Role of Solubilized Water in the Reverse Ionic ... - ACS Publications

2506

J. Phys. Chem. B 2007, 111, 2506-2513

Role of Solubilized Water in the Reverse Ionic Liquid Microemulsion of 1-Butyl-3-methylimidazolium Tetrafluoroborate/TX-100/Benzene Yan’an Gao, Na Li, Liqiang Zheng,* Xiangtao Bai, Li Yu, Xueyan Zhao, Jin Zhang, Mingwei Zhao, and Zhen Li Key Laboratory of Colloid and Interface Chemistry (Shandong UniVersity), Ministry of Education, Jinan 250100, China ReceiVed: December 4, 2006; In Final Form: January 23, 2007

The ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) can form nonaqueous microemulsions with benzene by the aid of nonionic surfactant TX-100. The effect of water on ionic liquidin-oil (IL/O) microemulsions was studied, and it was shown that the addition of small amount of water to the IL microemulsion contributed to the stability of microemulsion and thus increased the amount of solubilized bmimBF4 in the microemulsion. The conductivity measurements also showed that the attractive interactions between IL microdroplets were weakened, that is, the IL/O microemulsion becomes more stable in the present of some water. Fourier transform IR was carried out to analyze the states of the added water, and the result showed that these water molecules mainly behaved as bound water and trapped water, indicating that the water molecules are located in the palisade layers of the IL/O microemulsion. Furthermore, 1H NMR and 19F NMR spectra suggested that the added water molecules built the hydrogen binding network of imidazolium cations and H2O, BF4- anion and H2O, and at the same time the electronegative oxygen atoms of the oxyethylene units of TX-100 and water in the palisade layers, which made the palisade layers more firm and thus increased the stability of the microemulsion. The study can help in further understanding the formation mechanism of microemulsions. In addition, the characteristic solubilization behavior of the added water can provide an aqueous interface film for hydrolysis reactions and therefore may be used as an ideal medium to prepare porous or hollow nanomaterials.

Introduction Ionic liquids (ILs) are attracting significant attention as novel solvent systems for chemical reactions and separations.1,2 They have widely been regarded as “green solvents” because of their potential as a recyclable alternative to the traditional organic solvents.3,4 They are nonvolatile, thermal stable, nonflammable, and have extremely high ionic conductivity. Also, their physicochemical properties can be modulated by changing one of anions.5 These properties of ILs also make them highly desirable in many reactions of industrial importance.6 However, the one major issue that remains to be explored is the formation of supramolucular assemblies. The investigations of ILs formed micelles or microemulsions are of great interest and thus may widen their practical applications. The micellar aggregation behaviors of surfactants relative with ILs have been reported in recent years. The aggregation of surfactants to form micelles in an IL was first reported in 1983.7 Afterward, Merrigan et al. demonstrated that imidazolium cations with attached long fluorous tails acted as surfactants and appeared to self-aggregate within imidazolium-based ILs.8 A communication presenting the evidence of dry micelles formation of several traditional surfactants in 1-butyl-3-methylimidazolium chloride (bmimCl) and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) has also appeared in the literature. The authors demonstrated the presence of solvatophobic interactions between ILs and the hydrophilic portion of surfactants, which was derived from the polyethylene groups * To whom correspondence should be addressed. E-mail: lqzheng@ sdu.edu.cn. Fax: 86-531-88564750. Phone: 86-531-88366062.

containing oxygens with lone pair electrons.9 The aggregation behaviors of several common surfactants in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide have been intensively investigated.10 More recently, a clear demonstration of the existence of a micellar phase formed by alkyl poly(ethyleneglycol)-ethers in 1-butyl-3-methylimidazolium-type ILs has been reported,5 and besides, four amphiphilic poly((1,2butadiene)-block-ethylene oxide) (PB-PEO) diblock copolymers have been shown to aggregate strongly and form micelles in bmimPF6.4 More recently, microemulsions using ILs as a substitute for water or oil have been intensively investigated. Han and coworkers discovered that 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) acts as polar nanosized droplets dispersed in a continuous hydrocarbon solvent. Freeze-fracture electron microscopy (FFEM) indicated a droplet structure which takes the same shape as “classic” water-in-oil (W/O) microemulsions.11 Their subsequent reports have also demonstrated that IL bmimPF6 could be used as polar component to be solubilized in the continuous toluene.12 Eastoe et al. have investigated the bmimBF4/TX-100/cyclohexane microemulsion system by smallangle neutron scattering (SANS), which showed a regular increase in droplet volume as micelles were progressively swollen with added bmimBF4, a behavior consistent with “classic” W/O microemulsions.13 In addition, the effects of confining the IL bmimBF4 on salvation dynamics and rotational relaxation of Coumarin 153 in TX-100/cyclohexane microemulsions have been explored, using steady-state and picosecond time-resolved emission spectroscopy.14 In our recent studies,

10.1021/jp068299g CCC: $37.00 © 2007 American Chemical Society Published on Web 02/17/2007

Role of Solubilized Water we have discovered that IL bmimPF6 can substitute for organic solvents to form a “green” microemulsion with the aid of surfactant TX-100.15 An electrochemical cyclic voltammetry method is followed to recognize the microregions of the IL microemulsion.16,17 Besides, the formation mechanism of the IL microemulsions has been proposed and it was shown that the interaction between the electronegative oxygen atoms of the oxyethylene (OE) units of TX-100 and the electropositive imidazolium ring may be the driving force for the solubilization of bmimBF4 into the core of the TX-100 aggregates.18 These IL microemusions can be regarded as a new class of reaction media. They not only can overcome the solubility limitations of ILs in apolar solvents but also provide hydrophobic or hydrophilic nanodomains, thereby expanding potential uses of ILs in microheterogeneous systems as reaction and separation or extraction media.13 Also, these IL microemulsions may have some unknown properties and some potential applications owing to the unique features of ILs and microemulsions.14,15 For example, a recent report has shown that surfactant ionic liquid-based microemulsions can be used to produce polymer nanoparticles, gels, and open-cell porous materials.19 Using ILs instead of the traditional organic solvents to create an IL-in-oil (IL/O) nonaqueous microemulsion is also an interesting research topic, as the nonaqueous microemulsions not only have attracted much interest from both theoretical and practical viewpoints but also have been widely applied to solar energy conversion, semiconductors, microcolloids, and cosmetics.22,21 There seems to be a number of distinct advantage of nonaqueous microemulsion systems over the aqueous systems.22 Herein, we investigated the effect of the solubilized water on the bmimBF4-in-benzene (IL/O) microemulsion, and to our knowledge, there have been scarcely reports about the formation mechanism of the IL microemulsion. The phase behavior and conductivity measurements showed that more bmimBF4 can be solubilized into the IL/O microemulsion in the present of small amount of water. The solubilization mechamism of bmimBF4 was revealed by the use of Fourier transform (FT) IR and NMR spectra. The current studies can help in understanding the microstructure and formation mechanism of the microemulsions and thus in establishing a better way of using it as a new medium. Experimental Section Materials. Nonionic surfactant TX-100 was obtained from Alfa Aesar and evaporated under vacuum at 80 °C for 4 h to remove excess water before use. To avoid the atmospheric water, bmimBF4 was freshly made and used. The containers with the materials were sealed tightly to avoid any further contact with air before use. Benzene was purchased from Beijing Chemical Reagents Company. Hexadeuterobenzene (C6D6) was obtained from Aldrich. Water was doubly deionized and distilled. IL bmimBF4 was synthesized according to the standard method by a quaternization reaction of 1-methylimidazole using 1-chlorobutane.23 The imidazolium bromide salt was crystallized in ethyl acetate at -30 °C. The postmetathesis product was obtained by ion exchange of 1-butyl-3-methylimidazolium bromide and potassium tetrafluoroborate in distilled water and then washed with dichloromethane and dried under a high vacuum. The purity of the product was checked using 1H NMR spectroscopy. Apparatus and Procedures. A low-frequency conductivity meter (Model DDS-307, Shanghai Cany Precision Instrument Co., Ltd.) with a precision of (1% was used to measure the conductivities of the bmimBF4/TX-100/benzene microemulsions

J. Phys. Chem. B, Vol. 111, No. 10, 2007 2507

Figure 1. Phase diagram of the bmimBF4/TX-100/benzene threecomponent system at 25 °C. For the lines a and b, the initial benzene weight fraction is 0.65 and 0.72, respectively. For the line c, the bmimBF4/TX-100 molar ratio, R, is 0.4.

at various water content at 25.0 °C. The diameters of the bmimBF4/TX-100/benzene microemulsions were determined by dynamic light scattering (DLS, Brookhaven Instrument Co., BI-200SM goniometer and BI-9000AT correlator) with an argon-ion laser operating at 488 nm. The viscosity of the continuous phase for each mixture was first measured by a NDJ-1 Rotating Viscosimeter (Shanghai, China) with an accuracy of 1%. All measurements were made at the scattering angle of 90° at a temperature of 25 °C. FTIR absorption spectra of all the samples were recorded in the range of 400-4000 cm-1 with a Bio-Rad model FTS-165 spectrometer using a fixed path length of 0.025 mm cell equipped with AgCl windows. There was no obvious signal that the KBr pellets were eroded by the addition of small amount of water to the investigated system. To obtain a good quality of the spectra, each sample was recorded with 32 scans at an effective resolution of 2 cm-1. All spectra were recorded at room temperature (about 25 °C). The hydroxyl stretching vibration bands (3050-3700 cm-1) were fitted using the Marquardt algorithm with least-squares curve fitting. 1H NMR measurements were carried out with a Bruker AMX 400 NMR spectrometer at room temperature (about 25 °C). The instrument was operated at a frequency of 400.13 MHz. The 1H NMR spectra were obtained in the lock condition by hexadeuterobenzene, and tetramethylsilane was used as an internal reference. All 19F NMR spectra were recorded on a Bruker AMX 400 NMR spectrometer equipped with a 4-nucleus PFG AutoSwitchable probe. The 19F NMR spectra (64K data points) were run at 470.30 MHz. The 19F chemical shifts were reported relative to external CF3COOH at 0 ppm. Results and Discussion 1. Phase Behavior and Solubilization of bmimBF4. The phase behavior of bmimBF4/TX-100/benzene three-component system at 25.0 °C is shown in Figure 1. A large single isotropic region extending from the IL corner to the benzene corner is observed. The blank region marked “one phase” is the onephase microemulsion and the shadow region marked “twophase” is a turbid region, that is, a microemulsion in equilibrium with an excess benzene or bmimBF4 phase. The large single phase microemulsion region has been differentiated into the IL/O, bicontinuous, and oil-in-ionic liquid (O/IL) microemulsions according to the previous report.16,17 In the IL/O microemulsion region, which is shown in Figure 1, a series of samples

2508 J. Phys. Chem. B, Vol. 111, No. 10, 2007

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Figure 3. The solubilized bmimBF4 weight content (wt %) in the IL/O microemulsion (along the line b in Figure 1) as a function of added water.

Figure 2. Sizes and size distribution of the droplets of the bmimBF4in-benzene microemulsion with R ) 0.70 (1), 0.90 (2), 1.10 (3), 1.33 (4), and 1.55 (5).

in the IL/O microemulsion region along the dilute line a are chosen and characterized by DLS at various sample compositions (9). It is observed that the sizes of microemulisons increased from 60.2, 86.2, 106.8, 136.4, to 148.5 nm with increasing the [IL]/[T-X100] mole ratio, R, from 0.70, 0.90, 1.10, 1.33, to 1.55.(Figure 2) The result is similar to those of bmimBF4/TX-100/cyclohexane microemulsions, suggesting the formation of IL/O microemulsions,11 because this regular swelling behavior is consistent with the volume of dispersed nanodomains being directly proportional to the amount of added IL, which is common to many droplet microemulsions.13

In the IL/O microemulison region, we choose some microemulsion systems that are close to the phase boundary of singlephase microemulsion region and two-phase region. When a small amount of water is added to the microemulsion systems, the amount of solubilized bmimBF4 is much more than that without water. The variation of solubilized bmimBF4 as a function of added water weight percent content (relative to the IL microemulsion) for the IL/O microemulsion (along the line b in Figure 1) is shown in Figure 3. It can be seen that the amount of solubilized bmimBF4 is enhanced with increasing water content. Thus, it can be deduced that the addition of water perhaps contributes to the stability of the IL microemulsion and thus increase the amount of solubilized bmimBF4. By consideration that water is also polar and immiscible with apolar benzene, it is more possible to be solubilized into the TX-100 aggregates as a dispersed component. However, generally the increase of polar component would lead to the phase separation especially for the microemulsion near the phase boundary. In order to further confirm this point, we also investigated the bmimBF4/TX-100/cyclohexane system, the same phenomenon was observed. Therefore, it is interested for us to study the role of added water to the IL/O microemulsion. 2. Electrical Conductivity Measurements. It is known that electrical conductivity measurements can be used to predict qualitatively the interaction between droplets and thus the stability of microemulison. For example, appearance of an electrical percolation process indicates the existence of large attractive interaction between droplets.24,25 A quantitative relation between electrical percolation threshold and the strength of attractive interdroplet interactions has been made.26 Our recent study has shown that electrical conductivity can be used to identify the microregions of IL/O, bicontinuous, and O/IL microemulsions.16,17 In that work, the traditional organic solvents were used as titration phase and a percolation phenomenon appeared at a high organic solvent concentration. The percolation process and microstructural transition can be determined through the plots of the electrical conductivity versus oil weight content. Figure 4 shows the variations of electrical conductivity K as a function of the benzene weight content for the bmimBF4-inbenzene microemulsion (along the line c in Figure 1) at different added water content. It is obvious that all the investigated microemulsions present the electrical percolation phenomenon. In the absence of water, the electrical conductivity originally increases with increasing the benzene content, which is due to the successive increase of conductive O/IL microemulsion droplets. At this stage, the O/IL type microemulsion forms. The next nonlinear decrease shows that the medium underwent a structural transition and a bicontinuous microstructure appears owing to the progressive growth and interconnection of the O/IL microemulsion droplets. The following linear decrease of K lies

Role of Solubilized Water

Figure 4. Variations of the electrical conductivity K as a function of the benzene weight content for the bmimBF4-in-benzene microemulsion (along line c, i.e., R ) 0.4, in Figure 1) at the different water contents (relative to the original mixture of TX-100 and bmimBF4).

with the consequence of the formation of IL/O microemulsion, which results from the partial fusion of clustered inverse microdroplets. The final nonlinear decrease of K, with further increase of benzene content, corresponds to the appearance of the percolation phenomenon. The percolation phenomenon is related with an increase of attractive interactions between droplets, because these interactions produce an increase of the lifetime of two or more associated droplets which facilitate the migration of the electric ions along connected paths through the microemulsions.26 The process of microstructural transition is reversible to that with polar solvents as titration phase. In the presence of water, the electrical conductivities of the microemulsion increase greatly at the low benzene content, because, in this case, the IL bmimBF4 behaves as continuous phase and the addition of water makes the outer phase into the low-viscous electrolyte solution, which leads to a great increase of electrical conductivity. Also, the percolation threshold Φ(oil) decreases with the increase of water, i.e., increases the Φ(IL) accordingly, and as a consequence, decreases the number of attractive interactions. Therefore, the addition of water contributes to the stability of the IL microemulsion, which accords with the above conclusion that more IL can be solubilized into the IL microemulsion in the present of small amount of water. 3. FTIR Spectra. Over the last several years, efforts have been made to understand the details of the water molecules solubilized in the heterogeneous media.27,28 For the currently investigated microemulsion system, it is also important to study the states and properties of solubilized water in microemulsion media and further understand how the added water molecules improve the stability of the IL microemulsion. The characteristics of the water molecules incorporated to reverse micelles depend strongly on water content and the nature of surfactant headgroups.29 It has been reported, that when a small amount of water is added to the TX-100 reverse micelles, most of the water molecules are bound to the OE groups, but at higher water concentrations, more and more water molecules are present in its free form in water pools.30 In our recent work, the FTIR spectra have revealed that water molecules are inclined to be first trapped and bound into the polar outer shells of TX-100 supported microemulsion. Herein, the states of water added to the bmimBF4-in-benzene IL microemulsion have been investigated by FTIR spectra. The split of trapped water, bound water, and free water has been resolved by least-squares curve fitting. The investigated microemulsion composition was along the line a with R ) 1.33 and a typical O-H stretching spectra of bmimBF4-in-benzene microemulsion at the water content of 6.5 wt % (relative to the total microemulsion weight) was shown in Figure 5. For the traditional aqueous microemulsions, the states of solubilized water have been intensively investigated.31-35 In general, the solubilized water in microemulsions has three


Figure 5. A typical O-H stretching spectra of bmimBF4-in-benzene microemulsion along the line a, with the molar ratio of bmimBF4/TX100, R ) 1.33, at water content of 6.5 wt %.

Figure 6. Variations in the area fraction of different water species with water content in bmimBF4-in-benzene microemulsion (along line a with bmimBF4/TX-100 molar ratio R ) 1.33).

distinct states: trapped water, bound water, and free water. The trapped water, with O-H stretching vibration at about 3600 cm-1, is defined as the water species dispersing among long hydrocarbon chains of surfactant molecules.31 It exists as monomers (or dimers) and has no hydrogen bonding interaction with its surroundings. Besides, a small amount of water dissolving in nonpolar solvent is also considered as trapped water.32 As the trapped water molecules are matrix-isolated dimers or monomeric in nature, they absorb in the highfrequency region.31 The bound water molecules are hydrogen bonded with the polar headgroups of surfactants, which results in absorption in the low-frequency region of the IR spectrum. For the nonionic TX-100 formed W/O microemulsions, the O-H stretching vibration of the bound water appears at 3400 ( 20 cm-1.34 Apart from these two types of water species, the free water molecules, occupying the cores of surfactant aggregates, have strong hydrogen bonds among themselves, that is, have a similar bulk water properties, which shifts the O-H stretching band to lower frequency of 3220 ( 20 cm-1.34 Figure 6 plots the dependence of the area fraction for various water species as a function of water content in a TX-100stabilized IL/O microemulsion. As can be seen in Figure 6, the area fraction of free water is quite small compared with those of the bound water and trapped water and shows little increase with water content, which is different from the traditional W/O type microemulsions, where the free water would increase greatly with increasing water content. So, it can be deduced that these water molecules are mainly located in the palisade layer and either mechanically trapped within the microemulsion structure or thermodynamically bound to the OE groups via intermolecular hydrogen bonding. It also can be seen that at very low water content, water molecules mainly behave as the trapped water and with increasing water content, the trapped water fraction decreases and bound water fraction increases, respectively. The possible reason is that the equilibrium of the trapped water dispersing among the microemulsion system can be achieved in a moment. Further addition of water leads to


Gao et al.

CHART 1: Chemical Structure and Atom Numbering for bmimBF4 and Tx-100.

the saturation of such water species. The number of trapped water molecules is unchanged and thus cause a decrease in distribution fraction with increasing water content. It is well known that the nonionic surfactant TX-100 has a long hydrophilic tail and thus can bind a large amount of water molecules. So the bound water content always increases with water content. In addition, if water molecules hydrate bmimBF4, the bound water number would also increase with hydration between water and cation or anion of bmimBF4. FTIR spectra have revealed that when a small amount of water was added to the IL/O microemulsion, the water molecules are inclined to be first trapped and bound into the polar outer shells of the microemulsion. It seems that such trap or bound did not enhance the burden of solubilizing these water molecules into the dispersed polar domains or IL pools but helped to increase the stability of the IL/O microemulsion. In addition, it can be realized that this unique solubilization behavior provided an aqueous interface film and thus the IL/O microemulsion may be used as a medium to prepare porous or hollow nanomaterials by hydrolysis reactions. 4. NMR. NMR spectra give more detailed information about the interaction between molecules. They therefore can provide an insight into the solubilization information of added water molecules in the polar cores of the IL/O microemulsions. In our previous reports, 1H NMR spectra have been used to investigate the microstructure characteristics and the formation mechanism of the IL/O microemulsion.18 The experimental results revealed that the electron cloud density of the oxygen atoms of OE units electrostatically attracted the electropositive imidazolium rings and the electrostatic interaction was considered to be the driving force for solubilizing IL into the core of the TX-100 aggregates.18 In this part, the effect of water on the NMR microenvironment of the IL/O microemulsion was investigated and all protons of bmimBF4 and TX-100 in the microemulsion system were analyzed. The numbering of atomic positions was shown in Chart 1. Figures 7-11 show the 1H chemical shifts of imidazolium ring protons and TX-100 in the bmimBF4-in-benzene microemulsion along the line a with R )

Figure 7. Variatons of the 1H chemical shift of the IL bmimBF4 and surfactant TX-100 in the bmimBF4-in-benzene microemulsion (along line a, the molar ratio of bmimBF4/TX-100 R ) 1.33) with water content (relative to the total microemulsion weight). The 1H signals of H-2, H-3, H-6, H-7, H-8, H-a, H-b, and H-c are all multiple peaks; their average values were used in the figures.

Figure 8. Variations of the 1H chemical shift of the IL bmimBF4 and surfactant TX-100 in the bmimBF4-in-benzene microemulsion (along the line a, the molar ratio of bmimBF4/TX-100 R ) 1.33) with water content (relative to the total microemulsion weight). The 1H signals of H-2, H-3, H-6, H-7, H-8, H-a, H-b, and H-c are all multriple peaks; their average values were used in the figures.

Figure 9. Variatons of the 1H chemical shift of the IL bmimBF4 and surfactant TX-100 in the bmimBF4-in-benzene microemulsion (along line a, the molar ratio of bmimBF4/TX-100 R ) 1.33) with water content (relative to the total microemulsion weight). The 1H signals of H-2, H-3, H-6, H-7, H-8, H-a, H-b, and H-c are all multriple peaks; their average values were used in the figures.


1.33 at various water content. It is obvious that the proton signals of H-2, H-3, H-4, H-5, H-6, H-7, and H-8 on imidazolium ring shift downfield, while H-1 signal moves to a highfield position. Thus, it can be deduced that the addition of water has affected the initial structure of bmimBF4. The interior structures, including the interactions between cation and anion of ILs, have been intensively investigated by various techniques.36-45 The structural data of the ion pair show that the occurrence of hydrogen bonding between the fluorine atoms of the PF6- and the imidazolium cation.36 The hydrogen bonding has been considered to intensify the formation of ion pairs and may lead to the high viscosity and some of the other specific properties of ILs.37 NMR techniques were also used to confirm the formation of hydrogen bonding (C1H‚‚‚F) between

Role of Solubilized Water



Figure 12. A typical 19F NMR spectra of bmimBF4-in-benzene microemulsion along line a with the molar ratio of bmimBF4/TX-100 R ) 1.33; water weight percent content is 6.5 wt %.

the counterions, bmim+, and BF4-.42 It was proposed that the H-1 proton of imidazolium ring may act as a hydrogen-bond donors to F atoms of the counterion in ILs with a fluorinecontaining anion.42,43 However, a very recent study shows that, in pure bmimBF4, there are strong contacts from the BF4- to all of the imidazolium ring protons without selectivity. The anion “sees” all of the various protons more or less equally.44 Therefore, an individual cation is surrounded by more than one anion, which eliminates the hydrogen bonding as the primary source of the interaction between the anion and cation, as this would lead to selectivity in the HOESY contacts.44 So, the ionic interaction would be primarily responsible for the observed HOESY effects and the BF4- anion may straddles the imidazolium plane or floats around the cation but remains quite close.44 We are inclined to support the latter conclusion because 1H,19F-HOESY is a more preferable tool to characterize the interionic contacts and interactions than theoretical calculation and simulation. Furthermore, hydrogen bonding is not enough to explain the properties of ILs, such as higher viscosity, because there is not at all hydrogen bonding between cation and anion for many ILs, as it is well-known that only F, O, and N atoms can form hydrogen bondings with hydrogen proton. It should be high Coulombic forces (strong electrostatic attractions) that constrain the cation and anion of ILs and thus exert practically IL properties such as no vapor pressure above the liquid surface.46 However, the presence of water would destroy the structure of bmim+ and BF4- ion pair. It has been confirmed that methanol as a polar solvent can solvate the bmim+ and BF4and thus reduce anion-cation contacts.44 There has also been evidence of water-imidazolium cation interactions at very low water content.40,42,43 In addition, recent reports have also revealed the formation of hydrogen bonding between water and anions in many ILs, including bmimBF4.41 Most of the water molecules exist in symmetric 1:2 type H-bonded complexes, i.e., anion‚‚‚HOH‚‚‚anion. Intermolecular nuclear Overhauser enhancements (NOEs) have also suggested that water can act as hydrogen-bond donor toward the BF4- ion.42 Either solvation of ion-pair or hydrogen-bonding interaction with cation or anion suggested that the presence of water loosened the intimate contacts between imidazolium cation and BF4- anion. The decomposition of ion-pair remarkably decreases the electron cloud density of the whole imidazolium ring, which causes the chemical shifts of imidazolium protons magnetic resonance to locate at a lower field. The whole chemical shifts of all the imidazolium protons are in accordance with the 1H,19F-HOESY studies, in which it is shown that there are strong contacts from the BF4- to all of the imidazolium ring protons without

selectivity and the anion “sees” all of the various protons more or less equally.44 Thus, our result proves the conclusion that Coulombic forces dominate in the ILs and an individual cation is actually surrounded by more than one anion. By assumption that the hydrogen bonds between the BF4- and a certain imidazolium proton dominate, only one or few proton signals would be affected, rather than all imidazolium protons. However, the H-1 is much different from H-2 and H-3 because H-1 is bonded to a carbon that is located between two electronegative nitrogen atoms, and hence H-1 should be more acidic than H-2 and H-3.40 The appearance of water causes oxygen atoms of water to hydrogen bonding to H-1 proton,42 which would enhance the electron cloud density of H-1 proton and then makes the chemical shifts of the H-1 resonate at a relatively higher field.(see Figure 7) Although the imidazolium ring was deshielded by the decomposition of anion-cation ion pair due to the addition of water, the hydrogen bonding between H-1 and oxygen atoms of water seems to be regnant in these two opposite contributions. Furthermore, the addition of water also causes the downfield shifts of protons signal of OE units of TX-100, but no chemical shifts were observed for the other protons of TX-100 such as H-a, H-b, H-c, H-d, and H-e (see Figures 8, 10, and 11) The downfield shift experienced by the OE proton resonances may be induced by the added water molecules hydrogen bonding to the OE units,48,49 and thus the electron density of oxygen atoms of OE units is decreased. The electropositivity of the carbon atoms adjacent to the oxygen atoms is enhanced owing to the induction effect. As a consequence, the hydrogen atoms on the carbon atoms are deshielded and resonate in a downfield position. Besides, the benzene, water, and ether linkages all give rise to highly anisotropic shielding effects; the change of environment can lead to the chemical shift changes.50 Also, coordination at oxygen and so-called polar effect are expected to give rise to deshielding.50 In this part, the interaction between water and BF4- ion has been investigated by 19F NMR. Figure 12 shows a typical 19F NMR spectrium of the bmimBF4-in-benzene microemulsion at the water content of 6.5 wt %. It can be seen that there are two F signals in the spectra, which is due to the two isotopes of B element, 10B and 11B.44 The downfield shifts (Figure 13) of F signals indicated that the F atoms of BF4- were deshielded with the addition of water. The reason is similar with that of OE units: water molecules can hydrogen bond to the F atoms of BF4-, and thus reduce the electron cloud density. From the above analysis, it can be proposed that the addition of water separates the caiton-anion ion pair of bmimBF4 and at the same


Figure 13. Variations of the 19F chemical shift of BF4- in the bmimBF4-in-benzene microemulsion (along line a, the molar ratio of bmimBF4/TX-100 R ) 1.33) with water content.

Gao et al. resonate at a higher field.51 However, with increasing water content, rapid decrease in percentage of BF4--bound water due to the dilution deduces the electron cloud density of hydrogen atoms and therefore shifts the δ downfield. Moreover, in the palisade layer of TX-100, the water molecules either just mechanically trapped within the microemulsion structure, or thermodynamically bound to the OE units via intermolecules hydrogen bonding. It is also possible that some water molecules are intermolecularly hydrogen bonded with themselves.52 Both formation of hydrogen bonds among water molecules themselves and bound water can cause the chemical shift of water molecules downfield,40 which is in keeping with the result of FTIR, in which free water exhibits a lower stretching frequency, responding to a lower resonance position. The broad single peak for the solubilized water indicates the rapid exchange between water protons at various states.32 On the basis of the above analyses, a possible structure was described to illustrate arrangement of molecules inside and outside the microemulsion palisade layer (Figure 14). Conclusion

Figure 14. A possible structure of illustrating the arrangement of molecules inside and outside the microemulsion palisade layer.

time builds the hydrogen binding network of water-cation, water-anion, and water-TX-100. The formation of hydrogen binding network bridges the bmimBF4 and TX-100, which is stronger than the weak electrostatic interaction between imidazolium cation and electronegative oxygen atoms of OE units on TX-100, that has been regarded as the solubilization mechanism of bmimBF4 into the TX-100 aggregates.18 So, the stability of the IL/O microemulsion was enhanced and then more bmimBF4 was solubilized into the IL/O microemulsion in the presence of a small amount of water. In addition, it is also worth noticing that, on successively adding water to the investigated IL/O microemulsion, a new broad peak is observed and becomes more and more stronger. Also, the strong broad peak shifts downfield gradually with increasing the water content (not shown here). The new peak is actually ascribed to the added water. The reason for the downfield shift may be that, when water was originally added to the IL/O microemulsion, trapped water dominates among three state of water, as has been confirmed by FTIR spectra. Trapped water exists as monomers (or dimers) and has no hydrogen bonding interaction with its surroundings, which leads to the higher O-H stretching frequency corresponding to a strong bond strength and therefore a higher electron cloud density around the protons leading the magnetic resonance of water protons to the highfields.32 In addition, the hydration of water molecules to BF4- anion can enhance the electron cloud density of hydrogen atoms, which makes water proton magnetic

IL/O microemulsions consisting of bmimBF4, surfactant TX100, and benzene were prepared and the effect of water on the microstructure was studied. The results showed that the addition of small amount of water to the IL/O microemulsion increased the amount of solubilized bmimBF4. The conductivity measurements also revealed that the attractive interactions between IL microdroplets were decreased, reflecting that the IL/O microemulsion became more stable in the present of water. FTIR spectra were used to study the states of added water in order to explain the above phenomenon. The result showed that these added water molecules mainly behaved as bound water or trapped water, located in the palisade layers of the IL/O microemulsion. Such solubilization model of water did not enhance the burden of solubilizing the polar water molecules into the dispersed polar domains. Furthermore, 1H and 19F NMR spectra have suggested that the addition of water destroyed the original ion pair structure of pure IL and at the same time a hydrogen binding network of water-imidazolium cation, waterBF4- anion, and water-TX-100 was built. The hydrogen binding network is much stronger than the weak electrostatic interaction between imidazolium cation and electronegative oxygen atoms of OE units on TX-100 that has been regarded as the solubilization mechanism of bmimBF4 into the TX-100 aggregates.18 This makes the palisade layers of TX-100 more firm and accordingly enhances the stability of IL/O microemulsion. The special solubilization position of water molecules can provide an aqueous interface film for hydrolysis reactions and thus may be used as an ideal medium to prepare porous or hollow nanomaterials. Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (No. 50472069) and the National Basic Research Program (2007CB808004, Z2004B02). Prof. Jimao Lin of Shandong University is thanked for his help with 1H NMR analysis. References and Notes (1) Mehnert, C. P.; Cook, R. A.; Dispenziere, N. C.; Afeworki, M. J. Am. Chem. Soc. 2002, 124, 12932-12933. (2) Gao, Y. A.; Li, Z. H.; Du, J. M.; Han, B. X.; Li, G. Z.; Hou, W. G.; Shen, D.; Zheng, L. Q.; Zhang, G. Y. Chem.-Eur. J. 2005, 11, 58755880. (3) Seth, D.; Chakraborty, A.; Setua, P.; Sarkar, N. Langmuir 2006, 22, 7768-7775.

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Role of Solubilized Water in the Reverse Ionic ... - ACS Publications

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