Thermophysical properties of 4-dimethylaminopyridine based ionic liquids

Thermophysical properties of 4-dimethylaminopyridine based ionic liquids

Journal Pre-proof Thermophysical properties of 4-dimethylaminopyridine-based ionic liquids Dongren Cai, Nisakorn Saengprachum, Zhipeng Lin, Ting Qiu P...

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Journal Pre-proof Thermophysical properties of 4-dimethylaminopyridine-based ionic liquids Dongren Cai, Nisakorn Saengprachum, Zhipeng Lin, Ting Qiu PII:

S0167-7322(19)31186-9

DOI:

https://doi.org/10.1016/j.molliq.2019.111875

Reference:

MOLLIQ 111875

To appear in:

Journal of Molecular Liquids

Received Date: 28 February 2019 Revised Date:

27 September 2019

Accepted Date: 2 October 2019

Please cite this article as: D. Cai, N. Saengprachum, Z. Lin, T. Qiu, Thermophysical properties of 4-dimethylaminopyridine-based ionic liquids, Journal of Molecular Liquids (2019), doi: https:// doi.org/10.1016/j.molliq.2019.111875. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

1

Thermophysical properties of 4-dimethylaminopyridine-based

2

ionic liquids

3

Dongren Cai, Nisakorn Saengprachum, Zhipeng Lin, Ting Qiu∗

4

Fujian Universities Engineering Research Center of Reactive Distillation, College of Chemical

5

Engineering, Fuzhou University, Fuzhou 350116, Fujian, China

6

ABSTRACT: In this study, a molecular simulation (Gaussian03) was used to analyze the

7

chemical structure of 4-dimethylaminopyridine (DMAP) to investigate its use for the synthesis of

8

ionic liquids (ILs). Based on DMAP, a series of new ILs was prepared and characterized by fourier

9

transform infrared spectroscopy (FT-IR),

1

H nuclear magnetic resonance (1H-NMR),

10

thermogravimetric analysis (TGA), and differential scanning calorimeter (DSC). The density data

11

of the studied ILs were measured from 328.15 K to 358.15 K, p=0.101 MPa. It is shown that

12

density decreases with an increase in the temperature and the alkyl chain length of cations. In

13

addition, density is affected by the anion type. Furthermore, the volume of ions, NBO charges, and

14

the interactions of anions and cations were calculated by Gaussian03 to explain the obtained

15

density results. The second-order polynomial equation was adopted to correlate the relationship of

16

density and temperature. Then, an isobaric thermal expansion coefficient, molecular volume, and

17

lattice potential energy were obtained from the density data. Viscosity was determined from

18

323.15 K to 368.15 K, p=0.101 MPa. It is determined that the viscosity of ILs decreases with an

19

increase in the temperature but increases with an increase in the alkyl chain length of cations. The

20

temperature dependence of viscosity was described by the Vogel-Fulcher-Tamman (VFT) model,

21

and the activation energy of viscous flow (Eη) was obtained. Furthermore, the decomposition



Corresponding author. Tel.: +86 13705945511. E-mail address: [email protected] (T. Qiu)

22

temperature (Td), glass transition temperature (Tg), and melting temperature (Tm) of ILs were

23

obtained from the results of TGA and DSC.

24

Keywords: Ionic liquids, 4-Dimethylaminopyridine, Density, Viscosity, Thermal stability

25

1. Introduction

26

In recent years, ionic liquids (ILs) have received considerable attention in many research

27

fields due to their superior physicochemical properties [1-6]. ILs are composed of only cations and

28

anions; ILs are liquids at room temperature (melting point < 100 ), which is different from

29

conventional ionic compounds [7,8]. Because of their ionic nature, ILs always exhibit

30

nonvolatility, wide liquid range, good thermal stability, high conductivity, and tunable polarity.

31

These properties allow to use ILs as solvents in organic reactions and as efficient catalysts by

32

designing cations and anions. For example, by combining molecular simulations and experiments,

33

Harini et al. [9] designed task-specific ILs to be used as solvents for the extraction of a

34

pharmaceutical intermediate. Qiu et al. [10] prepared -SO3H-functionalized ILs using

35

N,N-dimethylcyclohexylamine as the matrix for biodiesel production from coconut oil. The result

36

showed that under optimal conditions, the biodiesel yield can reach 98.7%, and there was no

37

considerable decrease in the catalytic activity of IL after being used for 6 cycles. The design and

38

application of ILs need reliable and systematic data on thermophysical properties (e.g., density,

39

viscosity, and thermal stability) to better understand the behaviors and interactions of ions in ILs

40

[11,12]. However, currently, there is still a lack of systematical knowledge on thermophysical

41

properties, especially for 4-dimethylaminopyridine (DMAP)-based ILs. DMAP is essential in

42

organic synthesis and medicine [13,14]. Some studies have shown that DMAP is an efficient phase

43

transfer and nucleophilic acylation catalyst that can be used for interfacial polymerization

44

reactions [15]. DMAP contains pyridine and tertiary amino groups, which offers various

45

possibilities for the synthesis of ILs. Currently, there are few reports on DMAP-based ILs. Thus, it

46

is essential to develop DMAP-based ILs and further investigate their thermophysical properties,

47

which will expand the types of existing ILs and speeding up their practical application.

48

In this study, the structure of DMAP was carefully analyzed by molecular simulations, and a

49

series of new DMAP-based ILs was prepared and characterized. The thermophysical properties

50

(e.g., density, viscosity, and thermal stability) were investigated. The effect of cation and anion

51

types on density and viscosity were discussed. The isobaric thermal expansion coefficient,

52

molecular volume, lattice energy, and the activation energy of viscous flow, which can reflect the

53

interactions between cations and anions in ILs, were obtained from the experimental data.

54

Furthermore, the volume of ions, NBO charges, and interactions of anions and cations were

55

calculated by Gaussian03 to explain the abovementioned results.

56

2. Experimental

57

2.1 Computational details

58

Computational chemistry was used to analyze the related molecular structures. The

59

computational details were as follows. All structures were fully optimized using the Gaussian03

60

package at the B3LYP/6-311+G(d, p) level of theory with ultrafine integration grids [16].

61

2.2 Chemicals

62

All chemicals used in this experiment are shown in Table 1 Table 1. Chemicals used for this experiment a

63 Type

CAS

Purity

Source

4-dimethylaminopyridine

1122-58-3

>99.0%

Aladdin

1-butyl bromide

109-65-9

>99.0%

Aladdin

1-bromohexane

111-25-1

>99.0%

Aladdin

1-bromooctane

111-83-1

>98.0%

Aladdin

1,3-propanesulfonate

1120-71-4

>98.0%

Aladdin

trifluoromethanesulfonic acid

1493-13-6

>98.0%

Aladdin

methanesulfonic acid

75-75-2

>99.0%

Aladdin

sulfuric acid

7664-93-9

>98.0%

Aladdin

ethyl acetate

141-78-6

> 99.5%

Aladdin

deionized water

--

--

Our lab

64

a

65

2.3 Preparation and characterization of ILs

66

All chemicals were used without purification

The ILs investigated in this study are shown in Table 2. Two methods were adopted to

67

prepare ILs (i.e., non-SO3H and -SO3H functionalized ILs).

68

2.3.1 Preparation and characterization of non-SO3H functionalized ILs

69

1-butyl-4-dimethylaminopyridinium hydrogen sulfate [B-DMAP][HSO4]: A 0.1 mol

70

DMAP compound was dissolved in ethyl acetate. Equimolar 1-butyl bromide was added dropwise

71

into the mixture at room temperature under full stirring. The reaction was carried out at 70

72

stirred for 48 h. After suction filtration, repeated washing of ethyl acetate, and vacuum drying, a

73

white solid [B-DMAP][Br] [1H NMR (500 MHz, DMSO) δ ppm 8.34 (d, J = 7.8 Hz, 2H, cation

74

N=CH-C=C), 7.05 (d, J = 7.8 Hz, 2H, cation N=C-CH=C), 4.18 (t, J = 7.2 Hz, 2H, cation

75

N-CH2-C-C-C), 3.19 (s, 6H, cation N-CH3), 1.79-1.68 (m, 2H, cation N-C-CH2-C-C), 1.24 (dd, J

76

= 15.1, 7.5 Hz, 2H, cation N-C-C-CH2-C), 0.90 (t, J = 7.4 Hz, 3H, cation N-C-C-C-CH3) (Figure

77

2), FT-IR: v (cm-1) 2960, 2935, 2874, 1650, 1567, 1382, 660] was obtained. Then, the white solid

78

was dissolved in deionized water. Under vigorous stirring in an ice water bath, equimolar

79

concentrated sulfuric acid and one half of Ag2O was slowly added to the abovementioned solution,

80

and the mixture was stirred at room temperature. After the reaction, a yellowish solid was removed

81

by suction filtration, and the liquid was removed via rotary evaporation (363.15 K, -0.1 MPa).

82

After repeated washing with ethyl acetate and vacuum drying, [B-DMAP][HSO4] was obtained.

83

The obtained [B-DMAP][HSO4] was titrated by silver nitrate with nitric acid acidification, and

and

84

there was no yellowish precipitation in the titration process, which suggests that there is basically

85

no bromide ion left in [B-DMAP][HSO4]. The pH value of a 0.1 mol/L [B-DMAP][HSO4]

86

aqueous solution is 1.27, which suggests that a part of H+ from HSO4- combined with the N atoms

87

of tertiary amine to form [B-DMAPH][SO4] [Figure S1.(a) of the Supporting Information]. 1H

88

NMR (500 MHz, DMSO) δ ppm 8.32 (d, J = 7.8 Hz, 2H, cation N=CH-C=C), 7.04 (d, J = 7.8 Hz,

89

2H, cation N=C-CH=C), 4.17 (t, J = 7.2 Hz, 2H, cation N-CH2-C-C-C), 3.19 (s, 6H, cation

90

N-CH3), 1.77-1.70 (m, 2H, cation N-C-CH2-C-C), 1.28-1.19 (m, 2H, cation N-C-C-CH2-C), 0.90

91

(t, J = 7.4 Hz, 3H, cation N-C-C-C-CH3) (Figure S2 of the Supporting Information). FT-IR: v

92

(cm-1) 2958, 2926, 2872, 1648, 1566, 1382, 1171, 1033, 748.

93

1-hexyl-4-dimethylaminopyridinium

hydrogen

sulfate

[H-DMAP][HSO4]:

The

94

preparation steps of [H-DMAP][HSO4] were the same as those of [B-DMAP][HSO4] except that

95

n-butyl bromide was replaced with 1-bromohexane. The pH of a 0.1 mol/L [H-DMAP][HSO4]

96

aqueous solution is 1.28, which suggests that a part of H+ from HSO4- combined with the N atoms

97

of tertiary amine to form [H-DMAPH][SO4] [Figure S1.(b)]. 1H NMR (500 MHz, DMSO) δ ppm

98

8.31 (d, J = 7.8 Hz, 2H, cation N=CH-C=C), 7.04 (d, J = 7.8 Hz, 2H, cation N=C-CH=C), 4.16 (t,

99

J = 7.2 Hz, 2H, cation N-CH2-C-C-C-C-C), 3.19 (s, 6H, cation N-CH3), 1.78-1.71 (m, 2H, cation

100

N-C-CH2-C-C-C-C), 1.29-1.20 (m, 6H, cation N-C-C-CH2-CH2-CH2-C), 0.86 (dd, J = 9.0, 5.1 Hz,

101

3H, cation N-C-C-C-C-C-CH3) (Figure S2). FT-IR: v (cm-1) 2958, 2926, 2858, 1648, 1566, 1382,

102

1171, 1033, 728.

103

1-octyl-4-dimethylaminopyridinium

hydrogen

sulfate

[O-DMAP][HSO4]:

The

104

preparation steps of [O-DMAP][HSO4] were the same as those of [B-DMAP][HSO4] except that

105

n-butyl bromide was replaced with 1-bromooctane. The pH value of a 0.1 mol/L

106

[O-DMAP][HSO4] aqueous solution is 1.28, which suggests that a part of H+ from HSO4-

107

combined with the N atoms of tertiary amine to form [O-DMAPH][SO4] [Figure S1.(c)]. 1H NMR

108

(500 MHz, DMSO) δ ppm 8.31 (d, J = 7.7 Hz, 2H, cation N=CH-C=C), 7.04 (d, J = 7.7 Hz, 2H,

109

cation N=C-CH=C), 4.16 (t, J = 7.2 Hz, 2H, cation N-CH2-C-C-C-C-C-C-C), 3.19 (s, 6H, cation

110

N-CH3), 1.79-1.70 (t, 2H, cation N-C-CH2-C-C-C-C-C-C), 1.30-1.19 (m, 10H, cation

111

N-C-C-CH2-CH2-CH2-CH2-CH2-C), 0.86 (t, J = 6.9 Hz, 3H, cation N-C-C-C-C-C-C-C-CH3)

112

(Figure S2). FT-IR: v (cm-1) 2958, 2926, 2856, 1648, 1566, 1382, 1171, 1033, 724.

113

1-octyl-4-dimethylaminopyridinium

methanesulfonate

[O-DMAP][CH3SO3]:

The

114

preparation steps of [O-DMAP][CH3SO3] were the same as those of [O-DMAP][HSO4] except

115

that sulfuric acid was replaced with methanesulfonic acid. 1H NMR (500 MHz, DMSO) δ ppm

116

8.32 (d, J = 6.3 Hz, 2H, cation N=CH-C=C), 7.04 (d, J = 7.7 Hz, 2H, cation N=C-CH=C), 4.16 (t,

117

J = 7.2 Hz, 2H, cation N-CH2-C-C-C-C-C-C-C), 3.19 (s, 6H, cation N-CH3), 2.41 (s, 3H, anion

118

CH3-SO3), 1.79-1.69 (m, 2H, cation N-C-CH2-C-C-C-C-C-C), 1.29-1.19 (m, 10H, cation

119

N-C-C-CH2-CH2-CH2-CH2-CH2-C), 0.85 (t, J = 6.9 Hz, 3H, cation N-C-C-C-C-C-C-C-CH3)

120

(Figure S2). FT-IR: v (cm-1) 2955, 2924, 2859, 1650, 1565, 1382, 1167, 1030, 759, 725.

121

1-octyl-4-dimethylaminopyridinium trifluoromethanesulfonate [O-DMAP][CF3SO3]:

122

The preparation steps of [O-DMAP][CF3SO3] were the same as those of [O-DMAP][HSO4]

123

except that sulfuric acid was replaced with trifluoromethanesulfonic acid. 1H NMR (500 MHz,

124

DMSO) δ ppm 8.30 (d, J = 7.8 Hz, 2H, cation N=CH-C=C), 7.03 (d, J = 7.8 Hz, 2H, cation

125

N=C-CH=C), 4.15 (t, J = 7.2 Hz, 2H, cation N-CH2-C-C-C-C-C-C-C), 3.19 (s, 6H, cation N-CH3),

126

1.79-1.71

127

N-C-C-CH2-CH2-CH2-CH2-CH2-C), 0.86 (t, J = 7.0 Hz, 3H, cation N-C-C-C-C-C-C-C-CH3)

(m,

2H,

cation

N-C-CH2-C-C-C-C-C-C),

1.30-1.18

(m,

10H,

cation

128

(Figure S2). FT-IR: v (cm-1) 2964, 2929, 2859, 1650,1564, 1382, 1258, 1168, 1030, 758, 725, 632.

129

2.3.2 Preparation and characterization of -SO3H-functionalized ILs

130

1-propylsulfonate-4-dimethylaminopyridinium hydrogen sulfate [PS-DMAP][HSO4]: A

131

0.1 mol DMAP compound was dissolved in ethyl acetate. Equimolar 1,3-propanesulfonate was

132

added dropwise into the mixture at room temperature under stirring. The reaction was carried out

133

at 50

134

washing with ethyl acetate, and vacuum drying, the zwitterion was obtained. Then, the zwitterion

135

was dissolved in deionized water. Under vigorous stirring, equimolar concentrated sulfuric acid

136

was added to the abovementioned solution, and the mixture was stirred at 80

137

removing water, repeated washing with ethyl acetate, and vacuum drying, [PS-DMAP][HSO4] was

138

obtained. 1H NMR (500 MHz, DMSO) δ ppm 8.31 (d, J = 7.8 Hz, 2H, cation N=CH-C=C), 7.04

139

(d, J = 7.8 Hz, 2H, cation N=C-CH=C), 4.29 (t, J = 6.8 Hz, 2H, cation N-CH2-C-C-SO3), 3.19 (s,

140

6H, cation N-CH3), 2.39 (t, J = 7.3 Hz, 2H, cation N-C-C-CH2-SO3), 2.11-2.02 (m, 2H, cation

141

N-C-CH2-C-SO3) (Figure S2). FT-IR: v (cm-1) 2930, 1652, 1571, 1388, 1162, 1018, 775.

142

with stirring for 24 h to produce a white solid zwitterion. After suction filtration, repeated

for 8 h. After

1-propylsulfonate-4-dimethylaminopyridinium methanesulfonate [PS-DMAP][CH3SO3]:

143

The preparation steps of [PS-DMAP][CH3SO3] were the same as those of [PS-DMAP][HSO4]

144

except that sulfuric acid was replaced with methanesulfonic acid. 1H NMR (500 MHz, DMSO) δ

145

ppm 8.31 (d, J = 7.8 Hz, 2H, cation N=CH-C=C), 7.04 (d, J = 7.8 Hz, 2H, cation N=C-CH=C),

146

4.29 (t, J = 6.8 Hz, 2H, cation N-CH2-C-C-SO3), 3.19 (s, 6H, cation N-CH3), 2.46 (s, 3H, anion

147

CH3-SO3), 2.40 (t, J = 7.3 Hz, 2H, cation N-C-C-CH2-SO3), 2.12-2.02 (m, 2H, cation

148

N-C-CH2-C-SO3) (Figure S2). FT-IR: v (cm-1) 2960, 1652, 1571, 1388, 1162, 1018, 773, 594.

149

Table 2. Information on prepared ILs ILs

Chemical structure

Mass fraction purity a

Water content / ppm b

[B-DMAP][HSO4]

N

> 99%

200

> 99%

150

C8H17

> 99%

175

C8H17

> 99%

165

> 99%

200

C3H6SO3H

> 98%

230

C3H6SO3H

> 99%

210

N

C4H9

HSO4

[H-DMAP][HSO4]

N

N

C6H13

HSO4

[O-DMAP][HSO4]

N

N HSO4

[O-DMAP][CH3SO3]

N

N

CH3SO3

[O-DMAP][CF3SO3]

N

N

C8H17

CF3SO3

[PS-DMAP][HSO4]

N

N HSO4

[PS-DMAP][CH3SO3]

N

N

CH3SO3

150 151

a

The purities of the prepared ILs were estimated by 1H-NMR.

b

The water content was determined by Karl Fischer titration.

152

2.4 Measurement of density and viscosity

153

For the density measurement (328.15~358.15 K, p=0.101 MPa), the density of ILs was

154

determined via a vibrating tube densimeter Anton Paar DMA 5000 with ±5.0×10-3 kg/m3 of

155

repeatability. For the viscosity measurement (323.15~368.15 K, p=0.101 MPa), the viscosity of

156

ILs was measured by a DV-S digital display rotational viscometer with a relative precision and

157

reproducibility in dynamic viscosity of ±1.0% and ±0.2%, respectively.

158

2.5 Thermal stability and thermal behavior

159

The decomposition temperature of ILs was determined by the simultaneous thermal analyzer,

160

(Netzsch STA 449C Jupiter®), and the sensitivity of the balance was 0.1 µg in the full range. The

161

sample was put into a crucible with a continuous nitrogen flow (20 mL/min) and measured by

162

scanning the temperature from room temperature to 873 K at a heating rate of 10 K/min. The

163

obtained decomposition temperature (Td) is the onset temperature, which is the intersection of the

164

baseline below the decomposition temperature with the tangent to the mass loss versus the

165

temperature plots in the TGA profiles. The thermal behavior was analyzed using a differential

166

scanning calorimeter (Netzsch DSC214). The ILs were put into sealed crucibles and evaluated in

167

the temperature range of 193.15-333.15 K using the continuous method with a heating rate of 5

168

K/min and a nitrogen gas flow of 40 mL/min. Glass transition temperature (Tg) is the midpoint of

169

a small heat capacity change, and melting temperature (Tm) is the curve peak.

170

3. Results and discussion

171

3.1 Analysis of the DMAP structure

172

To better understand the DMAP structure, molecular simulations were used to obtain its

173

electrostatic potential (ESP) and nature bond orbital (NBO) charges, as shown in Figure 1.

174

Figure1.(a) shows that DMAP has pyridine and tertiary amino groups, which allows to synthesize

175

various ILs. In the ESP analysis [Figure 1.(b)], red color represents a negative charge (high

176

electron density, nucleophilic region), blue color indicates a positive charge (low electron density,

177

electrophilic region), and green and yellow represent a neutral level. The high electron density

178

around the N atom of pyridine suggests that it can combine with electrophilic reagents (e.g.,

179

1-butylbromide and 1,3-propanesulfonate) to prepare ILs. Meanwhile, the positive charge around

180

methyl hydrogen of tertiary amine indicates that the N atom of tertiary amine has a considerable

181

amount of negative charge, which leads to nucleophilic reactions. Figure 1.(c) shows the NBO

182

charges of all atoms in the DMAP molecule and the p-π conjugative effect between the N atom of

183

tertiary amine and pyridine ring. The value of negative charge of the N atom of pyridine is slightly

184

larger than that of the N atom of tertiary amine, and the steric hindrance of the N atom of pyridine

185

is much smaller than that of the N atom of tertiary amine. Thus, in nucleophilic reactions, the

186

reactivity of the N atom of pyridine is higher than that of the N atom of tertiary amine.

187

Furthermore, to investigate the number of reactive sites involved in the nucleophilic reaction,

188

1

189

with the structure, which suggests that only one site (N atom of pyridine) participates in the

190

reaction at the molar ratio (DMAP/1-butyl bromide) of 1:1. The abovementioned analysis shows

191

that DMAP can be used as the matrix for ILs preparation.

H-NMR of [B-DMAP][Br] was used as an example, as shown in Figure 2. The peak is consistent

(a) Molecular structure

(b) ESP

192 193

Figure 1. Structure of DMAP

194 195

Figure 2. 1H-NMR of [B-DMAP][Br]

(c) NBO charges

196

3.2 Density

197

The density (ρ) of ILs was measured in the temperature range of 328.15~358.15 K, p=0.101

198

MPa, and the results are shown in Table S1 and Figure 3. For all studied ILs, the density decreases

199

with an increase in the temperature, which agrees with the previous reports [11,17-19]. At the

200

same temperature, the density decreases in the following order: [PS-DMAP][HSO4] >

201

[PS-DMAP][CH3SO3] > [B-DMAP][HSO4] > [H-DMAP][HSO4] >

202

[O-DMAP][CH3SO3] > [O-DMAP][CF3SO3], which is primarily determined by the molar mass,

203

molar volume, and the interactions of cation and anion. For [PS-DMAP][HSO4] (342 g/mol) and

204

[PS-DMAP][CH3SO3] (340 g/mol), the interaction of cation and anion in [PS-DMAP][CH3SO3] is

205

slightly larger than that in [PS-DMAP][HSO4] [Figure 4.(A) and (B)]. However, the molar volume

206

of HSO4- (53.778 cm3/mol, calculated by Gaussian 03) is smaller than that of CH3SO3- (61.376

207

cm3/mol, calculated by Gaussian 03), which may make [PS-DMAP][HSO4] have higher density

208

compared to that of [B-DMAP][HSO4]. Although the cation of [PS-DMAP][HSO4] has larger

209

volume (182.793 cm3/mol for [PS-DMAP] and 150.405 cm3/mol for [B-DMAP], calculated by

210

Gaussian 03), it possesses greater molar mass (245 g/mol and 179 g/mol), which may be a more

211

significant factor that influences density. The density comparison of [O-DMAP][HSO4],

212

[O-DMAP][CH3SO3], and [O-DMAP][CF3SO3] indicates that anion type also affects density.

213

Although [O-DMAP][CF3SO3] has the highest molar mass among the three ILs, and it has been

214

reported by Zubeir et al. [20] that fluorine-based ILs possess the high structural organization

215

efficiency which makes ILs have higher density, e.g., [Cnmim][Tf2N] > [Cnmim][TCM]. In this

216

study, the F atom of [CF3SO3]- forms strong repulsive interaction with the N atom of tertiary amine

217

of [O-DMAP] and the F atom of other [CF3SO3] , which increases spacing between the ions and

+

-

[O-DMAP][HSO4] >

218

results in the lowest density value. In [O-DMAP][HSO4], the result of pH measurement indicates

219

that a part of H+ from HSO4- combines with the N atom of tertiary amine to form

220

[O-DMAPH][SO4] [Figure S1.(a)], which increases the electrostatic attraction of anion and cation

221

in the IL and results in the highest density. Furthermore, based on the density results of

222

[B-DMAP][HSO4], [H-DMAP][HSO4], and [O-DMAP][HSO4], it can be concluded that as the

223

alkyl chain length increases, the density decreases, which agrees with the previously published

224

results [11,21-24]. Gaussian 03 was used to calculate the NBO charges of the three cations, as

225

shown in Figure 5. The results suggest that the change in alkyl chain does not affect the charge

226

distribution of cations. This observation indicates that electrostatic fields produced by ions are

227

virtually unchanged. Thus, the steric hindrance due to the alkyl chain has the highest effect on the

228

density of ILs. With an increase in the alkyl chain length, the volume of cation (150.405 cm3/mol,

229

190.20 cm3/mol, 213.273 cm3/mol for [B-DMAP]+, [H-DMAP]+, and [O-DMAP]+, respectively)

230

and steric hindrance increase, which increases distance between ions [21-24] and lowers density at

231

the macro level. Compared with other ILs with different cations, the density of

232

[O-DMAP][CF3SO3] is lower than those of [bmim][CF3SO3] [19], [C2mim][CF3SO3] [22], and

233

[bpy][CF3SO3] [25], which may be due to the larger cationic volume and stronger electrostatic

234

repulsion between the cation and anion in [O-DMAP][CF3SO3]. The stronger electrostatic

235

interactions in [B-DMAP][HSO4] may make its density higher than those of [BMIM][HSO4]

236

[26,27] and [BMPy][HSO4] [26].

1400 1350

ρ / (kg/m3)

1300 1250 1200 1150 1100 325

330

335

239

345

350

355

360

T/K

237 238

340

Figure 3. Densities of the studied ILs at different temperatures:

[PS-DMAP][HSO4];

[B-DMAP][HSO4]; [H-DMAP][HSO4]; [O-DMAP][HSO4]; [O-DMAP][CH3SO3]; [O-DMAP][CF3SO3]

240 241 242 243 244 245 246 247 248 (A)

249 250 251 2.409

252 253 1.734

254 255 256 257 258 259

[PS-DMAP][CH3SO3];

2.926

2.195

1.954 [PS-DMAP][HSO4]

260 261 262 263 264 265 266 267 268

(B)

269

Figure 4. Interactions of the cation and anion in ILs

2.491

2.170

2.879 1.617

1.963 [PS-DMAP][CH3SO3]

[B-DMAP]+

[H-DMAP]+

[O-DMAP]+

270 271

Figure 5. NBO charges of [B-DMAP]+, [H-DMAP]+, and [O-DMAP]+

272

The second-order polynomial equation was adopted to correlate the density data with the

273

temperature as follows:

(

)

ln ρ / kg/m 3 = A1 + A2 × (T / K ) + A3 × (T / K )

2

(1)

274

where A1, A2, and A3 are the fitting constants. The fitting results are shown in Table 3. All

275

correlation coefficients (R2) are greater than 0.9999, and the values of all ARD are lower than

276

0.0055%, which indicates that the mathematical model can describe the relationship of density and

277

temperature well.

278

Table 3. Fitting parameters of Eq.(1) and average relative deviation A1 / (kg/m3

ILs [PS-DMAP][HSO4] [PS-DMAP][CH3SO3] [B-DMAP][HSO4]

A2 / [kg/(m3·K)]

A3 / [kg/(m3·K2)]

R2

ARD / % a

7.3507

-1.7193×10-4

-3.9239×10-7

0.9999

0.0030

7.3321

-4

-7

0.9999

0.0055

-8

0.9999

0.0015

-7

0.9999

0.0042

-7

1.0000

0.0009

-9

0.9999

0.0039

-7

0.9999

0.0050

7.3129

[H-DMAP][HSO4]

7.3011

[O-DMAP][HSO4]

7.2551

[O-DMAP][CH3SO3]

7.2493

[O-DMAP][CF3SO3]

7.2125

-2.7008×10

-4

-5.0907×10

-4

-7.1385×10

-4

-6.2035×10

-4

-5.8451×10

-4

-3.5722×10

-3.2816×10 9.7078×10 3.0150×10 1.2343×10

-6.3914×10 -3.9093×10

279

a

280 281

where n is the number of data points; the superscripts “exp” and “cal” represent the experimental values and

282

According to the results in Table 3, the isobaric thermal expansion coefficient (αp, K-1) can be

283

100 n ρ k − ρ k ∑k =1 n ρ kexp exp

ARD =

cal

calculated values of density, respectively.

obtained from Eq. (2):

1  ∂ρ   ∂ ln ρ  α p = −   = −  = −( A2 + 2 ⋅ A3 ⋅ T ) ρ  ∂T  p  ∂T  p

(2)

284

where ρ, T, and p indicate the density of the studied ILs, kg/m3; absolute temperature, K; and the

285

pressure, 0.101 MPa, respectively. A2 and A3 are the fitting parameters in Table 3.

286 287

Based on the density data, the molecular volumes (Vm, nm3) of ILs can be calculated by the following equation:

Vm =

288

M × 10 24 ρ ⋅ NA

(3)

where M is the molar mass, kg/kmol; NA indicates the Avogadro's constant, 6.02214129×1023

289

mol-1; ρ represents the density of the studied ILs, kg/m3. The lattice potential energy (UPOT, kJ/mol)

290

can be estimated according to the following relationship [11,28-31]:

 α  U POT = 2 × I ×  + β  3  Vm 

(4)

291

where α, β are the fitting parameters; I represents the ionic strength. For the simple

292

ionic compounds MX (cation:anion=1:1), the values of α, β, and I are 117.3 kJ·nm/mol, 51.9

293

kJ/mol, and 1, respectively [11,33]. The values of αp, Vm, and UPOT at 333.15 K are shown in Table

294

4.

295

Table 4. Values of αp, Vm, and UPOT at 333.15 K ILs

104αp / K-1

Vm / nm3

UPOT / (kJ/mol)

[PS-DMAP][HSO4]

4.33

0.4034±0.0020

421.3±0.7

[PS-DMAP][CH3SO3]

4.89

0.4191±0.0017

417.3±0.6

[B-DMAP][HSO4]

4.44

0.3584±0.0014

434.1±0.6

[H-DMAP][HSO4]

5.13

0.4179±0.0017

417.6±0.6

[O-DMAP][HSO4]

5.38

0.4724±0.0019

405.0±0.5

[O-DMAP][CH3SO3]

5.89

0.4734±0.0019

404.8±0.5

[O-DMAP][CF3SO3]

6.18

0.5530±0.0022

389.6±0.5

296 297

The uncertainties of the molecular volume and the lattice potential energy were calculated using uncertainty

298

The isobaric thermal expansion coefficients of studied ILs are in the range of 4.33 –

299

6.18×10-4 K-1. For -SO3H-functionalized ILs, the isobaric thermal expansion coefficient follows

300

the order: [PS-DMAP][HSO4] < [PS-DMAP][CH3SO3]. Despite the effect of an anion on the

301

isobaric thermal expansion coefficient, the cation moieties influence the isobaric thermal

302

expansion coefficient in the following order: [B-DMAP][HSO4] < [H-DMAP][HSO4] <

303

[O-DMAP][HSO4]. This result is obtained because the IL with a large cation has larger distance

304

between ions, which weakens electrostatic interaction and facilitates their thermal expansion

305

[32,33]. The isobaric thermal expansion coefficient increases in the following anion order:

306

[O-DMAP][HSO4] < [O-DMAP][CH3SO3] < [O-DMAP][CF3SO3], which is attributed to the

propagation.

307

interactions

between

anions

and

cations,

specifically,

the

attractive

308

[O-DMAP][HSO4] and the repulsive interaction of [O-DMAP][CF3SO3].

interaction

of

309

A molecular volume was used to express the volumetric behavior of studied ILs. The

310

molecular volume increases with an increase in the alkyl chain length (0.3584 to 0.4724 nm3),

311

with a contribution of (0.0285 ± 0.0013) nm3 per methylene group (-CH2-), which is comparable

312

with the increment of (0.0281 ± 0.0006) [11] and (0.0269 ± 0.0001) nm3 [34]. The effect of anion

313

on

314

[O-DMAP][CF3SO3]. It can be concluded that when the alkyl chain is sufficiently long (e.g.,

315

-C8H17), the electrostatic attraction of the cation and anion does not affect molecular volume,

316

which results in a similar molecular volume of [O-DMAP][HSO4] and [O-DMAP][CH3SO3].

317

However, the electrostatic repulsion of the cation and anion clearly affects the molecular volume.

molecular

volume

follows

the

order:

[O-DMAP][HSO4]≈[O-DMAP][CH3SO3]

<

318

It is observed that as the alkyl chain increases, the lattice potential energy decreases, which

319

indicates a lower structural organization efficiency when IL has a longer alkyl chain [35-37]. An

320

increase in the alkyl chain enhances steric hindrance and weaken the electrostatic interaction

321

between the cation and anion. The effect of anion on the lattice potential energy follows the order:

322

[O-DMAP][HSO4]≈[O-DMAP][CH3SO3] > [O-DMAP][CF3SO3], and the explanation is the same

323

as that for the molecular volume. For -SO3H-functionalized ILs, the lattice potential energy of

324

[PS-DMAP][HSO4] is higher than that of [PS-DMAP][CH3SO3]. In addition, the lattice potential

325

energies of studied ILs were determined to be much lower than those of fused salts, such as CsI

326

(613 kJ/mol at 298.15 K), which makes them have low melting points [35,36].

327

3.3 Viscosity

328

The viscosity of seven ILs was determined in the temperature range of 323.15-368.15 K,

329

0.101 MPa, and the results are shown in Table S2 and Figure 6. As expected, the monotonic

330

decrease in viscosity is observed with an increase in the temperature. Compared to the density, the

331

viscosity has a larger decrease degree with an increase in the temperature. It is clear that the

332

chemical structures of cations and anions considerably affect the strength of van der Waals

333

interactions and the capacity to form hydrogen bonds, which affects the viscosity of the studied

334

ILs [32,38]. For -SO3H-functionalized ILs, their viscosities are higher than those of non-SO3H

335

functionalized ILs, which is attributed to stronger electrostatic interactions between cations and

336

anions. It is determined that as the alkyl chain length of the cation increases, the viscosity

337

increases, which is similar to the previously published results [11,32,35]. The long alkyl chain

338

leads to the strong van dar Waals interactions as well as large bulkiness, which increases the

339

viscosity. For [O-DMAP][HSO4], [O-DMAP][CH3SO3], and [O-DMAP][CF3SO3], the long chain

340

of the cation (-C8H17) occupies enough space to reduce the effect of electrostatic repulsion on

341

viscosity, which results in similar viscosity values between [O-DMAP][CF3SO3] and

342

[O-DMAP][CH3SO3]. Different from electrostatic repulsion, the effect of electrostatic attraction

343

on viscosity is more clear, which makes [O-DMAP][HSO4] have maximum viscosity value among

344

the three ILs. Compared to the previously reported results, the viscosity of [O-DMAP][CF3SO3] is

345

higher than those of [bmim][CF3SO3] [19,39,40] and [EMIM][CF3SO3] [41] due to the longer

346

alkyl chain in [O-DMAP]+. Similarly, [O-DMAP][CH3SO3] has higher viscosity than that of

347

[EMIM][CH3SO3] [41]. The viscosity of [PS-DMAP][CH3SO3] is higher than that of

348

[EMIM][CH3SO3] [41] due to the stronger electrostatic interactions between the cation and anion.

18000 16000 14000

η / (mPa· s)

12000 10000 8000 6000 4000 2000 0 320

330

340

350

360

370

T/K

349 350

Figure 6. Viscosities of the studied ILs at different temperatures. The lines are the correlative values of the VFT

351

equation, and the symbols represent the experimental values:

352 353 354

[O-DMAP][CF3SO3];

[O-DMAP][CH3SO3];

[B-DMAP][HSO4]; [H-DMAP][HSO4]; [O-DMAP][HSO4]; [PS-DMAP][CH3SO3]; [PS-DMAP][HSO4]

The temperature dependence of viscosity can be satisfactorily described by the Vogel-Fulcher-Tamman (VFT) model [19,42,43]:  B2    T − B3 

η = B1 ⋅ exp

(5)

355

where η is the viscosity, mPa·s; T represents the absolute temperature, K; B1, B2, and B3 are the

356

fitting parameters that are estimated according to the experimental data shown in Table 5. All

357

correlation coefficients (R2) are greater than 0.9997, and the values of all ARD are lower than

358

1.9000% except for [PS-DMAP][CH3SO3] (4.4002%), which indicates a good agreement between

359

the experimental data and the VFT model.

360

Table 5. Fitting parameters of the VFT model, average relative deviation, and activation energy ILs

B1 / (mPa·s)

B2 / K

B3 / K

R2

ARD / % a

Eη / (kJ/mol) b

[PS-DMAP][HSO4]

0.06261

1616.44

193.35

0.9998

1.8712

76.32

[PS-DMAP][CH3SO3]

0.00125

2561.34

162.40

0.9997

4.4002

81.07

[B-DMAP][HSO4]

0.04255

1331.67

200.76

0.9999

1.0952

70.11

[H-DMAP][HSO4]

0.07955

1138.98

216.22

0.9999

0.8247

76.87

[O-DMAP][HSO4]

0.04369

1333.18

206.41

0.9999

0.4867

76.59

[O-DMAP][CH3SO3]

0.00054

2651.62

120.73

0.9999

0.5095

54.23

[O-DMAP][CF3SO3]

0.00378

2032.55

134.09

0.9999

0.4196

47.33

361

a

362 363 364

where n is the number of data points; the superscripts “exp” and “cal” represent the experimental and calculated

365

Based on the results of the VFT equation, the activation energy of viscous flow (Eη) was

366

ARD =

exp cal 100 n η k − η k ∑k =1 exp n ηk

values of viscosity, respectively. b

T=333.15 K

obtained using the following equation:  T  ∂(lnη )  Eη = R ⋅ = R ⋅ B2 ⋅  ∂(1/ T )  T − B3 

2

(6)

367

where B2, B3 are the fitting parameters of the VFT model; R represents the ideal gas constant,

368

8.314 J/(mol·K). The activation energy of viscous flow at 333.15 K is shown in Table 5. It is easy

369

to determine that the order of activation energy at 333.15 K does not follow the same trend as that

370

of the viscosity, and similar results have been reported by Rocha et al. [11] and Yadav et al. [19].

371

3.4 Thermal stability and thermal behavior

372

The thermogravimetric analysis (TGA) of the studied ILs was conducted from room

373

temperature to 873 K. The TGA curves are shown in Figure 7, and the decomposition

374

temperatures (Td) are shown in Table 6. For [B-DMAP][HSO4], [H-DMAP][HSO4], and

375

[O-DMAP][HSO4], Td increases with an increase in the alkyl chain length of the cation, which

376

agrees with the previously published reports [35,44]. In addition, it is determined that the IL based

377

on [CF3SO3]- has high Td (613 K), and the same finding ([C2mim][CF3SO3]: 713 K) is also

378

reported in the literature [45]. Furthermore, it can be concluded that the temperatures of density

379

measurements (328.15~358.15 K, p=0.101 MPa) and viscosity measurements (323.15 K~368.15

380

K, p=0.101 MPa) are much lower than the Td of all ILs (≥ 526.15 K), which meets the

381

requirements of thermophysical property measurement.

382

The glass transition temperature (Tg) and melting temperature (Tm) obtained from DSC are

383

also shown in Table 6. It can be concluded that the electrostatic interactions between anions and

384

cations in ILs considerably affect the Tg based on the results of -SO3H-functionalized ILs >

385

non-SO3H functionalized ILs, [PS-DMAP][CH3SO3] > [PS-DMAP][ HSO4] [Figures 4 (A) and

386

(B)],

387

[O-DMAP][CF3SO3] (electrostatic repulsion). As the alkyl chain length increases, the Tg increases

388

([O-DMAP][HSO4] > [H-DMAP][HSO4] > [B-DMAP][HSO4]) because the larger substituent

389

group leads to the higher internal rotation resistance for ILs. The effects of the electrostatic

390

interactions between anions and cations on Tm are the same as the trend of Tg. However, compared

391

with Tg, Tm decreases with an increase in the alkyl chain length. This can be explained by the

392

increasing disruption of crystal packing because when the chain length is extended, the increased

393

van der Waals interactions between larger components become overridden [45,46].

and

[O-DMAP][HSO4]

(electrostatic

attraction)

>

[O-DMAP][CH3SO3]

100

Mass fraction / %

80

60 [PS-DMAP][CH3SO3] [PS-DMAP][HSO4] 40

[O-DMAP][HSO4] [O-DMAP][CH3SO3]

20

[O-DMAP][CF3SO3] [H-DMAP][HSO4] [B-DMAP][HSO4]

0 400

500

600

700

800

T/K

394 395

Figure 7. TGA curves of the studied ILs

396

Table 6. Tg, Tm, and Td of the studied ILs ILs

Tg / K

Tm / K

Td / K

[B-DMAP][HSO4]

219.05

313.15

596.15

[H-DMAP][HSO4]

221.25

309.35

603.15

[O-DMAP][HSO4]

223.55

307.15

607.15

[O-DMAP][CH3SO3]

218.95

303.35

526.15

>

397

[O-DMAP][CF3SO3]

205.65

292.65

700.15

[PS-DMAP][HSO4]

243.85

314.45

563.15

[PS-DMAP][CH3SO3]

253.15

315.75

610.15

4. Conclusions

398

In this study, a series of new DMAP-based ILs was prepared and characterized, and their

399

thermophysical properties, density, and viscosity were investigated as a function of temperature.

400

By analyzing DMAP, it is determined that DMAP has pyridine and tertiary amino groups, which

401

allows to synthesize various ILs. However, due to the larger electron density and smaller steric

402

hindrance, the reactivity of the N atom of pyridine is higher than that of the N atom of tertiary

403

amine in nucleophilic reactions. Meanwhile, the 1H-NMR result of [B-DMAP][Br] reveals that

404

only one N atom (from the pyridine group) participates in the nucleophilic reaction, which indicates

405

that DMAP can serve as a matrix for the preparation of ILs. Empirical equations were adopted to

406

describe the temperature dependence of density and viscosity, specifically, the second-order

407

polynomial equation for density and the VFT model for viscosity. It is determined that both

408

density and viscosity decrease with an increase in the temperature. However, viscosity has a larger

409

decrease degree as the temperature increases. For density, at the fixed temperature, the decreasing

410

rank is as follows: [PS-DMAP][HSO4] > [PS-DMAP][CH3SO3] > [B-DMAP][HSO4] >

411

[H-DMAP][HSO4] > [O-DMAP][HSO4] > [O-DMAP][CH3SO3] > [O-DMAP][CF3SO3], which is

412

greatly affected by the molar mass, molar volume, and interactions of the cation and anion. Some

413

physicochemical properties (e.g., isobaric thermal expansion coefficient, molecular volume, and

414

lattice potential energy) were obtained from the density data, which are closely related to the

415

molar mass, molar volume, and interactions of the cation and anion. Regarding the viscosity, it can

416

be seen that as the alkyl chain length of the cation increases, viscosity increases. The long alkyl

417

chain leads to the strong van dar Waals interactions and large bulkiness, which increases viscosity.

418

The anion type affects the viscosity of ILs by attractive and repulsive interactions with the cation.

419

It is determined that the order of Eη derived from the VFT model does not follow the same trend as

420

that of the viscosity. Furthermore, all of the studied ILs possess high thermal stability (Td ≥ 526.15

421

K), and the alkyl chain length positively affects Tg and negatively affects Tm, respectively. In

422

conclusion, the investigation of thermophysical properties of DMAP-based ILs promotes their

423

application in many fields.

424

Acknowledgements

425

We acknowledge the financial support for this work from the National Natural Science

426

Foundation of China (Nos. 21576053 and Nos. 21878054) and the Natural Science Foundation of

427

Fujian Province (No. 2016J01689).

428

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Cation

with

Various

Anions

and

the

Highlights 

The structure of 4-dimethylaminopyridine was analyzed by molecular simulation.



A series of new DMAP based ILs were prepared and characterized.



The density and viscosity of prepared ILs were investigated for the first time.



The density data can be correlated well by second order polynomial equation.



The VFT model was used to fit viscosity data.

-1-

Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Thermophysical properties of 4-dimethylaminopyridine-based ionic liquids”.