Journal Pre-proof Thermophysical properties of 4-dimethylaminopyridine-based ionic liquids Dongren Cai, Nisakorn Saengprachum, Zhipeng Lin, Ting Qiu PII:
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.
Thermophysical properties of 4-dimethylaminopyridine-based
Dongren Cai, Nisakorn Saengprachum, Zhipeng Lin, Ting Qiu∗
Fujian Universities Engineering Research Center of Reactive Distillation, College of Chemical
Engineering, Fuzhou University, Fuzhou 350116, Fujian, China
ABSTRACT: In this study, a molecular simulation (Gaussian03) was used to analyze the
chemical structure of 4-dimethylaminopyridine (DMAP) to investigate its use for the synthesis of
ionic liquids (ILs). Based on DMAP, a series of new ILs was prepared and characterized by fourier
transform infrared spectroscopy (FT-IR),
H nuclear magnetic resonance (1H-NMR),
thermogravimetric analysis (TGA), and differential scanning calorimeter (DSC). The density data
of the studied ILs were measured from 328.15 K to 358.15 K, p=0.101 MPa. It is shown that
density decreases with an increase in the temperature and the alkyl chain length of cations. In
addition, density is affected by the anion type. Furthermore, the volume of ions, NBO charges, and
the interactions of anions and cations were calculated by Gaussian03 to explain the obtained
density results. The second-order polynomial equation was adopted to correlate the relationship of
density and temperature. Then, an isobaric thermal expansion coefficient, molecular volume, and
lattice potential energy were obtained from the density data. Viscosity was determined from
323.15 K to 368.15 K, p=0.101 MPa. It is determined that the viscosity of ILs decreases with an
increase in the temperature but increases with an increase in the alkyl chain length of cations. The
temperature dependence of viscosity was described by the Vogel-Fulcher-Tamman (VFT) model,
and the activation energy of viscous flow (Eη) was obtained. Furthermore, the decomposition
Corresponding author. Tel.: +86 13705945511. E-mail address: [email protected]
temperature (Td), glass transition temperature (Tg), and melting temperature (Tm) of ILs were
obtained from the results of TGA and DSC.
Keywords: Ionic liquids, 4-Dimethylaminopyridine, Density, Viscosity, Thermal stability
In recent years, ionic liquids (ILs) have received considerable attention in many research
fields due to their superior physicochemical properties [1-6]. ILs are composed of only cations and
anions; ILs are liquids at room temperature (melting point < 100 ), which is different from
conventional ionic compounds [7,8]. Because of their ionic nature, ILs always exhibit
nonvolatility, wide liquid range, good thermal stability, high conductivity, and tunable polarity.
These properties allow to use ILs as solvents in organic reactions and as efficient catalysts by
designing cations and anions. For example, by combining molecular simulations and experiments,
Harini et al.  designed task-specific ILs to be used as solvents for the extraction of a
pharmaceutical intermediate. Qiu et al.  prepared -SO3H-functionalized ILs using
N,N-dimethylcyclohexylamine as the matrix for biodiesel production from coconut oil. The result
showed that under optimal conditions, the biodiesel yield can reach 98.7%, and there was no
considerable decrease in the catalytic activity of IL after being used for 6 cycles. The design and
application of ILs need reliable and systematic data on thermophysical properties (e.g., density,
viscosity, and thermal stability) to better understand the behaviors and interactions of ions in ILs
[11,12]. However, currently, there is still a lack of systematical knowledge on thermophysical
properties, especially for 4-dimethylaminopyridine (DMAP)-based ILs. DMAP is essential in
organic synthesis and medicine [13,14]. Some studies have shown that DMAP is an efficient phase
transfer and nucleophilic acylation catalyst that can be used for interfacial polymerization
reactions . DMAP contains pyridine and tertiary amino groups, which offers various
possibilities for the synthesis of ILs. Currently, there are few reports on DMAP-based ILs. Thus, it
is essential to develop DMAP-based ILs and further investigate their thermophysical properties,
which will expand the types of existing ILs and speeding up their practical application.
In this study, the structure of DMAP was carefully analyzed by molecular simulations, and a
series of new DMAP-based ILs was prepared and characterized. The thermophysical properties
(e.g., density, viscosity, and thermal stability) were investigated. The effect of cation and anion
types on density and viscosity were discussed. The isobaric thermal expansion coefficient,
molecular volume, lattice energy, and the activation energy of viscous flow, which can reflect the
interactions between cations and anions in ILs, were obtained from the experimental data.
Furthermore, the volume of ions, NBO charges, and interactions of anions and cations were
calculated by Gaussian03 to explain the abovementioned results.
2.1 Computational details
Computational chemistry was used to analyze the related molecular structures. The
computational details were as follows. All structures were fully optimized using the Gaussian03
package at the B3LYP/6-311+G(d, p) level of theory with ultrafine integration grids .
All chemicals used in this experiment are shown in Table 1 Table 1. Chemicals used for this experiment a
2.3 Preparation and characterization of ILs
All chemicals were used without purification
The ILs investigated in this study are shown in Table 2. Two methods were adopted to
prepare ILs (i.e., non-SO3H and -SO3H functionalized ILs).
2.3.1 Preparation and characterization of non-SO3H functionalized ILs
1-butyl-4-dimethylaminopyridinium hydrogen sulfate [B-DMAP][HSO4]: A 0.1 mol
DMAP compound was dissolved in ethyl acetate. Equimolar 1-butyl bromide was added dropwise
into the mixture at room temperature under full stirring. The reaction was carried out at 70
stirred for 48 h. After suction filtration, repeated washing of ethyl acetate, and vacuum drying, a
white solid [B-DMAP][Br] [1H NMR (500 MHz, DMSO) δ ppm 8.34 (d, J = 7.8 Hz, 2H, cation
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
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
= 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
2), FT-IR: v (cm-1) 2960, 2935, 2874, 1650, 1567, 1382, 660] was obtained. Then, the white solid
was dissolved in deionized water. Under vigorous stirring in an ice water bath, equimolar
concentrated sulfuric acid and one half of Ag2O was slowly added to the abovementioned solution,
and the mixture was stirred at room temperature. After the reaction, a yellowish solid was removed
by suction filtration, and the liquid was removed via rotary evaporation (363.15 K, -0.1 MPa).
After repeated washing with ethyl acetate and vacuum drying, [B-DMAP][HSO4] was obtained.
The obtained [B-DMAP][HSO4] was titrated by silver nitrate with nitric acid acidification, and
there was no yellowish precipitation in the titration process, which suggests that there is basically
no bromide ion left in [B-DMAP][HSO4]. The pH value of a 0.1 mol/L [B-DMAP][HSO4]
aqueous solution is 1.27, which suggests that a part of H+ from HSO4- combined with the N atoms
of tertiary amine to form [B-DMAPH][SO4] [Figure S1.(a) of the Supporting Information]. 1H
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,
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
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
(t, J = 7.4 Hz, 3H, cation N-C-C-C-CH3) (Figure S2 of the Supporting Information). FT-IR: v
(cm-1) 2958, 2926, 2872, 1648, 1566, 1382, 1171, 1033, 748.
preparation steps of [H-DMAP][HSO4] were the same as those of [B-DMAP][HSO4] except that
n-butyl bromide was replaced with 1-bromohexane. The pH of a 0.1 mol/L [H-DMAP][HSO4]
aqueous solution is 1.28, which suggests that a part of H+ from HSO4- combined with the N atoms
of tertiary amine to form [H-DMAPH][SO4] [Figure S1.(b)]. 1H NMR (500 MHz, DMSO) δ 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), 4.16 (t,
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
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,
3H, cation N-C-C-C-C-C-CH3) (Figure S2). FT-IR: v (cm-1) 2958, 2926, 2858, 1648, 1566, 1382,
1171, 1033, 728.
preparation steps of [O-DMAP][HSO4] were the same as those of [B-DMAP][HSO4] except that
n-butyl bromide was replaced with 1-bromooctane. The pH value of a 0.1 mol/L
[O-DMAP][HSO4] aqueous solution is 1.28, which suggests that a part of H+ from HSO4-
combined with the N atoms of tertiary amine to form [O-DMAPH][SO4] [Figure S1.(c)]. 1H NMR
(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,
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
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
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)
(Figure S2). FT-IR: v (cm-1) 2958, 2926, 2856, 1648, 1566, 1382, 1171, 1033, 724.
preparation steps of [O-DMAP][CH3SO3] were the same as those of [O-DMAP][HSO4] except
that sulfuric acid was replaced with methanesulfonic acid. 1H NMR (500 MHz, DMSO) δ ppm
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,
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
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
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)
(Figure S2). FT-IR: v (cm-1) 2955, 2924, 2859, 1650, 1565, 1382, 1167, 1030, 759, 725.
1-octyl-4-dimethylaminopyridinium trifluoromethanesulfonate [O-DMAP][CF3SO3]:
The preparation steps of [O-DMAP][CF3SO3] were the same as those of [O-DMAP][HSO4]
except that sulfuric acid was replaced with trifluoromethanesulfonic acid. 1H NMR (500 MHz,
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
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),
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)
(Figure S2). FT-IR: v (cm-1) 2964, 2929, 2859, 1650,1564, 1382, 1258, 1168, 1030, 758, 725, 632.
2.3.2 Preparation and characterization of -SO3H-functionalized ILs
1-propylsulfonate-4-dimethylaminopyridinium hydrogen sulfate [PS-DMAP][HSO4]: A
0.1 mol DMAP compound was dissolved in ethyl acetate. Equimolar 1,3-propanesulfonate was
added dropwise into the mixture at room temperature under stirring. The reaction was carried out
washing with ethyl acetate, and vacuum drying, the zwitterion was obtained. Then, the zwitterion
was dissolved in deionized water. Under vigorous stirring, equimolar concentrated sulfuric acid
was added to the abovementioned solution, and the mixture was stirred at 80
removing water, repeated washing with ethyl acetate, and vacuum drying, [PS-DMAP][HSO4] was
obtained. 1H NMR (500 MHz, DMSO) δ 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), 4.29 (t, J = 6.8 Hz, 2H, cation N-CH2-C-C-SO3), 3.19 (s,
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
N-C-CH2-C-SO3) (Figure S2). FT-IR: v (cm-1) 2930, 1652, 1571, 1388, 1162, 1018, 775.
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]:
The preparation steps of [PS-DMAP][CH3SO3] were the same as those of [PS-DMAP][HSO4]
except that sulfuric acid was replaced with methanesulfonic acid. 1H NMR (500 MHz, DMSO) δ
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),
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
CH3-SO3), 2.40 (t, J = 7.3 Hz, 2H, cation N-C-C-CH2-SO3), 2.12-2.02 (m, 2H, cation
N-C-CH2-C-SO3) (Figure S2). FT-IR: v (cm-1) 2960, 1652, 1571, 1388, 1162, 1018, 773, 594.
Table 2. Information on prepared ILs ILs
Mass fraction purity a
Water content / ppm b
The purities of the prepared ILs were estimated by 1H-NMR.
The water content was determined by Karl Fischer titration.
2.4 Measurement of density and viscosity
For the density measurement (328.15~358.15 K, p=0.101 MPa), the density of ILs was
determined via a vibrating tube densimeter Anton Paar DMA 5000 with ±5.0×10-3 kg/m3 of
repeatability. For the viscosity measurement (323.15~368.15 K, p=0.101 MPa), the viscosity of
ILs was measured by a DV-S digital display rotational viscometer with a relative precision and
reproducibility in dynamic viscosity of ±1.0% and ±0.2%, respectively.
2.5 Thermal stability and thermal behavior
The decomposition temperature of ILs was determined by the simultaneous thermal analyzer,
(Netzsch STA 449C Jupiter®), and the sensitivity of the balance was 0.1 µg in the full range. The
sample was put into a crucible with a continuous nitrogen flow (20 mL/min) and measured by
scanning the temperature from room temperature to 873 K at a heating rate of 10 K/min. The
obtained decomposition temperature (Td) is the onset temperature, which is the intersection of the
baseline below the decomposition temperature with the tangent to the mass loss versus the
temperature plots in the TGA profiles. The thermal behavior was analyzed using a differential
scanning calorimeter (Netzsch DSC214). The ILs were put into sealed crucibles and evaluated in
the temperature range of 193.15-333.15 K using the continuous method with a heating rate of 5
K/min and a nitrogen gas flow of 40 mL/min. Glass transition temperature (Tg) is the midpoint of
a small heat capacity change, and melting temperature (Tm) is the curve peak.
3. Results and discussion
3.1 Analysis of the DMAP structure
To better understand the DMAP structure, molecular simulations were used to obtain its
electrostatic potential (ESP) and nature bond orbital (NBO) charges, as shown in Figure 1.
Figure1.(a) shows that DMAP has pyridine and tertiary amino groups, which allows to synthesize
various ILs. In the ESP analysis [Figure 1.(b)], red color represents a negative charge (high
electron density, nucleophilic region), blue color indicates a positive charge (low electron density,
electrophilic region), and green and yellow represent a neutral level. The high electron density
around the N atom of pyridine suggests that it can combine with electrophilic reagents (e.g.,
1-butylbromide and 1,3-propanesulfonate) to prepare ILs. Meanwhile, the positive charge around
methyl hydrogen of tertiary amine indicates that the N atom of tertiary amine has a considerable
amount of negative charge, which leads to nucleophilic reactions. Figure 1.(c) shows the NBO
charges of all atoms in the DMAP molecule and the p-π conjugative effect between the N atom of
tertiary amine and pyridine ring. The value of negative charge of the N atom of pyridine is slightly
larger than that of the N atom of tertiary amine, and the steric hindrance of the N atom of pyridine
is much smaller than that of the N atom of tertiary amine. Thus, in nucleophilic reactions, the
reactivity of the N atom of pyridine is higher than that of the N atom of tertiary amine.
Furthermore, to investigate the number of reactive sites involved in the nucleophilic reaction,
with the structure, which suggests that only one site (N atom of pyridine) participates in the
reaction at the molar ratio (DMAP/1-butyl bromide) of 1:1. The abovementioned analysis shows
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
Figure 1. Structure of DMAP
Figure 2. 1H-NMR of [B-DMAP][Br]
(c) NBO charges
The density (ρ) of ILs was measured in the temperature range of 328.15~358.15 K, p=0.101
MPa, and the results are shown in Table S1 and Figure 3. For all studied ILs, the density decreases
with an increase in the temperature, which agrees with the previous reports [11,17-19]. At the
same temperature, the density decreases in the following order: [PS-DMAP][HSO4] >
[PS-DMAP][CH3SO3] > [B-DMAP][HSO4] > [H-DMAP][HSO4] >
[O-DMAP][CH3SO3] > [O-DMAP][CF3SO3], which is primarily determined by the molar mass,
molar volume, and the interactions of cation and anion. For [PS-DMAP][HSO4] (342 g/mol) and
[PS-DMAP][CH3SO3] (340 g/mol), the interaction of cation and anion in [PS-DMAP][CH3SO3] is
slightly larger than that in [PS-DMAP][HSO4] [Figure 4.(A) and (B)]. However, the molar volume
of HSO4- (53.778 cm3/mol, calculated by Gaussian 03) is smaller than that of CH3SO3- (61.376
cm3/mol, calculated by Gaussian 03), which may make [PS-DMAP][HSO4] have higher density
compared to that of [B-DMAP][HSO4]. Although the cation of [PS-DMAP][HSO4] has larger
volume (182.793 cm3/mol for [PS-DMAP] and 150.405 cm3/mol for [B-DMAP], calculated by
Gaussian 03), it possesses greater molar mass (245 g/mol and 179 g/mol), which may be a more
significant factor that influences density. The density comparison of [O-DMAP][HSO4],
[O-DMAP][CH3SO3], and [O-DMAP][CF3SO3] indicates that anion type also affects density.
Although [O-DMAP][CF3SO3] has the highest molar mass among the three ILs, and it has been
reported by Zubeir et al.  that fluorine-based ILs possess the high structural organization
efficiency which makes ILs have higher density, e.g., [Cnmim][Tf2N] > [Cnmim][TCM]. In this
study, the F atom of [CF3SO3]- forms strong repulsive interaction with the N atom of tertiary amine
of [O-DMAP] and the F atom of other [CF3SO3] , which increases spacing between the ions and
results in the lowest density value. In [O-DMAP][HSO4], the result of pH measurement indicates
that a part of H+ from HSO4- combines with the N atom of tertiary amine to form
[O-DMAPH][SO4] [Figure S1.(a)], which increases the electrostatic attraction of anion and cation
in the IL and results in the highest density. Furthermore, based on the density results of
[B-DMAP][HSO4], [H-DMAP][HSO4], and [O-DMAP][HSO4], it can be concluded that as the
alkyl chain length increases, the density decreases, which agrees with the previously published
results [11,21-24]. Gaussian 03 was used to calculate the NBO charges of the three cations, as
shown in Figure 5. The results suggest that the change in alkyl chain does not affect the charge
distribution of cations. This observation indicates that electrostatic fields produced by ions are
virtually unchanged. Thus, the steric hindrance due to the alkyl chain has the highest effect on the
density of ILs. With an increase in the alkyl chain length, the volume of cation (150.405 cm3/mol,
190.20 cm3/mol, 213.273 cm3/mol for [B-DMAP]+, [H-DMAP]+, and [O-DMAP]+, respectively)
and steric hindrance increase, which increases distance between ions [21-24] and lowers density at
the macro level. Compared with other ILs with different cations, the density of
[O-DMAP][CF3SO3] is lower than those of [bmim][CF3SO3] , [C2mim][CF3SO3] , and
[bpy][CF3SO3] , which may be due to the larger cationic volume and stronger electrostatic
repulsion between the cation and anion in [O-DMAP][CF3SO3]. The stronger electrostatic
interactions in [B-DMAP][HSO4] may make its density higher than those of [BMIM][HSO4]
[26,27] and [BMPy][HSO4] .
ρ / (kg/m3)
1300 1250 1200 1150 1100 325
Figure 3. Densities of the studied ILs at different temperatures:
[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
260 261 262 263 264 265 266 267 268
Figure 4. Interactions of the cation and anion in ILs
Figure 5. NBO charges of [B-DMAP]+, [H-DMAP]+, and [O-DMAP]+
The second-order polynomial equation was adopted to correlate the density data with the
temperature as follows:
ln ρ / kg/m 3 = A1 + A2 × (T / K ) + A3 × (T / K )
where A1, A2, and A3 are the fitting constants. The fitting results are shown in Table 3. All
correlation coefficients (R2) are greater than 0.9999, and the values of all ARD are lower than
0.0055%, which indicates that the mathematical model can describe the relationship of density and
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)]
ARD / % a
-3.2816×10 9.7078×10 3.0150×10 1.2343×10
where n is the number of data points; the superscripts “exp” and “cal” represent the experimental values and
According to the results in Table 3, the isobaric thermal expansion coefficient (αp, K-1) can be
100 n ρ k − ρ k ∑k =1 n ρ kexp exp
calculated values of density, respectively.
obtained from Eq. (2):
1 ∂ρ ∂ ln ρ α p = − = − = −( A2 + 2 ⋅ A3 ⋅ T ) ρ ∂T p ∂T p
where ρ, T, and p indicate the density of the studied ILs, kg/m3; absolute temperature, K; and the
pressure, 0.101 MPa, respectively. A2 and A3 are the fitting parameters in Table 3.
Based on the density data, the molecular volumes (Vm, nm3) of ILs can be calculated by the following equation:
M × 10 24 ρ ⋅ NA
where M is the molar mass, kg/kmol; NA indicates the Avogadro's constant, 6.02214129×1023
mol-1; ρ represents the density of the studied ILs, kg/m3. The lattice potential energy (UPOT, kJ/mol)
can be estimated according to the following relationship [11,28-31]:
α U POT = 2 × I × + β 3 Vm
where α, β are the fitting parameters; I represents the ionic strength. For the simple
ionic compounds MX (cation:anion=1:1), the values of α, β, and I are 117.3 kJ·nm/mol, 51.9
kJ/mol, and 1, respectively [11,33]. The values of αp, Vm, and UPOT at 333.15 K are shown in Table
Table 4. Values of αp, Vm, and UPOT at 333.15 K ILs
104αp / K-1
Vm / nm3
UPOT / (kJ/mol)
The uncertainties of the molecular volume and the lattice potential energy were calculated using uncertainty
The isobaric thermal expansion coefficients of studied ILs are in the range of 4.33 –
6.18×10-4 K-1. For -SO3H-functionalized ILs, the isobaric thermal expansion coefficient follows
the order: [PS-DMAP][HSO4] < [PS-DMAP][CH3SO3]. Despite the effect of an anion on the
isobaric thermal expansion coefficient, the cation moieties influence the isobaric thermal
expansion coefficient in the following order: [B-DMAP][HSO4] < [H-DMAP][HSO4] <
[O-DMAP][HSO4]. This result is obtained because the IL with a large cation has larger distance
between ions, which weakens electrostatic interaction and facilitates their thermal expansion
[32,33]. The isobaric thermal expansion coefficient increases in the following anion order:
[O-DMAP][HSO4] < [O-DMAP][CH3SO3] < [O-DMAP][CF3SO3], which is attributed to the
[O-DMAP][HSO4] and the repulsive interaction of [O-DMAP][CF3SO3].
A molecular volume was used to express the volumetric behavior of studied ILs. The
molecular volume increases with an increase in the alkyl chain length (0.3584 to 0.4724 nm3),
with a contribution of (0.0285 ± 0.0013) nm3 per methylene group (-CH2-), which is comparable
with the increment of (0.0281 ± 0.0006)  and (0.0269 ± 0.0001) nm3 . The effect of anion
[O-DMAP][CF3SO3]. It can be concluded that when the alkyl chain is sufficiently long (e.g.,
-C8H17), the electrostatic attraction of the cation and anion does not affect molecular volume,
which results in a similar molecular volume of [O-DMAP][HSO4] and [O-DMAP][CH3SO3].
However, the electrostatic repulsion of the cation and anion clearly affects the molecular volume.
It is observed that as the alkyl chain increases, the lattice potential energy decreases, which
indicates a lower structural organization efficiency when IL has a longer alkyl chain [35-37]. An
increase in the alkyl chain enhances steric hindrance and weaken the electrostatic interaction
between the cation and anion. The effect of anion on the lattice potential energy follows the order:
[O-DMAP][HSO4]≈[O-DMAP][CH3SO3] > [O-DMAP][CF3SO3], and the explanation is the same
as that for the molecular volume. For -SO3H-functionalized ILs, the lattice potential energy of
[PS-DMAP][HSO4] is higher than that of [PS-DMAP][CH3SO3]. In addition, the lattice potential
energies of studied ILs were determined to be much lower than those of fused salts, such as CsI
(613 kJ/mol at 298.15 K), which makes them have low melting points [35,36].
The viscosity of seven ILs was determined in the temperature range of 323.15-368.15 K,
0.101 MPa, and the results are shown in Table S2 and Figure 6. As expected, the monotonic
decrease in viscosity is observed with an increase in the temperature. Compared to the density, the
viscosity has a larger decrease degree with an increase in the temperature. It is clear that the
chemical structures of cations and anions considerably affect the strength of van der Waals
interactions and the capacity to form hydrogen bonds, which affects the viscosity of the studied
ILs [32,38]. For -SO3H-functionalized ILs, their viscosities are higher than those of non-SO3H
functionalized ILs, which is attributed to stronger electrostatic interactions between cations and
anions. It is determined that as the alkyl chain length of the cation increases, the viscosity
increases, which is similar to the previously published results [11,32,35]. The long alkyl chain
leads to the strong van dar Waals interactions as well as large bulkiness, which increases the
viscosity. For [O-DMAP][HSO4], [O-DMAP][CH3SO3], and [O-DMAP][CF3SO3], the long chain
of the cation (-C8H17) occupies enough space to reduce the effect of electrostatic repulsion on
viscosity, which results in similar viscosity values between [O-DMAP][CF3SO3] and
[O-DMAP][CH3SO3]. Different from electrostatic repulsion, the effect of electrostatic attraction
on viscosity is more clear, which makes [O-DMAP][HSO4] have maximum viscosity value among
the three ILs. Compared to the previously reported results, the viscosity of [O-DMAP][CF3SO3] is
higher than those of [bmim][CF3SO3] [19,39,40] and [EMIM][CF3SO3]  due to the longer
alkyl chain in [O-DMAP]+. Similarly, [O-DMAP][CH3SO3] has higher viscosity than that of
[EMIM][CH3SO3] . The viscosity of [PS-DMAP][CH3SO3] is higher than that of
[EMIM][CH3SO3]  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
Figure 6. Viscosities of the studied ILs at different temperatures. The lines are the correlative values of the VFT
equation, and the symbols represent the experimental values:
352 353 354
[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
where η is the viscosity, mPa·s; T represents the absolute temperature, K; B1, B2, and B3 are the
fitting parameters that are estimated according to the experimental data shown in Table 5. All
correlation coefficients (R2) are greater than 0.9997, and the values of all ARD are lower than
1.9000% except for [PS-DMAP][CH3SO3] (4.4002%), which indicates a good agreement between
the experimental data and the VFT model.
Table 5. Fitting parameters of the VFT model, average relative deviation, and activation energy ILs
B1 / (mPa·s)
B2 / K
B3 / K
ARD / % a
Eη / (kJ/mol) b
362 363 364
where n is the number of data points; the superscripts “exp” and “cal” represent the experimental and calculated
Based on the results of the VFT equation, the activation energy of viscous flow (Eη) was
exp cal 100 n η k − η k ∑k =1 exp n ηk
values of viscosity, respectively. b
obtained using the following equation: T ∂(lnη ) Eη = R ⋅ = R ⋅ B2 ⋅ ∂(1/ T ) T − B3
where B2, B3 are the fitting parameters of the VFT model; R represents the ideal gas constant,
8.314 J/(mol·K). The activation energy of viscous flow at 333.15 K is shown in Table 5. It is easy
to determine that the order of activation energy at 333.15 K does not follow the same trend as that
of the viscosity, and similar results have been reported by Rocha et al.  and Yadav et al. .
3.4 Thermal stability and thermal behavior
The thermogravimetric analysis (TGA) of the studied ILs was conducted from room
temperature to 873 K. The TGA curves are shown in Figure 7, and the decomposition
temperatures (Td) are shown in Table 6. For [B-DMAP][HSO4], [H-DMAP][HSO4], and
[O-DMAP][HSO4], Td increases with an increase in the alkyl chain length of the cation, which
agrees with the previously published reports [35,44]. In addition, it is determined that the IL based
on [CF3SO3]- has high Td (613 K), and the same finding ([C2mim][CF3SO3]: 713 K) is also
reported in the literature . Furthermore, it can be concluded that the temperatures of density
measurements (328.15~358.15 K, p=0.101 MPa) and viscosity measurements (323.15 K~368.15
K, p=0.101 MPa) are much lower than the Td of all ILs (≥ 526.15 K), which meets the
requirements of thermophysical property measurement.
The glass transition temperature (Tg) and melting temperature (Tm) obtained from DSC are
also shown in Table 6. It can be concluded that the electrostatic interactions between anions and
cations in ILs considerably affect the Tg based on the results of -SO3H-functionalized ILs >
non-SO3H functionalized ILs, [PS-DMAP][CH3SO3] > [PS-DMAP][ HSO4] [Figures 4 (A) and
[O-DMAP][CF3SO3] (electrostatic repulsion). As the alkyl chain length increases, the Tg increases
([O-DMAP][HSO4] > [H-DMAP][HSO4] > [B-DMAP][HSO4]) because the larger substituent
group leads to the higher internal rotation resistance for ILs. The effects of the electrostatic
interactions between anions and cations on Tm are the same as the trend of Tg. However, compared
with Tg, Tm decreases with an increase in the alkyl chain length. This can be explained by the
increasing disruption of crystal packing because when the chain length is extended, the increased
van der Waals interactions between larger components become overridden [45,46].
Mass fraction / %
60 [PS-DMAP][CH3SO3] [PS-DMAP][HSO4] 40
[O-DMAP][CF3SO3] [H-DMAP][HSO4] [B-DMAP][HSO4]
Figure 7. TGA curves of the studied ILs
Table 6. Tg, Tm, and Td of the studied ILs ILs
Tg / K
Tm / K
Td / K
In this study, a series of new DMAP-based ILs was prepared and characterized, and their
thermophysical properties, density, and viscosity were investigated as a function of temperature.
By analyzing DMAP, it is determined that DMAP has pyridine and tertiary amino groups, which
allows to synthesize various ILs. However, due to the larger electron density and smaller steric
hindrance, the reactivity of the N atom of pyridine is higher than that of the N atom of tertiary
amine in nucleophilic reactions. Meanwhile, the 1H-NMR result of [B-DMAP][Br] reveals that
only one N atom (from the pyridine group) participates in the nucleophilic reaction, which indicates
that DMAP can serve as a matrix for the preparation of ILs. Empirical equations were adopted to
describe the temperature dependence of density and viscosity, specifically, the second-order
polynomial equation for density and the VFT model for viscosity. It is determined that both
density and viscosity decrease with an increase in the temperature. However, viscosity has a larger
decrease degree as the temperature increases. For density, at the fixed temperature, the decreasing
rank is as follows: [PS-DMAP][HSO4] > [PS-DMAP][CH3SO3] > [B-DMAP][HSO4] >
[H-DMAP][HSO4] > [O-DMAP][HSO4] > [O-DMAP][CH3SO3] > [O-DMAP][CF3SO3], which is
greatly affected by the molar mass, molar volume, and interactions of the cation and anion. Some
physicochemical properties (e.g., isobaric thermal expansion coefficient, molecular volume, and
lattice potential energy) were obtained from the density data, which are closely related to the
molar mass, molar volume, and interactions of the cation and anion. Regarding the viscosity, it can
be seen that as the alkyl chain length of the cation increases, viscosity increases. The long alkyl
chain leads to the strong van dar Waals interactions and large bulkiness, which increases viscosity.
The anion type affects the viscosity of ILs by attractive and repulsive interactions with the cation.
It is determined that the order of Eη derived from the VFT model does not follow the same trend as
that of the viscosity. Furthermore, all of the studied ILs possess high thermal stability (Td ≥ 526.15
K), and the alkyl chain length positively affects Tg and negatively affects Tm, respectively. In
conclusion, the investigation of thermophysical properties of DMAP-based ILs promotes their
application in many fields.
We acknowledge the financial support for this work from the National Natural Science
Foundation of China (Nos. 21576053 and Nos. 21878054) and the Natural Science Foundation of
Fujian Province (No. 2016J01689).
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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.
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”.