Water-in-Ionic Liquid Microemulsion Formation in Solvent Mixture of

Sep 16, 2014 - We report that water-in-ionic liquid microemulsions (MEs) are stably formed in an organic solvent-free system, i.e., a mixture of aprot...
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Water-in-Ionic Liquid Microemulsion Formation in Solvent Mixture of Aprotic and Protic Imidazolium-Based Ionic Liquids Takumi Kusano, Kenta Fujii,* Kei Hashimoto, and Mitsuhiro Shibayama* Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan S Supporting Information *

ABSTRACT: We report that water-in-ionic liquid microemulsions (MEs) are stably formed in an organic solvent-free system, i.e., a mixture of aprotic (aIL) and protic (pIL) imidazolium-based ionic liquids (ILs) containing the anionic surfactant dioctyl sulfosuccinate sodium salt (AOT). Structural investigations using dynamic light, smallangle X-ray, and small-angle neutron scatterings were performed for MEs formed in mixtures of aprotic 1-octyl-3-methylimidazolium ([C8mIm+]) and protic 1alkylimidazolium ([CnImH+], n = 4 or 8) IL with a common anion, bis(trifluoromethanesulfonyl)amide ([TFSA−]). It was found that the ME structure strongly depends on the mixing composition of the aIL/pIL in the medium. The ME size appreciably increases with increasing pIL content in both [C8mIm+][TFSA−]/ [C8ImH+][TFSA−] and [C8mIm+][TFSA−]/[C4ImH+][TFSA−] mixtures. The size is larger for the n = 8 system than that for the n = 4 system. These results indicate that the shell part of MEs is composed of both AOT and pIL cation, and the ME size can be tuned by pIL content in the aIL/pIL mixtures.

1. INTRODUCTION Surfactants can stabilize water droplets in nonpolar organic solvents, which is well-known as reverse micelles (RMs) or water-in-oil microemulsions (MEs). In a conventional organic solvent system, MEs have been widely applied in chemical reactions, for example, nanoparticle synthesis, separation, protein stabilization, or refolding and so on.1−9 However, organic solvents used as the “oil” phase in ME formation are generally volatile, flammable, and toxic, which are significant problems in the environmental aspect. An alternative to such conventional systems, formation of thermodynamically stable water-in-ionic liquid (IL) type MEs, has been reported by using room-temperature ILs.10−22 As is well-known now, ILs possess novel and unique properties such as nonvolatility, high thermal stability, high designability, etc., and are thus used as a green solvent in the field of chemistry.23,24 In particular, note that the solvent properties of ILs can be easily varied by changing the chemical structure of ions and the combination of cation and anion25−28 to give a water-in-IL ME with novel and unique physicochemical properties.29−33 It has been established that ILs having cations with long alkyl groups, n (n = 8−12), aggregate with each other to form micelles in aqueous solutions.34−38 This suggests that ILs show an interfacial ability as well as cationic surfactant. Focusing on water-in-IL systems, Gao et al. first reported that water-in-IL type MEs are formed in ternary system of 1-butyl-3methylimidazolium tetrafluoroborate ([C4mIm+][BF4−]), Triton X-100 (neutral surfactant), and cyclohexane (organic solvent).10 Such MEs in a ternary system, i.e., IL/organic solvent/neutral surfactant, have often been proposed by some researchers and characterized by varying cation and anion © 2014 American Chemical Society

species of ILs. For neutral surfactants such as Triton X-100 and Tween 20, MEs can be formed in a system of IL and neutral surfactant without organic solvent.10,11,14,39,40 Development of organic-solvent-free systems is very important for environmental points of view and is thus desired in the field of chemistry. Goto et al. also reported that water-in-IL MEs are stably formed in typical aprotic IL (aIL) by adding dioctyl sulfosuccinate sodium salt (AOT) as an anionic surfactant and organic solvent (hexanol),12,13 resulting in successful activation and stabilization of enzymes in the MEs.16,41 Rai et al. recently found formation of water-in-IL RMs stabilized by zwitterionic surfactants (SB-12) in aILs containing ethanol.21 IL-based MEs used cationic or anionic surfactants provide an excellent reaction medium relative to conventional organic solvent systems.13,41 To our knowledge, however, unlike those systems with neutral surfactants, organic-solvent-free systems have not been developed in an ME system with ionic surfactant. Furthermore, incorporation of organic solvents as one component in the system leads to a decrease in IL mole fraction, resulting in a limitation of unique and novel properties originated from ILs. In this work, we report formation of water-in-IL MEs without organic solvents in an ionic liquid mixture of 1-octylimidazolium-based aprotic (aIL) and protic ILs (pIL) with anionic surfactant AOT. In the organic-solvent-free system proposed here, we focused on the water-droplet size variation as a function of (1) the water concentration, CW, and of (2) the Received: July 21, 2014 Revised: September 16, 2014 Published: September 16, 2014 11890

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detector distance (SDD) of 1.2 and 4.2 m. The scattered X-rays were counted by an imaging plate detector (R-AXIS VII++, Rigaku Corporation, Tokyo, Japan) with 3000 × 3000 pixel arrays and a pixel size of 0.1 mm pixel−1. The obtained 2D data were circularly averaged and corrected for dark current, background (cell) scattering, and transmittance. The obtained scattering data were normalized to the absolute intensity scale using a glassy carbon secondary standard. 2.4. Small-Angle Neutron Scattering (SANS). SANS experiments were performed on High-flux Advanced Neutron Application Reactor (HANARO) at Korea Atomic Energy Research Institute (KAERI), Korea. A monochromated cold neutron beam with an average neutron wavelength 6.00 Å was irradiated to the samples, and the scattered neutrons were counted with a 2-dimensional area detector. Two SDDs, i.e., 2 and 17.5 m, were employed to cover the qrange from 0.003 to 0.4 Å−1, where the magnitude of the scattering vector q is defined by q = 4π sin(θ/2)/λ (λ and θ represent the wavelength and the scattering angle, respectively). All measurements were performed at room temperature. After necessary corrections for open beam scattering, transmission, and detector inhomogeneities, the corrected scattering intensity functions were normalized to the absolute intensity scale. The incoherent scattering intensity subtraction was conducted with the procedure reported in the literature.44

mole fraction of pIL, xpIL, which are characterized by dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS) techniques.

2. EXPERIMENTAL SECTION 2.1. Materials. Figure 1 shows the chemical structures of the samples used in this study. The aprotic IL (aIL) 1-octyl-3-

3. RESULTS AND DISCUSSION 3.1. Microemulsion Formation in AIL/PIL Mixtures. Figure 2 shows photographs of mixtures of water and

Figure 1. Chemical structures of imidazolium-based aprotic ([C8mIm+][TFSA−]) and protic ionic liquids ([C8mImH+][TFSA−] and [C4mImH+][TFSA−]) and ionic surfactant (AOT). methylimidazolium bis(trifluoromethanesulfonyl)amide, [C8mIm+][TFSA−], was synthesized from N-methylimidazole and 1-bromoalkane, which is a standard method reported previously.28,42 Protic ILs (pILs) 1-alkylimidazolium bis(trifluoromethanesulfonyl)amide, [CnImH+][TFSA−] (n = 4 and 8), were prepared by mixing 1alkylimidazole and HTFSA in acetonitrile, and the solutions were stirred for 24 h, followed by removing the solvent under a reduced pressure. For SANS measurements, the deuterated ionic liquid d20[C8mIm][TFSA] was used. The deuteration process of the IL is described in our previous paper.28 The synthesized d20-[C8mIm][TFSA] was characterized by 1H NMR, and the degree of deuteration was 98%. For SLS/DLS measurements, AOT was added into the solvent (neat IL or aIL/pIL mixtures) and was stirred for 1 day, followed by addition of water with rigorous stirring for 30 min. 2.2. Light Scattering. SLS/DLS measurements were performed using ALV5000 DLS/SLS apparatus, ALV, Germany. The temperature of the samples was maintained at 25 ± 0.03 °C. The light source was a 22 mW He−Ne laser, whose wavelength λ was 632.8 nm. The scattering angles were in the range of 30° ≤ θ ≤ 150°. In the DLS measurements, the intensity correlation functions were obtained during the duration of 60 s at θ = 90°. The hydrodynamic radius, Rh, of RMs was evaluated by both cumulant and CONTIN analyses43 with the Stokes−Einstein equation, where the viscosities and the refractive indexes of the mixtures were independently measured in our laboratory with an Abbe refractometer (DR-A1, ATAGO, Japan) and a rheometer (MCR-501, Anton Paar, Austria) as a function of the mole fraction of pIL, xpIL, to the total ILs (Figure S1). 2.3. Small-Angle X-ray Scattering (SAXS). SAXS experiments were performed on the BL03XU beamline at SPring-8, which is located in Sayo, Hyogo, Japan. A monochromated X-ray beam with the wavelength λ of 1.00 Å was used to irradiate the samples at room temperature, and the scattered X-rays were counted at the sample-to-

Figure 2. Solubilization of water at room temperature in the IL mixtures of (a) [C8mIm+][TFSA−] and [C8ImH+][TFSA−] with 0.075 M AOT and 0.83 M water and (b) [C8mIm+][TFSA−] and [C8ImH+][TFSA−] with 0.83 M water.

[C8mIm+][TFSA−] (aIL) and [C8ImH+][TFSA−] (pIL) (a) with and (b) without AOT after rigorous stirring. Figure 2a contains 0.075 M AOT and CW = 0.83 M (≈1.5 vol %) water, while Figure 2b does CW = 0.83 M water only, where CW is the water concentration. The mole fraction of pIL, xpIL, in the aIL/ pIL mixture was xPIL = 0.2, and the molar ratio of water to AOT was W0 = 11. It is clear in Figure 2a that the mixed aIL/pIL solvent with AOT can dissolve relatively large amounts of water to give rise to a transparent solution. On the other hand, the water and IL mixture without AOT (Figure 2b) shows a turbid dispersion. As mentioned in the Introduction, Goto et al. reported that water molecules stably exist as droplets in a ternary [C8mIm+][TFSA−]/hexanol/AOT system to form water-in-IL type MEs, while a system without AOT undergoes phase separation. Note that the typical droplet size of the MEs reported by Goto et al. was ca. 100 nm. In comparison with their work, it is deduced in our system that water molecules exist as water droplets also in the aIL/pIL mixture with AOT but without organic solvent (e.g., hexanol), and the MEs are composed of water core and AOT shell. 11891

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3.2. Water Concentration Dependence. Figure 3 shows the autocorrelation functions, g(2)(τ) − 1, obtained from DLS

M. These results are consistent with previous studies of waterin-IL ME systems.12 Here, note that [C8ImH+][TFSA−] (pIL) is hydrophilic and is miscible with water, whereas [C8mIm+][TFSA−] (aIL) is hydrophobic and is immiscible in water. Therefore, this DLS result implies that the MEs formed in the aIL/pIL mixture involve pIL cation (C8ImH+) in their shell parts as well as AOT, and then the ME size strongly depends on xpIL. Figures 4a and 4b show SAXS and SANS profiles of mixtures with different CWs together with those of neat IL (i.e., CW = 0), respectively. In this work, we obtained SANS data for the mixture of deuterated [C8mIm+][TFSA−] (d20-[C8mIm][TFSA]) and normal [C8ImH+][TFSA−] with the fixed concentrations of AOT (CAOT = 0.075 M) at various CWs. Note that the incoherent scattering arises mainly from pIL, AOT, and water, which was subtracted by the method reported elsewhere and by numerical estimation based on the chemical structures of the component.44 A broad peak of SAXS profiles in high q region (q = 0.31 Å−1) is attributed to the liquid structure of IL.28 In the SANS profiles, a SANS curve of neat IL with AOT (neat IL + AOT) is also shown. From Figures 4a and 4b one can learn the followings: (1) Neat IL gives similar SAXS and SANS profiles. (2) By adding water, scattering intensity increases exclusively at low q region (q ≤ 0.02 Å−1) in both SAXS and SANS. (3) There is significant difference in the intermediate q region (0.08 ≤ q ≤ 0.2 Å−1) between SAXS and SANS patterns, and the SANS profiles exhibit a plateau. This plateau appears by simply adding AOT. These results strongly suggest that the systems consist of two kinds of scattering: one from MEs and the other from AOT molecules dispersed in the IL matrix as shown in the cartoon of Figure 4b. In the case of SANS, there are two scattering contrasts, i.e., one between water and IL and the other AOT and IL. On the other hand, SAXS has only single contrast between water and IL because the electron density difference between AOT and IL is negligible (ρAOT = 1.0 × 10−5 Å−2, ρIL = 1.1 × 10−5 Å−2). Hence, the SANS scattering intensity from water/AOT/IL systems is given by

Figure 3. Water concentration (CW) dependence of correlation functions observed for aIL/pIL mixtures (aIL: [C8mIm+][TFSA−]; pIL: [C8ImH+][TFSA−]) with CAOT = 0.075 M and xpIL = 0.2 at 25 °C.

measurements in the aIL/pIL mixtures with varying CW (0.75 ≤ CW ≤ 1.0 M) with fixed concentrations of AOT and pIL, CAOT = 0.075 M and xPIL = 0.2. Note that it was impossible to obtain meaningful data at lower water concentrations (CW ≤ 0.6 M) because of too weak scattering. This could be due to that significant amount of water were dissolved in to the bulk phase and no ME formation occurred as will be discussed later. As described above, anionic surfactant AOT is needed to form MEs in this system. As can be seen in Figure 3, a clear single relaxation was observed in the aIL/pIL mixture with AOT system for all the CWs examined here. With increasing CW, the characteristic decay time seems to shift to larger τ side, indicating that the ME size becomes larger with increasing CW. By applying cumulant analysis to the observed data, i.e., ln[g(2)(τ) − 1] vs τ plot as shown in the inset, we estimated the hydrodynamic radius, Rhs, of the MEs. The Rh value was estimated to be 131 nm in the solution at CW = 0.75 M, and the value increased with increasing CW to reach 195 nm at CW = 1.0

I(q) = N1v12(Δρ1)2 Φ2(q) + N2v2 2(Δρ2 )2 FAOT(q)

(1)

where N1 and v1 are the number density and volume of the water droplets, respectively, and N2 and v2 are the number density and volume of the AOT molecules, respectively. Δρ1 is

Figure 4. SAXS (a) and SANS (b) profiles observed for the aIL/pIL mixtures of deuterated [d20-C8mIm+][TFSA−] (aIL) and normal [C8ImH+][TFSA−] (pIL) (xpIL = 0.2) with different water content, CWs, at 25 °C. Except for neat IL, AOT (CAOT = 0.075 M) is also present in the mixture. In the SANS profiles, SLS data of CW = 0.75 M are also shown. The dashed and solid curves are the fit with eqs 4 and 1, respectively. The inset cartoons schematically show the difference in the scattering contrasts between SAXS and SANS. 11892

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the neutron scattering length density difference between water and deuterated IL (Δρ1 = 4.84 × 10−6 Å−2), and Δρ2 is that between AOT and deuterated IL(Δρ2 = 3.66 × 10−6 Å−2). Φ2(q) and F(q) are the form factors of a spherical object and AOT molecule, respectively, and are given by Φ(q) ≡

9 [sin Rq − Rq cos Rq] (Rq)3

F(q) = exp[−R g,AOT 2q2 /3]

(2) (3)

Here, R is the radius of the water droplet and Rg,AOT is the radius of gyration of AOT. The SAXS scattering intensity from the same system, on the other hand, is given by I(q) = N1v12(Δρ1)2 Φ2(q)

(4)

where Δρ1 reads the electron density difference between water droplets and IL(Δρ1 = 2.05 × 10−6 Å−2; the X-ray scattering length density difference between water and IL). Note that there is no scattering contrast in X-ray between AOT and IL because these electron densities are very close. The dashed curve in Figure 4a and the solid curve in Figure 4b show the fits with eqs 4 and 1, respectively. As shown by the fitting results, the calculated SANS functions well reproduce the observed functions. Hence, the scattering systems can be schematically depicted by the cartoons in the insets, where letter “v” denotes an AOT molecule dispersed in the medium. The Rg obtained from SANS was evaluated to be 0.85 nm, which is in a good agreement of the van der Waals size of AOT.45 Furthermore, N2 values was 2.55 × 1019 cm−3, which is consistent with calculated values (1.94 × 1019 cm−3). With regard to the I(q) ∝ q−1 relationship in the high-q range above 0.1 Å −1, a similar behavior was reported in the ME formation in aqueous solutions according to previous SANS study by Mata et al.46 They pointed out that the I(q) ∝ q−1 scattering corresponds to a small rodlike component, i.e., monomer surfactant with no micelle formation. In the case of the aIL/pIL mixture system, AOT molecules may exist both at the interface of MEs and in the IL matrix as molecular dispersion. On the other hand, the obtained N1 values from the SAXS and SANS measurement 1.51 × 1011 and 6.19 × 1011 cm−3, respectively, at CW = 0.45 M were much smaller than the calculated value (N1 = 4.53 × 1013 cm−3). This indicates that a significant amount of water molecules must be dissolved in the bulk IL. We reexamine the N1 values so as to satisfy the absolute intensities of SANS and SAXS and determined the critical water concentration below which water is completely dissolved in the bulk phase, CW* = 0.444 M. In the case of CW = 0.45 M and above, the excess water molecules, i.e., (CW − CW*), form MEs and give rise to scattering. The blue solid line in Figure 4b shows the fitting line for CW = 0.75 M by using eq 1. In this fitting, we assume that the radius of ME and its polydispersity were the same as evaluated by DLS measurements (R = 130.6 nm and ΔR/R = 0.201). The fitting was not perfect but seems to be acceptable by considering that SANS results are reproduced by the same set of the structural parameters obtained by DLS. This indicates that it is necessary to add an excess amount of water to the sytem for stable formation of MEs in the aIL/pIL mixture. Figure 5 shows the variation of the radius of water droplets as a function of CW determined by SAXS (R) and DLS (Rh). For comparison of the radii with different physical meanings, the relationship Rh:R ≈ 1:1 (for a hard sphere) was used. The ME

Figure 5. CW dependence of the radius of RMs, R, in the C8mIm+(aIL)/C8ImH+(pIL) systems with CAOT = 0.075 M and xpIL = 0.2.

radius increases rather linearly with CW. This result is in good accordance with that reported by Goto et al.12 This result indicates that water-in-IL MEs are formed due to the presence of AOT even without organic solvent. 3.3. Protic IL Concentration Dependence. In this section, we discuss the role of pIL in microemulsion formation. Figure 6 shows a series of DLS correlation functions of water/

Figure 6. Correlation functions observed for water and aIL/pIL mixtures (aIL: [C8mIm+][TFSA−]; pIL: [C8ImH+][TFSA−]) with CAOT = 0.075 M at various xpILs from 0.1 to 1.0.

AOT/IL mixtures where the molar fraction of pIL was varied from xpIL = 0.1 to 1.0. Interestingly to note that the correlation functions show a single relaxation and the characteristic decay time shifts to larger time with increasing xpIL. The inset shows the semilogarithmic plot for evaluation of the characteristic decay rate. Figure 7 shows the Rh values observed for the aIL/pIL mixtures with alkyl chain length of pIL, n = 4 and 8. The dashed lines are guides for the eye. In the case of the [C8mIm+][TFSA−]/[C8ImH+][TFSA−] mixture system, i.e., the case where the alkyl chain lengths are the same for aIL and pIL, the Rh increases with xpIL. By replacing the pIL with n = 8 to that with shorter alkyl group (n = 4), the Rh also increases with xpIL. However, it was found that the Rh value is appreciably smaller 11893

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and they are stabilized by two types of interactions: (1) water− imidazolium headgroup interactions at the core−shell interface and (2) van der Waals interaction between alkyl chains of C8ImH+ and AOT. On the other hand, it is possible that aIL cation (C8mIm+) does not contribute to ME formation in the aIL/pIL mixture system. Indeed, the ME size becomes large depending on pIL concentration (see Figure 7). The same rule is also applied to the [C8mIm+][TFSA−]/[C4ImH+][TFSA−] system. However, note that the ME size for [C4ImH+][TFSA−] system is appreciably smaller than that for [C8ImH+][TFSA−] system at high pIL mole fraction (as shown in Figure 7). This could be explained by the fact that the interaction between C4ImH+ and AOT is weaker than that between C8ImH+ and AOT due to a weaker van der Waals interaction between the shorter alkyl chain of C4ImH+ and longer one of AOT. As the result, the number of the constituting C4ImH+ cations in ME formation is less than that of corresponding C8ImH+ to give a smaller ME size for the [C4ImH+][TFSA−] system.

Figure 7. pIL mole fraction dependence of the Rh values in the C8mIm+(aIL)/C8ImH+(pIL) and C8mIm+(aIL)/C4ImH+(pIL) systems with CAOT = 0.075 M and CW = 0.83 M.

for the n = 4 system than that for the n = 8 system in the high xpIL region. It has been established in the previous studies37 that the imidazolium-based IL with n = 8 has a high efficiency for lowering surface tension because of its long hydrophobic alkyl chain in comparison with that with n = 4. It is thus expected that IL cation (in this case, protic CnImH+) acts as a cosurfactant for ME formation, which plays a key role in the formation of MEs. 3.4. Role of Protic Ionic Liquid on Microemulsion Formation. On the basis of these results, we propose ME formation mechanism in the aIL/pIL mixture system, which is illustrated in Figure 8. In the [C8mIm+][TFSA−]/[C8ImH+]-

4. CONCLUSION We succeeded in preparation of water-in-ionic liquid type microemulsions (MEs) in the absence of organic solvent. The MEs are formed in a mixture of aprotic (aIL) and protic (pIL) imidazolium-based ionic liquids (ILs) containing anionic surfactant, AOT. From structural investigations with DLS, SAXS, and SANS, the following facts were disclosed: (1) The size of MEs increases linearly with the water content, i.e., CW. (2) SANS is able to distinguish two types scatterings: one from MEs and the other from AOT molecules dispersed in the aIL/ pIL mixtures. (3) The size of MEs increases with increasing the ratio of pIL, i.e., xpIL. (4) The alkyl chain length of pIL cation also influences the ME size, particularly, at high pIL mole fraction. On the basis of the above results, we pointed out that the size of MEs formed in this system strongly depends on pIL content in aIL/pIL mixtures. The results obtained here give valuable insights for controlling the size of MEs in ionic liquid media.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1: experimental refractive index and viscosity. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (K.F.). *E-mail [email protected] (M.S.).

Figure 8. Possible ME formation model on aIL/pIL mixture system at (a) low pIL and (b) high pIL contents.

Present Address

K.F.: Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan.

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[TFSA ] mixtures with AOT, water molecules are stably distributed in the mixtures as water-in-IL type MEs in addition to dissolution to the bulk phase. The ME size is determined by the mixing ratio of aIL/pIL in the medium, and it becomes larger with increasing xpIL. [C8ImH+][TFSA−] (pIL) is a hydrophilic ionic liquid and is thus miscible in water. In contrast, [C8mIm+][TFSA−] (aIL) is a typical hydrophobic ionic liquid, which is immiscible in water and gives rise to phase separation. This implies that the polar imidazolium headgroup of pIL cation (C8ImH+) could interact with water molecules at the interface with the water droplets, and it is not for the case of aIL cation (C8mIm+). Therefore, we conjecture that the shell parts of MEs are composed of both AOT and C8ImH+ cation,

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (No. 24750066 to K.F., No. 25248027 to M.S.). The SANS experiment was performed by using 40 m SANS at HANARO, KAERI, Daejeon, South Korea, and the authors acknowledge technical assistance by Tae-Hwan Kim and Young-Soo Han. The SAXS experiment was 11894

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conducted at the second hutch of the Frontier Soft Matter Beamline (FSBL; BL03XU), SPring-8, Hyogo, Japan, with the assistance of Atsushi Izumi, Sumitomo Bakelite, Co., Ltd. with proposal no. 2014A7210, 2013A7212. The SAXS experiment was also performed at BL40B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2013A1622).

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