Investigation of a family of structurally-related guanidinium ionic liquids through XPS and thermal analysis

Article abstract of DOI:10.1016/j.molliq.2018.12.083

A family of structurally-related guanidinium bistriflimide ionic liquids has been prepared and characterized. TGA analyses showed a high thermal stability for all the proposed ionic liquids while DSC and XPS analyses divided them into distinct subsets depending on whether one or more constraining cycles were present. The results obtained highlighted the influence of the cation structure on some of the physico-chemical properties and thus the possibility to tune them by selecting proper substituents. The solvatochromic parameters of a selected guanidinium IL have also been studied.

Full text of DOI:10.1016/j.molliq.2018.12.083

Journal of Molecular Liquids 277 (2019) 280–289
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Investigation of a family of structurally-related guanidinium ionic liquids
through XPS and thermal analysis
Aldo Moscardini a, Andrea Mezzetta a, Nicola Calisi b,c, Stefano Caporali b,c, Christian Silvio Pomelli a,
a
Lorenzo Guazzelli a, , Cinzia Chiappe
⁎
a
Department of Pharmacy, University of Pisa, Via Bonanno 33, 56126 Pisa, Italy
Department of Industrial engineering (DIEF), University of Firenze, 50139 Firenze, Italy
Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), 50123 Firenze, Italy
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 October 2018
Received in revised form 11 December 2018
Accepted 14 December 2018
Available online 15 December 2018
A family of structurally-related guanidinium bistriflimide ionic liquids has been prepared and characterized. TGA
analyses showed a high thermal stability for all the proposed ionic liquids while DSC and XPS analyses divided
them into distinct subsets depending on whether one or more constraining cycles were present. The results
obtained highlighted the influence of the cation structure on some of the physico-chemical properties and thus
the possibility to tune them by selecting proper substituents. The solvatochromic parameters of a selected
guanidinium IL have also been studied.
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
of selected ILs deriving from the combination of the most studied
1,3 dialkyl imidazolium cation and 16 anions, demonstrated that only
Ionic liquids (ILs) are salts which are, by definition, liquid at a tem-
perature lower than 100 °C. They are composed by an organic cation
and an organic or inorganic anion. During the last 20 years they
attracted a great deal of interest as neoteric solvents characterized by
remarkable and sometimes unique physico-chemical properties [1]. In
particular, the high thermal stability, negligible vapor pressure, low
flammability and wide electrochemical window suggested from the be-
ginning their potential use as alternatives to traditional volatile organic
solvents (VOCs). During the years, the idea of ILs as intrinsically green
species has been mitigated by investigating their toxicity [2,3] and deg-
radation [4], and looking at the synthetic strategies to prepare them in
more detail [5]. Recently, natural (e.g. amino acids, terpenes, fatty
acids) [6,7] or biobased ions [8] have been employed in the develop-
ment of new ILs as a way to reduce the use of fossil derived materials
and enhance their biodegradability. Nowadays, ILs are employed as sol-
vents or additives in a wide and disparate range of applications which
span for instance from organic synthesis [9], to biopolymers dissolution
and modification [10,11], from innovative separation techniques [12] to
biosensor development [13].
5% of the possible total structures under consideration are actually pres-
ent in the literature, even for this simple cation.
Guanidinium ILs have been far less investigated than the imidazolium
one, although they showed interesting potential applications as well. In
fact, they were used as electrolytes for lithium batteries [15,16], and as
possible media for CO2 capture [17], organic reactions [18,19], cellulose
dissolution [20], and electrochemical deposition of elemental titanium
[21]. Furthermore, a series of tetramethyl guanidinium ILs resulted as
biodegradable and non-toxic for normal HEK 293 and cancerous DLD-1
cell cultures [22].
A peculiar trait of guanidinium ILs is the charge delocalization of the
cation core where the positive charge is spread over three nitrogen
atoms. Instead, in pyrrolidinium and imidazolium ILs the positive
charge is localized on the sole nitrogen atom present in the first case
or distributed across the π-ring system in the latter case. As a conse-
quence of the different positive charge density, the interaction between
the guanidinium cation and its anion is weaker when compared to
pyrrolidinium and imidazolium ILs, as shown by X-ray photoelectron
spectroscopy (XPS) analysis [23]. XPS is a surface-sensitive spectro-
scopic technique which works under high vacuum and is particularly
suitable for studying ILs thanks to their negligible vapor pressure [24].
Several features of ILs have been investigated by means of XPS. For in-
stance, anion-cation interactions [25], in situ reaction monitoring [26],
surface composition [27], and even the different orientations of chiral
enantiopure ionic liquids at the IL/vacuum interface [28].
Another aspect of ILs which is often praised is the almost limitless
number of structures that can be prepared from known ions. This
means that their properties can be finely tuned to address a specific
problem, hence their reputation as designer solvents. Although this is
true, a recent work by Bara et al [14], focused on the frequency of use
To gain further insight into the guanidinium ILs family, and in partic-
ular into the correlation between cation structure and physico-chemical
⁎
Corresponding author.
E-mail address: lorenzo.guazzelli@unipi.it (L. Guazzelli).
https://doi.org/10.1016/j.molliq.2018.12.083
0167-7322/© 2018 Elsevier B.V. All rights reserved.

A. Moscardini et al. / Journal of Molecular Liquids 277 (2019) 280–289
281
properties, as well as bulk-surface composition, six structurally-related
guanidinium bistriflimide ILs, including also practically not yet explored
bicyclic systems, have been prepared. The obtained ILs have been char-
acterized by means of NMR, DFT calculations, TGA, DSC and resolved
angle XPS.
The hydrogen bond donating (HBD) acidity, α, is calculated using ET
(30) and π* values as follows:
α ¼ 0:0649ET ð30Þ−2:03−0:72πꢀ
ð2Þ
Finally, the hydrogen bond accepting (HBA) basicity, β, is deter-
mined on the basis of the absorption of 4 nitroaniline (probe 2) and N,
N diethyl 4 nitroaniline by using Eq. (3):
2. Experimental
2.1. Material and methods
1:035νð2Þmax−νð1Þmax þ 2:64
β ¼
ð3Þ
2:80
2.1.1. Material
Oxalyl
chloride
(≥99%),
tetramethylurea
(99%),
The ET(30) value was determined on the basis of the absorption max-
imum of the Reichardt's dye 30, with the polarity scale ET(30) being de-
fined as reported in following equation:
1,3 Dimethyl 2 imidazolidinone (≥99%), and N ethylbutylamine
(≥98%) were purchased from Sigma-Aldrich. Azepane (≥99%), Pi-
peridine (99%), and N butylmethylamine (98%) were obtained
from Acros. Lithium bis(trifluoromethylsulfonyl)imide (99%), was
purchased from io-li-tech. Rigorously dried organic solvents were
used. All amines were dried with KOH pellets and stored under an
argon atmosphere.
28591
ET ð30Þ ¼
λmaxðin nmÞ
2.1.2.2. DFT calculations. Calculations have been performed on a Linux
Workstation (Xeon E5-1660 (six cores), 32 Gb RAM) using Gaussian
16 rev. B.01 [29].
2.1.2. Characterizations
NMR spectra were recorded at room temperature and 70 °C using a
Bruker Instrument at 250 MHz (1H) and 62.9 MHz (13C) using deuter-
ated chloroform and dimethyl sulfoxide as solvent. Purity of the
guanidinium Tf2N ILs (1–6) was ascertained by 13C NMR on neat
compounds.
2.1.2.3. X-ray photoelectron spectroscopy. Ionic liquids were drop-casted
on a gold plate to form a thin layer. The gold plate with the ionic liquid
was then transferred into the XPS ultra-high vacuum (UHV) chamber.
In the UHV chamber is located a manipulator that permits to tilt, to ro-
tate, to warm up and to cool down samples. The XPS instrumentation
makes use of a non-monochromatic X-ray source (VSW Scientific In-
strument Limited model TA10, Mg Kα radiation, 1253.6 eV), operating
in this case at 120 W (12 kV and 10 mA), and of a hemispherical
analyser (VSW Scientific Instrument Limited model HA100). The
analyser was equipped with a 16-channel detector and dedicated differ-
ential pumping system that permits to work during the acquisition with
pressure in the main chamber up to 10−8 Torr range. The pass energy
was set to 22 eV. The obtained spectra were analyzed using CasaXPS
dedicated software. The inelastic background was subtracted using
Shirley's method [30] and a mixed Gaussian and Lorentzian contribu-
tions were used for each component. The calibration of spectra was ob-
tained shifting to 284.8 eV the lowest component relative to the 1 s
transition of carbon for adventitious carbon [31].
Infrared spectra were registered using an ATR-FTIR Agilent 660
(Agilent Technologies, Santa Clara, CA, USA).
The thermal stability of the synthesized ILs was investigated by ther-
mal gravimetric analysis (TGA), conducted in a TA Instruments Q500
TGA. IL (10–20 mg) was heated in a platinum crucible. First, the heating
mode was set to isothermal at 60 °C in N2 (100 mL/min) for 15 min.
Then, IL was heated from 45 to 700 °C with a heating rate of 5 °C/ min
under nitrogen (100 mL/min) and maintained at 700 °C for 5 min.
Mass change was recorded as a function of temperature and time. TGA
experiments were carried out in triplicate.
The thermal behavior of the ionic liquids was analyzed by a differen-
tial scanning calorimeter (TA DSC, Q250, USA). About 3–5 mg of sample
was loaded in hermetic aluminium crucibles and the phase behavior
was explored under nitrogen atmosphere in the temperature range of
90–130 °C. DSC experiments were carried out in duplicate.
Measurements were made at two temperatures: 40 °C (ionic liquids
at liquid state) and −140 °C (ionic liquids at solid state). The conforma-
tion of the bulk and the surface of the ionic liquids were determined by
means of angle resolved X-ray photoelectron spectroscopy (AR-XPS).
The measurements were performed at 4 take-off angles from 0° (normal
emission) to 80° (grazing angle).
2.1.2.1. Determination of Kamlet-Taft parameters. UV–vis absorption
spectra of the three solvatochromic dyes (Reichardt's betaine dye,
N,N diethyl 4 nitroaniline, and 4 nitroaniline) dissolved in 2 were
registered using a quartz cell with a light path length of 1 mm on a
Cary 300 UV–Vis spectrophotometer (250–800 nm). To remove
water and volatile impurities, before measurements, ILs were dried
by heating (≈80 °C), with constant stirring, and at high vacuum
(≈10−4 Pa) for at least 48 h. Proper amounts of Reichardt's betaine
dye (ET30), N,N diethyl 4 nitroaniline and 4 nitroaniline were added
to 2 (2 mL) in order to have absorptions in the range of 1–1.5 at
2.2. General procedure for the preparation of hexaalkyl guanidinium
chloride
A solution of urea (50 mmol) in dry CH2Cl2 (10 mL) was added
dropwise at 0 °C under an Argon atmosphere to a solution of freshly dis-
tilled oxalyl chloride (55 mmoL) in dry CH2Cl2 (25 ml). The reaction
mixture was allowed to reach rt. and was stirred at rt 6h. The solvent
was then evaporated under reduced pressure to afford a brown solid
which was washed with dry Et2O (4 × 20 mL) to give a white solid.
The white solid was dissolved in dry acetonitrile (40 mL) and then
added dropwise to a mixture of secondary amine (50 mmol) and
triethylamine (50 mmol) at 0 °C. The resulting solution was stirred at
rt. overnight with formation of a white solid. The obtained suspension
was filtered, and the resulting solution was concentrated under vac-
uum. A 0,1 M NaOH solution was added to the resulting oil until the
pH was slightly alkaline. The aqueous solution was washed with diethyl
the respective λmax
.
The solvent dipolarity/polarizability, π* was determined on the
basis of the wavelength maximum of the lowest energy band of N,
N diethyl 4 nitroaniline (probe 1), a nonhydrogen bond donor sol-
ute, using Eq. (1).
πꢀ ¼ 8:649−0:314 ν1
ð1Þ
where ν1 is the frequency of probe 1 at the absorption maximum,
expressed as wavenumber in kK.

282
A. Moscardini et al. / Journal of Molecular Liquids 277 (2019) 280–289
ether (3 × 50 mL) and then concentrated under reduced pressure to
2.3.2.
N,N,N′,N′ tetramethyl N″,N″ butylethylguanidinium
bis
give the final product.
(trifluoromethylsulfonyl) imide (2)
Clear liquid 0,916 g, yield 95%; 1H NMR (DMSO) δ 3.41–3.07 (m, 4H),
2.94–2.91 (d, 12H), 1.67–1.37 (m, 2H), 1.36–1.20 (m, 2H), 1.17 (t, J =
7.1 Hz, 3H), 0.91 (t, J = 7.2 Hz, 3H) ppm. 13C NMR (DMSO) δ 162.3,
126.7–111.3, 47.8, 43.4, 39.3, 39.2, 28.5, 19.0, 12.6, 12.0 ppm. IR (ATR):
ν = 2963 (b), 2938 (b), 2879 (b), 2816 (m), 1565 (i), 1411 (m), 1348
2.2.1. N,N,N′,N′ tetramethyl N″,N″ butylmethylguanidinium chloride (1a)
White solid 9,83 g, yield 89%; 1H NMR (CDCl3) δ 3.15 (t, J = 7.6 Hz,
2H), 3.07 (s, 3H), 3.00 (s, 6H), 2.97 (s, 6H), 1.50 (m, 2H), 1.34–1.07
(m, 2H), 0.85 (t, J = 7.2 Hz, 3H) ppm. 13C NMR (CDCl3) δ 163.4, 52.5,
40.8, 40.46, 38.3, 29.6, 19.9, 13.7 ppm. IR (ATR): ν = 2956 (m), 2931
(i), 1177 (i), 1135 (i), 1054 (i) cm−1
.
(m), 2872 (m), 1576 (i), 1468 (b), 1403 (i), 1254 (b) cm−1
.
2.3.3. N,N,N′,N′ tetramethyl N″,N″ pentamethyleneguanidinium bis
(trifluoromethylsulfonyl) imide (3)
2.2.2. N,N,N′,N′ tetramethyl N″,N″ butylethylguanidinium chloride (2a)
White solid 9,43 g, yield 80%; 1H NMR (DMSO) δ 2.86–2.65 (m, 4H),
2.53 (s, 12H), 1.10 (bs, 2H), 0.94–0.76 (m, 2H), 0.71 (t, J = 7.1 Hz, 3H),
0.48 (t, J = 7.2 Hz, 3H) ppm. 13C NMR (DMSO) δ 162.0, 47.7, 43.6, 38.7,
28.0, 18.9, 12.5, 10.9 ppm.
Clear liquid 0,910 g, yield 96%; 1H NMR (CDCl3) δ 3.22 (bm, 4H),
2.94–2.92 (d, 12H), 1.67 (s, 6H) ppm. 13C NMR (CDCl3) δ 162.8,
127.7–111.6, 49.9, 40.2, 40.2, 25.1, 23.4 ppm. IR (ATR): ν = 2944 (b),
2864 (b), 1619 (i), 1563 (i), 1416 (b), 1348 (i), 1175 (i), 1134 (i),
1052 (i) cm−1
.
2.3.4.
N,N,N′,N′ tetramethyl N″,N″ esamethyleneguanidinium
bis
2.2.3. N,N,N′,N′ tetramethyl N″,N″ pentamethyleneguanidinium chloride
(3a)
(trifluoromethylsulfonyl) imide (4)
Light yellow clear liquid 0,930 g, yield 95%; 1H NMR (CDCl3) δ 3.32
(s, 4H), 2.89 (s, 12H), 1.66 (bm, 8H) ppm. 13C NMR (CDCl3) δ 163.3,
128.1–117.8, 52.1, 40.3, 40.1, 29.5, 26.4 ppm. IR (ATR): ν = 2936 (b),
2863 (b), 1588 (m), 1562 (i), 1410 (m), 1347 (i), 1175 (i), 1134 (i),
White solid 8,27 g, yield 75%; 1H NMR (CDCl3) δ 3.18 (m, 4H), 2.83 (s,
6H), 2.81 (s, 6H), 1.57 (bs, 6H) ppm. 13C NMR (CDCl3) δ 162.8, 50.1, 41.0,
40.8, 25.3, 22.5 ppm. IR (ATR): ν = 2930 (m), 2856 (m), 1564 (i), 1435
(m), 1407 (i), 1277 (b), 1253 (b) cm−1
.
1053 (i) cm−1
.
2.2.4. N,N,N′,N′ tetramethyl N″,N″ esamethyleneguanidinium chloride (4a)
White solid 8,97 g, yield 77%; 1H NMR (CDCl3) δ 3.28 (bs, 4H), 2.94
(s, 6H), 2.89 (s, 6H), 1.44 (bm, 8H) ppm. 13C NMR (CDCl3) δ 163.1,
52.0, 40.6, 29.1, 26.5 ppm. IR (ATR): ν = 2912 (m), 2852 (m), 1579
2.3.5.
1 (1,3 Dimethylimidazolidin 2 yliden)
piperidinium
bis
(trifluoromethylsulfonyl) imide (5)
Yellow clear liquid 0,907 g, yield 96%; 1H NMR (DMSO) δ 3.14 (bs,
4H), 2.82 (bs, 4H), 2.43 (bs, 6H), 1.13 (bs, 6H) ppm. 13C NMR (DMSO)
δ 162.1, 126.6–111.3, 48.3, 48.1, 34.6, 24.5, 22.0 ppm. IR (ATR): ν =
2947 (b), 2864 (b), 1619 (i), 1563 (i), 1416 (b), 1348 (i), 1175 (i),
(i), 1479 (b), 1449 (b), 1408 (i), 1299 (b), 1233 (b) cm−1
.
1134 (i), 1052 (i) cm−1
.
2.2.5. 1 (1,3 Dimethylimidazolidin 2 yliden) piperidinium chloride (5a)
Amber clear oil 5,27 g, yield 48%; 1H NMR (CDCl3) δ 3.64 (s, 4H), 3.18
(bs, 4H), 2.86 (s, 3H), 2.86 (s, 3H), 1.45 (bs, 6H) ppm. 13C NMR (63 MHz,
CDCl3) δ 162.7, 49.5, 49.4, 36.4, 25.5, 22.8 ppm. IR (ATR): ν = 2937 (m),
2.3.6.
1 (1,3 Dimethylimidazolidin 2 yliden)
azepanium
bis
(trifluoromethylsulfonyl) imide (6)
Light yellow clear liquid 0,897 g, yield 92%; 1H NMR (CDCl3) δ 3.64
(bs, 4H), 3.45–3.33 (m, 4H), 2.91 (s, 6H), 1.68 (sm, 4H), 1.65–1.52
(sm, 4H) ppm. 13C NMR (CDCl3) δ 164.2, 127.4, 122.3, 117.2, 112.0,
52.0, 49.4, 36.4, 28.4, 27.4 ppm. IR (ATR): ν = 2938 (b), 2863 (b),
2857 (m), 1615 (i), 1557 (i), 1447 (b), 1414 (b), 1303 (i) cm−1
.
2.2.6. 1 (1,3 Dimethylimidazolidin 2 ylidene) azepanium chloride (6a)
Amber clear oil 5,01 g, yield 43%; 1H NMR (CDCl3) δ 3.73 (bs, 4H),
3.46–3.40 (m, 4H), 2.99 (s, 6H), 1.72–1.60 (s, 4H), 1.58–1.53 (m, 4H)
ppm. 13C NMR (CDCl3) δ 164.4, 52.3, 50.1, 37.2, 28.4, 27.9 ppm. IR
(ATR): ν = 2925 (m), 2856 (m), 1602 (i), 1556 (i), 1452 (b), 1413
1604 (i), 1562 (m), 1416 (b), 1348 (i), 1174 (i), 1134 (i), 1052 (i) cm−1
.
3. Results and discussion
(b), 1305 (m), 1299 (b), 1233 (b) cm−1
.
3.1. Synthesis and conformational analysis
A well-known procedure has been followed to prepare the six
guanidinium ILs (Fig. 1, top) [16]. Briefly, a commercial urea (either
tetramethyl urea or N,N′ dimethylethylene urea) was reacted with
oxalyl chloride to afford the corresponding chloroformamidinium chlo-
ride which was then treated with a secondary amine. The obtained
guanidinium chloride IL was converted into the corresponding
bistriflimide IL through a metathesis reaction with LiTf2N. The struc-
tures of the ILs 1–6 are summarized in Fig. 1 (bottom). It is worth men-
tioning that a drop in yield was observed in the first step for the cyclic
urea, from 75–80% (1–4) to 43–48% (5,6).
2.3. General procedure for the preparation of hexaalkyl guanidinium
bistriflimide
A solution of lithium bis(trifuoromethylsulfonyl)imide (0,59 g,
2 mmol) in water (5 mL) was added dropwise to a solution of
hexaalkylguanidinium chloride (2 mmol) in water (6 mL). The resulting
solution was stirred at room temperature for 3 h and then 30 mL of di-
chloromethane were added. The organic phase was separated and
washed with several portions of deionised water until chloride could
not be detected (AgNO3 test). The organic phase was dried over
Na2SO4, filtered, and the solvent was removed under reduced pressure.
The obtained product was analyzed by NMR and FTIR.
The identity of the obtained compounds has been ascertained by
means of NMR and IR analyses. IR spectra displayed the distinctive ab-
sorption band for the CN3+ at around 1561–1578 cm−1
.
As reported before for other guanidinium ILs [32], a split for the
methyl signals of compound 1–4 was observed in the 13C NMR spectra.
These were performed in co-axial tubes with DMSO d6 in the internal
capillary as a lock and chemical shift reference. The observed phenome-
non arises from the C-NR2 partial double bond character which hinders
the free rotation and thus renders the otherwise magnetically equivalent
symmetrical substituents on the nitrogen atoms magnetically non-
equivalent. By heating at 70 °C, the rotation barriers were overcome,
2.3.1.
N,N,N′,N′ tetramethyl N″,N″ butylmethylguanidinium
bis
(trifluoromethylsulfonyl) imide (1)
Light yellow clear liquid 0,903 g, yield 95%; 1H NMR (CDCl3) δ 3.12 (t,
J = 7.6 Hz, 2H), 2.91 (s, 12H), 2.88 (s, 3H), 1.55 (bs, 2H), 1.38–1.19 (m,
2H), 0.90 (t, J = 7.3 Hz, 3H) ppm. 13C NMR (CDCl3) δ 163.4, 128.1–112.4,
52.5, 40.1, 37.7, 29.5, 19.8, 13.5 ppm. IR (ATR): ν = 2963 (b), 2878 (b),
1578 (i), 1409 (m), 1348 (i), 1176 (i), 1134 (i), 1053 (i) cm−1
.

A. Moscardini et al. / Journal of Molecular Liquids 277 (2019) 280–289
283
Fig. 1. General synthetic procedure to prepare guanidinium Tf2N ILs (top); structures of the six guanidinium Tf2N ILs studied (bottom).
and coalescence of the signals was observed. Fig. 2 shows this behavior
for compound 3 (please refer to the supporting information file for com-
pounds 1, 4–6).
This behavior of hexaalkyl guanidinium salts is attributed to a pecu-
liar non-planar propeller-like shape which is favored over a planar ste-
rically crowded structure.
Substituted guanidinium cations are indeed sterically hindered spe-
cies. Except when almost all substituents are hydrogens, the steric re-
pulsion between sidechains placed on different nitrogen atoms lead to
a deviation from planarity and to hindering the rotation of the carbon
atoms that are in the α position with respect to the nitrogen ones.
Substituted guanidinium cations deviate from the ideal planar trigonal
Fig. 2. Enlargement of the 13C NMR spectrum of 3 at 20 °C (top) and at 70 °C (bottom).

284
A. Moscardini et al. / Journal of Molecular Liquids 277 (2019) 280–289
Table 1
form where the central carbon, the three nitrogen atoms and the six
atoms in α position with respect the nitrogen atoms lie on the same
plane to assume a propeller-like shape.
We have performed some DFT calculations to evaluate the interplay
between sterical hindering, delocalization of the π electrons, etc. The
cations considered are, among those examined in this work, those re-
ported in Fig. 3.
Energetic and geometrical results are reported in Table 1. All calcula-
tions have been performed at the B3LYP/6–311++G(d,p) level of the-
ory using the Gaussian 16 package [29].
We have neglected cations 2 and 6 that are very similar to cations 1
and 4, respectively.
To evaluate the extent of the sterical hindering effect we have first
optimized the structures of these ions and then optimized analogues
where the central moiety is planar, like the simple guanidinium ion.
The difference in energy between the free optimized cation and the
flattered one can be defined as sterical deformation energy. All the
flattered structures are not local minima since they present three imag-
inary frequencies each related to the rotation of one of the R-N-R′
groups. Cations 1, 3, 4 present a flattering energy of about 250 J/mol
(slightly lower in the case of 4 where the seven membered ring permits
a certain degree of relaxation) while, in the case of 5 it is about a quarter
(58.50 kJ/mol). This last one is the only cation where a cycle includes the
central carbon and the nitrogen atoms of the guanidinic group. The
resulting five-membered ring (that contains 3 sp2 atoms) is almost
flat. Furthermore, the three carbon-nitrogen bonds are shorter than in
the other substituted cations. The rotation of the R-N-R′ groups can be
evaluated by using torsional angles. In the case of guanidine, the torsion
between any of the N-C-N-R groups is 0.0° when the R group is in a cis-
like position with respect to the first nitrogen, 180° when it is in trans-
Energetic and geometric quantities related to cations. All calculations have been per-
formed at the B3LYP/6–311++G(d,p) level of theory. N1 is the nitrogen enclosed in the
6 or 7 membered rings (4, 5) or bounded to a methyl and a butyl group (1). Orbitals ener-
gies are relative to the HOMO-2 absolute value.
System
Guanidinium
1
3
4
5
ΔE/(kJ/mol)
R(C-N1)/Å
R(C-N2)/Å
R(C-N3)/Å
N1-C1-N2-X/°
0.00
246.70
1.351
1.352
1.351
−37.2
143.5
180.7
−33.5
145.2
178.7
−35.4
145.8
242.29
1.349
1.353
1.352
−33.7
145.2
178.9
−36.9
143.6
180.5
−35.6
143.8
210.07
1.352
1.352
1.353
−32.9
144.9
177.8
−36.4
143.8
180.2
−36.0
143.7
58.50
1.343
1.349
1.349
−18.8
174.6
193.4
−23.3
172.1
195.4
−6.8
157.7
164.5
0.30142
0.06562
0.06113
0.00000
1.335
1.335
1.335
0.0
180.0
180.0
0.0
180.0
180.0
0.0
180.0
180.0
0.46320
0.15899
0.15899
Sum
N1-C1-N3-/°
Sum
N2-C1-N3-X/°
Sum
LUMO (A2′)/Ha
HOMO (E″)/Ha
181.2
179.4
179.7
0.28886
0.05856
0.05328
0.00000
0.28888
0.06012
0.05362
0.00000
0.27548
0.04934
0.04138
0.00000
HOMO-2 (A2″)/Ha 0.00000
Absolute energy/Ha −0.65134
−0.45528 −0.45393 −0.44045 −0.45488
like position. A rigid rotation must lead to different torsion but the sum
of the two angles must be conserved. The data reported in Table 1 show
that acyclic or monocyclic cations have a torsion of about 35° and the
torsion is almost rigid. The bicyclic one presents deformations and
lower torsion angles. It is the most similar structure to the simple
guanidinium one.
It is interesting to note the effect of this deviation from planarity on
the orbitals of the π conjugated system. The orbitals of the guanidinium
cation and of 3 are reported in Fig. 4. Full size orbital figures are reported
in the supporting information file. In the case of 3, the no longer parallel
p orbitals superimpose with orbitals of nearby atoms with compatible
symmetry, like σ bonding orbitals of C\\H bonds. This effect is larger
in the HOMO-2 full symmetric orbital. Relative orbital energies show a
similar trend to bond distances: those of 5 are larger and more like
guanidinium than the other ones.
Finally, we studied the conformational barrier of rotation of one of
the CH3-N-CH3 group in 3. The geometry of the top of barrier structure,
that presents a notable degree of pyramidalization on the nitrogen
atom, is reported in Fig. 5. The conformation barrier is of 67.30 kJ/mol
(in the case of ethane is of 11.31 kJ/mol at the same level of theory).
This further confirms that we are in presence of a highly hindered
system.
A paper by Butschies et al. [33] presents some similar DFT calculation
on not too different guanidinium cations. Their calculations have been
performed at the B3LYP/6-31G(d,p) level. The cations studied have,
generally, an aromatic substituent. Given the similarities between the
structures and the level of calculation (the functional is the same, our
is a triple-zeta basis set with diffuse functions vs double-zeta without
diffuse functions, but both belong to the Pople's family) is possible to
qualitatively compare the results. The magnitude of torsional angles is
about 30° in both studies. They do not consider flattering energies, but
the torsional barriers calculated for similar systems lie in the range of
60–70 kJ/mol where our calculated value is of 67.30 kJ/mol.
3.2. Thermal analysis of prepared guanidinium ILs
TGA analysis were performed using the following experimental condi-
tions: mass at 15–18 mg, N2 atmosphere with flowing rate 100 mL/min,
platinum pans, and a 5 °C/min heating rate. Tonset and Tpeak values are re-
ported in Table 2 and are the average values of three temperature-
ramped TGA experiments.
The six guanidinium ILs display similar high thermal stability, ILs 1 and
5 as the lower and upper limits of the series, with a single degradation
Fig. 3. Guanidinium ions considered in the text. They can be structurally related by meta-
reactions.

A. Moscardini et al. / Journal of Molecular Liquids 277 (2019) 280–289
285
LUMO (A2')
HOMO(E”)
HOMO-2(A2”)
Fig. 4. Orbitals of the π conjugated system. Left: guanidinium cation. Right: IL 3. Symmetry symbols are related to the D3h guanidinium cation. Degeneration is removed in sterical hindered
compounds, but the energies of the related orbitals are still close.
Fig. 5. Structure of 3 at minimum (left) and at the top of the barrier structure for torsion of the CH3-N-CH3 group (right) with respect the guanidinium moiety. The torsion angle of the
minimum is −33.7° while the top of barrier structure is of −54.9°. The difference in energy is of 67.30 KJ/mol. It is evident the pyramidalization of the nitrogen atom.
event (Fig. 6). Cao and Mu [34] classified 66 ILs into five different levels on
the basis of their Tonest: least stable (200 °C b Tonest b 250 °C), less stable
(250 °C b Tonest b 300 °C), moderate (300 °C b Tonest b 350 °C), more stable
(350 °C b Tonest b 400 °C), most stable (400 °C b Tonest b 450 °C). All the
guanidinium Tf2N belong to the most stable level of this classification,
which is essential for some applications as for instance for their use as
electrolytes in batteries.
• cooling from 40 °C to −90 °C at 20 °C/min;
• heating from −90 °C to +130 °C at 20 °C/min;
• cooling from 120 °C to −90 °C at 10 °C/min;
• heating from −90 °C to +100 °C at 10 °C/min;
• cooling from 100 °C to −90 °C at 5 °C/min;
• heating from −90 °C to +100 °C at 5 °C/min
ILs 3,5 and 4,6 can also be seen as guanidinium-piperidinium and
guanidinium-azepanium mixed structures, respectively. Previous
works on piperidinium Tf2N ILs reported thermal stabilities both
lower [35] and higher [36] than the one obtained here for 3 and 5,
thus further studies are required to properly understand the contribu-
tion of the guanidinium frame to the decomposition events. Instead,
considering the lower thermal stability of the azepanium ILs reported
in the literature to date [37], it seems that the guanidinium structure
prevails over the azepanium one in determining the thermal stability
for 4 and 6.
As reported by Domínguez et al. [38], three thermal behaviors are
generally expected for ILs: structures without a true phase transition
that show only the formation of an amorphous glass (type I), ILs
which present a freezing transition, forming crystals upon cooling and
a melting transition upon heating (type II), and finally ILs that don't
show a crystallization upon cooling, but exhibit a cold crystallization
event upon heating (type III).
ILs 1 and 2 well fit into the first type of ILs as reversible glass transi-
tions are reproducibly observed at −78.9 °C and −81.7 °C, respectively,
in the form of an endothermic small peak in all heating half cycles.
ILs 3 and 4, which present the piperidinium and the azepanium cy-
cles respectively, display a cold crystallization transition just before
the melting transition (third type of thermal behavior). Both transitions
appear as the overlapping of two different events.
While TGA analysis didn't show any discernible trend throughout
the six guanidinium Tf2N, DSC curves allowed for their division into dis-
tinct groups. DSC study was performed at 10 °C/min in nitrogen flow,
according to the following protocol:
IL 5 instead behaves as a low melting salt, thus belonging to the sec-
ond type of ILs, and is characterized by a sharp endothermic melting
peak at about 25 °C in the heating half cycle (reproducible temperatures
for I and II cycles). In the cooling half cycle, a comparable freezing peak
is found at about −20 °C. The recorded shift is probably due to a signif-
icant supercooling effect.
Table 2
Tonset and Tpeak value for guanidinium Tf2N ILs 1–6 measured under a nitrogen atmosphere
and with a heating rate of 5 °C/min.
ILs
1
2
3
4
5
6
Tonset (°C)
Tpeak (°C)
413
451
415
449
422
449
425
461
434
473
419
459
Finally, the thermal behavior of IL 6 is a variation of the third type of
thermal behavior and presents two subsequent distinct transitions after

286
A. Moscardini et al. / Journal of Molecular Liquids 277 (2019) 280–289
Fig. 6. Temperature-ramped TGA thermographs for guanidinium Tf2N ILs 1,3,5 with heating rate of 5 °C/min.
the crystallization during the isothermal step: a solid/solid transition at
−20 °C and a melting transition at 32 °C. These transitions were
assigned by changing the heating rate from 5 °C/min to 10 °C/min; the
first peak moved which is typical for solid/solid transitions. The latter
transition is again the result of two overlapping events.
The picture obtained here divides the guanidinium Tf2N ILs on the
basis of the number of cycles present in their structures (at least for
the couples 1,2 and 3,4) and is in good agreement with previous find-
ings [39]. It is worth noting that in general the bistriflimide anion
plays a dual role on the thermal properties of ILs. In fact, because of its
low nucleophilicity and basicity, it increases the thermal stability of
the ILs in comparison at least to halide anions [40–42]. On the other
hand, it decreases the melting points widening the liquid range, as re-
ported before for other guanidinium salts [43].
outermost portion of the surface results fluorine depleted respect to al-
iphatic carbon. Accordingly, this trend is also displayed by the C-F signal
shown in Fig. 7b which confirms these observations. Two different phe-
nomena can account for this trend: 1) the absorption of tiny amount of
aliphatic contaminants which remain segregates in the outermost por-
tion of the IL and 2) the arrangements of the aliphatic chains which ori-
entate themselves to pointing towards vacuum. Data collected on the
other XPS signals support the second hypothesis. It is worth noting
that at 40 °C this trend is less evident, probably due to dynamic
equilibrium.
Fitting of the C1s peak resulted quite difficult due to overlapping of
the different types of carbon atoms. Anyway, it was possible to distin-
guish the aliphatic carbons (those not directly linked to the nitrogen
atom) and the C-F carbon already discussed.
It is evident that there is an enrichment of the aliphatic carbon as by
moving from the bulk to the surface for all the ILs both, at −140 °C and
at 40 °C (Fig. 8).
3.3. X-ray photoelectron spectroscopy
A similar behavior, namely the tendency of aliphatic carbons to
move towards the surface is not unusual and has been reported before
for imidazolium ILs [24].
The six guanidinium ILs were analyzed by angle resolved XPS (AR-
XPS) in order to elucidate the composition of the outmost regions of
the IL/air interface.
Again, the structural differences among the six guanidinium ILs
influence this trend. In fact, ILs 1 and 2, exhibit at 40 °C the most pro-
nounced aliphatic enrichment of the surface followed by azepanium
ILs 4 and 6, and by the piperidinium ILs 3 and 5. At low temperature,
instead, 4,6 gave the most prominent effect followed by 1,2 and then
3,5.
The N1s components, both from the anion and the cation (Fig. 9),
and the C-F component support the above conclusions for the surface al-
iphatic carbon component enrichment and inner positioning of the
anion, although caution is required when analyzing these components
which are characterized by weak peaks, just above the detection limit
of the technique.
Two sets of experiments were carried out either at −140 °C or at 40
°C to assess the surface composition in different conditions: glass and
liquid status. In the first case we can consider the surface “frozen”
while in liquid state we can consider the surface in a dynamic equilib-
rium with the bulk. For each sample the spectra were recorded at differ-
ent angles between the normal (0°) and the grazing (80°) take-off angle
in order to change the probing depth.
High resolution spectra were recorded for the individual elements
(C1s, F1s, and N1s).
The F1s electron emission spectrum is well resolved and since fluo-
rine is only present in the anion, this signal's relative intensity was
used to monitor the enrichment or depletion of anion at the surface
(Fig. 7a). The two ILs containing the piperidinium ring (3 and 5) display
fluorine content higher respect to the others ILs and, in some cases, it re-
sults superior to the stoichiometric amount. This trend can be reason-
ably explained by anion surface enrichment. For all the samples the
amount of fluorine decreases as function of take-off angle; i.e. the
3.4. Polarity
The solvatochromic parameters for IL 2 were also measured. Polarity
is indeed a fundamental property for a liquid to ascertain its potential as

A. Moscardini et al. / Journal of Molecular Liquids 277 (2019) 280–289
287
Fig. 8. Total carbon content in the samples as a function of the temperature and the take-
off angle.
acidity (α = 0.552) comparable to bistriflimide based pyridinium
salts [44]. Since this property is principally determined by the IL cat-
ion, although a strong hydrogen bonding acceptor anion can de-
crease the donor ability of the related cation, the obtained α value
suggests a not negligible acidity of some of the protons of 2, probably
of those on carbons adjacent to the positively charged sp2 nitrogen
atoms.
4. Conclusions
Six hexaalkyl guanidinium Tf2N ILs, characterized by a close struc-
tural relationship, have been prepared and studied to find differences
and similarities among them. On the basis of their structures, these ILs
may be divided into different subsets, as for instance two groups with
the same total number of carbon atoms (10 carbon atoms for odd num-
bered ILs, 11 carbon atoms for even numbered ILs) but a diminished de-
gree of freedom due to the presence of 1 or 2 cycles, or three groups
considering the number of cycles in the structure (1,2 no cycles; 3,4
one cycle; 5,6 two cycles) or the type of cycles (1,2 no cycles; 3,5
piperidinium cycle; 4,6 azepanium cycle).
Fig. 7. Total fluorine content (a) and carbon (C-F) (b) in the samples as a function of the
temperature and the take-off angle.
NMR analysis and DFT calculations confirmed the propeller-like
shape for all the ILs, which is however less marked for the bicyclic ILs.
Regarding the thermal stability, TGA analysis showed again a similar be-
havior for all ILs which resulted highly stable (Tonset N 400 °C) with a sin-
gle degradation event. Conversely, DSC analysis allowed to highlight
structural related thermal behaviors: only glass transitions were found
for 1 and 2, while 3 and 4 displayed a cold crystallization followed by
a melting transition in the heating half cycle. The two bicyclic ILs 5
and 6 displayed a different peculiar thermal behavior. Finally, XPS anal-
ysis showed an inner positioning of the anion and a surface carbon com-
ponent enrichment for all ILs, which decreases with the following order:
1,2 N 4,6 N 3,5 at 40 °C, and 4,6 N 1,2 N 3,5 at −140 °C. Solvatochromic
parameters for IL 2 were also measured.
a solvent, since it affects not only the dissolution ability but also dramat-
ically the kinetic and stereochemical behavior of the reactions occurring
inside. Thus, IL 2 was selected taking into account several aspects as for
instance the high synthetic yields, its stability and liquid state over a
wide range of temperatures, and the aliphatic carbon component sur-
face enrichment as obtained by XPS analysis. In particular, the
solvatochromic Kamlet Taft parameters, which have been largely
employed to define ILs polarity, were determined at 25 °C, on a previ-
ously accurately dried sample of 2.
As expected, 2 shows a high dipolarity/polarizability (π* = 0.943),
which fits perfectly into the relatively limited range characterizing ILs
(0.8–1.20) associated to a quite low hydrogen bond basicity (β =
0.296). This latter parameter depends indeed principally from the IL
anion and bistriflimide salts are normally characterized by values
lower than 0.3. Furthermore, IL 2 is characterized by a hydrogen bond
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2018.12.083.

288
A. Moscardini et al. / Journal of Molecular Liquids 277 (2019) 280–289
[8] J. Hulsbosch, D.E. De Vos, K. Binnemans, R. Ameloot, Biobased ionic liquids: solvent
for a green processing industry? ACS Sustain. Chem. Eng. 4 (2016) 2917–2931.
[9] C. Chiappe, C.S. Pomelli, Point-functionalization of ionic liquids: an overview of syn-
thesis and applications, Eur. J. Org. Chem. (28) (2014) 6120–6139.
[10] M. Gericke, P. Fardim, T. Heinze, Ionic liquids – promising but challenging solvents
for homogeneous derivatization of cellulose, Molecules 17 (2012) 7458–7502.
[11] A. Mezzetta, L. Guazzelli, C. Chiappe, Access to cross-linked chitosans by exploiting
CO2 and the double solvent-catalytic effect of ionic liquids, Green Chem. 19
(2017) 1235–1239.
[12] A. Berthod, M.J. Ruiz-Angel, S. Carda-Broch, Recent advances on ionic liquid uses in
separation techniques, J. Chromatogr. A1559 (2018) 2–16.
[13] X. Wang, J. Hao, Recent advances in ionic liquid-based electrochemical biosensor,
Sci. Bull. 61 (2016) 1281–1295.
[14] V.F. Scalfani, A.A. Alshaikh, J.E. Bara, Analysis of the frequency and diversity of 1,3 di-
alkylimidazolium ionic liquids appearing in the literature, Ind. Eng. Chem. Res. 57
(2018) 15971–15981.
[15] X. Zhang, S. Fang, Z. Zhang, L. Yang, Li/LiFePO4 battery performance with a
guanidinium-based ionic liquid as the electrolyte, Chin. Sci. Bull. 56 (2011)
290–2910.
[16] N. Bucher, S. Hartung, M. Arkhipova, D. Yu, P. Kratzer, G. Maas, M. Srinivasanac, H.E.
Hoster, A novel ionic liquid for Li ion batteries – uniting the advantages of
guanidinium and piperidinium cations, RSC Adv. 4 (2014) 1996–2003.
[17] X. Lei, Y. Xu, X. Wang, Highly efficient and reversible CO2 capture through
1,1,3,3 tetramethylguanidinium imidazole ionic liquid, RSC Adv.
4 (2014)
7052–7057.
[18] J. Porwal, S. Kumar, S. Kaul, S.L. Jain, Guanidine based task specific ionic liquids for
the synthesis of biolubricant range esters under solvent-free condition, RSC Adv. 6
(2016) 93640–93644.
[19] P. Li, Y. Li, C. Chen, L. Wang, J. Zhang, Catalytic performance of a series of
guanidinium-based ionic liquids in the coupling of carbon dioxide with epoxides,
RSC Adv. 6 (2016) 87036–87043.
[20] A.W.T. King, J. Asikkala, I. Mutikainen, P. Jarvi, I. Kilpeläinen, Distillable acid-base
conjugate ionic liquids for cellulose dissolution and processing, Angew. Chem. Int.
Ed. 50 (2011) 6301–6305.
[21] C.A. Berger, M. Arkhipova, A. Farkas, G. Maas, T. Jacob, Titanium deposition from
ionic liquids – appropriate choice of electrolyte and precursor, Phys. Chem. Chem.
Phys. 18 (2016) 4961–5965.
[22] A.P. Singh, D. Sithambaram, R. Sanghavi, P.K. Gupta, R.S. Verma, M. Doble, R.L.
Gardas, S. Senapati, Environmentally benign tetramethylguanidinium cation based
ionic liquids, New J. Chem. 41 (2017) 12268–12277.
[23] A.R. Santos, R.K. Blundell, P. Licence, XPS of guanidinium ionic liquids: a comparison
of charge distribution in nitrogenous cations, Phys. Chem. Chem. Phys. 17 (2015)
11839–18847.
[24] K.R.J. Lovelock, I.J. Villar-Garcia, F. Maier, H.-P. Steinrück, P. Licence, Photoelectron
spectroscopy of ionic liquid-based interfaces, Chem. Rev. 110 (2010) 5158–5190.
[25] T. Cremer, C. Kolbeck, K.R.J. Lovelock, N. Paape, R. Wçlfel, P.S. Schulz, P.
Wasserscheid, H. Weber, J. Thar, B. Kirchner, F. Maier, H.-P. Steinrück, Towards a
molecular understanding of cation-anion interactions-probing the electronic struc-
ture of imidazolium ionic liquids by NMR spectroscopy, X-ray photoelectron spec-
troscopy and theoretical calculations, Chem. Eur. J. 16 (2010) 9018–9033.
[26] M.T. Camci, B. Ulgut, C. Kocabas, S. Suzer, In-situ XPS monitoring and characteriza-
tion of electrochemically prepared Au nanoparticles in an ionic liquid, ACS Omega
2 (2017) 478–486.
[27] C. Kolbeck, M. Killian, F. Maier, N. Paape, P. Wasserscheid, H.-P. Steinrück, Surface
characterization of functionalized imidazolium-based ionic liquids, Langmuir 24
(2008) 9500–9507.
[28] S. Caporali, C. Chiappe, T. Ghilardi, A. Iuliano, G. Longhi, P. Margari, C.S. Pomelli,
Arrangements of enantiopure and racemic ionic liquids at the liquid/air inter-
face: the role of chirality on self-assembly and layering, RSC Adv. 6 (2016)
8053–8060.
[29] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G.
Scalmani, V. Barone, G.A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A.V. Marenich,
J. Bloino, B.G. Janesko, R. Gomperts, B. Mennucci, H.P. Hratchian, J.V. Ortiz, A.F.
Izmaylov, J.L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J.
Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V.G. Zakrzewski, J. Gao,
N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J.A.
Montgomery Jr., J.E. Peralta, F. Ogliaro, M.J. Bearpark, J.J. Heyd, E.N. Brothers, K.N.
Kudin, V.N. Staroverov, T.A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A.P.
Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, J.M. Millam, M. Klene, C.
Adamo, R. Cammi, J.W. Ochterski, R.L. Martin, K. Morokuma, O. Farkas, J.B.
Foresman, D.J. Fox, Gaussian 16, Revision B.01, Gaussian, Inc., Wallingford CT, 2016.
[30] D.A. Shirley, High-resolution x-ray photoemission spectrum of the valence bands of
gold, Phys. Rev. B 5 (1972) 4709–4714.
[31] T. Susi, T. Pichler, P. Ayala, X-ray photoelectron spectroscopy of graphitic carbon
nanomaterials doped with heteroatoms, Beilstein J. Nanotechnol. 6 (2015)
177–192.
[32] B. Oelkers, Jörg Sundermeyer, Pentaalkylmethylguanidinium methylcarbonates –
versatile precursors for the preparation of halide-free and metal-free guanidinium-
based ILs, Green Chem. 13 (2011) 608–618.
Fig. 9. Content of cationic (a) and anionic (b) nitrogen in the samples as a function of the
temperature and the take-off angle.
References
[1] T. Welton, Ionic liquid: a brief history, Biophys. Rev. 10 (2018) 691–706.
[2] T.P.T. Pham, C.-W. Cho, Y.-S. Yun, Environmental fate and toxicity of ionic liquids: a
review, Water Res. 44 (2010) 352–372.
[3] K.S. Quraish, M.A. Bustam, S. Krishnan, N.F. Aminuddin, N. Azeezah, N. Abd, Y.
Ghani, J.M. Leveque Uemura, Ionic liquids toxicity on fresh water microalgae,
Scenedesmus quadricauda, Chlorella vulgaris & Botryoccus; selection criterion
in a two-phase partitioning bioreactor (TPPBR), Chemosphere 184 (2017)
642–651.
[4] A. Jordan, N. Gathergood, Biodegradation of ionic liquids – a critical review, Chem.
Soc. Rev. 44 (2015) 8200–8237.
[5] P.G. Jessop, Fundamental properties and practical applications of ionic liquids: con-
cluding remarks, Faraday Discuss. 206 (2018) 587–601.
[6] A. Mezzetta, L. Guazzelli, M. Seggiani, C.S. Pomelli, M. Puccini, C. Chiappe, A general
environmentally friendly access to long chain fatty acid ionic liquids (LCFA-ILs),
Green Chem. 19 (2017) 3103–3111.
[7] I. Palazzo, A. Mezzetta, L. Guazzelli, S. Sartini, C.S. Pomelli, W.O. Parker Jr., C. Chiappe,
Chiral ionic liquids supported on natural sporopollenin microcapsules, RSC Adv. 8
(2018) 21174–21183.
[33] M. Butschies, S. Sauer, E. Kessler, H.-U. Siehl, B. Claasen, P. Fischer, W. Frey, S.
Laschat, Influence of N-alkyl substituents and counterions on the structural and me-
somorphic properties of guanidinium salts: experiment and quantum chemical cal-
culations, ChemPhysChem 11 (2010) 3752–3765.
[34] Y. Cao, T. Mu, Comprehensive investigation on the thermal stability of 66 ionic liq-
uids by thermogravimetric analysis, Ind. Eng. Chem. Res. 53 (2014) 8651–8664.

A. Moscardini et al. / Journal of Molecular Liquids 277 (2019) 280–289
289
[35] T. Yim, H.Y. Lee, H.-J. Kim, J. Mun, S. Kim, S.M. Oh, Y.G. Kim, Synthesis and properties
of pyrrolidinium and piperidinium bis(trifluoromethanesulfonyl)imide ionic liquids
with allyl substituents, Bull. Kor. Chem. Soc. 28 (2007) 1567–1572.
[36] M. Montaninoa, M. Carewskaa, F. Alessandrini, S. Passerini, G.B. Appetecchi, Therole of
the cation aliphatic side chain length in piperidinium bis(trifluoromethansulfonyl)
imide ionic liquids, Electrochim. Acta 57 (2011) 153–159.
[37] T. Belhocine, S.A. Forsyth, H.Q.N. Gunaratne, M. Nieuwenhuyzen, P. Nockemann, A.V.
Puga, K.R. Seddon, G. Srinivasan, K. Whiston, Azepanium ionic liquids, Green Chem.
13 (2011) 3137–3155.
[40] C. Maton, N. De Vos, C.V. Stevens, Ionic liquid thermal stabilities: decomposition
mechanisms and analysis tools, Chem. Soc. Rev. 42 (2013) 5963–5977.
[41] M.T. Clough, K. Geyer, P.A. Hunt, J. Mertes, T. Welton, Thermal decomposition of car-
boxylate ionic liquids: trends and mechanisms, Phys. Chem. Chem. Phys. 15 (2013)
20480–20495.
[42] Z. Xue, L. Qin, J. Jiang, T. Mu, G. Gao, Thermal, electrochemical and radiolytic stabil-
ities of ionic liquids, Phys. Chem. Chem. Phys. 20 (2018) 8382–8402.
[43] M. Butschies, M.M. Neidhardt, M. Mansueto, S. Laschat, S. Tussetschläger, Synthesis
of guanidinium–sulfonimide ion pairs: towards novel ionic liquid crystals, Beilstein
J. Org. Chem. 9 (2013) 1093–1101.
[38] E. Gómez, N. Calvarn, A. Domínguez, Thermal Behaviour of Pure Ionic Liquids Scott
Handy, Intechopen, 2015 199–228 (Charter 8).
[44] C. Chiappe, C.S. Pomelli, S. Rajamani, Influence of structural variations in cationic
and anionic moieties on the polarity of ionic liquids, J. Phys. Chem. B 115 (2011)
9653–9661.
[39] S. Fanga, L. Yanga, C. Wei, C. Jianga, K. Tachibanab, K. Kamijimac, Ionic liquids based
on guanidinium cations and TFSI anion as potential electrolytes, Electrochim. Acta
54 (2009) 1752–1756.