ARTICLE pubs.acs.org/EF
Effects of Nucleators on the Thermodynamic Properties of Seasonal Energy Storage Materials Based on Ionic Liquids Liguang Bai, Xuemei Li, Jiqin Zhu,* and Biaohua Chen State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: Seasonal energy storage based on phase-change materials (PCMs), long-chain alkylimidazolium bromide ionic liquids [C16MIM]Br and [C16MMIM]Br, are investigated in this paper. The structures of ionic liquids are measured by infrared (IR) spectra. Thermodynamic properties of ionic liquids, such as melting/freezing temperature and endothermic/exothermic heat, are measured by differential scanning calorimetry (DSC). The active use of supercooling of ionic liquids is proposed, which means that the thermal energy is stored in a supercooled liquid state and released by nucleating agents when needed. Copper powder and graphite powder, used as nucleating agents, are added with different mass fractions (025 wt %), and their effects on the thermodynamic properties of two ionic liquids are investigated. The results show that the nucleating agents have little effect on the thermodynamic properties of ionic liquids. The degree of supercooling of [C16MIM]Br is maintained at about 20 K when copper powder or graphite powder is added. The proper melting point and stable supercooling state of [C16MIM]Br indicates that it is more suitable for seasonal energy storage.
1. INTRODUCTION The efficient use of solar thermal energy is largely affected by seasonal variation. For example, the demand of heat supply is at a maximum in winter when the availability of solar energy is minimal, and the supply of solar energy is abundant in summer when there is little demand for heating. Therefore, the seasonal storage of solar energy is proposed to compensate for the seasonal discrepancy between solar energy supply and heating demand.1,2 Among the most frequently used phase-change materials (PCMs) for solar heating systems are inorganic PCMs (CaCl2 3 6H2O, Na2SO2 3 10 H2O, etc.) and organic PCMs (polyethylene glycol, paraffins, amides, etc.).3 Inorganic PCMs are prone to obtain phase separation and corrosion. Organic PCMs are highly volatile and flammable. Therefore, new PCMs overcoming the defects of inorganic and organic PCMs are urgently needed. Ionic liquids are salts that are composed of organic cations and inorganic anions. Unfamiliar to traditional high-temperature molten salts, their melting points are usually below about 100 °C and sometimes as low as 96 °C.4 As green functional materials owning negligible vapor pressure, high thermal stability, low viscosity, large liquidus range, and favorable solvation behavior, ionic liquids have been extensively investigated in many areas, such as catalysis, synthesis, extraction, and separation for the last several years,512 yet the application of ionic liquids in the field of thermal energy storage is still in its infancy. At present, some basic investigations have been intensively proposed in the thermodynamic properties of ionic liquids for application as liquid thermal storage media,13 heat-transfer fluids,14 and dyesensitized solar cell.15 The important properties include high heat capacity, high density, high thermal conductivity, extremely low volatility, nonflammability, high thermal stability, wide temperature range for liquid, and ease to be tailored, allowing for ionic liquids to be excellent thermal storage media or heattransfer fluids in solar energy storage. r 2011 American Chemical Society
Figure 1. Typical IR spectra obtained for (a) [C16MIM]Br and (b) [C16MMIM]Br.
Recently, the applications of long-chain alkylimidazolium ionic liquids ([CnMIM]Br) in the area of analytics and surfactants have attracted much attention.16,17 In our previous work,18 thermal properties of long-chain alkylimidazolium ionic liquids have been studied. The experimental results indicate that 1-hexadecyl-3-methylimidazolium bromide ([C16MIM]Br) and 1hexadecyl-2,3-dimethylimidazolium bromide ([C16MMIM]Br) with high latent heat and proper melting point are suited for PCMs. Ionic liquids exhibit a large degree of supercooling, which is favorable to the seasonal phase-change heat storage. Schultz et al. have proposed another creative opinion to actively use the Received: December 25, 2010 Revised: March 4, 2011 Published: March 07, 2011 1811
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Figure 2. DSC curves of pure [C16MIM]Br at the heating and cooling rate of 2 K min1.
supercooling of PCMs for seasonal solar energy storage.19 In their opinion, supercooling makes it possible to let storage parts already melted cool to the surrounding temperature without solidification. Thus, that part of the storage will be heat-loss-free but will still hold the latent heat in the form of the heat of fusion. Sodium acetate trihydrate has been chosen for the PCM storage in their work because of the convenient melting temperature at 58 °C, which makes it suitable for both domestic hot water preparation and space heating, and the ability of stable supercooling. Experiments on the melting process of sodium acetate trihydrate were performed in a stainless-steel mantle tank with circulating water (80 °C), and additional water was added to increase the water content above 40% to increase the solubility. In the freezing process, solidification was activated by an electromagnetically controlled piston injecting a salt crystal or clicking a metal disk. The active use of supercooling showed a high potential for achievement of 100% solar fraction in a lowenergy house located in a Danish climate. Until now, a lot of studies, which concerned the mechanism of melting and freezing behaviors of ionic liquids, have been performed by researchers.2022 However, studies related to seasonal heat storage using supercooling of ionic liquids have not been found in the literature. In addition, it is necessary to consider the effect of nucleating agents on the solidification process. Therefore, two ionic liquids, [C16MIM]Br and [C16MMIM]Br, are chosen as seasonal energy storage materials, and the effects of nucleating agents, copper powder and graphite powder, on thermodynamic properties of ionic liquids are investigated and reported in this paper.
2. EXPERIMENTAL SECTION 2.1. Ionic Liquid Preparation. 1-Methylimidazole (g99 wt %) was purchased from the Linhai Kaile Chemical Factory. 1,2-Dimethylimidazole (g98 wt %) was purchased from the Changzhou Zhongkai Chemical Co., Ltd.. 1-Bromohexadecane (g98 wt %) and copper powder (99.7 wt %, 200 mesh) were purchased from the Sinopharm Chemical Reagent Co., Ltd. Toluene (g99 wt %) and ethyl acetate (g99 wt %) were purchased from the Beijing Chemical Plant. Graphite powder (g98 wt %, 200 mesh) was purchased from the Tianjin Fuchen Chemical Reagent Factory. [C16MIM]Br and [C16MMIM]Br were synthesized as reported previously.23 Doubled 1-bromohexadecane and 1-methylimidazole or 1,2-dimethylimidazole were mixed, followed by gentle reflux at about
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Figure 3. DSC curves of pure [C16MMIM]Br at the heating and cooling rate of 2 K min1.
Figure 4. Melting curves of ionic liquid [C16MIM]Br with different mass fractions of copper powder: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 15 wt %, (e) 20 wt %, and (f) 25 wt %. 343.15 K for 48 h. After washing by toluene, recrystallization from ethyl acetate solution, and vacuum distillation for 24 h at 373 K, the pure products were obtained. [C16MIM]Br and [C16MMIM]Br were white crystalline powders at ambient temperature. The water contents in the ionic liquids were determined by the Karl Fischer method. The water contents in the two samples were less than 500 ppm. Then, copper powder or graphite powder, as nucleating additives, were added to the ionic liquids, with mass fraction varied from 0 to 25 wt %. Finally, the samples were obtained by thoroughly mixing in an agate ball mill for 5 min. 2.2. Ionic Liquid Characterization. Fourier transform infrared (FTIR) spectra of the samples were obtained using a Bruker Tensor-27 FTIR spectrophotometer at a scanning number of 30 with the KBr sampling method. Measurements of melting and freezing behaviors of [C16MIM]Br and [C16MMIM]Br were performed with Pyris I differential scanning calorimetry (DSC), provided by PerkinElmer. The samples inside the DSC furnace were exposed to a N2 atmosphere. The sample was typically 67 mg and enclosed in an aluminum pan. The standard heating or cooling rate of the present DSC measurement was set at 2 K min1. The measuring temperature ranged from 298.15 to 373.15 K.
3. RESULTS AND DISCUSSION 3.1. IR Spectra. The infrared (IR) spectra of the ionic liquids [C16MIM]Br and [C16MMIM]Br are presented in the range 1812
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from 500 to 4000 cm1 (Figure 1). The peaks from 2800 to 3000 cm1 originate from the cetyl chain attached to the imidazolium ring, which are attributed to the stretch mode and Fermi resonance mode of CH2 and CH3.2426 Features between 3000 and 3200 cm1 are from the CH vibration modes of the imidazolium ring.27 The peaks that appear around 3500 cm1 are assigned to the hydrogen bond C2HBr and C2methylBr for [C16MIM]Br and [C16MMIM]Br, respectively. 3.2. Thermodynamic Properties of Pure Ionic Liquids. About 6 mg of [C16MIM]Br was placed in a DSC aluminum pan. The sample was heated from 298.15 to 343.15 K at a rate of 2 K min1 and then cooled back to 298.15 K at the same rate. As shown in Figure 2, an endothermic peak appears at about 336.46 K in the process of heating, which corresponds to the melting of [C16MIM]Br. Upon the following cooling process, an exothermic peak also appears at about 316.25 K, which attributes to the freezing of ionic liquid. When the two peaks are compard, it is evident that the freezing peak is sharper than the melting peak. For the broad melting peak, the top of the peak is considered as the melting point.21 In addition, the freezing peak appears at about 20 K lower than the melting peak. It is considered to be supercooling, which has been reported by many researchers.2831 The same heating and cooling operations have been performed to [C16MMIM]Br with the temperature range from 330.15 to 380.15 K (Figure 3). The supercooling phenomenon also has been found for [C16MMIM]Br. However, both the melting and freezing temperatures of [C16MMIM]Br are higher than those of [C16MIM]Br, which should be ascribed to the introduction of a methyl group on the 2 position of the imidazolium ring that elevates the melting point of the corresponding ionic liquids.3236 In the melting process of
Figure 5. Freezing curves of ionic liquid [C16MIM]Br with different mass fractions of copper powder: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 15 wt %, (e) 20 wt %, and (f) 25 wt %.
Table 1. Thermodynamic Properties of [C16MIM]Br Ionic Liquid with Different Mass Fractions of Copper Powder mass fraction of copper powder (wt %)
Tm (K)
Tf (K)
ΔT (K)
ΔHm
ΔHf
(J g1)
(J g1)
0
336.46
316.25
20.20
144.37
87.32
5
336.37
315.64
20.73
135.48
79.88
10
336.14
315.23
20.92
128.33
75.73
15
336.11
315.79
20.32
118.83
69.78
20
336.07
314.65
21.42
117.42
65.58
25
336.13
314.92
21.21
110.40
61.17
Figure 6. Melting curves of ionic liquid [C16MIM]Br with different mass fractions of graphite powder: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 15 wt %, (e) 20 wt %, and (f) 25 wt %.
Figure 7. Freezing curves of ionic liquid [C16MIM]Br with different mass fractions of graphite powder: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 15 wt %, (e) 20 wt %, and (f) 25 wt %.
Table 2. Thermodynamic Properties of [C16MIM]Br Ionic Liquid with Different Mass Fractions of Graphite Powder mass fraction of graphite powder (wt %)
Tm (K)
Tf (K)
ΔT (K)
ΔHm (J g1)
ΔHf (J g1)
ΔHγ (J g1)
0
336.46
316.25
20.20
144.37
87.32
5
336.40
316.08
20.33
137.51
92.11
1.094
10 15
336.09 336.00
316.07 316.09
20.02 19.91
130.35 120.23
79.28 72.54
3.34 3.98
0
20
336.10
316.47
19.64
115.82
70.65
4.55
25
336.03
316.95
19.08
104.62
65.11
21.388
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[C16MMIM]Br, two peaks rather than one are found. The front peak can be named as R, and the back peak can be named as β (see Figure 3). In fact, peak R denotes that a glass transition happens during the melting process and is affected by the heating rate. 3.3. [C16MIM]Br þ Copper Powder. The melting processes of [C16MIM]Br with various mass fractions of copper powder are represented in Figure 4. As shown in Figure 4, the area of the melting peak decreased with the increase of the copper powder content; however, the melting peak positions of six samples appear in the same temperature, and their shapes are also similar.
Therefore, it can be concluded that copper powder has no effect on the melting temperature of [C16MIM]Br. Figure 5 shows the exothermic curves of the mixture in the vicinity of the freezing point. It can be seen that the freezing point decreases from 316.25 to 314.92 K as the addition of copper increases from 0 to 25 wt %. The freezing point decreased only 1.33 K, so that the effect of copper powder can be neglected. When the fraction of copper powder is in the range of 515 wt %, the exothermic peak is not regular. This phenomenon may be attributed to the uneven distribution of the nucleating additive.37 It is known that the copper powder has a better thermal conductivity than organic materials. In the process of cooling, copper powder may be sedimentary at the bottom of the bulk liquid phase, and thereby the uneven heat release leads to a slight shift in the exothermic peak. In Table 1, the specific thermodynamic properties of [C16MIM]Br with added copper powder, including melting temperature (Tm), freezing temperature (Tf), supercooling degree (ΔT), endothermic heat (ΔHm), and exothermic heat (ΔHf), are listed. With the increase of the mass fraction of the copper powder (WCu), both endothermic and exothermic heat decrease proportionally, while the degree of supercooling expands a little rather than shrinks. As a result, we conclude that the copper powder has little influence on the heat storage properties of [C16MIM]Br. 3.4. [C16MIM]Br þ Graphite Powder. Graphite powder as another nucleating additive is added to [C16MIM]Br. Figure 6 shows the heating curves of [C16MIM]Br adding various mass fractions of graphite powder (Wgr) at a heating rate of 2 K min1 from 298.15 to 343.15 K. The experimental data and calculated
Figure 8. Melting curves of ionic liquid [C16MMIM]Br with different mass fractions of copper powder: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 15 wt %, (e) 20 wt %, and (f) 25 wt %.
Figure 10. Melting curves of ionic liquid [C16MMIM]Br with different mass fractions of graphite powder: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 15 wt %, (e) 20 wt %, and (f) 25 wt %.
Figure 9. Freezing curves of ionic liquid [C16MMIM]Br with different mass fractions of copper powder: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 15 wt %, (e) 20 wt %, and (f) 25 wt %.
Table 3. Thermodynamic Properties of [C16MMIM]Br Ionic Liquid with Different Mass Fractions of Copper Powder mass fraction of copper powder (wt %) Tm(R) (K) Tm(β) (K) Tf (K) ΔT(R) (K) ΔT(β) (K) ΔHm(R) (J g1) ΔHm(β) (J g1) ΔHm(Rþβ) (J g1) ΔHf (J g1) 0
366.48
370.68
343.22
23.26
27.46
2.56
121.12
123.68
117.90
5
366.66
371.03
344.72
21.94
26.31
2.62
113.92
116.54
110.06
10
366.49
370.97
346.48
20.01
24.49
1.82
108.30
110.11
103.11
15
366.65
371.07
346.50
20.14
24.56
1.17
106.02
107.19
100.64
20
366.67
370.71
345.09
21.58
25.62
2.40
98.44
100.84
93.65
25
366.50
370.98
344.19
22.31
26.80
1.59
89.24
90.83
71.04
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Table 4. Thermodynamic Properties of [C16MMIM]Br Ionic Liquid with Different Mass Fractions of Graphite Powder mass fraction of graphite powder (wt %) Tm(R) (K) Tm(β) (K) Tf (K) ΔT(R) (K) ΔT(β) (K) ΔHm(R) (J g1) ΔHm(β) (J g1) ΔHm(R þ β) (J g1) ΔHf (J g1) 0
366.48
370.68
343.22
23.26
27.46
2.56
121.12
123.68
117.90
5
366.50
371.09
351.18
15.33
19.91
1.70
116.91
118.61
112.48
10
366.52
371.05
354.79
11.73
16.26
1.57
111.97
113.53
108.02
15
366.61
371.04
353.63
12.99
17.41
1.37
104.73
106.10
101.07
20
366.64
370.96
356.22
10.41
14.74
1.03
101.62
102.64
99.65
25
366.58
370.98
349.25
17.33
21.73
0.66
92.96
93.62
89.42
Figure 11. Freezing curves of ionic liquid [C16MMIM]Br with different mass fractions of graphite powder: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 15 wt %, (e) 20 wt %, and (f) 25 wt %.
results are listed in Table 2. The melting temperature of [C16MIM]Br has almost no change with the increase of the mass fraction of graphite powder, while endothermic heat decreases gradually. For the cooling curves shown in Figure 7, a new broad peak (named γ) appears at 319.58 K before the freezing point. With the addition of graphite powder from 5 to 25 wt %, the area of γ (ΔHγ) peak increases from 1.09 to 21.39 J/g. In addition, the position of the melting peak shifts from 316.25 to 316.95 K. However, the variation of the freezing process is inconspicuous compared to that of the pure ionic liquids. 3.5. [C16MMIM]Br þ Copper Powder. The effects of copper powder on the thermodynamic properties of [C16MMIM]Br are also investigated. Figure 8 shows the melting process of [C16MMIM]Br with different mass fractions of copper powder. In the heating process, no variation is found, except that the total endothermic heat decreases. In the cooling process shown in Figure 9, the freezing point changes slightly. At first, the freezing point of [C16MMIM]Br increases with the mass fraction of copper (WCu). When WCu reaches 15 wt %, the freezing point of [C16MMIM]Br is 346.35 K, an improvement of about 4 K. It is not true that, the more copper powder added, the higher the freezing point can be improved. Subsequently, the freezing point begins to decrease with the further addition of copper powder (see Table 3). When the percentage of copper powder is above 15 wt %, the distribution of nucleating agents in ionic liquids may be non-uniform, which is unfavorable to heat transfer. The temperature differences between Tm(R) and Tf and ΔT(β) and Tf are represented by ΔT(R) and ΔT(β), respectively.
3.6. [C16MMIM]Br þ Graphite Powder. As shown in Figure 10, the addition of graphite powder affects the melting process of [C16MMIM]Br obviously, except the position of peaks. The results are given in Table 4. For the R peak, the area reduces gradually, from 2.56 J/g at 0 wt % to 0.66 J/g at 25 wt %. The area proportion of the β peak in the endothermic process increases correspondingly. It means that the glass transition can be eliminated partially when using graphite powder as a nucleating additive. On the other hand, the freezing points shift to a higher temperature. When the mass fraction of graphite powder (Wgr) is 20 wt %, Tf of [C16MMIM]Br is 356.22 K and is about 13 K enhanced, as compared to that of pure [C16MMIM]Br. It can be seen from Figure 11 that the freezing point varies irregularly as the addition of graphite powder increases. This can also be ascribed to the fact that the random distribution of nucleating agents leads to uneven heat transfer.
4. CONCLUSIONS In this paper, two ionic liquids, [C16MIM]Br and [C16MMIM]Br, are synthesized, which are expected to be potential seasonal energy storage materials. Their structures are determined by IR spectra. In addition, the thermodynamic properties, such as melting temperature, freezing temperature, heat of phase transition, and degree of supercooling, are measured by DSC. The effects of copper powder and graphite powder as nucleating additives in ionic liquids are also investigated. The results show that the heat store or release of the phase transition decreases with the increase of the mass fraction of nucleating additives and the melting temperature as well as the freezing temperature changes slightly. The degree of supercooling of [C16MIM]Br remains at about 20 K, regardless of the addition of copper powder or graphite powder. Comparing the thermodynamic properties of the two ionic liquids, specifically the proper melting point and stable supercooling state, indicates that [C16MIM]Br is more suitable for seasonal solar energy storage. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The financial support by the National Scientific Fund of China (20706005 and 21046009) and the International Science and Technology Cooperation Program of China (2010DFA62530) is gratefully acknowledged. 1815
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’ REFERENCES (1) Chung, M.; Park, J.; Yoon, H. Sol. Energy 1998, 64, 163–178. (2) Ucar, A.; Inalli, M. Renewable Energy 2005, 30, 1005–1019. (3) Kenisarin, M.; Mahkamov, K. Renewable Sustainable Energy Rev. 2007, 11, 1913–1965. (4) Holbery, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 13, 2133–2139. (5) Lin, X.; Xu, Y. Electrochim. Acta 2008, 53, 4990–4997. (6) Poole, C. F.; Poole, S. K. J. Chromatogr., A 2010, 1217, 2268–2286. . J. Chem. (7) Gonzalez, E. J.; Calvar, N.; Gomez, E.; Domínguez, A Eng. Data 2010, 55, 3422–3427. (8) Sun, X.; Peng, B.; Ji, Y.; Chen, J.; Li, D. Sep. Purif. Technol. 2008, 63, 61–68. (9) Pereiro, A. B.; Rodriguez, A. Sep. Purif. Technol. 2008, 62, 733–738. (10) Abai, M.; Holbrey, J. D.; Rogers, R. D.; Srinivasan, G. New J. Chem. 2010, 34, 1981–1993. (11) Painter, P.; Williams, P.; Lupinsky, A. Energy Fuels 2010, 24, 5081–5088. (12) Samadi, A.; Kemmerlin, R. K.; Husson, S. M. Energy Fuels 2010, 24, 5797–5804. (13) Wu, B. Q.; Reddy, R. G.; Rogers, R. D. Novel ionic liquid thermal storage for solar thermal electric power systems. Proceedings of the Solar Forum 2001 Solar Energy; The Power to Choose, Washington, D.C., 2001; pp 445451. (14) Valkenburg, M. E. V.; Vaughn, R. L.; Williams, M.; Wilkes, J. S. Thermochim. Acta 2005, 425, 181–188. (15) Fan, L.; Kang, S.; Wu, J.; Hao, S.; Lan, Z.; Lin, J. Energy Sources, Part A 2010, 32, 1559–1568. (16) Soukup-Hein, R. J.; Warnke, M. M.; Armstrong, D. W. Annu. Rev. Anal. Chem. 2009, 2, 145–168. (17) Blesic, M.; Marques, M. H.; Plechkova, N. V.; Seddon, K. R.; Rebelo, L. P. N.; Lopes, A. Green Chem. 2007, 9, 481–490. (18) Zhu, J.; Bai, L.; Chen, B.; Fei, W. Chem. Eng. J. 2009, 147, 58–62. (19) Schultz, J. M.; Furbo, S., Solar heating systems with heat of fusion storage with 100% solar fraction for solar low energy buildings. In Proceedings of the International Solar Energy Society (ISES) World Congress 2007; Goswami, D. Y., Zhao, Y., Eds.; Springer: New York: 2009; Vol. 15, pp 27212725. (20) Yamamuro, O.; Minamimoto, Y.; Inamura, Y.; Hayashi, S.; Hamaguchi, H. Chem. Phys. Lett. 2006, 423, 371–375. (21) Nishikawa, K.; Wang, S.; Katayanagi, H.; Hayashi, S.; Hamaguchi, H.-o.; Koga, Y.; Tozaki, K.-i. J. Phys. Chem. B 2007, 111, 4894–4900. (22) Ngo, H. L.; LeCompte, K.; Hargens, L.; McEwen, A. B. Thermochim. Acta 2000, 357358, 97–102. (23) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Phys. Chem. Chem. Phys. 2001, 3, 5192–5200. (24) Berg, R. W.; Deetlefs, M.; Seddon, K. R.; Shim, I.; Thompson, J. M. J. Phys. Chem. B 2005, 109, 19018–19025. (25) Jeon, Y.; Sung, J.; Kim, D.; Seo, C.; Cheong, H.; Ouchi, Y.; Ozawa, R.; Hamaguchi, H. J. Phys. Chem. B 2008, 112, 923–928. (26) Brubach, J. B.; Mermet, A.; Filabozzi, A.; Gerschel, A.; Lairez, D.; Krafft, M. P.; Roy, P. J. Phys. Chem. B 2001, 105, 430–435. (27) Talaty, E. R.; Raja, S.; Storhaug, V. J.; Dolle, A.; Carper, W. R. J. Phys. Chem. B 2004, 108, 13177–13184. (28) Chowdhury, P. K.; Halder, M.; Sanders, L.; Calhoun, T.; Anderson, J. L.; Armstrong, D. W.; Song, X.; Petrich, J. W. J. Phys. Chem. B 2004, 108, 10245–10255. (29) Marsh, K. N.; Boxall, J. A.; Lichtenthaler, R. Fluid Phase Equilib. 2004, 219, 93–98. (30) Sato, T.; Masuda, G.; Takagi, K. Electrochim. Acta 2004, 49, 3603–3611. (31) Kulkarni, P. S.; Branco, L. C.; Crespo, J. G.; Numes, M. C.; Raymundo, A.; Afonso, C. A. M. Chem.—Eur. J. 2007, 13, 8478–8488.
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(32) Bonh^ ote, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gr€atzel, M. Inorg. Chem. 1996, 35, 1168–1178. (33) Hagiwara, R.; Ito, Y. J. Fluorine Chem. 2000, 105, 221–227. (34) Mukai, T.; Yoshio, M.; Kato, T.; Ohno, H. Chem. Lett. 2004, 33, 1630–1631. (35) Endo, T.; Kato, T.; Nishikawa, K. J. Phys. Chem. B 2010, 114, 9201–9208. (36) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S. N. V. K.; Brennecke, J. F. J. Chem. Eng. Data 2004, 49, 954–964. (37) Lane, G. A. Sol. Energy Mater. Sol. Cells 1991, 27, 135–160.
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