Synthesis and properties of seven ionic liquids containing 1-methyl-3-octylimidazolium or 1-butyl-4-methylpyridinium cations

Article abstract of DOI:10.1021/je060096y

Syntheses are reported for ionic liquids containing 1-methyl-3- octylimidazolium or 1-butyl-4-methylpyridinium cations and trifluoromethyl sulfonate, dicyanamide, bis(trifluoromethylsulfonyl)imide, or nonafluorobutyl sulfonate anions. Densities, melting points, glass-transition temperatures, solubilities in water, and polarities have been measured. Ionic liquids containing pyridinium cations exhibit higher melting points, lower solubility in water, and higher polarity than those containing imidazolium cations.

Full text of DOI:10.1021/je060096y

J. Chem. Eng. Data 2006, 51, 1389-1393
1389
Synthesis and Properties of Seven Ionic Liquids Containing
1-Methyl-3-octylimidazolium or 1-Butyl-4-methylpyridinium Cations
Nicolas Papaiconomou,†,‡ Neal Yakelis,§ Justin Salminen,†,| Robert Bergman,‡,§ and John M. Prausnitz*,†,‡
Department of Chemical Engineering, 201 Gilman Hall, University of California, Berkeley, California 94720-1462,
Department of Chemistry, Room 419 Latimer Hall, University of California, Berkeley, California 94720-1460, Chemical
Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, and Environmental
Energy Technology Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720
Syntheses are reported for ionic liquids containing 1-methyl-3-octylimidazolium or 1-butyl-4-methylpyridinium
cations and trifluoromethyl sulfonate, dicyanamide, bis(trifluoromethylsulfonyl)imide, or nonafluorobutyl sulfonate
anions. Densities, melting points, glass-transition temperatures, solubilities in water, and polarities have been
measured. Ionic liquids containing pyridinium cations exhibit higher melting points, lower solubility in water,
and higher polarity than those containing imidazolium cations.
[MBPYR]+ cations. Figure 1 shows their structures. The
[OMIM]+ cation was chosen because ionic liquids containing
this cation exhibit low melting points.18-24
The hydrophobicity of an ionic liquid increases with the
length of the alkyl chain on the imidazolium ring, as shown
elsewhere.24,25 Regrettably, this increase tends to raise viscos-
ity.25
Introduction
Room-temperature ionic liquids (RTILs) provide a new class
of solvents where molecules are composed of ions. At normal
temperatures, ionic liquids have essentially zero vapor pressure
and are thermally stable over a wide range of temperature.
Therefore, they are considered as environmentally friendly
alternatives to classical organic solvents.
Four anions were studied: trifluoromethyl sulfonate (TfO),
nonafluorobutyl sulfonate (NfO), bis(trifluoromethylsulfonyl)-
imide (Tf2N), and dicyanamide (N(CN)2). TfO and NfO anions
were chosen because they are fluorinated anions with high
hydrophobicity. Several authors16,25-27 have reported first that
ionic liquids containing 1-methyl-3-butylimidazolium cation and
NfO anion, such as [BMIM]+[NfO]-, are immiscible with water
and, second, that ionic liquids containing TfO anions are
miscible with water but less viscous than ionic liquids containing
NfO anions.
RTILs are also tunable solvents as some of their properties
(e.g. melting points, immiscibility in other solvents, or polarity)
can be changed by modifying the structure of the ionic liquid’s
cation or anion.
During the past decade, numerous publications have discussed
properties and applications of RTILs.1,2 Ionic liquids can serve
as solvents in organic chemistry,3-5 in electrochemistry,6,7 as
catalysts8-12 or as extracting agents for organic or ionic
solutes.13-17 Four ionic liquids synthesized here contain pyri-
dinium cations. These ionic liquids are used as catalysts or
solvents in organic chemistry.3,9
For extraction of heavy-metal ions from an aqueous phase,
ionic liquids need to meet specific criteria: low water solubility,
low viscosity, and high affinity for the ion to be extracted.
Because these properties pertain to only a limited number of
known ionic liquids, it is desirable to synthesize promising new
ionic liquids and to measure their physicochemical properties.
To enhance fundamental understanding concerning ionic
liquids potentially suitable for extraction of heavy-metal ions,
this work reports the synthesis of seven ionic liquids and some
of their physicochemical properties. Three of the seven ionic
liquids studied here have also been synthesized previously:
1-methyl-3-octylimidazolium trifluoromethyl sulfonate,18 1-methyl-
3-octylimidazolium bis(trifluoromethylsulfonyl)imide,15,19-21 and
1-butyl-4-methylpyridinium bis(trifluoromethylsulfonyl)imide.22
All Tf2N-based ionic liquids studied previously16,25-27 are
immiscible with water. Moreover, they exhibit lower viscosities
-
than water-immiscible ionic liquids containing PF6 anions or
BF4- anions and long alkyl chains, such as [OMIM]+[BF4]-.28
Ionic liquids with PF6- may not be useful for extraction of ions
-
from water because PF6 slowly reacts with water.29
Previous work reports that ionic liquids containing dicyana-
mide anion have low viscosity.30 However, they are water
soluble.
As discussed below, a two-step metathesis procedure was
used to synthesize seven new ionic liquids.
[OMIM]+[Tf2N]-. In a flask containing 150 mL of acetoni-
trile, 6.27 g (26.7 mmol) of [OMIM]+[Cl]- was mixed with
9.02 g (28.3 mmol, 6 % excess) of potassium bis(trifluoro-
methylsulfonyl)imide, used as received from Wako Chemicals
(98 % purity).
Synthesis of Ionic Liquids
After being stirred for 4 h, the solution was filtered to remove
precipitated KCl. The solvent was then removed by rotary
evaporation. Traces of the remaining acetonitrile were removed
by placing the yellow-white product under high vacuum for 30
min. The product was washed with methylene chloride and set
in a freezer for 4 h at -18 °C. The solution was filtered to
remove the remaining precipitated potassium Tf2N. Methylene
We have synthesized seven ionic liquids containing 1-methyl-
3-octylimidazolium [OMIM]+ or 1-butyl-4-methylpyridinium
* Corresponding author. E-mail: prausnit@cchem.berkeley.edu.
† Department of Chemical Engineering.
‡ Chemical Sciences Division.
§ Department of Chemistry.
| Environmental Energy Technology Division.
10.1021/je060096y CCC: $33.50 © 2006 American Chemical Society
Published on Web 06/27/2006

1390 Journal of Chemical and Engineering Data, Vol. 51, No. 4, 2006
Figure 1. Structures of the cations and anions used in this work.
chloride was removed by rotary evaporation, and the product
was set under high vacuum for 30 min. The resulting ionic solid
was colorless. The mass of [OMIM]+[Tf2N]- was 10.75 g,
giving a yield of 87 %.
1H NMR (400 MHz, CDCl3, δ): 9.157 (1H, s, N-CH-N),
7.330 (1H, s, N-CH), 7.266 (1H, s, CH-N), 4.173-4.136 (2H,
t, CH2), 3.971-3.954 (3H, d, CH3), 1.836 (2H, s, CH2), 1.281-
1.218 (10H, m, C5H10), 0.856-0.822 (3H, t, CH3). Anal.
Calcd: C, 35.33; H, 5.58; N, 8.83. Found: C, 35.28; H, 5.08;
N, 8.98.
[OMIM]+[TfO]-. 5.91 g (25.2 mmol) of [OMIM]+[Cl]- was
mixed in 150 mL of acetonitrile with 6.22 g (30.24 mmol) of
potassium trifluoromethyl sulfonate, used as received from
Sigma (98 % purity). After filtering, removing acetonitrile by
rotary evaporation, and washing with methylene chloride, 6.86
g (20.0 mmol) of [OMIM]+[TfO]- was obtained as a yellow
liquid. The yield was 79 %.
1H NMR (400 MHz, CDCl3, δ): 9.157 (1H, s, N-CH-N),
7.330 (1H, s, N-CH), 7.266 (1H, s, CH-N), 4.173-4.136 (2H,
t, CH2), 3.971-3.954 (3H, d, CH3), 1.836 (2H, s, CH2), 1.281-
1.218 (10H, m, C5H10), 0.856-0.822 (3H, t, CH3). 19F NMR
(300 MHz, CDCl3, δ): -77.86 (3F, s, CF3). Anal. Calcd: C,
45.30; H, 6.70; N, 8.10. Found: C, 44.80; H, 6.72; N, 8.12.
[OMIM]+[NfO]-. 2.33 g (10.1 mmol) of [OMIm+] [Cl-],
used as received from Solvent Innovation (98 % purity), and
4.27 g (12.6 mmol, 25 % excess) of potassium nonafluorobutyl
sulfonate, used as received from Sigma (98 % purity), were
mixed in a flask containing 150 mL of acetonitrile. After
filtering, removing acetonitrile by rotary evaporation, and
washing with methylene chloride, 4.56 g of [OMIM]+[NfO]-
was obtained as a light-orange liquid. The yield was 91 %.
1H NMR (400 MHz, CDCl3, δ): 9.157 (1H, s, N-CH-N),
7.330 (1H, s, N-CH), 7.266 (1H, s, CH-N), 4.173-4.136 (2H,
t, CH2), 3.971-3.954 (3H, d, CH3), 1.836 (2H, s, CH2), 1.281-
1.218 (10H, m, C5H10), 0.856-0.822 (3H, t, CH3). Anal.
Calcd: C, 38.87; H, 4.70; N, 5.66. Found: C, 38.66; H, 4.84;
N, 5.64.
(2H, t, CH2), 1.300-1.216 (10H, m, C5H10), 0.840-0.806 (3H,
t, CH3). 13C NMR (300 MHz, CDCl3, δ): 136.51, 123.95,
122.50, 120.04, 50.53, 36.75, 31.76, 30.22, 29.09, 28.98, 26.34,
22.69, 14.19. Anal. Calcd: C, 64.33; H, 8.81; N, 26.80.
Found: C, 61.108; H, 8.48; N, 25.50.
[MBPYR]+[Tf2N]-. 4.36 g (18.9 mmol) of 1-butyl-4-meth-
ylpyridinium bromide, used as received from Solvent Innovation
(98 % purity), was mixed with 7.56 g (24.0 mmol, 25 % excess)
of potassium Tf2N in a flask containing 150 mL of acetonitrile.
After filtering, removing acetonitrile by rotary evaporation, and
washing with methylene chloride, 7.53 g (17.5 mmol) of
[MBPYR]+[Tf2N]- was obtained as a light-yellow liquid. The
yield was 93 %.
1H NMR (400 MHz, CDCl3, δ): 9.299-9.283 (2H, d, pyr-
CHm), 7.844-7.829 (2H, d, pyr-CHo), 4.933-4.896 (2H, t,
CH2), 2.647 (3H, s, CH3), 2.012-1.936 (2H, m, CH2), 1.434-
1.340 (2H, m, CH2), 0.956-0.919 (3H, t, CH3). Anal. Calcd:
C, 33.46; H, 3.70; N, 6.50. Found: C, 33.13; H, 3.53; N, 6.46.
[MBPYR]+[TfO]-. 2.34 g (10.0 mmol) of 1-butyl-4-meth-
ylpyridinium bromide, from Solvent Innovation, washed with
ethyl acetate, was mixed with 2.30 g (12.2 mmol, 20 % excess)
of potassium trifluoromethyl sulfonate in a flask containing 100
mL of acetonitrile. After filtering, removing acetonitrile by
rotary evaporation, and washing with methylene chloride, 2.70
g (8.9 mmol) of [MBPYR]+[TfO]- was obtained as a light-
yellow liquid. This ionic liquid was further purified by mixing
100 mg of silver trifluoromethyl sulfonate with [MBPYR]+[TfO]-
in 15 mL of methylene chloride. After filtering and removing
the methylene chloride, a white-yellow solid was obtained. The
yield was 89 %.
1H NMR (400 MHz, CDCl3, δ): 8.846-8.831 (2H, d, pyr-
CHm), 7.787-7.771 (2H, d, pyr-CHo), 4.601-4.563 (2H, t,
CH2), 2.603 (3H, s, CH3), 1.939-1.883 (2H, m, CH2), 1.395-
1.301 (2H, m, CH2), 0.944-0.907 (3H, t, CH3). 19F NMR (300
MHz, CDCl3, δ): -77.74 (3F, s, CF3). Anal. Calcd: C, 43.30;
H, 5.39; N, 4.68. Found: C, 41.20; H, 5.11; N, 4.95.
[MBPYR]+[NfO]-. 5.16 g (22.5 mmol) of 4-methyl-N-
pyridinium bromide, from Solvent Innovation, washed with
acetyl acetate, was mixed with 10.0 g (30.0 mmol, 33 % excess)
of potassium nonafluorobutyl sulfonate in a flask containing
150 mL of acetonitrile. After filtering, removing acetonitrile
by rotary evaporation, and washing with methylene chloride,
9.02 g (20 mmol) of [MBPYR]+[NfO]- was obtained as a light-
yellow liquid. The yield was 90 %.
[OMIM]+[N(CN)2]-. 10.55 g (45.7 mmol) of [OMIM]+[Cl]-
was mixed in 150 mL of acetonitrile with 4.99 g (56.0 mmol,
23 % excess) of sodium dicyanamide, used as received from
Sigma (98 % purity). After filtering, removing acetonitrile by
rotary evaporation, and washing with methylene chloride, 9.82
g (37.5 mmol) of [OMIM]+[N(CN)2]- was obtained as a pale
yellow liquid. The yield was 82 %.
1H NMR (400 MHz, CDCl3, δ): 9.123 (1H, s, N-CH-N),
7.404-7.401 (1H, d, N-CH), 7.353-7.344 (1H, t, CH-N),
4.201-4.164 (2H, t, CH2), 3.989 (3H, s, CH3), 1.883-1.847
1H NMR (400 MHz, CDCl3, δ): 8.785-8.769 (2H, d, pyr-
CHm), 7.787-7.771 (2H, d, pyr-CHo), 4.601-4.563 (2H, t,
CH2), 2.627 (3H, s, CH3), 1.959-1.883 (2H, m, CH2), 1.387-

Journal of Chemical and Engineering Data, Vol. 51, No. 4, 2006 1391
Table 1. Densities G at 25 °C, Melting Points tm, Glass-Transition Temperatures tg, Decomposition Temperatures td, and Mass Fraction
Solubilities w in Water at 25 °C for Several Ionic Liquids
ionic liquid
F/g‚mL-1
tm/°C
tg/°C
td/°C
100 wb
[OMIM]+[Tf2N]-
[OMIM]+[TfO]-
[OMIM]+[NfO]-
[OMIM]+[N(CN)2]-
[MBPYR]+[Br]-
[MBPYR]+[Tf2N]-
[MBPYR]+[TfO]-
[MBPYR]+[NfO]-
1.31, 1.3219, 1.3615
a
5
-86
-80
-86
-89
-38
325
250
250
150
225
275
200
275
0.21
S
0.16
S
S
0.48
S
2.29
1.12
1.33
1.00
solid
1.35
1.17
solid
32
-49
78
10
38
70
-80, -8415
-68
32
[BMIM]+[Tf2N]-
[BMIM]+[TfO]-
[BMIM]+[NfO]-
[BMIM]+[N(CN)2]-
1.4325,34, 1.4431
1.3034,31,1.2925
1.4725
-4, -325,34, -425, -231
-91, -8734, -8631
330, 33031
34031, 40934
225
0.71, 0.8015
S
1.69
S
15, 1615, 1734, 1331
2025
nd
1.0631
-631
-9031
24031
a [OMIM]+[Tf2N]- has no melting point. b S indicates an ionic liquid miscible in water. nd indicates no data available.
1.293 (2H, m, CH2), 0.938-0.901 (3H, t, CH3). Anal. Calcd:
Thermal Stability. Thermogravimetric analysis (TGA) pro-
vides information concerning thermal stability. Perkin-Elmer
TGA 7 was used. The dried ionic liquid samples were stored
and sampled in a dry helium glovebox. The TGA sample holder
was under dry-nitrogen flow. Similar size samples, near 12 mg,
and the same heating rate, 10 °C/min, were used for all our
ionic liquids. Values for the temperatures are given ( 5 °C.
For most of our dried ionic liquids, the TGA curve exhibits
some minor weight loss, < 1 %, below 200 °C. Two of our
samples show lower thermal stability than 200 °C; [OMIM]+-
[N(CN)2]- shows an onset temperature at 150 °C and
[MBPYR]+[N(CN)2]- at 180 °C.
C, 37.43; H, 3.78; N, 3.12. Found: C, 37.47; H, 3.37; N, 3.10.
Physicochemical Properties
Melting and Glass-Transition Temperatures. Table 1 shows
melting points and glass-transition temperatures measured by
differential scanning calorimetry (Perkin-Elmer DSC 7). Ionic
liquids were dried in a drying pistol at 1 mbar and 60 °C or in
an antechamber for 3 days in the presence of P2O5. The ionic
liquids were stored and sampled in a dry-helium glovebox prior
to DSC measurements. The DSC apparatus was under a dry-
nitrogen hood. The samples were cooled to -120 °C and kept
at this temperature for 30 min, heated well above the melting
point, and cooled again. The scanning rate used was 10 °C/
min. Melting points and glass-transition temperature were taken
from the second heating curve. Five of the seven ionic liquids
synthesized here are liquid at room temperature. Values for the
temperatures are given ( 1 °C.
Ionic liquids synthesized in this work containing the NfO
anion have melting points above 25 °C. [OMIM]+[Tf2N]-
exhibits no melting points. Instead, a glass-transition temperature
is observed at -86 °C.
To compare the properties of ionic liquids containing
[OMIM]+ and [MBPYR]+ cations with those of ionic liquids
containing [BMIM]+ cations, Table 1 shows densities, melting
points, glass-transition temperatures, and solubilities of ionic
liquids containing [BMIM]+ cations, as reported in the literature.
We also show our melting points and glass-transition temper-
atures.
Thermal stability is primarily related to the nature of the
anion. For every cation studied here, the thermal stability follows
the trend: Td(Tf2N) > Td(NfO) > Td(TfO) > Td(N(CN)2).
Density. Densities of ionic liquids were measured at 20 °C
gravimetrically with a 1 mL volumetric flask. Values for the
densities are given ( 0.01 g‚mL-1. Table 1 shows the results.
All ionic liquids at room temperature are more dense than
water due to the anions used. The density of [OMIM]+[Tf2N]-
has been reported previously.15,19 Reported values are 1.3615
and 1.32.19 The latter density is in accord with our measurement,
(1.31 ( 0.01) g‚cm-3
.
Compared to ionic liquids containing [BMIM]+, [OMIM]+
cations lead to less dense ionic liquids due to an increase in the
alkyl chain length, as reported previously.12,19,24 Replacing the
imidazolium cation with [MBPYR]+, on the other hand, leads
to an increase in density when the imidazolium cation is
[BMIM]+ or [OMIM]+.
Solubility in Water. Only ionic liquids containing NfO and
Tf2N anions appear to be essentially immiscible with water.
Solubilities of ionic liquids in water were obtained by recording
the UV spectra (Beckman DU640) of an aqueous phase in
equilibrium with the ionic liquid. The intensity of the absorption
of ionic liquids containing [OMIM]+ cations was recorded at
211 nm; this wavelength corresponds to the wavelength of
maximum absorption for the 1-methylimidazole group. For ionic
liquids containing [MBPYR]+ cations, the intensity of absorption
was recorded at 254 nm, corresponding to the wavelength of
maximum absorption of 4-methylpyridine.
Dilute aqueous solutions containing a known mass fraction
of ionic liquid, between (0.001 and 0.02) % were used for
calibration. An aqueous solution saturated with ionic liquid was
prepared and diluted 100 times. Table 1 shows the solubilities
of ionic liquids in water. Values for the mass fraction solubilities
are given ( 0.01 %. [OMIM]+[NfO]- appears to be a strongly
hydrophobic ionic liquid; its solubility in water is lower than
that of any ionic liquid reported in the literature.
For all anions except NfO, melting points for ionic liquids
containing [OMIM]+ cations are lower than those for ionic
liquids containing 1-methyl-3-butylimidazolium, [BMIM+] cat-
ions. This is in agreement with results for ionic liquids
containing tetrafluoroborate anions.28
When the [BMIM]+ cation is replaced by a [MBPYR]+
cation, the latter ionic liquids exhibit higher melting points. In
the case of [BMIM]+[Tf2N]-, for example, our measurements
show a glass-transition at -91 °C as compared to -81 °C
obtained by Crosthwaite et al.,22 -86 °C by Tokuda et al.,19
and -87 °C by Bonhote et al.25 and a melting point at -5 °C
as compared to -3 °C by Tokuda et al.,19 -4 °C by Bonhote
et al.,25 and -2 °C by Fredlake et al.31 Our 1-butyl-4-
methylpyridinium Tf2N shows a glass-transition temperature at
-80 °C and a melting point at 10 °C. No melting point was
observed for 1-butyl-3-methylpyridinium Tf2N by Crosthwaite
et al.22 For all other ionic liquids, it appears that substitution of
imidazolium with pyridinium yields ionic liquids melting above
room temperature.

1392 Journal of Chemical and Engineering Data, Vol. 51, No. 4, 2006
[MBPYR]+[TfO]-, [MBPYR]+[NfO]- and [MBPYR]+[N(CN)2]-
are solid at ambient temperature, their polarities were not
measured.
Table 2. Solvatochromic Shifts of Reichardt’s Dye in Several Ionic
N
Liquids and the Corresponding ET Scalea
N
liquid
λ
max/nm
ET
As reported previously,24,33 the polarities of ionic liquids
containing imidazolium cations mostly depend on the cation
and on the length of the cation’s alkyl chain. For example,
previous results show that ionic liquids containing [BMIM]+
cations are more polar than those containing [OMIM]+ cations,
regardless of the anion. The results obtained here are consistent
with these earlier observations; [OMIM]+[Tf2N]- and [OMIM]+-
[BMIM]+ [PF6]-
[OMIM]+ [Tf2N]-
[OMIM]+[N(CN)2]-
[OMIM]+[TfO]-
[MBPYR]+[Tf2N]-
546 24
559
559
558
481
0.667
0.629
0.629
0.631
0.884
water24
453
551
634
1.000
0.653
0.443
ethanol24
DMSO24
[N(CN)2]- have essentially the same ET value.
N
a For comparison, results are also shown for three common solvents.
Table 2 shows that pyridinium ionic liquid [MBPYR]+[Tf2N]-
has a higher value for the ET(30) scale than ionic liquids
Ionic liquids containing dicyanamide or TfO anions are sol-
uble in water. For NfO or Tf2N anions, ionic liquids containing
[OMIM]+ and [MBPYR]+ cations are essentially insoluble in
water. For the same anion (Tf2N- or NfO -), ionic liquids con-
taining [OMIM]+ or [MBPYR]+ cations synthesized in this work
have mostly mass fraction in water lower than those for ionic
liquids containing the [BMIM]+ cation. For Tf2N anions, the
solubility in water decreases from 0.71 % (or 0.80 %15) for
[BMIM]+[Tf2N]- to 0.22 % (0.21 %15) for [OMIM]+-
[Tf2N]-, and 0.48 % for [MBPYR]+[Tf2N]-. For NfO anions,
the solubility in water is 0.16 % for [OMIM]+[NfO]- and 2.29
% for [MBPYR]+[NfO]-. The mass fraction of [BMIM]+[NfO]-
in water is 1.69 %, which is lower than the value of 2.29 % for
[MBPyr]+[NfO]- synthesized in this work. In general, longer
alkyl chains lead to lower solubilities in water, in accord with
literature.16, 24, 25
N
containing imidazolium cations studied in this work. The ET
value for [OMIM]+[Tf2N]- is 0.629, whereas that for [MBPYR]+-
[Tf2N]- is 0.884.
A previous attempt to increase the polarity of imidazolium
ionic liquids has been made by inserting ether, ketone or alcohol
groups into the alkyl chain of the cation.27 The normalized
polarity value for [BMIM]+[Tf2N]-, 0.64, increases to 0.93 for
1-(2-hydroxyethyl)-3-methyl imidazolium Tf2N.
Considering the ET(30) value obtained for [MBPYR]+[Tf2N]-,
substitution of hydroxyl groups into the alkyl chain of the
pyridinium cation may lead to a new class of solvents with high
polarities.
Conclusions
Seven ionic liquids have been synthesized; they contain
1-methyl-3-octylimidazolium or 1-butyl-4-methylpyridinium
cations. Their densities, glass-transition temperatures, melting
points, and decomposition temperatures have been measured.
We find a general trend in the influence of the alkyl chain
length and the nature of the cation on the physicochemical
properties of ionic liquids: with similar anions, ionic liquids
containing 1-butyl-4-methylpyridinium cations exhibit higher
melting points, higher densities, but lower water solubilities than
those for ionic liquids containing 1-methyl-3-butylimidazolium
or 1-methyl-3-octylimidazolium cations. On the other hand, ionic
liquids containing 1-methyl-3-octylimidazolium have lower
melting points, lower densities and lower water solubilities than
those containing 1-methyl-3-butylimidazolium ions.
N
SolWent Polarity: ET Scale with Reichardt’s Dye. The
polarity of a solvent provides a measure of a solvent’s overall
solvation capability. A common polarity scale is the ET(30) scale
where 30 corresponds to the number assigned to this dye by
Reichardt while studying a wide set of solvatochromic dyes.32
Reichardt’s dye is a zwitterion exhibiting negative solvato-
chromic properties (i.e., the wavelength corresponding to
maximum absorption shifts to lower wavelength as the polarity
of the solvent increases). The solvatochromism of the dye is
due to a change from the ground state to an excited state that
is influenced by the polarity of its surroundings (i.e., the solvent).
For Reichardt’s dye, the more polar the solvent, the more stable
the ground state of the dye, and the more energy is required to
move to the excited state. The ET(30) scale is usually in
kcal‚mol-1 units, corresponding to the energy required to go
from the ground state to the excited state.
Polarities were measured spectroscopically using Reichardt’s
dye. The polarities of ionic liquids are mostly determined by
their cation. Ionic liquids containing 1-methyl-3-octylimidazo-
lium cations have polarities similar to that of ethanol.
[MBPYR]+[Tf2N]- has higher polarity.
N
N
For convenience a normalized ET scale is used, where ET
N
) 1.0 for water (ET(30) ) 63.1) and ET ) 0.0 for tetrameth-
N
ylsilane (ET(30) ) 30.7). The ET scale is obtained from the
Literature Cited
wavelength of maximum absorption λmax of Reichardt’s dye
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({28591}/{λmax }) - 30.7
N
ET
with λmax in nm.
)
(1)
32.4
N
ET values have been reported in the literature24 for several
ionic liquids containing imidazolium cations but, to the best of
our knowledge, there are no published data for ionic liquids
containing pyridinium cations and [Tf2N]- anions. Polarities
for these ionic liquids are reported here.
Table 2 gives polarity data for four new ionic liquids:
[OMIM]+[Tf2N]-, [OMIM]+[N(CN)2]-, [OMIM]+[TfO]-, and
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with the resulting ET values. Because [OMIM]+[NfO]-,
N

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Received for review March 2, 2006. Accepted May 21, 2006. The
authors are grateful to the Office for Basic Sciences, U.S. Department of
Energy for financial support.
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