Effect of Hydroxyl Groups in a Cation Structure on the Properties of Ionic Liquids

Article abstract of DOI:10.1134/S0036024418120245

Two series of imidazolium ionic liquids with the bis(trifluoromethylsulfonyl)imide anion were synthesized, which differ by the presence of a hydroxyl group at the ω-position of the alkyl substituent in the cation structure (n_C = 2–8). The properties of the liquids were studied by DSC, TGA, and IR and NMR spectroscopy. Their thermal stability was studied, and the melting points, viscosity, and volatility in vacuum were measured. The effect of OH groups in the structure of the ionic liquid on its properties was evaluated.

Full text of DOI:10.1134/S0036024418120245

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2018, Vol. 92, No. 12, pp. 2379–2385. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © V.G. Krasovskiy, E.A. Chernikova, L.M. Glukhov, G.I. Kapustin, A.A. Koroteev, 2018, published in Zhurnal Fizicheskoi Khimii, 2018, Vol. 92, No. 12,
pp. 1851–1858.
PHYSICAL CHEMISTRY OF HYBRID NANOMATERIALS
AND MULTICOMPONENT SYSTEMS
Effect of Hydroxyl Groups in a Cation Structure
on the Properties of Ionic Liquids
a,
a
a
a
b
V. G. Krasovskiy *, E. A. Chernikova , L. M. Glukhov , G. I. Kapustin , and A. A. Koroteev
a
Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
b
Moscow Aviation Institute (National Research University), Moscow, 125993 Russia
*
е-mail: miyusha@mail.ru
Received May 21, 2018
Abstract—Two series of imidazolium ionic liquids with the bis(trifluoromethylsulfonyl)imide anion were syn-
thesized, which differ by the presence of a hydroxyl group at the ω-position of the alkyl substituent in the cat-
ion structure (n = 2–8). The properties of the liquids were studied by DSC, TGA, and IR and NMR spec-
C
troscopy. Their thermal stability was studied, and the melting points, viscosity, and volatility in vacuum were
measured. The effect of OH groups in the structure of the ionic liquid on its properties was evaluated.
Keywords: ionic liquids, bis(trifluoromethylsulfonyl)imide, imidazole, OH groups, viscosity, thermal stabil-
ity, volatility
DOI: 10.1134/S0036024418120245
Ionic liquids (ILs) are melts of organic salts with viscosity, evaporation enthalpy, etc. [12]. The imidaz-
bulky cations and anions characterized by two main olium ILs in both solid and liquid state are character-
specific properties: high polarity and low volatility. ized by strong hydrogen bonding between hydrogen at
High polarity is determined by the ionic character of the carbon atom in the position 2 of imidazolium ring
ILs, and low volatility is due to the Coulomb interac- and the anion, which determines many of their prop-
tions between charged cations and anions. It was erties [13, 14]. The presence of OH groups in the alkyl
believed for a long time that ionic liquids, like inor- substituent in the imidazolium cation promotes the
ganic salts, hardly evaporate. That is why they were intermolecular interactions in the IL due to the
considered promising for use as solvents that do not appearance of a mobile hydrogen atom at the oxygen
pollute the atmosphere in industry (“green chemis- atom in the cation structure so that the interaction
try”) [1–4]. However, further studies showed that at between the OH group and the anion becomes prefer-
relatively high temperatures (200°C), the saturated able to that between the OH group and water [15]. The
vapor pressure of ILs is ~0.01–0.05 Pa depending on studies of solvatochromism in various ILs having OH
the structure [5, 6], and their evaporation is character- groups in their structure also show that the introduc-
–1
ized by enthalpies of 130–190 kJ mol at 25°C [7–9]. tion of OH groups in the cation structure increases
The volatility of conventional imidazolium ILs in vac- their polarity [16] and significantly increases the
uum at high temperatures (~200°C and higher) is esti- hydrophilicity of imidazolium ILs [17].
–
1
–2
mated at 20–30 mg h cm [6, 10, 11]. It can be
reduced by increasing the molecular mass of ILs or by
promoting the intermolecular interaction due to the
introduction of polar groups in the structure of IL. An
increase in the molecular mass of organic substituents
in imidazolium cation leads to lower thermal stability
and higher viscosity of IL. The introduction of the
second ion pair in the structure of IL generally leads to
an increase in the melting point of IL. The volatility
can also be reduced by introducing polar groups capa-
ble of interacting with ions to form hydrogen bonds in
the structure of IL.
A considerable number of imidazolium, pyridin-
ium, pyrrolidinium, morpholinium, and ammonium
liquids containing hydroxyl groups in both cations and
anions have already been synthesized [18]. ILs with
hydroxyl groups have found use in various fields of sci-
ence and technology. Ionic liquids with OH groups are
widely used as polar solvents and catalysts in organic
synthesis. 1-(2-Hydroxyethyl)-3-methylimidazolium
chloride catalyzes the condensation of aromatic alde-
hydes with compounds containing active methylene
groups (Knoevenagel condensation) with an up to
97% yield of the desired products [19]. The use of IL
Although Coulomb forces are the dominant type of with hydroxyl groups in the cation structure makes it
interaction in ILs, hydrogen bonds also have a signifi- possible to obtain cyclic carbonates with up to 99%
cant effect on their properties such as melting point, selectivity and yield by cycloaddition of epoxides with
2379

2380
KRASOVSKIY et al.
CO [20–22]. Ionic liquids with OH groups in the cat- in vacuum was evaluated using a McBain quartz bal-
2
ion are successfully used for utilization of CO ; the ance. A sample (~0.2 g) was placed in a quartz cup
2
fixed to the movable end of the helical spring of the
polymer material obtained with participation of the
,3-bis(2-hydroxyethyl)imidazolium
phosphate IL has the highest absorption capacity The cartridge with the sample was placed in a thermo-
2
1
hexafluoro- balance. The surface area of the liquid was ~1.7 cm .
(
10 mol %) among all ionic liquids [23]. Hydroxyl-con- statted aluminum box. The spring extension was deter-
taining imidazolium ILs have good electrochemical mined from the change in the position of the reference
properties. 1-(2-Hydroxyethyl)-3-methylimidazolium marks using a KM-8 cathetometer with an accuracy of
tetrafluoroborate and hexafluorophosphate have wide ±0.02 mm. The quartz helical spring used had a sensi-
electrochemical windows (5.4–6.4 V) and high ionic tivity of 0.3709 mm/mg. The unit was evacuated using
–1
a diffusion pump. Before the measurements, the sam-
ples were dried to constant weight (~15 h) in vacuum
conductivity (2.1–4.6 mS cm ) at 25°C [24].
In the analysis of alkanes, alcohols, ethers, etc., the
capillary columns for GLC containing 1-methyl-2-
butyl-3-(6-hydroxyhexyl)imidazolium and 1-methyl-
–4
better than 10 Torr at 100°C.
Commercial 1,2-dimethylimidazole (Sigma-
2
-butyl-3-(8-hydroxyoctyl)imidazolium bis(triflu- Aldrich) was dried over KOH at 50°C and then dis-
romethylsulfonyl) imides as the stationary phase show tilled in vacuum. Chloroalcohols and alkyl halides:
high thermal stability, symmetric chromatographic 2-chloroethanol (99%), 3-chloro-1-propanol (98%),
peaks, and selectivity compared with conventional 6-chloro-1-hexanol (96%), 8-chloro-1-octanol
ionic liquids [25]. In the synthesis of nanoobjects, (98%), bromoethane (98%), 1-chloropropane (98%),
OH-ILs can be used as both reducing agents and stabi- 1-chlorobutane (99%), 1-chlorohexane (99%), and
+
lizers of metal nanoparticles [26]. When Ag is reduced 1-chloroctane (99%), as well as lithium bis(trifluoro-
0
methanesulfonyl)imide (99%), were purchased from
Acros and Sigma-Aldrich. All the organic solvents
used in the synthesis were preliminarily dried over
to Ag in a hydroxyl-containing ionic liquid, the OH
group is oxidized to the aldehyde one [27]. Recently,
the use of hydroxyl-containing ionic liquids as heat car-
riers in high vacuum was also reported [28, 29].
CaH and distilled.
2
The changes in the properties of OH-containing
ionic liquids were discussed only based on the general
principles or comparison of only one pair of samples:
alkyl–IL and OH–IL. The presence of OH groups in
the structure of ionic liquids due to hydrogen bonding
General Procedure for the Synthesis
of Hydroxyl-Containing Ionic Liquids (1, 2, 4, 5)
Quaternization of 1,2-dimethylimidazole with
leads to an increase in the density of IL [30], and the chloroalcohols was performed in acetonitrile (50%
increase in the length of the alkyl radical in the cation solution) at the boiling point of the solvent and an
leads to higher viscosity [31, 32].
equimolar ratio of the starting reagents for 48 h. After
the solvent was distilled off in vacuum, lithium bis(tri-
fluoromethylsulfonyl)imide (10% excess) in water
The present paper is devoted to a study of the prop-
erties of two series of imidazolium ionic liquids with
different lengths of alkyl substituent in the cation,
which differ in the presence of OH group in theω-posi-
tion of the substituent.
(
30% solution) was added to the solid residue, and the
reaction mixture was stirred for 30 min. The desired
product was separated as a water-insoluble lower liq-
uid phase. The resulting IL was repeatedly washed
with small portions of distilled water until negative
EXPERIMENTAL
reaction to the chloride ion with AgNO and then
3
1
13
dried in vacuum, gradually increasing the temperature
The H and C NMR spectra were measured on a
Bruker AM-300 NMR spectrometer. The thermo-
gravimetric analysis was performed on a Derivato-
to 100°C for 10 h.
1,2-Dimethyl-3-(2-hydroxyethyl)imidazolium bis(tri-
1
graph-C instrument (MOM, Hungary) in an argon fluoromethylsulfonyl)imide (1). Yield 91%. H NMR
–1
atmosphere at a heating rate of 10 K min (~20 mg (DMSO-d , 300.13 MHz), δ, ppm: 2.58 s (3Н, СН -
6
3
samples). The glass transition temperatures were С), 3.70 m (2Н, СН О), 3.76 s (3Н, СН -N), 4.18 m
2
3
determined by DSC using a DSC-822e differential
scanning calorimeter (Mettler-Toledo, Switzerland)
in the temperature range from –100 to 100°C at a
(
2Н, СН -N), 5.08 br. s (1Н, ОН), 7.60 m (2Н
).
2
imidaz
13
С NMR (DMSO-d , 75.47 MHz), δ, ppm: 9.65,
6
3
4.90, 50.71, 60.04, 113.53, 117.79, 121.61, 122.05,
–1
heating rate of 10 K min in an argon atmosphere.
The kinematic viscosity was measured with an Ost-
wald viscosimeter with a capillary diameter of 1.2 mm.
The viscosimeter was calibrated at 25°C using ethylene
glycol (Aldrich, 99.8%, <0.01% water) as the reference
liquid. The density was determined using a pycnome-
ter with a nominal volume of 1 mL. The pycnometer
1
22.48, 126.31, 145.22. Found, %: C 25.57, H 3.04, F
2
7.15, S 15.05, N 10.01. Calculated, %: С 25.66, H
.11, F 27.05, S 15.22, N 9.97.
3
1
,2-Dimethyl-3-(3-hydroxypropyl)imidazolium bis(tri-
1
fluoromethylsulfonyl)imide (2). Yield 88%. H NMR
(
DMSO-d , 300.13 MHz), δ, ppm: 1.88 m (2Н,
6
was calibrated with distilled water. The volatility of IL СН СН О), 2.58 s (3Н, СН -С), 3.42 m (2Н,
2 2 3
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 92
No. 12
2018

EFFECT OF HYDROXYL GROUPS IN A CATION STRUCTURE
2381
СН О), 3.75 s (3Н, СН -N), 4.17 m (2Н, СН -N), was then removed, and lithium bis(trifluoromethyl-
2
3
2
1
3
sulfonyl)imide (10% excess) in water (30% solution)
was added to the solid residue, and the reaction mix-
ture was stirred for 30 min. The desired product pre-
cipitated from water into the lower liquid phase. The
upper aqueous layer was decanted, methylene chloride
4
.70 br. s (1Н, ОН), 7.60 m (2Н
DMSO-d , 75.47 MHz), δ, ppm: 9.43, 32.41, 35.02,
6
). С NMR
imidaz
(
4
5.22, 57.34, 113.53, 117.80, 121.32, 122.06, 122.73,
26.32, 144.79. Found, %: C 27.56, H 3.48, F 26.19, S
4.79, N 9.59. Calculated, %: C 27.59, H 3.47, F 26.18,
S 14.73, N 9.65.
,2-Dimethyl-3-(4-hydroxybutyl)imidazolium bis(tri-
1
1
(
equal volume) was added to the lower layer, and the
resulting solution was washed again with distilled
1
–
water till negative reaction to Cl with AgNO . The
1
3
fluoromethylsulfonyl)imide (3). Yield 84%. H NMR
ionic liquids were dried by azeotropic distillation of
absolute methylene chloride, then the traces of the
organic solvent were removed in vacuum by heating
(3 h at 100°C).
(
DMSO-d , 300.13 MHz), δ, ppm: 1.42 m (2Н,
6
СН СН О), 1.76 m (2Н, СН СН N), 2.58 s (3Н,
2
2
2
2
СН -С), 3.44 m (2Н, СН О), 3.75 s (3Н, СН -N),
3
2
3
4
(
.13 m (2Н, СН -N), 4.50 br. s (1Н, ОН), 7.60 m
2
1
,2-Dimethyl-3-ethylimidazolium
methylsulfonyl)imide (I). Yield 89%. H NMR
DMSO-d , 300.13 MHz), δ, ppm: 1.36 m (3Н,
bis(trifluoro-
1
3
2Н
). С NMR (DMSO-d , 75.47 MHz), δ,
1
imidaz
6
ppm: 9.37, 26.49, 29.19, 32.93, 47.93, 60.46, 113.51,
17.79, 121.20, 122.05, 122.65, 126.31, 144.57. Found,
: C 29.33, H 3.87, F 25.35, S 14.29, N 9.40. Calcu-
lated, %: C 29.40, H 3.81, F 25.37, S 14.27, N 9.35.
,2-Dimethyl-3-(6-hydroxyhexyl)imidazolium bis(tri-
(
6
1
%
СН СН -N), 2.59 s (3Н, СН -С), 3.76 s (3Н, СН -
3
2
3
3
13
N), 4.16 m (2Н, СН -N), 7.63 m (2Н
). С NMR
2
imidaz
(
DMSO-d , 75.47 MHz), δ, ppm: 9.30, 15.02, 34.93,
6
1
4
3.21, 113.51, 117.81, 120.65, 122.07, 122.75, 126.33,
1
fluoromethylsulfonyl)imide (4). Yield 92%. H NMR
1
44.43. Found, %: C 26.73, H 3.29, F 28.15, S 15.77,
(
DMSO-d , 300.13 MHz), δ, ppm: 1.29 m (4Н, О-
6
N 10.31. Calculated, %: С 26.67, H 3.23, F 28.12, S
СН СН (СН ) СН СН -N), 1.39
m
m
(2Н, О-
(2Н, О-
2
2
2 2
2
2
15.82, N 10.37.
СН СН (СН ) СН СН -N), 1.70
2
2
2 2
2
2
1
,2-Dimethyl-3-propylimidazolium bis(trifluoro-
СН СН (СН ) СН СН -N), 2.57 s (3Н, СН -С),
1
2
2
2 2
2
2
3
methylsulfonyl)imide (II). Yield 88%. H NMR
3
(
.38 m (2Н, О-СН СН (СН ) СН СН -N), 3.74 s
2
2
2 2
2
2
(
DMSO-d , 300.13 MHz), δ, ppm: 0.88 m (3Н,
6
3Н,
СН -N),
4.09
m
(2Н,
О-
3
СН СН СН -N), 1.76 m (2Н, СН СН СН -N),
3
2
2
3
2
2
СН СН (СН ) СН СН -N), 4.35 br. s (1Н, ОН),
2
2
2 2
2
2
2
.58 s (3Н, СН -С), 3.75 s (3Н, СН -N), 4.08 m (2Н,
3 3
13
7
.62 m (2Н
). С NMR (DMSO-d , 75.47 MHz),
13
imidaz
6
СН -N), 7.63 m (2Н
). С NMR (DMSO-d ,
6
2
imidaz
δ, ppm: 9.25, 25.32, 25.81, 29.55, 32.61, 34.85, 48.00,
1.01, 113.52, 117.78, 121.13, 122.04, 122.60, 126.31,
44.49. Found, %: C 32.47, H 4.28, F 23.94, S 13.48,
N 9.01. Calculated, %: С 32.70, H 4.43, F 23.88, S
3.43, N 8.80.
,2-Dimethyl-3-(8-hydroxyoctyl)imidazolium bis(tri-
7
1
5.47 MHz), δ, ppm: 9.50, 10.80, 22.99, 35.06, 49.36,
6
13.52, 117.81, 121.31, 122.07, 122.73, 126.36, 144.68.
1
Found, %: C 28.59, H 3.68, F 27.19, S 15.23, N 9.98.
Calculated, %: C 28.64, H 3.61, F 27.18, S 15.29, N
1
1
0.02.
1
1
,2-Dimethyl-3-butylimidazolium
bis(trifluoro-
1
fluoromethylsulfonyl)imide (5). Yield 94%. H NMR
methylsulfonyl)imide 1,2-dimethyl-3-buthylimidazo-
(
DMSO-d , 300.13 MHz), δ, ppm: 1.27 m (8Н, О-
1
6
lium (III). Yield 93%. H NMR (DMSO-d ,
6
СН СН (СН ) СН СН -N), 1.41
m
m
(2Н, О-
(2Н, О-
2
2
2 4
2
2
300.13 MHz), δ, ppm: 0.91 m (3Н, СН СН СН СН -
3
2
2
2
СН СН (СН ) СН СН -N), 1.71
2
2
2 4
2
2
N), 1.32 m (2Н, СН СН СН СН -N), 1.71 m (2Н,
3 2 2 2
СН СН (СН ) СН СН -N), 2.57 s (3Н, СН -С),
2
2
2 4
2
2
3
СН СН СН СН -N), 2.58 s (3Н, СН -С), 3.75 s
3 2 2 2 3
3
.39 m (2Н, О-СН СН (СН ) СН СН -N), 3.75 s
2
2
2 4
2
2
(3Н, СН -N), 4.11 m (2Н, СН -N), 7.62 m (2Н
).
imidaz
3
2
(
3Н,
СН -N),
4.09
m
(2Н,
О-
13
3
С NMR (DMSO-d , 75.47 MHz), δ, ppm: 9.45,
6
СН СН (СН ) СН СН -N), 4.31 br. s (1Н, ОН),
2
2
2 4
2
2
1
3.64, 19.28, 31.58, 35.01, 47.78, 113.53, 117.81, 121.25,
122.08, 122.71, 126.35, 144.61. Found, %: C 30.43, H
.01, F 26.29, S 14.74, N 9.66. Calculated, %: C 30.48,
H 3.95, F 26.30, S 14.80, N 9.70.
,2-Dimethyl-3-hexylimidazolium
methylsulfonyl)imide (IV). Yield 94%. H NMR
13
7
.62 m (2Н
). С NMR (DMSO-d , 75.47 MHz),
imidaz 6
δ, ppm: 9.49, 25.84, 25.98, 28.95, 29.18, 29.59, 32.91,
5.04, 47.97, 61.14, 113.87, 117.81, 121.26, 122.07,
22.72, 126.79, 144.61. Found, %: C 35.67, H 4.91, F
2.41, S 12.67, N 8.23. Calculated, %: С 35.64, H 4.98,
F 22.55, S 12.69, N 8.31.
4
3
1
1
bis(trifluoro-
2
1
(
DMSO-d , 300.13 MHz), δ, ppm: 0.87 m (3Н,
6
СН СН СН СН СН СН -N), 1.29
m
m
(6Н,
(2Н,
3
2
2
2
2
2
General Procedure for the Synthesis of Ionic Liquids with СН СН СН СН СН СН -N), 1.72
3
2
2
2
2
2
an Alkyl Substituent in the Cation (I–V)
СН СН СН СН СН СН -N), 2.58 s (3Н, СН -С),
3
2
2
2
2
2
3
3
.76 s (3Н, СН -N), 4.11 m (2Н, СН -N), 7.62 m
Quaternization of 1,2-dimethylimidazole with
3
2
13
alkyl halides was performed in acetonitrile (50% solu- (2Нimidaz). С NMR (DMSO-d
, 75.47 MHz), δ,
6
tion) at the boiling point of the solvent and an equim- ppm: 9.44, 14.08, 22.30, 25.65, 29.53, 31.06, 35.01,
olar ratio of the starting reagents for 48 h. The solvent 47.99, 113.51, 117.82, 121.24, 122.08, 122.70, 126.34,
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 92 No. 12 2018

2382
KRASOVSKIY et al.
Cl
n
OH
LiN(SO2CF3)2
−LiCl
N
N
N
N
+
N
N
N
+
+
n
OH
n
OH
Cl−
−
N(SO CF )
2
3 2
n = 1−7
(
1, 2, 4, 5)
Cl
n
LiN(SO2CF3)2
N
N
N
N
N
+
n
−LiCl
n
Cl−
−
N(SO CF )
2
3 2
n = 1−7
(I, II, III, IV, V)
Scheme 1.
1
44.59. Found, %: C 35.32, H 4.93, F 23.94, S 13.45, performed in water. An aqueous solution of lithium
N 8.79. Calculated, %: С 35.37, H 4.87, F 23.97, S bis(trifluoromethylsulfonyl)imide (Tf NLi) was
2
13.49, N 8.84.
added to the corresponding chloride salt, and the
–
1,2-Dimethyl-3-octylimidazolium
bis(trifluoro- resulting ionic liquid with the Tf
2
N anion precipi-
1
methylsulfonyl)imide (V). Yield 93%. H NMR tated from water into a separate phase. Due to this, the
(
DMSO-d , 300.13 MHz), δ, ppm: 0.87 m (3Н, desired products were obtained in high yield and with
6
high purity because the impurities (excess Tf NLi and
СН (СН ) СН СН –N)
1.29
1.72
m
(10Н,
(2Н,
2
3
2 5
2
2
the resulting lithium chloride) are soluble in water. In
contrast to hydroxyl-containing ILs, alkyl ionic liq-
uids are miscible with dichloromethane. Therefore,
purification of these ILs from lithium salts by
extraction with water was performed after dissolving
them in CH Cl (30% solution), and subsequent dry-
СН (СН ) СН СН –N),
m
3
2 5
2
2
СН (СН ) СН СН –N), 2.58 s (3Н, СН -С), 3.76 s
3
2 5
2
2
3
(
3Н, СН -N), 4.11 m (2Н, СН (СН ) СН СН –N),
3
3
2 5
2
2
13
7.63 m (2Н
). С NMR (DMSO-d , 75.47 MHz),
imidaz 6
δ, ppm: 9.43, 14.18, 22.45, 26.01, 28.87, 28.93, 29.60,
2
2
3
1.59, 35.00, 47.99, 113.51, 117.81, 121.24, 122.08,
22.70, 126.35, 144.59. Found, %: C 36.77, H 5.20, F
3.31, S 13.04, N 8.55. Calculated, %: С 36.81, H 5.15,
F 23.29, S 13.10, N 8.58.
ing was performed by azeotropic distillation of the
absolute solvent.
1
2
The preparation of ionic liquid 3 according to the
above scheme is impossible because of cyclization of
4
-chlorobutanol in the presence of 1,2-dimethylimid-
RESULTS AND DISCUSSION
,2-Dimethyl-3-alkylimidazolium ionic liquids
azole to form tetrahydrofuran. Therefore, IL 3 was
synthesized by the procedure of [33] using a trimeth-
ylsilyl protecting group.
1
–
with the bis(trifluoromethylsulfonyl)imide (Tf N )
anion were chosen as the objects of our study. The
2
–
Nine of ten obtained ILs with Tf N anions are liq-
2
replacement of the mobile hydrogen atom in the sec- uids under normal conditions (Table 1). 1,2-
ond position in imidazole by the methyl group will Dimethyl-3-ethylimidazolium bis(trifluoromethyl-
preclude the effect on the intermolecular hydrogen sulfonyl)imide (I) melts at 28°C.
bonding interactions of this hydrogen atom with the
Crystallization of hydroxyl-containing ionic liq-
–
anion. Due to the Tf N anion, the studied ILs have
2
uids 1–5 under the conditions of DSC study was not
comparatively low viscosity and high thermal stability
observed even at very low temperatures (to ~–60°C);
[28].
Table 1 shows their glass transition temperatures
The ILs (except 3) were synthesized in two stages determined by this method. The alkylimidazolium liq-
(
Scheme 1). At the first stage, 1,2-dimethylimidazole uids not containing hydroxyl groups can be divided
was quaternized with ω-chloroalcohols (1, 2, 4, and 5) into two groups: (1) I, IV, and V have pronounced
or alkyl halides (I, II, III, IV, and V). At the second melting peaks on the DSC curve (Table 1); (2) II and
stage, the chloride anion was exchanged by the bis(tri- III do not crystallize under the experimental condi-
fluoromethylsulfonyl)imide anion. Both processes are tions and are characterized only by glass transition
well studied, proceed completely, and almost without temperatures. The crystallization of IL I is determined
formation of by-products. The chlorides synthesized by strong Coulomb interaction of the cation with the
at the first stage are solids. As all imidazolium halides bulky fluorinated anion, which is apparently not hin-
are readily soluble in water, the anion exchange was dered by the three short alkyl substituents in the cat-
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EFFECT OF HYDROXYL GROUPS IN A CATION STRUCTURE
Table 1. Physicochemical properties of ionic liquids
2383
IL
Structural formula
tglass, °С (DSC)
tdestr, °С (TGA)
η, cSt (30°С)
1
[НОC MMIm]Tf N
–69
–66
–63
–62
–58
28*
–77
–75
0*
428
414
403
399
417
446
446
451
447
426
84.5
100.2
134.5
186.4
217.2
38.9
2
2
2
[НОC MMIm]Tf N
3 2
3
[НОC MMIm]Tf N
4 2
4
[НОC MMIm]Tf N
6 2
5
[НОC MMIm]Tf N
8
2
I
[C MMIm]Tf N
2
2
II
III
IV
V
[C MMIm]Tf N
50.6
3
2
[C MMIm]Tf N
54.5
4
2
[C MMIm]Tf N
72.4
6
2
[C MMIm]Tf N
4*
93.9
8
2
*
t .
m
–
ion. Therefore, even with the Tf N anion in its struc- liquids, which reduce the thermal stability of IL. The
2
thermal destruction of imidazolium cations occurs
with formation of amines during the cleavage of the
aromatic ring as a result of the nucleophilic attack of
the anion [34, 35]. The decrease in the decomposition
temperature of ionic liquids resulting from the intro-
duction of OH groups in the cation structure can be
attributed either to the effect of association (formation
of hydrogen bonds) on the thermal destruction by this
mechanism or to the acidity of the OH groups and the
possible start of thermal destruction involving the
hydroxyl fragment of the cation.
Figure 1 shows the temperature dependences of the
kinematic viscosity of the synthesized ILs. As the ionic
liquids being compared have the same anion and the
cation structure differs only by the presence of a
hydroxyl group, the obtained data allow us to directly
ture, the ionic liquid is a solid under normal condi-
tions. When the length of the substituent in
imidazolium cation is increased to six or eight methy-
lene groups (IV and V), nonpolar and polar micro-
phases are likely to form, which consist of methylene
groups and ions. The presence of these phases leads to
crystallization of ionic liquids, but already at tempera-
tures below room temperature, alkylimidazolium
ionic liquids II and III occupy an intermediate posi-
tion: the substituents in the cation consisting of three
and four methylene units hinder the interaction
between the ions, but are not long enough to form a
separate phase. Therefore, these ILs are characterized
only by glass transition temperatures, which are even
lower than those of OH-IL. A comparison of the DSC
(
t and t ) data for two series of ionic liquids shows
m glass
that the presence of hydroxyl groups in the structure of trace its effect on the viscosity of the IL. Table 1 shows
the IL cation inhibits their crystallization. The differ- the viscosity values of ionic liquids 1–5 and I–V at
ence of 10 K between the glass transition temperatures 30°C. The presence of a polar group capable of hydro-
of alkyl ILs II and III and OH-ILs 2 and 3, whose gen bonding in the IL structure leads to a 2–2.5-fold
alkyl chains have the same length, suggests that the increase in the viscosity of the IL at this temperature
mechanisms of their glass transition are different.
depending on the length of the alkyl radical. This
increase in viscosity is difficult to explain only by the
intermolecular interaction of OH groups with one
another. The temperature dependences of the kine-
matic viscosity of ionic liquids presented in Fig. 1 are
The glass transition temperature determines the
lower limit of the working temperature range of the
ionic liquid. The upper limit is determined by the
decomposition temperature of the IL. The thermal
stability of the synthesized ILs was studied by thermo-
gravimetric analysis (TGA) (heating rate 10 K/min,
argon medium). The ionic liquids are thermally stable,
and their destruction temperatures are >400°C (in
2
well approximated (R > 0.99) by the Vogel–Tam-
mann–Fulcher (VTF) equation, which describes the
temperature dependences of the viscosity of melts
[
36]:
argon). A comparison of the t
of OH-IL and ILs
destr
a
ln ν =
+ b.
without hydroxyl groups in the cation shows (Table 1)
that the decomposition temperatures of alkyl ILs are
T −T0
2
0–50 K higher. The differences are explained by the An analysis of the parameters of the VTF equation
presence of polar OH groups in the structure of ionic (Table 2) shows that two ionic liquids are notable:
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 92 No. 12 2018

2384
KRASOVSKIY et al.
η, cSt
Density, g/mL
.55
2
50
00
50
00
1
5
4
1.50
1.45
1.40
2
1
1
3
2
HO-IL
IL
V
1
1.35
1.30
IV
III
II
50
I
2
3
4
5
6
7
8
n
C
0
2
0
40
60
80
100
120
Т, °С
Fig. 2. Dependences of the density of IL (23°C) on the
length of the substituent in the cation.
Fig. 1. Temperature dependences of the kinematic viscos-
ity (η) of ionic liquids.
1
(a = 251.6) and V (a = 401.5). The obtained data are the aliphatic “microphase” itself. The difference in
evidently explained by the structural peculiarities of the densities of ionic liquids with substituents of the
these liquids. As the substituents in the imidazolium same length is determined by the denser packing of IL
cation are the shortest (n = 1 and 2), in ionic liquid 1 due to hydrogen bonding with OH groups.
C
the interactions due to Coulomb forces and hydrogen
The low volatility of ionic liquids is also caused by
bonding are most pronounced. For ionic liquid V,
the strong intermolecular Coulomb interaction. The
hydroxyl groups are involved in this interaction due to
hydrogen bonding, which leads to a three- or fourfold
decrease in the volatility of IL (Fig. 3). The differences
in the volatility are slightly reduced when the substitu-
ent length in the cation increases due to the emerging
steric effect.
whose cation structure contains the longest alkyl sub-
stituent (without the OH group) with n = 8, the Cou-
C
lomb interaction between the cations and anions is
weakest (due to the steric effect), and the contribution
of van der Waals interactions between cations is high-
est because of the long alkyl radical. These data, along
with the viscosities of OH-IL and alkyl ILs, indicate
that the Coulomb interaction is critical to the viscosity
of ionic liquids and involves the polar groups present
in the structure of IL.
To summarize, the introduction of a hydroxyl
group in the structure of the cation of 1,2,3-trialkylim-
idazolium ILs with the bis(trifluoromethylsulfo-
nyl)imide anion changes the whole set of properties of
ILs relative to those of analogous alkylimidazolium
The density of ionic liquids is determined both by
the presence of polar groups in the cation structure
and by the length of the side chain. The dependences ILs. Depending on the length of the ω-hydroxyalkyl
of the IL density on the length of the substituent for fragment, the presence of the OH group leads to a
alkyl and hydroxyalkyl ILs are nearly parallel (Fig. 2). decrease in the thermal stability of the IL by 10–20°C
The density decreases when the length of substituent (according to TGA data), a two- or threefold increase
increases because of the decrease in the intermolecu- in viscosity at 30°C, an increase in the density by 3–
lar interaction in ILs caused by the increased steric 4%, and a three- or fourfold decrease in the volatility
effect of substituent and because of the low density of in vacuum.
Table 2. Parameters of the Vogel–Tammann–Fulcher equation for the ionic liquids
IL
1
2
3
4
5
I
II
III
IV
V
а
251.6
343.0
341.2
298.7
343.3
318.2
329.8
326.7
355.3
401.5
–
b
0.603
203.7
0.920
185.4
0.852
188.6
0.685
202.2
0.818
194.5
0.782
169.0
0.860
174.5
0.823
175.4
0.913
174.9
1.038
T0
169.8
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 92
No. 12
2018

EFFECT OF HYDROXYL GROUPS IN A CATION STRUCTURE
2385
1
2. K. Fumino, T. Peppel, M. Geppert-Rybczyńska, et al.,
−1
−2
Volatility, mg h cm
Phys. Chem. Chem. Phys. 13, 14064 (2011).
3
0
5
0
5
0
13. Y. Deng, P. Besse-Hoggan, M. Sancelme, et al.,
J. Hazard. Mater. 198, 165 (2011).
1
1
4. J. Dupont, J. Braz. Chem. Soc. 15, 341 (2004).
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0496 (2010).
6. C. Chiappe, C. S. Pomelli, and S. Rajamani, J. Phys.
Chem. B 115, 9653 (2011).
2
2
1
1
1
1
1
7. J. D. Holbrey, L. B. Turner, W. M. Reichert, and
1
R. D. Rogers, Green Chem. 5, 731 (2003).
1
8. S. Tang, G. A. Baker, and H. Zhao, Chem. Soc. Rev.
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2
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Acta 50, 5399 (2005).
Foundation (grant no. 14-19-00503).
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Translated by L. Smolina
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 92 No. 12 2018