Do all the protic ionic liquids exist as molecular aggregates in the gas phase?

Article abstract of DOI:10.1039/c1cp21817d

According to an EI-MS study of 1,1,3,3-tetramethylguanidium-based protic ionic liquids (PILs), it has been concluded that not all PILs exist as molecular aggregates in the gas phase. The detection of both ions of m/z 115.0 and m/z 116.0 for the 1,1,3,3-tetramethylguanidinium trifluoromethylsulfonate (TMGS) protic ionic liquid indicates that both the molecular and ionic aggregates co-exist in the gas phase, which is to say that the TMGS may also evaporate via the ionic aggregates just like aprotic ionic liquids. Furthermore, investigation on triethylamine-based and 1-methylimidazole-based PILs confirmed that the gas phase structure of PILs depends on both the acidity and basicity of the corresponding acid and base. This journal is the Owner Societies.

Full text of DOI:10.1039/c1cp21817d

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
PCCP
Cite this: Phys. Chem. Chem. Phys., 2011, 13, 17445–17448
www.rsc.org/pccp
COMMUNICATION
Do all the protic ionic liquids exist as molecular aggregates in the gas
phase?
Xiao Zhu,a Yong Wangb and Haoran Li*b
Received 4th June 2011, Accepted 19th August 2011
DOI: 10.1039/c1cp21817d
According to an EI-MS study of 1,1,3,3-tetramethylguanidium-
based protic ionic liquids (PILs), it has been concluded that not
all PILs exist as molecular aggregates in the gas phase. The
detection of both ions of m/z 115.0 and m/z 116.0 for the 1,1,3,3-
tetramethylguanidinium trifluoromethylsulfonate (TMGS) protic
ionic liquid indicates that both the molecular and ionic aggregates
co-exist in the gas phase, which is to say that the TMGS may
also evaporate via the ionic aggregates just like aprotic ionic
liquids. Furthermore, investigation on triethylamine-based and
1-methylimidazole-based PILs confirmed that the gas phase
structure of PILs depends on both the acidity and basicity of
the corresponding acid and base.
aprotic ILs by earlier mass spectrometric studies14 and also the
recent atmospheric-pressure thermal desorption ionization
experiments,15 atmospheric-pressure chemical ionization mass
spectrometry experiments,16a and the electrospray ionization
mass spectrometry.17
Protic ionic liquids (PILs) are a subset of ionic liquids
formed by the equimolar mixing of a Brønsted acid and a
Brønsted base, and they have been categorized as poor ionic
liquids.18 The proton transfer mechanism has been uncovered
during the evaporation of the PILs.10,19 For example, a typical
protic ionic liquid, 1-methylimidazoium ethanoate, has been
confirmed to exist as neutral molecules in the gas phase.10
However, there is also substantial disagreement over the
nature of species of PILs that are present in the gas phase.
In the experiments of Rebelo et al., in the PILs [BH]+XÀ there
appears to be no association between the acidic HX and basic
B molecules. They suggested that protic ILs exist as fully
dissociated neutral molecules in the vapor phase.10 However,
in recent works by Ludwig, Eberlin, and Dupont et al., the ion
clusters have also been detected in the PILs.16
Introduction
In recent years, ionic liquids (ILs) have received considerable
attention due to their unique properties, such as non-
flammability, thermal stability, high conductivity, and excellent
solubility. Thus they are considered to be very promising
replacements for traditional volatile organic solvents and
playing important roles in catalysis,1 synthesis,2 separations,3
and nano-materials.4 In addition, ILs have been widely
regarded as the ‘‘green solvents’’, due to their negligible vapor
pressure.5 However, very recently, the general belief that
ILs exert no measurable vapor pressure was shown to be
erroneous. It has been demonstrated that some ILs are volatile,
and they can be distilled at low pressure without decomposition.6
Despite all recent reports on their stable thermal evaporation,7
the nature of the species that are present in the gas phase has
remained controversial. Vaghjiani et al.8 demonstrated the
presence of intact ion pairs in the gas phase using mass
spectrometry. Jones et al.,9 Rebelo et al.,10 and Ballone
et al.11 have revealed that the vapor phase of aprotic ILs
consists of ion pairs, with no detectable concentration of
either free ions or larger clusters. Dai et al.12 indicated that
the IL vapors consist of ion pairs based on the direct UV-
spectroscopic measurements. Leone et al.13 obtained isolated
ion pairs of a hypergolic ionic liquid using aerosol particle
vaporization as a source. However, supra-molecules or ion
clusters have been suggested to exist in the gas phase of the
In this paper, the electron ionization mass spectrometry
(EI-MS) experiments have been carried out to investigate the
gas phase structures of 1,1,3,3-tetramethylguanidinium-based
PILs including 1,1,3,3-tetramethylguanidinium lactate (TMGL),
1,1,3,3-tetramethylguanidinium formate (TMGF), 1,1,3,3-
tetramethylguanidinium trifluoroacetate (TMGA), and 1,1,3,3-
tetramethylguanidinium trifluoromethylsulfonate (TMGS);
triethylamine-based PILs including triethylammonium lactate
(N222L), triethylammonium formate (N222F), triethylammonium
trifluoroacetate (N222A), and triethylammonium trifluoromethyl-
sulfonate (N222S); and 1-methylimidazole-based PILs including
1-methylimidazolium lactate (MimL), 1-methylimidazolium
formate (MimF), 1-methylimidazolium trifluoroacetate (MimA),
and 1-methylimidazolium trifluoromethylsulfonate (MimS). The
EI-MS results of these PILs clearly illustrate that the gas phase
structure of the PILs depends on both the acidity and basicity of
the acid and base corresponding to the anion and cation.
Experiments
a School of Chemistry and Chemical Engineering,
Qufu Normal University, Qufu 273165, P. R. China
Synthesis of the protic ionic liquids
TMG-based PILs are synthesized according to the literature.20
N222 and Mim-based PILs are synthesized as follows.
b Department of Chemistry, Zhejiang University, Hangzhou 310027,
P. R. China. E-mail: lihr@zju.edu.cn; Fax: +86-571-8795-1895
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 17445–17448 17445

View Article Online
N222A. The synthesis of the ionic liquid was carried out in a
100 ml round-bottomed flask, which was immersed in a water-
bath and fitted with a reflux condenser. In a typical reaction,
trifluoroacetic acid (28.8 g, 0.25 mol) was dropped into the
diethylamine (15.0 g, 0.25 mol) at 25 1C for 2 hours (the
dropping of the trifluoroacetic acid should be slow to avoid
temperature runaway). After the addition, the reaction
mixtures were stirred for an additional period of 2 hours at
60 1C to ensure that the reaction had proceeded to completion.
The impurities were removed by extraction using ether for
several times and then heating the residue at 80 1C in high
vacuum (5 mmHg) until the weight of the residue remained
EI-MS spectra
The EI-MS spectra were obtained from a Thermofisher
DSQ instrument and collected at 70 eV, all the samples were
dissolved in chloroform.
Results and discussion
The EI-MS spectra of the TMG-based PILs are depicted in
Fig. 1. The peak corresponding to the ionized molecules of m/z
115.0 is observed in TMGL, TMGF, and TMGA respectively.
The ion of m/z 116.0 is also present. However, in the spectra of
TMGL, TMGF, and TMGA, the ions m/z 116.0 : 115.0 ratio
are 4.88%, 5.86%, and 6.05%, respectively, indicating that the
m/z 116.0 might be the isotopic peak. This shows that the
TMGL, TMGF, and TMGA PILs exist mainly as molecular
1
constant. The yield of N222A was 95% (41.6 g). H NMR d
(500 MHz, CDCl3): 12.75 (s, 1H, NH), 8.48 (s, H, CH), 3.16
(m, 6H, CH2), 1.35 (t, 9H, CH3).
The following PILs were synthesized similarly to N222A.
N222L. 22.5 g (0.25 mol) of lactic acid, and 15.0 g (0.25 mol)
of triethylamine, and the product N222L was obtained as
a colorless or pale yellow liquid. Yield: 96%. 1H NMR d
(500 MHz, CDCl3): 8.53 (s, 1H, NH), 4.72 (s, 1H, OH),
3.80 (s, 1H, CH), 2.80 (m, 6H, CH2), 1.10 (s, 3H, CH3), 1.01
(t, 9H, CH3).
N222F. 11.5 g (0.25 mol) of formic acid, and 15.0 g
(0.25 mol) of triethylamine, and the product N222F was
obtained as a colorless liquid. Yield: 90%. 1H NMR d
(500 MHz, CDCl3): 12.65 (s, 1H, NH), 3.21 (m, 6H, CH2),
1.39 (t, 9H, CH3).
N222S. 37.5 g (0.25 mol) of trifluoromethylsulfonic acid,
and 15.0 g (0.25 mol) of triethylamine, and the product
1
N222S was obtained as a white solid. Yield: 97%. H NMR d
(500 MHz, CDCl3): 3.18 (m, 6H, CH2), 1.36 (t, 9H, CH3).
MimL. 22.5 g (0.25 mol) of lactic acid, and 20.5 g (0.25 mol)
of 1-methylimidazole, and the product MimL was obtained
1
as a colorless or pale yellow liquid. Yield: 96%. H NMR d
(500 MHz, CDCl3): 8.77 (s, 1H, NH), 7.95 (s, 1H, CH), 7.23
(s, H, CH), 7.01 (s, H, CH), 3.76 (s, 3H, CH3), 1.45 (s, 3H, CH3).
MimF. 11.5 g (0.25 mol) of formic acid, and 20.5 g (0.25 mol)
of 1-methylimidazole, and the product MimF was obtained as a
1
colorless liquid. Yield: 90%. H NMR d (500 MHz, CDCl3):
13.88 (s, 1H, NH), 8.42 (s, 1H, CH), 8.01 (s, H, OH), 7.15
(s, H, CH), 7.06 (s, H, CH), 3.80 (s, 3H, CH3).
MimA. 28.8 g (0.25 mol) of trifluoroacetic acid, and 20.5 g
(0.25 mol) of 1-methylimidazole, and the product MimA
was obtained as a colorless liquid. Yield: 90%. 1H NMR d
(500 MHz, CDCl3): 12.67 (s, 1H, NH), 8.95 (s, 1H, CH), 7.38
(s, H, CH), 7.16 (s, H, CH), 3.96 (s, 3H, CH3).
MimS. 37.5 g (0.25 mol) of trifluoromethylsulfonic acid,
and 20.5 g (0.25 mol) of 1-methylimidazole, and the product
1
MimS was obtained as a white solid. Yield: 97%. H NMR d
(500 MHz, CDCl3): 10.53 (s, 1H, NH), 8.46 (s, 1H, CH), 7.31
(s, 2H, CH), 3.81 (s, 3H, CH3).
Fig. 1 The EI-MS spectra of the TMG-based ILs.
c
17446 Phys. Chem. Chem. Phys., 2011, 13, 17445–17448
This journal is the Owner Societies 2011

View Article Online
aggregates in the gas phase and the vaporization of these PILs
involves a proton transfer mechanism with formation of two
volatile neutral molecules. Actually it has been previously
demonstrated that no charged species or ionic pairs were
detected in the gas phase of 1,1,3,3-tetramethylguanidinium
chloride and a proton transfer led to the formation of the
neutral acid and base precursors.21
Compared with the above three kinds of PILs, in the case of
TMGS, the m/z 116.0 : 115.0 ratio is 71.5 : 100.0, hence it is
apparently not an isotopic peak. To eliminate the effect of the
chemical ionization, we increased the concentration of the
samples and re-measured their EI-MS spectra. If the peak
m/z 116 is formed by the chemical ionization, the intensity of
m/z 116 would increase with the increasing of the amount of
TMGS. However, it was found that the intensity of m/z 116
decreased from 71.50 to 63.06 when we increased the TMGS
concentration (one times higher than the former). This indi-
cates that the peak m/z 116 is not caused by the chemical
ionization. Besides, no significant changes have been found in
EI-MS spectra of other three kinds of PILs when their
concentration changed. Therefore the ion peak of m/z 116.0
corresponds to the [TMG]+ cation, which strongly indicates
the presence of the ionic aggregates in the gas phase of
TMGS. In general belief, PILs evaporate via the neutral
products of the proton transfer, while the aprotic ionic liquids
evaporate via ion pairs (cation–anion).6 The detection of both
ions peak of m/z 115 and 116 indicates that the molecular and
ionic aggregates co-exist in TMGS and the TMGS may also
evaporate via ion pairs.
Fig. 2 The EI-MS spectra of the N222-based ILs.
To examine the effect of various cations, the triethylamine
and 1-methylimidazole-based PILs were synthesized and investi-
gated. Similar to 1,1,3,3-tetramethylguanidinium (pKa = 13.6),
triethylamine is also a strong base (pKa = 10.6), but Mim is a
weak base (pKa = 7.4). The EI-MS spectra of these two kinds of
PILs are shown in Fig. 2 and 3. According to Fig. 2, the peak
corresponding to the ionized molecules of m/z 101.0 is observed
in N222L, N222F, N222A, and N222S, respectively. The peak
corresponding to the N222 cation of m/z = 102.0 is present in
spectra of N222S (the m/z 102.0 : 101.0 ratio is 44.5 : 13.8), but
absent in the spectra of N222L, N222F, and N222A. This
reveals that equilibrium between the ion aggregates and mole-
cular aggregates exists in the gas phase of N222S. In addition,
molecular aggregates may dominate in the gas phase of N222L,
N222F, and N222A. Furthermore, as shown in Fig. 3, the peak
corresponding to the Mim cation of m/z = 83 is relatively
weaker. The m/z 83.0 : 82.0 ratio is 5.4 : 100.0 even in the MimS,
which indicates that molecular aggregates may be the primary
formation existing in the gas phase of the Mim-based PILs.
To our knowledge, the basicity of the cations and the acidity
of the anions may significantly affect the gas phase structure of
the PILs. 1,1,3,3-Tetramethylguanidinium and triethylamine
are strong bases and are relatively easier to be protonated
than 1-methylimidazole (a relatively weaker base). The acidity
indices (pKa) for lactic acid, formic acid, trifluoroacetic acid,
and trifluoromethylsulfonic acid are 3.86, 3.75, 0.23, and
oÀ1, respectively. This suggests that the trifluoromethyl-
sulfonic acid is the strongest acid among the four acids mentioned
above and the deprotonation of trifluoromethylsulfonic acid is
easier than that of the other three acids. Therefore, the TMGS
Fig. 3 The EI-MS spectra of the Mim-based ILs.
and N222S are combinations of a strong Brønsted base and a
strong Brønsted acid, and proton transfer from cations to
anions may not be so easy in these kinds of PILs, which may
explain the co-existence of molecular and ionic aggregates in
the gas phase of these PILs.
Conclusion
In conclusion, both molecular aggregates and ionic aggregates
may co-exist in the gas phase for TMGS protic ionic liquid,
c
This journal is the Owner Societies 2011
Phys. Chem. Chem. Phys., 2011, 13, 17445–17448 17447

View Article Online
and this finding shows that some PILs may also evaporate via
both ion-pairs form and neutral molecules. To examine the
effect of different cations, the triethylamine and 1-methyl-
imidazole-based PILs were synthesized and investigated.
Similar phenomena have also been found in the strong
Brønsted base triethylamine-based PILs, but no evidence
proved the presence of ionic aggregates in the weak Brønsted
base 1-methylimidazole-based PILs. Further analysis of the
pKa values suggested that the gas phase structure of the PILs is
governed by both the acidity and basicity of the acid and base
corresponding to the anion and cation. The results will help us
to extensively understand the behavior of vaporized PILs and
offer us some valuable information for the design of PILs.
More experiments are underway in our laboratory to explore
the structure characters of other PILs.
7 (a) Y. U. Paulechka, D. H. Zaitsau, G. J. Kabo and A. A. Strechan,
Thermochim. Acta, 2005, 439, 158; (b) D. H. Zaitsau, G. J. Kabo,
A. A. Strechan and Y. U. Paulechka, J. Phys. Chem. A, 2006,
110, 7303; (c) J. H. Gross, J. Am. Soc. Mass Spectrom., 2008,
19, 1347; (d) A. Nobyuki, P. David, K. Akio and S. Kazuhiko,
J. Phys. Chem. B, 2009, 113, 4756.
8 S. D. Chambreau, J. A. Boatz, G. L. Vaghjiani, J. F. Friedman,
N. Eyet and A. A. Viggiano, J. Phys. Chem. Lett., 2011, 2, 874.
9 J. P. Armstrong, C. Hurst, R. G. Jones, P. Licence, K. R. J.
Lovelock, C. J. Satterley and I. J. Villar-Garcia, Phys. Chem.
Chem. Phys., 2007, 9, 982.
10 J. P. Leal, J. M. S. S. Esperanca, M. E. Minas da Piedade, J. N. C.
Lopes, L. P. N. Rebelo and K. R. Seddon, J. Phys. Chem. A, 2007,
111, 6176.
11 P. Ballone, C. Pinilla, J. Kohanoff and M. G. Del Popolo, J. Phys.
Chem. B, 2007, 111, 4938.
12 C. Wang, H. Luo, H. Li and S. Dai, Phys. Chem. Chem. Phys.,
2010, 12, 7246.
13 C. J. Koh, C. Liu, C. W. Harmon, D. Strasser, A. Golan,
O. Kostko, S. D. Chambreau, G. L. Vaghjiani and S. R. Leone,
J. Phys. Chem. A, 2011, 115, 4630.
Acknowledgements
14 (a) A. K. Abdul-Sada, A. E. Elaiwi, A. M. Greenway and K. R.
Seddon, Eur. J. Mass Spectrom., 1997, 3, 245; (b) F. C. Gozzo,
L. S. Santos, R. Augusti, C. S. Consorti, J. Dupont and M. N.
Eberlin, Chem.–Eur. J., 2004, 10, 6187.
15 H. Chen, Z. Ouyang and R. G. Cooks, Angew. Chem., Int. Ed.,
2006, 45, 3656.
16 (a) B. A. S. Neto, L. S. Santos, F. M. Nachtigall, M. N. Eberlin
and J. Dupont, Angew. Chem., Int. Ed., 2006, 45, 7251;
(b) R. Ludwig, J. Phys. Chem. B, 2009, 113, 15419.
17 A. M. Fernandes, J. A. P. Coutinho and I. M. Marrucho, J. Mass
Spectrom., 2009, 44, 144.
This work was supported by the National Natural Science
Foundation of China (No. 20806065, No. 20990221) and
Doctoral Start-up Scientific Research Foundation of Qufu
Normal University (No. BSQD20090117). The authors thank
Mr Kezhi Jiang for the measurement of the EI-MS.
Notes and references
1 (a) M. Sobota, M. Schmid, M. Happel, M. Amende, F. Maier,
H. P. Steinruck, N. Paape, P. Wasserscheid, M. Laurin, J. M.
Gottfried and J. Libuda, Phys. Chem. Chem. Phys., 2010,
12, 10610; (b) Y. Wang, H. Li, C. Wang and H. Jiang, Chem.
Commun., 2004, 1938.
18 (a) B. Nuthakki, T. L. Greaves, I. Krodkiewska, A. Weerawardena,
M. I. Burgar, R. J. Mulder and C. J. Drummond, Aust. J. Chem.,
2007, 60, 21; (b) Y. Wang, H. Pan, H. Li and C. Wang, J. Phys.
Chem. B, 2007, 111, 10461; (c) Y. Wang, C. Wang, L. Zhang and
H. Li, Phys. Chem. Chem. Phys., 2008, 10, 5976; (d) D. R.
MacFarlane and K. R. Seddon, Aust. J. Chem., 2007, 60, 3;
(e) L. Zhang, H. Li, Y. Wang and X. Hu, J. Phys. Chem. B, 2007,
111, 11016; (f) X. Zhu, Y. Wang and H. Li, AIChE J., 2009, 55,
198.
19 M. Yoshizawa, W. Xu and C. A. Angell, J. Am. Chem. Soc., 2003,
125, 15411.
20 (a) H. Gao, B. Han, J. Li, T. Jiang, Z. Liu, W. Wu, Y. Chang and
J. Zhang, Synth. Commun., 2004, 34, 3083; (b) W. Wu, B. Han,
H. Gao, Z. Liu, T. Jiang and J. Huang, Angew. Chem., Int. Ed.,
2004, 43, 2415.
2 S. Narayanaperumal, E. E. Alberto, K. Gul, O. E. D. Rodrigues
and A. L. Braga, J. Org. Chem., 2010, 75, 3886.
3 F. Agel, F. Pitsch, F. F. Krull, P. Schulz, M. Wessling, T. Melin
and P. Wasserscheid, Phys. Chem. Chem. Phys., 2011, 13, 725.
4 (a) S. Perkin, T. Albrecht and J. Klein, Phys. Chem. Chem. Phys.,
2010, 12, 1243; (b) Y. Wang, J. Zhang, X. Wang, M. Antonietti
and H. Li, Angew. Chem., Int. Ed., 2010, 49, 3356; (c) Y. Wang,
X. Wang, M. Antonietti and Y. Zhang, ChemSusChem, 2010,
3, 435.
5 R. D. Rogers and K. P. Seddon, Science, 2003, 302, 792.
6 M. J. Earle, J. M. S. S. Esperanca, M. A. Gilea, J. N. Canongia
Lopes, L. P. N. Rebelo, J. W. Magee, K. R. Seddon and J. A.
Widegren, Nature, 2006, 439, 158.
21 J. Vitorino, J. P. Leal, M. E. Minas da Piedade, J. N. C. Lopes, J.
M. S. S. Esperanca and L. P. N. Rebelo, J. Phys. Chem. B, 2010,
114, 8905.
c
17448 Phys. Chem. Chem. Phys., 2011, 13, 17445–17448
This journal is the Owner Societies 2011