Templating and phase behaviour of the long chain ionic liquid C 16mimCl

Article abstract of DOI:10.1524/zpch.2006.220.10.1455

In the light of recent studies about the favourable sol-gel templating behaviour of the long chain ionic liquids (ILs) 1-hexadecyl-3-methylimidazolium chloride and -bromide (C₁₆mimCl and C₁₆mimBr) the phase behaviour of these ILs is investigated in detail and compared to common surfactants such as hexadecyltrimethylammonium chloride and -bromide (CTAC and CTAB). Specially developed small angle X-ray scattering (SAXS) evaluation approaches confirm a lamellar double-layer structure for crystalline C ₁₆mimCl of the ABACAB type. Presumably, the orientation of the head groups repeats in every second head group layer. Therefore, the imidazolium head group exhibits long range interactions on packing and templating, in contrast to the ammonium head group. The lyotropic phase behaviour is investigated via SAXS, isothermal titration calorimetry (ITC) and surface tension measurements. Interestingly, the phase diagrams of C₁₆mimCl and CTAC are quite different, especially at higher concentration of surfactant: instead of a gel phase in the phase diagram of CTAC, a lamellar-hexagonal two phase region as well as a pure lamellar phase is found in the phase diagram of C ₁₆mimCl. The critical micellar concentration (cmc) and enthalpy of micellization (Δ_micH) show that the cmc for imidazolium based surfactants is slightly lower and Δ_micH is more exothermic than for the ammonium type surfactants. These results are able to account for the specialties observed in the sol-gel templating behavior. by Oldenbourg Wissenschaftsverlag.

Full text of DOI:10.1524/zpch.2006.220.10.1455

Z. Phys. Chem. 220 (2006) 1455–1471 / DOI 10.1524/zpch.2006.220.10.1455
by Oldenbourg Wissenschaftsverlag, München
Templating and Phase Behaviour of the Long
Chain Ionic Liquid C16mimCl
∗
By Helena Kaper and Bernd Smarsly
Max Planck Institute of Colloids and Interfaces, Research Campus Golm,
D-14424 Potsdam, Germany
(Received August 3, 2006; accepted August 15, 2006)
Ionic Liquids / Phase Behavior / Sol–Gel Templating
In the light of recent studies about the favourable sol–gel templating behaviour of the
long chain ionic liquids (ILs) 1-hexadecyl-3-methylimidazolium chloride and -bromide
(C16mimCl and C16mimBr) the phase behaviour of these ILs is investigated in detail
and compared to common surfactants such as hexadecyltrimethylammonium chloride and
-bromide (CTAC and CTAB). Specially developed small angle X-ray scattering (SAXS)
evaluation approaches confirm a lamellar double-layer structure for crystalline C16mimCl
of the ABACAB type. Presumably, the orientation of the head groups repeats in every
second head group layer. Therefore, the imidazolium head group exhibits long range
interactions on packing and templating, in contrast to the ammonium head group. The
lyotropic phase behaviour is investigated via SAXS, isothermal titration calorimetry (ITC)
and surface tension measurements. Interestingly, the phase diagrams of C16mimCl and
CTAC are quite different, especially at higher concentration of surfactant: instead of
a gel phase in the phase diagram of CTAC, a lamellar-hexagonal two phase region as
well as a pure lamellar phase is found in the phase diagram of C16mimCl. The critical
micellar concentration (cmc) and enthalpy of micellization (∆mic
H show that the cmc
for imidazolium based surfactants is slightly lower and ∆mic H is more exothermic than
for the ammonium type surfactants. These results are able to account for the specialties
observed in the sol–gel templating behavior.
1. Introduction
The synthesis of inorganic mesoporous materials (e.g. SiO2 using ionic am-
phiphiles as structure directing agent has been in the focus of research in the
last decade [1, 2]. Two possible mechanisms have been elaborated, one being
the nanocasting “true liquid templating”, in which a one-to-one replica of the
initial lyotropic mesostructure is created [3]. The other one is the hydrother-
*
Corresponding author. E-mail: smarsly@mpikg.mpg.de

1456
H. Kaper and B. Smarsly
mal approach, where the cooperative interaction between the template and the
precursor leads to the desired structure [4, 5]. In both cases, mesoporosity is
created by decomposition of the template at elevated temperatures (calcina-
tion) or extraction. A variety of templates such as colloidal particles, block
copolymers, surfactants, here especially CTAB and CTAC have been applied.
However, exploring new templates is still of great interest and significance in
this field.
In our approach, we study ILs as possible candidates as templates for di-
verse mesostructured metal oxides. Ionic liquids are salts with a low melting
point, usually below 100 ◦C. Therefore they have characteristics of common
molten salts, e.g. wide liquidus range (up to 400 ◦C), negligible vapour pres-
sure, electrochemical stability, catalytical activity, or ion conductance [6]. As
such, they have gained interest in a lot of fields, especially as reaction me-
dia due to their low vapour pressure, which makes them interesting as “green
solvents” [7]. For the same reason they were employed as electrolytes in en-
ergy conversion devices such as solar cells and batteries [8]. Since certain ionic
liquids exhibit lyotropic and thermotropic phase behaviour [9–11], they are
interesting candidates for templates in sol–gel chemistry.
Very recently, several unique templating features of C16mimCl and
C16mimBr have been observed in comparison to other surfactants:
1.) C16mimCl has been used for the preparation of high ordered silica via
nanocasting [12–14], with an unusually high degree of order.
2.) In addition, C16mimCl and C16mimBr enable the preparation of meso-
porous hexagonal [15] and cubic silica (MCM-41 [1] and MCM-48) using
a hydrothermal synthesis. The synthesis of MCM-48 using CTAB or
CTAC as structure-directing agent still involves certain problems, espe-
cially regarding the reproducibility. Recently, we were able to show that
using C16mimCl or C16mimBr as template not only allows a synthesis at
lower surfactant concentration, but moreover offers a more reproducible
pathway to extremely highly ordered MCM-48 (submitted to Langmuir).
3.) The exceptional templating behaviour of C16mimCl has been demonstrated
in the preparation of hierarchical mesoporous systems using C16mimCl and
a block copolymer [16] or polystyrene beads [17] as templates. Interest-
ingly, such hierarchical porous systems cannot be created using CTAB or
CTAC.
These examples show the potential of ionic liquids as templates in sol–gel
chemistry, and the question arises whether there is a relationship between the
molecular structure, the phase behaviour in water and the sol–gel templating
features of the ionic liquids C16mimCl and C16mimBr in comparison to CTAB
and CTAC, which are used as reference templates in our study. The only differ-
ence between these two types of templates is their head group, the one of CTAB
and CTAC being a substituted ammonium head group and the one of C16mimCl
and C16mimBr being a substituted imidazolium ring (Fig. 1).

Templating and Phase Behaviour of the Long Chain Ionic Liquid C16mimCl
1457
Fig. 1. Molecular structure of C16mimCl (a) and CTAB (b).
Thus, detailed SAXS studies of the pure crystalline C16mimCl have been
carried out, and suitable fitting approaches for experimental SAXS curves with
suitable model functions are used. In addition, the special phase behaviour
of the ionic liquid confined in their silica mesopores is studied for different
morphologies (hexagonal, gyroid, lamellar) to shed light on the influence of
confinement on the phase behaviour of ILs.
Furthermore, also the lyotropic phase behaviour in water is examined via
SAXS measurements and appropriate numerical analysis. The particular ther-
motropic and lyotropic liquid-crystalline phase behaviour of C16mimCl is fi-
nally discussed in the light of the superior sol–gel templating behaviour of this
ionic liquid.
2. Experimental
2.1 Preparation of C16mimCl and C16mimBr
C16mimCl and C16mimBr ionic liquid were synthesized according to the pro-
cedures published in the literature [13, 18]. All chemicals were purchased from
Aldrich and used as received. As a typical synthesis, an excess of 1-hexa-
decylchloride (or 1-hexadecylbromide) (33 41 g, 0 128 mol) was mixed with
1-methylimidazole (10 26 g, 0 125 mol). The mixture was put into a 250 ml
flask, refluxed at 90 ◦C for 72 h, and then cooled down to room temperature.
The product was further purified by recrystallization in THF. After washing
several times with THF, the crystalline C16mimCl white powder was collected
by vacuum filtration, and dried in air at room temperature.
1H-NMR (CDCl3, ppm): 10.97 (s, 1H), 7.27 (s, 1H), 7.25 (s, 1H), 4.29 (t, 2H),
4.11 (s, 3H), 1.87(m, 4H), 1.23 (m, 24H), 0.86 (s, 3H).
Elemental analysis: C (theoretical: 70 07 wt %) 66.45, H (theoretical:
11 39 wt %) 11.95), N (theoretical: 8 17 wt %) 7.78. The purity is thus 94.9%.
C16mimCl is a very hygroscopic ionic liquid. Prior to phase investigations,
the material was dried under vacuum at 60 ◦C for three days. The dried sam-
ple contained 0 9 ppm of water, while a usual sample kept on air contains
4649 ppm of water, as determined by Karl-Fischer titration.

1458
H. Kaper and B. Smarsly
CTAB and CTAC were purchased from Sigma Aldrich and used without
further purification.
2.2 Synthesis of the SiO2-nanocomposites
High-quality cubic and hexagonal mesoporous SiO2 samples were prepared by
a hydrothermal procedure, which will be published separately.
The lamellar nanocomposite was prepared via evaporation induced self-
assembly (EISA) by dip-coating on Si-wafers. The solution was prepared
by dissolving 0 388 g C16mimCl in 2 5 ml stock solution (A2**, containing
tetraethylorthosilicate (TEOS), ethanol and hydrochloric acid, details for the
preparation are given in [19]), 0 1 ml H2O, 6 0 ml EtOH and 0 3 ml 0 07 M
HCl. The mixture was stirred at room temperature for 4 h and then dip-coated
with a speed of 37 cm min under a humidity of 10%. In order to obtain the
powder used for differential scanning calorimetry (DSC) and wide angle X-ray
scattering (WAXS) measurements, ca. 30 films were scratched of the Si-wafer
with a razor blade.
2.3 Methods
Elemental analysis was performed on a Vario EL Elementar (Elementar Analy-
sensysteme, Hanau, Germany). The samples were tested for carbon, hydrogen
and nitrogen content. 1H nuclear magnetic resonance (NMR) spectra were
recorded on a 400 MHz Bruker spectrometer equipped with a 9.395 Bruker
magnet. Samples of ca. 5 mg were analysed at room temperature in deuter-
ated chloroform. The signal of CHCl3 was used as chemical shift reference.
Surface tension measurements were performed in double distilled water at
room temperature with a K 12 tensiometer (Krüss) using the Du Nouy ring
method. For IR spectroscopy, a Bio-Rad 6000 FT-IR spectrometer equipped
with a single reflection diamond ATR was employed for recording IR spec-
tra. Small angle X-ray scattering (SAXS) measurements were performed using
a rotating anode device (Cu K ) with a 3-pinhole-collimation and equipped
with a 2D MARCCD detector. Isothermal titration calorimetry (ITC) meas-
urements were performed with a VP-ITC microcalorimeter from MicroCal
(Northhampton, MA). The reference and sample cells (both have a volume of
1 4424 mL) were enclosed in an adiabatic jacket and filled with degassed wa-
ter. The surfactant solution was injected in 2–10 µl steps into the working cell
via a syringe of total volume 288 µl and constant stirring rate. Thus, the cmc
was determined via demicellization as a reversed process. The concentration
in the syringe was chosen in such a way that during the experiment, the cmc
is reached inside the working cell. At each injection step, the heat produced or
consumed is measured as a function of time (Fig. 9a). The heat of each injec-
tion is then determined by integration of the heat flow peak and division by the
injected mole number (Fig. 9b). The difference between the two plateaus, that

Templating and Phase Behaviour of the Long Chain Ionic Liquid C16mimCl
1459
is the difference between the heat of demicellization and the heat from diluting
the micelles, gives the enthalpy of micellization, ∆mic H. The cmc can be de-
termined from the first derivative, which displays a sharp minimum at the cmc
(Fig. 9c).
3. Results and discussion
3.1 Structure of crystalline ionic liquid
The synthesis of lamellar and hexagonal silica using C16mimCl as template via
the nanocasting technique was described in [20]. The hexagonal phase is cre-
ated at higher water content, while the lamellar phase is synthesized at very low
water content. This lamellar phase (consisting of alternating layers of IL and
silica) is special not only because of its extreme high order (peaks up to the
23rd order are observed), but also because the SAXS pattern of the silica is quite
similar to the SAXS pattern of the pure ionic liquid in its crystalline state. In the
following the crystalline material and the mesoporous lamellar silica are inves-
tigated via SAXS measurements and fitting of a model to the experimental data
for a better understanding of the structural peculiarities of this IL.
From differential scanning calorimetry (DSC) and temperature-dependent
in-situ wide angle X-ray scattering (WAXS) experiments different morpholo-
gies of C16mimCl can be observed. In peculiar, the morphology of C16mimCl
depends strongly on its thermal history, as also observed by Downard et
al. [21]. A stable modification of crystalline C16mimCl exhibits a SAXS peak at
a quite low angle (s = 0 18 nm−1) which corresponds to a quite large d-spacing
of 5 6 nm. Heating the material above 220 ◦C and cooling down to room tem-
perature leads to a metastable, not fully crystalline modification of C16mimCl.
SAXS experiments reveal that this modification lacks the peak at very low
angle and only with time the low angle peak and crystallinity develop. Thus,
also modifications exist where the first peak at very low angle is smaller than
the second peak.
The large d-spacing of crystalline, imidazolium-based, long chain ionic li-
quids was already discussed for C18mimCl and C14mimCl as a double bilayer
structure, where in every second head group layer the orientation of the head
groups is repeated [21]. However, no direct evidence has been reported so far
based on suitable SAXS evaluation approaches. Hence, we applied two com-
plementary SAXS analyses, namely fitting of suitable model functions to the
SAXS curves in reciprocal space and also Fourier transformation (FT) into real
space.
First, a SAXS model function corresponding to the scattering of an
ABACAB layer structure that is a double bilayer with a layer sequence of
A-B-A-C-A-B was fitted to experimental data of crystalline C16mimCl. Within
this model, the fit parameters are the thickness of the lamellae B, dB, and
of the lamellae C, dC, the relative electron densities of the layers
and
1
2

1460
H. Kaper and B. Smarsly
(with
dC ,
=
B − A and
and
=
C − A ) as well as their variance dB ,
1
2
. Next to that, the long-period, the height of the lamel-
2
1
lae stack N and the corresponding variances d and
N are taken into
account.
The fit was carried out for three polymorphs of C16mimCl: a fully crys-
talline modification where the first peak is bigger than the second (X), a half
crystalline modification where the first peak is smaller than the second (Y) and
finally a metastable modification where the first peak at low scattering vectors
is not present (Z). In Table 1, the fit parameters for all three possible crystal
modifications are compared.
It is seen that the SAXS curve can be well modelled (Fig. 2) with respect
to the positions and relative intensities of the reflections, providing the afore-
mentioned parameters with good accuracy. Thus, all peaks belonging to the
lamellar double bilayer structure can be found in the fitted plot. In essence,
the relative intensities are sensitive towards the layer thicknesses and the rela-
tive electron densities. Details of this approach will be presented in a separate
publication.
Interestingly, in order to create the peak at very low angle, the relative elec-
tron densities 1 and 2 have to be sufficiently different. If the difference is big,
the first peak is actually higher than the second (case X), while in the case of
a smaller first peak (case Y), the difference in electron density is accordingly
smaller, and finally, without observing the first peak (case Z) they are identi-
cal. Regarding the thickness of the layers dB and dC no trend is observed. For
X and Y, the difference between dB and dC is quite small and for Z dB and dC
are nearly identical.
Another way to learn about order in crystalline materials is Fourier trans-
formation (FT) of the X-ray data, proving relative differences in the electron
density profile in the direction perpendicular to the layers. The FT has been car-
ried out using the following equation for the calculation of the electron density
ꢀ
h=m
profile: ∆ x =
Fh cos 2 xh d , with h, the order of reflection, Fh,
h=1
the form factor of the hth peak, and d the long-period. The form factors are
derived from the square root of the integrated areas of the respective peak mul-
tiplied with s2. The sign of each diffraction order can be − or +. The major
problem in FT analysis is the “phase problem”, i.e. the choice of the signs in
the Fourier sum of a quite large number of SAXS reflections. A significant
number of possible combinations of signs of the form factors can be excluded
because they produce obsolete electron density profiles. Our criteria for rea-
sonable electron density profiles were combinations providing a structure with
relatively smooth and reasonable electron density plateaus, in our case a se-
quence of signs + + − + − + −+. In Fig. 3, the electron density profile of
the pure crystalline C16mimCl is shown, and it is seen that three plateaus are
present, corresponding to layers of the corresponding relative electron densi-
ties. According to the assumed ABACAB
double bilayer structure, these
three plateaus belong to the alkyl chains and the two different head group

Templating and Phase Behaviour of the Long Chain Ionic Liquid C16mimCl
1461
orientations. However, the exact determination of the thickness of the layers is
difficult based on the FT and much more accurate using the fitting approach
presented above, which demonstrates that in general in such cases the analysis

1462
H. Kaper and B. Smarsly
Fig. 2. Experimental SAXS curve of C16mimCl (a) and fitted curve (b). The peaks indi-
cated with a star do not belong to the lamellar double-bilayer structure.
Fig. 3. Electron density profile (in arbitrary units) obtained from Fourier transformation of
the SAXS data of pure C16mimCl.
in reciprocal space can be superior to transformations into real space. In con-
clusion, the FT contains valuable information such as the d-spacing and the
three plateau structure, but lacks details about the concrete structure. Also, it
should be noted that the relative differences in the electron density among the
three types of layers are similar for both approaches.
In conclusion, the fitting is able to account for the main features of the
scattering curves with a small number of parameters, thus strongly support-
ing the presence of an ABACAB layered structure for C16mimCl, and provides
interesting insights into the relationship between the molecular structure of
the IL and the mesoscopic superstructure: Taking into account the geometry
and size of this IL, A can be attributed to the alkyl chain, while B and C

Templating and Phase Behaviour of the Long Chain Ionic Liquid C16mimCl
1463
Fig. 4. SAXS pattern of pure C16mimCl (a) compared to the SAXS pattern of SiO2 (b).
The SiO2 was prepared via nanocasting with C16mimCl as template under low water con-
ditions.
could correspond to two different orientations of the head group. The fact that
in C16mimCl the layers B and C obey a strict sequence of ABACAB
,
i.e. an alternation of B and C in each second layer, points to a long-range
interaction not only within the layers, but also perpendicular to them. It is
therefore worthy to note that the head-groups in one particular layer experi-
ence the influence of the imidazolium rings in the next but one layer, leading
to an accordingly high mutual orientation. This strong dipolar coupling of
head groups over distances of ca. 6 nm (repeat-unit perpendicular to the layers)
could explain the high degree of order in the previously reported lamellar SiO2-
IL-nanocomposites.
Next to that, we also studied lamellar C16mimCl–SiO2 nanocomposites
by SAXS, using the ABACAB fitting approach and FT. The FT approach
did not provide a reasonable result, and also the fitting method did not al-
low modelling the SAXS curve reasonably in terms of the relative peak in-
tensities. We interpret these difficulties by the presence of pillars between
the lamellae, which could create such a deviation from the ideal SAXS
ABACAB motif. This interpretation also explains that the lamellar structure
does not collapse upon calcination, as expected for common lamellar struc-
tures.
In Fig. 4 the SAXS pattern of the pure C16mimCl as well as the SAXS pat-
tern of the lamellar SiO2 nanocomposite are shown together with the proposed
layer order. In the case of the nanocomposite, alternating layers of SiO2 and
C16mimCl lead to the lamellar order. In comparison, for the pure C16mimCl
layers of alkyl chains and head groups with two different orientations organize
into the ABACAB double bilayer structure.

1464
H. Kaper and B. Smarsly
Fig. 5. DSC curve of cubic SiO2-C16mimCl nanocomposite heating range: −20 ◦C–
220 ◦C, heating rate: 10 K min.
3.2 Phase behaviour confined in pores
Since it is possible to synthesize hexagonal, lamellar and bicontinuous cubic
ordered silica, it is interesting to study the phase behaviour of the ionic liquid
confined in pores, e.g. whether crystallization from the ionic liquid takes place
in the same way as in the bulk. Thus, hexagonal, cubic (gyroid) and lamel-
lar silica were synthesized with C16mimCl as template, and the materials were
studied by DSC and in situ temperature dependent WAXS experiments. For the
hexagonal and the lamellar nanocomposite, the phase behaviour of C16mimCl
is completely suppressed, that is no phase transitions are observed. Only the
bicontinuous cubic silica shows a transition at ca. 25 ◦C in both, cooling and
heating curves (Fig. 5), which does not depend on the heating range or rate.
The material was then studied by in-situ temperature-dependent WAXS experi-
ments. No peaks from crystalline ionic liquid can be seen in the data (Fig. 6),
and the SAXS peaks belong to the cubic gyroid phase. With temperature, the
relative height of these SAXS peaks alters which can be explained by changes
in the form factor due to the rearrangement of the ionic liquid inside the pores.
However, no phase transition corresponding to a liquid-crystalline phase tran-
sition is observed. Instead, in the wide angle region a small peak on top of the
halo (originating from amorphous SiO2 disappears by heating the material to
50 ◦C (Fig. 6). This peak might be due to a packing of the alkyl tails as in a ro-
tator phase [22], which upon heating melts into a complete disordered phase. In
conclusion, these experiments prove that the liquid-crystalline phase transition
of this IL is completely suppressed in mesopores of ca. 3–5 nm in size, and the
rotator-phase transition is only observable for the IL being confined in a bicon-
tinuous gyroid structure. Again, the latter result demonstrates that in sol–gel
templating C16mimCl has a certain tendency to create mesostructures with low
curvature, because the long-range interaction favours flat mesostructures.

Templating and Phase Behaviour of the Long Chain Ionic Liquid C16mimCl
1465
Fig. 6. WAXS pattern of cubic C16mimCl SiO2 nanocomposite at two temperatures. a):
nanocomposite at 10 ◦C, b): nanocomposite at 50 ◦C. The peaks indicated by stars origi-
nate from the sample holder.
3.3 Lyotropic phase behaviour
In order to elucidate the templating behaviour of the long chain ionic li-
quids C16mimCl and C16mimBr compared to CTAB and CTAC, the phase
behaviour in water (“lyotropic phases”) of these templates is of particular
interest. In nanocasting, the phase behaviour of the template under reaction
conditions determines the morphology of the finally created inorganic oxide
phase. Thus, investigation of the phase behaviour in water might help to un-
derstand the sol–gel templating features, even though the conditions during
nanocasting are different, e.g. pH, presence of other solvents and the precursor.
The situation is quite different for hydrothermal synthesis, where coopera-
tive interaction between the precursor and the template lead to the desired
structure. In this case, investigation of the cmc and micellization enthalpy
helps to understand the templating behaviour (see below). In the following,
first the lyotropic phase behaviour in water is investigated and then the cmc
and micellization enthalpy are discussed with respect to the templating be-
haviour.
The long chain ionic liquids C16mimCl and C16mimBr show lyotropic su-
perstructures in water, similar to CTAB and CTAC. The phase behaviour of
C16mimCl in water was determined via SAXS measurements at room tempera-
ture (Fig. 7). At low concentrations of C16mimCl (below 55 wt %) a micellar
phase L1 is present. In comparison, the phase diagram of CTAC presented
by Blackmore et al. [23] shows a much smaller micellar region, and already
at ca. 32 wt % a hexagonal H1 phase is formed. In the case of C16mimCl,
the hexagonal phase occurs only at ca. 55 wt % C16mimCl. Around 60 wt %
a hexagonal–lamellar two phase region exists, which is followed by a pure
lamellar phase at ca. 80 wt %. It is interesting to note that the phase diagram
of CTAC shows a gel phase between the hexagonal and lamellar phase, which
is not present in the phase diagram of C16mimCl.

1466
H. Kaper and B. Smarsly
Fig. 7. Phase diagram of C16mimCl in water at room temperature with corresponding
SAXS measurements.
Fig. 8. Experimental and fitted curve of a sample containing 67% of C16mimCl, as well as
the form factor and the lattice factor. a) form factor, b) fitted curve, c) experimental curve,
d) d lattice factor.
A fitting approach that is based on randomly oriented cylindrical stacks was
fitted to the SAXS patterns of the lyotropic phases with 60 wt % of C16mimCl,
67 wt % and 70 wt % [24]. The major fit parameters are the d-spacing d10, the
average radius of the cylinders R, the stack height N as well as the correspond-
ing variances (d10
, (R) and (N). In Fig. 8 experimental and fitted curve
of a sample containing 67 wt % of C16mimCl are shown. Typically, the SAXS
√
√
√
pattern of a hexagonal phase has a peak sequence of 1 : 3 : 2 : 7 : 3 : 12 :

Templating and Phase Behaviour of the Long Chain Ionic Liquid C16mimCl
1467
Table 2. Parameters obtained from fitting Rfit: radius; R : variance of R, d10: d-spacing,
d10 : variance of d10, N: stack height, a: lattice parameter, Rcalc: calculated radius ob-
tained from volume fraction.
wt. % of
Rfit
(R)
d10
d10
N
a
Rcalc
C16mimCl
nm
nm
nm
60
67
70
2 1 0 03 0 3 0 05 4 4 0 05
2 5 0 03 0 3 0 05 4 4 0 05 0 13 0 05
3 3 0 03 0 2 0 05 4 4 0 05 0 07 0 05
0 2 0 05
8
8
8
4
1
2
3.8
3.9
3.8
1.55
1.65
1.66
√
13 , but interestingly the third and fifth peaks are missing, which can be
attributed to the form factor of cylindrical micelles having a minimum at these
positions, as illustrated in Fig. 8. At lower concentration (60 wt %) and higher
concentration (70 wt %), the third peak is present, while the fifth peak is miss-
ing in all curves. Thus, changing the concentration of C16mimCl has a great
influence on the peak sequence in SAXS curves due to its influence on the form
factor. Since the SAXS curves are quite sensitive towards changes in R, the
micellar radius can be determined with high accuracy based on the algorithm
described in Ref. [24].
The results of the fits are presented in Table 2. As can be seen, increas-
ing the concentration of C16mimCl leads to an increase in the radius R and the
d-spacing d10.
The theoretical radius of the hexagonal phase can also be calculated from
the volume fraction based on the weight percentage. It is remarkable that this
radius is always much lower than the radius obtained from the SAXS fit, and
also its relative change with increasing C16mimCl concentration is much lower
compared to the SAXS results. Such a substantial increase in the micellar ra-
dius as a function of concentration is unusual for short-chain surfactants [25]
and points to a special behaviour of C16mimCl. Since the incorporation of sig-
nificant amounts of water into the micelles can be excluded, this IL exerts
a strong tendency to reduce the curvature of the cylindrical micelles at higher
concentrations in water. Taking into account the peculiar head group, this be-
haviour can be attributed to a stronger interaction of the imidazolium head
groups, compared to CTAC. Furthermore, this observation confirms the trends
that have been observed when using C16mimCl as template, namely the forma-
tion of an extremely high ordered lamellar phase synthesized via nanocasting
and the preference of building up a bicontinuous cubic phase with low cur-
vature instead of a hexagonal phase when used in a hydrothermal synthesis.
Apparently, above a certain concentration the head groups of C16mimCl are
able to strongly interact and tend to align in a parallel fashion as predetermined
by the head group’s shape and mediated by the counter ions.
The cmc of C16mimCl, C16mimBr, CTAB and CTAC was determined by
surface tension measurements and ITC and compared to literature values,

1468
H. Kaper and B. Smarsly
Fig. 9. ITC curve of C16mimCl in water a) raw ITC b) integrated c) first derivative.
where possible. From these data, the influence of the head group on the cmc
seems to be the same for C16mimCl and C16mimBr. In comparison to the
respective hexadecyltrimethylammonium surfactants, the cmc of the imida-
zolium based surfactant is slightly smaller (Table 3). Thus, the interaction
between the charged head groups should be larger for imidazolium ions than
for trimethylammonium ions. In the micellization process, the counterion of

Templating and Phase Behaviour of the Long Chain Ionic Liquid C16mimCl
1469
Table 3. CMC and ∆mic H values for CTAB, CTAC, C16mimCl, and C16mimBr.
CTAB
CTAC
C16mimCl
C16mimBr
∆
mic H kJ mol
−8.1
−3.5
−5.9
−11.2
(Ref. [26]: −5.1)
cmc mol l
8 6×10−4
1 3×10−3
1 1×10−3
5 4×10−4
(Ref. [27]:
(Ref. [26]:
8 9×10−4
)
1 1×10−3
)
ionic surfactants binds to the micelles and therefore screens the electrostatic
repulsion between the head groups. Micellization is facilitated for counterions
with high binding affinity. Therefore, the cmc for surfactants with chloride as
counterion is smaller than that for bromides – as also observed in this study.
As for the cmc, also for the micellization enthalpy ∆mic H trends for
changing the counterion or cationic head group can be observed, even though
the presented values have certain uncertainty due to experimental conditions
(Table 3). However, these data at least allow a relative comparison of the mi-
cellization enthalpies, and it is seen that ∆mic H is significantly smaller for the
surfactants with bromide as anion. More importantly, changing the surfactant
head group from ammonium to imidazolium substantially decreases the micel-
lization enthalpy (i.e. the modulus increases). In total, in our case the influence
of the head group is larger than the influence of the counterion. With respect to
the templating behaviour, a more exothermic micellization observed for the im-
idazolium based surfactants supports the idea of stronger interaction between
the head groups and therefore also possibly stronger interaction between the IL
and silica precursors, e.g. through hydrogen bonds. These experiments are also
in full agreement with the finding that, as observed in the hydrothermal syn-
thesis of mesoporous silica, stronger binding between the SiO2 and the ionic
liquids leads to precipitation of the SiO2 at lower absolute concentration of IL
in comparison to CTAB.
4. Conclusion
The present study was intended to clarify the phase behaviour of C16mimCl,
especially in the light of the previously observed peculiar behaviour in sol–
gel templating of silica mesostructures. In essence, we addressed the basic
question, if an IL such as C16mimCl possesses observable differences in its
phase behaviour in water, in comparison with well-known surfactants only dif-
fering in the head groups. Our studies have indeed revealed that C16mimCl
bears subtle differences under certain experimental conditions. First, it should
be pointed out that in various situations C16mimCl behaves almost identically

1470
H. Kaper and B. Smarsly
to CTAC. In particular, they have a similar cmc, suggesting that in dilute so-
lutions the interaction of the imidazolium groups does not significantly come
into play. Second, already the different micellization enthalpy indicates the
specialty of the head group, which becomes more important at higher concen-
trations of this IL: in essence, various experiments (lyotropic phase behaviour,
nanocomposites with silica, etc.) have demonstrated a strong tendency to adopt
a mesostructure with low curvature, owing to the stronger interaction between
the imidazolium groups and their preference of parallel alignment. Another
important aspect with respect to the templating behaviour of ammonium- and
imidazolium type surfactants is the observation that C16mimCl itself adopts
a ABACAB double bilayer lamellar structure, while CTAB and CTAC have
a more simple ABAB bilayer structure. Therefore, as a significant difference
between these two types of surfactants, the imidazolium head group exerts
a long-range interaction through the mesostructure. Thus, there is a direct link
between the SiO2 mesostructures and the general phase behaviour: at high
ratios C16mimCl : SiO2 the lamellar nanocomposite mesostructure is highly
ordered and shows similarly exotic SAXS features as the pure IL. Moreover,
the alignment of the imidazolium groups can also account for the success in
generating hierarchical mesopore structures via the usage of template mix-
tures, especially block copolymers in combination with C16mimCl. While
normally small surfactants like CTAB form mixed micelles with block copoly-
mers, possibly the strong interaction of the C16mimCl head groups with silica
and also among themselves results in the formation of separate C16mimCl
micelles.
In conclusion, the careful study of the phase behaviour of C16mimCl helps
to understand diverse aspects of the peculiar templating features of IL surfac-
tants. Further studies will be dedicated to other fundamental issues, e.g. the
factors determining the micellization of C16mimCl (entropy versus enthalpy,
etc.).
Acknowledgement
The author thank Prof. P. Wasserscheid for the help with the Karl-Fischer
titration.
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