The Polarity of Ionic Liquids: Relationship between Relative

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Thermodynamics, Transport, and Fluid Mechanics

The Polarity of Ionic Liquids: relationship between relative permittivity and spectroscopic parameters of probe Xinyu Wang, Songna Zhang, Jia Yao, and Haoran Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00485 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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Industrial & Engineering Chemistry Research

The Polarity of Ionic Liquids: relationship between relative permittivity and spectroscopic parameters of probe Xinyu Wanga,//, Songna Zhanga,//, JiaYaoa, and Haoran Li*a,b aDepartment

of Chemistry, ZJU-NHU United R&D Center, Zhejiang University, Hangzhou 310027 (P.R. China); bState Key Laboratory of Chemical Engineering Department of Chemical and Biological Engineering Zhejiang University, Hangzhou 310027 (P.R. China)

Abstract: Polarity is one of the most important properties of Ionic liquids (ILs) and an essential requirement when choosing an IL for a specific industrial application. Up to now, several empirical solvent polarity scales, such as Hildebrand solubility parameter, relative permittivity, the electronic transition energy of the longest-wavelength Vis absorption band of betaine dye no. 30 [ET(30) value] and the hyperfine coupling constant (AN) etc. have been applied to ILs to provide quantitative evaluation of the polarity of ILs. Among them, the ET(30) value is widely used to reflect the polarity of ILs, however, it cannot be determined for opaque solvents and for solvents in which betaine dye no. 30 is insoluble. To broaden its scope of application and uniform the polarity standard, many approaches were used to predict the values of ET(30). Herein, we managed to apply a modified semi-empirical reaction field of molecular solvents to predict the ET(30) and AN values of spin probe in ILs. Based on the experimental and estimated ET(30) values of 791 data entries, 240 ILs, 108 cations, and 34 anions of ILs, an “overall” polarity sequence of ILs can be obtained: primary, secondary and tertiary alkylammonium salts > heterocyclic salts > quaternary alkylammonium salts ≈ phosphonium salts ≈ guanidinium salts.

Introduction Ionic liquids (ILs) with many attractive properties are considered as potential substitutes to many traditional organic solvents in synthesis, catalysis, separation,1, 2 extraction,3 and other chemical processes, which attracted the attention of a growing number of researchers recently.4-16 The high possibility for structure variations makes them potential “designer solvents”.17-23 Their physicochemical properties can be tailored to suit a target function by changing the ion chemical structures or the incorporation of functional groups.24-29 The solvent polarity is the most important property of ILs for choosing an IL for a specific industrial application.30-37 For example, solubility of the proteins depends on the polarity of ILs.38 Recently, many empirical molecular solvent polarity scales39 have been applied to ILs, attempting to provide quantitative estimates of their polarities, such as the Hildebrand solubility parameter,40-42 kinetic rate constant,15, 43-46 relative permittivity,42, 47-56 and several spectroscopic parameters of the solute.57-64 However, these empirical polarity scales always have their own limitations when applied to ILs, and can even show controversial results.15, 27, 32, 65 Among these polarity scales, the Hildebrand solubility parameter (  H ) is not a good ILs polarity parameter because of its harsh experimental conditions or low accuracy calculated by other parameters.55 The relative permittivity is the most commonly used solvent polarity parameter in molecular solvents which is also used to ILs. The relative permittivity of ILs has been obtained from the microwave dielectric spectra and the values are remarkably lower than those of common polar molecular solvents, which often lead to confusion.53, 66 Since the molar electronic transition energy of the longest-wavelength Vis absorption band of betaine dye no. 30 [ET(30) value] exhibits a good, generally linear correlation with a large number of other solvent-sensitive spectral parameters, such as some UV-Vis absorption bands, the hyperfine coupling constant (AN), and the stokes shift of fluorescence spectra in molecular solvents,67, 68 and also has the same trend with Z-, Ω-, and S-values as well as

many thermodynamic and dynamic data,69-73 ET(30) values are widely used as a polarity parameter for ILs.74-86 Reichardt et al. have collected the ET(30) values of nearly 210 ILs before. Therefore, in this work, ET(30) was chosen as a standard and generally applicable solvent polarity parameter of ILs.87 Due to the limitations of various measurement methods, it is significant to predict ET(30) values accurately for us to study the polarity of ILs.88 Recently, many researchers tried to predict the ET(30) values, however, only multiparameter equations can correlate different ILs polarity parameters well for a wide range of ILs.89-92 Many groups attempted to use the equation derived by Marcus93 from the Kamlet-Abboud-Taft (KAT) equation of 166 organic molecular solvents to predict the ET(30) values of ILs.94-98 However, it consists of four empirical parameters which cannot allow us to directly compare the polarity of ILs.99-107 This may hinder the complete and most efficient utilization of ILs. Other dyes whose electronic transition energies corresponding to their longest-wavelength Vis absorption band and having an excellent linear correlation with the ET(30) values, are also used to calculate the ET(30) values of ILs.57, 86, 108 Amongst these dyes, the ET(33) values (derived from betaine dye no. 33) are widely used.109-114 Moreover, because AN correlates satisfactorily with the ET(30) values of molecular solvents,67 EPR spectroscopy has also been used to obtain ET(30) values indirectly.61 Furthermore, Baker et al. used the classic Onsager reaction field model to study polarity but found that it is not applicable to ILs polarity research.49 In the previous work, we proposed a modified semi-empirical reaction field to relate ET(30), AN, and relative permittivity, which worked well for both aprotic and protic solvents. Here, we try to estimate ET(30) and AN values from the relative permittivity by using this modified reaction field on ILs. Consequently, 791 data entries, 240 ILs, 108 cations, and 34 anions of both experimental and predicted polarity parameters are summarized and a whole picture about the polarity sequence of ILs was obtained according to their experimental and estimated ET(30) values. The abbreviations of the terms used in this work are given in Table S1.

1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Results and Discussion

solvents such as N,N-dimethylformamide [DMF, ET(30)=43.8]. Besides, the ET(30) values of 1-(2-methoxyethyl)-1-methylpyrrolidinium ILs cover a range of 42.7-64.1 kcal/mol which is a very wide range, suggesting the importance of hydrogen bonding for the polarity of ILs. A discussion of how the molecular structure affects the value of ET(30) was reported by Reichardt.57 Furthermore, the data exhibited in Table S2 shows that the ET(30) values obtained for the same ILs are sometimes different from each other as measured by various research groups. The error of measurements is within 5 kcal/mol except entry 104 [BuMim][Tf2N] whose error is 5.4 kcal/mol and entry 160 [Bupy][BF4] whose error is 6.5 kcal/mol. The reason is probably that it is difficult to obtain ILs with the required purity, and very small amounts of impurities, such as water, can cause differences of the polarity of an IL. For example, the ET(30) value of [BuMim][PF6] is 52.9 kcal/mol with a concentration of water (CH2O) of 0.15 mol/L while the ET(30) value is 52.3 kcal/mol with a CH2O of lower than 6×10-3mol/L.129

1 Collection of experimental polarity parameter values Although many ILs have been reported, their polarity has not been fully understood. To have a better understanding of polarity of ILs, we collected some polarity parameters containing permittivity, ET(30), and AN values of 240 ILs, consisting of 108 cations and 34 anions, from the references given in Table S2. 1.1 Relative Permittivity (εr) In these polarity scales, the permittivities of more than 40 ILs have been reported so far47, 48, 50-53, 115 based on the method of the microwave dielectricspectra.47, 54 The permittivities of many aprotic ILs range from 9 to 16, i. e. they are smaller than that of polar molecular solvents, which is against what we have always known that a electriferous system will form a highly polar environment. In addition, Ekimova et al.116 used the Kirkwood theory117 and Onsager model,118 which are suitable for molecular solvents, to estimate the permittivity of ILs, however, Jin. et al indicated that these theories are not applicable to ILs.119 1.2 Hyperfine Coupling Constant (AN) The hyperfine coupling constant (A) of electron paramagnetic resonance (EPR) can also reflect the polarity of ILs.120, 121 Scheme 1 shows four commonly used EPR probes and the isotropic A (14N) values of them are collected in Table S2.60, 61, 67, 121, 122 According to the known A values which ranged from 15 N to 16.5, we can learn that the polarity of these ILs are similar to that of alcohols. However, the available data of AN is limited under the present experimental conditions, and high accuracy is required because of the small range of AN values measured in ILs. OH

NH2

N

N

N

N

N

O

Scheme 2. Molecule structure of betaine dye no.30.

2 Predicted values for the polarity of ILs

I N O

O

O

O

TEMPO

TEMPOL

ATEMPO

CAT-1

Scheme 1. Molecular structures of four commonly used EPR probes.

1.3 Molar electronic transition energy of betaine dye no. 30 [ET(30) value] Compared with the relative permittivity, the ET(30) scale (or the alternative dimensionless normalized ETN scale), is a better parameter to reflect the polarity of ILs,69, 123-127 which is not only easily measured but also exhibits a good linear correlation with other solvent-sensitive spectral parameters.128 The available ETN values of five common families of ILs were reported by Reichardt before.57 The known ET(30) values of ILs are within 42-63 kcal/mol which is in the range of common molecular solvents rather than super polarity. We can learn that the polarities of [CnNH3]+X-, [(Cn)2NH2]+X- and [(Cn)3NH]+X- are in the range of protic molecular solvents, e.g. short-chain alcohols. In contrast, the polarities of [(Cn)4N]+X- ILs such as [(C6)4N][H5C6CO2] are similar to that of aprotic molecular

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Recently, many approaches were used to predict the ET(30) values, using corresponding ET(33) and AN values; these predicted values are included in Table S2. In the previous work, our group proposed a modified semi-empirical reaction field to relate ET(30), AN, and relative permittivity through redefining the volume of the Onsager reaction field (Scheme 3), which worked well for both aprotic and protic solvents.130 Herein, we attempt to apply this modified Onsager reaction field model to ILs.

Scheme 3. Molecular model of the modified reaction field.

According to the modified reaction field we introduced before, the ET(30) and AN values are proportional to the reaction field at 298 K according to Eqs. (1) and (2):

2


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ET (30) / kcal mol1  26.556  36466 mol cm 3  ( R / a 3 )

(1)

AN / Gauss  15.083  903.31 mol cm 3  ( R / a 3 )

(2)

same phenomenon occurred with AN in the modified reaction field [Fig. 2(a)]. Actually, ionic interactions are much stronger than hydrogen bonds. Then, the r1 of ILs might be smaller than that of protic solvents based on the rule of protic and aprotic solvents. The length of the first chemical bond remote from the interaction site was found to be an appropriate r1 to ILs systems.

where

a  2r1  r2

(3)

4 V1   r13 3

(4)

4 V2   r23 3

(5)

Major ionic interactions

H 3C N

r2 denotes the radius of the solute molecule (labeled by 2), V1 and V2 are the molar volumes of the solvent molecule (labeled by 1) and the solute molecule 2, r1 is the radius of the primary solvation shell in aprotic solvents, which equals to the length of three chemical bonds remote from the hydrogen atom, where is the most susceptible site to form hydrogen bonds with the solute in ILs, which can be calculated by the UNIFAC method.131 R is defined as:

R

r 1 2 r  1

O

Dye no. 30

N O

(6)

ATEMPO

N CnH2n+1 H

ILs

H 3C H 2N

X

X N CnH2n+1 H ILs

Scheme 4. The major ionic interaction between probes and ILs.

2.1 Applying the modified reaction field on IL systems When the modified reaction field model is used on IL systems, the pyridinium N-phenolate betaine dye (dye no. 30) and 4-amino-2,2,6,6-tetramethyl-piperidine-1-oxyl (ATEMPO) are selected as probes in ILs. Since dye no. 30 is a zwitterionic molecule and the O atom carries a negative charge, the cation of ILs would have a strong ionic interaction with it (Scheme 4). ATEMPO and ILs also display a similar interaction as dye no. 30 (Scheme 4).132, 133 The anion of ILs can also simultaneously interact, to a less extent, with the cationic part of the zwitterionic probes. To simplify the model, the interactions between the anion of ILs and the probe is not considered in this work. Consequently, we applied the modified reaction field to those ILs with both ET(30) and relative permittivity as protic solvents. The results are shown in Fig. 1(a) and manifest clearly that all the ILs points are well above the line of molecular solvents. The

NH3

2.2 Molar volumes obtained according to the UNIFAC method When calculating the r1 of ILs systems, we found that r1 equals to the length of the first chemical bond remote from the interaction site, which leads to the best results, since the ionic interaction is much stronger than the van der Waals' forces and hydrogen-bond interactions. Scheme 5 shows the segmentation of ILs. From the UNIFAC volume parameter VK, the V1 of ion fragments can be calculated [Eq. (7)] and are shown in Tables 1 and 2. All the used VK are the molecular groups due to the complexity and few data capacity of ionic groups, which makes the calculated value a little smaller. n

V1  15.17  Vk

(7)

k 1

N

NO3

BF4

1

N(CF3SO2)2

N

N 2

3 O

N(CF3SO2)2

N

N(CF3SO2)2

N

N(CF3SO2)2 N

4

5

6

N

N

Cl

P

N N

N(CF3SO2)2

7

N 10

8

N(CN)2

N

N(CN)2

9

N(CF3SO2)2

S

11

Scheme 5. Rules for calculating volumes of ILs.

3



For example, V1([EtNH3] NO3) = 15.17VK(CH2NH2) = 20.77 cm3 mol-1 (Entry 1 in Scheme 5); V1([BuMim]BF4) = 15.17[VK(CH3N)+2VK(ACH)] = 34.12 cm3 mol-1 (Entry 2 in Scheme 5); V1([MeBuPyrr] Tf2N) = 15.17[VK(CH3N)+3VK(CH2)] = 48.69 cm3 mol-1 (Entry 4 in Scheme 5). Because the positive charge can be delocalized in aromatic rings, and many theoretical works on the analysis of the electron density distribution in imidazole IL systems have clearly shown that the positive charge is delocalized around the imidazolium ring, the VK of imidazole and pyridine are chosen to calculate V1 of corresponding ILs. Phosphorus-containing groups are not common UNIFAC groups, thus the VK of this kind of groups are

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hard to find. We used the VK of sulfur-containing groups in place of those of phosphorus-containing groups. In addition, the volumes of dye no. 30 and ATEMPO were obtained based on our quantum chemical calculations, because the volumes of probes we used should be that in the vacuum without any influence of other molecules. The ET(30) values, relative permittivities, and V1 values calculated from the UNIFAC method of 17 ILs are shown in Table 1. The AN values, relative permittivities, and V1 value calculated from the UNIFAC method of 5 ILs are shown in Table 2.

Table 1. Solvent polarity parameters ET(30), measured at 298.15K or room temperature at 1013 hPa, for 17 ILs, taken from refs.57, 87, relative permittivities εr for 17 ILs, taken from refs.15, 48, and vander Waals volumes V1 of ion fragments of the ILs, calculated from the UNIFAC volume parameters, taken from ref.134 -ET(30)’ values are the values predicted by the modified reaction-field model, and σ is the error in ET(30)’ relative to ET(30). No.

ILs

ET(30) kcal mol-1

εr F m-1

R/a3 ×103

V1 cm3 mol-1

ET(30)’ kcal mol-1

σ kcal mol-1

1

[EtMim] BF4

53.7

12.8

0.68812

31.25

51.65

-2.05

2

[BuMim] BF4

52.5

13.9

0.69481

31.25

51.89

-0.61

3

[BuMMim] BF4

49.4

13.3

0.69129

31.25

51.76

2.36

4

[BuMim] PF6

52.3

14.0

0.69536

31.25

51.91

-0.39

5

[EtMim] Tf2N

52.0

12.3

0.68471

31.25

51.52

-0.48

6

[PrMim] Tf2N

51.9

13.3

0.69129

31.25

51.76

-0.14

7

[BuMim] Tf2N

51.5

14.0

0.69536

31.25

51.91

0.41

8

[BuMMim] Tf2N

48.6

14.0

0.69536

31.25

51.91

3.31

9

[BuMim] TfO

52.3

13.2

0.69068

31.25

51.74

-0.56

10

[EtMMim] Tf2N

50.0

12.8

0.68812

31.25

51.65

1.65

11

[EtNH3] NO3

60.0

26.2

0.87462

20.77

58.45

-1.55

12

[HO(CH2)2NH3] HCO2

59.5

61.0

0.90408

20.77

59.52

0.02

13

[Oct3NMe] Tf2N

45.9

8.5

0.52454

48.69

45.68

-0.22

14

[BuPy] Tf2N

51.7

15.3

0.63037

39.44

49.54

-2.16

15

[MeEtPyrr] N(CN)2

48.7

41.5

0.60696

48.69

48.69

-0.01

16

[MeBuPyrr] Tf2N

48.3

14.7

0.56733

48.69

47.24

-1.06

17

[MePentPyrr] Tf2N

49.0

12.5

0.55682

48.69

46.86

-2.14

Table 2. Solvent polarity parameters AN, measured at 298.15K or room temperature at 1013 hPa, for 5 ILs, taken from refs.61, 122, relative permittivities εr for 5 ILs, taken from refs.15, 48, and van der Waals volumes V1 of ionic fragments of the ILs, calculated from the UNIFAC volume parameters, taken from ref.134 -AN’ values are the values predicted by the modified reaction-field model, and σ is the error in AN’ relative to AN. No.

ILs

AN Gauss

εr F m-1

R/a3 ×103

V1 cm3 mol-1

AN’ kcal mol-1

σ kcal mol-1

1

[EtMim] BF4

15.91

12.8

0.001165

31.25

16.14

0.23

2

[BuMim] BF4

16.26

13.9

0.001176

31.25

16.15

-0.11

3

[BuMim] PF6

16.30

14.0

0.001177

31.25

16.15

-0.15

4

[EtMim] EtOSO3

15.82

35.0

0.001257

31.25

16.22

0.40

5

[EtMim] TfO

16.32

16.5

0.001197

31.25

16.16

-0.16

4


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Fig. 1 Plot of ET(30) vs. modified reaction fields. (a) a = the length of three chemical bonds; (b) a = the length of only one chemical bond. The solid line is the fitting of Eq. (1) using data of molecular solvents. Cross dots are the data of molecular solvents and solid dots are the data of ILs.

Fig. 2 Plot of AN vs. modified reaction fields. (a) a = the length of three chemical bonds; (b) a = the length of only one chemical bond. The solid line is the fitting of Eq. (2) using data of molecular solvents. Cross dots are the data of molecular solvents and round dots are the data of ILs.

The relationship between ET(30) and the modified Onsager reaction field of ILs is presented in Fig. 1 (b). Obviously, ET(30) values give a better linear correlation with R/a3 as shown in Fig. 1(b), compared with that in Fig. 1(a). The deviations between experimental values and estimated results (σ) are within the region of molecular solvents. Therefore, this modified semi-empirical model is applicable to IL systems, and ET(30) values increase with increasing relative permittivity of the solvents and with decreasing volume of the primary solvation shell. The ILs [BuMMim][BF4], [BuMMim][Tf2N], and [EtMMim][Tf2N] (Entries 3, 8, and 10 in Table 1) have a substituent group on C-2 position of the imidazoliium ring, thus V1 of them should be larger than that of imidazole, which leads to the estimated larger ET(30). Moreover, a linear relationship is also observed between AN and R/a3 of ILs as shown in Fig. 2 (b). The deviations between experimental values and estimated results (σ) are also within the region found for molecular solvents. In Fig. 1 (b), we can see that the polarity sequence of ILs is: primary alkylammonium salts > pyridinium salts ≈ imidazolium salts > pyrrolidinium salts ≈ quaternary alkylammonium salts.

The result reveals that the structure of cations, to a large extent, influences the polarity of ILs. The strongest polarity similar to that of water [ET(30)=63.1 kcal mol-1] was found for the primary alkylammonium salts. The cations with an aromatic ring, such as the imidazole and pyridine ring, lead to a high polarity which approximates that of ethanol [ET(30)=51.9 kcal mol-1]. The cations with a saturated heterocyclic ring such as pyrrole and quaternary alkylammonium make a medium polarity close to that of 2-propanol [ET(30)=48.6 kcal mol-1]. The anions and substituents can affect the polarity slightly. According to the results, ET(30) and AN increase with the increasing relative permittivity of the solvent and with decreasing volume of the primary solvation shell. Furthermore, there are inconsistencies in order between ET(30) and relative permittivity. For example, according to literature in reaction systems,43 the order of reaction rate for the reaction of Cl- with methyl-p-nitrobenzenesulfonate in ILs and molecular solvents is same with that of ET(30) (Table 3). The order is DCM > DMSO > [BuMPy][Tf2N] > [BuMim][Tf2N] > MeOH. However, the trend in relative permittivity is quite different with reaction rate, especially when comparing ILs with molecular solvents.

5



Besides, the εr of [BuMim][Tf2N] is 11.7 which is equal to that of [BuMim][BF4]. However, the ET(30) value of [BuMim][Tf2N] is 50.0kcal mol-1, and that of [BuMim][BF4] is 52.5kcal mol-1, which is in good agreement with the order of  ( =20.42 J1/2 cm-3/2 for [BuMim][BF4] and  = 10.23 J1/2 cm-3/2 for [BuMim][Tf2N].55 Moreover, the relative permittivity value of [EtMim][BF4] is 12.8 and that of [BuMMim][BF4] is 13.3. This result is not in

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agreement with our common sense, that is, increasing the alkyl-chain length or the number of alkyl chains should lead to the decrease of the ILs polarity. According to the ET(30) data of these two ILs, the polarity of [EtMim][BF4] is larger than that of [BuMMim][BF4], which indicates that ET(30) can be a better polarity parameter.

Table 3. Experimental or predicted values of the relative permittivity (εr values) of solvents, the molar electronic transition energy of the longest-wavelength Vis absorption band of betaine dye no. 30 [ET(30) values], and the reaction rate for the reaction of Cl- with Methyl-p-nitrobenzenesulfonate in ILs and molecular solvents. Solvent

DCM

DMSO

[BuMPy][Tf2N]

[BuMim][Tf2N]

MeOH

k2/M-1s-1

1.07a

(9.0×10-1)a

(3.9×10-2)a

(1.2×10-2)a

(4.0×10-4)a

ET(30)

41.1b

45b

50.2c

51.5c

55.5b

εr

8.704b

46.68b

21.3d

14±0.5e

32.7b

a was obtained from reference43; b was obtained from reference130; c was obtained from reference57; d was calculated by eq.(1); e was obtained from reference48.

2.3 Prediction of polarity parameters of ILs via the modified reaction-field model Because the modified reaction field works well for IL systems, we can use this model to predict ET(30) and AN values as well as relative permittivities from one to another according to Eqs. (1), (2), and (6). The experimental and estimated values of these polarity parameters are shown in Table S2. The ET(30) values estimated by the relative permittivity of ILs are within the data range of corresponding ILs summarized by Reichardt.87 The AN values estimated by the relative permittivity of a set of ILs are also reasonable. Thus, this modified reaction field is a good model for the prediction of ET(30) and AN values with relative permittivity. Moreover, based on the data we can learn that this model is suitable to ILs, especially for less polar ILs whose predicted error is nearly 0.1 such as piperidinium and morpholinium salts. Comparing the ET(30) values predicted by ET(33) with those calculated by the modified Onsager reaction model (entries 53, 70, and 90), ET(33)’s prediction is closer to experimental values but as for entry 76 it is on the contrary. As result, using the modified Onsager reaction field model may be an excellent complementary approach to predict the ET(30) values of ILs.

Conclusions In this work, we proposed that ET(30) and AN values are more suitable to reflect the polarity of ILs, because ET(30) and AN have advantages for the measurement of ILs’ data, they can be converted into many other parameters, and they are comparable with that of molecular solvents. ET(30) and AN values cannot be only directly measured experimentally, they can also be predicted by means of the solvent’s relative permittivity. By defining the radius of the modified Onsager reaction field as the length of the first chemical bond remote from the interaction site of IL plus the radius of the probe, this semi-empirical model can be applied on a variety of IL systems. The results of correlation analysis suggest that ET(30) or AN could be estimated by relative

permittivity, which breaks the limits of application scope of these polarity scales and opens up the possibility of routine screening of ILs for a target application. Then, we used this modified Onsager reaction-field models to predict ET(30) and AN values of ILs, as a reference, both the experimental and predicted values of some polarity parameters of 240 ILs such as permittivity, ET(30) and AN values are summarized. Based on a large number of experimental and estimated values of ET(30), a unified polarity sequence of ILs sorting by cation was obtained (Scheme 6): primary, secondary, and tertiary alkylammonium salts > pyridinium salts ≈ imidazolium salts ≈ pyrrolidinium salts ≈ piperidinium salts ≈ morpholinium salts > quaternary alkylammonium salts ≈ phosphonium salts ≈ guanidinium salts. The polarities of all the ILs are larger than that of acetone. Addition of hydrogen-bond donor functional groups such as hydroxyl increases the polarity of ILs significantly, to a range between that of ethanol and water. Moreover, increasing the alkyl-chain length or the number of alkyl chains on the cations causes a slight decrease in the polarity of ILs. The polarity ranges of different kinds of cations have some overlaps, which can be ascribed to the effects of anions and substituents. These results are more in agreement with our common sense compared with relative permittivity. However, the cases of solvent polarity probes with a positive charge are not discussed in this modified model, which needs further research. Furthermore, due to the noticeable lack of data of polarity parameters, the determination of ET(30) and AN values for new ILs should be still preferential done by direct measurements if possible. Nevertheless, the proposed method for our calculation can be useful in case of probe insolubility, in case of acidic solvents (which protonate betaine dye B30), and because the number of ILs composed of a great variety of cations and anions is practically infinite. The calculated values can then help to design new ILs with the desired solvent polarity.

Computational Section The volume of probes were all calculated by using the Gaussian 09 program.135 In addition, B3LYP hybrid functional with the

6


Page 7 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6-31G basis set is used to fully optimize the molecular geometries and volumes of probes. The vibrational frequencies

have been also analysed to make sure that the optimized structures are at a minimum energy.

Scheme 6. The polarity sequence of ILs according to the ET(30) values. Y=N, P, or S, m=1, 2, 3, n=3, 2, 1.

neoteric solvents, Green Chem., 2002, 4, 73-80.

Associated Content

(5) H. Olivier-Bourbigou, L. Magna and D. Morvan, Ionic liquids and

Supporting information available: Glossary of terms of abbreviations and experimental and estimated values of ET(30) (Table S1 and S2), This material is available free of charge via the Internet at http://pubs.acs.org.

catalysis: Recent progress from knowledge to applications, Appl. Catal. A: Gen., 2010, 373, 1-56. (6) D. M. D'Alessandro, B. Smit and J. R. Long, Carbon Dioxide Capture: Prospects for New Materials, Angew. Chem. Int. Ed., 2010,

Author information

49, 6058-6082.

Corresponding author

(7) V. I. Parvulescu and C. Hardacre, Catalysis in ionic liquids,

*(H. Li) E-mail: [email protected]

Chem. Rev., 2007, 107, 2615-2665.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. // these authors contributed equally.

(8)

T. Welton, Ionic liquids in catalysis, Coord. Chem. Rev., 2004,

248, 2459-2477. (9) D. B. Zhao, M. Wu, Y. Kou and E. Min, Ionic liquids: applications in catalysis, Catal. Today., 2002, 74, 157-189.

Acknowledgements

(10) S. Zhang, N. Sun, X. He, X. Lu and X. Zhang, Physical

This work was supported by the National Natural Science Foundation of China (No. 21573196), the Fundamental Research Funds for the Central Universities and the National High Technology Research and Development Program (863 Program) of China (Grant No. SS2015AA020601).

Ref. Data., 2006, 35, 1475-1517.

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13


The Polarity of Ionic Liquids: Relationship between Relative

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