Comprehensive Refractive Index Property for Room-Temperature

Jul 13, 2012 - ABSTRACT: Refractive index, an optical property, and liquid density were ... to the refractive index of the room-temperature ionic liqu...
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Comprehensive Refractive Index Property for Room-Temperature Ionic Liquids Shiro Seki,*,† Seiji Tsuzuki,*,‡ Kikuko Hayamizu,‡ Yasuhiro Umebayashi,§,∥ Nobuyuki Serizawa,† Katsuhito Takei,† and Hajime Miyashiro† †

Materials Science Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-11-1, Iwado-kita, Komae, Tokyo 201-8511, Japan ‡ Research Initiative of Computational Sciences (RICS), Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan § Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan ABSTRACT: Refractive index, an optical property, and liquid density were measured for 17 types of room-temperature ionic liquids at various temperatures. Ab initio calculations were performed to yield theoretical polarizability of the respective ionic liquid composing ions. A highly linear correlation was found between the experimental refractive index and the predicted polarizability normalized in terms of the molar volume. This suggests that the electronic polarization of the ionic liquid composing ions predominantly contributes to the refractive index of the room-temperature ionic liquids.

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INTRODUCTION Room-temperature ionic liquids (RTILs), which are composed of many types of cation and anion, show a wide range of physicochemical properties because of the particular types of interaction between certain cation and anion species. RTILs have many important physicochemical properties such as low flammability, low volatility, low viscosity, and high electrochemical stability. In recent years, many scientists and engineers have conducted the research and development of RTILs for many basic and applied fields. For example, RTILs have been applied in various electrochemical devices (e.g., lithium secondary batteries, fuel cells, electric double-layer capacitors, dye-sensitized solar cells, and field-effect transistors, etc.),1 tribology,2 synthetic production,3 and as biomass solvent.4 Their physicochemical properties depend on the combination of their cations and anions. RTILs are so-called “designer solvents” owing to their semi-infinite chemical structures and combinations of cations and anions. However, their physicochemical uniformity has not yet been reported. We have reported a unique phase transition (phosphonium-cation-based RTIL)5 and the regularity of the ratio between their molecular volumes (i.e., frame volume and van der Waals volume).6 In this study, we measured the refractive index and density of 17 types of RTILs, which were composed of six cations and ten anions, and studied the cation and anion dependences on the physical properties of these RTILs. We investigated the optical properties of the RTILs by experimental and computational (ab initio) methods. The extractions of universal properties of physicochemical parameters (universalization) are very important © 2012 American Chemical Society

for the future molecular design of RTILs because of the semiinfinite cation and anion combinations possible for RTILs.

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EXPERIMENTAL SECTION Samples. Six bis(fluorosulfonyl)-amide-anion-based RTILs were used to investigate the cationic species of RTILs. The cation species were 1-ethyl-3-methylimidazolium (1-Et-3-Me-Im), N-methyl-N-propylpyrrolidinium (N-Me-Pr-Pyrr), N-ethylpyridinium (Et-Py), N-butyl-pyridinium (Bu-Py), N-hexylpyridinium (He-Py), 1-ethyl-3-methyl-pyridinium (1-Et-3-MePy), and 1-butyl-3-methylimidazolium (1-Bu-3-Me-Im), respectively. On the other hand, to investigate the anionic species of RTILs, 10 EMIm-cation-based RTILs were used. The anion species were bis(trifluoromethanesulfonyl)amide (N(SO2CF3)2), bis(fluorosulfonyl)amide (N(SO2F)2), bis(nonafluorobutanesulfonyl)amide (N(SO2C4F9)2), tetrafluoroborate (BF4), dicyanamide (N(CN) 2 ), tris(pentafluoroethyl)trifluorophosphate ((C2F5)3PF3, FAP), tetracyanoborate (B(CN)4, TCB), thiocyanate (SCN), hydrogen sulfate (SO3OH), methanesulfonate (SO3CH3), and ethyl sulfate (SO3OC2H5), respectively. The measurement matrix for the cations and anions of all of the RTILs used in this study is summarized in Table 1. All of the samples were dried in a vacuum chamber at 323 K for more than Received: January 25, 2012 Accepted: June 20, 2012 Published: July 13, 2012 2211

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Table 1. Measurement Matrix of Refractive Indexes for Room-Temperature Ionic Liquids Used in This Study abbreviation

Mw

A B C D E

1-ethyl-3-methylimidazolium 1-ethyl-3-methylimidazolium 1-ethyl-3-methylimidazolium 1-ethyl-3-methylimidazolium 1-ethyl-3-methylimidazolium

1-Et-3-Me-Im 1-Et-3-Me-Im 1-Et-3-Me-Im 1-Et-3-Me-Im 1-Et-3-Me-Im

111.17 111.17 111.17 111.17 111.17

abbreviation

Mw

N(SO2CF3)2 N(SO2F)2 BF4 N(CN)2 FAP

280.15 180.13 86.80 66.04 445.01

Dai-ichi Kogyo Seiyaku Dai-ichi Kogyo Seiyaku Aldrich Aldrich Merck

F G H I J K L M

1-ethyl-3-methylimidazolium 1-ethyl-3-methylimidazolium 1-ethyl-3-methylimidazolium 1-ethyl-3-methylimidazolium 1-ethyl-3-methylimidazolium N-methyl-N-propylpyrrolidinium N-methyl-N-propylpyrrolidinium N-ethyl-pyridinium

1-Et-3-Me-Im 1-Et-3-Me-Im 1-Et-3-Me-Im 1-Et-3-Me-Im 1-Et-3-Me-Im N-Me-Pr-Pyrr N-Me-Pr-Pyrr Et-Py

111.17 111.17 111.17 111.17 111.17 128.24 128.24 108.16

TCB SCN SO3OH SO3CH3 SO3OC2H5 N(SO2CF3)2 N(SO2F)2 N(SO2F)2

114.88 58.08 97.07 95.10 125.12 280.15 180.13 180.13

136.21 bis(fluorosulfonyl)amide

N(SO2F)2

180.13

164.27 bis(fluorosulfonyl)amide

N(SO2F)2

180.13

1-ethyl-3-methyl-pyridinium

1-Et-3-Me-Py 122.19 bis(fluorosulfonyl)amide

N(SO2F)2

180.13

1-butyl-3-methylimidazolium

1-Bu-3-Me-Im 139.22 bis(nonafluorobutanesulfonyl)amide

N(SO2C4F9)2 580.19

Merck Merck Tokyo Chemical Industry Tokyo Chemical Industry Tokyo Chemical Industry Dai-ichi Kogyo Seiyaku Dai-ichi Kogyo Seiyaku Mitsubishi Materials Electronic Chemicals Mitsubishi Materials Electronic Chemicals Mitsubishi Materials Electronic Chemicals Mitsubishi Materials Electronic Chemicals Mitsubishi Materials Electronic Chemicals

N

N-butyl-pyridinium

Bu-Py

O

N-hexyl-pyridinium

He-Py

P Q

code

cation

anion bis(trifluoromethanesulfonyl)amide bis(fluorosulfonyl)amide tetrafluoroborate dicyanamide tris(pentafluoroethyl) trifluorophosphate tetracyanoborate thiocyanate hydrogen sulfate methanesulfonate ethyl sulfate bis(trifluoromethanesulfonyl)amide bis(fluorosulfonyl)amide bis(fluorosulfonyl)amide

supplier

Table 2. Experimental Refractive Index Data of 17 Kinds of Room-Temperature Ionic Liquids temp./RTILs 283.15 K 288.15 K 293.15 K 298.15 K 303.15 K 308.15 K 313.15 K 318.15 K 323.15 K 328.15 K 333.15 K 338.15 K 343.15 K 348.15 K 353.15 K temp./ RTILs 283.15 K 288.15 K 293.15 K 298.15 K 303.15 K 308.15 K 313.15 K 318.15 K 323.15 K 328.15 K 333.15 K 338.15 K 343.15 K 348.15 K 353.15 K temp./RTILs 283.15 K 288.15 K

1-Et-3-Me-Im-N(SO2CF3)2 1.42738 1.42594 1.42451 1.42309 1.42166 1.42022 1.41881 1.41740 1.41597 1.41458 1.41319 1.41177 1.41037 1.40902 1.40760 1-Et-3-Me-ImSCN

1-Et-3-Me-Im-BF4

1.45179 1.45035 1.44891 1.44747 1.44605 1.44462 1.44320 1.44179 1.44039 1.43899 1.43758 1.43619 1.43481 1.43342 1.43207

1.41546 1.41423 1.41300 1.41177 1.41055 1.40934 1.40810 1.40690 1.40570 1.40452 1.40327 1.40211 1.40093 1.39975 1.39858

1-Et-3-Me-ImSO3OH

1.55600 1.55450 1.55299 1.55149 1.54999 1.54850 1.54702 1.54557 1.54409 1.54263 1.54119 1.53973 1.53831 1.53687 1.53545 Et-Py-N(SO2F)2 1.47477 1.47331

1-Et-3-Me-Im-N(SO2F)2

1-Et-3-Me-ImSO3CH3

1.50042 1.49931 1.49820 1.49710 1.49601 1.49489 1.49379 1.49269 1.49161 1.49052 1.48944 1.48836 1.48727 1.48623 1.48512 Bu-Py-N(SO2F)2 1.47569 1.47420

1.51774 1.51605 1.51441 1.51278 1.51121 1.50963 1.50808 1.50655 1.50502 1.50351 1.50191 1.50030 1.49865 1.49708 1.49557 1-Et-3-Me-ImSO3OC2H5

1.49994 1.49855 1.49718 1.49582 1.49444 1.49308 1.49172 1.49039 1.48905 1.48768 1.48638 1.48498 1.48367 1.48230 1.48095 He-Py-N(SO2F)2 1.47548 1.47398 2212

1-Et-3-Me-Im-N(CN)2

1-Et-3-Me-Im-FAP

1.37305 1.37176 1.37043 1.36916 1.36783 1.36656 1.36525 1.36399 1.36267 1.36141 1.36011 1.35887 1.35756 1.35635 1.35509 N-Me-Pr-PyrrN(SO2CF3)2

1.48317 1.42474 1.48185 1.42336 1.48053 1.42201 1.47921 1.42064 1.47789 1.41925 1.47657 1.41792 1.47528 1.41656 1.47395 1.41519 1.47262 1.41389 1.47130 1.41252 1.46994 1.41119 1.46850 1.40986 1.46723 1.40853 1.46596 1.40718 1.46468 1.40589 1-Et-3-Me-Py-N(SO2F)2 1.47636 1.47489

1-Et-3-Me-Im-TCB 1.45284 1.45108 1.44936 1.44763 1.44592 1.44421 1.44252 1.44081 1.43913 1.43745 1.43578 1.43412 1.43248 1.43085 1.42922 N-Me-Pr-PyrrN(SO2F)2

1.44659 1.44522 1.44385 1.44250 1.44114 1.43977 1.43846 1.43710 1.43577 1.43446 1.43312 1.43182 1.43052 1.42919 1.42792 1-Bu-3-Me-Im-N(SO2C4F9)2 1.39249 1.39104

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Table 2. continued temp./RTILs 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

K K K K K K K K K K K K K

Et-Py-N(SO2F)2

Bu-Py-N(SO2F)2

He-Py-N(SO2F)2

1-Et-3-Me-Py-N(SO2F)2

1-Bu-3-Me-Im-N(SO2C4F9)2

1.47185 1.47038 1.46890 1.46745 1.46602 1.46458 1.46315 1.46174 1.46032 1.45895 1.45754 1.45610 1.45476

1.47271 1.47123 1.46975 1.46828 1.46683 1.46537 1.46390 1.46247 1.46103 1.45963 1.45823 1.45677 1.45536

1.47248 1.47098 1.46948 1.46798 1.46651 1.46502 1.46355 1.46210 1.46063 1.45920 1.45776 1.45628 1.45489

1.47341 1.47194 1.47048 1.46897 1.46754 1.46611 1.46466 1.46322 1.46180 1.46036 1.45899 1.45757 1.45612

1.38955 1.38807 1.38662 1.38516 1.38367 1.38222 1.38078 1.37930 1.37783 1.37638 1.37496 1.37350 1.37206

Figure 1. Temperature dependence of refractive indexes for N(SO2F)2-anion-based RTILs (353.15 K to 283.15 K).

Figure 2. Temperature dependence of density (ρ) for N(SO2F)2anion-based RTILs (353.15 K to 283.15 K).

48 h and stored in a dry argon-filled glovebox ([O2] < 0.4 ppm, [H2O] < 0.1 ppm, Miwa Mfg. Co., Ltd.) before measurement. Measurements. Density (ρ/g·cm−3) measurements were carried out using a thermo-regulated Stabinger-type viscosity and density/specific gravity meter (Anton Paar, SVM3000G2, accuracy: 5·10−4 g·cm−3). The measurements were performed during cooling from (353.15 to 283.15) K at 5 K intervals with a stopper to avoid moisture and air contamination. Refractive index measurements were carried out using a thermo-regulated refractive index measurement system (Anton Paar, ABBEMAT, measured wavelength: 589.3 nm, accuracy: 4·10−5) in the air conditions. Measurements were also performed during heating from (283.15 to 353.15) K at 5 K intervals. Computational Methods. The Gaussian 03 program7 was used for the ab initio molecular orbital calculations. The basis sets implemented in the Gaussian program were used. The geometries of isolated ions were optimized at the MP2/ 6-311G** level. The molecular polarizabilities of isolated ions were calculated at the MP2/aug-cc-pVDZ level using the optimized geometries, unless otherwise noted. The MP2/aug-cc-pVDZ level

molecular polarizability calculations of large ions require huge CPU time and are not practical at present. Therefore the molecular polarizabilities of He-Py cation and FAP anion were calculated at the B3LYP/aug-cc-pVDZ level. The polarizabilities summarized in Table 4 are the averages of the calculated αxx, αyy, and αzz. The polarizabilities of ionic liquids shown in Figure 5 were obtained as the sum of the calculated molecular polarizabilities of isolated cations and anions in Table 4.

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RESULTS AND DISCUSSION The refractive indexes of the 17 RTILs were measured in the temperature range of (283.15 to 353.15) K with intervals of 5 K. All experimental refractive index values of RTILs are given in Table 2. Figure 1 shows the temperature dependences of the refractive indexes of the N(SO2F)2 anion based RTILs. A highly linear relationship with temperature was obtained for all samples in the temperature range examined in this study. The tendency of the temperature dependences of refractive indexes is closely related to that of the density of RTILs (Figure 2). All experimental density values of RTILs are given in Table 3. 2213

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Table 3. Experimental Density Values of 17 Kinds of RTILs temp./RTILs 353.15 K 348.15 K 343.15 K 338.15 K 333.15 K 328.15 K 323.15 K 318.15 K 313.15 K 308.15 K 303.15 K 298.15 K 293.15 K 288.15 K 283.15 K temp./ RTILs 353.15 K 348.15 K 343.15 K 338.15 K 333.15 K 328.15 K 323.15 K 318.15 K 313.15 K 308.15 K 303.15 K 298.15 K 293.15 K 288.15 K 283.15 K temp./RTILs 353.15 K 348.15 K 343.15 K 338.15 K 333.15 K 328.15 K 323.15 K 318.15 K 313.15 K 308.15 K 303.15 K 298.15 K 293.15 K 288.15 K 283.15 K

1-Et-3-Me-Im-N(SO2CF3)2 1.4639 1.4687 1.4736 1.4785 1.4834 1.4884 1.4933 1.4983 1.5033 1.5084 1.5134 1.5185 1.5236 1.5287 1.5338 1-Et-3-Me-ImSCN

1-Et-3-Me-Im-BF4

1-Et-3-Me-Im-N(CN)2

1.3919 1.3963 1.4007 1.4051 1.4095 1.4139 1.4184 1.4229 1.4275 1.4321 1.4367 1.4414 1.4462 1.4509 1.4556

1.2392 1.2428 1.2465 1.2502 1.2540 1.2578 1.2616 1.2653 1.2691 1.2729 1.2767 1.2805 1.2843 1.2882 1.2920

1.0658 1.0689 1.0720 1.0752 1.0783 1.0815 1.0848 1.0880 1.0913 1.0946 1.0979 1.1012 1.1046 1.1079 1.1114

1-Et-3-Me-ImSO3OH

1.0826 1.0855 1.0884 1.0914 1.0944 1.0973 1.1003 1.1033 1.1063 1.1094 1.1124 1.1155 1.1186 1.1217 1.1247 Et-Py-N(SO2F)2 1.4108 1.4151 1.4194 1.4237 1.4281 1.4324 1.4367 1.4411 1.4455 1.4499 1.4543 1.4587 1.4631 1.4675 1.4719

1-Et-3-Me-Im-N(SO2F)2

1-Et-3-Me-ImSO3OC2H5

1-Et-3-Me-ImSO3CH3

1.3334 1.3364 1.3394 1.3424 1.3454 1.3485 1.3515 1.3546 1.3578 1.3611 1.3643 1.3674 1.3705 1.3736 1.3767 Bu-Py-N(SO2F)2

1.2048 1.2081 1.2115 1.2148 1.2181 1.2215 1.2248 1.2282 1.2315 1.2349 1.2382 1.2416 1.2450 1.2484 1.2520 He-Py-N(SO2F)2

1.3240 1.3281 1.3322 1.3363 1.3404 1.3445 1.3486 1.3528 1.3569 1.3611 1.3652 1.3694 1.3736 1.3777 1.3819

1.2586 1.2625 1.2664 1.2703 1.2742 1.2782 1.2821 1.2861 1.2900 1.2940 1.2979 1.3019 1.3058 1.3098 1.3137

1-Et-3-Me-Im-FAP

1.6435 1.6493 1.6551 1.6609 1.6667 1.6726 1.6784 1.6843 1.6902 1.6961 1.7020 1.7079 1.7139 1.7198 1.7258 N-Me-Pr-PyrrN(SO2CF3)2

1.2009 1.3786 1.2042 1.3829 1.2076 1.3872 1.2109 1.3916 1.2142 1.3960 1.2176 1.4004 1.2209 1.4048 1.2243 1.4093 1.2276 1.4138 1.2310 1.4183 1.2343 1.4228 1.2376 1.4274 1.2410 1.4319 1.2443 1.4364 1.2477 1.4410 1-Et-3-Me-Py-N(SO2F)2 1.3605 1.3647 1.3689 1.3731 1.3774 1.3816 1.3858 1.3901 1.3943 1.3986 1.4029 1.4071 1.4114 1.4157 1.4200

1-Et-3-Me-Im-TCB 0.9860 0.9898 0.9936 0.9974 1.0013 1.0052 1.0091 1.0130 1.0169 1.0209 1.0249 1.0289 1.0329 1.0370 1.0410 N-Me-Pr-PyrrN(SO2F)2

1.2951 1.2989 1.3027 1.3066 1.3104 1.3143 1.3182 1.3222 1.3261 1.3301 1.3341 1.3381 1.3421 1.3462 1.3503 1-Bu-3-Me-Im-N(SO2C4F9)2 1.5468 1.5526 1.5584 1.5642 1.5700 1.5758 1.5816 1.5874 1.5932 1.5991 1.6050 1.6111 1.6173 1.6234 1.6295

of the temperature dependence of density (Figure 4). The range of the obtained refractive indexes is 1.35 to 1.60, which shows that the anion dependence of the refractive index is stronger than the cation dependence shown in Figure 1. The order of the refractive indexes is SCN ≫ N(CN)2 > SO3OH > SO3CH3 > SO3OC2H5 ≫ N(SO2F)2 > TCB ≫ N(SO2CF3)2 > BF4 ≫ FAP at all temperatures. However, the order of the refractive indexes did not have perfect uniformity of the chemical structure of the anions and decreased with anionic molecular weight. This tendency is opposite to the tendency of

The range of the obtained refractive indexes (1.42 to 1.48) is quite narrow. The order of the refractive indexes is 1-Et-3-MePy > Bu-Py > He-Py > Et-Py ≫ 1-Et-3-Me-Im > N-Me-Pr-Pyrr at all temperatures. The refractive indexes are clearly independent from the molecular weight of the cation, while dependent on the cationic structure, as in the case of pyridinium-cation based RTILs with various substitutions on the pyridinium ring. Figure 3 shows the temperature dependences of the refractive indexes of the 1-Et-3-Me-Im-cation based RTILs. A highly linear temperature dependence was found, as in the case 2214

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Table 4. Polarizabilities Calculated for Cations and Anions (in a.u.) at the MP2/aug-cc-pVDZ Level

a

cation

polarizability

anion

polarizability

1-Et-3-Me-Im 1-Bu-3-Me-Im Et-Py Bu-Py He-Py 1-Et-3-Me-Py N-Me-Pr-Pyrr

79 105 82 106 137a 137 95

N(SO2CF3)2 N(SO2F)2 BF4 N(CN)2 TCB SCN FAP SO3OC2H5 SO3CH3 SO3OH

98 69 23 53 79 52 118a 68 53 43

Calculated at the B3LYP/aug-cc-pVDZ level.

Figure 3. Temperature dependence of refractive indexes for 1-Et-3Me-Im-cation-based RTILs (353.15 K to 283.15 K).

Figure 5. Relationship between polarizability/molecular volume and refractive index for RTILs at 303.15 K.

agreed with previous reports (in the case of 1-Et-3-Me-ImN(SO2CF3)2).8 The relationship between the refractive indexes of RTILs and other physicochemical properties of RTILs attracted much attention. Previously, Shimizu et al. reported the relationship between the molecular refractive indexes and the cohesive molecular internal energies.9 Moreover, many researchers reported the relationship of the refractive indexes of RTILs with various physicochemical parameters (e.g., density, viscosity, surface tension, heat capacity, etc.).10−14 The refractive index indicates the dielectric response to an electrical field induced by electromagnetic waves (light). Therefore, the refractive index can be considered as the response to electronic polarization, within an instantaneous time scale as the firstorder approximation, so that it could be represented by using the polarizabilities of isolated cations and anions. Therefore, in this study, to analyze the refractive index of RTILs, the molecular polarizabilities of ions were calculated and are summarized in Table 4. The molecular polarizabilities of the He-Py cation and FAP anion were calculated at the B3LYP/ aug-cc-pVDZ level. Large ions have generally large molecular polarizabilities. Molecular polarization originates from the polarization of atoms. Therefore, molecular polarizability is approximately the sum of atomic polarizabilities. Molecular polarizability depends on the number of atoms in the molecule and atomic polarizabilities. The refractive index depends on the polarizability of materials per volume. We investigated the

Figure 4. Temperature dependence of density (ρ) for 1-Et-3-Me-Imcation-based RTILs (353.15 K to 283.15 K).

density (Figures 2 and 4), and the reason for this will be described later from ab initio calculations. The 1-Et-3-Me-ImSCN ionic liquid has the highest refractive index (1.55 at 303.15 K) in this study, which is close to that of quartz crystal (1.54), a high-refraction material. The refractive index of 1-Et3-Me-Im-SCN is larger than that of the liquid immersion oil (1.51), which was used to increase the resolution of the optical microscope. The high refractive index suggests that many applications are expected for the ionic liquid as a new optical material. Also, observed density and refractive index values 2215

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relationship between the refractive index of ionic liquid and the calculated polarizability of ion pairs/molecular volume as shown in Figure 5. The polarizabiliy of ion pairs is the sum of the calculated polarizabilities of isolated cations and anions. A high correlation (R2 > 0.98) was obtained for the 17 RTILs, which suggests that a good estimation of refractive index is possible from the polarizability and vice versa, and followed with Lorentz−Lorenz equation. This result implies the possibility of other universal physicochemical parameters of RTILs in addition to the molecular volume (occupied, van der Waals) as previously reported.6 However, no simple estimation method for the refractive index of RTILs has yet been reported. This is the first report on the combined analysis of optical properties of ionic liquids and the molecular polarizabilities of cations and anions. We found that there exists a linear relationship between the refractive index of ionic liquid and the molecular polarizability of ion pair, which suggests that we can design RTILs with high refractive indexes using the polarizabilities calculated for ions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.S.), [email protected] (S.T.). Fax: +81-3-3480-3401. Tel.: +81-3-3480-2111. Present Addresses ∥

Graduate School of Science and Technology, Niigata University, 8050, Igarashi, 2-no-cho, Nishi-ku, Niigata City, 950-2181, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors sincerely acknowledge Daizo Kameoka (Electric Power Engineering Systems Co., Ltd.) for technical support in the experiments.

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REFERENCES

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dx.doi.org/10.1021/je201289w | J. Chem. Eng. Data 2012, 57, 2211−2216