Formation of diverse mesophases templated by a diprotic anionic surfactant

Article abstract of DOI:10.1002/chem.200800766

The synthesis system for mesophase formation, using the diprotic anionic surfactant N-myristoyl-L-glutamic acid (C₁₄GluA) as the structure-directing agent (SDA) and N-trimethoxylsilylpropyl-N,N,N-trimethyl- ammonium chloride (TMAPS) as the co-structure-directing agent (CSDA), has been investigated and a full-scaled synthesis-field diagram is presented. In this system we have obtained mesophases including three-dimensional (3D) micellar cubic Fm3m, Pm3n, Fd3m, micellar tetragonal P4 ₂/mnm, two-dimensional (2D) hexagonal p6mm and bicontinuous cubic Pn3m, by varying the C₁₄GluA/NaOH/TMAPS composition ratios. From the diagram it can be concluded that the mesophase formation is affected to a high degree by the organic/inorganic-interface curvature and the mesocage-mesocage electrostatic interaction. Bi-continuous cubic and 2D-hexagonal phases were found in the low organic/inorganic-interface curvature zones, whereas micellar cubic and tetragonal mesophases were found in the high organic/inorganic-interface curvature zones. Formation of cubic Fm3m and tetragonal PA₂/mnm was favoured in highly alkaline zones with strong mesocage-mesocage interactions, and formation of cubic Pm3n and Fd3m was favoured with moderate mesocage-mesocage interactions in the less alkaline zones of the diagram.

Full text of DOI:10.1002/chem.200800766

FULL PAPER
DOI: 10.1002/chem.200800766
Formation of Diverse Mesophases Templated by a Diprotic Anionic
Surfactant
Chuanbo Gao,[a, b] Yasuhiro Sakamoto,[b] Osamu Terasaki,[b] and Shunai Che*[a]
Abstract: The synthesis system for
mesophase formation, using the diprot-
ic anionic surfactant N-myristoyl-l-glu-
tamic acid (C14GluA) as the structure-
directing agent (SDA) and N-tri-
two-dimensional
(2D)
hexagonal
continuous cubic and 2D-hexagonal
phases were found in the low organic/
inorganic-interface curvature zones,
whereas micellar cubic and tetragonal
mesophases were found in the high or-
¯
p6mm and bicontinuous cubic Pn3m,
by varying the C14GluA/NaOH/
TMAPS composition ratios. From the
diagram it can be concluded that the
mesophase formation is affected to a
high degree by the organic/inorganic-
interface curvature and the mesocage–
mesocage electrostatic interaction. Bi-
A
ganic/inorganic-interface
curvature
¯
ammonium chloride (TMAPS) as the
co-structure-directing agent (CSDA),
has been investigated and a full-scaled
synthesis-field diagram is presented. In
this system we have obtained meso-
phases including three-dimensional
zones. Formation of cubic Fm3m and
tetragonal P42/mnm was favoured in
highly alkaline zones with strong meso-
cage–mesocage interactions, and for-
¯
¯
mation of cubic Pm3n and Fd3m was
favoured with moderate mesocage–
mesocage interactions in the less alka-
line zones of the diagram.
Keywords: formation mechanism ·
mesophases · mesoporous materials ·
surfactants · synthesis-field diagram
¯
¯
(3D) micellar cubic Fm3m, Pm3n,
¯
Fd3m, micellar tetragonal P42/mnm,
Introduction
mechanisms of these mesophases, even though a simplified
surfactant packing parameter g[15] was employed to predict
the mesophase.[16]
Since the discovery of mesoporous silica in the last
decade,[1,2] various mesophases have been found and re-
solved including the cage-type tetragonal (P42/mnm,[3] etc.),
Anionic-surfactant templating method, first reported by
Che et al.,[17] affords diverse mesoporous silicas with high
crystallinity, including anionic-surfactant templated mesopo-
rous silica (AMS), (AMS)-1~10,[17–20,3,13] and a chiral meso-
porous silica with helical morphology.[21–23] Therefore, this
system is a good candidate for the investigation of the for-
mation mechanisms and the relationships of different meso-
phases. Moreover, these kinds of mesoporous material are
homogeneously functionalised with organic moieties, and
have a high loading efficiency through the interactions be-
tween surfactant headgroups and the amino or quaternary
3D hexagonal (P63/mmc[4]), cubic (Pm3n,
Fd3m,
[5,6]
[7,8]
¯
¯
[6,9]
[10,11]
[2,12]
¯
¯
¯
Fm3m, etc), bicontinuous cubic (Ia3d,
etc), 2D hexagonal (p6mm[2,14]), and lamellar
Im3m,
[13]
¯
Pn3m,
phase.[2] The mesoporous silicas that have these mesostruc-
tures were obtained from different synthesis systems by
using cationic, anionic or non-ionic surfactants as a struc-
ture-directing agent (SDA). Because of the complexity of
the synthesis systems it is difficult to find the formation
ammonium groups of
a
co-structure-directing agent
[a] C. Gao, Prof. S. Che
(CSDA). Therefore, AMS has attracted much attention of
the researchers with different scientific interests including
material science, structural physics, supermolecular chemis-
try and pharmacy.
The capacity of the AMS synthesis system to form diverse
and highly-ordered mesophases can be mainly attributed to
the introduction of an additional co-structure-directing
effect by the inclusion of aminopropylsiloxane or quater-
nised aminopropylsiloxane in the inorganic precursors to act
as a CSDA. The negatively charged headgroups of the
School of Chemistry and Chemical Technology
State Key Laboratory of Composite Materials
Shanghai Jiao Tong University, Shanghai 200240 (China)
Fax : (+86)21-5474-1297
E-mail: chesa@sjtu.edu.cn
[b] C. Gao, Dr. Y. Sakamoto, Prof. O. Terasaki
Structural Chemistry
Arrhenius Laboratory
Stockholm University, 10691 Stockholm (Sweden)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800766.
Chem. Eur. J. 2008, 14, 11423 – 11428
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11423

anionic surfactants interact electrostatically with the posi-
tively charged ammonium sites of the CSDAs, while the al-
koxysilane groups of the CSDA co-condense with tetraal-
koxysilane and are subsequently assembled to form the
silica framework.[17] The interaction between the wall-form-
ing precursors and the surfactant by this method is much
stronger than that in the conventional synthesis of mesopo-
rous silicas, examples including MCM-41 and SBA-15.[2,14]
This strong organic/inorganic electrostatic interaction serves
as the driving force of the formation of highly ordered AMS
mesostructures.
Results and Discussion
Diverse mesostructures in the synthesis-field diagram: Sixty-
four compositions were carefully chosen to complete the
synthesis-field diagram of the C14GluA/NaOH/TMAPS syn-
thesis system (Figure 1). The determination of the meso-
It is also found that well-ordered mesophases in decreas-
ing order of organic/inorganic-interface curvature from
¯
cage-type (tetragonal P42/mnm; cubic Pm3n with modula-
¯
tions and cubic Fd3m), to 2D hexagonal (p6mm), bicontinu-
¯
¯
ous cubic (Ia3d and Pn3m) and lamellar, can be easily con-
trolled by decreasing the ionization degree of the weakly
acidic anionic surfactant.[19] It has been considered that a de-
crease in the degree of ionization of the surfactant leads to
a reduction in the negative-charge density on the micelles,
which further results in a decrease in the electrostatic repul-
sion of the surfactant headgroups and the headgroup area in
micelles. The smaller headgroup area leads to a larger sur-
factant-packing parameter, g, which gives rise to mesophases
with smaller organic/inorganic interface curvatures. More-
over, it is also reported that by controlling the reaction ki-
netics, different cage-type mesostructures including
¯
¯
P42/mnm, Pm3n, Fd3m and a modulated structure can be
obtained.[3] Nevertheless, the precise mechanism of the mes-
ophase determination is not well understood.
Figure 1. Synthesis-field diagram (mole fraction) of the C14GluA/NaOH/
TMAPS synthesis system.
Herein we present a full-scaled synthesis-field diagram of
the mesoporous silicas prepared with diprotic anionic surfac-
tant N-myristoyl-l-glutamic acid (C14GluA) as the SDA and
N-trimethoxylsilylpropyl-N,N,N-trimethylammonium chlo-
ride (TMAPS) as the CSDA. We chose diprotic C14GluA as
the referenced SDA, because it has two carboxylate groups,
which creates the possibility of forming different mesophas-
es with highly curved organic/inorganic interface curvatures
as well as mesophases with reduced curvature. Water soluble
TMAPS, which has a permanent positive charge, interacts
with anionic surfactant in an electrostatic way and serves as
the counterion of the anionic surfactant. The amount of tet-
raethyl orthosilicate (TEOS) in the synthesis of AMS within
a certain range does not have significant effects on the cur-
vature of the resultant mesophase. Therefore, the key fac-
tors of the mesophase formation has been determined as
SDA C14GluA, CSDA TMAPS, and NaOH, the composi-
tions of which have been changed, while keeping other fac-
tors (the concentration of surfactant, the TEOS/C14GluA
ratio and temperature) constant to get the full picture of the
diagram. On the basis of the synthesis-field diagram the for-
mation mechanism and the structural relationship of differ-
ent mesophases have been further discussed in details.
structure was on the basis of X-ray diffraction (XRD) pat-
terns and HRTEM images. The synthesis conditions of the
mesophases are summarised in Table 1.
Table 1. Synthesis conditions of the mesophases in the C14GluA/NaOH/
TMAPS system.
Mesophase
Mesopore
type
pH[a]
NaOH/
C14GluA
TMAPS/
C14GluA
¯
Pn3m
bicontinuous
cylindrical
micellar cubic
–
micellar cubic
micellar tetragonal
micellar cubic
5.0–5.2
5.2–5.6
5.6–6.2
6.2–7.4
6.1
0.45–0.62
0.62–0.86
0.86–1.25
1.25–1.7
1.16
0.84–3.8
0.35–3.8
1.1–4.5
1.6–3.8
1.0
p6mm
¯
Fd3m
undefined
¯
Pm3n
P42/mnm
6.7
7.4–11.0
1.56
1.7–2.7
1.2
1.6–3.3
¯
Fm3m
[a] The pH of the reaction solution was measured at 708C before the ad-
dition of TMAPS and TEOS.
The XRD pattern of the mesoporous silica synthesised
¯
with the space group of Pn3m (Supporting Information, Fig-
ure S1), as the composition shown in Figure 1, has two well-
pffiffiffi pffiffiffi
resolved peaks with the reciprocal d-spacing ratio of 2/ 3,
which can be assigned to 110 and 111 reflections. According
to the analysis of its high-resolution transmission electron
microscopy (HRTEM, images not shown), the mesostruc-
¯
ture is confirmed to be bicontinuous cubic Pn3m, composed
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Chem. Eur. J. 2008, 14, 11423 – 11428

Mesophases Templated by Surfactant
FULL PAPER
of an enantiomeric pair of 3D-
mesoporous networks that are
interwoven with each other.[13]
The three peaks of the typical
XRD pattern of the mesopo-
rous silica with the 2D-hexago-
nal symmetry (Supporting In-
formation, Figure S1) can be
well assigned to 10, 11, and 20
reflections of the p6mm space
group. These two structures,
Figure 2. a) XRD pattern, b) HRTEM image, and c) SEM image of the cage-type mesoporous silica that has
the space group cubic Pm3n: C14GluA/NaOH/TMAPS 0.317:0.367:0.317.
¯
¯
bicontinuous Pn3m and p6mm,
which show a low organic/inor-
ganic-interface curvature and
the bicontinuous/cylindrical mesopore geometry, were
formed within a pH range of as low as 5.0–5.6 and NaOH/
C14GluA ratio of 0.45–0.86 (Table 1).
gram. The mesostructure was formed with a NaOH/
C14GluA ratio of 1.16 showing a pH of 6.1, and the TMAPS
content in the system was lower (TMAPS/C14GluA=1.0)
¯
¯
The cage-type mesophases, including cubic Fd3m, cubic
than that of the Fd3m synthesis system.
¯
¯
Pm3n, tetragonal P42/mnm and cubic Fm3m, have been pre-
pared by changing the C14GluA/NaOH/TMAPS composi-
tions. These mesophases are formed in the systems with a
high ionization degree of surfactant (NaOH/C14GluA=
0.86–2.7) showing a relatively high pH of 5.6–11.0. The oc-
currence of each mesophase in the synthesis-field diagram
has been described in Figure 1 and Table 1.
The typical XRD pattern, HRTEM image, and SEM
image of the mesophase P42/mnm are collected in the Sup-
porting Information (Figure S3). The XRD pattern is diffi-
cult to index, because no detailed peaks are presented
within the range of 1–6o 2q. However, from the HRTEM
image taken along [001] zone axis, the mesostructure can be
determined as tetragonal P42/mnm. The SEM image shows
that the sample possesses tetragonal morphology and has
4/mmm point group symmetry, which confirms the tetrago-
nal mesostructure. The structure shows many X-ray reflec-
tions, which makes the XRD pattern hard to index. The tet-
ragonal mesophase was formed at a TMAPS/C14GluA ratio
of 1.2 and a NaOH/C14GluA ratio of 1.56 showing a pH of
6.7.
The typical XRD pattern, HRTEM and scanning electron
¯
microscopy (SEM) images of the cubic Fd3m mesophase are
shown in the Supporting Information (Figure S2). The
HRTEM image taken along [110] direction clearly reveals
the mesostructure, and the two well-resolved peaks in the 1–
pffiffi pffiffiffi
o
6
2q of the XRD pattern with a 2/ 3 reciprocal d-spac-
ing ratio can be indexed to 220 and 222 reflections of the
Fd3m space group. An existence of twin planes, which can
¯
The typical XRD pattern, HRTEM, and SEM images of
¯
be clearly observed in the HRTEM image, produces streaks
in the Fourier diffractogram inserted in the image. The mor-
phology of the sample observed from the SEM is crystal-
the face-cantered cubic Fm3m mesostructure is shown in
Figure 3. The XRD peaks can be indexed to 111, 200, 220,
and 311 reflections, indicating a high crystallinity. The
HRTEM image taken in [110] direction clearly confirms the
mesostructure. Figure 3c shows a polyhedron crystal of the
¯
like and is typically cubic having m3m symmetry. This meso-
phase was formed within a TMAPS/C14GluA range of 1.1–
4.5 and a NaOH/C14GluA range of 0.86–1.25 showing a pH
of 5.6–6.2.
¯
structure, which does not fit m3m point group symmetry.
This morphology can be explained by twinning commonly
[24]
¯
The typical XRD pattern, HRTEM, and SEM images of
observed in face cantered cubic Fm3m structure.
This
¯
the cubic Pm3n mesophase are shown in Figure 2. The XRD
mesophase was formed at a high pH of 7.4–11.0 with a
TMAPS/C14GluA ratio of 1.6–3.3 and a NaOH/C14GluA
ratio of 1.7–2.7.
pattern showing three diffraction peaks with a reciprocal d-
pffiffi pffiffiffi pffiffiffi
spacing ratio of 4/ 5/ 6 can be well indexed as the 200,
210, and 211 reflections, con-
¯
sistent with the Pm3n space
group. The HRTEM images
taken along [100] zone axis
and the dodecahedron crystal
morphology demonstrated by
the SEM image strongly con-
firm the mesostructure. It
¯
shows m3m point group sym-
metry. In the HRTEM image
we can observe planar defects,
which give rise to diffuse scat-
tering in the Fourier diffracto-
Figure 3. a) XRD pattern, b) HRTEM image, and c) SEM image of the cage-type mesoporous silica that has
the space group cubic Fm3m: C14GluA/NaOH/TMAPS 0.200:0.400:0.400.
¯
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11425

S. Che et al.
A zone around C14GluA/NaOH/TMAPS 0.2:0.3:0.5 in the
diagram gives rise to a undefined cage-type mesophase,
which was formed at a pH of 6.2–7.4. The hydrolysis of
TMAPS and silica source TEOS was very low within this
range of pH, which hindered the self-organization of the or-
ganic/inorganic species and the formation of stable ordered
mesophases. The mesophase formed is unstable and easily
collapses during the thermal treatment at 5508C. The struc-
ture is not fully understood and is under further investiga-
tion. (see the HRTEM images in Figure S4 of the Support-
ing Information)
a line in direction c in the diagram, and the structure of the
mesoporous silica obtained changes from bicontinuous cubic
¯
¯
Pn3m to 2D hexagonal p6mm, cubic Fd3m, an undefined
¯
cage-type phase, and cubic Fm3m; from 2D hexagonal
p6mm to cubic Pm3n and tetragonal P42/mnm. These
¯
changes of mesophases show a dramatic mesopore geometry
change from bicontinuous to cylindrical and further cage-
types, with an increase of the organic/inorganic interface
curvature.
Mesophase formation dominated by the organic/inorganic
interface curvature: As a conclusion of the above discus-
sions, the bicontinuous cubic phase, which has a low organ-
ic/inorganic-interface curvature (1/2<g<1), has been ob-
tained in the zones with low surfactant ionization degree
(NaOH/C14GluA=0.45–0.62) showing an initial pH of 5.0–
5.2. The 2D-hexagonal phase, whose organic/inorganic-inter-
face curvature (g=1/2) is larger than bicontinuous cubic,
was synthesised in the zones with a moderate ionization
degree of surfactant (NaOH/C14GluA=0.62–0.86), with an
initial pH of 5.2–5.6. Increasing the ratio of NaOH/C14GluA
to 0.86–2.7, or increasing the pH of synthesis system to 5.6–
11.0 lead to the formation of cage-type mesophases with the
largest organic/inorganic-interface curvature (g=1/3).
Notably, in Figure 1, composition points in zone a gives
¯
¯
¯
rise to a coexistence of Fd3m, Pm3n, P42/mnm, and Fm3m
(Supporting Information, Figure S5); composition points in
zone b give a mixture of cubic and 2D-hexagonal phase
(Supporting Information, Figure S6); and a mixed phase
¯
comprising bicontinuous cubic Pn3m and 2D hexagonal
p6mm could be found in zone g (Supporting Information,
Figure S7). However, in the zones d, e, or z, mesoporous sili-
cas could not be formed because the mole fraction of
NaOH, C14GluA or TMAPS, respectively, was too high.
The diverse mesophases, as have been described above,
were achieved in the same synthesis system but different
composition ratios. It proved the versatility of the meso-
phase formation by using the diprotic anionic surfactant
CnGluA as the SDA by means of the co-structure-directing
method. On the other hand, the diverse mesophases formed
in the same system make it possible for the study on the
structure occurrence, dependence, and formation mecha-
nism.
In our earlier works[13,19] we have described that in the
synthesis of AMS, higher alkalinity favours the formation of
mesophases with high organic/inorganic-interface curvature,
owing to the lager ionization degree of the anionic surfac-
tant. From the full-scaled synthesis-field diagram shown in
Figure 1, this change of mesophase from bicontinuous cubic
to 2D hexagonal and further cage-type, triggered by the in-
crease of the ionization degree of the surfactant, proves the
dominating rule of the organic/inorganic-interface curvature
on the mesophase determination.
Effect of the synthesis composition on the mesostructure:
From the diagram shown in Figure 1, it can be inferred that
the phase fields are strip-like, lie in the sector zones, and
employ C14GluA/NaOH/TMAPS 0:0:1 as the vertex (as the
shadow zones in Figure 1 shows). It is known that when
changing the composition point down a line, starting at the
point C14GluA/NaOH/TMAPS 0:0:1 in the synthesis-field
diagram, the ratio of NaOH/C14GluA is kept constant and
the ratios of TMAPS/NaOH and TMAPS/C14GluA are de-
creasing. Therefore, the strip-like distribution of the meso-
phases in the diagram indicates that the determining factor
of the mesophase formation is the ionization degree of the
surfactant (NaOH/C14GluA); the effect of the amount of
TMAPS in the synthesis gel on the mesophase formation is
not prominent. The change in the fraction of TMAPS in the
synthesis gel could not dramatically change the mesophase
obtained, especially in the zones of bicontinuous cubic and
2D hexagonal. Notably, the mesophase changes could be
also observed in the zones of the cage-type mesophase from
tetragonal P42/mnm to an undefined cage-type phase and
Mesophase formation affected by the mesocage–mesocage
electrostatic interactions: The organic/inorganic-interface
curvature dominates the change of mesophase from bicon-
tinuous cubic to 2D hexagonal and further cage-type; how-
ever, it cannot explain the packing manner of the mesocages
in the cage-type zone and the formation mechanism of dif-
ferent 3D mesostructures, which has been long pursued by
scientists. Our system affords various cage-type mesostruc-
tures by simply varying the mole fraction, which provides
experimental foundations to propose a possible formation
mechanism as described in Scheme 1.
In the formation of cage-type mesostructures anionic sur-
factant molecules form micelles to reduce the free energy,
which makes a negatively charged surface. On the outside of
this surface, to balance the negative charge, positively
charged CSDA distributes in both the Stern layer in which
immobile CSDA cations are tightly bonded to the surface
and the diffuse layer in which mobile CSDA cations weakly
interact with the charged surface (Scheme 1). The micelles
and the inorganic species in the Stern layer, which together
are defined as a “mesocage”, are then packed by certain
means with the hydrolyzed TEOS to form the mesostruc-
¯
¯
from cubic Pm3n to cubic Fd3m, as a result of an increase
of the mole fraction of TMAPS in the C14GluA/TMAPS/
NaOH tri-component system.
When keeping the mole fraction of TMAPS constant in
the C14GluA/TMAPS/NaOH system and increasing the
NaOH/C14GluA ratio, the composition point changes down
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Chem. Eur. J. 2008, 14, 11423 – 11428

Mesophases Templated by Surfactant
FULL PAPER
caused by the highly charged
mesocages, give rise to a face-
cantered close-packed cubic
¯
Fm3m mesophase. It can be in-
ferred that the charging state
of the mesocages or the meso-
cage–mesocage interactions is
the determining factor of the
packing of the mesocages and
formation of cage-type meso-
structures.
It can be also inferred from
¯
the results that cubic Pm3n
and tetragonal P42/mnm were
formed in the synthesis system
with low mole fraction of
TMAPS (TMAPS/C14GluA=
1.0 and 1.2, respectively), and
Scheme 1. Different charging states of mesocage: a) The mesocage is mildly negatively charged when the sur-
factant micelle shows low-charge density; b) The mesocage is highly negatively charged when the micelle
shows high-charge density. The different charging state leads to different interactions between the mesocages
and different packing manners.
¯
¯
cubic Fd3m and Fm3m were
formed with a high content of
TMAPS
in
the
system
(TMAPS/C14GluA=1.1–4.5
and 1.6–3.3, respectively). It in-
ture. In this model, the “mesocage” acts as a “brick” and
the hydrolyzed TEOS acts as “mortar”, and the interactions
between the “bricks” dominate the structure of the formed
composite matrix.
These building blocks of the cage-type mesostructures,
mesocages, are negatively charged, because the positive
charges in the Stern layer could not balance the negative
charges of the micelles, which could only be achieved at an
infinite distance. The electrostatic interaction between the
mesocages changes with the charging state of the micelles
and further determines the packing manner of these meso-
cages. As described in Scheme 1, qualitatively speaking,
when the charge density of the micelles is low, the meso-
dicates that the mole fraction of CSDA also affects the
structure of the mesoporous silica to an extent, which may
be attributed to the screening of the mesocage–mesocage
electrostatic repulsion by the diffused CSDA cations in the
synthesis system.
The packing of charged identical hard spheres has been
investigated and the particle volume fraction, particle charg-
ing and dipoles have been considered the factors of the hard
sphere packing.[25] To a great extent the hard sphere packing
system is similar to the synthesis system of mesoporous
silica and the results are fairly consistent. However, in our
system, because of the soft nature of the synthesis approach
and the need to minimise the free energy and occupy the
space, the system itself would adopt a more versatile strat-
egy and more abundant phases could be obtained, for exam-
ACHTUNGTRENNUNGcages are mildly charged, and the mesocage–mesocage elec-
trostatic interaction is weak; on the contrary, when the
charge density of the micelles gets higher, the mesocages
are highly charged, and the mesocage–mesocage electrostat-
ic interaction gets strong.
¯
¯
ple, Pm3n, Fd3m and P42/mnm, in which there are at least
two types of micelles with different sizes and shapes.[6,8]
Furthermore, from the perspective of the mesocage–
mesocage interaction we can propose a reasonable mecha-
nism of the mesophase change through varying the delayed
time (t) of the addition of TEOS after 3-aminopropyltri-
From our results shown in Figure 1 and Table 1 it can be
inferred that, in the mildly alkaline zones of the diagram
(the initial pH of the C14GluA solution is 5.6–6.2), the ioni-
zation degree of surfactant is relatively low (NaOH/
C14GluA=0.86–1.25), and therefore the mesocages are mod-
erately charged and the interactions are relatively weak;
AHCTUNGTRENNUNG
ethoxylsilane (APES) reported by Garcia-Bennett et al.[3] It
is known that the rate of hydrolysis and condensation of or-
ganotriethylsilane is lower than TEOS, and a larger value of
the delayed time t makes a pre-hydrolyzed APES and thus
facilitates the co-condensation of APES and TEOS. It leads
to a Stern layer with lower positive charge density and
therefore larger mesocage–mesocage interactions. However,
the further elongation of the time t results in a self-conden-
sation of APES, which leads to a higher positive charge den-
sity of the Stern layer and therefore lower mesocage–meso-
cage interactions. As a result, mesophase changes from
¯
¯
cubic Pm3n and Fd3m were formed when packing of these
mesocages. If the ionization degree of the surfactant gets
higher (NaOH/C14GluA=1.56) at more alkaline conditions
(the initial pH of the C14GluA solution is 6.7), the meso-
ACHTUNGTRENNUNGcages are highly charged because of the high charge density
on the micelle surfaces, and therefore the mesocage–meso-
cage interactions are strong; tetragonal P42/mnm can be
found in these zones of the diagram. If the reaction is car-
ried out under even stronger alkaline conditions (the initial
pH is 7.4–11.0), the strong mesocage–mesocage interactions,
¯
¯
¯
Fd3m to P42/mnm, and further from P42/mnm to Pm3n and
Fd3m were observed by delaying the addition of TEOS.
Chem. Eur. J. 2008, 14, 11423 – 11428
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11427

S. Che et al.
Conclusion
Acknowledgements
This work was supported by National Natural Science Foundation of
China (Grant No. 20425102 and 20521140450) and the China Ministry of
Education. OT and YS thank Swedish Research Council (VR) and Japan
Science and Technology Agency (JST) for financial support.
The formation of the mesophases in the C14GluA/NaOH/
TMAPS synthesis system of AMS was discussed in detail,
and the full-scaled synthesis-field diagram was presented.
This synthesis system can form diverse mesophases, which
include cubic Fm3m, Pm3n, Fd3m, Pn3m, tetragonal
P42/mnm, and 2D hexagonal p6mm. According to the occur-
rence of each of the mesophase, it can be inferred that the
mesophase formation is affected by both the organic/inor-
ganic-interface curvature and the mesocage–mesocage inter-
actions. The organic/inorganic-interface curvature deter-
mines if the mesopore geometry is layered, bicontinuous, cy-
lindrical, or cage-type. Lower charging of the mesocage, and
therefore weaker mesocage–mesocage interactions, results
¯
¯
¯
¯
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Experimental Section
Synthesis of diprotic anionic surfactant C14GluA: Anionic surfactant N-
myristoyl-l-glutamic acid C14GluA was synthesised by a previously re-
ported method.[13] To a solution prepared from l-glutamic acid (35.5 g,
0.24 mol) in water (140 mL), acetone (120 mL), and sodium hydroxide
(19.2 g) myristoyl chloride (49.3 g, 0.2 mol), and sodium hydroxide (8.0 g,
0.2 mol) in water (20 mL) were added with stirring at 08C and at pH 12
over a period of 20 min. The reaction mixture was stirred for one addi-
tional hour, and acidified to pH 1 with sulfuric acid. Crude crystals of
C14GluA precipitated, were washed in petroleum ether, and deionised
water to obtain the pure crystals, which were then dried in vacuum.
Synthesis of mesoporous materials: When preparing the C14GluA/NaOH/
TMAPS triangle synthesis-field diagram, each of the three chemical com-
ponents has been changed in the synthesis, keeping the following condi-
tions constant: concentration of the surfactant in water=1 wt%; TEOS/
surfactant=15; synthesis temperature: 708C.
¯
Typical synthesis of mesoporous silica (mesostructure: Pm3n): C14GluA
(0.357 g, 1 mmol) was first dispersed in deionised water at 708C. To the
solution, 1m NaOH (1.16 g) was added with stirring. A mixture of TEOS
(3.12 g, 15 mmol) and TMAPS (0.515 g, 1 mmol, 50 wt% in methanol)
was added with stirring. After 10 minutes the stirring was stopped and
the reaction mixture was aged at 708C for 2 days. The precipitate was fil-
tered, dried at 508C, and calcined at 5508C for 6 h to give surfactant-free
mesoporous silica.
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Characterizations: Powder X-ray diffraction (XRD) patterns were re-
corded by using
a
Rigaku X-ray diffractometer D/MAX-2200/PC
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equipped with CuKa radiation (40 kV, 20 mA) at the rate of 1.0o/min over
the range of 1–68 (2q). High-resolution transmission electron microscopy
(HRTEM) was performed by using a JEOL JEM-3010 microscope oper-
ating at 300 kV (Cs=0.6mm, resolution 1.7 ꢁ). Images were recorded by
means of a CCD camera (MultiScan model 794, Gatan, 1024ꢂ1024
pixels, pixel size 24ꢂ24 mm) at 50–80 k magnification under low-dose
conditions. The microscopic morphological features were observed by
using a SEM (JEOLJSM-7401F). An accelerating voltage, 1 kV (resolu-
tion: ꢀ1.4 nm) was chosen for all mesoporous silica samples.
Received: April 22, 2008
Published online: November 14, 2008
11428
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 11423 – 11428