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Non-aqueous microemulsions based on N,N’alkylimidazolium alkylsulfate ionic liquids Oscar Rojas, Brigitte Tiersch, Christian Rabe, Ralf Stehle, Armin Hoell, Bastian Arlt, and Joachim Koetz Langmuir, Just Accepted Manuscript • DOI: 10.1021/la401080q • Publication Date (Web): 16 May 2013 Downloaded from http://pubs.acs.org on May 22, 2013
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Non-aqueous microemulsions based on N,N’-alkylimidazolium alkylsulfate ionic liquids Oscar Rojas1,2, Brigitte Tiersch1, Christian Rabe3, Ralf Stehle3, Armin Hoell3, Bastian Arlt4, Joachim Koetz1* 1
Universität Potsdam, Institut für Chemie, Karl-Liebknecht-Strasse 24-25, Haus 25, 14476 Potsdam (Golm), Germany 2 Laboratorio de Polímeros Universidad Nacional (POLIUNA), Heredia, Costa Rica 3 Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner Platz 1, 14109 Berlin, Germany 4 Anton Paar Germany GmbH, Hellmuth-Hirth-Str. 6, 73760 Ostfildern, Germany The ternary system composed of the ionic liquid surfactant (IL-S) 1-butyl-3methylimidazolium dodecylsulfate ([Bmim][DodSO4]), the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium ethylsulfate ([Emim][EtSO4]) and toluene has been investigated.
Three major mechanisms guiding the structure of the
isotropic phase were identified by means of conductometric experiments, which have been
correlated
to
the
presence
of
oil-in-IL,
bicontinuous,
and
IL-in-oil
microemulsions. IL-S forms micelles in toluene, which swell by adding RTIL as to be shown by Dynamic Light Scattering (DLS) and Small Angle X-ray Scattering (SAXS) experiments. Therefore, it is possible to form water-free IL-in-oil reverse microemulsions ≤ 10 nm in size as a new type of nanoreactor.
Keywords: ionic liquids, water-free microemulsions, swollen micelles.
* Corresponding author
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1. INTRODUCTION Ionic liquids are defined as organic salts with a melting temperature below 100°C, consisting of an organic cation and an organic or inorganic anion.1 The first report concerning the preparation of ionic liquids is dated to 1914 with the synthesis of ethylammonium nitrate (EAN).2 Since then, an increasing number of reports are published every year.3-8 The great diversity of ionic liquid structures offering versatile and tunable physicochemical properties makes them attractive for applications in several chemical processes as a potential alternative to conventional solvents.4 Self assembly of amphiphilic molecules in room temperature ionic liquids (RTIL) has been recently reviewed.5-8 The ability to form micelles, vesicles, liquid crystal and microemulsions opens a wide range of applications. Microemulsions are thermodynamically stable and optically clear isotropic solutions of one liquid dispersed in another one, surrounded by a surfactant film and/or cosurfactant molecules. Among different types of microemulsions can be clearly distinguished,
i.e.
oil-in-water
microemulsions
(L1),
reverse
water-in-oil
9
microemulsions (L2), and bicontinuous microemulsions. The particular advantage of using ionic liquids in the formulation of microemulsions is their versatile use as a polar phase (substituent of water), as an organic solvent (substituent of oil), as a cosurfactant and/or as an amphiphilic component. The compartmentalization of ILs in a core of a micelle can be applied as a nanoreactor of high thermal stability10 for organic reactions,11,12 polymerizations,13,14 synthesis of inorganic nanoparticles,15 enzymatic reactions,16,17 or extraction processes.18 Another interesting approach is to use these systems for drug delivery.19 Reverse
IL-in-oil
microemulsions
composed
of
1-butyl-3-methylimidazolium
tetrafluoroborate [Bmim][BF4] in cyclohexane stabilized by the non-ionic surfactant (Triton X-100) have been extensively studied by Gao et al.20-24 Relatively large elliptical shaped aggregates were determined by means of scattering techniques and 1
H NMR spectroscopy.
The use of ionic surfactants to form microemulsions have also been documented.25-29 Falcone et al. have investigated the influence of ionic and non-ionic surfactants (i.e. benzyl-n-hexadecyldimethylammonium chloride (BHDC) micellization
of
1-butyl-3-methylimidazolium
and Triton X-100) in the
bis(trifluoromethylsulfonyl)imide 2
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([Bmim][Tf2N]) and [Bmim][BF4] in benzene.27 Moreover, comparative studies between cationic (BHDC) and anionic sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) surfactants with ionic liquids were presented.28 The authors have suggested that the interactions between the ionic liquids and the type of surfactant strongly influence the size of the micelle and the distribution of the different compounds in self-assembled structures. A similar approach was presented by Rabe and Koetz obtaining
reverse
micelles
of
1-ethyl-3-methylimidazolium
ethylsulfate 29
([Emim][EtSO4]) in presence of cetyltrimethylammonium bromide (CTAB).
More recently, the preparation of ionic liquids with certain surface activity due to the incorporation of long alkyl-chain substituents have expanded the applications range. The so called “IL-like surfactants” (IL-S) based on N,N’-dialkylimidazolium cations may have considerable advantage in terms of the formation of stable interfaces due to the electrostatic interactions between the anion and the cation. However, reports about the formulation of microemulsions using this kind of surfactants are rather scarce and only a few have been published so far.39-32 It has to be mentioned here that two general strategies can be used to come to IL-S. On the one hand, long alkyl chains can be incorporated into the imidazolium cation33 and on the other hand, the counteranion can contain a long alkyl chain.34-37 Room temperature (RTIL) as well as surfactant-like (IL-S) ionic liquids can be formed in absence of halogen components. So called halogen-free ILs are of interest due to their relatively low melting temperature and a high stability with regard to hydrolysis. The preparation of these ILs can be considered relatively easy and environmentally sustainable.38 The use of [Emim][EtSO4] as polar solvent can have technical advantages due to the relatively low viscosity, the broad window of fluidity, low toxicity and commercial availability on ton-scale.39 The combination with IL-S, e.g. 1butyl-3-methylimidazolium octylsulfate ([Bmim][OctSO4]), can result in different interesting phenomena of self-assembly, recently shown by us.32 However, the isotropic phase range is limited. The aim of this work was to increase the range of isotropic microemulsions by making the IL-S component more surfactant-like. One possibility to do that is to increase the length of the hydrophobic tail of the anion. Therefore, we synthesized an ionic liquid with a long alkyl chain anion, i.e. 1-butyl-3-methylimidazolium dodecylsulfate ([Bmim][DodSO4]). The isotropic water-free ternary system consisting of
two
IL-components
and
one
oil
component;
i.e. 3
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[Emim][EtSO4]/toluene/[Bmim][DodSO4], was investigated by means of different techniques, i.e. conductometry and rheology. For a more comprehensive characterization of the inverse microemulsion droplets DLS, SAXS and Cryo-SEM were used.
2. EXPERIMENTAL SECTION 2.1. Materials The ionic liquids 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) and 1-ethyl-3methylimidazolium ethyl sulfate ([Emim][EtSO4]) were obtained from Iolitec (Denzlingen, Germany). Sodium dodecylsulfate (SDS) (>99%), toluene (99%) were purchased
from
Roth
(Karlsruhe,
Germany).
The
ionic
liquid
1-butyl-3-
methylimidazolium dodecyl sulfate was synthesized according the protocol reported by Wasserscheid.409 First 15.3 g of 1-butyl-3-methylimidazolium chloride and 26.6 g of sodium dodecylsulfate were dissolved in 50 ml of hot water and stirred for 2h at 65°C. Subsequently, the water was removed using a rotational evaporator and the resulting solid was dissolved in 50 ml of dichloromethane and filtrated afterwards. The filtrate was washed with water until the water phase was colorless (at least 5 times). The organic phase was concentrated and dried under high vacuum at 80°C for 48 h. The product (65 % of the theoretical yield, based on [Bmim][Cl]) consist of a beige waxy-gel. The ionic liquid was characterized by DSC, 1H and
13
C NMR. The
water content was determined by Karl Fisher titration (< 0.3 %).
2.2. Apparatus and Procedures The ternary system consisting of [Emim][EtSO4]/toluene/[Bmim][DodSO4] was investigated at 25°C. The isotropic phase was determined by visual inspection titrating the toluene/IL-S mixture with [Emim][EtSO4] at 25°C. After adding each drop, the mixture was equilibrated in a thermostatic bath to guarantee steady state conditions of the optically clear solution. 4 ACS Paragon Plus Environment
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Conductivity measurements were carried out in the isotropic range of the ternary mixture in order to determine structural transitions in the microemulsion system. A microprocessor conductometer LF 2000 (WTW) was used to perform the measurements at 25°C. Dynamic viscosity studies at different microemulsion compositions were performed with the Gemini 150 Rheometer (Malvern Instruments) with cone-plate (1/40) geometry at 25 °C. The viscosity was calculated by linear regression of the Newtonian flow curves over the shear rate between 1 s−1 and 1000 s−1. Dynamic Light Scattering (DLS) measurements were carried out with a Nano Zeta sizer 3600 (Malver Instruments) at a fixed angle of 173° (backscattering detection) equipped with a He-Ne laser (λ = 633 nm; 4 mW) and a digital autocorrelator. Small Angle X-ray Scattering (SAXS) measurements were conducted at the beam line 7TMPW-SAXS at the BESSY II storage ring in Berlin, Germany. Microemulsion samples were enclosed in 2 mm quartz mark tubes from Hilgenberg and placed in a tempered sample holder. All measurements were carried out at 25 °C. The photon energy was selected by a silicon double crystal monochromator (Si 110) and was set to 12 keV corresponding to a wavelength of λ = 1 Å. By using sample detector distances of 1.2 m and 3 m a q-range between 0.1nm-1 and 3 nm-1 was covered. Sample and empty cell transmissions were determined using a photo sensitive diode inserted behind the sample. The individual sample path length was determined by transmission measurements of the samples moved perpendicular to the incident beam. The intensity normalization was performed using glassy carbon with a thickness of 90 µm as reference. Silver behenate was used for the radial calibration. The data reduction including corrections of the background, detector noise, sample thickness, radial and absolute calibration was done with the software package SASredTool (version V1.1.) developed by Haas. The microstructure of the optically clear microemulsion was examined by Cryo-high resolution Scanning Electron Microscopy (Cryo-SEM). The samples were plunged into liquid nitrogen, freeze fractured at −180 °C, etched for 60 s at −98 °C, sputtered with platinum in the GATAN Alto 2500 Cryo preparation chamber, and then transferred into the Cryo-SEMS-4800 (Hitachi).
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3. RESULTS AND DISCUSSION 3.1. Phase diagram, conductometric and rheological measurements Room temperature ionic liquid (RTIL), i.e. [Emim][EtSO4], and the surfactant-like ionic liquid (IL-S), i.e. [Bmim][DodSO4], have been mixed together with toluene and the resulting ternary system has been studied. The phase diagram was determined by preparing
different
compositions
of
[Bmim][DodSO4]/toluene
and
[Bmim][DodSO4]/RTIL into glass tubes. It has to be noted that also [Bmim][DodSO4] can be classified as a room temperature ionic liquid. Nevertheless in the following discussion [Emim][EtSO4] is called “RTIL” and [Bmim][DodSO4] “IL-S” respectively. The binary mixtures, initially transparent, were titrated with the RTIL or toluene respectively, until the system becomes turbid. Subsequently, the tubes were weighted and the amount of RTIL or toluene was calculated. The phase diagram resulting from an optical inspection of the ternary mixture is illustrated in the Supplementary Information. Two regions can be distinguished. The grey area corresponds to the turbid or two phase region, whereas the surrounded white area corresponds to the isotropic phase. Starting from the EmimEtSO4 corner and going along the base lines one can see that EmimEtSO4 is completely soluble in BmimDodSO4, but only up to 10% soluble in toluene. The relatively large isotropic region can be related to a major packing of the ionic liquid surfactant molecules at the interface. One can assume the formation of a palisade layer of the dodecyl sulfate anion possibly influenced by the presence of [Bmim] cation, which can acts as an electrolyte, co-surfactant and/or polar phase.41 In this context the particular arrangement may remarkably affect the spontaneous curvature of the interfacial film, but also could influence the droplet size of the formed micelles. In order to investigate the isotropic area regarding the different types of self assembling structures several techniques were employed. In the following experiments an alternative conductometric strategy was used, already suggested by several other groups.20,22,42-45 By this approach the oil component was added to the polar phase/surfactant mixture under registration of the conductance. To study this 6 ACS Paragon Plus Environment
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system in more detail, toluene was added to a given mixture of ionic liquids expressed in molar ratio (W0) of [Emim][EtSO4] in [Bmim][DodSO4], represented in supporting information (line b to f, for W 0 = 0.43, 0.74, 1.14, 1.71 and 2.58). Note that W0 is the molar ratio between two ionic liquids in contrast to water-based microemulsions.
0.20 Conductivity, κ / S/m
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Molar ratio EmimEtSO4:BmimDodSO4 W 0= 0.43 W 0= 0.74
0.15
W 0= 1.14 W 0= 1.71
0.10
W 0= 2.58
0.05 Oil-in-IL µE
0.00 0
10
Bicontinuous µE
20
30
IL-in-oil µE
40
50
60
70
80
90 100
Toluene wt.%
Figure 1. Conductivity of the EmimEtSO4/toluene/BmimDodSO4 systems at 25 °C and different EmimEtSO4:BmimDodSO4 molar rations as a function of toluene content.
For low [Emim][EtSO4] : [Bmim][DodSO4] molar ratios (W 0 = 0.43, 0.74), four regions can be identified. In the range of low toluene content (up to 20 wt. %), an increase in κ can be registered until a maximum is reached. This behavior was explained in terms of the formation of microdroplets of toluene dispersed in the ionic liquids in similarity to the previous investigated [Bmim][OctSO4]-based microemulsion,46 and to other studied systems.20,43 After this point, a second zone of non-linearity of decreasing κ can be correlated to the formation of a bicontinuous microstructure, where IL and oil micro channels coexist. The subsequent addition of toluene leads to a linear decline of the conductivity which can be associated to the formation of IL droplets dispersed in the continuous toluene phase (up to 70 wt. %). Finally, the curve at very high toluene content (>70 wt. %) shows a region of constant κ, which might be related to the existence of isolated IL-in-oil droplets. On basis of the conductometric break points, shown in Figure 1, three different microemulsion phases can be assumed, i.e. oil-in-IL, bicontinuous and IL-in-oil microemulsions. The resulting phase diagram showing the different microemulsion zones is given in Figure 2. 7 ACS Paragon Plus Environment
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Toluene 0.0
1.0
0.2
0.8
0.4
0.6
Turbid phase IL/oil
0.6
0.4
bicontinuous
0.8
oil/IL
1.0 0.0
0.2
0.2
Isotropic phase
0.4
0.6
0.0
0.8
1.0
BmimDodSO4
EmimEtSO4
Figure 2. Resulting phase diagram of the EmimEtSO4/toluene/BmimDodSO4 system obtained at showing the different types of microemulsion zones 25 °C.
Additional electrical conductivity experiments were performed following the experimental pathway corresponding to line a (shown in supporting information) to be sure that we are still in the isotropic phase range. In the experiment, a [Bmim][DodSO4]/toluene solution of 30/70 wt. %, was titrated by an IL-mixture, i.e. [Emim][EtSO4]/[Bmim][DodSO4] (70/30 wt. %). Knowing the amount of IL mixture added, the resulting [Emim][EtSO4] content was calculated and plotted against the registered conductance at 25 °C.
0.100
0.25
Shear Viscosity, η / (Pa s)
(a) Conductivity, κ (S/m)
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0.20 0.15 0.10 Point A
0.05 0.00
(b) 0.075
0.050 Point A
0.025
0.000 0
10
20 30 40 50 60 EmimEtSO4 content/ wt. %
70
0
10
20 30 40 50 60 EmimEtSO4 content / wt. %
70
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Figure 3. (a) Conductivity and (b) shear viscosity measurements along line a (according to supporting information) of the EmimEtSO4/toluene/BmimDodSO4 system at 25 °C at a fixed IL-S concentration of 30 wt. %.
As to be seen in Figure 3a, low conductivity values (below 0.05 S cm-1) are registered for RTIL contents up to 10 wt. %. Assuming the formation of diluted IL-in-oil droplets in this concentration range, one can suggest that the formed micelles dispersed in the continuous toluene phase are stabilized by a DodSO4 -based surfactant layer. A significant increase in κ (above point A) might indicates the motion of relatively small ions (i.e. [Emim]+, [EtSO4]- or possibly [Bmim]+ ions) from droplet to droplet. The transfer of ions may be similar to analogous aqueous systems where the exchange process is understood in terms of the formation of progressive droplet interlinking and clustering processes47,48 which implies certain flexibility of the surfactant film. However, in the investigated system, the determination of a clear percolation threshold is difficult to establish under the investigated conditions. Further addition of EmimEtSO4 presents only a slight variation in the conductance at about 50 wt. %, which suggests a change in the conduction mechanism associated by structural transformations. Complementary, rheological measurements were performed over a series of compositions on the [Emim][EtSO4]/toluene/[Bmim][DodSO4] system along the same experimental pathway at 25 °C. A Newtonian flow behavior and low viscosity are some of the features of the system, combined by a moderated increase of the shear viscosity in dependence on the RTIL content (Figure 3b). However, a significant change in the slope is observed at about 50 wt. % RTIL. On the one hand, the sharp increase of the viscosity can be interpreted in terms of increasing interactions between the droplets. On the other hand, the increase in the viscosity could also be explained by the presence of anisotropic droplets. Noteworthy that the presence of isolated droplets composed of IL-in-oil assemblies surrounded by a surfactant film, this means swollen micellar structures, was already suggested by conductivity measurements and the relatively low shear viscosity determined in the concentration range ≤10 wt. % RTIL. In order to characterize these colloidal structures in more detail, this means their size and size distribution, Dynamic Light Scattering (DLS) and Small Angle X-ray Scattering (SAXS) measurements were performed.
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3.2. Dynamic Light Scattering (DLS) and Small Angle X-ray Scattering (SAXS) For a more comprehensive characterization the experimental pathway shown in supporting information (line a) at low RTIL concentrations (below point A) was investigated by means of DLS and SAXS. Surprisingly, already in absence of RTIL a Bragg peak is observed in SAXS suggesting the formation of micelles. This means [Bmim][DodSO4] is able to form micelles in toluene. Figure 4 shows dynamic light scattering
data
of
different
microemulsion
compositions
at
a
constant
[Bmim][DodSO4] surfactant concentration (30 wt. %). A slight shift to a larger droplet size is observed when [Emim][EtSO4] is added, indicating a swelling behavior, which is a characteristic feature of aqueous-based systems, but also in non aqueous microemulsions containing ionic liquids.31,49 30 25
Intensity / %
20 15 10
10
0
5
1
Size /
10
nm
0
tSO w t.% 4
15 5
EmimE
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Figure 4. Size distribution of the ternary EmimEtSO4/toluene/BmimDodSO4 s in dependence on the [Emim][EtSO4] concentration at constant IL surfactant concentration (30 wt. %) determine by DLS.
The increase in the hydrodynamic diameter indicates that the interfacial area occupied per surfactant molecule; in this case occupied by the dodecyl sulfate anion, tends to be less affected by the addition of RTIL. At 15 wt. % a small decrease of the hydrodynamic radius can be observed. An explanation therefore can be the increased ionic strength in the whole system.
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Studies on microemulsion systems containing ionic liquids have shown some similarities to our results with regard to the size of the micellar aggregates. Zech and co-workers have investigated a microemulsion system composed of ethylammonium nitrate (EAN) as a polar phase, 1-hexadecyl-3-methylimidazolium chloride as a surfactant and dodecane and decanol as oil and co-surfactant, respectively.48 From the DLS data, it is shown that the droplet size is varied only slightly at low RTIL content. It might also be considered that an increase in the number density of droplets may also increase the interaction and polydispersity in the system creating certain problems in the DLS data interpretation and originates deviations in the droplet
size
determination.
Noteworthy,
that
Kazuba
et
al.
have
shown
subnanometer measurements by means of dynamic light scattering with a precision of 0.1 nm.50 To reinforce this interpretation, SAXS experiments were additionally performed. Figure 5 shows a single broad peak, similar to SAXS intensities of aqueous microemulsions. The presence of a peak maximum in absence of [Emim][EtSO4], suggest that [Bmim][DodSO4] might form micelles in toluene as already concluded from DLS. Note that the position of the peak maximum and the corresponding intensity varies conform with the amount of [Emim][EtSO4] (Table 1). As expected, an increase in the domain size of the resulting structure can be correlated to the shift of the peak maximum to smaller q values by increasing the RTIL content up to 10 wt.%. Assuming the presence of droplets at low RTIL content, one can explain that the increase of the domain size is caused by the swelling process of the formed structure due to the incorporation of [Emim][EtSO4] into the droplet core.
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I(q) Teubner Strey Fit 1
-1 I(q) / [cm ]
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0.1
10 1
q/
5
[nm -1 ]
0
im Em
SO Et
t. /w
%
4
Figure 5. SAXS spectra of EmimEtSO4/toluene/BmimDodSO4 microemulsion system in dependence of [Emim][EtSO4] weight fraction plotted and given in logarithmic scale. Full lines represent the fit by applying the Teubner-Strey model.
In order to obtain additional structural parameter that can describe the studied system, the resulting small angle scattering intensities from microemulsions were fitted by using the Teubner-Strey (T-S) model.51 According to the T-S model, the resulting scattering intensities (Figure 5) can be fitted applying the following equation (1): (1)
) ݍ( ܫൌ
ଵ
మ ାభ
మ ା ర మ
ܫ
This expression reveals two measures length scales by fitting three parameters a2, c1 and c2. From the peak position the periodicity of the oil and water domains, d, is determined, whereas from the width of the peak the correlation length ξ is obtained: (2) (3)
ଵ మ
݀ ൌ 2ߨ ቈଶ ቀ ቁ మ
ߦൌ
ଵൗ ଶ ଵ ቈଶ ቀ మ ቁ మ
ଵൗ ଶ
ଵ భ
ିଵൗ ଶ
െ ସ మ
ଵ భ
ିଵൗ ଶ
ସ మ
Additionally, one can obtain information regarding to the amphiphilic strength by the calculation of the amphiphilic factor using equation:5,52,53 12 ACS Paragon Plus Environment
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(4)
݂ ൌ
భ
భ (ସ మ మ ) ൗమ
Table 1 summarized the different calculated length scales, (the periodicity d, and the correlation length (ξ) obtained from the fitted parameters. Additionally, the amphiphilic factor fa was determined. Table 1.Values of peak positions qmax, scattering intensities I(qmax) from experimental SAXS data. Characteristic length scales d and ξ and dimensionless quantities calculated from the resulting fit parameter applying the T-S model for the EmimEtSO4/toluene/BmimDodSO4 microemulsion system.
rmaxa
[Emim][EtSO4]
qmax
I (qmax)
d
ξ
(wt.%)
(nm-1)
(cm-1)
(nm)
(nm)
0
1.78
0.60
3.34
1.92
-0.86
4
5
1.76
0.73
3.44
2.08
-0.87
6
10
1.64
0.81
3.67
2.19
-0.87
7
a
fa
(nm)
Obtained from the p(r) function.
From Table 1, a slightly increase of d and ξ can be correlated to the swelling of IL droplets dispersed in toluene, in similarity to other investigations.30 As already pointed out, fa can provide additional information about the microemulsion structure. Taking this into account, the relatively low values in the amphiphilic factor (fa) varying between -0.86 and -0.87 over the investigated range of investigation, suggest that the region studied is close to bicontinuous microemulsions. Similar results has been reported by Zech in microemulsions containing [Bmim][BF4] as polar phase.30 However, the results obtained by this model may contrast the results obtained by DLS and conductivity measurements with regard to the formation of IL-in-oil droplets at low RTIL content. An alternative strategy to describe the small angle scattering data is to use the freeform model approach proposed by Glatter.54 Hereby, a generalized indirect Fourier transformation (GIFT) was used. This method allows to determine the form factor P(q) and the structure factor S(q), simultaneously.55 Therefore, a description of the scattering intensity can be proximate by the product of the form and structure factors. (5)
) ݍ(ܫൌ ܰܲ ()ݍ(ܵ )ݍ 13 ACS Paragon Plus Environment
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Figure 6 illustrates that the scattering curves can be evaluated by applying the GIFT method in a satisfactory manner.
In addition the scattering curves can be
represented in real space in terms of the pair distance distribution function p(r) (PDDF). For globular micelles with constant contrast the PDDF can be represented as a bell-shaped function with its maximum at about half of the diameter.56
0.2
1 µE-0 wt. % EmimEtSO4
(a)
µE-5 wt. % EmimEtSO4
(b)
-1
µE-10 wt. % EmimEtSO4
p (r) / a.u.
I(q) / cm
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0.1
0.1
0.0 1
0 -1
q / nm
2
4
6
8
10
12
r / nm
Figure 6. (a) SAXS curves fit using GIFT evaluation (b) pair distance distribution function p(r) for microemulsion at point A (10 wt. % EmimEtSO4).
The calculations of the pair distance distribution function suggest the formation of core shell spheres at point A. This means that at a RTIL concentration of 1 wt% microemulsion droplets with a total radius of about 7 nm and inner core approximated to 3 or 4 nm are formed. Summarizing the results from the model approach proposed by Teubner-Strey51 and Glatter54 respectively, one can conclude that core-shell swollen micelles are formed at the borderline to bicontinuous microemulsions.
3.3. Cryo-scanning electron microscope (Cryo-SEM) In order to visualize the microemulsion droplets Cryo-Scanning Electron Microscopy was performed. The micrograph at Point A (10 wt % of [Emim][EtSO4]) is shown in the Supporting Information.
Based on our own experience by using cryo-SEM for characterizing IL-based microemulsions (compare References 29, 32, 46) one can identify spherical 14 ACS Paragon Plus Environment
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aggregates in the 10nm size range. However, the low contrast between the ionic liquid and oil can be related to the core-shell structure proposed by SAXS data.
4. CONCLUSIONS A non-aqueous microemulsion combining the amphiphilic/solvent features of the ionic liquids, i.e [Bmim][DodSO4] and [Emim][EtSO4] respectively, and toluene as the oil component was formulated. In contrast to previous relevant literature3,6-8,20-24 the isotropic phase range has been observed in absence of a “classical” surfactant component. That means [Bmim][DodSO4] fulfill IL-like properties as well as interfacial-like properties in the ternary system. The results show, that the isotropic area can be significantly increased by replacing the anion octylsulfate (compare Reference 32) with dodecylsulfate. The large isotropic phase range has been investigated by means of different methods. In particular, conductometric experiments were successfully applied to determine three different regions, correlating to the presence of oil- in-IL, bicontinuous and IL-in-oil microemulsions. The results were reinforced by additional conductometric
and rheological
measurements at constant IL-S concentration, suggesting a percolation boundary at about 15 wt. % RTIL. Dynamic light scattering (DLS) of systems before reaching the percolation boundary indicate the presence of droplets ≤10nm. A single Bragg peak, determined by SAXS experiments, supports the formation of ionic liquid micelles in toluene. For a more comprehensive characterization SAXS data were fitted by using two appropriate microemulsion models, i.e. the model proposed by Teubner and Strey51 and O. Glatter54. Based on the calculations one can conclude that a core shell aggregate structure is formed. Cryo-SEM provides valuable evidence about the formation of such spherical aggregates in the [Bmim][DodSO4] based system. DLS and SAXS data show that already in absence of RTIL the surfactant-like ionic liquid forms micellar aggregates in the oil component. These micelles containing dodecylsulfate as main surfactant component can solubilize [Emim][EtSO4] and microemulsion droplets with a core shell structure are formed. 15 ACS Paragon Plus Environment
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These water- and halogen-free reverse microemulsion droplets, containing two ionic liquids and oil, seem to be of interest as alternative nanoreactors for the preparation of nanoparticles or to perform chemical reactions.
ASSOCIATED CONTENT
Supporting Information. Figure S1 shows the phase diagram of the ternary system EmimEtSO4/toluene/BimimDodSO4 including the base lines a-f. Figure S2 shows the cryo-SEM micrograph at point A. This material is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgments We acknowledge the National Science Bureaus of Costa Rica (MICIT and CONICIT) and the DAAD for the financial support. The authors would like to thank Dr. Guenter Goerigk for the kind help with the SAXS measurements, as well as for the help during the preparation of the beamtime and helpful discussion.
AUTHOR INFORMATION
Corresponding Author *E-mail:
[email protected]. Phone: +49-331-9775220. Fax: +49-331-9775054
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Graphical abstract
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