Understanding and Optimizing Microemulsions with Magnetic Room

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Understanding and Optimising Microemulsions with Magnetic Room Temperature Ionic Liquids (MRTIL) Andreas Klee, Sylvain Prevost, Urs Gasser, and Michael Gradzielski J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp512545c • Publication Date (Web): 13 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Understanding and Optimising Microemulsions with Magnetic Room Temperature Ionic Liquids (MRTIL) Andreas Klee,† Sylvain Prevost,†,‡ Urs Gasser,¶ and Michael Gradzielski∗,† Stranski-Laboratorium f¨ ur Physikalische und Theoretische Chemie, Institut f¨ ur Chemie, Straße des 17. Juni 124, Sekr. TC7, Technische Universität Berlin, 10623 Berlin, Germany., Helmholtz-Zentrum Berlin f¨ ur Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany., and Laboratory for Neutron Scattering, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland. E-mail: [email protected] Phone: +49 30 314 24934. Fax: +49 30 314 26602

Abstract

microemulsions and a rather large amount of alcohol is required for stabilization, where the effectiveness of the alcohol increases with increasing chain length of the alcohol. From this comprehensive investigation systematic trends can be deduced in order to formulate correspondingly structured microemulsions with MRTIL as polar phase.

Non-aqueous microemulsions containing the magnetic room temperature ionic liquid (MRTIL) bmimFeCl4 as polar phase were studied with respect to their macroscopic phase behavior and structure by means of small angle neutron scattering (SANS). The phase behaviour was studied in detail for different alcohols as cosurfactant, different oils as non-polar phase and mainly by varying the chain length of the used ionic surfactant (Cn mimCl with n=14,16,18). In general, phase behaviour and structural ordering in the mesophases were found to be comparable to water systems where with increasing content of MRTIL the microemulsions seems to become less and less structured leading to a rough and softer interface with less long range ordering. The extent of structuring increases with increasing chain length of the surfactant. However, the pure surfactant is not able to form

Introduction Room temperature ionic liquids (RTIL) are molten salts with a melting point under or near room temperature (often ≤ 100◦ C is used as an arbitrary definition) which is usually realized by bulky organic ions, which render crystallization difficult. The possibility of a modular combination of plenty known ions makes a great pool of properties accessible 1 and RTILs are interesting for various applications as solvent, lubricant, additive or catalyst. 2–4 Forming selfassembled structures in these solvents e. g. by adding surfactant a mesoscopic structuring can be achieved which makes these systems even more diverse. In former studies structures such as micelles, 5,6 microemulsions, 6–8 emulsions, 9,10 vesicles 11,12 or liquid crystals 5,6 (LC) have been found which makes them interesting e. g. as

∗

To whom correspondence should be addressed Stranski-Laboratorium f¨ ur Physikalische und Theoretische Chemie, Institut f¨ ur Chemie, Straße des 17. Juni 124, Sekr. TC7, Technische Universit¨ at Berlin, 10623 Berlin, Germany. ‡ Helmholtz-Zentrum Berlin f¨ ur Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany. ¶ Laboratory for Neutron Scattering, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland. †

ACS Paragon Plus Environment

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templates in particle synthesis, 13,14 polymerization or heterogeneous catalysis. 14–16 As just stated RTILs can also be a component in forming microemulsions, where compared to classical microemulsions they can take the part of the water and/or the oil, depending on whether one employs a hydrophilic or hydrophobic RTIL. 14,17–20 The first case is the normal one but there are also examples known where an RTIL substitutes the oil phase in a microemulsion. 21 The formation of RTIL containing microemulsions has mostly been investigated for bmim based ILs and with nonionic surfactants, where in particular TX-100 has been shown to be quite effective. 22,23 For these nonionic surfactants one can observe as a function of temperature and surfactant concentration in the phase behaviour the classical ”Kahlweit-Fisch”, 24 as for instance it has been demonstrated for the case of C14 E4 and various alkanes. 7 For the case of ionic surfactants one typically has to resort to adding a cosurfactant in order to raise the solubilisation capacity of the hydrocarbon in the IL microemulsion (in this respect microemulsions in polar ILs behave similar to ones in water). For instance this has been successfully done for C16 mimCl in bmimBF4 , where the solubilisation of dodecane could be facilitated by the presence of decanol as cosurfactant. 25 Similarly for a system of CTAB in 1ethyl-3-methylimidazolium hexylsulfate (emim hexSO4 ) the solubilisation of toluene could be increased substantially by the use of pentanol as cosurfactant and one observes the percolation phenomenon as in similar microemulsions in water. 26 Here in the droplet regime also an increase of the droplet size with increasing content of IL was observed. Such microemulsions can be quite robust with respect to temperature changes, as demonstrated for the case of C16 mimCl in bmimBF4 with decanol as cosurfactant and dodecane as continuous oil phase, where stability up to 150 ◦ C has been shown, 27 obviously much higher than it can be achieved with similar water based systems. So, there exists already quite a bit of knowledge regarding the formulation of microemulsions with ILs but the situation is in general more complex than in

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water as it depends also subtlely on the precise type of IL employed. One RTIL with quite special properties is 1-butyl-3-methylimidazolium tetrachloroferrate (bmimFeCl4 ) which include a wide liquid range, paramagnetic behavior 28 and catalytic activity. 29,30 This paramagnetic room temperature ionic liquid (MRTIL) was already proven to be able to form water free microemulsions which retain the magnetic properties. 8 This makes these systems interesting for potential future application based upon a manipulation via an external field. That our microemulsion can be manipulated by an external magnetic field is demonstrated in Fig. S9, which shows that our microemulsion can be moved into a certain direction by the presence of a gradient of the magnetic field. It might be noted in that context that recently other self-assembled systems containing magnetic ionic liquid surfactant (MILS) with the ability to respond to a magnetic field have received some attention. Their behavior in the presence of magnetic fields could be explained well by the bulk paramagnetism of the samples and the forces acting on volumes and interfaces of surfactant solutions depend simply on the concentration of the paramagnetic Fe ions. 31 However, so far the conditions for forming such microemulsions are not really well understood and accordingly this point was now investigated in detail by using several different alcohols as cosurfactant, an imidazolium chloride based surfactant with different alkyl chain lengths and a systematic variation in composition with respect to the oil and cosurfactant employed. This extends the concept of structural control by variation of the chain length of the surfactant and gives a more complete view on the system providing thereby the possibility to draw more general conclusions on self-assembly in non-aqueous media. Hereby the main focus lies on elucidating the role of the surfactant chain length in stabilizing microemulsions with MRTILs.


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Experimental Section

served q-range of 0.09 nm−1 ≤ q ≤ 3.04 nm−1 . Incoherent scattering of water was used to correct the detector efficiency. 33 The data were brought to absolute scale by comparing with the scattering intensity of glassy carbon 34 for a sample-to-detector distance of 5 m. Data reduction was done with BerSANS 35 and fitting with SASfit. 36

bmimFeCl4 was synthesized as described in literature. 8,28 C14 mimCl, C16 mimCl and C18 mimCl were synthesized as described by Kunz et al. 25 Oils and alcohols were used as purchased without further purification. For details see ESI (S1). The pseudo ternary phase diagrams were recorded at a temperature of 24±0.5 ◦ C. Mixtures of different ratios between cyclohexane and surfactant/cosurfactant were titrated with bmimFeCl4 . In all cases the molar ratio of Cn mimCl/decanol was 1:2, as we show in this work that this is a good ratio for forming microemulsions. The pseudo ternary phase diagrams to determine the cosurfactant influence were recorded by the same titration method but starting with a constant mass fraction of 86.4 % cyclohexane in all samples and varying ratio between C16 mimCl and alcohol. The phase boundary was detected visually. Kahlweit fish diagrams 24 were recorded by titrating samples with a volume ratio of bmimFeCl4 /oil=1:1 and different surfactant concentrations with decanol. For details see ESI (S2). Surface tension measurements were done by the pending drop method (Dataphysics Contact angle System OCA 15plus) by using a needle with an outer diameter of 1.83 mm (NE45, Kr¨ uss) and a home build temperature control cell. All samples were measured with different droplet sizes and the values were extrapolated to large droplet volumes to cancel out effects arising from needle tilting. Surface tension values were corrected by the calculated sample density which is in good agreement with experimental values. For details see reference. 32 Small angle neutron scattering (SANS) experiments were performed at PSI, Swizerland (SANSII). For better contrast the samples were prepared with D12-cyclohexane. (For consistency mass fractions in the phase diagrams were recalculated to H12-cyclohexane with the same volume.) Samples were measured in quartz cuvettes (Hellma, 1 mm thickness) at 24.0 ◦ C with an incoming beam of 6 ˚ A wavelength. Scattered neutrons were recorded for sample-to-detector distances of 1.2 m and 5 m resulting in an ob-

Results and Discussion Micellization with decanol In a first step we investigated the binary or pseudo-binary (containing cosurfactant in addition) systems in order to elucidate their potential as a basis for microemulsion formation. As shown earlier by recording temperature dependent binary phase diagrams, 32 the Krafft points of the pure surfactants in bmimFeCl4 are, dependent on the chain length, above or around 24 ◦ C and with that no microemulsion formation is expected at ambient conditions. Fig. 1 shows surface tension measurements at 45 ◦ C as a function of surfactant+decanol concentration of solutions of Cn mimCl/decanol in bmimFeCl4 . In the decanol free binary systems the reduction of the surface tension depends on the surfactant chain length (as already discussed earlier 32 ) but by adding decanol this effect is first damped (1 mol decanol/surfactant) and vanishes completely by adding more cosurfactant. This can be explained by the rather high amount of decanol used, i. e., the decanol is effectively determining the amphiphilic strength in these mixtures. In the following microemulsion systems at ambient conditions (24 ◦ C) will be discussed, which works out despite the Krafft point issue, as all formulations studied contain rather large amounts of cosurfactant which reduce the Krafft temperature correspondingly. Although these surface tension measurements shown here were recorded at higher temperatures (to avoid problems with the Krafft point) we may conclude from that data to the aggregation behavior at room temperature as it does not change much in this temperature range. This was


3


concluded from surface tension measurements at room temperature (Fig. S9, with systems including enough decanol to lower the Krafft point below room temperature) and having obtained comparable SANS spectra at 24 and 36 ◦ C (see ESI, Fig. S8). The surface tension measurements at ambient conditions also show a significant decrease of the Krafft point by adding alcohol which makes it possible to formulate systems at room temperature.

45

γ / mN/m

40 35

no C10OH

Microemulsions

C14mimCl

Adding an oil (which is insoluble in the MRTIL) to the system MRTIL/Cn mimCl can lead to the formation of microemulsions if the amphiphilic strength of the surfactant Cn mimCl is high enough to stabilize the MRTIL/oil interface. As discussed in the preceding section, surface tension measurements predict no microemulsion formation with pure surfactant at room temperature as the cmc goes beyond the solubility of the surfactant. However, by adding the cosurfactant decanol micelle formation is much facilitated and accordingly they might be swollen by adding an oil, thereby leading to the formation of microemulsions.

30 C16mimCl C18mimCl 45

γ / mN/m

40 35

1n C10OH C14mimCl

30 C16mimCl C18mimCl 45 40 γ / mN/m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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35

Macroscopic observations.

2n C10OH

The main characteristics of a microemulsion system can already be noticed by the simple observation of macroscopic phase separation. Fig. 2 shows the ability to solubilize bmimFeCl4 in cyclohexane at 24 ◦ C dependent on the alcohol/C16 mimCl ratio for different aliphatic alcohols as cosurfactant. For our experiments we chose cyclohexane as oil as it demands a reduced need of surfactant to form monophasic systems and shows a higher solubilisation capacity compared to other oils, such as isooctane and several linear alkanes, see Fig. S1, ESI. As clearly seen by extrapolating to an alcohol free system, the ability of MRTIL uptake is zero which means that no microemulsion formation takes place. Adding alcohol supports an uptake of MRTIL what can be interpreted as the formation of microemulsions. Independent on which alcohol was used qualitatively

C14mimCl 30 C16mimCl C18mimCl 0.01

0.1 1 10 CnmimCl (+C10OH) / wt%

100

Figure 1: Surface tension measurements at 45 ◦ C for binary mixtures of bmimFeCl4 /Cn mimCl (top), bmimFeCl4 /Cn mimCl+1 mol decanol (middle) and bmimFeCl4 /Cn mimCl+2 mol decanol (bottom) by using C14 mimCl (cubes), C16 mimCl (circles) or C18 mimCl (triangles) as surfactant.


4

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reduced by the addition of shorter chain alcohols but not so for longer alcohols like octanol or decanol. 38 As for the stability of the microemulsions a certain rigidity of the amphiphilic film might be required this is better achieved for the longer chain alcohols. Both effects (maximum shift and lower efficiency) are consistent with both proposed explanations for the maximum: A growing solubility of alcohol in the MRTIL will as well reduce the entropy/synergism effect mentioned before (leading as well to a lower amplitude) and lowers the effective ratio in the interface (maximum shift to higher concentrations). Similarly the packing parameter will be shifted less by a shorter alcohol and therefore a larger amount would be needed to achieve a balanced microemulsion (where one expects the peak of solubility). In addition, with increasing alcohol chain length the amphiphilic system is rendered more hydrophobic and having a stiffer monolayer, which apparently favors oil solubilisation. 38 At higher alcohol/surfactant ratios the uptake ability declines due to a lack of amphiphilicity as decanol itself is not a feasible amphiphile in this system. In addition to the linear aliphatic alcohols several other alcohols (3,7-dimethyloctanol, geraniol, cis-nerolidol, 2-butoxyethanol) were tested but gave no improvements in MRTIL solubilization (Fig. S1, ESI), only the 3,7dimethyloctanol had a similar performance as the 1-octanol. Due to its good ability to function as cosurfactant, decanol was used for further investigations. Although dodecanol enhances even more the MRTIL uptake it was not taken into account as its melting point is above/around room temperature caused solubility problems which lead to solid precipitate at low MRTIL concentrations (not shown in Fig. 2). Employing alkanes of different chain length as oils was also investigated and showed that the extent of the monophasic microemulsion region become smaller with increasing chain length (Fig. S1, ESI). Fig. 3 shows Kahlweit-fish diagrams for bmimFeCl4 /cyclohexane systems for all three surfactant chain lengths. As a control parameter to modulate the packing parameter not the temperature was used (as known for non-

C12OH C10OH C8OH C6OH C4OH

14 12 bmimFeCl4 / wt%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 8 6 4 2 0

0

1

2

3 4 5 6 7 n(CiOH)/n(C16mimCl)

8

9

Figure 2: Maximum MRTIL uptake at a constant ratio of cyclohexane/(cyclohexane+alcohol+C16 mimCl) of 86.4 wt%. for different aliphatic alcohols as cosurfactant as a function of alcohol/C16 mimCl mole ratio at 24 ◦ C. Lines are guides to the eye. the uptake capacity is first enhanced by enhancing the alcohol ratio in the mixture and then passes through a maximum which indicates an optimum cosurfactant/surfactant ratio. This phenomenon could be explained either by entropy/synergism effects due the preferred solubility of surfactant and alcohol mainly in the MRTIL and oil, respectively, as proposed by Huibers et al. for mixed surfactant systems 37 or by geometric considerations, as the surfactant/cosurfactant ratio influences the packing parameter and the maxima in MRTIL-uptake shown in Fig. 2 are located at the resulting optimal interfacial curvature for MRTIL uptake. The shift of the maximum to higher alcohol content for shorter chain alcohols can on one side be explained by a growing solubility in the MRTIL and with that a growing part of alcohol which is not acting as a cosurfactant but is solubilized monomerically (or at least as aggregates too small for a solvent uptake) in the MRTIL. In addition, for conventional microemulsions it has been observed before that the rigidity of the amphiphilic monolayer becomes substantially


5


ionic surfactants in water) but the cosurfactant/surfactant molar ratio δ (as similarly done when investigating the effect of medium chain alcohols as cosurfactants on the phase behavior of nonionic surfactants. 39 δ=

0.80 0.75 0.70

n(C10 OH) n(C10 OH) + n(Cn mimCl)

(1)

0.50

14

16

18

23 0.68

19 0.64

15 0.61

2

0.45 10

15

20

25

30

35

40

(CnmimCl+C10OH) (surfactant+decanol)i/iwt%

Figure 3: Phase digrams observed by plotting surfactant/cosurfactant ratio δ (eq. 1) vs. wt% surfactant+cosurfactant at equal cyclohexane and MRTIL volumes (Kahlweit fish) for C14 mimCl (straight line), C16 mimCl (dotted line) and C18 mimCl (broken line) at 24 ◦ C. Red line gives the experimental path for SANS experiments done at the C14 mimCl system shown in Fig. 5. as here maximum solubilization occurs according to Fig. 2. With a solid precipitate at low MRTIL-content, a multi-phase region in the low surfactant region and a broad mono-phasic region above a certain surfactant concentration all three surfactants show similarities. An increasing need of MRTIL to dissolve all surfactant in the oil rich region with longer surfactant chain is due to a decreasing solubility in bmimFeCl4 . The region at high surfactant and MRTIL concentrations was not investigated in detail but gives qualitatively a multi-phase area increasing in size with longer surfactant chain. The only small differences in the size of the multi phase region at low surfactant concentrations could be misinterpreted as only a weak enhancement of the ability to form microemulsions with longer alkyl chains. However a comparison with Fig. 3 can explain this phenomenon with the fact that a Cn mimCl/decanol mole ratio of 1:2 (δ = 0.67) is rather ideal for the C14 mimCl but becomes increasingly less so for the longer chain surfactants which illustrates the essential need of both MRTIL/oiland surfactant/cosurfactant-ratio variation to

Table 1: Characteristic parameters extracted from Fig. 3 giving the fishtail position (lowest surfactant amount needed to form the monophasic region) and the required cosurfactant content δ.

C10 OH+Cn mimCl [wt%] δ

1

0.60 0.55

As already shown in Fig. 2 for C16 mimCl, one finds for all three surfactant systems that the presence of alcohol to form microemulsions is crucial. The efficiency decreases from C18 - over C16 - to C14 mimCl due to a rising monomeric surfactant solubility in the MRTIL. Nevertheless the formation of microemulsions instead of pure molecular solutions is definitely proven by observing a three phase region. Characteristic parameters for the position of the fishtail (minimum amount of amphiphilic material and surfactant/cosurfactant ratio δ required for forming a single phase microemulsion) are summarized in table 1. For the C14 mimCl system no three phase system was observed but SANS measurements (Fig. 5, along the path shown in Fig. 3) proof the existence of mesoscopic structuring. Apparently the C14 mimCl is a much weaker structuring amphiphile and the 3-phase region was either to small to be detected (hindered also by the rather high concentrations and corresponding slowness of the phase separation) or is simply no longer appearing.

Cn mimCl

2

3

0.65

δ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Beside the phase behavior induced by surfactant/cosurfactant variation, in addition compositions with different oil/MRTIL ratios were investigated. Fig. 4 shows the pseudo ternary phase diagram of these systems with a constant Cn mimCl/decanol mole ratio of 1:2 (equals to δ = 0.67), this value being chosen


6

)

lti p mu

1:2 /n=

ha

8 · π · c2 · ⟨η 2 ⟩ /ξ + BG a 2 + c1 · q 2 + c2 · q 4 [ √ ]−1/2 Ds 1 a2 1 c1 = − 2π 2 c2 4 c2 ]−1/2 [ √ 1 a 2 1 c1 + ξ= 2 c2 4 c2 ⟨ 2⟩ η = ΦIL · Φoil · (∆ρ)2 = Qinv /2π 2 I(q) =

)

e an

C l/

ex

C 10 O Hn

loh

cyc

w( C nm im

length ξ (eq. 2c) of the structural units. Here ⟨η 2 ⟩ is directly related to the scattering invariant (Qinv ) and accounts for the contrast ∆ρ and volume fractions Φ of the oil and MRTIL phase (eq. 2d).

w(

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sic

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mono phasic

multi phasic w (bmimFeCl4)

(2a) (2b) (2c) (2d)

Next to the background, Ds , ξ and ⟨η 2 ⟩ were free parameters during fitting even though the latter value derives directly from the sample composition and the distribution of the components in the two phases. As the distribution of surfactant and decanol between polar, oily and interface domains is not clear this approach is justified. More details on this are given in the last part of this article. From the fit parameters the amphiphilicity factor fa and the renormalized mean bending modulus κ were calculated according to eqs. 3 and 4, respectively. 42,43

Figure 4: Pseudo-ternary phase diagrams for C14 mimCl (straight line, circles), C16 mimCl (dotted line, squares) and C18 mimCl (broken line, triangles) at 24 ◦ C by weight. Thick red line/crosses gives the sample position/experimental path for SANS experiments done with the C14 mimCl system shown in Fig. 5, thick dotted line for SANS experiments done at all three surfactant system shown in Fig. 7 get a full picture of the surfactant efficiency. Mesoscopic structure. To investigate the system on the mesoscopic scale SANS measurements were done, one of the key methods to study microemulsions. 40 The scattering curves in Fig. 5 show that the intensity increases largely upon reducing the content of amphiphile in the system and for the highest amphiphile content only very little coherent scattering is seen. Also interesting to note is that only at intermediate amphiphile concentration a correlation peak is seen that vanishes again upon further dilution. In order to deduce quantitative structural information from the SANS curves we applied the phenomenological Teubner-Strey (TS) model, 41 in which the scattering intensity is given by eq. 2a and is basically determined by the quasiperiodic repeat distance Ds (eq. 2b) and the correlation

√ fa = c1 / 4a2 c2

(3)

√ κ 10 3π ξ = kB T 64 Ds

(4)

Table 2: Teubner-Strey fit parameter derived from SANS measurements shown in Fig. 5 and calculated amphiphilicity factor fa and bending rigidity κ. ⟨

η2

⟩

C14 mimCl +C10 OH [wt%]

ξ

Ds

BG

[nm]

[nm]

[ cm 1nm3 ]

1 [ cm ]

61 53 49 42 37 29 26

1.31 1.34 1.43 1.50 1.50 1.55 1.51

3.66 3.86 4.04 4.63 5.38 8.64 16.26

0.02 0.04 0.06 0.11 0.16 0.26 0.30

0.69 0.66 0.65 0.64 0.64 0.61 0.59

fa

κ [kT]

-0.67 -0.65 -0.67 -0.61 -0.51 -0.12 0.49

0.31 0.30 0.30 0.28 0.24 0.15 0.08

Fig. 5 shows measurements at constant oil/MRTIL volume ratio of 1:1 along the exper-


7


1.7 1.6 ξ / nm

10

1

0.1

ξ Ds cube model

1

18 16 14

1.5

12

1.4

10

1.3

8

1.2

6

1.1

4

1

2 20 25 30 35 40 45 50 55 60 65 C14mimCl+C10OH / wt%

Ds / nm

1.8 C14mimCl+C10OH / wt% 26 29 37 42 49 53 61

Intensity / cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 16

Figure 6: Teubner-Strey fit parameters ξ (open squares) and Ds (filled squares) for C14 mimCl derived from curves displayed in Fig. 5. Dashed line displays the cube model (eq. 5) with Σ = 1.0 nm2 , cmon = 0.17 and xc = 0.5. For details see ESI (S7).

Scattering vector q / nm-1

Figure 5: SANS data (symbols) for microemulsions formulated with C14 mimCl and a constant volume ratio oil/MRTIL=1:1. For sample positions see Fig. 3. Straight lines are results from fits with eq. 2a.

6ϕp ϕap (5) nΣ Here n is the number of surfactant molecules in the interfacial film, ϕp and ϕap are the volume fractions of polar and apolar phase, respectively, and Σ gives the area per surfactant/decanol-unit. For details see ESI (S7). The model with suitable parameters is plotted in Fig. 6. Despite its simplicity it suffices to describe the experimental values quite well with values for Σ around 1 nm2 which is a reasonable value for the size of the surfactant head group. Similarly the monomeric concentration of 17 wt% which is needed to describe the data corresponds well to the cmc measurements described before. This findings accord with the preliminary made interpretations and yields a coherent model. ξ increases as well by lowering the amphiphile amount explained by more defined aggregates with a lower polydispersity. Calculated values for the amphiphilicity factor and the mean bending modulus as listed in table 2 are as well in a good agreement with the here proposed trends. κ increases with increasing surfactant concentration as the structures are expected to become stiffer, fa Ds =

imental path shown in Fig. 3 and 4. Decreasing the amphiphile amount while keeping the oil/MRTIL ratio constant at 1:1 leads to an increase in scattering intensity due to an increase of the size of structures present. The data were fitted with the TS model (eq. 2a) and the obtained parameters are summarized in Fig. 6 and table 2. The domain size Ds increases with decreasing surfactant concentration which can be explained such that less amphiphile per fluid is available to form an oil/MRTIL interface and with that the oil and MRTIL domains have to grow to house the volume of the two solvents with less surfactant stabilized interface available. Ds increases largely upon reaching the emulsification failure (cf. Fig. 3 and 4) and appears to be diverging there. A simple geometrical model proposed by Jouffroy et al. 44 describing the microemulsion by a lattice of cubes filled with either polar or apolar solvent and a separating surfactant layer between differently filled cubes was slightly modified to meet the characteristics of the actual system resulting in eq. 5.


8

Page 9 of 16

is located well under the Lifshitz-line 45 as expected for microemulsion structures at higher surfactant concentrations. Only near the phase boundary at low surfactant concentration a positive value points to a less structured system. To compare structures formed with the different surfactants additional SANS measurements were done along the experimental path shown in Fig. 4 (thick dotted line). For all samples the ratio between the molar quantity of amphiphile and the solvent volume was held constant at n(Cn mimCl)/(VM RT IL + Voil ) = 0.68 mol/L. Then the only parameter varying is the ratio between oil and MRTIL volume defined as

a) C14mimCl

xMRTIL 0.05 0.12 0.20 0.42 0.54 0.72 0.77 1.00

a) C16mimCl

xMRTIL 0.05 0.19 0.35 0.50 0.61 0.71 0.79 0.94 1.00

a) C18mimCl

xMRTIL 0.12 0.19 0.26 0.40 0.54 0.65 0.75 0.79 0.94

Intensity / cm-1

10

1

VM RT IL (6) VM RT IL + Voil In Fig. 7 the obtained SANS curves are shown and it is interesting to note that the overall scattering intensity becomes lower with increasing length of the surfactant. So apparently the structural units are the largest for the shortest chain surfactant. In contrast, the correlation peak at intermediate mixing ratio of MRTIL/oil becomes more prominent with increasing surfactant chain length, which indicates that the degree of ordering increases correspondingly. Teubner-Strey fits as described above were carried out and the results are summarized in table 3. In all three surfactant systems the amplitude (quantitatively expressed by ⟨η 2 ⟩) is decreasing with increasing xM RT IL due to a vanishing contrast by substituting deuterated cyclohexane with hydrogenated MRTIL. Following the picture of an inversion of mean curvature while going from the oil-rich to the MRTIL-rich side of the phase diagram a maximum in domain sizes Ds is expected for intermediate xM RT IL and can indeed be observed for all three surfactants as seen in Fig. 8. Surprisingly the correlation length ξ is not following simultaneously the same trend as known from water systems. 41,46 Instead after a similar rise up to xM RT IL ≈ 0.3, ξ decreases again for higher MRTIL content. This can be interpreted by a more flexible and interpenetrating mesoscopic structure in the MRTIL-dominated region which is supported by an increasing value xM RT IL =

Intensity / cm-1

10

1

10 Intensity / cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

0.1

1 scattering vector q / nm-1

Figure 7: Selected SANS data (symbols) for microemulsions formulated with C14 mimCl (a), C16 mimCl (b) and C18 mimCl (c) along the experimental path displayed in Fig. 4. Straight lines are fits with eq. 2a. Further spectra can be found in Fig. S6, ESI.


9

The Journal of Physical Chemistry 7

16

C14mimCl C14mimCl 6 C16mimCl C16mimCl C18mimCl 5 C18mimCl

14

ξ / nm

Table 3: Teubner-Strey fit parameters derived from SANS measurements shown in Fig. 7 and calculated amphiphilicity factor fa and bending rigidity κ calculated fith eq. 3 and 4, respectively. ⟩

6

2

4

0

Ds [nm]

BG 1 ] [ cm

fa

[ cm 1nm3 ]

κ [kT]

C14 mimCl

η2

3

2

0.05 0.12 0.19 0.20 0.37 0.42 0.49 0.54 0.60 0.72 0.77 1.00

1.67 1.74 1.79 1.82 1.77 1.69 1.54 1.36 1.27 0.90 0.87 0.78

4.86 5.67 6.15 7.19 8.82 9.88 10.77 12.48 13.85 16.04 8.95 5.41

0.60 0.57 0.54 0.51 0.44 0.41 0.33 0.30 0.22 0.08 0.03 0.03

0.29 0.38 0.48 0.54 0.59 0.63 0.65 0.70 0.66 0.60 0.58 0.54

-0.65 -0.58 -0.54 -0.43 -0.23 -0.07 0.11 0.36 0.50 0.78 0.45 0.10

0.29 0.26 0.25 0.21 0.17 0.15 0.12 0.09 0.08 0.05 0.08 0.12

C16 mimCl

⟨

8

1

ξ [nm]

0.05 0.12 0.19 0.27 0.35 0.42 0.50 0.55 0.61 0.71 0.79 0.94 1.00

1.69 1.91 2.05 2.20 2.15 2.11 1.95 1.87 1.71 1.53 1.56 1.41 1.07

4.76 5.38 5.91 6.42 6.90 7.24 7.54 7.57 7.45 6.84 5.96 4.85 4.80

0.56 0.51 0.46 0.43 0.36 0.30 0.24 0.21 0.15 0.05 0.01 0.01 0.03

0.29 0.41 0.47 0.57 0.58 0.62 0.63 0.68 0.66 0.63 0.57 0.54 0.55

-0.67 -0.66 -0.65 -0.65 -0.59 -0.54 -0.45 -0.41 -0.35 -0.32 -0.46 -0.54 -0.33

0.30 0.30 0.29 0.29 0.27 0.25 0.22 0.21 0.19 0.19 0.22 0.25 0.19

0.12 0.19 0.26 0.33 0.40 0.47 0.54 0.59 0.65 0.75 0.79 0.94

2.09 2.27 2.37 2.35 2.36 2.19 2.03 1.92 1.77 1.58 1.57 1.52

5.21 5.70 6.11 6.51 6.81 7.12 7.07 7.18 6.89 6.14 5.75 5.52

0.44 0.38 0.38 0.32 0.29 0.24 0.19 0.14 0.09 0.03 0.01 0.01

0.44 0.48 0.57 0.59 0.68 0.68 0.68 0.68 0.67 0.59 0.55 0.55

-0.73 -0.72 -0.71 -0.67 -0.65 -0.58 -0.53 -0.48 -0.44 -0.45 -0.49 -0.50

0.34 0.34 0.33 0.31 0.29 0.26 0.24 0.23 0.22 0.22 0.23 0.23

xM RT IL

4

Ds / nm

12 10

C18 mimCl

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 16

0

0.2

0.8

1

Figure 8: Teubner-Strey fit parameters ξ (open symbols) and Ds (filled symbols) for C14 mimCl (square), C16 mimCl (circle) and C18 mimCl (triangle) as a function of xM RT IL derived from curves displayed in Fig. 7. for fa . A second observation not being in accordance with common water systems is the fact, that the structures are getting bigger by shortening the alkyl chains. Intuitively one would expect stiffer and bigger domains with longer chains. While a higher stiffness is indeed confirmed by higher values for ξ and κ, the contrary behavior of the domain size can be explained by a better molecular solubility of C14 mimCl compared to the longer chain surfacants in bmimFeCl4 leading to a smaller interface to volume ratio and a tendency for building bigger structures as expected from eq. 5. Again some limitations of comparability may arise from the fact that the samples have different positions relatively to the fishtail position (compare Fig. 3), a complication not to be avoided as within a 4-component system such compromises regarding the composition have to be done, in order to have better comparability for other aspects. While the C16 - and C18 mimCl containing systems are placed in a region relatively far from the fishtail, the C14 mimCl containing system is located very close to its optimal δ-value to produce bigger structures.


10

0.4 0.6 xMRTIL

Page 11 of 16 10

∫

C14mimCl C16mimCl C18mimCl

8 Qinv / cm-1nm-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Qinv =

MRTIL N

+

N

+

N

MRTIL N

2

0

+

FeCl4-

0

N

N

+

oil

HO 1, 3, 5

oil

HO N

HO

0.2

1, 3, 5

0.4 0.6 xMRTIL

0.8

(7)

where extrapolation to zero and infinity was done by the Guinier and Porod approximation, respectively (see ESI for details). In the oil rich region (small xM RT IL values) a very good agreement is obtained for the case were the surfactant/alcohol alkyl chains are divided almost equally (6 and 7 C atoms of decanol and Cn mimCl surfactant, respectively, are counted into the polar phase; solid lines in Fig. 9), while making this division at the head group leads to largely different values. This is in a good agreement with earlier work. 8 Going to the MRTIL richer region (xM RT IL ≥ 0.4) the experimental values start to deviate and have smaller values as predicted by this model. This can be explained by a less and less defined interface caused by a weaker mesoscopic structuring which is in good agreement with findings for ξ discussed above. For xM RT IL values near 1 the experimental values can be better described by a model which counts all CH2 -groups to the oil phase. This is plausible as the portion of cyclohexane is getting more and more negligible compared to the amount of surfactant/decanol. Additionally to the Teubner-Strey model a clipped random wave (CRW) model 47,48 was applied to the data. As the derived values for the lengthscale parameters Ds and ξ are nearly identical with the ones obtained by TS it is only described in detail in the ESI. Nevertheless applying this model to the scattering data gives additional information as it delivers a third lengthscale (c) which accounts for the interfacial roughness. In the oil rich region (low xM RT IL ) this roughness parameter shows values comparable to water systems 48–50 (in our system they are slightly higher due to a higher surfactant concentration) and the trend of a growing roughness value with longer surfactant chains is as expected. Increasing the MRTIL content in the system gives continuously bigger c-values for all three systems which fits well to the general picture of a weakening of the mesoscopic structuring by increasing the MRTIL ratio. Above xM RT IL ≈ 0.4 the CRW-fits give random high numbers for c. This is due to the

N

HO

FeCl4-

I(q)q 2 dq

0

6

4

∞

1

Figure 9: Scattering invariant Qinv (open symbols) for C14 mimCl (square), C16 mimCl (circle) and C18 mimCl (triangle) as a function of xM RT IL derived from curves displayed in Fig. 7 with eq. 7. Small filled symbols are values calculated from sample composition with eq. 2d for the two cases of all (broken line) and only parts (straight lines) of the surfactant/alcohol CH2 -units counted to the oil phase. Inset cartoons illustrate the separation between oily and aqueous phase for these two cases. To get an even more detailed insight into the microemulsion structure, values for the theoretical invariant Qinv were calculated with eq. 2d. Although the sample compositions are fixed the result is highly dependent on the assumption of the partitioning of the surfactant/cosurfactant chains between MRTIL and oil domain as this effects volume ratio and average scattering length density of both domains. Different possible distributions were calculated (for details see ESI) that differ with respect to how much of the alkyl chain of the surfactant is counted into the hydrophobic part, and a comparison with experimental values is shown in Fig. 9. As the invariant value obtained by the Teubner-Strey fit is strongly effected by the fit quality instead the experimental invariant was calculated by integration of the measured data by


11


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fact that the roughness parameter is not necessary anymore to simulate the SANS data (i. e. the fit quality is independent from c) as the TSmodel itself gives already excellent fit results. Also remarkable is the fact that the simple two phase model to explain the invariant (compare Fig. 9) as well fails above xM RT IL ≈ 0.4 which gives a hint to structural changes at this point.

Page 12 of 16

less structured systems with interpenetrating phases leading to a rough and less stiff interface with less pronounced long range ordering. The here presented broad investigation yields quantitative information on the compositionstructure relationship and therefore gives recipes to design magnetic microemulsions with optimised properties and structures, as it has not yet been done for such systems that can be manipulated, e. g. become moved by a magnetic field. These findings are useful for designing strategies for formulating microemulsions of a given structure with MRTILs as polar component. This is important as such microemulsions could in the future be employed as interesting reaction media which contain also a component for separation via magnetic forces.

Conclusion The here presented study gives a detailed view on the phase behavior of bmimFeCl4 (a magnetic room temperature ionic liquid; MRTIL) containing microemulsions. As surfactants we employed different 1-alkyl-3methylimidazolium chlorides with C14 , C16 and C18 chains, but as alone its amphiphilic strength was not high enough to form microemulsions it was employed in a 1:2 molar ratio with decanol as cosurfactant. Studies of the phase behavior showed that alcohols become increasingly effective as cosurfactants with increasing chain length, while the range of having monophasic microemulsions becomes at the same time smaller upon increasing the chain length of the oil. The variation of the surfactant chain length shows on the one hand a classical behavior expressed by an enhancement of solubilization strength or film rigidity with increasing chain length. On the other hand the effect is damped by the influence of the high amount of cosurfactant so that the surfactant chain length has nearly no effect on surface tension. The SANS data can be well described with the Teubner-Strey model and show that microemulsion structures form most prominently in the region of xM RT IL = 0.2 − 0.6. The degree of structuring increases with increasing chain length of the surfactant and the size of the structural domains increases largely upon approaching the emulsification failure. Values for κ, c (mean bending modulus and roughness parameter from the CRW model) and ξ are comparable with water systems in the oil rich region. With an increasing content of MRTIL all this parameters point to less and

Acknowledgement SINQ at PSI is thanked for allocation of SANS beam time, Lisa Reile and Miriam Simon for helping to record ternary phase diagrams and surface tension, respectively. This research project has been supported by the European Commission under the 7th Framework Programme through the ’Research Infrastructures’ action of the ’Capacities’ Programme, contract No: CP-CSA INFRA2008-1.1.1 Number 226507-NMI3. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org/.

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Graphical TOC Entry

The developement of structuring in MRTIL based microemulsions displayed in its long range ordering and interface roughness is triggered by its MRTIL content and surfactant chain length.


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Understanding and Optimizing Microemulsions with Magnetic Room

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