Facile Preparation of Hybrid Fluids from Ionic Liquid-Inorganic

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 22, 2013 | http://pubs.acs.org Publication Date (Web): December 18, 2009 | doi: 10.1021/bk-2009-1030.ch015

Chapter 15

Facile Preparation of Hybrid Fluids from Ionic Liquid-Inorganic Nanoparticles: Focus on Surface Modification of the Nanoparticles Asako Narita,a,c Eisuke Miyoshi,a Kensuke Naka,b and Yoshiki Chujoa a

Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto, 615-8510, Japan b Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan c Advanced Software Technology & Mechatronics Research Institute of Kyoto (ASTEM)

Hybrid fluids formed from inorganic nanoparticles and ionic liquids have been prepared. Although nanoparticles covalently modified with 1-methylimidazolium chloride were uniformly dispersed in water containing 1-butyl-3-methylimidazolium chloride at first, they formed hydrophobic dispersions after potassium bis{(trifluoromethyl)sulfonyl}amide was added, due to anion exchange at the surface of the nanoparticles. The concentration method needs no evaporation, centrifuging, filtration, or dialysis techniques, even if the nanoparticles were dispersed at a low level in ionic liquids.

© 2009 American Chemical Society In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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Background Preparation of inorganic nanoparticles is one of the recent key technologies of nanoscience. In most cases, nanoparticles are prepared in liquid media, and frequently nanoparticles are used as dispersion in liquids. Thus, dispersion techniques are quite important for a nano-sized material chemistry. Usually, inorganic nanoparticles need surface stabilisers to keep their size and shape, because the crystal surfaces have high potential energy compared to the body of the crystal. Specifically, nanoparticles have a large surface area compared to their inner volume. For example, the surface area of a 1 cm cube is 6 cm2. In contrast, the total surface area of nanoparticles (10 nm diameter) with a total volume of 1 cm3 is approximately 600 m2. The large surface area of the nanoparticles is explains the importance of the use of surface stabilisers compared to bulk materials. The surface stabiliser also affects the dispersibility of nanoparticles in liquid media. For example, nanoparticles stabilised by lipophilic molecules show a high dispersibility in hydrophobic oils. In contrast, nanoparticles coated by molecules having hydrophilic moieties are easy to disperse in aqueous media. Ionic liquids are of interest as dispersion media for nanoparticles. If the dispersibility of nanoparticles in ionic liquids can be well controlled, the application could be expanded to more extreme environments, such as under reduced pressure, or at high temperature. The methodology for the concentration of inorganic nanoparticles into ionic liquids is an important subject, especially the relationship between dispersant and stabiliser on the surface of nanoparticles. Recently, methods for the direct synthesis of nanoparticles in ionic liquids have been developed (1). Such nanoparticles do not need any surface stabiliser. However, such methods are specific to each class of nanoparticles. On the other hand, focusing on the surface stabiliser has advantages, because the knowledge about surface properties can be generically applied to any components of the nanoparticles. Furthermore, classical synthetic methods for nanoparticles in aqueous or organic media can be applied. If there is simple method to disperse nanoparticles into ionic liquids, such hybrid fluids are promising materials for various fields.

Objective The objective of this study is the facile preparation of inorganic nanoparticles-ionic liquid hybrid fluids. The nanoparticles were prepared in aqueous solution; they were covalently modified with ionic liquid-derived organic salts as surface stabilisers. These nanoparticles are concentrated in a simple procedure without any centrifuging, filtration or evaporation, utilising the surface properties of the surface stabiliser.

In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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Experimental


Materials Dry 1-methylimidazole and 1-chlorobutane were purchased from Aldrich Chemical Co. Dry pyridine, tetrachloroauric(III) acid, sodium tetrahydroborate, sodium hydroxide and iron(III) chloride hexahydrate were purchased from Wako Pure Chemical Industries, Ltd. Oleic acid, (3chloropropyl)trimethoxysilane and octyltrimethoxysilane were purchased from Tokyo Kasei Co. Ltd. Iron(II) chloride tetrahydrate, potassium and lithium bis{(trifluoromethyl)sulfonyl}amide (K[NTf2]), bis{(trifluoromethyl)sulfonyl}amide (Li[NTf2]) were purchased from Kanto Chemical Co. Ltd. 1-Butyl-3-methylimidazolium chloride ([C4mim]Cl) was prepared by heating under reflux an equimolar mixture of 1-methylimidazole and 1-chlorobutane at 90 °C for three days (2). The mixture was washed with dry ethyl ethanoate three times, and dried under reduced pressure. 1-Butylpyridinium chloride ([C4py]Cl) was prepared similarly (2). Preparation of organic salts-modified iron oxide nanoparticles 1-Methylimidazolium-modified iron(II,III) oxide nanoparticles ([Rmim]ClFe3O4-NPs) were prepared by the method described in our earlier report (3). Initially, a silane coupling agent was prepared by mixing 1-methylimidazole and (3-chloropropyl)trimethoxysilane at 90 °C for three days. The resulting viscous liquid was washed with dry ethyl ethanoate three times, and then dried under reduced pressure. Iron(II,III) oxide (Fe3O4) nanoparticles were prepared by hydrolysis of mixed aqueous solutions of iron(II) and iron(III) chloride (respectively 2 mmol and 4 mmol in 120 cm3) using 15 cm3 of 28 % aqueous ammonium hydroxide solution with vigorous mechanical stirring. After dispersant water was replaced by ethanol, the iron(II,III) oxide nanoparticles and silane coupling agent were mechanically stirred in ethanol containing a small amount of water at 70 °C. After six hours, the nanoparticles were washed three times with ethanol by centrifuge (9000 rpm; 1 h), and then ethanol was removed by pipette. The dispersant was replaced by water, and the nanoparticles were well dispersed with an ultrasonic wave bath. Pyridinium chloride modified iron oxide nanoparticles ([Rpy]Cl-Fe3O4NPs) were prepared by similar procedures, replacing 1-methylimidazole with pyridine (vide supra). The structures of the silane coupling agents were confirmed by 1H NMR spectroscopy. 1H NMR (dmso) spectrum of coupling agent [Rmim]Cl, which is a yellow viscous liquid: δ/p.p.m.: d 9.29 (s, 1H), 7.81 (s, 1H), 7.75 (s, 1H), 4.15 (t, 2H), 3.88 (s 3H), 3.46 (s, 9H), 1.83 (m, 2H), 0.54 (t, 2H). Yield: 95%. 1H NMR spectrum (dmso) of coupling agent [Rpy]Cl, which is a brown viscous liquid: δ/p.p.m.: d 9.17 (d, 2H), 8.62 (t, 1H), 8.17 (t, 2H), 4.61 (t, 2H), 3.47 (m 9H), 1.96 (m, 2H), 0.59 (t, 2H). Yield: 98%.


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Preparation of alkyl chain-modified iron oxide nanoparticles Octyl chain-modified iron(II,III) oxide nanoparticles (C8-Fe3O4-NPs) were prepared by stirring of prepared iron(II,III) oxide nanoparticles and octyltrimethoxysilane in ethanol in the same procedures as [Rmim]Cl-Fe3O4NPs. Surface modification was confirmed by FT-IR spectroscopy, TGA weight loss, and the observation of totally insolubility in water. Oleic acid coated iron(II,III) oxide nanoparticles (OA-Fe3O4-NPs), modified via hydrogen bonding, were prepared according to earlier reports (4,5). Oleic acid (0.2 cm3) was added to mixed aqueous solutions of iron(II) and iron(III) chloride (respectively 2 mmol and 4 mmol in 120 cm3) with vigorous stirring. After addition of 28 % aqueous ammonium hydroxide solution (15 cm3), the mixture was stirred for 30 min at 80 °C. Oleic acid (0.2 cm3) was then added to the stirring mixture (four times at 5 min intervals). The resulting aqueous dispersion was used for experiments without washing. This modification by hydrogen bonding is weaker than that by covalent bonding using silane coupling agents, hence washing using a centrifuge tended to cause breakdown of the modified layer and high aggregation. Preparation of imidazolium cation-modified gold nanoparticles Preparation of 1-methylimidazolium chloride-modified gold nanoparticles ([Rmim]Cl-Au-NPs) was reported elsewhere (6,7). Gold nanoparticles were prepared by reduction of H[AuCl4] using sodium tetrahydroborate or sodium hydride in water in the presence of 3,3′-[disulfanylbis(hexane-1,6-diyl)]-bis(1methyl-imidazol-3-ium) dichloride. The dispersant was replaced with pure distilled water by dialysis several times. The presence of the imidazolium salt on the surface was obtained by 1H NMR spectroscopy. Preparation of nanoparticle-ionic liquid hybrid fluids Hybrid mixtures of nanoparticles and ionic liquid were prepared by the same procedure for both iron(II,III) oxide and gold nanoparticles. At first, [C4mim]Cl or [C4py]Cl were dissolved in an aqueous dispersion of surface modified nanoparticles in a screw-capped vial. An aqueous solution of K[NTf2] was added to the mixture, which was then shaken until phase separation occurred. Then, the upper aqueous phase was removed by pipette, and the remaining hydrophobic ionic liquid phase was washed three times with distilled water. Details about volumes are shown in each Figure. Measurements 1

H NMR spectra were obtained with a JOEL EX-400 spectrometer (400 MHz). Transmission electron microscopy (TEM) measurements were performed using a JOEL JEM-100SX instrument, operated at 100 kV electron


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beam accelerating voltage. One drop of the sample solution was deposited onto a copper grid and the excess of the droplet was dried under ambient conditions. The TEM images (shown in the following section) are of a methanol dispersion of [Rmim]Cl-Fe3O4-NPs and an aqueous dispersion of [Rmim]Cl-Au-NPs, respectively. FT-IR spectra were recorded on a Perkin Elmer 1600 infrared spectrophotometer using a KBr disc dispersed with the powdered sample. Dynamic light scattering (DLS) was measured with a FPAR-1000 (Otsuka Electronics Co., Ltd.) instrument. Thermogravimetric analysis (TGA) was carried out using an Exster-6000 system (Seiko Instruments Inc.).

Results & Discussion Characterisation of surface modified Fe3O4-NPs and Au-NPs The diameter of [Rmim]Cl-Fe3O4-NPs was found to average at ca. 8 nm (Figure 1, left). The diameter measured by DLS was slightly larger (around 11 nm) (3). The diameter measured by DLS is normally found to be larger than measured by TEM (3,5). The difference could be due to organic salts and an electric bilayer around the particles (5). Organic components were around 5 wt % according to the TGA curve, in both the cases of [Rmim]Cl-Fe3O4-NPs and [Rpy]Cl-Fe3O4-NPs. The number of attached 1-methylimidazolium chloride units was calculated as around 440 based on the nitrogen:carbon ratio measured by elemental analysis. The average diameter of the gold nanoparticles was found to be 4.8 nm in a TEM image (Figure 1, right). The diameter measured by DLS was 5.4 nm. Facile preparation of ionic liquid based magnetic fluids Preparation of ionic liquid based hydrophobic magnetic iron(II,III) oxide fluids is demonstrated in Figure 2. As reported previously, anion exchange occurred at the surface of the nanoparticles (3,5,6). The anion exchange a

50 nm Figure 1. TEM images of [Rmim]Cl-Fe3O4-NPs (left) and [Rmim]Cl-Au-NPs (right).



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Figure 2. Preparation of ionic liquid based hydrophobic magnetic fluids and the importance of imidazolium salt layers on the surface. easily turned the hydrophilic iron(II,III) oxide nanoparticles hydrophobic (3,5), and caused concentration of iron(II,III) oxide nanoparticles in the hydrophobic ionic liquid phase. In our previous report about [Rmim]Cl-Au-NPs, phase transfer of the [Rmim]Cl-Au-NPs from the aqueous phase to the hydrophobic [C4mim][PF6] phase was observed when hexafluorophosphoric acid was added to an aqueous phase containing [Rmim]Cl-Au-NPs (6). The phase transfer indicated that this is an easy method to concentrate nanoparticles modified with organic salts having the same structure as the ionic liquid dispersant. Similar concentration to the hydrophobic ionic liquid [C4mim][NTf2] was reported using ammonium salt modified quantum dots (8). Also, in this system, the anion on the surface was exchanged, and the change of surface properties improved the miscibility with the [C4mim][NTf2] as dispersant. However, removal of generated small ion pairs and using the same structure for both surface stabiliser and dispersant should be important for making high purity systems. As a reference experiment, OA-Fe3O4-NPs was also used with the same procedure. OA-Fe3O4-NPs did not concentrate to the [C4mim][NTf2] phase, mainly remaining in the aqueous phase. The result indicates that concentration of the [Rmim]Cl-Fe3O4-NPs was due to anion exchange of the surface modified organic salt moiety. Figure 3 shows the comparison of the miscibility of [Rmim]Cl-Fe3O4-NPs and C8-Fe3O4-NPs in aqueous and hydrophobic phases. Although both of these nanoparticles were hydrophobic, neither of them could be dispersed in a hexane phase. This result implies the key factor to dominate the dispersibility is not only hydrophobicity, but also good compatibility between the surface stabiliser and dispersant. Additionally, insufficient dispersibility of C8-Fe3O4-NPs in hexane may suggest that the low density and viscosity of hexane make it difficult to keep a high dispersibility of large volumes of iron oxide nanoparticles with a high density comparing to dispersant.



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Figure 3. Comparison of [Rmim]Cl-Fe3O4-NPs and C8-Fe3O4-NPs preparations of ionic liquid based hydrophobic magnetic fluids, illustrating the importance of imidazolium salt layers on the surface. Suitable combination of modified organic salts and ionic liquid dispersant The importance of our method to concentrate nanoparticles relies on the similarity of the molecular structure of the surface stabiliser and ionic liquid as dispersant. If the combination is a mismatch, what will occur? Figure 4 shows the results of concentration using mismatched structures between the modified organic salt and the dispersant. Interestingly, in a case that [Rpy]Cl-Fe3O4-NPs was concentrated in [C4mim][NTf2], good dispersibility was observed (Figure 4, upper). In contrast, when [Rmim]Cl-Fe3O4-NPs were concentrated in [C4py][NTf2], nanoparticles were not transferred to the hydrophobic phase, as shown in the lower pictures of Figure 4. Such experiments with “false” combinations were reported also in a case of gold nanoparticles (9). However, in our result, the difference of miscibility, even if the combination has the same surface and dispersant, merits further discussion focusing on the different amounts of each. The amount of surface modified organic salt is only around 5 wt% of the total components of the nanoparticles. The organic salt layer would be nearly monolayer, meaning quite a low volume compared to that of the dispersant. The miscibility of [C4mim][NTf2] and [C4py][NTf2] should depend on this ratio (10-12). In the upper part of Figure 4, the amount of pyridinium salt is approximately 0.2 mg (supposing 5 wt% of 4 mg is equal to the amount of organic salt), meaning a quite small quantity of [C4py][NTf2] was dissolved in [C4mim][NTf2]. In contrast, in the lower part of Figure 4, [C4py][NTf2] could not displace [C4mim][NTf2] even in low concentration. The fact suggests that dispersibility of ionic liquid-like organic salt-modified nanoparticles in ionic liquids should be strongly dominated by the miscibility of two-phase ionic liquids. In other words, knowledge about the miscibility of ionic liquids is useful to control the dispersibility of nanoparticles. We can choose ionic liquid as dispersants for nanoparticles without studying the dispersibility of the target nanoparticles.



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Figure 4. Mismatch of molecular structure between surface modified salts and ionic liquids as dispersant. Easy preparation of gold nanoparticle-ionic liquid hybrid fluids One advantage of our method is the easy control of the nanoparticle concentration. Figure 5 illustrates concentration control of the gold nanoparticles. If the concentration of the starting aqueous dispersion is known, the final concentration of ionic liquid based fluid can be controlled by changing the amount of [C4mim]Cl. Generally, although dilution of the dispersion is easy, concentration needs evaporation, centrifuging, filtration, or dialysis. However, the concentration method described here needs none of these procedures, saving a lot of time, manpower, energy and materials. Moreover, it is quite hard to concentrate ionic liquid based dispersions because many of ionic liquids have remarkably low vapour pressure. Indeed, gold nanoparticles synthesised by simply reduction in an aqueous solution are generally too small to concentrate by centrifuging. Actually, the gold nanoparticles in this study could not be concentrated by centrifuging, even at the maximum speed of our machine (20000 rpm). Our concentration method solves the difficulties of concentration, both in ionic liquids and with very small nanoparticles.



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Figure 5. Concentration of [Rmim]Cl-Au-NPs in to [C4mim][NTf2] in different concentrations, depending on the volume of [C4mim]Cl .

Conclusions In summary, we have reported a facile procedure to prepare inorganic fluids based on ionic liquids using inorganic nanoparticles covalently modified with ionic liquid-like organic salts on the surface. At first, nanoparticles modified with chloride salts of organic cations are dispersed in water containing the same salt. Then, after addition of a salt containing a hydrophobic anion such as bis{(trifluoromethyl)sulfonyl}amide or hexafluorophosphate, hydrophobic ionic liquid phases containing inorganic nanoparticles were formed. This is due to anion exchange between the dispersant and the surface of the nanoparticles. This method requires no evaporation, centrifuging, filtration, or dialysis to regulate the concentration, even if the nanoparticles are very small (being impossible to collect by centrifuging), and is based on ionic liquids with vanishingly small vapour pressures.

Acknowledgement This study is a part of a joint research programme, focussed on the development of basic technology for establishing the COE of nano-medicine, carried out through Kyoto City Collaboration of Regional Entities for Advancing Technology Excellence (CREATE), assigned by the Japan Science and Technology Agency (JST).


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Facile Preparation of Hybrid Fluids from Ionic Liquid-Inorganic

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