Chemical Analysis of Ionic Liquids Using Photoelectron Spectroscopy

Article abstract of DOI:10.1002/bkcs.10683

The feasibility of utilizing X-ray photoelectron spectroscopy (XPS) to analyze roomerature ionic liquids (RTILs) was investigated in this study. Conventionally, the chemical structure of organic compounds is identified by nuclear magnetic resonance (NMR) spectroscopy. The properties of RTILs, especially their low vapor pressure, make it possible to analyze RTILs by using XPS. The usefulness of XPS on RTILs was confirmed by commercial RTILs. All atoms in RTILs were detected in survey XPS spectra, and the calculated atomic percentages matched well with theoretical values. After the verification of commercial RTILs by XPS, we synthesized three RTILs and investigated them with XPS. The atomic ratio and chemical environment of carbon in RTILs were verified by XPS. By adapting XPS to the investigation of RTILs, carbon atoms in different chemical environments were distinguishable by the binding energy shift, and the atomic ratio of the constituent atoms was identifiable after peak deconvolution. In addition, inorganic constituents were detected by XPS unlike in the case of NMR spectroscopy.

Full text of DOI:10.1002/bkcs.10683

Article
DOI: 10.1002/bkcs.10683
BULLETIN OF THE
S. Y. Seo et al.
KOREAN CHEMICAL SOCIETY
Chemical Analysis of Ionic Liquids Using Photoelectron Spectroscopy
†
†
*
SungYong Seo, Juyun Park, and Yong-Cheol Kang
Department of Chemistry, Pukyong National University, Busan 608-737, Republic of Korea.
E-mail: yckang@pknu.ac.kr
*
Received October 14, 2015, Accepted November 23, 2015, Published online February 22, 2016
The feasibility of utilizing X-ray photoelectron spectroscopy (XPS) to analyze room-temperature ionic liquids
(RTILs) was investigated in this study. Conventionally, the chemical structure of organic compounds is iden-
tified by nuclear magnetic resonance (NMR) spectroscopy. The properties of RTILs, especially their low vapor
pressure, make it possible to analyze RTILs by using XPS. The usefulness of XPS on RTILs was confirmed by
commercial RTILs. All atoms in RTILs were detected in survey XPS spectra, and the calculated atomic per-
centages matched well with theoretical values. After the verification of commercial RTILs by XPS, we synthe-
sized three RTILs and investigated them with XPS. The atomic ratio and chemical environment of carbon in
RTILs were verified by XPS. By adapting XPS to the investigation of RTILs, carbon atoms in different chem-
ical environments were distinguishable by the binding energy shift, and the atomic ratio of the constituent
atoms was identifiable after peak deconvolution. In addition, inorganic constituents were detected by XPS
unlike in the case of NMR spectroscopy.
Keywords: X-ray photoelectron spectroscopy, Nuclear magnetic resonance, Room-temperature ionic liquid
Introduction
XPS technique is limited to analyzing solid samples because
it is performed under ultrahigh vacuum (UHV) conditions.
However, the constraint for using XPS to analyze RTILs
can be overcome because of their extremely low vapor
pressure.
Room-temperature ionic liquids (RTILs) are remarkable
materials due to their unique characteristics including
extremely low vapor pressure at room temperature, high ionic
mobility, high electrical conductivity, low flammability, low
melting point, and superior chemical and thermal
stabilities. These properties result from the irregular config-
uration between organic cations, which generally contain
nitrogen or phosphorous atoms, and large organic or inorganic
In this study, XPS was used to identify the chemical struc-
ture of RTILs. Commercially available RTILs (1-butyl-3-
1–3
methylimidazolium tetrafluoroborate ([BMIM] BF ), 1-
4
butyl-3-methylimidazolium hexafluorophosphate ([BMIM]
PF ), 1-butyl-3-methylimidazolium trifluoromethanesulfo-
6
4
anions. Because of these properties, RTILs are useful as sol-
nate ([BMIM] OTf ), 1-hexyl-3-methyl imidazolium hexa-
5
6
vents in organic reactions and synthesis of metallic particles,
fluorophosphate ([HMIM] PF ), and 1-ethyl-3-methyl
6
7
8,9
as catalysts in dye-sensitized solar cells, in tribology, and
as electrolytes in batteries or double-layer capacitors.
imidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]
10
NTf ) were qualitatively and semiquantitatively investigated
2
The physicochemical properties of RTILs are dependent on
the chemical structures of the cation and anion of RTILs.
Many research groups have investigated the effect of different
cations with same anions or various anions with same
by XPS. In order to confirm whether the results of XPS were
correct, nuclear magnetic resonance (NMR) spectroscopy
was performed on the commercially available RTILs. After
the XPS results were confirmed by NMR spectroscopy,
we synthesized the RTILs 1-butyl-3-(4-sulfobutyl)-3-
imidazolium trifluoromethanesulfonate ([BSBIM] OTf ), 1-
methyl-3-(3-sulfopropyl)-3-imidazolium trifluoromethane-
sulfonate ([MSPIM] OTf ), and 1-butyl-3-(4-sulfopropyl)-3-
imidazolium trifluoromethanesulfonate ([BSPIM] OTf ) and
analyzed them with both XPS and NMR spectroscopy.
We used XPS and NMR spectroscopy to analyze commer-
cially available and synthesized RTILs in this work. NMR
spectroscopy is a useful tool to analyze organic compounds
in the conventional sense. In order to identify the chemical
environment of the elements in an RTIL by NMR spectros-
1
1–13
cations on the properties of RTILs.
To verify the proper-
ties of RTILs, many techniques such as liquid
4
9
chromatography, grazing incidence X-ray diffraction, soft
1
4
X-ray emission spectroscopy, high-resolution electron
1
5
energy-loss spectroscopy (HREELS), low-energy ion scat-
1
6,17
tering (LEIS),
(
high-resolution Rutherford backscattering1
13
HRBS), metastable impact electron spectroscopy (MIES),
ultraviolet photoelectron spectroscopy (UPS),
photoelectron spectroscopy (XPS)
1
,18
and X-ray
have been utilized.
1
,18,19
Among them, photoelectron emission spectroscopy (XPS or
UPS) would be a useful technique to confirm the qualitative
and semiquantitative properties of solid materials in general.
1
13
copy, H-NMR and C-NMR analyses should be performed.
However, XPS is a useful technique to analyze both organic
and inorganic compounds. In addition, NMR spectroscopy
†
These authors contributed equally to this work.
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has to be carried out several times depending on the kinds of
constituents of the organic compound, while XPS needs to be
performed only once to analyze all constituents in the organic
and/or inorganic RTILs.
J = 9.1, 6.8 Hz, 2H), 1.62–1.49 (m, 2H), 1.30–1.19 (m,
2H), 0.89 (t, J = 7.4 Hz, 3H).
13
C NMR (101 MHz, DMSO-d )δ137.38, 124.00, 123.84,
6
120.42 (q, JCF = 324.4 Hz), 51.74, 50.00, 49.87, 32.64,
9.72, 22.73, 20.14, 14.57.
2
Experimental
[MSPIM] OTf. 1-Methylimidazole (0.97 mL, 12.2 mmol)
and 1,3-propanesultone (1.07 mL, 12.2 mmol) were dis-
ꢀ
General Methods. Air- and/or moisture-sensitive reactions
were carried out under an argon atmosphere in oven-dried
glassware and with anhydrous solvents. All compounds were
purchased from commercial sources unless otherwise noted
and used without further purification. Solvents were freshly
distilled (1,4-dioxane and toluene over sodium) or dried by
passing through an alumina column. Thin-layer chromatogra-
phy was carried out on glass plates coated with silica gel SiO2
solved in toluene (3.0 mL), and stirred at 60 C for 24 h.
ꢀ
The mixture was cooled to 25 C. The precipitate was washed
with ether and toluene to remove non-ionic residues, and dried
in vacuo. 3-(1-Methyl-1H-imidazol-3-ium-3-yl)propane-1-
sulfonate zwitterion was obtained as a white solid
(2.41 g, 96.8%).
Triflic acid (1.04 mL, 11.8 mmol) was added to 3-(1-
methyl-1H-imidazol-3-ium-3-yl)propane-1-sulfonate zwit-
terion (2.41 g, 11.86 mmol) at 25 C. The reaction mixture
was stirred at 60 C for 18 h. After cooling to 25 C, Et O
(2 mL) was added to the reaction mixture. The ionic liquid–
Et O mixture was stirred for 2 min, whereupon mixture clearly
ꢀ
60 F254 (Merck: St. Louis, MO, USA), with visualization
ꢀ
ꢀ
using a UV lamp (254 nm) or by staining with a p-
anisaldehyde or potassium permanganate solution. Flash
chromatography was performed with silica gel SiO2
2
2
6
0 (0.040–0.063 μm, 230–400 mesh), technical solvents,
showed two separate layers. The upper Et O layer was care-
2
1
and a head pressure of 0.2–0.4 bar. Proton ( H) and carbon
fully removed by decantation. This procedure was repeated
five times to remove non-ionic residues. The residue was dried
in vacuo to afford title compound as a colorless oil (4.12 g,
1
3
(
C) NMR spectroscopy were performed (JEOL ECP-400
1
spectrometer: Tokyo, Japan) at 400 MHz ( H) and
13
22,23
100 MHz ( C) at 294 K. Chemical shifts are reported in
98.6%). Spectral data matched those in the literature.
1
ppm relative to the residual protiated solvent (CDCl : δH =
7
H NMR (400 MHz, D O) δ 8.64 (s, 1H), 7.42
3
2
1
3
.26 ppm, δC = 77.16 ppm). All C NMR spectra are
(t, J = 1.8 Hz, 1H), 7.35 (q, J = 1.6 Hz, 1H), 4.26
(t, J = 7.2 Hz, 2H), 3.80 (s, 3H), 2.82 (t, 7.2 Hz, 2H),
2.28–2.15 (m, 2H).
proton-decoupled. The resonance multiplicity is described
as s (singlet), d (doublet), t (triplet), q (quartet), p (pentet),
dd (doublet of doublet), dt (doublet of triplet), td (triplet of
doublet), m (multiplet), and br (broad). The notation and
chemical structure of RTILs studied in this work are listed
in Table 1.
13
C NMR (101 MHz, D O) δ 137.08, 124.86, 123.24,
2
120.76 (q, JCF = 319.7 Hz), 48.83, 48.35, 36.77, 26.11.
[BSPIM] OTf. 1-Butylimidazole (1.06 mL, 8.05 mmol) and
1,3-propanesultone (0.702 mL, 8.05 mmol) were dissolved
ꢀ
Synthesis of RTILs
in toluene (2.0 mL), and stirred at 60 C for 24 h. The mixture
ꢀ
[BSBIM] OTf. 1-(4-Sulfonic acid) butyl-3-butyl-3-
was cooled to 25 C. The precipitate was washed with ether
butylimidazolium was synthesized according to the literature
procedure. 1-Butylimidazole (1.06 mL, 8.05 mmol) and
and toluene to remove non-ionic residues, and dried in vacuo.
3-(1-Butyl-1H-imidazol-3-ium-3-yl)propane-1-sulfonate
zwitterion was obtained as a white solid (1.95 g, 98.5%).
Triflic acid (0.699 mL, 7.9 mmol) was added to 3-(1-butyl-
20
1
,4-butanesultone (0.824 mL, 8.05 mmol) were dissolved in
ꢀ
toluene (2.0 mL), and stirred at 60 C for 24 h. The mixture
was cooled to at 25 C. The precipitate was washed with ether
ꢀ
1H-imidazol-3-ium-3-yl)propane-1-sulfonate
zwitterion
ꢀ
and toluene to remove non-ionic residues, and dried in vacuo.
(1.95 g, 7.9 mmol) at 25 C. The reaction mixture was stirred
ꢀ
ꢀ
1-Butyl-3-(butyl-4-sulfonate)imidazolium zwitterion was
at 60 C for 18 h. After cooling 25 C , Et O (2 mL) was
2
obtained as a white solid (2.09 g, 99.7%).
added to the reaction mixture. The ionic liquid–Et O mixture
2
Triflicacid(0.710 mL, 8.03 mmol) wasaddedto1-butyl-3-
was stirred for 2 min, whereupon the mixture clearly showed
(
8
6
butyl-4-sulfonate)imidazolium
zwitterions
(2.09 g,
two separate layers. The upper Et O layer was carefully
2
ꢀ
.03 mmol) at 25 C. The reaction mixture was stirred at
removed by decantation. This procedure was repeated five
times to remove non-ionic residues. The residue was dried
in vacuo to afford title compound as a colorless oil (3.03 g,
ꢀ
ꢀ
0 C for 18 h. After cooling to 25 C, Et O (2 mL) was
2
added to the reaction mixture. The ionic liquid–Et O mixture
was stirred for 2 min, whereupon mixture clearly showed two
separate layers. The upper Et O layer was carefully removed
2
2
4
96.7%). Spectral data matched those in the literature.
1
H NMR (400 MHz, D O) δ 8.79 (s, 1H), 7.51 (t, J = .9,
2
2
by decantation. This procedure was repeated five times to
remove non-ionic residues. The residue was dried in vacuo
to afford the title compound as a colorless oil (3.10 g,
1H), 7.49 (t, J = 1.8 Hz, 1H), 4.34 (t, J = 7.1 Hz, 2H), 4.18
(t, J = 7.2 Hz, 2H), 2.89 (t, J = 7.4 Hz, 2H), 2.30 (q,
J = 7.2 Hz, 2H), 1.82 (q, J = 7.4 Hz, 2H), 1.36–1.21 (m,
2
0,21
9
4%). Spectral data matched those in the literature.
2H), 0.89 (t, J = 7.4 Hz, 3H).
1
13
H NMR (400 MHz, DMSO-d ) δ 9.21 (t, J = 1.7 Hz, 1H),
C NMR (101 MHz, D O) δ 136.23, 123.55, 123.23,
6
2
7.79 (q, J = 1.7 Hz, 2H), 4.17 (dt, J = 8.7, 7.1 Hz, 4H), 2.58
120.59 (q, JCF = 319.7 Hz), 50.34, 48.71, 48.19, 32.03,
(td, J = 7.4, 1.1 Hz, 2H), 1.89 (p, J = 7.3 Hz, 2H), 1.77 (tt,
26.00, 19.64, 13.49.
Bull. Korean Chem. Soc. 2016, Vol. 37, 355–360
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Chemical Analysis of RTILs
KOREAN CHEMICAL SOCIETY
Table 1. Commercial and synthesized RTILs and their chemical structures.
Ionic Liquids
Full name
Structure
[
[
[
HMIM] PF
6
1-Hexyl-3-methyl imidazolium hexafluorophosphate
EMIM] NTf
2
1-Ethyl-3-methyl imidazolium bis((trifluoromethyl)sulfonyl))imide
1- Butyl-3-(4-sulfobutyl) imidazolium trifluoromethanesulfonate
BSBIM] OTf
[
[
[
[
BSPIM] OTf
MSPIM] OTf
BMIM] OTf
1-Butyl-3-(3-sulfopropyl) imidazolium trifluoromethanesulfonate
1-Butyl-3-(4-sulfopropyl) imidazolium trifluoromethanesulfonate
1-Butyl-3-methyl imidazolium trifluoromethanesulfonate
1-Butyl-3-methyl imidazolium hexafluorophosphate
BMIM] PF
6
[BMIM] BF4
1-Butyl-3-methyl imidazolium tetrafluoroborate
X-ray Photoelectron Spectroscopy. The commercially avail-
concave to prevent the spilling of RTILs in the analysis cham-
ber. RTILs in the sample holder were pre-pumpedovernight in
the load-lock chamber for to evacuate the dissolved gas in the
RTILs. RTILs in the preparation chamber were transferred to
the analysis chamber by two transfer rods and two wobble
sticks. The working pressure of the analysis chamber was
able RTILs ([HMIM] PF , [EMIM] NTf , [BMIM] BF ,
6
2
4
[
BMIM] PF , and [BMIM] OTf ) were analyzed by an XPS
6
(VG ESCALab 2000: St Leonards-on-Sea, East Sussex,
UK) system using the Mg Kα source (1253.6 eV) with a high
voltage of 15 kV and a beam current of 15 mA. The survey
and high-resolution XPS spectra were obtained from a pass
energy of 50 and 20 eV and a scan step of 1 and 0.05 eV,
respectively. The dwell time for both spectra was 50 ms.
The synthesized RTILs, namely [BSBIM] OTf, [MSPIM]
OTf, and [BSPIM] OTf, were investigated by a modified
XPS (based on the VG ESCALab MKII) system, which con-
sisted of four components: a load-lock chamber, a preparation
chamber equipped with two multi-sample carousels, an anal-
ysis chamber, and a sputter chamber for deposition of thin
films in the UHV environment. The base pressure of the cus-
tomized electron emission chamber was maintained at below
−
6
maintained below 1 × 10
Pa. An Al Kα source
(1486.6 eV) with an anode voltage of 7.5 kV and a beam cur-
rent of 20 mA was used in this work. The survey XPS spectra
were obtained with a pass energy of 100 eV, a dwell time of
100 ms, and a step size of 0.5 eV. The high-resolution XPS
spectra were collected with a pass energy of 50 eV, a dwell
time of 100 ms, and a step size of 0.02 eV. The peak-fitting
of the XPS spectra was achieved by the XPSPEAK program
(version 4.1). The background type applied was Shirley mode,
and the Gaussian/Lorentzian ratio was 30:70. The charge
accumulation effect was corrected on the basis of the aliphatic
carbon at the binding energy of 284.6 eV.
−7
~
1.0 × 10 Pa using two rotary vane pumps, two turbo
molecular pumps, three ion getter pumps, and two Ti-
sublimation pumps. The analysis chamber was equipped with
a dual X-ray anode for XPS, a UV lamp for UPS, an electron
gun for Auger electron spectroscopy (AES), an Ar ion sputter
gun for cleaning the sample and depth profiling, a concentric
hemispherical analyzer (CHA), and a channeltron for the
detection of the photoemitted electrons. To investigate the
chemical property of RTILs using XPS, many researchers
Results and Discussion
RTILs investigated in this work consist of organic cations
(imidazolium) and various inorganic anions. In ususal circum-
stances, organic compounds are investigated by NMR spec-
troscopy. Therefore, the chemical information of cations is
successfully obtained, while information on anions remains
unknown because of the inorganic character of the anions.
To overcome this limitation, XPS analysis was adapted in this
work. The feasibility of using XPS for RTIL analysis was
tested on commercially available RTILs. The representative
1
,19,25,26
have tried to form RTIL films.
This method, however,
is cumbersome for analyzing RTILs. To overcome this prob-
lem, we designed a customized sample holder made of stain-
less steel for RTILs. The center region of the sample holder is
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survey XPS spectra and deconvoluted C 1s spectra of [HMIM]
information. The peak-fitting of C 1s was achieved with six
PF are shown in Figure 1(a) and (b), respectively. In Figure 1
singlet peaks, as shown in Table 3. The C 1s peak with the low-
est binding energy (284.6 eV) is assigned to aliphatic sp car-
6
3
(
[
a), all characteristic XPS peaks of the constituents of the
HMIM] PF were detected. XPS peaks of P 2p, C 1s, N 1s,
bon (denoted as C1 in the molecular structure of [BSPIM]
6
3
and F 1s were observed at the binding energies of 150, 285,
99, and 650 eV, respectively. From the survey spectra of
HMIM] PF , the constituent elements in the RTIL were suc-
OTf ). The sp carbon bonded with sulfonyl group is denoted
3
as C2 at 285.2 eV. The carbon (C3) located at the alkyl chain
andbonded with nitrogen is positioned at285.7 eV. In theimi-
dazolium ring, two carbons with different chemical environ-
ments exist. The C4 and C5 peaks are positioned at 286.4
and 287.4 eV, respectively. The C6 is used for denoting the
carbon in triflate ions (OTf: CF SO or NTf : N(SO CF ) ).
[
6
cessfully identified. The purity of commercial RTILs was also
corroborated by NMR spectroscopy. From the XPS results, all
atoms consisting of [HMIM] PF were detected by survey
6
XPS spectra, and the atomic percentages of the RTILs are
listed in Table 2. The atomic percentages were calculated by
3
3
2
2
3 2
Four carbon species were assigned after deconvolution of
27
considering the peak areas and atomic sensitivity factors.
the C 1s XPS spectra of [HMIM] PF as shown in Figure 1
6
It shows that the atomic percentages obtained by XPS match
(b). The atomic ratio of C1:C3:C4:C5 in [HMIM] PF is
6
well with the theoretical values of [HMIM] PF . This result
5:2:2:1,
the
obtained
experimental
value
was
6
confirmed the reliability of XPS in the investigation of RTILs.
Furthermore, the information about the chemical environment
of specific atoms makes XPS a powerful investigating tool for
RTILs. In this study, high-resolution C 1s spectra were taken,
and deconvolution process was performed to obtain detailed
5.0:1.6:2.0:1.2. It revealed that the qualification and semi-
quantitative analysis of RTILs by means of XPS is feasible.
Figure 2 shows the survey XPS spectra of all RTILs ana-
lyzed in this work. The characteristic XPS peaks of fluorine
centered at 685 eV, oxygen at 531 eV, nitrogen at 400 eV,
Table 2. Atomic percentage in various RTILs. The numbers in
parenthesis are the theoretical values.
Ionic liquids
C
N
F
[
[
[
[
[
[
[
[
HMIM] PF6
EMIM] NTf
56.1 (56)
48.3 (47)
68.1 (71)
64.4 (69)
65.9 (62)
69.1 (64)
56.7 (50)
62.5 (57)
10.2 (11)
16.1 (18)
12.0 (12)
12.7 (13)
11.5 (15)
11.8 (14)
9.2 (13)
33.7 (33)
35.6 (35)
19.9 (18)
22.9 (19)
22.6 (23)
19.1 (21)
34.1 (38)
27.2 (29)
2
BSBIM] OTf
BSPIM] OTf
MSPIM] OTf
BMIM] OTf
BMIM] PF6
Figure 1. (a) Survey XPS spectra and (b) the deconvoluted C 1s spec-
tra of [HMIM] PF .
6
BMIM] BF
4
10.3 (14)
Table 3. Ratio of carbon species in different chemical environments obtained after peak deconvolution of C 1s spectra. The numbers without
parentheses indicate the experimental values, and the numbers in parenthesis mean the theoretical value of RTILs
.
Carbon species
C1
284.6
C2
285.2
1.5 Æ 0.1
C3
285.7
C4
286.4
C5
287.4
C6
292.2
1.7 Æ 0.3
Binding energy (eV)
FWHM (eV)
1.6 Æ 0.3
5.0 (5)
1.4 (1)
5.0 (5)
3.6 (4)
1.0 (1)
3.0 (3)
3.2 (3)
3.2 (3)
1.5 Æ 0.2
1.6 (2)
2.2 (2)
2.2 (2)
1.6 (2)
1.8 (2)
2.0 (2)
2.0 (2)
2.0 (2)
1.5 Æ 0.1
2.0 (2)
2.0 (2)
2.0 (2)
2.0 (2)
2.0 (2)
2.0 (2)
2.0 (2)
2.0 (2)
1.9 Æ 0.2
1.2 (1)
1.0 (1)
1.2 (1)
1.0 (1)
1.0 (1)
1.0 (1)
1.2 (1)
1.0 (1)
[
[
[
[
[
[
[
[
HMIM] PF
EMIM] NTf
BSBIM] OTf
BSPIM] OTf
MSPIM] OTf
BMIM] OTf
6
2
1.6 (2)
0.8 (1)
0.8 (1)
0.8 (1)
0.6 (1)
1.0 (1)
0.8 (1)
1.2 (1)
BMIM] PF
BMIM] BF
6
4
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Figure 2. Survey XPS spectra of RTILs. The star indicates the F KLL
Auger lines.
carbon at285 eV, sulfur at231and 167 eV, and phosphorus at
191 eV were detected. It shows that the analysis of RTILs by
XPS is simple without any sample preparation processes, such
as the formation of RTIL thin films.
Figure 3(a) shows the representative deconvoluted high-
Figure 3. Deconvoluted high-resolution XPSspectra of C 1s(a), N 1s
(
b), and F 1s (c).
resolution the C 1s XPS spectra of [BMIM] BF and [BMIM]
4
PF as well as [EMIM] NTf for commercial RTILs and
energy than that in [BSPIM] OTf. The ratio of carbon species
obtained by peak deconvolution is listed in Table 3. The exper-
imental ratios agree reasonably well with the theoretical values
not only for commercial also for synthesized RTILs.
Figure 3(b) shows the high-resolution N 1s XPS spectra
after the deconvolution process. Two nitrogen peaks are fitted
at 401.5 and 399.3 eV, which correspond to the nitrogen in
6
2
[BSPIM] OTf for synthesized RTILs. The peak profiles of
C1sin[BMIM]BF and[BMIM] PF aresimilar. Thisimplies
4
6
that the cation effect on the chemical environment of the anion
in RTILs is weak. In order to get detailed chemical informa-
tion, peak deconvolution of C 1s was performed. The peak-
fitting for two RTILs containing [BMIM] anion was per-
formed with four singlet peaks (C1, C3, C4, and C5), like
imidazolium cations (Ncation) and the NTf anion (Nanion),
2
for [HMIM] PF . The result of peak deconvolution shows that
respectively. Nitrogen in cations is positioned at the imidazo-
lium ring in all cations, and singlet spectra are detected in
[BMIM] BF , [BMIM] PF , and [BSPIM] OTf; however, a
6
[
BMIM] BF and [BMIM] PF have similar parameters, such
4 6
as a full width at half maximum of each peak and the ratio of
peak area. In [BSPIM] OTf, two more peaks (C2 and C6 as
shown in Table 3) are added in C 1s spectrum for peak decon-
volution. Comparing the carbon species in [BSPIM] OTf with
those in [BMIM] BF and [BMIM] PF , the carbon bonded
4
6
shoulder appears in the case of [EMIM] NTf . A peak shift
2
to the lower binding energy region in [BSPIM] OTf is
observed, which is caused by the charge distribution of imida-
12
zolium ring to the long alkyl chain. The theoretical and
experimental atomicratio of Ninimidazoliumcations toanion
4
6
with sulfonic acid in cation (C2) and carbon in triflate anions
C6) were detected (285.2 and 292.2 eV) from [BSPIM] OTf.
(
N in [EMIM] NTf is 67:33 and 71:29, respectively. The
2
And the other peaks (C1, C3, C4, and C5) are not significantly
experimental value agrees with the theoretical value within
the range of experimental error. The deconvoluted high-
resolution XPS spectra of F 1s are shown in Figure 3(c). All
RTILs contain fluorine atoms, and in this study, the F 1s peaks
for four RTILs appeared at different binding energies. This is
due to the different anions. As the anion basicity is increased,
the binding energy of fluorine increases from 685.6 to
688.8 eV. Therefore, we can infer that the basicity of the anion
increases in the order BF , PF , OTf, and the NTf anion. This
different from those in [BMIM] BF and [BMIM] PF . The
4
6
carbons in the cation of [EMIM] NTf have a similar chemical
2
environment as those in [BMIM] cation. However, the peak
position of C3, C4, and C5 are shifted to a higher binding
energy region. This phenomenon is caused by the anion effect.
The basicity of the anion in [EMIM] NTf is higher than that of
2
other anions. Consequently, electrons in the cation can be
dragged to the anion, NTf . Therefore, the binding energy of
2
4
6
2
carbons in anion in [EMIM] NTf has a higher value than those
result is in agreement with the observation of Santos et al. in
2
12
in[BMIM]BF and[BMIM]PF . Thesizeof[EMIM]cationis
guanidinium ILs. The atomic percentages of atoms in
RTILs, carbon, nitrogen, and fluorine are listed in Table 2.
The achieved values match well with the theoretical values
for all RTILs. From this study, the application of XPS to the
analysis of RTILs was confirmed using commercial RTILs,
4
6
smaller than that of [BSPIM] cation which has longer alkyl
chains compared to the [EMIM] cation. Thus, the anion effect
more strongly influences [EMIM] NTf than [BSPIM] OTf.
2
The carbon (C6) in [EMIM] NTf has also a higher binding
2
Bull. Korean Chem. Soc. 2016, Vol. 37, 355–360
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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BULLETIN OF THE
Article
Chemical Analysis of RTILs
KOREAN CHEMICAL SOCIETY
and then the synthesized RTILs were evaluated by XPS and
NMR techniques.
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1
1
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We focused on the performance of XPS in terms of the iden-
tification of the RTILs. Two types of RTILs were investigated:
commercially available and synthesized ones. The commer-
cial RTILs ([HMIM] PF , [EMIM] NTf , [BMIM] OTf,
6
2
[
BMIM] PF , and [BMIM] BF ) were analyzed by XPS,
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6
4
and the results were cross-checked with NMR techniques.
All constituent atoms in the RTILS were detected in the survey
XPS spectra. Thisisan advantage compared toNMR spectros-
copy. After deconvolution of the C 1s peak, we could identify
the atomic ratio of carbon with different chemical environ-
ments in the RTILs. The synthesized RTILs ([BSBIM] OTf,
1
1
4. K. Kanai, T. Nishi, T. Iwahashi, Y. Ouchi, K. Seki,
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2
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[
BSPIM] OTf, and [MSPIM] OTf ) were analyzed by XPS,
and the experimentally obtained atomic ratios concurred with
the theoretical values. XPS was therefore proved to be a useful
technique to identify the chemical environment of organic
cations and inorganic anions in RTILs as well.
1
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1
Acknowledgments. This research was supported by the
Basic Science Research Program through the National
Research of Korea (NRF) funded by the Ministry of
Education (2010–0021332), and by the Basic Science
Research Program through the National Research Foundation
of Korea (NRF) funded by the Ministry of Education (NRF-
1
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Bull. Korean Chem. Soc. 2016, Vol. 37, 355–360
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
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