Triethylammonium-based protic ionic liquids with sulfonic acids: Phase behavior and electrochemistry

Article abstract of DOI:10.1016/j.molliq.2018.06.059

Six triethylammonium-based protic ionic liquids (PILs) and two molten salts were synthesized via a proton transfer reaction from sulfonic acid to triethylamine (TEA). The PILs were characterized by ¹H NMR, ¹³C NMR, ¹H/¹⁵N NMR and FT-IR spectroscopic methods. The phase behavior of the PILs was studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The temperature dependences of the PILs electrical conductivity, viscosity, and electrochemical stability window (ECW) were studied. The highest electrical conductivity and ECW values are registered in triethylammonium triflate. The thermal and electrochemical characteristics of the salts obtained in this work have been analyzed in comparison with the literature data by the properties of triethylammonium-based salts with sulfonic acids.

Full text of DOI:10.1016/j.molliq.2018.06.059

Accepted Manuscript
Triethylammonium-based protic ionic liquids with sulfonic acids:
Phase behavior and electrochemistry
L.E. Shmukler, M.S. Gruzdev, N.O. Kudryakova, Yu A. Fadeeva,
A.M. Kolker, L.P. Safonova
PII:
DOI:
Reference:
S0167-7322(18)32162-7
doi:10.1016/j.molliq.2018.06.059
MOLLIQ 9257
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
24 April 2018
13 June 2018
15 June 2018
Please cite this article as: L.E. Shmukler, M.S. Gruzdev, N.O. Kudryakova, Yu A. Fadeeva,
A.M. Kolker, L.P. Safonova , Triethylammonium-based protic ionic liquids with sulfonic
acids: Phase behavior and electrochemistry. Molliq (2018), doi:10.1016/
j.molliq.2018.06.059
This is a PDF file of an unedited manuscript that has been accepted for publication. As
a service to our customers we are providing this early version of the manuscript. The
manuscript will undergo copyediting, typesetting, and review of the resulting proof before
it is published in its final form. 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.

ACCEPTED MANUSCRIPT
Triethylammonium-based protic ionic liquids with sulfonic acids: phase
behavior and electrochemistry
L.E. Shmukler*, M.S. Gruzdev, N.O. Kudryakova, Yu. A. Fadeeva, A.M. Kolker, L.P. Safonova
G.A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences, 1 Akademicheskaya
St., Ivanovo, 153045 Russia
Corresponding author. E-mail address: les@isc-ras.ru (L.E. Shmukler).
Abstract
Six triethylammonium-based protic ionic liquids (PILs) and two molten salts were synthesized
via a proton transfer reaction from sulfonic acid to triethylamine (TEA). The PILs were
characterized by 1H NMR, 13C NMR, 1H/15N NMR and FT-IR spectroscopic methods. The phase
behavior of the PILs was studied using differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA). The temperature dependences of the PILs electrical
conductivity, viscosity, and electrochemical stability window (ECW) were studied. The highest
electrical conductivity and ECW values are registered in triethylammonium triflate. The thermal
and electrochemical characteristics of the salts obtained in this work have been analyzed in
comparison with the literature data by the properties of triethylammonium-based salts with
sulfonic acids.
Keywords: Triethylammonium salts, Ionic liquid, Synthesis, Thermal properties,
Electrochemistry
1. Introduction
Protic ionic liquids (PILs) are one of the most important subclasses of ionic liquids. PILs are
obtained via a neutralization reaction between Brønsted acids and Brønsted bases. These
substances are salts containing a large organic cation and an organic or inorganic anion. The key
feature of PILs is the presence of an exchangeable proton on the cation enabling the formation of
hydrogen bonds with the anion. Ionic liquids are called “Designer solvents” [1] because
variations of the cation and anion nature allow synthesizing salts with different physico-chemical
properties. The persistently high interest in the PILs studies in the last few decades has been
caused by a wide range of their possible applications. Thanks to the combination of some useful
properties, such as low vapour pressure, high thermal stability, high electrical conductivity, large
electrochemical window, PILs are used as the so-called “green solvents” [2–5], supercapacitors
[6], and electrolytes for fuel cells [7–13].
In this work, we study PILs based on triethylamine with sulfonic acids. At present there are
publications devoted to investigation of triethylamine-based PILs with trifluoroacetic [14–17],
benzoic [14,16], formic [18,19], nitric [19], acetic [19–25], phosphoric [10,20,21,23,26,27],
dibutylphosphoric [28,29], monofluorophosphoric [26], and phosphorous [10,15,17] acids. The
most studied PILs among those with sulfonic acids are the salts with methanesulfonic
[10,14,16,26,28,30–34], trifluoromethanesulfonic [10,14,16,26,30,31–33,35,36] and sulfuric
[10,20–24,26] acids, and bis(tri(tetra)fluoromethylsulfonyl)imide [7,33,35,37–40]. There are also
few works devoted to the synthesis and study of the properties of triethylamine-based PILs with
p-toluenesulfonic [15,17], benzenesulfonic, as well as of fluorinated aromatic sulfonic acids
(fluorobenzenesulfonic,
trifluorobenzenesulfonic,
perifluorobenzenesulfonic,
perifluorobutanesulfonic, perfluoroctanesulfonic, and pentafluorobenzenesulfonic ones)
[16,34,35]. It should be noted that the ionic liquids studied in most of the abovementioned works
belong to the group of room temperature PILs.

ACCEPTED MANUSCRIPT
The size of the electrochemical window (ECW) of ammonium PILs varies greatly (from 0.66
to 5.57 V) depending on the cation and anion nature and measurement conditions (temperature
range, scanning speed) [5,18,41–43]. In PILs with di/triethylammonium cations, the ECW size
ranges between 1.23–4.73 V [43,44]. Besides, work [44] shows that the ECW values for
TEA/MsO obtained with glassy carbon, gold and platinum electrodes equaled 4.73, 2.83, and
2.47 V, respectively.
The experimental material accumulated so far allows us to find correlations between
individual thermal and physico-chemical characteristics of PILs and to use them for predicting
the properties of the PILs. It has been established that in many ionic liquids, the ratio of the glass
transition temperature to the melting temperature Tg/Tm = 2/3 (the “2/3 law” for glass transition
temperatures). In our earlier studies of triethanolamine-based protic ionic liquids with various
sulfonic acids, we obtained that the Tg/Tm values of the PILs are located in the range of 0.64–
0.69, which is in good agreement with the “2/3 law” [45]. At the same time, the authors of work
[26] studied the properties of a lot of PILs based on aliphatic and aromatic amines with different
acids (trifluoromethanesulfonic, difluorophosphoric, formic, phosphoric, sulfuric, nitric,
methanesulfonic, tetrafluoroboric, hydrofluoric ones) and found that this rule could be easily
applied only to a limited number of PILs in the ethylammonium family. However, when they
tried to obtain a correlation between Tg and Tm for all the salts that were studied in the work and
whose temperatures were determined, the deviation from the 2/3 ratio became quite significant.
The authors concluded that this law was only true for the salts, whose melting points were less
than 50% above their glass transition temperatures.
The authors of work [33] obtained a correlation of viscosities of triethylammonium
methylsulfonate, trifluoromethyl sulfonate and imidazolium bis(trifluoromethylsulfonyl)imide
with their vaporization enthalpies.
A correlation of different properties of PILs that is often tested by scientists is their dependence
on ΔpKa (the difference between the dissociation constant of the protonated base and acid in
aqueous solution). Works [46–48] have determined a dependence of the thermal stability of PILs
based on secondary and/or tertiary ammonium cations on the pKa value of the original acid
anion. According to work [46], the thermal stability of PILs with ΔpKa > 15 is similar to that of
aprotic ionic liquids. Besides, a ΔpKa threshold level has been found at which PILs have both
good thermal stability and high ionicity. The authors of [49] who studied five protic ionic liquids
with propylammonium, 3-hydroxy propylammonium as cations, and formate, acetate,
trifluoroacetate as anions have established that PILs with high ΔpKa values have a high density
and low viscosity. At the same time, in work [47], the authors who studied the electrochemical
properties of the PILs based on different amines and bis(trifluoromethanesulfonyl)amide acid
have come to a conclusion that conductivity, density and viscosity by and large depend on the
structure of amines rather than the ΔpKa. Electrochemical measurements for a series of protic
ionic liquids based on 1,8-diazabicyclo[5.4.0]-7-undecene and tertiary amines (diethylmethyl
amine and ethylmethylpropyl amine) with different Brønsted acids have revealed that the open
circuit potential for H2/O2 cells grows with an increase in ΔpKa of the constituent acid and base
and passes through a maximum to gradually fall with a further increase in the ΔpKa [50]. To sum
up, it should be noted that the pKa values determined for aqueous solutions may be incorrect for
analysis of nonaqueous PILs [14]. The authors of work [51] used the Hammet acidity function
values (H0 values), calculated based on the H0 of the pure forms of sulfuric and fluorosulfuric
acids as the criterion of acidity evaluation of the original acids for PILs synthesis. They also
emphasized that this analysis was correct for the members of the sulfuric acid family.
Thus, it remains urgent to determine physico-chemical and electrochemical properties of
PILs and to find the correlation dependences between the different characteristics in order to
select the optimal composition of new salts for their further effective applications.
2. Experimental part

ACCEPTED MANUSCRIPT
2.1. Materials
Triethylamine was obtained from Acros organics, the sulfonic acids [sulfuric,
methanesulfonic, trifluoromethanesulfonic (triflic), amidosulfonic (sulfamic), benzene sulfonic,
3-nitrobenzenesulfonic, 2-sulfobenzoic (o-sulfobenzoic), 5-sulfosalicylic] were of the highest
purity available (Tokyo Chemical Industry, Acros organics and Sigma-Aldrich) and were used
without further purification.
2.2. General procedure of PILs synthesis
The PILs were prepared via a triethylamine neutralization reaction directly with sulfonic
acids in an argon atmosphere. A detailed description of the procedure of synthesizing ionic
liquids is shown in work [15].
2.3. Methods
A detailed description of the experimental techniques is given in work [15]. The FT-IR spectra
of the compounds were recorded on a Bruker Vertex 80V device in the region of 7500 – 370 cm-
1 from pellets with KBr. The NMR spectra were recorded on a Bruker Avance-500 spectrometer
1
in DMSO-d6 solution. All the H, 13C NMR chemical shifts were referenced against TMS set as
0 ppm. The melting point, crystallization point and glass transition temperature were determined
on a NETZCH DSC 401 F1-calorimeter at the rate of 10 °C min-1. The TGA of the PILs was
performed on a NETZCH TG 209 F1 analyzer in a flow of argon (20 ml/min) at a heating rate of
10 K/min. The PILs resistance was determined by the electrochemical impedance method with a
Solartron 1260A complete with the electrochemical interface Solartron 1287 over the frequency
range of 0.1 Hz – 1 MHz with the signal amplitude of 10 mV and with the accuracy higher than
0.2%. The viscosities (η) of the PILs were measured with a rotational programmable viscometer
(Brookfield DV-II, spindle S18); the measurement accuracy was equal to ± 1 %. The
electrochemical windows were obtained by using a PI 50-Pro-3 pulse potentiostat with automatic
registration, and the data were processed using the PS Pack 2 software. The electrochemical
investigations were carried out with a three-electrode cell with a platinum wire as the working
electrode in the temperature range of 50–120° С. To maintain the temperature, we used a
thermostat (TERMEX, Russia) in which the thermostating liquid was silicone oil PMS 20; the
accuracy of maintaining the temperature was ± 0.1oC. The density was measured with an Anton
Paar DMA 5000M densimeter equipped with an Xsample 352 H automatic filling unit. The
densimeter was calibrated with dry air and bidistilled water before every measurement.
The water contents of PILs at room temperature, determined by Karl–Fischer titration (V30,
Mettler Toledo), were below 1 wt%.
3. Results and discussion
The structural formulas of the salts, aqueous dissociation constants of the original acids, and the
abbreviations of the synthesized PILs are summarized in Table 1.
Table 1
Name, structural formulae and abbreviations of synthesized triethylammonium ILs.
pKa
starting acids
Salts
abbreviation
Name
Structural formulas
O
NH+
O
S
OH
1.99 [52]
hydrogen sulfate
TEA/HSO4
O

ACCEPTED MANUSCRIPT
CH3
NH+
O
S O
O
-1.92 [53]
mesylate
triflate
TEA/MsO
TEA/TfO
TEA/SAM
TEA/BSu
F3C
−14.2 [54]
−12 [55]
−5.2 [56]
NH+ O
S
O
O
O
NH+
O
S
NH2
0.99 [57]
0.70 [52]
sulfamate
O
O
besylate
(benzene sulfonate)
NH+
O
S
O
O
NH+
O
S
-7.12 [58]
-1.21 [59]
3-nitrobenzenesulfonate
2-sulfobenzoate
TEA/NBSu
O
NO2
OH
O
O
TEA/SBA
NH+
S
O
O
OH
O
O
2.13 [60]
5-sulfosalicylate
TEA/SSA
NH+
S
O
OH
O
pKa (TEA) = 10.75 [52]
3.1. IR NMR spectroscopy characterization
15
Nitrogen atom protonation in tertiary amines, as a rule, is accompanied by a downfield N
resonance shift [61]. The chemical shifts of the onium cation nitrogen in the 1Н/15N salts spectra
are displaced 9 ppm downfield compared to the pure triethylamine signal (Table 2). The
1
resonance signals of the protons of the NCH2 and CH3 groups in the Н NMR spectra of salt
solutions are also shifted downfield relative to the original TEA. The frequency shift does not
exceed 0.6 ppm. Besides, the 13С NMR chemical shifts of the cation are 2-3 ppm shifted upfield
compared to the original TEA for the amine methyl fragment signal (Table 2).
Table 2
1H, 13C and HMBC-1H/15N chemical shifts of triethylamine and triethylammonium cation in
DMSO-d6.
δH (ppm)
CH3
δC (ppm)
NCH2
δN (ppm)
NH+
PILs
NCH2
NH+
-
CH3
11.77
8.64
8.9
TEA
2.43-2.39
3.08-3.03
3.13-3.08
0.93-0.9
1.18-1.15
1.33 1.3
45.73
45.84
46.5
-334.77
-325.32
-326.5
TEA/H2SO4
TEA/MsO
4.31
10.2

ACCEPTED MANUSCRIPT
TEA/TfO
TEA/SAM
TEA/BSu
TEA/NBSu
TEA/SBA
TEA/SSA
3.12-3.07
3.09-3.05
3.09-3.05
3.12-3.08
3.08-3.04
3.13-3.06
1.19-1.16
1.19-1.16
1.19-1.16
1.18-1.15
1.16-1.13
1.18-1.15
8.82
8.74
9.15
8.83
11.12
9.02
45.8
45.8
45.8
45.9
45.8
45.7
8.6
8.6
-325.2
-325.6
-325.1
-325.3
-325.3
-325.3
8.76
8.8
8.76
8.5
A detailed comparison of the bands in the IR- and NMR- spectra is presented in ESI.
The IR-spectra of the salts, in contrast to the spectra of the original compounds, contain a
number of bands which we classify as the NH stretching vibrations in the region of 2533-2677
cm-1 for aliphatic and 2257-2678 cm-1 for aromatic acids [62–64]. Fig. 1 shows the IR-spectra of
the synthesized salts in comparison with the pure TEA spectrum.
Fig. 1. FT-IR-spectra of the original triethylamine and salts with different acids.
Thus, the NMR and IR spectroscopy studies have confirmed the presence of protonated
amine.
3.2 Phase behavior of the synthesized compounds
The temperatures of phase transitions and thermal stability of materials are important
physical properties of PILs determining their working temperature range.
The melting temperature, the cool crystallization temperature, the glass transition
temperature and the decomposition temperature of the synthesized PILs in comparison with the
literature data are given in Table 3.
Table 3

ACCEPTED MANUSCRIPT
Key properties of the synthesized PILs.
Tcr
(oC)
Tm
Tg / (Δcp)
(oC) / (J∙g-
1K-1)
Tdec
(oC)
Н2О
(wt%)
 Hm
(J∙g-1)
PILs
(oC)
-22.9
-24 [14]
-31 [30]
-31.6 [28]
33.1
–
–
–
203
0.81
liq
89.67
33 [14]
25 [30]
24.3 [28]
21.6 [26]
33 [31]
17.4 [10]
305 [14] 50 ppm
218 [30] 2 mol%
-78.9 [28]
-96.5 [26]
-75 [31]
-62.1 [10] 290 [31]
225 [10]
–
100
ppm
<1 ppm
–
TEA/MsO
–
–
–
269.7
[26]
–
liq
cr
-29.3
-20 [14]
-13 [30]
–
–
26.8
33.2
32 [14]
26 [30]
34.3 [10]
36 [35]
66.8
35.55
13.95
–
-58 [14]
–
315.4
376 [14] 50 ppm
322 [30] 2 mol%
358 [10]
380 [35]
229
0.50
TEA/TfO
–
–
–
0.25
-58 [35]
-35.5
(0.162)
–
TEA/SAM
-14.6 (10K/min) 69.8 (10K/min) 37.87
310
0.77
TEA/BSu cr
11 (5K/min)
106 [16]
0.9
76.3 (5K/min)
119 [16]
75.9
330 [16] 50 ppm
cr
58.27
-42.7
(0.140)
298
0.57
TEA/NBSu
TEA/SBA cr
54.0
117.3
–
106.5
147.0
140 [65]
105.92
49.03
–
–
–
279
278
–
0.57
0.74
–
solid
TEA/SSA
cr
60.65
–
82.42
74.7 [10]
84.2 [26]
41.26
-60.7
(0.046)
–
259
0.72
–
<1 ppm
270 [10]
262.8
[26]
TEA/HSO4
–
-100.1 [26]
Тcr is the crystallization temperature; Tm is the melting temperature; Tg is the glass transition temperature; Tdec is the
decomposition temperature, which are stated as the temperatures corresponding to a 5% loss of the sample weight.
The measured temperatures of phase transitions for TEA/MsO(TfO) agree well with the
literature data (Table 3). A certain deviation of the Tcr value obtained in this work for TEA/TfO
from the literature data may be caused by the DSC experiment conditions, in particular, by the
different scanning speed. The values of Tcr and Tm obtained in this work for TEA/BSu and the Tg
value for TEA/HSO4 differ a lot from the literature data (Table 3). The reason for such
discrepancy cannot be easily identified.
As Table 3 shows, the TEA/SAM(NBSu, HSO4) salts exhibit crystallization/melting and
glass transition. The thermograms of the other salts show only the crystallization/melting
transitions. It is possible that if the DSC thermograms of some PILs do not have glass transitions,
the Tg value can be beyond the limits of the measured temperature range. As an example, Fig. 2
shows DSC thermograms of the TEA/NBSu and TEA/SBA. The exotherm (Tcr) observed on the
heating cycle is associated with the crystallization of the supercooled liquid at 54oC for
TEA/SBA and at 0.9oC for TEA/NBSu. Further heating of the salts results in the observation of
the main endotherm associated with the melting point (Tm) at 106.5oC for TEA/SBA and at
75.9oC for TEA/NBSu. For TEA/SBA a small endothermic process (Tcc) is also observed at
83.9oC, associated with the crystal–crystal phase transition between two metastable phases [66].
The TEA/NBSu thermogram shows a glass transition.

ACCEPTED MANUSCRIPT
Fig. 2. Differential scanning calorimeter (DSC) thermograms of the TEA/NBSu and TEA/SBA
for the second heating steps.
The synthesized triethylammonium mesylate and triflate are in liquid state at ambient
temperature and, thus, can be classified as room temperature ionic liquids. The melting point of
the TEA/SBA(SSA) salts is above 100oC, which allows us to classify them as molten salts.
Substitution of the hydroxyl group in the hydrogen sulphate anion with different functional
groups (CH3, NH2 CF3) and benzene ring reduces the melting temperature of
triethanolammonium mesylate, sulfamate, triflate and besylate, respectively. In [67], it has been
found that the melting points vary depending on the nature of the anions. It has been determined
that an increase in the anion radius in imidazole-based aprotic ionic liquids leads to a decrease in
the salts melting points as the electrostatic force between the imidazolium cation and the anions
becomes weaker. In some of the studied anions, such trend is observed for the salts with a
sulfuric acid anion and its derivatives: MsO~TfO < SAM <BSu < HSO4.
When studying the thermal characteristics of the PILs based on primary, secondary and
tertiary ammonium cations with fluorinated (heptafluorobutyrate, pentadecafluorooctanoate) and
non-fluorous (octanoate) anions, we found that the salts with fluorinated anions have higher
melting points compared to those with hydrocarbons [68]. The TEA/TfO(MsO) synthesized in
this work have similar Tm values. The reason why we did not observe any changes in the melting
point under the transition from mesylate to triflate may be the small number of fluorine atoms in
the TEA/TfO in comparison with the salts from reference [68].
Introduction of substituents in the BSu benzene ring and changes in their positions increase
the melting points of TEA/NBSu(SBA, SSA). A comparison of the obtained characteristics for
the TEA salts with the analogous properties for triethanolammonium-based (ТЕОА) salts [45]
shows that almost all TEOA salts with sulfonic acids undergo glass transition, which is evidently
associated with their viscosity. Besides, cooling causes only glass transition in
triethanolammonium hydrogen sulphate, triflate, sulfobenzoate and sulfosalicylate, but does not
lead to melt phase transition to crystalline state. The salts based on TEA with the same anions
(except TEA/HSO4) do not undergo glass transition, whereas the crystallization and melting
phase transitions are observed. The DSC curve of the TEA/HSO4 shows the temperatures of the
three phase transitions. Among the studied PILs, only triethyl- and triethanolammonium
sulfamates demonstrate similar thermal behavior. The decomposition temperatures of
o
triethanolamine salts are about 100 C lower than those of triethylamine salts [45]. In the
functionalized four IL, the ILs containing TfO anion shows a higher stability than HSO4 anion
[69]. Thus, the thermal behavior of PILs depends both on the cation and anion nature.
The TG plots are shown in Fig. 3.

ACCEPTED MANUSCRIPT
Fig. 3. TG analysis of the PILs.
The thermal stability of the studied triethylammonium-based ionic liquids lies within the
o
temperature range of 200–320 C. No correlation is observed between the Tdec and ΔрКa values
for the PILs synthesized in this work. For the TEA-based salts, we earlier found a correlation
between the temperatures of crystallization and melting [15]. The values of Tc and Tm obtained in
this work are also quite well described by the proposed correlation equation.
3.3. Electrical сonductivity and electrochemical window
Electrical conductivity is a very important parameter for the assessment of the potential
application of PILs in electrochemical devices [8,12,70–73]. The electrical conductivity of the
synthesized PILs has been evaluated from their impedance spectra. As an example, the spectra of
TEA/TfO at different temperatures are shown in Fig. 4. The spectra of the other PILs, except
TEA/SAM, look similar. The spectra correspond to the equivalent circuit consisting of volume
resistance of the electrolyte (R) connected in series with a constant phase element (СPEdl). The
impedance hodograph for TEA/SAM consists of two separated typical regions: an arc at high
frequencies and a line in the low-frequency region. The electrode equivalent circuit consists of a
constant phase element connected in series with a parallel combination of the bulk electrolyte
resistance and a second CPE representing the cell geometric capacitance. The appearance of
geometric capacitance is associated with the fact that the cell electrodes are separated by a low
conductivity electrolyte. It should be noted that a temperature increase reduces the semicircle
diameter and the arc gradually disappears [30].
Fig. 4. Impedance spectra for TEA/TfO at selected temperatures.

ACCEPTED MANUSCRIPT
Fig. 5 presents temperature dependences of electrical conductivity of TEA salts studied in
this work and TEOA salts obtained by us earlier in work [45].
Fig. 5. Arrhenius plot of electrical conductivity for TEA (a) and TEOA (b) based salts. The data
on the electrical conductivity of TEA/PTSA and triethanolamine-based salts were taken from our
previous works [15,45].
The obtained values of electrical conductivity of the ТЕА salts lie within the range of 10-4 ÷
10-2 Ohm-1∙cm-1. Triethylammonium triflate has the highest electrical conductivity value. The
limited temperature range of electrical conductivity measurements for the salts with sulfuric,
benzenesulfonic, 3-nitrobenzenesulfonic and amidosulfonic acids is associated with the melting
temperature of these ionic liquids. It is possible to measure the electrical conductivity of the salts
with TfO, MsO and PTSA [15] at the temperatures lower than their Tm as these salts are in
metastable liquid state. The TEA/TfO(MsO) electrical conductivity values obtained in this work
are in good agreement with the literature data presented in [28,30], the difference in the values at
the same temperatures does not exceed 0.1 mS∙cm-1. These electrical conductivity values differ
slightly more significantly from the data of work [10]. For example, for TEA/MsO and
TEA/HSO4, the difference in the electrical conductivity values at 120 oC was 5.3 and 2.8 mS∙cm-
1, respectively.
The value of the PILs specific electrical conductivity depends on different factors: viscosity,
ion size, ion aggregation and proton transfer mechanism (Grotthus proton-hopping and ion
migration (vehicle)). It is known that if the proton transfer mechanism is ion migration, the
electrical conductivity mainly depends on viscosity. For the PILs synthesized in this work, the
electrical conductivity reduction correlates with the increase in the salts viscosity (see the Table
S1 and Fig. S2 in the ESI). The electrical conductivity of the triethylammonium based salts with
different sulfonic acids increases in the anion series in the following order: SAM < NBSu < BSu
< HSO4 < PTSA < MsO < TfO at 75 oС.
In work [28] devoted to studying the properties of PILs containing low-molecular-weight
Bronsted acids and amine bases it was concluded that electrical conductivity of the salts did not
solely depend on viscosity. It was found that the ethylenediammonium and di-n-butylammonium
formate salts had similar viscosity values but different electrical conductivity. Besides,
pyrrolidinium o-benzoic sulfimide and acetate had similar electrical conductivity values, whereas
their viscosity differed by almost two orders of magnitude. Having studied the properties of PILs
based on di(tri)ethylamine, diethanolamine, pyrrolidine, ethylenediamine, bis(2-
methoxyethyl)amine with formic, acetic, methylsulfonic acids and di-n-butylphosphate, the
authors [44] came to a conclusion that salt electrical conductivity depended to a greater extent on
the anion type rather than on the viscosity value. We did not observe such dependences in the
salts that we studied.
A comparison of the electrical conductivity of TEA salts with the similar values for TEOA
obtained by us earlier in [45] shows (Fig. 5) that the electrical conductivity of the TEA PILs is

ACCEPTED MANUSCRIPT
mostly higher than that of the TEOA salts, which can be also associated with a higher viscosity
of the latter. The electrical conductivity values of the ТЕOА salts lie within the range of 10−5 ÷
10−3 Ohm-1∙cm-1. The electrical conductivity of the TEOA-based salts also decreases with the
increase in their viscosity.
The temperature dependent electrical conductivity values for TEA/(MsO, TfO) were fitted to
the nonlinear Vogel–Tamman–Fulcher (VTF) equation [74,75]:


 B
T To
  o exp
,




where кo is the electrical conductivity at infinitely high T; B is related to the activation energy Ea;
and To is the ideal glass transition temperature. The VFT parameters calculated from the
electrical conductivities of the salts are summarized in Table 4.
Table 4
VFT parameters for electrical conductivity and ECW of the PILs.
PILs
кo (S∙cm-1)
0.35 ± 0.09
0.80 ± 0.04
0.06 ± 0.01
B (K)
To (K)
R2
0.999
0.999
0.998
TEA/MsO
TEA/TfO
TEA/PTSA*
556 ± 81
925 ± 25
285 ± 37
191 ± 11.5
116 ± 3.2
246 ± 6.3
 the data of the work [15].
The Walden's rule is widely used for analyzing electroconductive properties of liquid systems. If
a liquid consists of independent ions and their diffusion depends on the liquid viscosity, the
Walden plot of such system shows an “ideal” line with the slope close to 1 (we used dilute
aqueous solutions of KCl as the reference system). According to the positions of the
dependences for this compound – above or below this “ideal” line – the ionic liquids are divided
into “poor”, “good” и “superionic” types [76]. The Fig. 6 shows a Walden plot for TEA/TfO and
TEA/MsO.
Fig. 6. Walden plot for TEA/TfO and TEA/MsO. The equivalent conductivity and viscosity
values for 0.01M KCl were taken from [77,78] and [79], respectively.
As the Fig. 6 shows, the dependences for triethylammonium mesylate and triflate are close to the
“ideal” KCl line but are still below it, which can, first and foremost, be explained by the ionic
association of these salts.
Table 5 shows the ECW values for the studied PILs at the temperature of 120 ºС.
Table 5
The ECW values for the PILs at 120 ºС.
PILs
ECW (V) (120 ºC)
TEA/TfO
TEA/MsO
TEA/SAM
TEA/BSu
2.80
2.50
2.0
1.95

ACCEPTED MANUSCRIPT
TEA/NBSu
TEA/SBA
TEA/PTSA*
TEA/HSO4
TEA/SSA
1.85
2.30
2.1
2.5
No well-defined potential
window can be obtained
* the data of the work [15]
For the salts synthesized in this work and triethylammonium tosylate [15], the
electrochemical window increases in the series of the salts as follows: NBSu < BSu < SAM <
PTSA < SBA < HSO4(MsO) < TfO (Table 5, Fig. 7). The ECW for TEA/MsO agrees well with
the data of [44] obtained in similar conditions (2.47 V). In ESI section (see Fig. S3) the cyclic
voltammograms of the TEA/MsO at a high number of cycles are shown. The little shifts of both
cathodic and anodic potentials were observed when the number of cycles was increasing. The
triflate PIL has a wider electrochemical window than the mesylate PIL, because the TfO anion is
more stable against oxidation than the MsO anions. The improved stability is attributed to the
greater delocalization of the negative charge in the triflate anion than in the mesylate anion [30].
o
Fig. 7. Cyclic voltammograms of the PILs at 120 C (a) and of the TEA/TfO at different
temperatures (b). The potential scan rate is 10 mV∙sec-1.
The reduction potential of the TEA cation depends on the anion, lies within the range of
(0.2÷1.1) V and grows in the following order: SAM < BSu(NbSu) < SBA(MsO) < PTSA <
HSO4 < TfO. The TEA/MsO salt has the biggest oxidation potential of the anion (1.9 V), which
is 0.1–0.6 V higher than in the other PILs studied in this work. In all the PILs, a temperature
increase reduces the ECW, both in the anode and the cathode regions. As an example, Fig. 7b

ACCEPTED MANUSCRIPT
shows сyclic voltammograms of the TEA/TfO at different temperatures. The cathodic peak
current at -0.6 V can be associated with the presence of dissolved oxygen in the PIL and, as a
result, with the oxygen reduction reaction (ORR). The authors of [80], supposed that the
possibility of an irreversible two-electron reaction leading to the formation of H2O2/H2O on Pt.
We did not control the water content in the sample after the CV experiment. The increase in the
peak current and the shift of the ORR initial potential to the positive region caused by the
temperature growth are attributed to enhanced kinetics for the ORR [81].
The ECW for triethylammonium salts is smaller than that for similar triethanolammonium PILs
[45], in which the cation reduction potential also depends on the counterion nature. In reference
[82] it is shown that every structure of the triethanolammonium-based PILs is stabilized through
multiple H-bonds between the protons in the N-H and O-H groups of the cation and different
oxygen atoms of the anion acid and the strong electrostatic interactions. It probably leads to
higher TEОA cation decomposition overvoltage, making the ECW value for
triethanolammonium PILs larger than for triethylammonium salts.
Thus, the electrochemical properties of the salts and their thermal behavior are determined by
the nature of both the anion and the cation.
4. Conclusions
Six triethylammonium-based protic ionic liquids (triethylammonium mesylate, triflate,
sulfamate, besylate, nitrobenzenesulfonate, hydrogen sulfate) and two molten salts
(triethylammonium sulfobenzoate and sulfosalicylate) have been synthesized. The PILs have
been characterized by 1H NMR, 13C NMR, 1H/15N NMR and FT-IR spectroscopic methods. The
obtained spectral characteristics confirm salt formation.
In the TEA/SAM(NBSu, HSO4) salts, we observed crystallization/melting processes and
glass transition, while in the TEA/MsO(TfO, BSu, SBA, SSA) salts – only
crystallization/melting transitions. It has been established that the TEA/MsO(TfO) salts have
melting temperatures ~ 33oC and belong to room temperature PILs. The TEA/SBA(SSA) salts
o
melt at the temperatures above 100 C and belong to molten salts. The melting temperatures of
the TEA/SAM(BSu, NBSu, HSO4) are within the range of 66 - 83 oC.
The thermal stability Td of the studied triethylammonium-based ionic liquids lies within the
temperature range of 200–320 oC.
The obtained electrical conductivity values of ТЕА salts are within the range of 10-4 ÷ 10-2
Ohm-1∙cm-1 depending on the temperature and the anion. In the PILs synthesized in this work,
electrical conductivity decrease correlates with the increase in the salt viscosity.
The electrochemical window of the studied PILs at the temperature of 120 ºС ranges from
1.85 to 2.8 V. A temperature increase narrows the ECW in all the studied PILs.
Funding
This work was supported by the Russian Science Foundation (grant No. 16-13-10371).
Acknowledgment
The investigations were carried out on the equipment of the Interlaboratory scientific center “The
Upper Volga Region Center of Physicochemical Research”.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
References

ACCEPTED MANUSCRIPT
[1] M. Freemantle, Chem. Eng. News 76 (1998) 32–37.
[2] M.J. Earle, K. R. Seddon, Pure Appl. Chem. 72 (2000) 1391–1398.
[3] D. Kogelnig, A. Stojanovic, M. Galanski, M. Groessl, F. Jirsa, R. Krachler, B.K. Keppler,
Tetrahedron Lett. 49 (2008) 2782–2785.
[4] R. Kordala-Markiewicz, H. Rodak, B. Markiewicz, F. Walkiewicz, A. Sznajdrowska, K.
Materna, K. Marcinkowska, T. Praczyk, J. Pernak, Tetrahedron 70 (2014) 4784–4789.
[5] Z. Chen, Sh. Liu, Z. Li, Q. Zhang, Y. Deng, New J. Chem. 35 (2011) 1596–1606.
[6] L. Timperman, F. Beguin, E. Frackowiak, M. Anouti, J. Electrochem. Soc. 161 (2014)
A228–A238.
[7] Md.A.B.H. Susan, A. Noda, S. Mitsushima, M. Watanabe, Chem. Commun. 8 (2003) 938–
939.
[8] A. Fernicola, B. Scrosati, H. Ohno, Ionics 12 (2006) 95–102.
[9] K. Wippermann, J.Wackerl, W. Lehnert, B. Huber, C. Korte, J. Electrochem. Soc. 163
(2016) F25–F37.
[10] H. Nakamoto, M. Watanabe, Chem. Commun. 43 (2007) 2539−2541.
[11] S.S. Sekhon, P. Krishnan, B. Singh, K. Yamada, C.S. Kim, Electrochim. Acta 52 (2006)
1639–1644.
[12] D.R. MacFarlane, N. Tachikawa, M. Forsyth, J.M. Pringle, P.C. Howlett, G.D. Elliott, J.H.
Davis, M. Watanabe, P. Simon, C.A. Angell, Energy Environ. Sci. 7 (2014) 232−250.
[13] J.-P. Belieres, D. Gervasio, C.A. Angell, Chem. Commun. 46 (2006) 4799–4801.
[14] M. Martinez, Y. Molmeret, L. Cointeaux, C. Iojoiu, J.-C. Lepretre, N. El Kissi, P.
Judeinstein, J.-Y. Sanchez, J. Power Sources 195 (2010) 5829–5839.
[15] L.E. Shmukler, M.S. Gruzdev, N.O. Kudryakova, Yu.A. Fadeeva, A.M. Kolker, L.P.
Safonova, RSC Adv. 6 (2016) 109664–109671.
[16] M. Martinez, C. Iojoiu, P. Judeinstein, L. Cointeaux, J.-C. Lepretre, J.-Y. Sanchez, ECS
Trans. 25 (2009) 1647–1657.
[17] I.V. Fedorova, M.A. Krestyaninov, L.P. Safonova, J. Phys. Chem. A 121 (2017) 7675–
7683.
[18] Ch. Zhao, G. Burrell, A.A.J. Torriero, F. Separovic, N.F. Dunlop, D.R. MacFarlane, A.M.
Bond, J. Phys. Chem. B 112 (2008) 6923–6936.
[19] T.L. Greaves, A. Weerawardena, I. Krodkiewska, C.J. Drummond, J. Phys. Chem. B 112
(2008) 896–905.
[20] P. Attri, P.M. Reddy, P. Venkatesu., A. Kumar, T. Hofman, J. Phys. Chem. B 114 (2010)
6126–6133.
[21] P. Attri, P.M. Reddy, P. Venkatesu, Indian J. Chem. 49A (2010) 736–742.
[22] T. Kavitha, P. Attri, P. Venkatesu, R.S. Rama Devi, T. Hofman, J. Chem. Thermodyn. 54
(2012) 223–237.
[23] V. Govinda, P.M. Reddy, I. Bahadur, P. Attri, P. Venkatesu, P. Venkateswarlu,
Thermochim. Acta 556 (2013) 75–88.
[24] R. Umapathi, P. Attri, P. Venkatesu, J. Phys. Chem. B 118 (2014) 5971−5982.
[25] V. Govinda, P. Attri, P. Venkatesu, P. Venkateswarlu, J. Mol. Liq. 164 (2011) 218–225.
[26] J.-Ph. Belieres, C.A. Angell, J. Phys. Chem. B. 111 (2007) 4926–4937.
[27] T. Kavitha, P. Attri, P. Venkatesu, R.S. Rama Devi, T. Hofman, Thermochim. Acta 545
(2012) 131–140.
[28] G.L. Burrell, I.M. Burgar, F. Separovic, N.F. Dunlop, Phys. Chem. Chem. Phys. 12 (2010)
1571–1577.
[29] G.L. Burrell, I.M. Burgar, Q. Gong, N.F. Dunlop, F. Separovic, J. Phys. Chem. B 114
(2010) 11436–11443.
[30] J.L. Lebga-Nebane, S.E. Rock, J. Franclemont, D. Roy, S. Krishnan, Ind. Eng. Chem. Res.
51 (2012) 14084−14098.
[31] C. Iojoiu, M. Martinez, M. Hanna, Y. Molmeret, L. Cointeaux, J.-C. Lepretre, N. El Kissi, J.
Guindet, P. Judeinstein, J.-Y. Sanchez, Polym. Adv. Technol. 19 (2008) 1406–1414.

ACCEPTED MANUSCRIPT
[32] K. Fumino, A.-M. Bonsa, B. Golub, D. Paschek, R. Ludwig, ChemPhysChem 16 (2015)
299–304.
[33] A.-M. Bonsa, D. Paschek, D.H. Zaitsau, V.N. Emel’yanenko, S.P. Verevkin, R. Ludwig,
ChemPhysChem 18 (2017) 1242–1246.
[34] V. Di Noto, M. Piga, G.A. Giffin, S. Lavina, E.S. Smotkin, J.-Y. Sanchez, C. Iojoiu, J.
Phys. Chem. C 116 (2012) 1361–1369.
[35] C. Iojoiu, P. Judeinstein, J.-Y. Sanchez, Electrochim. Acta 53 (2007) 1395–1403.
[36] C. Iojoiu, M. Hana, Y. Molmeret, M. Martinez, L. Cointeaux, N. El Kissi, J. Teles, J.-C.
Lepretre, P. Judeinstein, J.-Y. Sanchez, Fuel Cells 10 (2010) 778–789.
[37] H. Matsumoto, H. Sakaebe, K. Tatsumi, J. Power Sources 146 (2005) 45–50.
[38] L. Timperman, P. Skowron, A. Boisset, H. Galiano, D. Lemordant, E. Frackowiak, F.
Beguin, M. Anouti, Phys. Chem. Chem. Phys. 14 (2012) 8199–8207.
[39] A. Brandt, J. Pires, M. Anouti, A. Balducci, Electrochim. Acta 108 (2013) 226–231.
[40] Md.A.B.H. Susan, A. Noda, M. Watanabe, In Book Ionic Liquids IIIB: Fundamentals,
Progress, Challenges, and Opportunities, ACS Symposium Series, American Chemical
Society, Washington, DC, 2005.
[41] M. Anouti, M. Caillon-Caravanier, C. Le Floch, D. Lemordant, J. Phys. Chem. B 112
(2008) 9406–9411.
[42] B. Gorska, L. Timperman, M. Anouti, J. Pernak, F. Béguin, RSC Adv. 6 (2016) 55144–
55158.
[43] T.L. Greaves, C.J. Drummond, Chem. Rev. 115 (2015) 11379–11448.
[44] X. Lu, G. Burrell, F. Separovic, Ch. Zhao, J. Phys. Chem. B 116 (2012) 9160−9170.
[45] M.S. Gruzdev, L.E. Shmukler, N.O. Kudryakova, A.M. Kolker, Yu.A. Sergeeva, L.P.
Safonova, J. Mol. Liq. 242 (2017) 838–844.
[46] M.Sh. Miran, H. Kinoshita, T. Yasuda, Md.A.B.H. Susan, M. Watanabe, Phys. Chem.
Chem. Phys. 14 (2012) 5178–5186.
[47] M.Sh. Miran, T. Yasuda, Md.A.B.H. Susan, K. Dokko, M. Watanabe, ECS Trans. 50 (2012)
285-291.
[48] T.A. Siddique, S. Balamurugan, S.M. Said, N.A. Sairi, W.M.D.W. Normazlan, RSC Adv. 6
(2016) 18266–18278.
[49] P.K. Chhotaray, R.L. Gardas, J. Chem. Thermodyn. 72 (2014) 117–124.
[50] M.Sh. Miran, T. Yasuda, Md.A.B.H. Susan, K. Dokko, M. Watanabe, RSC Adv. 3 (2013)
4141–4144.
[51] M. Hasani, J.L. Yarger, C.A. Angell, Chem. Eur. J. 22 (2016) 13312–13319.
[52] CRC Handbook of Chemistry and Physics, 95th Edition, Editor W. M. Haynes, CRC Press,
2014.
[53] A.K. Covington, R. Thompson, J. Solution Chem. 3 (1974) 603–617.
[54] K.E. Gutowski, D.A. Dixon, J. Phys. Chem. A 110 (2006) 12044–12054.
[55] E. Raamat, K. Kaupmees, G. Ovsjannikov, A. Trummal, A. Kütt, J. Saame, I. Koppel, I.
Kaljurand, L. Lipping, T. Rodima, V. Pihl, I.A. Koppel, I. Leito, J. Phys. Org. Chem. 26
(2013) 162–170.
[56] J.P. Guthrie, Can. J. Chem. 56 (1978) 2342–2354.
[57] E.J. King, G.W. King, J. Am. Chem. Soc. 74 (1952) 1212–1215.
[58] D.C. French, D.S. Crumrine, J. Org. Chem. 55 (1990) 5494–5496.
[59] Advanced Chemistry Development (ACD/Labs) Software V11.02 (© 1994-2017
ACD/Labs) (predict).
[60] A. Agren, Acta Chem. Scand. 8 (1954) 266–279.
[61] M. Witanowski, L. Stefaniak, G. Webb, In: Annual reports on NMR spectroscopy, Editor
G.A. Webb, Academic Press, London, 1981.
[62] K. Mori, S. Hashimoto, T. Yuzuri, K. Sakakibara, Bull. Chem. Soc. Jpn. 83 (2010) 328–
334.
[63] R.R. Pinto, S. Mattedi, M. Aznar, Chem. Eng. Trans. 43 (2015) 1165–1170.

ACCEPTED MANUSCRIPT
[64] V.H. Paschoal, L.F.O. Faria, M.C.C. Ribeiro, Chem. Rev. 117 (2017) 7053−7112.
[65] G. Smith, U.D. Wermuth, D.S. Sagatys, J. Chem. Crystallogr. 41 (2011) 17–25.
[66] A.R. Neale, S. Murphy, P. Goodrich, C. Hardacre, J. Jacquemin, ChemPhysChem 18 (2017)
2040–2057.
[67] S. Zhang, N. Sun, X. He, X. Lu, X. Zhang, J. Phys. Chem. Ref. Data 35 (2006) 1475–1517.
[68] Y. Shen, D.F. Kennedy, T.L. Greaves, A. Weerawardena, R.J. Mulder, N. Kirby, G. Song,
C.J. Drummond, Phys. Chem. Chem. Phys. 14 (2012) 7981–7992.
[69] Z. Ullah, M.A. Bustam, Z. Man, N. Muhammad, A.S. Khan, RSC Adv. 5 (2015) 71449–
71461.
[70] T. Yasuda, M. Watanabe, MRS Bull. 38 (2013) 560–566.
[71] M. Galinski, A. Lewandowski, I. Stepniak, Electrochim. Acta 51 (2006) 5567–5580.
[72] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Nat. Mater. 8 (2009) 621–
629.
[73] K. Mishra, S.A. Hashmi, D.K. Rai, J. Solid State Electrochem. 18 (2014) 2255–2266.
[74] H. Vogel, Phys. Z. 22 (1921) 645–646.
[75] G. Tammann, W. Hesse, Z. Anorg. Allg. Chem. 156 (1926) 245–257.
[76] M. Yoshizawa, W. Xu, C.A. Angell, J. Am. Chem. Soc. 125 (2003) 15411–15491.
[77] B.B. Owen, H. Zeldes, J. Chem. Phys. 18 (1950) 1083–1085.
[78] H.E. Gunning, A.R. Gordon, J. Chem. Phys. 10 (1942) 126–131.
[79] M. Kaminsky, Z. Phys. Chem. 12 (1957) 206–231.
[80] A. Khan, X. Lu, L. Aldous, Ch. Zhao, J. Phys. Chem. C 117 (2013) 18334−18342.
[81] L. Johnson, A. Ejigu, P. Licence, D.A. Walsh, J. Phys. Chem. C 116 (2012) 18048−18056.
[82] I.V. Fedorova, L.P. Safonova, J. Phys. Chem. A 112 (2018) 4562–4570.

ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Highlights
 Protic ionic liquids based on triethylamine with sulfonic acids were synthesized.
 The PILs were characterized by NMR and FT-IR spectroscopic methods.
 The phase behavior of the PILs using DSC and TGA was studied.
 The conductivity of salts are within the range of 10-4 ÷ 10-2 Ohm-1∙cm-1.
 The electrochemical window of PILs at 120 ºС lies within the range of 1.85–2.8 V.