Infrared spectroscopic study on chemical and phase equilibrium in triethylammonium acetate

Article abstract of DOI:10.1007/s11426-012-4634-6

Protic ionic liquid (PIL) triethylammonium acetate was prepared by mixing equimolar amounts of acetic acid and triethylamine, and then studied using the combination of the Attenuated Total Reflection Fourier Transform Infrared spectroscopy, in-situ infrared spectroscopy, pH, and conductivity titration measurements. It was found that the equimolar synthesized triethylammonium acetate was separated into two layers, which suggesting that there were both chemical and phase equilibrium in this solution. Molecular species could be directly observed in the IR spectra over the range of 1200-1800 cm?1 and also checked by 1H NMR. Based on analysis, the upper layer was rich in amine with little acid and PIL, and the down layer was rich in PIL with residual acetic acid and amine. And single PIL-rich layer could be separated into two layers again when the mole ratio of newly added triethyamine to the theoretical produced triethylammonium acetate reached 0.12.

Full text of DOI:10.1007/s11426-012-4634-6

SCIENCE CHINA
Chemistry
August 2012 Vol.55 No.8: 1688–1694
doi: 10.1007/s11426-012-4634-6
• ARTICLES •
· SPECIAL ISSUE · Ionic Liquid and Green Chemistry
Infrared spectroscopic study on chemical and phase equilibrium in
triethylammonium acetate
LV YiQi1, GUO Yan1, LUO XiaoYan1 & LI HaoRan1,2*
1Department of Chemistry, Zhejiang University, Hangzhou 310027, China
2State Key Laboratory of Chemical Engineering; Department of Chemical and Biological Engineering, Zhejiang University,
Hangzhou 310027, China
Received February 23, 2012; accepted April 17, 2012; published online June 25, 2012
Protic ionic liquid (PIL) triethylammonium acetate was prepared by mixing equimolar amounts of acetic acid and triethyla-
mine, and then studied using the combination of the Attenuated Total Reflection Fourier Transform Infrared spectroscopy,
in-situ infrared spectroscopy, pH, and conductivity titration measurements. It was found that the equimolar synthesized tri-
ethylammonium acetate was separated into two layers, which suggesting that there were both chemical and phase equilibrium
in this solution. Molecular species could be directly observed in the IR spectra over the range of 1200–1800 cm1 and also
1
checked by H NMR. Based on analysis, the upper layer was rich in amine with little acid and PIL, and the down layer was
rich in PIL with residual acetic acid and amine. And single PIL-rich layer could be separated into two layers again when the
mole ratio of newly added triethyamine to the theoretical produced triethylammonium acetate reached 0.12.
chemical equilibrium, phase equilibrium, triethylammonium acetate, IR spectroscopy
mechanical calculations [24].
AH(bronsted acid)  B(bronsted base)  [BH]  A
1 Introduction
(1)
Protic ionic liquids (PILs), which were classified as “poor”
ionic liquids [1, 2], were rendered one of the most attractive
ionic liquids as their widely uses [3–5] ranging from batter-
ies [6], fuel cells [7, 8], double-layer capacitors [9, 10], ca-
talysis [11–16], and solvent [17, 18]. A fraction of the re-
searchers approved of the presenting of neutral acid and
base species due to a chemical equilibrium (eq. (1)) [19–21]
in PILs through different techniques. For example the neu-
tral formic acid was believed to be exsting in the N-methyl-
2-hydroxyethylammonium formate according to the faster
diffusion obtained for the anion than the cation (D>D+)
[22]. And one kind neutral ion pair “molecules” held to-
gether through a strong hydrogen bond [23] was found in
[{(CH3)2N}2CNH2]Cl by Raman spectroscopy and quantum
In case the equilibrium constant is very large, there will
be little non-negligible amount of neutral molecular species
in the PIL, however, when the equilibrium constant is not
very large, the PIL contains not only ionic but also neutral
species to form a complex system. MacFarlane and Seddon
[25] discussed the question of “Is the PILs true ionic liquids
or liquid mixture?”, and the equilibrium in PILs caught
much attention by many researchers [26–31].
In the investigation of the equilibrium, the NMR meas-
urement (1H NMR, 13C NMR, 15N NMR) [27, 28, 32] is the
commonly used technique, as the 13C, H chemical shifts,
1
relaxation times, and diffusion coefficients, can be used to
study the interaction between cation and anion [33]. Origi-
nally, NMR spectroscopy was adopted to distinguish the
neutral and ionized proton in the study of the ionic pairs,
*Corresponding author (email: lihr@zju.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

Lv YQ, et al. Sci China Chem August (2012) Vol.55 No.8
1689
molecules, and aggregation components in ILs. Especially,
2 Experimental
after Tokuda et al. [29, 34–36] successfully proposed the
Λimp/ΛNMR ratio to describe the ionicity of APILs (aprotic
ionic liquids), many researchers tried to apply NMR spec-
troscopy to investigate the properties and equilibrium in ILs.
2.1 Synthesis and characterization
Triethylammonium acetate was synthesized by equimolar
amounts of acid and base. The acetic acid (anhydrous,
GR99.8%, Aladdin) was added very slowly while stirring
to triethylamine (anhydrous, GR99.0%, Aladdin) con-
tained in a round-bottom flask over water at room tempera-
ture. And then the mixture was stirring continuously at
25 °C for 5 h. However, further purity measurements were
not applied on the equimolar synthesized solution, as we
believed that there were no impurities existing in the reac-
tants, even the residual neutral ones were considered as
molecular species to maintain the well-known chemical
equilibrium. The purpose of protecting the structure and
properties of the equimolar synthesized solution could give
us a chance to investigate the true contents of the PIL. The
water content was checked by Carl-Fischer titration, 659
ppm.
1
Tubbs et al., for instance, applied H NMR spectroscopy to
explore the chemical equilibrium and thermodynamic prop-
erties in imidazolium based ILs [37]. However, in ammo-
nium-based PILs, since one proton belonging to the acid in
the PIL may transfer fast between the acid and the amine,
only an average signal of the protons was observed in the
spectra as reported by Nuthakki et al. [27] due to the fact
that the lifetime of ion-pairs (20×1014 s) was too short to
monitor on a 1011–109 s timescale NMR spectroscopy [12,
29]. Even in 15N NMR spectra, which exhibits a high polar-
ity and is much more sensitive than the 13C or 1H NMR, also
only one moved resonance was observed in the study of
diethanolamine samples with different acetic acid concen-
trations [32]. In other words, it is impossible to figure out
two different chemical shifts belonged to the neutral and
ionized nitrogen to describe the chemical equilibrium di-
rectly by NMR spectroscopy. Besides this, Raman [38], IR,
[39–41] and viscosity compared to ionic conductivity
measurement (the classic Walden rule) [42–45] have also
been used to investigate the equilibrium, but little specific
quantitative data has been ever got. Furthermore, IR spec-
troscopy was usually used to investigate the general interac-
tions that exist in ILs at molecular level [46–49]. For exam-
ple, Shimomura et al. [50] applied ATR-IR spectroscopy to
study the effect of the alkyl-chain length on the mixing state
of imidazolium-based ionic liquid-methanol solutions. And
the thermal stability of N-alkyl-N-alkyl-pyrrolidinium im-
ides was also investigated by FT-IR spectroscopy [51].
Mashkovsky et al. [52, 53] studied the hydrogen bonding in
triethylammonium salts by IR spectroscopy. And Wulf et al.
[54] reported that both IR and NMR spectroscopic proper-
ties reflected a similar type of electronic perturbation caused
by hydrogen bonding, but take place on different timescales.
Compared to a 1011–109 s timescale NMR spectroscopy,
IR spectroscopy is more suited to investigate the equilibri-
um as the lifetime of ion-pairs is 20×1014 s.
It is interesting to find that the equimolar synthesized
solution is separated into two layers (Figure 1). The down
layer is recognized as the PIL-rich layer and the upper layer
as amine-rich layer by 1H NMR method.
The PIL-rich layer was characterized by NMR (1H NMR
and 13C NMR) spectra (500 MHz, CDCl3, TMS) [55],
DSC-TG (from room temperature to 500 °C at a cooling and
heating rate of 10 °C/min in nitrogen flow on NETZSCH
STA 409 PG/PC), and MS (used ESI as the ion source type
by positive and negative ion polarity respectively for cation
and anion) (Supporting Information).
2.2 Conductivity measurements
The conductivity of any solution depends not only on the
number of charge carriers but also on their mobility. The
ionic conductivity measurements of the PIL-rich layer were
conducted on a Model DDS-307 conductometer with a
DJS-10C type platinum black electrode at 25 °C. The cell
constant was determined by calibration using an aqueous
In this paper, IR spectroscopy is chosen to investigate the
equilibrium of triethylammonium acetate. We present the
direct evidence of the existence of molecular species in tri-
ethylammnium acetate by IR spectroscopy rather than NMR
spectroscopy to note that there is a chemical equilibrium as
eq. (2) in the PIL.
NX + HAc  [NXH][Ac ]
(2)
And the results obtained from IR spectroscopy, in-situ IR
spectroscopy, pH, and conductivity titration measurements
indicate that there are both chemical and phase equilibrium
in the equimolar synthesized solution.
Figure 1 The picture of two-layer solution with three components. The
upper layer is recognized as amine-rich layer and the down one is PIL-rich
layer.

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Lv YQ, et al. Sci China Chem August (2012) Vol.55 No.8
0.01 M KCl solution. In the process of conductivity titra-
tions, the solutions were being equilibrated for 2 h before
each measurement.
book of Chemistry and the IR results got from acetic and
ammonium salts, peaks in Figure 3(a) of the PIL layer can
be observed associated with: (1) ionic peaks: the peaks in
the range of 1350–1450 cm1 are attributed to both COO
and N+-H, and in 1500–1600 cm1 is assigned to COO. (2)
molecular peaks: significant COOH modes in the range of
1680–1750 cm1, and one peak in the rang of 1200–1300
cm1 could be assigned to the C–O vibration in the COOH.
The molecular and ionic species of the PIL-rich layer are
directly observed in the IR spectra, suggesting that there is
indeed a chemical equilibrium (eq. (2)) in it. And the dis-
tinct two-layer solution illustrates that there is also a phase
equilibrium in the two-layer solution. In a word, the
equimolar synthesized solution exhibits a distinct phenom-
enon: two-layer, three components, and there are both
chemical and phase equilibrium in it.
Why is the equimolar synthesized solution separated into
two layers as shown in Figure 1? This is probably attributed
to the spatial structure of triethyammonium (with three alkyl
around the nitrogen atoms), which makes it difficult for the
proton transferring to produce triethylammonium acetate,
and the amount of the produced PIL exceeds the dissolva-
bility of neutral molecular species to the PIL. Figure 3(b)
describes that the area of molecular species (C–O and
COOH) is much larger than that of ionic species (COO and
N+–H), indicating that the amount of molecular species is
much larger than ionic ones, and this could also attribute to
the two separated layers phenomenon.
2.3 pH measurements
The pH measurements of the solution were conducted on a
portable acidity meter PHB-2 with composite electrode at
25 °C, which was purchased from Hangzhou Leici analyti-
cal instrument company. In the process of pH titrations, the
solutions were being equilibrated for 2 h before each meas-
urement.
2.4 IR measurements
Infrared spectroscopy experiments were performed on a
Nicolet FT-IR/Nexus 470 spectrometer in the attenuated
total reflection mode at 25 °C. A small droplet of the
PIL-rich layer was placed on top of Ge crystal. Spectra were
obtained in the range of 4000–600 cm1 at a resolution of 4
cm1, and accumulated for over 32 scans. Besides this, in
situ Fourier transform infrared spectroscopy (BRUKER
OPTIK GmbH, Matrix-MX) was used to explore the effect
of the addition of triethylamine to single PIL-rich layer and
two-layer solution (equimolar synthesized solution).
3 Results and discussion
MacFarlane and Seddon [25] reported that below 1% of
neutral species one could consider PILs as “pure” ILs. And
Lopes et al. [21] concluded that the term “PIL” should defi-
nitely be replaced by the term “PIL solution”. In other
words, the PIL-rich layer could be recognized as PIL solu-
tion. As a result, both chemical and phase equilibrium ex-
isting in the two-layer solution. The structure and properties
of that need further investigation.
3.1 Both chemical and phase equilibrium exist in
two-layer solution
The equimolar synthesized solution surprises us as it is sep-
arated into two layers, and called two-layer solution, as
shown in Figure 1. With the purpose to identify the compo-
nents of those two layers, 1H NMR spectroscopy was
adopted to check the two layers compared to material tri-
ethylamine. The results are listed in Table 1, which indi-
cates that both layers include triethylammonium acetate
(PIL), triethylamine, and acetic acid. The upper layer is rich
in amine with little acid and PIL, and the down layer is rich
in PIL with residual acetic acid and amine. Furthermore, the
single upper layer could stay stable as the two-layer solution
was separated, and the single PIL-rich layer stay stable in a
short period and would be separated into two layers again
after a long period.
To confirm the characteristic absorption peaks of the
PIL-rich layer, we conducted IR spectroscopy measure-
ments on sodium acetate, acetic acid, sodium chloride, and
the PIL-rich layer. Figure 2 shows the absorption spectra of
HAc, NaAc/H2O, and NH4Cl/H2O in the range of
1000–1800 cm1, with typical COO and N+-H bands in the
range of 1350–1450 cm1, COO 1500–1600 cm1, and
significant COOH modes in the range of 1680–1750 cm1
with C–O (1200–1300 cm1). According to Lange’s Hand
Figure 2 The IR spectra of HAc, NaAc/H2O, NH4Cl/H2O, and H2O in the
range of 1000–1800 cm1.

Lv YQ, et al. Sci China Chem August (2012) Vol.55 No.8
1691
Table 1 The 1H NMR results of upper layer and down layer of the
two-layer solution compared to triethylamine
investigation of equilibrium. The results were shown in
Figure 4.
1H NMR chemical shifts
As shown in Figure 4(a), the addition of triethylamine
causes the conductivity of the PIL-rich layer decrease firstly
due to the fact that ionic species is diluted when the amine
is added. Furthermore, triethylammonium acetate is not
easily produced as the spatial structure of cation, let alone
adding amine into the PIL-rich layer. All of this could at-
tribute to the decrease of the conductivity before 0.16 mole
ratio. Then adding amine to the PIL-rich layer continually
causes the conductivity increase as a result of the single
PIL-rich layer started to stratify. In other words, along with
adding the amine, the single PIL-rich layer gradually sepa-
rates into two layers again, and redundant amine goes to the
new upper amine-rich layer, leading to the increase of the
conductivity (after mole ratio 0.16, Figure 4), and the de-
crease of pH (mole ratio between 0.12–0.16, Figure 4).
Furthermore, the two-layer phenomenon was observed dur-
ing the experiment when the mole ratio of triethylamine to
theoretical triethylammonium acetate (n(N222):n(theoret-
ical N222HAc)) reached 0.12. After stratification, we still
measured the conductivity and pH of PIL-rich layer to ex-
PIL
0.99–1.03(CH3, 9H)
2.48–2.53(CH2, 6H)
0.99–1.03(CH3, 9H
2.48–2.53(CH2, 6H)
1.22–1.25(CH3, 9H),
2.00(CH3, 3H),
Triethylamine
Upper layer
Down layer
2.97–3.02(CH2, 6H),
10.8(H)
3.2 The equilibrium in the single PIL-rich layer com-
pared to the two-layer solution
Conductivity and pH measurements as macroscopic tech-
niques were used to monitor the movements of equilibrium
during the process of single PIL-rich layer which was taken
out of the two-layer solution changed into a new two-layer
solution, along with triethylamine added. And all the meas-
urements were focused on the PIL-rich layer. The conduc-
tivity could reflect the amount of moveable ions and the pH
could give information about the change in the concentra-
tion of [H]+ in that solution, and both of that are useful for
Figure 3 The attenuated total reflection (ATR) FTIR spectra and peak area of the PIL-rich layer in the range of 1200–1800 cm1. (a) The IR spectra with
typical COO and N+-H bands in the range of 1350–1450 cm1, COO1500–1600 cm1, and significant COOH modes in the range of 1680–1750 cm1 with
C–O (1200–1300 cm1), (b) the area of the four typical peaks diagram. The area of molecular species is much larger than the ionic ones, indicating there is
much more molecular species than ionic ones in the PIL-rich layer.
Figure 4 The conductivity and pH titration diagrams of PIL-rich layer separated out from two-layer solution along with the amine was added into the sin-
gle layer (N222: (CH3CH2)3N , N222HAc: [(CH3CH2)3NH][CH3COO], the molar amount of theoretical N222HAc is calculated by the mole of raw material
amine and acid: 0.5 mole amine mixed with 0.5 mole acid produce 0.5 mole PIL).

1692
Lv YQ, et al. Sci China Chem August (2012) Vol.55 No.8
plore the equilibria in the solution. All the results demon-
strated that at the first inflection point in Figure 4(b), the
molecular amine was separated out from the previous single
PIL-rich solution, and then the single layer gradually
reached a new two-layer solution.
which provides valuable insight into the fact that the
amount of acetic acid may increase. In Figure 5(c) the area
of COOH decreases along with the increase of n(N222):
n(theoretical N222HAc) without the effect of the amine-rich
layer. This is because the addition of N222 causes the right
shift of the chemical equilibrium to produce PIL, reflecting
the area of COOH decreases. In other words, eliminating
the amine-rich layer, the single PIL-rich layer has a normal
chemical equilibrium property. But in the two-layer solution,
there is a balance between the chemical and phase equilib-
rium, which makes this solution quite distinctive. And in the
process, the phase equilibrium plays a more important role.
The competition between the chemical and phase equilib-
rium could be the key point, and this needs further investi-
gation.
All the IR spectroscopy, conductivity, and pH titrations
results tell us that the equimolar synthesized triethylammo-
nium acetate solution is a two-layer solution with three
components-PIL, amine, and acetic acid, existing both
chemical and phase equilibrium. Therefore, we could not
get the expected PIL by equimolar acid and amine. The
down layer of the solution could be recognized as PIL solu-
tion according to MacFarlane and Seddon [24], and the PIL
solution will be separated into two layers again when the
amount of added amine reaches n(N222):n(theoretical
N222HAc) of 0.12.
In-situ IR spectroscopy was used to explore equilibrium
in both two-layer solution and single PIL-rich layer, along
with triethylamine added into those solution, and the results
are shown in Figure 5. In the two-layer solution, the trend of
changes in the area of COOH of the amine-rich layer which
was calculated by the IR auto-software is shown in Figure
5(a). It changes as “N” letter with two inflection points at
0.12 and 0.16. Before 0.12, the amount of acetic acid in the
amine-rich layer increases along with the added amine as
the dominant effect of phase equilibrium. Triethylammo-
nium acetate is produced between 0.12 and 0.16 due to the
chemical equilibrium. After 0.16 mole ratio, the effect of
phase equilibrium on the two-layer solution makes the
amount of acetic acid in the amine-rich layer increase again.
The properties of equilibria in single PIL-rich layer are
quite different from the above mentioned two-layer solution
with both chemical and phase equilibrium, which is demon-
strated by the opposite trend of changes in the area of
COOH of the two in Figure 5(b,c). In Figure 5(b), there is a
significant increase in the area of COOH following the
increased mole ratio of n(N222):n(theoretical N222HAc),
Figure 5 The relative area change of COOH peak by in-situ IR spectroscopy at various ratios of n(N222):n(theoretical N222HAc) in two-layer solution
and single PIL-rich layer (N222: (CH3CH2)3N, N222HAc: [(CH3CH2)3NH][CH3COO], the molar amount of theoretical N222HAc is calculated by the mole of
raw material amine and acid: 0.5 mole amine is mixed with 0.5 mole acid to produce 0.5 mole PIL).

Lv YQ, et al. Sci China Chem August (2012) Vol.55 No.8
1693
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4 Conclusions
In this work, we propose that there are both chemical and
phase equilibrium in equimolar synthesized triethylammo-
nium acetate solution by IR spectroscopy, in-situ IR spec-
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And this equimolar synthesized triethylammonium acetate
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