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”.