This journal is cthe Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 15355–15360 15355
Cite this:
Phys. Chem. Chem. Phys
., 2012, 14, 15355–15360
Magnetic microemulsions based on magnetic ionic liquidsw
Andreas Klee,*
a
Sylvain Prevost,
ab
Werner Kunz,
c
Ralf Schweins,
d
Klaus Kiefer
b
and Michael Gradzielski*
a
Received 10th July 2012, Accepted 13th September 2012
DOI: 10.1039/c2cp43048g
Microemulsions with magnetic properties were formed by employing a magnetic room
temperature ionic liquid (MRTIL) as polar phase, cyclohexane as oil, and an appropriate mixture
of ionic surfactant and decanol as a cosurfactant. By means of small-angle neutron scattering
(SANS) and electric conductivity the microemulsion structure could be confirmed, where the
classical structural sequence of oil-continuous–bicontinuous–polar phase continuous is observed
with increasing ratio [polar phase]/[oil]. Accordingly a maximum of the structural size is observed
at about equal volumes of oil and MRTIL contained. Therefore this system is structurally the
same as normal microemulsions but with the magnetic properties added to it by the incorporation
into the systems formulation.
1 Introduction
Microemulsions are thermodynamically stable systems typically
formed from oils and water by the presence of a surfactant.
Normally they are low viscous and can be of the water-in-oil
(W/O), oil-in-water (O/W), or bicontinuous structural type.
1
Room temperature ionic liquids (RTILs) have been intensely
investigated during recent years and also microemulsions
have been formulated with them. This has mostly been done
for RTIL replacing water
2–11
but it is also possible that the
RTIL functions as the hydrophobic component of the micro-
emulsion.
12,13
However, in any case the range of applicable
surfactant is much more restricted than in the case of water. A
rather novel subclass of RTILs are the so-called magnetic
RTILs (MRTILs)z, which were first described in 2004
14
and so
far only few publications regarding their properties exist.
15–27
In particular, none in which they have been employed as a
main component in self-aggregating systems, despite the fact
that, due to their magnetic properties, they are very interesting
and promising compounds. In that context it might also be
mentioned that ‘‘magnetic surfactants’’ have received high
attention very recently.
28
Accordingly, self-assembly in MRTIL is an interesting
question and in this work we describe the formulation of a
magnetic microemulsion based on a MRTIL, which to the best
of our knowledge, is the first time that such a system is described.
Other microemulsions with magnetic properties are all based on
microemulsions containing magnetic inorganic nanoparticles, which
is a very different type of formulation.
29–31
Therefore this magnetic
microemulsion based on a MRTIL is a novel type of self-assembled
system which is formed by employing an appropriate combination
of surfactant and cosurfactant. In the following we describe
the phase behavior observed, the magnetic properties and
a thorough structural characterisation of this magnetically
responsive colloidal system.
2 Experimental section
bmim[FeCl
4
] was synthesized as described in the literature.
14
Briefly bmimCl (24.8 g, 141.9 mmol, Aldrich, Z95%) was
heated to 90 1C in a glass flask under stirring. After adding an
equal amount of FeCl
3
6H
2
O (38.4 g, 141.9 mmol, Sigma
Aldrich) the mixture was cooled to room temperature and
stirred for 1 h. Removal of the water phase leads to 41.7 g
(124.0 mmol, 87.4%) product. C
16
mimCl was synthesized as
described in ref. 32.
The pseudo ternary phase diagram was recorded at a
temperature of 24 0.5 1C. Mixtures of different ratios between
cyclohexane and surfactant–co-surfactant were titrated with
bmim[FeCl
4
]. In all cases the molar ratio of C
16
mimCl/decanol
was 1 : 2.
Conductivity titration was done at a temperature of 24 1C
with a home-build Pt-electrode connected to a Methrom 712
conductometer at 2.4 kHz, starting with an oil rich micro-
emulsion sample by stepwise addition of an oil free sample
with a syringe.
a
Stranski-Laboratorium fu
¨r Physikalische und Theoretische Chemie,
Institut fu
¨r Chemie, Straße des 17. Juni 124, Sekr. TC7, Technische
Universita
¨t Berlin, 10623 Berlin, Germany.
Tel: +49 30 314 24934
b
Helmholtz-Zentrum Berlin fu
¨r Materialien und Energie,
Hahn-Meitner-Platz 1, 14109 Berlin, Germany
c
Institute of Physical and Theoretical Chemistry,
University of Regensburg, 93040 Regensburg, Germany
d
Institute Laue Langevin, F-38042 Grenoble, France
wElectronic supplementary information (ESI) available: Experimental
section, viscosity data, detailed description of Teubner–Strey and
core–shell fits. See DOI: 10.1039/c2cp43048g
zIt should be noted that they are paramagnetic compounds.
PCCP Dynamic Article Links
www.rsc.org/pccp PAPER
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Small angle neutron scattering (SANS) experiments were
performed at ILL, Grenoble (D11). For better contrast the samples
were prepared with D12-cyclohexane and for the measurements
placed in cuvettes (Quarz, Hellma) of 1 mm thickness and studied
with an incoming beam of 6 A
˚wavelength and a collimation length
of 8 m. Scattered neutrons were recorded using a 2D-detector for
sample-to-detector distances of 8 m and 1.2 m resulting in an
observed q-range of 0.09 nm
1
rqr5.12 nm
1
. Incoherent
scattering of water was used to correct the detector efficiency
and to bring the data to an absolute scale. Data reduction was
done with LAMP
33
and fitting with SASfit.
34
Magnetic susceptibility measurements were done at 300 K
with a MPMS (Quantum Design, located at the Laboratory
for Magnetic Measurements, HZB). Samples were placed in a
home-made vacuum-sealed sample chamber and scanned in a
range from 5 to 5 Tesla. The empty cell signal was subtracted
from each measurement. All samples showed a linear dependency
of magnetization vs. magnetic field in the measured range.
Examples are shown in Fig. S1 (ESIw).
Viscosity measurements (Fig. S2, ESIw) were done at 25 1C
using a micro-Ostwalt viscosimeter (Ic or IIc, SI Analytics,
Mainz). The obtained kinematic viscosity was multiplied with
the density of the same sample (Density meter DMA 4500,
Anton Paar) to calculate the dynamic viscosity.
3 Results and discussion
For our experiments we employed 1-butyl-3-methylimidazolium
tetrachloroferrate (bmim[FeCl
4
]) as MRTIL and cyclohexane
as a simple oil, which has been shown before to have rather
solubilisation capacities in microemulsion systems,
35,36
which
also applies to ones in RTIL.
37
As surfactants different types
(such as SDS, AOT, and various C
i
E
j
) were initially tested but
by far the best performance was observed for 1-hexadecyl-3-
methylimidazolium chloride (C
16
mimCl) which is an ionic
liquid itself but due to the long alkyl chain also a cationic
surfactant.
32
With the C
16
mimCl the formulation of a micro-
emulsion with the aim of using as little surfactant as possible
was achieved. However, by itself it is not able to form extended
microemulsion areas with bmim[FeCl
4
] and cyclohexane.
Therefore a cosurfactant, in our case decanol (which is insoluble
in bmim[FeCl
4
]), had to be added to the formulation as is a
typical recipe for microemulsions.
1,32
3.1 Phase behavior
The phase behavior of the quaternary system bmim[FeCl
4
]–
C
16
mimCl–decanol–cyclohexane was studied in thorough
detail and Fig. 1 shows the pseudo ternary phase diagram of
this system for a constant C
16
mimCl/decanol mole ratio of
1 : 2. This particular ratio gave a maximum for the mixing
efficiency of cyclohexane and bmim[FeCl
4
] and was therefore
chosen. In the following we consider C
16
mimCl and decanol as
a combined effective amphiphile. Compared to the examples
of IL containing microemulsions in the literature,
4,5,10
which
require 40 wt% more of amphiphile for the formation of a
balanced microemulsion, our formulation requires much less
stabilizing amphiphile and microemulsions for surfactant–
cosurfactant amounts less than 20 wt% are observed. The
phase diagram shows a very large and optically clear single
phase region. Viscosities (see ESIw) are low and in the range
obtained for assuming a linear relation for the MRTIL–
cyclohexane mixture viscosities, i.e., they are determined simply
by the liquid components of the microemulsion. The low
viscosity already hints at the presence of a microemulsion in
this range and against formation of a liquid crystalline phase,
which is further confirmed by the fact of optical isotropy. Two
multi phase regions are observed. One at low MRTIL content
due to a limited solubility of the surfactant in the oil resulting
in a coexistence of microemulsion and solid C
16
mimCl. However,
this region becomes monophasic at higher temperatures. The
multiphase region at low amphiphile concentrations consists of
coexisting oil- and MRTIL-rich phases, a behavior which is well
known in classical water containing microemulsions as emulsifica-
tion failure.
38
Here it might be noted that the phase behavior and
the microemulsions themselves are not sensitive to water,
which means they are stable under moist conditions.
3.2 Magnetic behavior
Of course, the most interesting aspect of this microemulsion is
its magnetic properties, which are depicted in Fig. 2, which
shows how the meniscus of the magnetic microemulsion is moved
by the presence of the magnetic field, i.e., it is magnetically
responsive and this applies to the whole microemulsion range
that contains at least a few percent of MRTIL. A more
quantitative insight has been gained by measurements of the
magnetic susceptibility, which was done along the dashed line
given in Fig. 1 covering the full range from the pure oil to pure
MRTIL as solvent. All samples show a paramagnetic behavior
indicated by a linear field dependency of the magnetization (see
Fig. S1, ESIw). The magnetic susceptibility (inset in Fig. 3)
increases in an almost perfectly linear fashion with the increase
in weight content of the MRTIL, which means that the magnetic
Fig. 1 Pseudo ternary phase diagram (by weight) for mixtures of
MRTIL (bmim[FeCl
4
]) and cyclohexane formulated with C
16
mimCl and
decanol (molar ratio 1 : 2) as surfactant and co-surfactant, respectively.
The filled squares mark the detected phase transitions and the filled circles
indicate the SANS samples. The dashed line gives the experimental path
for conductivity titration and magnetic measurements.
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properties of the microemulsion are not affected by its mesoscopic
structure. Extrapolation to pure bmim[FeCl
4
] gives a magnetic
susceptibility of 3.6 10
4
(SI units). For comparison the initial
susceptibility of ferrofluids can be found to be of the order of 1.
39
3.3 Mesoscopic structure
A first insight into this mesoscopic microemulsion structure can
be deduced from the conductivity titration shown in Fig. 3. The
position in the phase diagram is given by the MRTIL volume
ratio x
MRTIL
relative to the volume of cyclohexane (V
oil
)and
MRTIL (V
MRTIL
), defined as:
xMRTIL ¼VMRTIL
VMRTIL þVoil ð1Þ
With increasing MRTIL content the conductivity increases by
one order of magnitude, which is due to the formation of a
structure continuous in MRTIL. In water and oil containing
microemulsions as well as in systems with an IL substituting
the water such a percolation behavior of the conductivity is
well known and can be explained by structural transitions
between a droplet and a bicontinuous structure.
5
The behavior
of our MRTIL system is analogous and shows a percolation
point of conductivity around x
MRTIL
E0.1, determined by
plotting the specific conductivity with an exponent of 5/8
versus x
MRTIL
(Fig. 3).
32,40
In order to gain a more detailed structural insight we
performed small-angle neutron scattering (SANS). Experiments
were performed with D12-cyclohexane in order to have good
contrast between oil and MRTIL (the macroscopic phase
behavior was not affected by this isotopic substitution). The
obtained scattering curves (Fig. 4) show weak correlation peaks
for small amounts of MRTIL contained and continuously
decreasing intensity curves for higher MRTIL content. The
high scattering intensity, especially for about equal contents of
oil and MRTIL, and the moving of the scattering curves to
lower qprove the existence of relatively large, microphase
seperated, domains, as typically observed for microemulsions.
Similar scattering patterns have been observed for conventional
microemulsions
41,42
and also for ones containing RTIL.
2–4,32
It
is interesting to note that the degree of ordering is substantially
Fig. 2 Response of the MRTIL containing microemulsion to the field
gradient of an electromagnet. The sample shown consists of 31.2 wt%
D12-cyclohexane, 46.1 wt% bmim[FeCl
4
], 11.8 wt% C
16
mimCl
and 10.9 wt% decanol. The magnetic field is oriented parallel to the
liquid surface.
Fig. 3 Specific electric conductivity as a function of the MRTIL
volume ratio (eqn (1)) (filled diamonds) and magnetic susceptibility
as a function of wt% MRTIL (inset, open squares). Measurements
were done along the experimental path shown in Fig. 1.
Fig. 4 SANS curves with D12-cyclohexane as oil phase. The samples
are along the experimental path shown in Fig. 1. Labels are values
for x
MRTIL
(eqn (1)), arrows indicating growing x
MRTIL
. Symbols
are measured scattering intensities, lines are fits performed with the
TS-model (filled circles) and the sphere model (open symbols).
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lower for the MRTIL rich region as evidenced by the absence
of a correlation peak. This is different from water-based
microemulsions and can be explained by the much reduced
electrostatic repulsion as the RTIL screens effectively the
charge of the microemulsion droplets formed.
In order to deduce quantitative structural information from
the SANS curves we applied two models to describe the data:
first the phenomenological Teubner–Strey (TS) model,
43
in
which the scattering intensity is given by eqn (2a) and is
basically determined by the quasiperiodic repeat distance D
s
(eqn (2b)) and the correlation length x(eqn (2c)) of the
structural units, where hZ
2
iaccounts for the contrast and
volume fractions Fof the oil and MRTIL phase (eqn (2d)).
IðqÞ¼ 8pc2hZ2i=x
a2þc1q2þc2q4ð2aÞ
Ds¼2p1
2ffiffiffiffiffi
a2
c2
r1
4
c1
c2
1=2
ð2bÞ
x¼2p1
2ffiffiffiffiffi
a2
c2
rþ1
4
c1
c2
1=2
ð2cÞ
hZ
2
i=F
IL
F
oil
(Dr)
2
(2d)
As second model we employed a core–shell model of spherical
droplets, described by a particle form factor Pand a structure
factor S(in our case for hard spheres
44
) (eqn (3)), which are
determined by core radius R
c
, shell thickness dR, polydisper-
sity index p, and hard sphere radius R
HS
that accounts for the
repulsive interparticle interactions (for details see ESIw).
I(q)=NP(q,R
c
,dR,p)S(q,R
HS
) (3)
Both models allow for a reasonable description of the SANS
data but the quality of the agreement of the respective model
depends on the location of the sample in the phase diagram
(see ESIw).
The TS model works best in the range of intermediate
compositions (x
MRTIL
= 0.3–0.65), where a bicontinuous
structure is to be expected. For locations in the phase diagram
rich in MRTIL or oil the droplet picture of the core–shell
model is clearly superior. For the droplet regime one finds core
radii that increase with increasing content of solubilisate,
being somewhat larger on the oil-rich side. This can be
explained by the fact that here the head group of the surfactant
has to be counted into the core (see Fig. 5). This explains also
why here the shell thickness is correspondingly smaller and it
should also be kept in mind that this thickness is not the
thickness of the amphiphilic monolayer, as it corresponds only
to a part of the length of the ionic surfactant. In general, of
course, the core–shell model is superior for large q, as it captures
better the local structure than the TS model that assumes a
sharp interface. The parameters obtained are summarized in
Table 1 and nicely show the increase in structural size upon
approaching a balanced microemulsion.
In comparison, the TS analysis yields values of 1.7 to 2.3 nm
for the correlation length x. In contrast the repeat distance D
s
increases from 5.4 to 60 nm (the 60 nm is a lower realistic
estimate as this corresponds to the maximum of the experimental
observation window) (Table 1) with increasing x
MRTIL
until
a maximum is reached for the balanced microemulsion
(x
MRTIL
= 0.5). From this relatively low value of xone can
conclude that the bending rigidity of the amphiphilic interface
in the bicontinuous phase must be rather low as xis approaching
theeffectivelengthofthesurfactant.
45
This can be quantified
further by calculating the renormalized bending rigidity via
46,47
k
kBT¼10 ffiffiffi
3
pp
64
x
Dsð4Þ
which shows that kgoes down from 0.26 k
B
Tfor x
MRTIL
=
0.17 to 0.058 k
B
Tfor x
MRTIL
= 0.48 (balanced microemulsion).
Fig. 5 hZ
2
ias deduced from the TS fits (filled squares) compared to
values calculated with eqn (2d) from the real sample compositions for
three possible devisions of the interface: The whole interface is counted
into the IL phase (dashed line), all hydrocarbon chains of the
surfactant–cosurfactant belong to the oil phase (dotted line), and
partial partitioning of the hydrocarbon chains (full line).
Table 1 Parameters for the TS fits, correlation length x, repeat
distance D
s
and calculated amphiphilicity factor fa¼c1=ffiffiffiffiffiffiffiffiffiffiffi
4a2c2
p.
In addition, core radius R
c
, shell thickness dRand hard-sphere
radius R
HS
of the core–shell model are given. For samples with
x
MRTIL
=0.85 and 1.00 no structure factor S(q) was necessary.
Parameters for fits not shown in Fig. 4 are in italics
x
MRTIL
TS model Core–shell model
x/nm D
s
/nm f
a
R
c
/nm dR/nm R
HS
/nm
0.05 2.05 5.44 0.70 1.13 0.58 2.11
0.11 1.90 6.53 0.54 1.21 0.57 2.46
0.17 2.29 7.48 0.58 1.32 0.58 2.87
0.25 2.28 8.59 0.47 1.50 0.45 3.37
0.33 2.29 10.76 0.28 1.57 0.43 3.98
0.40 2.30 13.39 0.07 2.80 0.95 4.92
0.48 1.86 27.36 0.69 2.53 0.97 6.16
0.53 1.95 60.00 0.92 2.41 1.07 6.01
0.70 1.74 60.00 0.94 2.38 2.18 1.03
0.77 1.14 60.00 0.97 1.49 1.36 0.94
0.85 0.36 6.00 0.75 0.44 1.03 —
1.00 0.65 5.64 0.32 0.00 0.89
a
—
a
This value was described by a lognormal distribution and is calculated
from the 3rd moment similar to Rc¼ffiffiffiffiffiffiffiffiffiffi
hR3i
3
p(for details see ESI).
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This very low value for kexplains why we do not observe a
structural correlation peak in the SANS curves of the balanced
microemulsion as simply the amphiphilic interface is very soft.
This difference to conventional microemulsions can first be
attributed to having a very different solvent than water, where
the RTIL is much more efficient in reducing long range
electrostatic interactions. This is corroborated further by the
large increase of the amphiphilicity factor f
a
(see Table 1)
which indicates that structural ordering of the microemulsion
becomes less and less pronounced and for the balanced micro-
emulsion even approaches values that are not so far away from the
disorder line, where an unstructured liquid would be formed.
48,49
Nonetheless in our system large domain structures are present in
these solutions as evidenced by the high scattering intensity. In
general, this simply means that the surfactant employed here is not
a strong amphiphile for the cyclohexane–MRTIL mixture, which
is in agreement with the fact that rather large amounts of it are
required for forming a microemulsion, as is typically observed for
microemulsions with RTIL.
4,5,10
However, the increase of the structural size and the passing
from a MRTIL-in-oil (MRTIL/O) over a bicontinuous to
an oil-in-MRTIL (O/MRTIL) structure occurs in our
MRTIL microemulsion in a similar way as in conventional
microemulsions
38
(but with a more pronounced increase for
high MRTIL content). It must also be noted that the TS model
is very sensitive to the detailed structural features that deter-
mine the contrast conditions of the structure (eqn (2d)). In
Fig. 5 we compare the experimental value of hZ
2
iwith the ones
calculated such that all of the alkyl chains belongs either to the
oil phase or the MRTIL phase. Both assumptions show strong
deviations from the experimental data and only for a dividing
surface placed in the middle, which corresponds exactly to the
length of the decanol molecule (see Fig. 5), the intensity is
correctly accounted for. This means that our quantitative
analysis of the SANS curves allows us to deduce a rather
refined picture about the mesoscopic structure, as described by
hZ
2
iand D
s
, as well as about the precise local seperation
between the hydrophilic and hydrophobic domains.
4 Conclusion
In summary, our experiments demonstrate that with the
MRTIL bmim[FeCl
4
] we are able to form microemulsions
that are similar to classical oil–water microemulsions with
respect to their phase behavior and structure, but show weaker
amphiphilicity, softer amphiphilic films and accordingly less
structured microemulsions, particularly in the range of balanced
microemulsions. Nonetheless large size domains are present as
evidenced by the high SANS intensity observed. The most
interesting property is that, due to the MRTIL, this is a low viscous
‘‘magnetic microemulsion’’ over the whole range of composition,
which can be moved and manipulated by a magnetic field. By
proper choice of the surfactant–cosurfactant mixture the amount of
amphiphile required for the formation of a large single phase
microemulsion region can be kept relatively small, thereby leading
to these correspondingly large microemulsion structures. In this
region the typical phase inversion (O/MRTIL)–bicontinuous
(MRTIL/O) takes place as one changes the ratio between
ionic liquid and oil. Such a magnetic microemulsion is a novel
type of self-assembled system which due to its magnetic proper-
ties may open interesting paths for formulating functional and
responsive systems. This even more so as its formulation can be
achieved in a very simple fashion. Its properties are very
promising as in our system we combine the low viscosity and
structural versatility of a microemulsion with magnetic properties
without having to resort to incorporating magnetic nano-
particles, which would alter the whole structuring and behavior
of the given systems and render it much more complex to handle.
Accordingly we expect magnetic microemulsions to become a
class of colloidal systems which will have a large impact on future
research activities and applications.
Acknowledgements
The ILL is thanked for allocation of SANS beam time and the
Helmholz Zentrum Berlin for performing SQUID measure-
ments. Sebastian Gerischer and Robert Wahle are thanked for
their help with the construction of the SQUID liquid cell.
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