Article pubs.acs.org/jced
Determination and Prediction for the Polarity of Ionic Liquids Wei Guan,†,‡ Ning Chang,† Lili Yang,‡ Xiaoxue Bu,† Jie Wei,*,† and Qingshan Liu*,§ †
College of Chemistry, and ‡College of Environmental Science, Liaoning University, Shenyang 110036, P. R. China College of Science, Shenyang Agricultural University, Shenyang 110866, P. R. China
§
S Supporting Information *
ABSTRACT: Ionic liquids (ILs) have attracted remarkable attention as a new class of novel reaction medium and soft functional materials in the framework of Green Chemistry, and the polarity of ionic liquids is one of the primary concerns in the scientific community. Here, we introduce a new and simple scale of polarity, δμ, for ILs. Initially, two kinds of representative ionic liquids, {1-alkyl-3-methyllimidazolium acetate} [Cnmim][AcO] (n = 2,4,5,6) and {1-alkyl-2,3-dimethyimidazolium-N, N-bis(trifluoromethyl sulfonyl)imide} [Cnmmim][NTf2] (n = 2,4) are prepared and confirmed by 1H NMR spectroscopy and 13C NMR spectroscopy, respectively. Furthermore, a spectroscopic method based on solvatochromic probes is used to obtain the polarity of room temperature ILs (RTILs). In this work we mainly introduce that the scale of solvent polarity, ET(30) values, is determined by means of Reichardt’s dye (30), and π* values are determined by N,N-diethyl-4-nitroaniline. Simultaneously, in terms of the values of enthalpy of vaporization, δμ values of these ILs are predicted. Moreover, the trend of δμ values of the ionic liquids with the different alkyl chains are the same as the ET(30) and π* values. In addition, the new scale of polarities δμ of some molecular liquids are also predicted, and the results are in good agreement with relative permittivity, εr. vaporization, Δgl Hom(298) at 298 K, for ionic liquids was determined, and then based on Hildebrand’s theory,18 the new scale of polarity δμ was obtained and the polarity of ionic liquids could be predicted. As a continuation of our previous investigations,16,17 this article reports the following: (1) Preparation of ionic liquids 1alkyl-3-methylimidazolium acetate [Cnmim][AcO] (n = 2,4,5,6) and 1-alkyl-2,3-dimethylimdazolium-N,N-bis(trifluorosulfonyl)imide [Cnmmim][NTf2](n = 2,4) and confirmation by 1H NMR and 13C NMR spectroscopy; (2) determination solvent polarity parameter ET(30) and ENT of these ionic liquids by using Reichardt’s Dye (30), and the dipolarity of the solvent π* by N,N-diethyl-4-nitroaniline; (3) prediction the new scale of polarity δμ for these ionic liquids based on their vaporization enthalpies reported in the literature; and (4) the prediction of the new scale of polarities δμ of some molecular liquids, the results of which are in good agreement with relative permittivity εr.
1. INTRODUCTION Room temperature ionic liquids (RTILs) have been widely used in the areas of chemistry and chemical engineering because of their unique outstanding properties, such as negligible vapor pressure, high reusability, nonflammability, high thermal stability, and excellent solvation capacity.1−10 In recent years, ionic liquids as environmentally more benign new reaction media in the framework of Green Chemistry have attracted attention, and the polarity of the liquids is the crucial index. In general, the relative permittivity εr is often used as a scaling of molecular liquid polarity; however, εr cannot suitably show the polarity of ionic liquids. For example, the εr value of [C4mim][NTF2] reported by Daguenet et al.11 is 11.7 which is quite the similar to that of [C4mim][BF4] reported by Wakai et al.12 However, as we know, the polarities of [C4mim][NTF2] and [C4mim][BF4] are completely different. Reichardt13 has determined empirically the polarity of ca. 80 kinds of ionic liquids with the help of solvatochromic pyridinium N-phenolate betaine dyes, called the ET(30) or ENT scale, which is widely accepted. The π* value developed by Kamlet and Taft14 is a combined measure of the solvent’s polarizability and dipolarity. The measurements of π* for a series of alkylammonium nitrates and thiocyanates using a different set of dyes was carried out by Poole et al.,15 and π* values were still higher, which probably gives an index to the possible closeness of access to the charge centers of these salts and hence greater ion−dye Coulombic interactions. Recently, we have brought forward a new scale of polarity δμ for ILs,16,17 which can be predicted easily. First, the enthalpy of © 2017 American Chemical Society
2. EXPERIMENTAL SECTION Chemicals and Reagents. All the ionic liquids are prepared in our laboratory according to referencing reported methods,6 and the purities of these ionic liquids are listed in the Special Issue: Memorial Issue in Honor of Ken Marsh Received: January 25, 2017 Accepted: August 16, 2017 Published: August 29, 2017 2610
DOI: 10.1021/acs.jced.7b00082 J. Chem. Eng. Data 2017, 62, 2610−2616
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define empirically a solvent polarity scale, called ET(30) scale.19 The ET(30) scales were calculated using the following equation:13
Supporting Information Table S1. The structures of all the prepared compounds were well-confirmed by 1H NMR and 13C NMR spectroscopy (See Figures S1−S8 in Supporting Information). Reichardt’s betaine dye (30) (A) and N,Ndiethyl-4-nitroaniline (B) were purchased from Sigma-Aldrich and J&K, respectively, and used without further purification. The water contents and the halogen contents were determined by a Karl Fischer moisture titrator (ZSD-2 type) and ion selective electrode, respectively, and all are listed in Table S1. Analytical Methods. For the measurements of ET(30) and ENT scales and dipolarity π*, a moderate amount of the individual dye stock solution is prepared, that is, Reichardt’s dye (30) (A) with a concentration of 3.0 × 10−4 mol·L−1 and N,N-diethyl-4-nitroaniline (B) with a concentration of 1.0 × 10−3 mol·L−1 in methanol. The dye/methanol solutions were stored in brown sample bottles in the dark overnight for the sake of achieving equilibriums, and then they were added to the ionic liquids using a micropipette, respectively. The methanol was carefully eliminated by use of vacuum drying at 40 °C for 48 h, and then the dye/ionic liquid solutions were put into quartz cells, which were sealed with a silicon cork. In the next moment, they were placed at the holder fixed to the light-path. The absorption spectra were recorded with UV−vis spectrometer Lambda 35 (PerkinElmer) at room temperature, and the UV spectra of Reichardt’s dye (30) (A) and N,N-diethyl-4nitroaniline (B) in the six RTILs were obtained (see Figures S9 and S10 in Supporting Information). The wavelength corresponding to maximum absorption of Reichardt’s dye (30) (A) and N,N-diethyl-4-nitroaniline (B) was listed in Tables 1 and 2.
E T(30) = 28591/λmax (dye)
where λmax(dye) is the wavelength corresponding to maximum absorption of Reichardt’s dye (30) (A). From Figure S9, the maximum absorbance wavelengths, λmax were listed in Table 1. According to eq 1, ET(30) values of the ionic liquids were calculated and also listed in Table 1. Actually, the ET(30) scale ranges from 63.1 kcal mol−1 for water, the most polar solvent, to 30.7 kcal mol−1 for TMS, the least polar solvent, for which ET(30) values are experimentally available. However, ET(30) has the non-SI unit “kcal·mol−1” and in order to make ET(30) values into the SI unit “kJ·mol−1”, the dimensionless normalized ENT scale was introduced, using water (ENT = 1.00) and TMS (ENT = 0.00) as reference solvents to fix the scale, according to eq 2:13 E TN = [E T(solvent) − E T(TMS)]/[E T(water) − E T(TMS)] = [E T(solvent) − 30.7]/32.4
λmax /nm
ET(30)/kcal·mol−1
ENT
[C2mim][AcO] [C4mim][AcO]
562.5 565.0
[C5mim][AcO] [C6mim][AcO] [C2mmim][NTf2]
566.0 568.0 584.0
[C4mmim][NTf2]
589.0
50.8 50.6 50.522 50.5 50.3 49.0 50.023 48.5 48.623 50.822
0.621 0.614 0.611 0.612 0.606 0.563 0.59623 0.551 0.55223 0.620
[C4mim][NTf2]
Table 2. Values of λmax of N,N-Diethyl-4-nitroaniline (B) in the Six Ionic Liquids and the Values of π* λmax
π*
[C2mim][AcO] [C4mim][AcO]
417.6 412.4
[C5mim][AcO] [C6mim][AcO] [C2mmim][NTf2] [C4mmim][NTf2]
409.8 408.6 412.6 411.6
1.122 1.027 1.0422 0.979 0.957 1.031 1.013
ILs
(2)
The ETN values of the solvent were be obtained and are listed in Table 1. The ENT scale is considered to be a good general scale of the solvating ability of a liquid, and it has been applied to a large number of ionic liquids.15,20,21 Measurement of the position of the absorption maximum of Reichardt’s dye (30) (A) is a necessary part of the measurement as well as the source of the ENT scale. Hence we discuss the qENT values of the ionic liquids here. From Table 1, it can be seen that ET(30) and the ENT values decrease with the increasing length of the alkyl chains in the cations, the ET(30) value of [C4mim][AcO] determined in this work is very close to the reported one,22 and the determined ET(30) values of [Cnmmim][NTf2](n = 2,4) are in agreement with Machado’s results.23 In addition, the average contribution to ET(30), per the addition of methyl group (−CH2) in an alkyl chain for [Cnmim][AcO] (n = 2,4,5,6) is 0.13 kcal·mol−1, which is smaller than that of [Cnmmim][NTf2](n = 2,4). Moreover, the average contribution to ET(30), per the addition of a methylene group (−CH3) in the imidazolium ring for [C4mmim][NTf2] vs. [C4mim][NTf2],22 is 2.3 kcal·mol−1, and they are much larger than that per methyl group (−CH2) in the alkyl chain, meaning that the addition of a methylene group (−CH3) in the imidazolium ring of [C4mmim][NTf2] could have a stronger impact on ET(30), and the same effect on ETN. The order of ET(30) is as follow: ET(30) [C2mim][AcO] > ET(30) [C4mim][AcO] > ET(30) [C 5 mim][AcO] > E T (30) [C 6 mim][AcO] > E T (30) [C2mmim][NTf2] > ET(30) [C4mmim][NTf2], which is also as the same as the trend of ENT . π* Polarity Scales for Ionic Liquids. In fact, π* is derived from the change in the energy of the absorption maximum of the dye that is induced by the local electric field generated by the solvent. Therefore, it is no surprise that π* has been greatly affected by the ion−dye interactions now possible in the ionic liquid. π* values were calculated using the following equation:13
Table 1. Values of λmax of Reichardt’s dye (30) (A) in the Six Ionic Liquids and the Values of ET(30) and ENT ILs
(1)
π * = 0.314 × (27.52 − ν(dye))
3. RESULTS AND DISCUSSION ET(30) and ETN Solvent Polarity Scales for Ionic Liquids. The solvatochromic visible absorption of betaine dye (30) was used as a solvent-dependent reference process to
(3)
−4
where ν(dye) = 1/λmax(dye) × 10 kK (kK is kilokeyser, 10−3 cm−1), “dye” represents N,N-diethyl-4-nitroaniline. From Figure S10, the maximum absorbance wavelength, λmax(dye) of N,N-diethyl-4-nitroaniline (B) were listed in Table 2. 2611
DOI: 10.1021/acs.jced.7b00082 J. Chem. Eng. Data 2017, 62, 2610−2616
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Table 3. Values of nD, M, ρ, Rm, Δgl Homn, Δgl Hom(298), Δgl Homμ, and δμ for [Cnmim][AcO](n = 2,4,5,6), [Cnmmim][NTf2](n = 2,4), [Cnmim][[Lact]](n = 2,3,4,5), [Cnmim][NTf2](n = 2,4,6,8,10), and Others at 298 K ρ
M IL
nD
[C2mim][AcO] [C4mim][AcO] [C5mim][AcO] [C6mim][AcO] [C2mmim] [NTf2] [C4mmim] [NTF2] [C6mmim] [NTf2] [C2mim][Lact] [C3mim][Lact] [C4mim][Lact] [C5mim][Lact] [C2mim][NTf2] [C4mim][NTf2] [C6mim][NTf2] [C8mim][NTf2] [C10mim][NTf2] [C2mim][BF4] [C4mim][BF4] [C2C1im][SCN] [C2C1im] [N(CN)2] [C2C1im] [C(CN)3] [C2C1im] [B(CN)4]
1.493811 1.486911 1.484511 1.482911 1.429627 1.433427 1.436027 1.500430 1.496830 1.492730 1.491430 1.42131 1.42731 1.43131 1.43431 1.43631 1.40931 1.42031 1.550632 1.511232 1.512432 1.446932
−1
g·mol
170.213 198.263 212.29 226.316 408.369 436.423 464.476 200.209 214.222 228.233 242.239 391.31 419.37 447.42 475.47 503.53 197.97 226.02
cm ·mol 3
g·cm
48.3234 48 × 1034 55.8434 58.2834
(4)
(5)
where Δgl Hom(298) is enthalpy of vaporization at 298 K, which it can be experimental determination or calculated. Δgl Homn is the contribution part from the induced dipole moment of the ILs, and its value can be calculated by the Lawson−Ingham equation:26 Δlg Hmon = C[(nD2 − 1)/(nD2 + 2)]Vm = CR m
−1
kJ·mol
58.3 70.6 76.5 82.4 91.7 103.8 115.9 64.4 70.4 76.2 82.3 85.8 98.4 112.9 122.6 141.1 85.8 98.4 62.7 62.4 72.4 75.6
ΔglHom(298) −1
kJ·mol
131.228 134.817 137.4 28 140.7 17 156.529 160.529 168.728 138.835 140.835 143.328 145.435 132.736 137.836 142.336 147.036 147.536 132.736 137.836 153.737 156.437 138.537 135.637
ΔglHomμ
δμ
kJ·mol−1
J1/2·cm−3/2
72.9 63.8 60.9 58.3 64.8 56.7 52.8 74.4 70.3 67.1 63.1 46.9 39.5 29.4 24.4 6.4 46.9 39.5 91.0 94.0 66.1 60.0
21.5 18.217 17.0 16.017 15.3 13.4 12.3 20.435 18.935 18.0 16.435 13.28 11.45 9.26 8.13 3.88 23.35 21.72 24.3 24.0 18.7 16.4
Using the reported data,11,16,17,27−29 the values of δμ for [Cnmim][AcO] (n = 2,4,5,6) and [Cnmmim][NTf2] (n = 2,4) were calculated according to eq 4 and listed in Table 3. As listed in the table, the values of δμ decrease with increasing number of methylene (−CH2−) groups in the alkyl chains of the ILs, that is the order: δμ[C2mim][AcO] > δμ[C4mim][AcO] > δ μ [C 5 mim][AcO] > δ μ [C 6 mim][AcO] > δμ[C2mmim][NTf2] > δμ[C4mmim][NTf2]. In addition, the reported values of refractive index nD, molar mass M, density ρ, molar refraction Rm, the contribution part from the average o , enthalpy of vaporization permanent dipolmoment Δgl Hmn g o Δl Hm (298), contribution part from the induced dipole o , and the new scale of polarity δμ for moment ΔlgHmμ [Cnmim][Lact](n = 2,3,4,5), [Cnmim][NTf2](n = 2,4,6,8,10), and others at 298 K have been collected from the literature,30−37 and on the basis of eq 4, δμ values were predicted and listed in Table 3. From the table, it can been seen that anions have a great effect on the polarity of δμ for ionic liquids, and the quantitative relation will continue to be investigated in future work. From Tables 1−3, it can be seen that the order of δμ polarity scales we have put forward is the same as ET(30), ENT , and π* polarity scales; that is, the ET(30) and ETN, π* and δμ values decrease with the increasing length of the alkyl chains in the cations. In addition, small amounts of remaining water can change the polarity of an ionic liquid considerably; it has a negative effect on ET(30) and the determination for the same ionic liquid by various research groups differ sometimes considerably. However, based on the enthalpy of vaporization, polarity scales δμ could avoid these problems easily, because prior to a stepwise temperature-programmed run, the isothermal TGA is performed at a certain temperature for a long static hold period (1.5 to 4) h in order to remove volatile impurities and traces of water, thus avoiding the negative effect of the volatile impurities and traces of water on the polarity.
where Vm is molar volume and ΔglHomμ is the contribution part from the average permanent dipole moment of the ion pair in the ILs. Δgl Hmoμ = Δlg Hmo(298) − Δlg Hmo n
−1
1.1019011 1.0447411 1.0301311 1.0170011 1.491116 1.418516 1.359116 1.18630 1.154530 1.128230 1.105930 1.5033 1.4233 1.3333 1.3133 1.2133 1.279834 1.201534
According to eq 3, π* values were obtained and listed in Table 2. It should be noted that the ionic liquids used in this study are all composed of relatively similar cations and that π* values decrease with the increasing length of the alkyl chains in the cations, although differences between the ionic liquids are small, and the π* values listed in Table 2 for the ionic liquids are high in comparison with nonaqueous molecular solvents reported in the literature.24,25 From Table 2, the π* value of [C4mim][AcO] determined is in agreement with the one reported by Wu.22 The order of π* is also obtained as follow: π*[C2mim][AcO] > π*[C4mim][AcO] > π*[C5mim][AcO] > π*[C6mim][AcO] > π*[C2mmim][NTf2] > π*[C4mmim][NTf2]. δμ Polarity Scales for Ionic Liquids. In previous papers,16,17 we have put forward the new scale of polarity δμ based on Hildebrand’s theory,18 for ILs: δμ 2 = Δlg Hmoμ/Vm − (1 − xn)RT/Vm
ΔglHomn
Rm −3
(6)
where nD is the refractive index and C is an empirical constant which is equal to 1.297 kJ·cm−3 for a van der Waals liquid. In addition, xn = Δgl Homn/Δgl Hom(298) in eq 4. 2612
DOI: 10.1021/acs.jced.7b00082 J. Chem. Eng. Data 2017, 62, 2610−2616
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Table 4. Values of nD, V, ΔHvmn, Δgl Hom(298), ΔHvmμ, δμ, and εr for Molecular Solvents 106Vm molecular solvent trichloromethane pyridine 1-hexanol benzyl alcohol 1-cyclohexanol 2-propanol 1-propanol ethanol methanol propane-1,2,3-triol water 4-methyl-2-pentanone 2-hexanone 2-pentanone 3-pentanone cyclohexanone 2-butanone acetone methyl butanoate methyl propanoate
m ·mol 3
nD 38
1.4421 1.507139 1.415840 1.583141 1.464542 1.374941 1.383040 1.359441 1.328141 1.472243 1.332540 1.396544 1.399245 1.388246 1.389747 1.448238 1.376148 1.356142 1.38649 1.37649
−1
50
80.69 80.8651 125.3252 103.8353 105.9154 76.9541 75.1755 58.6356 40.7357 62.4558 18.0759 125.36344 124.0760 107.3461 106.4762 104.0963 90.1760 74.0364 114.5665 96.9566
ΔHvmn
Δgl Hom(298)
−1
−1
kJ·mol
27.70 31.22 40.77 45.01 37.94 22.84 22.74 16.76 10.72 22.69 4.81 39.11 38.94 32.87 32.71 36.16 26.84 20.98 34.90 28.85
That is, it could be said that δμ is more convenient and appropriate to predict the polarity scales of ionic liquids. δμ Polarity Scales for Molecular Liquids. The relative permittivity εr is the crucial parameter in the molecular solvent system and it usually can be used to measure the polarity of molecular solvents. In this work, the values of refractive index, nD, molar volume, Vm, and Δgl Hom(298) were collected from the literature,38−74 and according to eqs 4−6 δμ of some molecular solvents were obtained. These are listed in Table 4, and the corresponding relative permittivity εr for these molecular solvents were collected75−87 and also listed in Table 4. From Table 4, it can been seen that the polarity trends of δμ and εr for molecular solvents are consistent, that is, using δμ could be used to obtain the polarity of molecular solvents.
kJ·mol
67
■
31.14 40.2768 61.7169 65.9470 62.0071 43.9972 45.7272 41.6872 37.9772 91.773 43.9974 40.6175 43.175 38.4376 38.5276 45.1375 34.9277 30.878 39.2879 36.3280
ΔHvmμ kJ·mol−1
δμ (J·cm
3.44 9.05 20.94 20.93 24.06 21.15 22.98 24.92 27.25 69.01 39.18 1.5 4.16 5.56 5.81 8.97 8.08 9.82 4.38 7.47
−3 1/2
)
6.26 10.25 12.66 13.93 14.77 16.10 17.00 19.99 25.01 31.19 45.23 3.35 6.05 6.96 7.15 9.02 9.124 11.04 5.99 8.47
εr 81
4.76 12.5082 13.3073 13.4082 15.0082 18.3053 20 × 1053 24.5581 32.6083 42.5073 80.0084 13.1181 14.6081 16 × 1081 17.4585 18.2086 18.5087 20.7081 5.5049 6.2349
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00082. NMR and UV spectra; water content, halogen content, and purities of [Cnmim][AcO](n = 2,4,5,6) and [Cnmmim][NTf2] (n = 2,4) (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Jie Wei: 0000-0002-0042-2284 4. CONCLUSION
Funding
First, two kinds of representative ionic liquids, {1-alkyl-3methyllimidazolium acetate} [Cnmim][AcO](n = 2,4,5,6) and {1-alkyl-2,3-dimethyimidazolium-N,N-bis(trifluoromethyl sulfonyl)imide} [Cnmmim][NTf2] (n = 2, 4) have been prepared and confirmed by 1H NMR spectroscopy and 13C NMR spectroscopy, respectively. Second, the new scale of polarities δμ we have proposed in a previous work, has been predicted for the six ionic liquids, and the results are agreement with those of the ET(30), ENT , and π* polarity scales determined by means of Reichardt’s dye (30) and N,N-diethyl-4-nitroaniline in the experiment. The trend of δμ values of the ionic liquids with the different alkyl chains are the same as the ET(30) and ENT and π* polarity scales. In addition, the new scale of polarities δμ of some molecular liquids are also predicted in the work, and the results are in good agreement with relative permittivity, εr. Additionly, using the scale of polarity δμ we could avoid some negative effect on polarity such as polar impurities and trace amounts of water. Therefore, it is concluded that δμ can be used as a criterion to simply determine the polarity of ionic liquids and molecular liquids.
The project was supported by the National Natural Science Foundation of China (21673107, 21703090) and Liaoning Excellent Talents in University (LR2015025 and LJQ2015099), Peoples Republic of China. Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.jced.7b00082 J. Chem. Eng. Data 2017, 62, 2610−2616
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