Decrease of Droplet Size of the Reverse Microemulsion 1-Butyl-3

Mar 5, 2008 - ... in the periphery of the polar core of the microemulsion droplets and behave like a chock being inserted in the palisade layer of the...
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J. Phys. Chem. B 2008, 112, 3711-3719

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Decrease of Droplet Size of the Reverse Microemulsion 1-Butyl-3-methylimidazolium Tetrafluoroborate/Triton X-100/Cyclohexane by Addition of Water Yanan Gao,† Liane Hilfert,‡ Andreas Voigt,† and Kai Sundmacher*,†,§ Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, 39106 Magdeburg, Germany, Chemical Institute, Otto-Von-Guericke-UniVersity Magdeburg, UniVersita¨tsplatz 2, 39106 Magdeburg, Germany, and Process Systems Engineering, Otto-Von-Guericke-UniVersity Magdeburg, UniVersita¨tsplatz 2, 39106 Magdeburg, Germany ReceiVed: NoVember 20, 2007; In Final Form: December 21, 2007

In the present contribution, results concerning the role of small amounts of water in the 1-butyl-3methylimidazolium tetrafluoroborate (bmimBF4)-in-cyclohexane ionic liquid (IL) reverse microemulsions are reported. Dynamic light scattering (DLS) revealed that the size of microemulsion droplets decreased remarkably with increasing water content although water is often used as a polar component to swell reverse microemulsions. It was thus deduced that the number of microemulsion droplets was increased which was confirmed by conductivity measurements. The states of dissolved water were investigated by Fourier transform IR (FTIR) spectroscopic analysis showing that water molecules mainly act as bound water. 1H NMR along with two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (ROESY) further revealed that water molecules were mainly located in the periphery of the polar core of the microemulsion droplets and behave like a chock being inserted in the palisade layer of the droplet. This increased the curvature of the surfactant film at the IL/cyclohexane interface and thus led to the decrease of the microemulsion droplet size. The order of surfactant molecules arranged in the interface film was increased and thus induced a loss of entropy. Isothermal titration calorimetry (ITC) indicated that an enthalpy increase compensates for the loss of entropy during the process of microstructural transition.

Introduction Ionic liquids (ILs) that melt near ambient temperatures have enormous potential as alternative solvents and extracting agents.1 A unique property of ILs is that each molecule of the liquid exists as an ion resulting in liquids with high polarities and extremely low vapor pressures.2 Such characteristics turn ILs into attractive alternatives to traditional organic solvents in chemical reactions and also for separations.3,4 In addition, they are thermally stable, nonflammable, and their physicochemical properties can be adjusted by changing anions or cations.5 In many cases, ILs can be regarded as environmental benign, that is, “green” chemicals, making them highly desirable in many processes of industrial importance.6 Of particular interest in this regard is the self-assembly of surfactants in contact with ILs thereby forming micelles, microemulsion, liquid crystals, and so forth.7 The micellar aggregation behavior of surfactants in ILs has been investigated in recent years. It has been found that ILtype surfactants appear to self-aggregate within imidazoliumbased ILs.8 Evidence for micelle formation of some conventional surfactants in ILs was also presented, and solvatophobic interactions between ILs and nonionic surfactants were identified.9 Moreover, several often used surfactants show aggregation behavior in 1-ethyl-3-methylimidazolium bis(trifluoromethyl* To whom correspondence should be addressed. Tel.: +49 391 6110351; fax: +49 391 6110353; e-mail: [email protected]. † Max Planck Institute for Dynamics of Complex Technical Systems. ‡ Chemical Institute, Otto-von-Guericke-University Magdeburg. § Process Systems Engineering, Otto-von-Guericke-University Magdeburg.

sulfonyl)-imide.10 Recently, micellization of amphiphilic diblock copolymers in ILs has been reported; the universal micellar structures (spherical micelle, wormlike micelle, and bilayered vesicles) were all accessed by varying the length of the corona block.11 Patrascu et al. described the micellar aggregation behavior of alkyl poly(ethyleneglycol)-ethers in 1-butyl-3methylimidazolium type ILs.12 The addition of ILs to aqueous surfactant solutions of Triton X-100 has been studied too,13 and it was concluded that the added IL molecules partition into the Triton X-100 micellar phase close to the core as well as in the palisade layer of the micelles.13 It has also been demonstrated that 1-butyl-3-methylimidazolium bromide (bmimBr) has a pronounced effect on the aggregation behavior of Pluronic P104 aqueous solution; bmimBr originally prefers to be embedded into the micellar core, whereas above a critical concentration, micelles and large IL clusters coexist in the system.14 Tang et al. reported the temperature-dependent self-assembly process of polyoxyethylene-10-stearyl-ether surfactant (Brij 76) in bmimBF4. A transition process from nanofiber structure to gel and then to vesicle at increasing temperature was observed.15 In addition, self-assemblies of surfactants within microemulsions using ILs as a substitute for water or organic solvents were investigated intensively. Gao et al. were the first to discover that bmimBF4 assembles in polar nanosized droplets when dispersed in cyclohexane as solvent.16 Very recently, Liu et al. discovered that 1,1,3,3-tetramethylguanidinium type ILs can be dissolved in supercritical CO2 reverse micelles.17 They also reported that 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) can be dispersed in propylammonium formate (another type IL) or ethylene glycol.18,19 Small-angle neutron scattering (SANS) was used to investigate the bmimBF4/Triton

10.1021/jp711033w CCC: $40.75 © 2008 American Chemical Society Published on Web 03/05/2008

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X-100/cyclohexane microemulsion, and a swelling behavior consistent with water-in-oil microemulsions was observed.20 In addition, Seth and co-workers published a series of works on solvent and rotational relaxation of coumarin fluorescence probes in various IL microemulsions or micellar solutions.21-29 An environmentally affable microemulsion was also created by using bmimPF6 as a substitute for organic solvents.30,31 The driving force of IL microemulsion formation was considered to be the electrostatic interaction between the electronegative oxygen atoms of the oxyethylene (OE) units of Triton X-100 and the electropositive imidazolium ring.32 It was also demonstrated that a hydrogen-bonding network within the palisade layer is formed when small amounts of water are added to the bmimBF4-in-benzene microemulsion, which makes the microemulsion system more stable.33 The self-assembled structures of surfactants with ILs have shown different advantages over the traditional ones notably in the preparation of nanostructures. For example, a recent study has shown that IL-based microemulsions can be used to produce polymer nanoparticles, gels, and open-cell porous materials.34 Silica microrods with nanosized pores were prepared in a waterin-bmimPF6 microemulsion, whereas only silica nanoparticles were observed in water-in-oil microemulsions.35 Also, dispersed tetragonal ZrO2 nanoparticles can be obtained in a water-inbmimPF6 microemulsion, while significant agglomeration takes place in conventional water-in-oil microemulsions.36 In the present work, we wish to extend earlier studies on the microemulsion bmimBF4/Triton X-100/benzene to the system bmimBF4/Triton X-100/cyclohexane under addition of added water.33 Our basic interest lies in finding out how the added water influences the microstructures in these reverse IL microemulsions. The aim of our study is to improve the basic understanding of the microstructure and of the formation mechanism of IL-based microemulsions. Thereby, we want to contribute to the establishment of IL-based microemulsions as potential reaction media.

ature of the solution was controlled by a thermostat (F31C, Julabo) with an accuracy of (0.1 °C. Fourier transform IR (FTIR) spectra were collected on a Nicolet Nexus 670 with continuum microscope using OMNIC software. The microscope was equipped with a 15_cassegrain objective and a liquidnitrogen-cooled mercury-cadmium-telluride (MCT) detector. The view mode aided in locating a single bead. The transmission mode was used for the whole bead measurement. Beads flattened with a diamond window (SpectraTech, Shelton, CT) were used for all experiments in transmission mode. A clean diamond window (SpectraTech, Shelton, CT) was used to collect the background spectrum. Data were collected at 2 cm-1 resolution, and 32 scans were averaged. The spectrum deconvolution into trapped water, bound water, and free water was achieved by least-square curve fitting on the base of Gauss software. 1H NMR measurements were carried out with a Bruker AVANCE 600 NMR spectrometer at 300 K. The instrument was operated at a frequency of 600.13 MHz using tetramethylsilane as an internal reference. The spectrometer was fitted with a 5 mm CPTXI-1H-13C/15N/2H probe head with zgradients. The standard two-dimensional ROESY pulse sequence was used with a low-power spin-lock pulse. The relaxation delay was 2 s. Complex data (2k) were collected in 256 increments with eight transients each. The spin-lock field strength was 3200 Hz with a mixing time of 200 ms. Phase-sensitive twodimensional time domains were recorded and were processed using TPPI protocol. A pure squared cosine window function was used in both dimensions prior to filling and Fourier transformation. Isothermal titration calorimetry (ITC) experiments were carried out using the model Μrcsys-0011 titration calorimeter from Thermal Hazard Technology (THT). A 100 µL injection syringe was used for all studies. Water was used as titrant, and 4 µL of water was injected into a reaction vessel (1.8 mL) eight times with a 1500 s interval between each injection, that is, every time 0.625 wt % water was injected into the reaction vessel. The reaction vessel was filled with the studied bmimBF4-in-cyclohexane microemulsion for all runs.

Experimental Section

Results and Discussion 1. DLS Measurements. DLS is a powerful tool to characterize the size and size distribution of microemulsion droplets. Triton X-100 based bmimBF4-in-cyclohexane (IL) microemulsion has shown a swelling behavior similar to water-in-oil microemulsions, that is, the volume of the dispersed nanodroplets is directly proportional to the amount of added IL.16 An elliptical droplet structure was also proposed for the microemulsion on the base of the SANS experiments.20 Figure 1 shows the phase behavior of the three-component system bmimBF4/Triton X-100/cyclohexane at 23 °C. The single-phase region was differentiated into the bmimBF4-in-cyclohexane microemulsion, the bicontinuous phase, and the cyclohexanein-bmimBF4 microemulsion according to previous reports.31,38,39 In the ionic liquid-in-oil (IL/O) microemulsion region, the studied composition at the [bmimBF4]/[Triton X-100] molar ratio, R ) 0.53, is marked in Figure 1. The chosen sample was characterized by DLS at different water contents (Figure 2). Unexpectedly, the diameter of the microemulsion droplets remarkably decreases rather than increases with increasing water content. It is known that water is often used as a polar component to swell reverse microemulsions leading to the increase of droplet size. We argue that the role of added water in the bmimBF4-in-cyclohexane microemulsion is much different from that in water-in-oil (W/O) microemulsions. Water is insoluble in cyclohexane but soluble in bmimBF4 and also in pure Triton X-100 suggesting that the added water molecules

Materials. The nonionic surfactant Triton X-100 that was used in the experiments was obtained from Sigma-Aldrich. It was evaporated under vacuum at 80 °C for 4 h to remove any excess water before use. Cyclohexane (quality, 99.5%) was purchased from Merck. IL bmimBF4 was synthesized by the quaternization of 1-methylimidazole with 1-chlorobutane.37 The imidazolium chloride salt was crystallized in ethyl acetate at -30 °C. The post-metathesis product was obtained by ion exchange of 1-butyl-3-methylimidazolium chloride and potassium tetrafluoroborate in distilled water and then was washed with dichloromethane and was dried under high vacuum. The purity of the product was checked using 1H NMR spectroscopy. To avoid water, the containers with the materials were sealed tightly to avoid any further contact with air before use. Water was doubly deionized and distilled. Cyclohexane-D12 (99.5%) was provided by Merck and was used as received. Apparatus and Procedures. The droplet size distributions of the investigated microemulsions were determined by dynamic light scattering (DLS) using Nanotrac Particle Size Analyzer (Nanotrac NPA 250) and the microtrac FLEX application software program. All measurements were made with laser diode (780 nm wavelength, 3 mW nominal, Class IIIB at the scattering angle of 180°). A conductivity pocket meter (Model Cond 340i, Werksta¨tten GmbH & Co.KG) with an accuracy of (0.5% was used to measure the conductivities of the bmimBF4-in-cyclohexane microemulsions at various water contents. The temper-

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Figure 3. Variation of electrical conductivity of bmimBF4-in-cyclohexane microemulsion in dependence on the content of added water.

Figure 1. Phase diagram of the mixture bmimBF4/Triton X-100/ cyclohexane at 23 °C; the studied microemulsion composition is marked with 9: 49.9 wt % Triton X-100, 40.8 wt % cyclohexane, 9.3 wt % bmimBF4, i.e. [bmimBF4]/[Triton X-100] molar ratio, R ) 0.53.

Figure 2. Average size and size distributions of the bmimBF4-incyclohexane microemulsion at various water content: 0 (a), 1.0 (b), 2.0 (c), 3.0 (d), 4.0 (e), and 5.0 wt % (f) of the bmimBF4-in-cyclohexane microemulsion.

must be dissolved into the polar core of the microemulsion droplets. The total amount of polar components, that is, water and bmimBF4, is increased, whereas the size of microdroplets is decreased. Therefore, it can be deduced that the total number of microemulsion droplets is increased by increasing water content. 2. Electrical Conductivity. IL-based microemulsions exhibit a similar structure as water-based microemulsions.16,20 The percolation theory was successfully applied to characterize the microstructure of the IL microemulsions.31,38 The static percolation model attributes percolation to the appearance of a bicontinuous water structure in which open water channels are responsible for electrical conduction.40 The IL/O microemulsion can be regarded as a two-component system in which conducting IL droplets covered by a surfactant film are embedded in a continuous insulative oil medium. A high electrical conductivity of such a microemulsion can be explained by continuous droplet exchange.32 Figure 3 shows the electrical conductivity of the bmimBF4-in-cyclohexane microemulsion solution as a function of the content of added water. It can be seen that the conductivity, K, is significantly increased with increasing water content. It was measured by DLS that the addition of water causes a decrease of the IL microemulsion

droplet size. The increase of conductivity therefore suggests that this effect is due to the increasing number of IL droplets per unit volume of the microemulsion solution. In addition, according to the dynamic percolation model,40 the decrease of IL droplet size should decrease the attractive interactions between the IL droplets and thus the microemulsion conductivity. Thus, the observed increase in conductivity is definitely caused by the increasing droplet number. 3. FTIR Spectra. To clarify the role of added water in the bmimBF4-in-cyclohexane microemulsion, it is important to study the states and properties of the water molecules in the microenvironment. This will also help to understand how the added water molecules decrease the droplet size. The characteristics of the water molecules being incorporated in the reverse microemulsion depend strongly on the water content and the nature of the surfactant head groups.41 The states of dissolved water in water-based microemulsions were intensively investigated in the past.41-45 In general, the trapped water, with the O-H stretching vibration at about 3600 cm-1, is defined as the water species which are dispersed among long hydrocarbon chains of surfactant molecules43 existing as monomers without hydrogen-bonding interaction with the surroundings. In contrast to this, the bound water molecules are hydrogen-bonded to the polar head groups of the surfactants. For nonionic Triton X-100based W/O microemulsions, the O-H stretching vibration of the bound water appears at 3400 ( 20 cm-1.45 As the third possible state of dissolved water, there are free water molecules which have strong hydrogen bonds among themselves, with an O-H stretching bond at 3220 ( 20 cm-1.46 These three different states of water being added to the bmimBF4-in-cyclohexane microemulsion were analyzed by means of FTIR. The splitting of trapped water, bound water, and free water was achieved by least-square curve fitting on the basis of Gaussian peak shape. The effect of the terminal O-H group of Triton X-100 was eliminated by subtracting the baseline that was obtained from the spectrum of an IL-based microemulsion without water. As an example, the O-H stretching spectrum of bmimBF4-in-cyclohexane microemulsion with 4.0% added water is shown in Figure 4. It can be seen that the fitted curve fits the experimental one quite well. Similar fittings were performed for water contents of 1%, 2%, 3%, and 5%. Figure 5 gives the dependence of the area fractions for the three water species (trapped, bound, free water) as a function of the overall water content in the bmimBF4-in-cyclohexane microemulsion. It is obvious that the area fraction of bound water is relatively large compared with that of free and trapped water. The bound water molecules will be mainly located in the palisade layer of the microemulsion and either mechanically entrapped or bound to the OE groups via intermolecular hydrogen bonding.33 The weight fraction of Triton X-100 in the microemulsion is much higher than the fraction of added water. Consequently, there are always enough Triton X-100

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Figure 4. Deconvolution of the FTIR spectrum of bmimBF4-incyclohexane microemulsion with R ) 0.53 at a water content of 4.0 wt %.

Figure 5. Variation of area fractions of different water species in bmimBF4-in-cyclohexane microemulsion at R ) 0.53 with increasing water content.

molecules to bind the added water. Therefore, one can argue that the fraction of bound water always dominates among the three water species. Besides, water molecules can hydrate bmimBF4, and the number of bound water molecules will also increase with the hydration between water and the cation or anion of bmimBF4.33 4. 1H NMR Measurements. NMR spectra give more detailed information about the interaction between the molecules. They therefore can provide an insight into the dissolution behavior of added water molecules in the polar cores of the droplets of the bmimBF4-in-cyclohexane microemulsion. Thus, 1H NMR spectra were used to investigate the microstructure characteristics32 and the effect of water on the microenvironment of bmimBF4-in-benzene microemulsions.33 The results revealed that a hydrogen binding network is created between bmimBF4, water, and the OE groups of Triton X-100, which makes the palisade layer more firm.33 Figure 6 shows 1H NMR spectra for the bmimBF4-incyclohexane microemulsion at various water content. The numbering of atomic positions is shown in Chart 1. It can be seen from Figure 6a that the H-2 proton signal of the imidazolium ring shifts high field with increasing water content, which is similar to the behavior of bmimBF4-in-benzene microemulsions.33 Also, H-4 and H-5 of the imidazolium ring slightly shift to a high field position, indicating that the ion pairs of bmimBF4 are not separated. Moreover, the peaks of H-4 and H-5 gradually change into one peak with increasing water content because the oxygen atoms of water form hydrogen bonds to the acidic H-2, H-4, and H-5 protons, which in turn decreases the electropositivity of the imidazolium ring of bmimBF4, resulting in a close chemical environment of H-4 and H-5. However, the other proton signals of the imidazolium cation, that is, H-6, H-7, H-8, H-9, and H-10, obviously did not shift with the water content (not shown here). This result is also different from that of the bmimBF4-in-benzene microemulsion.33 There, the added water molecules can bind to the imidazolium cation by

Gao et al. hydrogen-bonding interaction,46-48 and these water molecules can form hydrogen bonds with the anions of bmimBF4 in a symmetric 1:2 type H-bonded complex.49 Intermolecular nuclear Overhauser enhancements (NOEs) have also suggested that water can act as a hydrogen-bonding donor toward the BF4ion.47 The hydrogen-bonding interaction with either cations or anions implied that the presence of water may loosen the intimate contacts between the imidazolium cation and the BF4anion. The decomposition of the ion pair remarkably decreases the electron cloud density of the imidazolium ring, which causes a relocation of the chemical shifts of the imidazolium proton magnetic resonance to a lower field.33 Therefore, the current result further suggests that the water molecules do not completely dissociate the ion pair of bmimBF4. The proton signals of the OE units of Triton X-100 were also analyzed. There are altogether eight resolvable resonance peaks for the surfactant Triton X-100, namely, (CH3-)3- (Ha), CH2- (H-c), (CH3-)2- (H-b), the ortho- (H-e) and meta(H-d) protons on the phenyl group of Triton X-100, OCH2(H-f), CH2O- (H-g), and (CH2CH2O)8H- (H-h).50,51 The chemical environments of the protons in the (CH2CH2O)8 group (labeled as H-h) are very close, as indicated by the broad signals in the spectra of Figure 6b. It was reported that small amounts of cyclohexane can penetrate the palisade layer of Triton X-100 to a certain extent.52 These cyclohexane molecules are perhaps located near the H-f and H-g protons of the OE units of Triton X-100 because the addition of water does not change their chemical shifts as reflected in Figure 6b. However, when the water content is 3 wt % of the IL microemulsion, we can see that a new peak is split apart from the broad signals of H-h and overlaps with the H-g peak. The proton number of the new peak corresponds to about four hydrogen atoms of Triton X-100. At water content up to 4 wt % of the IL-based microemulsion, the new peak shifts significantly downfield to a position at δ ) 3.99 ppm and the proton number of the peak reaches six hydrogen atoms. Further addition of water leads to a successive downfield shift and a further increase of the proton number. The result suggests that the hydration first takes place between the water molecules and the second OE units next to the phenoxy rings of Triton X-100 and then the third OE units along the hydrophilic chain of Triton X-100. Thus, one can imagine that these water molecules penetrate gradually from the outer shell into the inner polar core of the microemulsion droplets. The downfield shift experienced by the hydrated OE proton resonances is induced by the hydrogen bonding of the added water molecules to the OE units.53,54 Thereby, the electron density of oxygen atoms in the OE units is decreased. Because of induction effects, the electropositivity of the carbon atoms adjacent to the oxygen atoms is enhanced. As a consequence, the hydrogen atoms on the carbon atoms are shielded and resonate in a downfield position.20 In addition, the remaining OE proton signals only shift downfield very slightly, which is different from the observations at bmimBF4-in-benzene microemulsions, where all the OE proton signals shift downfield remarkably with the water content as the added water strongly binds all the OE units of Triton X-100.33 In the here investigated system, most water molecules mainly bind to the periphery (second and third OE unit) of the polar core, and only very small amounts of water enter the inner region to bind the remaining OE units. In addition, another new broad peak appears when water is added to the bmimBF4-in-cyclohexane microemulsion. The broadening becomes more and more pronounced, accompanied by a gradual shift downfield (from δ ) 4.17 to 4.87 ppm, not

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Figure 6. Variation of 1H chemical shift of bmimBF4 and Triton X-100 in bmimBF4-in-cyclohexane microemulsion at different water content.

CHART 1: Chemical Structure and Atom Numbering for BmimBF4 and Triton X-100

shown here). The new peak can be only ascribed to the added water.33 The broad single peak for the dissolved water indicates the rapid exchange between water protons in various states. 5. 2D ROESY Spectra. To further clarify the solubilization sites of the added water molecules, 2-dimensional rotational nuclear Overhauser effect spectroscopy (2D ROESY) was used to study the role of water in the bmimBF4-in-cyclohexane

microemulsion. ROESY is a widely accepted method to measure dipolar interactions between protons. The microstructure and structural change of the IL-based microemulsion can be outlined by 2D NMR to a certain extent although it is difficult to determine the precise interproton distances between each individual proton from the cross-peak intensities of the ROESY spectrum.50 Figure 7 describes the variations of ROESY spectra of the bmimBF4-in-cyclohexane microemulsion with water content. For clarity, the whole spectra were split into five sections (shown in Figure 7a-d). We can deduce the relative molecular arrangement of Triton X-100, bmimBF4, and added water in the IL-based microemulsion by 2D NMR. The original structure of the bmimBF4-in-cyclohexane microemulsion without water in the system was first analyzed. It can be seen that there are many interproton correlations among bmimBF4, namely, H-2/H-7, H-2/H-8, H-4/H-7, H4/H-8 (Figure 7a), H-5/ H-10, H-4/H-6 (Figure 7b), H-6/H-9, H-7/H-10, H-8/H-10, and H-9/H-10 (Figure 7c). These proton interactions were already observed in pure bmimBF4.47 This means that the IL structure

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Figure 7. Variation of 2D NMR spectra of bmimBF4-in-cyclohexane microemulsion at different water content. H-h2 and H-h3 denote hydrogen of the second OE unit and hydrogen of the third OE unit of Triton X-100, respectively.

was almost kept and a similar bulk IL environment was present in the bmimBF4-in-cyclohexane microemulsion. In other words, IL pools are present in the investigated IL microemulsion. There are no remarkable interactions between the hydrophobic tail protons (H-a, H-b, H-c) of Triton X-100 and the imidazolium

ring protons (Figure 7a, c, e) suggesting that the surfactant Triton X-100 separates the polar bmimBF4 and the apolar cyclohexane which again confirms that the IL microemulsion was really formed. Figure 7a and b shows that the H-d of phenoxy ring can interact with H-a, H-b, and H-c of the hydrophobic tail of

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Figure 8. Schematic of the arrangement of Triton X-100 at the bmimBF4/cyclohexane interface film of the considered microemulsion with and without addition of water.

Figure 9. ITC thermogram of bmimBF4-in-cyclohexane microemulsion at 25 °C.

Triton X-100 and even with some terminal H-h protons of the OE chain. Moreover, the H-e of the phenoxy ring interacts with either H-f and H-g that are attached to the hydrophobic tail of Triton X-100 and even with the terminal H-h protons of OE units or H-a of hydrophobic group of Triton X-100. These results show that the hydrophobic region extends at least one OE chain length beyond the hydrophilic polar core, which consequently results in a larger size of the microemulsion droplets. A similar phenomenon was also obtained from the computer simulation of the behavior of Triton X-100 micelles.55 We can also notice that H-a, H-b, and H-c of the hydrophobic tail interact with some terminal H-h protons (Figure 7c), indicating that some hydrophobic tails extend several OE chain lengths beyond the hydrophilic polar core. Thus, a staggered arrangement of Triton X-100 at the surfactant interface film can be assumed. The schematic diagram of the molecular arrangement of Triton X-100 is given in Figure 8. The staggered arrangement of Triton X-100 at the surfactant interface film corresponds to an ellipsoidal droplet structure.55-60 Furthermore, all protons of bmimBF4 can interact with H-h of Triton X-100 but not with H-f and H-g (Figure 7b-d). An exception is the interaction of H-10 and H-g that suggests that the IL molecules are dissolved into the whole polar core and that the IL molecules in the palisade layer are orderly arranged with the methyl group toward the hydrophobic phase. The effect of water on the structure of pure bmimBF4 was thoroughly analyzed by various techniques.46-49,61-66 It was shown that rotating frame NOEs’ (ROE) intensity decreases with an increasing water content for those interactions involving the

imidazolium ring protons H-2, H-4, and H-5, whereas the opposite is the case for interactions between H-10 and the protons of the n-butyl group (H-6, H-7, H-8, and H-9).47 These intensity changes suggested that water dissociated the ion pairs of bmimBF4 and formed the hydrogen-bonding interactions with aromatic ring protons H-2, H-4, and H-5 as water can behave as acceptor toward the imidazolium cation.47 Meanwhile, the addition of water made the methyl group shift toward the most hydrophobic part of the imidazolium ion, namely, the n-butyl chain.47 For the bmimBF4-in-cyclohexane microemulsion, many obvious changes of NOEs’ intensity were also observed after the addition of water. First, a new, very strong cross-peak attributed to the interaction of water/hydrogen atoms of the second OE unit and water/ hydrogen atoms of the third OE unit proton appears and becomes more and more strong with increasing water content. In the same case, only a very weak cross-peak due to the rest H-h/water correlation was observed (Figure 7d), although the rest H-h has more OE protons. This indicates that the added water molecules are mainly located near the periphery of the polar core of the microemulsion droplets, and only very small amounts of water penetrate into the droplet core, which corresponds to the result of the 1H NMR spectroscopic analysis. Second, the interaction intensities of H-2/H-h, H-4/H-h, and H-5/H-h are slightly decreased (Figure 7b), which happens because small amounts of water molecules enter the polar palisade layer of the microemulsion droplets and weaken the electrostatic interaction between the electronegative H-h and the electropositive imidazolium ring. This is due to the hydration of H-2, H-4, and H-5 and the hydrogen bonding of trace amounts of water to the OE units of H-h. Either the water/imidazolium interaction or the water/H-h interaction decreases the correlation of the imidazolium ring and the OE units of H-h. Moreover, the H-2/H-7 and H-2/H-8 cross-peaks disappear gradually with increasing water content (Figure 7a), which may also be attributed to the fact that trace amounts of water bond to acidic H-2 which leads to the decreased correlation. Third, three new peaks of H-d/H-f (Figure 7b), H-e/H-b, and H-e/H-c (Figure 7a) appear, indicating that the interaction between the hydrophobic tails was enhanced. Furthermore, the correlation intensities of H-e/H-h, H-e/H-g (Figure 7b), H-f/H-

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Figure 10. Schematic diagram of the bmimBF4-in-cyclohexane microemulsion. A large elliptical droplet structure can be assumed in the absence of water, while the addition of water results in small-size spherical microemulsion droplets.

a, H-f/H-b, H-g/H-a, H-g/H-b, H-h/H-a, H-h/H-b, H-h/H-c (Figure 7c), and H-f/H-h (Figure 7d) are all decreased suggesting that the interactions between the hydrophobic tails and also between the hydrophilic groups of Triton X-100 are enhanced. Therefore, we argue that the staggered structure of Triton X-100 in the interface film is replaced by a more ordered arrangement (see Figure 8). Fourth, the correlation intensities of H-10 and H-7, H-8, and H-9 (Figure 7c) decrease slightly, which is much different from the behavior in pure bmimBF4, where one sees an increase because of the water making the methyl group shift toward the hydrophobic n-butyl chain. Therefore, we conclude that here water does not enter the IL pools of the microemulsion, which corresponds to the argument that the added water molecules are mainly located near the periphery of Triton X-100 aggregates. The slightly depressed correlations of H-10 with H-7, H-8, and H-9 can be interpreted as follows: The IL molecules which are dispersed among the palisade layer of Triton X-100 are now separated by the small amounts of water molecules which are dissolved in the palisade layer and which act as a buffer among the IL molecules via hydrogen bonding to the imidazolium ring of bmimBF4. Fifth, the interactions of H-h with H-7, H-8, and H-9 (Figure 7c) were decreased, which is because trace amounts of water in the palisade layer can form hydrogen bonds to the OE units, which in turn decrease their interactions. In addition, the hydrogen of water bonds the imidazolium ring of bmimBF4, and as the volume of the imidazolium cation becomes larger, the sterical hindrance also would lead to the interaction decrease of H-h with H-7, H-8, and H-9. 6. ITC and Structural Analysis. DLS measurements along with the NMR spectroscopic analyses revealed that the surfactant molecules move from a large aggregate into the confines of a small aggregate and that the molecular arrangement becomes more and more ordered by the addition of water. The entropy must make an unfavorable contribution to the free energy of this microemulsion transition processes.67 Thus, one can conclude that the addition of water should lead to a significant decrease of enthalpy to compensate for the loss of entropy. The enthalpy of the microemulsion transition process was directly measured by isothermal titration calorimetry (ITC) as shown in Figure 9. The spontaneous transition from larger droplet sizes to small ones is an endothermic process on the molecular level. This enthalpy change is caused by the formation of hydrogen bonds between the OE units of Triton X-100 and water molecules as well as bmimBF4 and water molecules. 2D NMR spectroscopic analysis revealed an elliptical droplet structure of the bmimBF4-in-cyclohexane microemulsion with the staggered arrangement of Triton X-100 at the IL/cyclohexane interface film. Such a special arrangement leads to a large size

of the microemulsion droplets. The addition of water molecules causes a structural change of the IL microemulsion. The water molecules are mainly distributed at the periphery of the microemulsion droplets and behave like a chock to be inserted into the palisade layer of the microemulsion. This increases the curvature of the surfactant film at IL/cyclohexane interface and thus leads to a decrease of microemulsion droplet size. By addition of water, the staggered structure of the Triton X-100 interface film is replaced by a more ordered structure with hydrophobic tails close to hydrophobic tails and hydrophilic chains close to hydrophilic chains. A schematic of this IL microemulsion transition process is given in Figure 10. Conclusions The effect of adding small amounts of water to the bmimBF4in-cyclohexane microemulsion was investigated. The addition of water decreases the microemulsion droplet size and thereby increased the number of microemulsion droplets. The water molecules are mainly hydrogen bonded to the peripheral second and third oxyethylene (OE) units of Triton X-100 and behave like a chock to be inserted into the surfactant interface film. The ionic liquid (IL)/cyclohexane interface is bent toward the IL, and thus the microemulsion droplet size is decreased. A staggered arrangement of Triton X-100 at the IL/cyclohexane interface film is proposed for the original bmimBF4-in-cyclohexane microemulsion on the basis of 2D NMR spectroscopic analysis. The addition of water changes the elliptical droplet structure and leads to a more ordered arrangement of Triton X-100 molecules at the IL/cyclohexane interface. The structural change of the IL microemulsion induces a loss of entropy, while the enthalpy change derived from hydrogen bonding compensates for the entropy loss. The microstructural change can be interpreted as a transition from a large-size elliptical structure to a small-size spherical structure. Compared with the previously reported bmimBF4-in-benzene microemulsion, the role of water is different in the here studied microemulsion system. In the former case, it was found that no dried reverse micelles were formed as Triton X-100 is also soluble in the highly polarizable benzene.52 The addition of bmimBF4 induces the formation of IL microemulsion by weak electrostatic attraction between the electropositive bmim+ and the electronegative oxygen atoms of the OE units of Triton X-100.32 However, for the bmimBF4in-cyclohexane microemulsion, dried reverse micelles of Triton X-100 are formed before the addition of bmimBF4.52,58 The micellar polar core consists of OE units of Triton X-100 via hydrogen bonding between OE units and the terminal OH group of Triton X-100. It is relatively difficult for water molecules to enter the total palisade layer of the bmimBF4-in-cyclohexane microemulsion. They are located mainly at the periphery of the

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