in (b). The vertical lines are guides for the
eyes.
stereochemistry of the metal ion and the associated anions,
and the experimental conditions.57 TGA and DSC measure-
ments were used to study the thermal behaviour of the M(DS)3
complexes examined in the present work. The results for
Gd(DS)3 are very similar to those reported by Lee et al. for their
sample prepared at pH 6.18
The TGA curves for all the samples over the 25–900 uC range
(Fig. 7) show different trends of weight loss which can been
divided into three main temperature ranges, designated as
regions I, II and III. Region I is reproduced in detail in Fig. 8;
Regions II and III are represented in Fig. S3(a) and S3(b),
respectively.
The analysis of Region I, which corresponds to the first step
of weight loss taking place from room temperature to 140 uC,
3
Fig. 8 TGA curves (left axis) of the metal dodecyl sulfate complexes M(DS)n in
the 20–140 uC temperature range (region I of Fig. 7) and corresponding
derivative curves (right axis).
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Table 2 Relevant TGA data of the metal dodecyl sulfate complexes
Region
I
SDS Al(DS)3 Cr(DS)3 La(DS)3 Gd(DS)3
T/uC
40
63
72
84
40
99
40
72
72
90
98
0.8
220
253
100
112
8.3
148
164
112
6.3
Weight loss (%)
T/uC
1.3
182
206
3.7
162
186
II
156
76.5
Weight loss (%) 72.7 53.0
57.2
70.0
III
T/uC
358
338
422
482
465
541
521
588
648
521
560
626
720
5.9 13.4
Weight loss (%)
10.9
17.3
21.0
determined for La(DS)3 and Gd(DS)3 are in perfect agreement
with those previously reported for other lanthanide surfactant
complexes.59 On the basis of these findings, the formulae of
the
compounds
are
SDS?0.1H2O,
Al(DS)3?4.1H2O,
Cr(DS)3?3.2H2O, La(DS)3?0.7H2O and Gd(DS)3?2.0H2O.
In an attempt to deduce the thickness of the water layer
between two consecutive bilayers dW (Fig. 3 and 9, and
Table 1), the dilution law employed by Pacios et al. for the
lamellar liquid crystal formed by the surfactant sodium bis-2-
ethylhexyl sulfosuccinate (AOT) in water60 was adopted:
Fig. 9 Schematic view of the characteristic distances of the lamellar bilayer of
the metal dodecyl sulfate complexes M(DS)n.
dehydration onset temperature (Tonset) because of the complex
endotherm profile. For the other four complexes, Tonset values
of approximately 71, 99, 111 and 98 uC, respectively, were
determined (Table 3). The good agreement between these
values and those obtained from TGA data (Fig. 8 and Table 2),
i.e., 72, 99, 100 and 98 uC, is satisfying. Other observable
endothermic peaks discerned in the DSC curves of Fig. 10
correspond to formation of typical intermediate liquid crystal-
line49,53 or other61 mesophases. We also note that the melting
1/d = (1/d0)QS
d = d0 + dW
(1)
(2)
where d is the repeat distance between lamellae and d0 is the
thickness of the AOT bilayer. In the present case d0
corresponded to the spacing d1 and QS represented the molar
fraction of the metal dodecyl sulfate complex in the (metal
dodecyl sulfate complex+water) mixture instead of the volume
fraction of the surfactant in the (surfactant+water) mixture in
the original equation. Fig. 3 shows that the variation of the
thickness of the interlamellar water layer dW with the ionic
radii follows a linear relationship in the case of the complexes
incorporating a trivalent metal ion. The formation of super-
structures in the case of the Cr3+- and Al3+-based compounds is
understandable considering the magnitude of dW (see a
schematic representation in the Fig. 9).
The dehydration and other thermal processes of all the
metal dodecyl sulfate complexes were also followed by means
of DSC measurements. An endothermic peak correlated with
water loss was detected in the DSC curves of Al(DS)3 (broad
and highly structured), Cr(DS)3 (broad and sharp), La(DS)3
(sharp and intense), Gd(DS)3 (structured) and SDS (sharp with
shoulder) (Fig. 10). On second heating, this peak was absent,
in agreement with its attribution to dehydration. In the case of
the Al3+-based complex it was not possible to deduce the
Fig. 10 DSC curves of the metal dodecyl sulfate complexes M(DS)n.
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Table 3 Relevant DSC and PLTM data of the metal dodecyl sulfate complexes.
vS – very strong, S – strong, m – medium, w – weak, vw – very weak, sh –
shoulder
M(DS)n
DSC
PLTM
n = 1
n = 3
Tpeak (uC)
Tonset (uC)
Tonset (uC)
a
Na+
18 (m)
100(m)
108 (sh)
28 (vw)
49 (vw)
59(sh)
98
b
Al3+
67
64(S)
71(S)
28 (vw)
79 (S)
79 (vw)
115 (vS)
108 (m)
115 (m)
130 (sh)
Cr3+
La3+
Gd3+
71
99
75
96
111
104
a
b
Not analyzed. Not calculated.
temperatures of the complexes occur at temperatures greater
than 140 uC.
The thermal behaviour of the samples below 140 uC was
further evaluated by PLTM. Fig. 11 shows that Al(DS)3 and
Cr(DS)3 dehydrate at lower temperatures than La(DS)3 and
Gd(DS)3, in agreement with the TGA and DSC results. The
images obtained by PLTM allow us to easily follow the
dehydration process in Al(DS)3 and Cr(DS)3 by the appearance
of rounded boundaries (Fig. 12(a) and 12(b) and Fig. 12(c) and
12(d), respectively). In contrast, the changes in the PLTM
images of La(DS)3 and Gd(DS)3 are very subtle (Fig. 12(e) and
12(f) and Fig. 12(g) and 12(h), respectively), although they are
very well detected using light intensity signal measurement
(Fig. 11(b)), a particularly useful technique in these cases.
Another clear advantage of the PLTM measurements lies in the
fact that the calculation of the Tonset for dehydration is
considerably easier than with DSC. It was possible to deduce
the Tonset value for Al(DS)3 (67 uC) (Table 3), which was
unreliable in the DSC measurements.
Fig. 11 PLTM curves of the trivalent metal dodecyl sulfate complexes M(DS)3
accompanied by (a) DSC signal and (b) light intensity signal.
has the highest maximum mass loss temperature for all the
compounds studied (Table 2). Interestingly, the global trend
observed for the degradation Tonset of the anhydrous com-
plexes (Al(DS)3 , Cr(DS)3 , Gd(DS)3 , La(DS)3 , SDS) is
exactly the inverse order found for the water release Tonset from
the aquocomplexes. The fact that the temperatures corre-
sponding to the cleavage of the alkyl chains of trivalent metal-
based complexes M(DS)3 complexes are lower than the
temperature with the monovalent salt SDS is in agreement
with previous reports for cerium tri(dodecyl sulfate),6 for a
MCM-41-type material,62 and for SDS strongly bound to the
surface of mesoporous alumina.63 This suggests that there is a
stronger interaction between the surfactant headgroup and the
metal ions in the M(DS)3 compounds than in SDS. We will
return to this particular aspect below, in the section devoted to
the FT-IR analysis of the local environment of the sulfate
headgroup. Whilst no attempts were made at product
identification, based on reports on basic lead(II) dodecylsul-
fate, the degradation of the metal dodecyl sulfate complexes in
Region II is likely to lead to the formation of dodecanol64 and
the metal sulfates, M2(SO4)3. This latter assertion was
confirmed by FT-IR spectroscopy, where the spectra of the
The decomposition of the anhydrous complexes takes place
above 140 uC. This occurs in the temperature ranges
corresponding to Regions II (140 , T ¡ 300 uC) and III (300
, T ¡ 900 uC) (Fig. 7). A sharp mass loss is evident in Region
II (Fig. S3(a) and Table 2). The degradation process that
3
occurs here is likely to be associated either with cleavage of the
bond between the alkyl chains and the sulfate group or the
sulfate ester bond.58
The decomposition for Cr(DS)3 takes place abruptly in a
single step, with a mass loss maximum at 156 uC (Fig. S3(a) ).
3
In contrast, with all the other metal dodecyl sulfates, the
decomposition in Region II involves two-step processes (Fig.
S3(a) ). The temperatures of maximum mass loss depend on
3
the nature of the compound, and is lowest for Al(DS)3 and
highest for La(DS)3 among the trivalent metal salts, while SDS
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analyzed: the symmetric and asymmetric stretching CH2
modes (nsCH2 and naCH2, respectively) and the CH2 bending
(dCH2) mode.
The frequency, width and height of the FT-IR nsCH2 and
naCH2 bands are sensitive to the gauche/trans conformer ratio
and to intermolecular interactions between the alkyl chains.
While the position and intensity of the nsCH2 mode depend
on Fermi resonance between the nsCH2 fundamental with the
overtones of the dCH2 vibrations, the naCH2 mode is sensitive
to coupling of the chain torsional and rotational motions.68–72
The FT-IR nsCH2 and naCH2 bands of all-trans conformers of
alkylchains are expected in the 2846-2849 and 2919 cm21
ranges, respectively.70–76 In the case of alkyl chains adopting
gauche conformations, the FT-IR nsCH2 and naCH2 bands are
upshifted and emerge at 2856–2858 and 2924–2928 cm21
respectively.75,76
,
The FT-IR spectra of all the metal dodecyl sulfate complexes
except Cr(DS)3 are dominated by two bands at 2918 cm21 (very
strong, vS) and 2850 cm21 (strong, S) (Fig. S4(b) and Table 4),
3
assigned to the naCH2 and nsCH2 modes, respectively,
indicating that a considerable fraction of the alkyl chains are
fully stretched (all-trans) and densely packed. However, the
shoulder which emerges at about 2928 cm21 in the spectra of
all the complexes (Fig. S4(b) and Table 4) demonstrates that
3
gauche and all-trans conformers coexist in these compounds.
The band profiles in the FT-IR spectra of Cr(DS)3 are
significantly different: the naCH2 and nsCH2 bands are
markedly broader and their intensity maxima are shifted to
higher wavenumbers (2922 and 2854 cm21, respectively) (Fig.
S4(b) and Table 4). These findings prove that the proportion
3
of partially disordered alkyl chains is much higher in the Cr3+
based complex than in the other four complexes studied.
-
Fig. 12 PLTM images of Al(DS)3 (30 uC (a) and 70 uC (b)), Cr(DS)3 (30 uC (c) and
80 uC (d)), La(DS)3 (30 uC (e) and 120 uC (f)) and Gd(DS)3 (30 uC (g) and 120 uC
(h)).
Similar conclusions may be drawn from inspection of the
signature of the metal dodecyl sulfate complexes in the dCH2
region of the FT-IR spectra. The analysis of this band is
particularly useful, because its position, intensity and band
shape are sensitive to interchain interactions and chain
packing.76,77 A higher frequency (y1472 cm21) indicates the
presence of ordered alkyl chains along extended, all-trans zig-
zag, conformations. In contrast, lower frequencies (y1466
cm21), broadening and weakening of the intensity provide
evidence of a larger gauche/trans conformer ratio and conse-
quently the occurrence of larger population of disordered alkyl
chains.76,77 The FT-IR spectra in the dCH2 region of all the
metal dodecyl sulfate complexes display a prominent band
centered at about 1468 cm21 and a shoulder at 1478 cm21 (Fig.
M(DS)3 complexes after treatment at 900 uC (data not shown)
only exhibited a characteristic band ascribed to the sulfate
group.
Following carbon chain cleavage, the final degradation of
all the metal dodecyl sulfate complexes occurs in Region III
(Fig. 7). At the highest temperature studied, the Cr3+- and Gd3+-
based complexes are fully decomposed to form the metal
sulfates. However, with the other compounds studied, a
considerable amount of residue still remains (20–26% of the
sample mass) at the same temperature (Fig. 7). The series of
peaks detected in the TGA derivative curves of Fig. S3(b) above
3
550 uC might be linked to the combustion of residual organic
matter. Above 700 uC no signals are observed in the TGA
S4(c) and Table 4), thus confirming that the alkyl chains of all
3
these compounds contain simultaneously all-trans and gauche
conformers. It has been reported that the presence of gauche
bonds is accompanied by the development of a band at ca.
derivative curves (Fig. S3(b) ).
3
The FT-IR spectra obtained for the samples in the present
1440 cm2178,79 which is not the case here (Fig. S4(c) ). While
study (Fig. S4 and S5 ) are similar to those reported for related
3
3
materials.6,,44,49,65–67
the FT-IR data indicate some conformational disordering of
the alkyl chains in these metal dodecyl sulfates, they do not
indicate either the extent or location. However, further
information can be obtained from molecular dynamics
simulations on n-alkanes,80 which have demonstrated the
Rich information on the degree of chain packing and the
trans/gauche conformational ratio in alkyl chains of the solid
metal dodecyl sulfate complexes can been retrieved from the
FT-IR spectra. Three diagnostic modes of the alkyl chains were
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68–77,81–86,89–95
Table 4 Frequency (in cm21) and assignment of characteristic bands of the FT-IR the metal dodecyl sulfate complexes M(DS)n
n = 1
Assignmenta
Conformations
n = 3
Na+
Al3+
Cr3+
La3+
Gd3+
2928 sh
2918
2928 sh
2918
2922 sh
2854
2928 sh
2919
na(CH2)
ns(CH2
gauche
all-trans
gauche
2918
2850
1478 sh
1468
2851
1478 sh
1469
2851
1478 sh
1468
2851
1478 sh
1468
all-trans
all-trans
gauche
1478 sh
1468
1458 sh
1459 sh
1458
1458 sh
1458 sh
not assigned
1271 sh
1257 vS
1237 sh
1225 vS
1272 S
1272 sh
1259 sh
1237 sh
1213 vS
1273 sh
1257 sh
1237 vS
1222 vS
1273 sh
1257 sh
1237 vS
1215 vS
na(SO4) coordinated
1257 sh
1237 sh
1211 vS
n21
…
1185 vS
1172 sh
1157 sh
1189 vS
1172 sh
1157 S
1186 sh
1173 sh
1163 sh
1184 S
1172 sh
1163 sh
1193 vS
1173 vS
1157 S
M–O( H)SO3
1090 S
1077 m
1090 S
1077 sh
ns(SO4) coordinated
1077 m
1077 sh
1063 S
1077 sh
1065 S
n21
…
1045 m
891 w
1045 m
891 vw
1045 sh
1044 m
1044 m
891 vw
M–O( H)SO3
832 sh
ns(C–OSO3)
a
vS – very strong; S – strong; m – medium; w – weak; sh – shoulder.
various types of defects possible. To be consistent with the
XRD long spacings, it is not possible to have random
disordering along the alkyl chains, otherwise the reflections
would not be sharp. Instead, it is probable that relatively
localized disordering occurs close to the terminal sulfate
group. This suggests end-gauche, kink or jog structures.80 The
NMR results presented earlier are fully consistent with this
view.
series of alkali palmityl (hexadecyl) sulfates,84 which may be
considered as close analogs of the metal dodecyl sulfate
complexes.
Globally, the profile of the FT-IR spectrum of Al(DS)3 closely
resembles that of Gd(DS)3, while strong similarities are also
observed between the Cr(DS) /La(DS) pair (Fig. S5 ). The most
3
3
3
noteworthy aspect of the FT-IR spectrum of SDS is that the low-
frequency side of the naSO4 band differs considerably from
Although it would be valuable to carry out detailed analysis
of the 3700–3000 cm21 interval of the FT-IR spectra of SDS,
Al(DS)3, Cr(DS)3, La(DS)3 and Gd(DS)3, where the characteristic
OH stretching modes of water absorb, this is complicated by
bands from adsorbed water from the potassium bromide
pellets used, and the results for all the systems given in Fig.
those of the trivalent metal dodecyl complexes (Fig. S5 ).
3
The strong feature that emerges at 1107 cm21 in the spectra
of SDS?0.1H2O is assigned to the naSO4 mode, indicating the
presence of ‘‘free’’ anions.85,86 This event is also detected in
the spectra of the trivalent metal-based analogues (Table 4),
and is consistent with the XRD results discussed earlier.
The strong chemical resemblance between SDS and sodium
palmityl sulfate76 allowed us to associate the shoulder at 1271
cm21, the very strong events at 1257, 1237 and 1225 cm21 and
the intense event at 1090 cm21 to the naSO4 mode and the
strong band at 1090 cm21 and the medium intensity band at
1077 cm21 to the nsSO4 mode of the anion coordinated with
Na+ (Table 4). The various naSO4 components correspond to
different directions of the transition dipole moment.82
Comparison of these results with those of the trivalent metal
docecyl sulfates (Table 4) suggests that the naSO4 mode is
correlated with the nature of the cation, unlike in the case of
metal palmityl sulfates.84 The shift of the strong SDS naSO4
component at 1225 cm21 to lower wavenumbers for the
trivalent metal dodecyl complexes indicates that the dodecyl
S4(a) should only be considered for reference purposes.
3
The FT-IR spectral signature of the SDS, Al(DS)3, Cr(DS)3,
La(DS)3 and Gd(DS)3 complexes in the region where the main
2
absorption bands of the RSO4 ion are expected (1350–750
cm21 interval) is shown in Fig. S5 . A general discussion of the
3
vibrations of the sulfate group is given in the ESI. The analysis
3
of these spectra is complicated by the fact that some of the
most characteristic sulfate modes are superimposed with the
skeletal C–C stretching (nC–C) vibration modes of the alkyl
2
chains (Table 4). The attribution of the RSO4 bands of the
metal dodecyl sulfates (Table 4) will essentially rely on
published infrared spectroscopic studies dealing with SDS, a
Pr-based surfactant mesophase templated by the dodecyl
sulfate ion (DS2),81–83 gadolinium dodecyl sulfate18 and on a
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sulfate is complexed by the metal. This is similar to what is
observed in metal complexes of sulfonic acid salts (R–
SO32:M+),87 and suggests strong, essentially electrostatic,
interactions between trivalent metal ions and dodecyl sulfate.
In the nsSO4 region the 1090 cm21 feature undergoes a shift to
1063 and 1065 cm21 in the case of Cr(DS)3 and La(DS)3,
good evidence for slight conformational disordering of the
methylene groups close to the sulfate head group. The lamellar
structuring evidenced by XRD was corroborated by AFM
studies. Dehydration, followed at higher temperatures by alkyl
chain cleavage, of the trivalent metal decylsulfates is observed
upon heating. These results show a simple route under mild
conditions for preparation of layered structures of trivalent
metal ions of electronic, magnetic, optical and catalytic
interest.
respectively (Fig. S5 and Table 4). This downshift may be
3
interpreted as an indication, not only that the M–O bond is
about the same order of magnitude in both compounds, but
also that the M–O bond is stronger in these two complexes
than in SDS, Al(DS)3 and Gd(DS)3.
The presence of hydrogen bonding interactions between
the water molecules belonging to the first coordination sphere
of the metal ions and the sulfate ions along a monodentate
fashion (C3v) is highly likely. Based on the assignment
proposed by Hug for the [Fe(III)(H2O)5SO4]+ complexes formed
in hematite,85 the prominent band at 1185 cm21, the shoulder
at 1172 cm21, the medium intensity event located at 1045
cm21 and the weak feature detected at 891 cm21 detected in
Acknowledgements
˜
ˆ
MEC, FEDER, Fundaçao para a Ciencia e a Tecnologia (FCT,
Portugal) and COMPETE are thanked for financial support
through the projects MAT2008-06079/MAT and Pest-C/CTM/
LA0011/2011). R.F.P.P. thanks FCT for a Ph.D. grant (SFRH/
BD/38696/2007).
the spectrum of SDS?0.1H O (Fig. S5 ) are associated with the
3
2
n
2
1
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