Physicochemical properties of hydrophobic ionic liquids containing 1-octylpyridinium, 1-octyl-2-methylpyridinium, or 1-octyl-4-methylpyridinium cations

Article abstract of DOI:10.1021/je060440r

This paper reports the synthesis of some ionic liquids containing cations 1-octylpyridinium [OPYR]⁺, 1-octyl-2-methylpyridinium [2MOPYR] ⁺, or 1-octyl-4-metnylpyridinium [4MOPYR]⁺ and anions dicyanamide [N(CN)₂]^-, bis(trifluoromethylsulfonyl)imide [Tf₂N]^-, bis(pentafluoroethylsulfonyl)imide [BETI] ^-, trifluoromethyl sulfonate [TfO]^-, nonafluorobutyl sulfonate [NfO]^-, tetrafluoroborate [BF₄]^-, trifluorophenylborate [BF₃PH]^-, tetraphenylborate [BPh₄] or hexafluoroarsenate [AsF₆]^-. Melting points, decomposition temperatures, densities, mutual solubilities with water, and viscosities have been measured. Unlike similar ionic liquids containing imidazolium cations, pyridinium ionic liquids studied here are nearly immiscible with water. Viscosities are similar, and water contents are slightly lower than those for ionic liquids containing imidazolium cations.

Full text of DOI:10.1021/je060440r

J. Chem. Eng. Data 2007, 52, 833-840
833
Physicochemical Properties of Hydrophobic Ionic Liquids Containing
1-Octylpyridinium, 1-Octyl-2-methylpyridinium, or 1-Octyl-4-methylpyridinium
Cations
Nicolas Papaiconomou,†,‡ Justin Salminen,†,§ Jong-Min Lee,†,‡ and John M. Prausnitz*,†,‡
Department of Chemical Engineering, University of California, Berkeley, California 94720, Chemical Sciences Division,
Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Environmental Energy Technology Division, Lawrence
Berkeley National Laboratory, Berkeley, California 94720
This paper reports the synthesis of some ionic liquids containing cations 1-octylpyridinium [OPYR]+, 1-octyl-
2-methylpyridinium [2MOPYR]+, or 1-octyl-4-methylpyridinium [4MOPYR]+ and anions dicyanamide [N(CN)2]-,
bis(trifluoromethylsulfonyl)imide [Tf2N]-, bis(pentafluoroethylsulfonyl)imide [BETI]-, trifluoromethyl sulfonate
[TfO]-, nonafluorobutyl sulfonate [NfO]-, tetrafluoroborate [BF4]-, trifluorophenylborate [BF3Ph]-, tetraphe-
nylborate [BPh4]-, or hexafluoroarsenate [AsF6]-. Melting points, decomposition temperatures, densities, mutual
solubilities with water, and viscosities have been measured. Unlike similar ionic liquids containing imidazolium
cations, pyridinium ionic liquids studied here are nearly immiscible with water. Viscosities are similar, and water
contents are slightly lower than those for ionic liquids containing imidazolium cations.
of the symmetry and the nature of the cation. Several anions
were used: tetrafluoroborate [BF4]-, hexafluoroarsenate [AsF6]-,
trifluoromethyl sulfonate [TfO]-, nonafluorobutyl sulfonate
[NfO]-, dicyanamide [N(CN)2]-, bis(trifluoromethylsulfonyl)-
imide [Tf2N]-, bis(pentafluoroethylsulfonyl)imide [BETI]-,
trifluorophenylborate [BF3Ph]-, and tetraphenylborate [BPh4]-
(see Figure 1 for the structures). Tetrafluoroborate, bis(trifluo-
romethylsulfonyl)imide, and nonafluorobutyl sulfonate anions
are useful because they are known to be hydrophobic.17,19,24
Figure 1 shows the structures of cations and anions studied in
this work.
To elucidate the influence of the cation on the properties of
ILs, trifluoromethyl sulfonate and dicyanamide anions were used
with octylpyridinium cations; their properties are compared with
those of 1-methyl-3-octylimidazolium trifluoromethyl sulfonate
and 1-methyl-3-octylimidazolium dicyanamide, previously re-
ported.29 ILs containing dicyanamide anions exhibit low viscosi-
ties.30 Trifluorophenylborate and tetraphenylborate anions were
used to investigate the substitution of phenyl and fluoride on a
borate anion and to establish their influence on the properties
of ILs containing borate anions.
Introduction
Ionic liquids (ILs) are room temperature molten salts with
melting points near ambient temperatures. During the past
decade, ILs have become a popular research topic in chemistry.
Many studies have reported applications, for example in organic
chemistry,1-5 electrochemistry,6,7 catalysis,8-11 and extraction.12-16
ILs are tunable because there are many combinations of
cations and anions with distinct physicochemical properties. To
date, only a few ILs, mostly those based on ammonium and
imidazolium cations, have been investigated.1-26 Recently, other
types of ILs, containing pyridinium cations were studied.27-29
However, the physicochemical properties of ILs containing
pyridinium cations are yet to be fully investigated. To increase
our understanding of ILs and to extend the database, we report
here synthesis and selected properties of some ILs that contain
pyridinium cations.
For metal-ion extraction from wastewaters, ILs need to be
immiscible with water. Also such liquids need low or moderate
viscosity. Several authors report possible application of ILs to
metal-ion extraction.12-16 However, ILs previously studied were
primarily based on imidazolium cations. Solubilities of imida-
zolium ILs in water are too high for industrial extraction
processes.14,16,23 The solubility of an IL in water depends on
the length of the alkyl chain and on the anion. Recently, we
have shown that ILs containing pyridinium cations exhibit lower
solubilities in water than those of imidazolium homologues.
Here, we report the synthesis and some physicochemical
properties of hydrophobic ILs based on octylpyridinium cations.
Three different cations were used (1-octylpyridinium [OPYR]+,
1-octyl-2-methylpyridinium [2MOPYR]+, and 1-octyl-4-meth-
ylpyridinium [4MOPYR]+) to obtain insight into the influence
Experimental Section
Synthesis. 1-Octyl-3-methylpyridinium tetrafluoroborate was
used as received from Solvent Innovation. All other ILs were
obtained by a two-step synthesis. First, 1-octylpyridinium
bromide [OPYR]+[Br]-, 1-octyl-2-methylpyridinium bromide
[2MOPYR]+[Br]-, and 1-octyl-4-methylpyridinium bromide
[4MOPYR]+[Br]- were synthesized by reacting bromooctane
with pyridine, 2-methylpyridine, and 4-methylpyridine, respec-
tively, in acetonitrile at 70 °C. All syntheses were done under
nitrogen atmosphere. Then, a metathesis procedure was carried
out to substitute selected other anions for bromide anions.
The purity of ILs synthesized was checked with NMR and
elemental analysis. Except for [OPYR]+[Br]-, [2MOPYR]+[Br]-,
and [4MOPYR]+[Br]-, the absence of halide impurities was
checked with silver nitrate: 100 mg of each IL was mixed with
* Corresponding author. E-mail: prausnit@cchem.berkeley.edu.
† Department of Chemical Engineering, University of California, Berkeley.
‡ Chemical Sciences Division, Lawrence Berkeley National Laboratory.
§ Environmental Energy Technology Division, Lawrence Berkeley National
Laboratory.
10.1021/je060440r CCC: $37.00 © 2007 American Chemical Society
Published on Web 03/07/2007

834 Journal of Chemical and Engineering Data, Vol. 52, No. 3, 2007
methylpyridinium bromide, [4MOPYR]+[Br]-, was obtained as
a white solid (yield: 97 %).
1H NMR (400 MHz, CDCl3, δ): 9.328-9.311 (2H, d,
CHpyr-o), 7.865-7.849 (2H, d, CHpyr-m), 4.897-4.860 (2H, t,
N-CH2), 2.632 (3H, s, CH3-Cpyr-p), 2.002-1.931 (2H, m, CH2),
1.317-1.182 (10H, m,C5H10), 0.827-0.793 (3H, t, oct-CH3).
[2MOPYR]+[Br]-. Under nitrogen atmosphere, 13.1 g
(67.2‚10-3 mol) of bromooctane, used as received from Sigma
(99 % purity), was added dropwise to a solution of 4.5 g
(47.4‚10-3 mol) of 4-methylpyridine, used as received from
Sigma (98 % purity), and 25 cm3 of acetonitrile. After cooling
to room temperature, the solvent was removed by rotary
evaporation, and the light yellow solid product was washed with
ethyl acetate (3× 100 cm3) and filtered. Ethyl acetate was
removed by rotary evaporation, and the resulting solid was set
under vacuum for 12 h; 13 g (45.9‚10-3 mol) of 1-octyl-2-
methylpyridinium bromide, [4MOPYR]+[Br]-, was obtained as
a white solid (yield: 97 %).
1H NMR (400 MHz, CDCl3, δ): 9.652-9.632 (H, d,
CHpyr-o), 8.365-8.313 (H, t, CHpyr-m), 7.984-7.939 (H, t,
CHpyr-p), 7.886-7.860 (H, d, CHpyr-m-C), 4.905-4.853 (2H,
t, N-CH2), 2.940 (3H, s, CH3-Cpyr-m), 1.964-1.86 (2H, m, CH2),
1.448-1.227 (10H, m,C5H10), 0.863-0.817 (3H, t, oct-CH3).
[OPYR]+[Tf2N]-. 3.47 g (12.9‚10-3 mol) of [OPYR]+[Br]-
and 4.79 g (16.9‚10-3 mol) of bis(trifluoromethylsulfonyl)imide
acid salt, used as received from Wako Chemicals (99 % purity),
were mixed in a flask containing 100 cm3 of water for 24 h.
The resulting IL was extracted in 50 cm3 of methylene chloride.
Excess salt and HBr were removed by mixing the resulting
liquid with water and methylene chloride (20 cm3 of methylene
chloride and 3× 20 cm3 of water). The methylene chloride phase
was extracted each time. After removal of methylene chloride
by rotary evaporation, the product was set under high vacuum
for 24 h: 5.21 g (yield: 86 %) of [OPYR]+[Tf2N]- was obtained
as a colorless liquid.
1H NMR (400 MHz, CDCl3, δ): 9.652-9.632 (H, d,
CHpyr-o), 8.365-8.313 (H, t, CHpyr-m), 7.984-7.939 (H, t,
CHpyr-p), 7.886-7.860 (H, d, CHpyr-m-C), 4.905-4.853 (2H,
t, N-CH2), 2.940 (3H, s, CH3-Cpyr-m), 1.964-1.86 (2H, m, CH2),
1.448-1.227 (10H, m,C5H10), 0.863-0.817 (3H, t, oct-CH3).
[2MOPYR]+[Tf2N]-. 3.47 g (12.3‚10-3 mol) of [2MOPYR]+-
[Br]- and 4.79 g (14.9‚10-3 mol) of potassium bis(trifluorom-
ethylsulfonyl)imide, used as received from Wako Chemicals (99
% purity), were mixed in a flask containing 100 cm3 of
acetonitrile for 24 h. After filtering, removal of acetonitrile by
rotary evaporation, and washing with methylene chloride (20
cm3) and water (3× 20 cm3), the product was set under high
vacuum for 24 h; 5.21 g (yield: 88 %) of [2MOPYR]+[Tf2N]-
was obtained as a light-yellow liquid.
Figure 1. Structures of cations and anions used in this work.
1 cm3 of dichloromethane and 1 cm3 of an aqueous solution of
silver nitrate. If a white-yellow precipitate (corresponding to
the formation of AgBr) appeared, ILs were washed with water
or extracted with dichloromethane; the silver nitrate test was
then repeated until no AgBr precipitate was observed.
[OPYR]+[Br]-. Under nitrogen atmosphere, 25 g (128‚10-3
mol) of bromooctane, used as received from Sigma (99 %
purity), was added dropwise to a solution of 7.91 g (100.0‚10-3
mol) of pyridine, used as received from Sigma (99.9 % purity),
and 50 cm3 of acetonitrile. After cooling to room temperature,
the solvent was removed by rotary evaporation, and the light
yellow solid product was washed with ethyl acetate (3× 100
cm3) and filtered. Ethyl acetate was removed by rotary evapora-
tion, and the resulting solid was set under vacuum for 12 h:
20.6 g (76.8‚10-3 mol) of 1-octylpyridinium bromide, [OPYR]+
[Br]-, was obtained as a white solid (yield: 77 %).
1H NMR (400 MHz, CDCl3, δ): 9.460-9.447 (2H, d,
CHpyr-o), 8.513-8.475 (H, t, CHpyr-p), 8.132-8.097 (2H, t,
CHpyr-m), 5.039-5.002 (2H, t, N-CH2), 2.055-2.019 (2H, m,
CH2), 1.338-1.228 (10H, m,C5H10), 0.870-0.837 (3H, t, oct-
CH3).
Anal. Calcd for C16H24N2O4S2F6: C, 39.50; H, 4.97; N, 5.76.
Found C, 39.62; H, 5.19; N, 5.71.
[4MOPYR]+[Tf2N]-. 2.52 g (8.9‚10-3 mol) of [4MOPYR]+-
[Br]- and 3.48 g (10.8‚10-3 mol) of potassium bis(trifluorom-
ethylsulfonyl)imide, used as received from Wako Chemicals (99
% purity), were mixed in a flask containing 100 cm3 of
acetonitrile for 24 h. After filtering, removal of acetonitrile by
rotary evaporation, and washing with methylene chloride (20
cm3) and water (3× 20 cm3), the product was set under high
vacuum for 24 h; 3.75 g (yield: 87 %) was obtained as a
colorless liquid.
[4MOPYR]+[Br]-. Under nitrogen atmosphere, 14.43 g
(74.0‚10-3 mol) of bromooctane, used as received from Sigma
(99 % purity), was added dropwise to a solution of 5.0 g
(53.7‚10-3 mol) of 4-methylpyridine, used as received from
Sigma (98 % purity), and 25 cm3 acetonitrile. After cooling to
room temperature, the solvent was removed by rotary evapora-
tion, and the light yellow solid product was washed with ethyl
acetate (3× 100 cm3) and filtered. Ethyl acetate was removed
by rotary evaporation, and the resulting solid was set under
vacuum for 12 h; 14.77 g (52.1‚10-3 mol) of 1-octyl-4-
1H NMR (400 MHz, CDCl3, δ): 8.615-8.599 (2H, d,
CHpyr-o), 7.812-7.797 (2H, d, CHpyr-m), 4.502-4.464 (2H, t,
N-CH2), 2.656 (3H, s, CH3-Cpyr-p), 2.000-1.945 (2H, m, CH2),
1.310-1.234 (10H, m, C5H10), 0.863-0.830 (3H, t, oct-CH3).

Journal of Chemical and Engineering Data, Vol. 52, No. 3, 2007 835
Anal. Calcd for C16H24N2O4S2F6: C, 39.50; H, 4.97; N, 5.76.
Found: C, 39.38; H, 5.11; N, 5.73.
Anal. Calcd for C18H24NO3SF9: C, 34.61; H, 3.92; N, 2.45.
Found: C, 34.52; H, 3.75; N, 2.37.
[4MOPYR]+[BF4]-. 2.57 g (9.1‚10-3 mol) of [4MOPYR]+-
[Br]- and 1.47 g (13.1‚10-3 mol) of sodium tetrafluoroborate,
used as received from Sigma (98 % purity), were mixed in a
flask containing 100 cm3 of acetonitrile for 24 h. After filtering,
removal of acetonitrile by rotary evaporation, and washing with
methylene chloride, the product was set under high vacuum for
24 h; 2.34 g (yield: 89 %) was obtained as a white-yellow
liquid.
[4MOPYR]+[NfO]-. 1.97 g (7.0‚10-3 mol) of [4MOPYR]+-
[Br]- and 3.24 g (9.4‚10-3 mol) of potassium nonafluorobutane
sulfonate, used as received from Sigma (98 % purity), were
mixed in a flask containing 60 cm3 of acetonitrile for 24 h.
After filtering, acetonitrile was removed by rotary evaporation.
The resulting white solid was then washed with methylene
chloride and then water (3× 25 cm3). After 24 h under high
vacuum, 2.94 g (yield: 84 %) was obtained as a white solid.
1H NMR (400 MHz, CDCl3, δ): 8.658-8.641 (2H, d,
CHpyr-o), 7.794-7.778 (2H, d, CHpyr-m), 4.511-4.473 (2H, t,
N-CH2), 2.596 (3H, s, CH3-Cpyr-p), 1.927-1.891 (2H, m, CH2),
1.260-1.166 (10H, m,C5H10), 0.804-0.770 (3H, t, oct-CH3).
Anal. Calcd for C14H24NBF4: C, 57.36; H, 8.25; N, 4.78.
Found: C, 57.25; H, 8.64; N, 4.90.
1H NMR (400 MHz, CDCl3, δ): 8.858-8.842 (2H, d,
CHpyr-o), 7.795-7.774 (2H, m, CHpyr-m), 4.591-4.540 (2H, t,
N-CH2), 2.623 (3H, s, CH3-Cpyr-o), 1.949-1.925 (2H, m, CH2),
1.276-1.203 (10H, m, C5H10), 0.845-0.801 (3H, t, oct-CH3).
Anal. Calcd for C18H24NO3SF9: C, 42.78; H, 4.79; N, 2.77.
Found: C, 43.18; H, 4.96; N, 2.69.
[OPYR]+[TfO]-. 3 g (11.2‚10-3 mol) of [OPYR]+[Br]- and
2.57 g (13.4‚10-3 mol) of potassium trifluoromethane sulfonate,
used as received from Sigma (98 % purity), were mixed in a
flask containing 100 cm3 of acetonitrile. After filtering, removal
of acetonitrile by rotary evaporation, and washing with meth-
ylene chloride, the product was set under high vacuum for 24
h; 3.7 g (yield: 98 %) was obtained as a white solid.
Anal. Calcd for C14H24NO3SF3: C, 49.25; H, 6.50; N, 4.10.
Found: C, 49.83; H, 7.26; N, 3.90.
[4MOPYR]+[TfO]-. 3 g (10.6‚10-3 mol) of [4MOPYR]+[Br]-
and 2.40 g (12.5‚10-3 mol) of potassium trifluoromethane
sulfonate, used as received from Sigma (98 % purity), were
mixed in a flask containing 100 cm3 of acetonitrile. After
filtering, removal of acetonitrile by rotary evaporation, and
washing with methylene chloride, 3.45 g (yield: 92 %) was
obtained as a light yellow liquid.
1H NMR (400 MHz, CDCl3, δ): 8.858-8.842 (2H, d,
CHpyr-o), 7.879-7.804 (2H, m, CHpyr-m), 4.621-4.585 (2H, t,
N-CH2), 2.624 (3H, s, CH3-Cpyr-o), 2.005-1.933 (2H, m, CH2),
1.272-1.194 (10H, m, C5H10), 0.814-0.796 (3H, t, oct-CH3).
Anal. Calcd for C15H24NO3SF3: C, 50.69; H, 6.81; N, 3.94.
Found: C, 47.78; H, 6.89; N, 4.24.
[2MOPYR]+[TfO]-. 3.3 g (11.7‚10-3 mol) of [2MOPYR]+-
[Br]- and 2.80 g (14.6‚10-3 mol) of potassium trifluoromethane
sulfonate, used as received from Sigma (98 % purity), were
mixed in a flask containing 100 cm3 of acetonitrile. After
filtering, removal of acetonitrile by rotary evaporation, and
washing with methylene chloride, 3.66 g (yield: 89 %) was
obtained as a light yellow liquid.
[2MOPYR]+[NfO]-. 2.99 g (10.6‚10-3 mol) of [2MOPYR]+-
[Br]- and 3.65 g (10.6‚10-3 mol) of potassium nonafluorobutane
sulfonate, used as received from Sigma (98 % purity), were
mixed in a flask containing 60 cm3 of acetonitrile for 24 h.
After filtering, acetonitrile was removed by rotary evaporation.
The resulting white solid was then washed with methylene
chloride and then water (3× 25 cm3). After 24 h under vacuum,
4.05 g (yield: 76 %) was obtained as a white solid.
1H NMR (400 MHz, CDCl3, δ): 7.862-7.822 (H, d,
CHpyr-o), 8.315-8.279 (H, t, CHpyr-p), 7.862-7.822 (2H, m,
CHpyr-m), 4.590-4.537 (2H, t, N-CH2), 2.853 (3H, s, CH3-
C
pyr-o), 1.911-1.808 (2H, m, CH2), 1.389-1.223 (10H, m,
C5H10), 0.857-0.812 (3H, t, oct-CH3).
Anal. Calcd for C18H24NO3SF9: C, 42.78; H, 4.79; N, 2.77.
Found: C, 42.81; H, 4.87; N, 2.70.
[4MOPYR]+[N(CN)2]-. 2.10 g (7.4‚10-3 mol) of [4MOPYR]+-
[Br]- and 1.90 g (10.9 ‚10-3 mol) of silver dicyanamide,
synthesized from silver nitrate and sodium dicyanamide, were
mixed for 24 h in a flask containing 75 cm3 of water. After
filtering and removal of the solvent by rotary evaporation, the
resulting colorless liquid was washed with methylene chloride
(3× 25 cm3). After 24 h under vacuum, 1.49 g (75 %) of
[4MOPYR]+[N(CN)2]- was obtained as a colorless liquid.
1H NMR (400 MHz, CDCl3, δ): 8.733-8.712 (2H, d,
CHpyr-o), 7.830-7.810 (2H, d, CHpyr-m), 4.560-4.510 (2H, t,
N-CH2), 2.643 (3H, s, CH3-Cpyr-p), 1.960-1.915 (2H, m, CH2),
1.274-1.171 (10H, m, C5H10), 0.797-0.757 (3H, t, oct-CH3).
13C NMR (400 MHz, CDCl3, δ): 159.99, 143.70, 129.39,
120.16, 61.89, 31.84, 31.77, 29.17, 29.11, 26.30, 22.78, 22.56,
14.30.
1H NMR (400 MHz, CDCl3, δ): 8.842-8.826 (H, d,
CHpyr-o), 8.315-8.279 (H, t, CHpyr-p), 7.879-7.804 (2H, m,
CHpyr-m), 4.591-4.552 (2H, t, N-CH2), 2.863 (3H, s, CH3-
Anal. Calcd for C16H24N4: C, 70.55; H, 8.89; N, 19.37.
Found: C, 68.63; H, 9.08; N, 19.23.
C
pyr-o), 1.917-1.840 (2H, m, CH2), 1.426-1.240 (10H, m,
[4MOPYR]+[BF3Ph]-. 2.02 g of [4MOPYR]+[Br]- (7.1‚10-3
mol) was mixed for 10 h with 2.0 g of potassium phenyltrif-
luoroborate (9.2‚10-3 mol) in 100 cm3 of acetonitrile. After
filtering and removal of the acetonitrile by rotary evaporation,
the resulting white powder was washed with methylene chloride
and put in a freezer for 24 h. After filtering, methylene chloride
was removed by rotary evaporation, and the product was set
under vacuum for 24 h; 2.14 g of [4MOPYR]+[BF3Ph]-
(5.7‚10-3 mol) was recovered as a white-yellowish solid
(yield: 80 %).
1H NMR (400 MHz, CDCl3, δ): 8.262, 8.246 (2H, d,
N-CH2), 7.507, 7.491 (2H, d, CH-N-CHpyr), 7.414-7.399 (2H,
d, Ph-Hm), 7.141-7.054 (3H, m, Ph-H), 4.154-4.117 (2H, t,
CH-B-CH), 2.415 (3H, s, CH3-Cpyr), 1.698-1.623 (2H, m, CH2),
1.240-1.105 (10H, m, C5H10), 0.849-0.815 (3H, t, oct-CH3).
C5H10), 0.869-0.834 (3H, t, oct-CH3).
Anal. Calcd for C15H24NO3S F3: C, 50.69; H, 6.81; N, 3.94.
Found: C, 50.49; H, 6.97; N, 3.83.
[OPYR]+[NfO]-. 2.10 g (7.8‚10-3 mol) of [OPYR]+[Br]-
and 3.01 g (8.7‚10-3 mol) of potassium nonafluorobutane
sulfonate, used as received from Sigma (98 % purity), were
mixed in a flask containing 60 cm3 of acetonitrile for 24 h.
After filtering, acetonitrile was removed by rotary evaporation.
The resulting white solid was then washed with methylene
chloride and then water (3× 25 cm3). After 24 h under high
vacuum, 3.44 g (yield: 91 %) was obtained as a white solid.
1H NMR (400 MHz, CDCl3, δ): 8.976-8.962 (2H, d,
CHpyr-o), 8.466-8.427 (H, t, CHpyr-p), 8.057-8.022 (2H, t,
CHpyr-m), 4.740-4.703 (2H, t, N-CH2), 1.991-1.980 (2H, t,
CH2), 1.302-1.216 (10H, m, C5H10), 0.858-0.824 (3H, t, CH3).

836 Journal of Chemical and Engineering Data, Vol. 52, No. 3, 2007
Table 1. Air-Saturated Densities G of Selected Ionic Liquidsa
Anal. Calcd for C20H29NF3B: C, 68.38; H, 8.32; N, 3.99.
Found: C, 68.07; H, 8.49; N, 3.93.
ionic liquid
F/g‚cm-3
[4MOPYR]+[BPh4]-. 1.78 g of [4MOPYR]+[Br]- (6.3‚10-3
mol) was mixed for 10 h with 3.28 g of sodium tetraphenylbo-
rate (9.5‚10-3 mol) in 50 cm3 of acetonitrile. After filtering and
removal of the acetonitrile by rotary evaporation, the resulting
white powder was washed with methylene chloride and put in
a freezer for 24 h. After filtering, methylene chloride was
removed by rotary evaporation, and the product was washed
with water. After removal of water by rotary evaporation, the
product was set under vacuum for 24 h; 3.11 g of [4MOPYR]+-
[BPh4]- (6.0‚10-3 mol) was recovered as a white-yellowish solid
(yield: 95 %).
1H NMR (400 MHz, CDCl3, δ): 7.487 (8H, s, Ph-Ho),
6.927-6.878 (8H, t, Ph-Hm), 6.745-6.698 (4H, t, Ph-Hp), 6.282,
6.261 (2H, d, CH-N-CHpyr), 5.721, 5.700 (2H, d, 2CHpyr),
2.963-2.912 (2H, t, N-CH2), 2.144 (3H, s, CH3), 1.310-1.051
(10H, m, C5H10), 0.961-0.848 (5H, m, CH2-CH3).
[4MOPYR]+[BF4]-
[4MOPYR]+[AsF6]-
[4MOPYR]+[Tf2N]-
[4MOPYR]+[BETI]-
[4MOPYR]+[TfO]-
[4MOPYR]+[N(CN)2]-
1.08
1.33
1.29
1.39
1.17
0.98
a The relative humidity (RH) was 49 %.
rate was 10 °C/min. Melting point and glass transition temper-
atures were taken from the second heating curve. The uncertainty
of the measurement is ( 1 °C.
Thermogravimetric analysis (TGA) provides information
concerning thermal stability. Perkin-Elmer TGA 7 was used.
The dried IL samples were stored in a dry-helium glovebox.
The TGA sample holder was under dry-nitrogen flow during
the sampling. Similar size samples, approximately 8 mg, were
used at the same heating rate, 10 °C/min. The precision of the
measurement was ( 5 °C.
Anal. Calcd for C38H44NB: C, 86.84; H, 8.44; N, 2.66.
Found: C, 87.04; H, 8.68; N, 2.73.
[4MOPYR]+[AsF6]-. 1.80 g (6.4‚10-3 mol) of [4MOPYR]+-
[Br]- and 1.72 g (8.8‚10-3 mol) of lithium hexafluoroarsenate
were mixed in a flask containing 75 cm3 of water for 24 h. The
product was then extracted by adding 50 cm3 of dichloromethane
to the solution. After evaporation of the solvent by rotary
evaporation, the resulting viscous liquid was washed with water
(3× 25 cm3). After 24 h under vacuum, 2.29 g (yield: 91 %)
of [4MOPYR]+[AsF6]- was obtained as a light-yellow viscous
liquid.
1H NMR (400 MHz, CDCl3, δ): 8.486-8.469 (2H, d,
CHpyr-o), 7.776-7.760 (2H, d, CHpyr-m), 4.470-4.432 (2H, t,
N-CH2), 2.626 (3H, s, CH3-Cpyr-p), 1.943-1.907 (2H, m, CH2),
1.288-1.209 (10H, m, C5H10), 0.841-0.807 (3H, t, oct-CH3).
Anal. Calcd for C14H24NF6As: C, 42.54; H, 6.13; N, 3.54.
Found: C, 42.56; H, 6.20; N, 3.39.
[4MOPYR]+[BETI]-. 1.65 g (5.8‚10-3 mol) of [4MOPYR]+-
[Br]- and 2.47 g (6.4‚10-3 mol) of lithium bis(pentafluoroet-
hylsulfonyl)imide were mixed in a flask containing 50 cm3 of
water for 24 h. The product was then extracted by adding 50
cm3 of dichloromethane to the solution. Dichloromethane phase
was collected, and the solvent was removed by rotary evapora-
tion. The resulting viscous liquid was then washed with water
(3× 25 cm3). After 24 h under vacuum, 3.01 g (yield: 89 %)
of [4MOPYR]+[BETI]- was obtained as a light-yellow viscous
liquid.
Solubility in Water. Solubilities of ILs in water were obtained
by recording the UV spectra (Beckman DU640) of an aqueous
phase in equilibrium with the IL. Intensities of the absorption
of ILs containing [OPYR]+ or [4MOPYR]+ cations were
recorded at 254 nm; this wavelength corresponds to the
wavelength of maximum absorption for [4MOPYR]+ and
[OPYR]+. For ILs containing [2MOPYR]+ cations, the intensity
of absorption was recorded at 265 nm, corresponding to the
wavelength of maximum absorption of [2MOPYR]+ cations.
Dilute aqueous solutions containing a known amount of IL,
between (0.001 and 0.02) wt %, were used for calibration. An
aqueous solution saturated with IL was prepared and diluted
by a factor of 100. Table 2 shows the solubilities of ILs in water.
The mass fraction solubilities have a precision of ( 5 %.
Water Content of ILs. The water content of an IL was
measured by the Karl Fischer method using a Metler-Toledo
coulometer. Water contents were measured at two different
conditions.
The water content of an air-saturated IL was measured first.
The IL was exposed one week to air. Then, a sample of the IL
was taken, and the water content was measured at least three
times. Air humidity was also recorded.
Second, the water content of a water-saturated IL in equi-
librium with liquid water was measured by mixing water and
IL for 24 h. After that, the two-phase system was centrifuged
and left in contact for another 24 h to reach equilibrium. Then,
samples of the IL were taken, and its water content was
measured. All measurements were repeated at least three times.
The water contents of ILs are reported in mass fraction; they
are precise to ( 7 %.
1H NMR (400 MHz, CDCl3, δ): 8.594-8.578 (2H, d,
CHpyr-o), 7.778-7.762 (2H, d, CHpyr-m), 4.461-4.423 (2H, t,
N-CH2), 2.620 (3H, s, CH3-Cpyr-p), 1.920-1.899 (2H, m, CH2),
1.278-1.200 (10H, m, C5H10), 0.835-0.800 (3H, t, oct-CH3).
Anal. Calcd for C18H24N2O4S2F10: C, 36.87; H, 4.13; N, 4.78.
Found: C, 36.94; H, 4.16; N, 4.89.
Viscosities. Viscosities of selected ILs were measured using
an ARES Rheometric Scientific apparatus. Samples were dried
and stored in a helium glovebox prior to measurement. Sampling
and measurements were carried out under dry nitrogen. Before
each measurement, the sample was left at least 30 min inside
the rheometer at 100 °C under dry-nitrogen flow. The procedure
was tested with 1-octyl-3-methylpyridinium tetrafluoroborate,
used as received from Solvent Innovation. Our measured
viscosities of [OMPYR]+[BF4]- at (25 and 75) °C agree within
1 % with those reported in the literature.31
Densities. Densities were measured with a 1 cm3 volumetric
flask in air at 25 °C and relative humidity (RH) of 48 %. The
uncertainty of the density measurement is ( 0.01 g cm-3
.
Melting Points, Glass Transition, and Decomposition Tem-
peratures. Melting points and glass transition temperatures were
measured by differential scanning calorimetry (Perkin-Elmer
DSC 7). ILs were dried in a drying pistol at 1 mbar and 60 °C
or in an antechamber for at least 48 h in the presence of P2O5.
The ILs were stored and sampled in a dry-helium glovebox prior
to DSC measurements. The DSC was under a dry-nitrogen hood
with continuous flushing. The samples were cooled with liquid
nitrogen to -120 °C, kept at this temperature for 30 min, heated
well above the melting point, and cooled again. The scanning
Results
Densities. Table 1 shows densities for selected ILs containing
[4MOPYR]+ cations. The samples were exposed to air at RH

Journal of Chemical and Engineering Data, Vol. 52, No. 3, 2007 837
Table 2. Melting Points (tm), Glass Transition Temperatures (tg),
Onset Decomposition Temperatures (td), and Mass-Fraction
Solubilities in Water (w) for Several Ionic Liquids
points from (68 to 78) °C and no glass transition point. As for
ILs containing [BF4]- or [Tf2N]- anions, a methyl group on
the octylpyridinium cation has no effect on melting points.
For ILs containing [TfO]- anions, on the other hand, melting
points are influenced by a methyl group on the cation. For
[4MOPYR]+[TfO]-, for example, no melting point is observed,
whereas [OPYR]+[TfO]- and [2MOPYR]+[TfO]- have melting
points at (49 and 40) °C, respectively.
For ILs containing 1-methyl-3-alkylimidazolium, previous
reports showed that an increase in alkyl chain length leads to a
decrease in the melting point.18,19 This observation holds for
ILs containing pyridinium cations. With the same anion, Table
2 shows that ILs containing octylpyridinium ([OPYR]+ or
[4MOPYR]+) cations show lower melting points than those for
ILs containing 1-butyl-4-methylpyridinium cations.29
All ILs containing 1-methyl-3-octylimidazolium cations and
[Tf2N]-, [N(CN)2]-, [TfO]-, or [NfO]- anions are liquid at
room temperature.29 The results obtained here for ILs containing
octylpyridinium cations are different. Unlike liquids con-
taining [OMIM]+[NfO]- or [OMIM]+[TfO]-, ILs containing
[4MOPYR]+[NfO]- and [OPYR]+[TfO]- are solid at room
temperature.
ionic liquid
tm/°C
tg/°C
td/°C
100 w
1.68
[4MOPYR]+[BF4]-
[3MOPYR]+[BF4]-
[OPYR]+[Tf2N]-
a
a
-64
-70
-78
-77
-72
b
290
310
320
300
290
280
296
275
294
298
292
228
205
190
305
325
1.65
-12
0.035
0.022
0.10
1.46
1.59
[4MOPYR]+[Tf2N]-
[2MOPYR]+[Tf2N]-
[OPYR]+[TfO]-
7
a
49
a
[4MOPYR]+[TfO]-
[2MOPYR]+[TfO]-
[OPYR]+[NfO]-
-65
40
73
68
78
a
a
130
8
b
b
b
b
c
0.046
0.065
0.14
>5.0
0.49
<0.0005
0.027
0.013
[4MOPYR]+[NfO]-
[2MOPYR]+[NfO]-
[4MOPYR]+[N(CN)2]-
[4MOPYR]+[BF3Ph]-
[4MOPYR]+[BPh4]-
[4MOPYR]+[AsF6]-
[4MOPYR]+[BETI]-
-69
-57
b
-64
-75
2
a No melting point observed. b No glass-transition point observed. c Not
measured.
Table 3. Water Content in Mass-Fraction 100 w of Some Selected
Air-Saturated and Water-Saturated Ionic Liquids at 25 °C
Glass transition temperatures for all ILs reported here are (10
or 15) °C higher than those for ILs containing 1-methyl-3-
octylimidazolium cations. Glass transition temperatures are -64
°C for [4MOPYR]+[BF4]- and [4MOPYR]+[AsF6]-, -65 °C
for [4MOPYR]+[TfO]-, -69 °C for [4MOPYR]+[N(CN)2]-,
-75 °C for [4MOPYR]+[BETI]-, -57 °C for [4MOPYR]+-
[BF3Ph]-, and -77 °C for [4MOPYR]+[Tf2N]-. For ILs
containing [OMIM]+ cations, glass transition temperatures are
between -80 °C for [OMIM]+[TfO]- and -89 °C for
[OMIM]+[N(CN)2]-.29
Thermal Stability. Table 2 shows onset decomposition
temperatures for all ILs studied here. ILs containing phenylbo-
rate anions exhibit the lowest thermal stability. Our previous
work has shown that the thermal stabilities of ILs containing
borate anions increase with the number of fluorides and decrease
with the number of phenyls appended to boron. The onset
decomposition temperatures of [4MOPYR]+[BPh4]- and
[4MOPYR]+[BF3Ph]- are, respectively, (190 and 205) °C.
Previous work29 showed that [OMIM]+[N(CN)2]- has a lower
thermal stability than those of [OMIM]+[TfO]-, [OMIM]+[NfO]-,
[OMIM]+[BF4]-, or [OMIM]+[Tf2N]- anions. In agreement
with our previous results, onset decomposition temperature (td)
for [4MOPYR]+[N(CN)2]- is lower (228 °C) than those for
[4MOPYR]+[TfO]- (296 °C), [4MOPYR]+[NfO]- (298 °C),
[4MOPYR]+[BF4]- (290 °C), or [4MOPYR]+[Tf2N]- (300 °C).
For the same anion, the methyl group on the octylpyridinium
cation has only minor influence on the thermal stability of ILs.
For ILs containing [NfO]- anions, td varies from (298 to 294)
°C and 292 °C for [4MOPYR]+[NfO]-, [OPYR]+[NfO]-, and
[2MOPYR]+[NfO]-, respectively. For ILs containing [TfO]-
anions, td varies from (280 to 296) °C for [OPYR]+[TfO]- and
[4MOPYR]+[TfO]-, respectively.
ionic liquid
RH/%
100 w(air-sat)
100 w(wat-sat)
[4MOPYR]+[BF4]-
[3MOPYR]+[BF4]-
[OPYR]+[Tf2N]-
49
48
49
49
49
49
49
49
49
1.81
1.25
0.13
0.31
0.10
0.20
0.23
1.84
0.25
11.3
13.6
0.86
0.78
0.73
0.52
17.2
54.9
0.90
[2MOPYR]+[Tf2N]-
[4MOPYR]+[Tf2N]-
[4MOPYR]+[BETI] -
[4MOPYR]+[TfO]-
[4MOPYR]+[N(CN)2]-
[4MOPYR]+[AsF6]-
of (48 or 49) % at 25 °C. The corresponding water contents for
each IL are shown in Table 3. For the same cation, ILs
containing dicyanamide anion exhibit the lowest density (0.98
g cm-3) while [4MOPYR]+[BETI]- has the highest density
(1.39 g cm-3), followed by [4MOPYR]+[AsF6]- (1.33 g cm-3
and [4MOPYR]+[Tf2N]- (1.29 g cm-3).
)
The density of [4MOPYR]+[AsF6]- appears to be higher than
that of [4MOPYR]+[BF4]-, in agreement with the observations
that densities for perfluoroarsenate liquids are higher than those
liquids containing perfluorophosphates or perfluoroborate.
For the same anion, densities of ILs containing [4MOPYR]+
cation are lower or similar to those liquids containing 1-butyl-
4-methylpyridinium homologues.29 As expected, density de-
creases with the length of the alkyl chain. For [Tf2N]- and
[N(CN)2]- anions, ILs containing [OMIM]+ cation appear to
be more dense than [4MOPYR]+ homologues, in agreement
with our previous report showing that ILs containing 1-methyl-
3-butylimidazolium cations are more dense than those containing
1-butyl-4-methylpyridinium cations.29
Melting Points and Glass Transition Temperatures. Table
2 shows melting points (tm) and glass-transition temperatures
(tg) from differential-scanning calorimetry. Most ILs reported
here are liquid at room temperature.
The onset decomposition temperature for [3MOPYR]+[BF4]-
measured here is 310 °C, higher than 274 °C previously reported
by Crosthwaite et al.27 With a decomposition temperature of
290 °C, [4MOPYR]+[BF4]- appears to have a slightly lower
thermal stability than [3MOPYR]+[BF4]-. As expected, con-
sidering the large symmetric shape of the anion, [4MOPYR]+-
[AsF6]- exhibits higher thermal stability with a decomposition
temperature of 305 °C.
Due to the size and symmetry of the [BPh4]- anion,
[4MOPYR]+[BPh4]- has the highest melting point measured
here, 130 °C. When phenyls are substituted with fluorides, as
in the trifluorophenylborate anion, the resulting IL, [4MOPYR]+-
[BF3Ph]-, shows no melting point and a tg at -57 °C.
In agreement with results obtained for ILs containing 1-octyl-
3-methylpyridinium cations,27 ILs containing [BF4]- or [Tf2N]-
anions show no or low melting points. [OPYR]+[Tf2N]- shows
a melting point at -12 °C and a glass transition temperature at
-78 °C. All ILs containing [NfO]- anions exhibit high melting
ILs containing bis(perfluoroalkylsulfonyl)imide anions exhibit
the highest thermal stabilities measured here: td for [OPYR]+-

838 Journal of Chemical and Engineering Data, Vol. 52, No. 3, 2007
[Tf2N]- and [4MOPYR]+[BETI]- are (320 and 325) °C,
respectively. [4MOPYR]+[BETI]- is more thermally stable than
[4MOPYR]+[Tf2N]- due to the increased number of fluorinated
carbons. [BETI]- shows the highest thermal stability of all ILs
containing [4MOPYR]+ studied here. Thermal stabilities (td)
of ILs containing [4MOPYR]+ cations are in the order: [BETI]-
> [AsF6]- > [Tf2N]- > [NfO]- > [TfO]- ∼ [BF4]-
>
[N(CN)2]- > [BF3Ph]- > [BPh4]-.
Solubility in Water. All ILs studied here are nearly im-
miscible with water. This result is surprising because previous
studies of ILs containing 1-methyl-3-octylimidazolium cations
did not always yield water-immiscible ILs.19,29 For example,
[OMIM]+[TfO]- and [OMIM]+[N(CN)2]- are miscible with
water. Here, [4MOPYR]+[TfO]- and [4MOPYR]+[N(CN)2]-
are nearly immiscible with water. The solubilities in mass
fraction (w) of [4MOPYR]+[TfO]- and [4MOPYR]+[N(CN)2]-
in water are 1.59 % and above 5 %, respectively, the highest
measured in this work. The solubility in water for [4MOPYR]+-
[TfO]- is very close to that for [4MOPYR]+[BF4]-. Considering
the water solubility of [OMIM]+[TfO]- as opposed to
[OMIM]+[BF4]-, [4MOPYR]+[BF4]-, and [4MOPYR]+[TfO]-,
it appears that the higher solvation energy of the [TfO]- anion
is compensated by the decreased solvation energy of [4MOPYR]+
cation, as compared to that of the [OMIM]+ cation.
Due to the high hydrophobicity of the tetraphenylborate anion,
[4MOPYR]+[BPh4]- has the lowest solubility in water. Surpris-
ingly, the solubility of [4MOPYR]+[BF4]- in water is very close
to that for [OMIM]+[BF4]-. The solubility in water of
[4MOPYR]+[BF3Ph]- (0.49 %) is similar to that for [OMIM]+-
[BF3Ph]- (0.51 %). The borate anions appear to determine the
solubility of ILs in water.
As expected, low solubilities in water are obtained for ILs
containing [Tf2N]-, [NfO]-, [BETI]-, or [AsF6]- anions. The
mass fractions w of ILs in water are 0.013 %, 0.022 %, 0.065
%, and 0.027 % for [4MOPYR]+[BETI]-, [4MOPYR]+[Tf2N]-,
[4MOPYR]+[NfO]-, and [4MOPYR]+[AsF6]-, respectively.
Due to the increased hydrophobicity of the [BETI]- anion,
[4MOPYR]+[BETI]- is the room temperature IL with the lowest
solubility in water found in this work. The solubilities shown
here are an order of magnitude lower than those previously
reported for ILs containing 1-octyl-3-methylimidazolium cat-
ion.29
The water solubilities w obtained for [OPYR]+[Tf2N]- and
[OPYR]+[NfO]- are 0.035 % and 0.046 %, respectively. ILs
containing [OPYR]+ cations appear to have similar solubilities
in water as compared to those of their [4MOPYR]+ homologues.
On the other hand, ILs containing [2MOPYR]+ cations show
higher solubilities in water as compared to ILs containing
[OPYR]+ or [4MOPYR]+ cations. For example, 0.10 % is
obtained for the solubility in water of [2MOPYR]+[Tf2N]- as
compared to (0.035 and 0.022) % obtained for [OPYR]+[Tf2N]-
and [4MOPYR]+[Tf2N]-. Similarly, the solubility in water for
[2MOPYR]+[NfO]- is 0.14 % as compared to (0.046 and 0.065)
% for [OPYR]+[NfO]- and [4MOPYR]+[NfO]-, respectively.
The significant influence of a minor change in molecular
structure is clearly evident.
Figure 2. Viscosities η for selected ionic liquids containing 1-octylpyri-
dinium cations.
saturated samples of [3MOPYR]+[BF4]- and [4MOPYR]+[BF4]-
are lower than those for [4MOPYR]+[TfO]- (17.2 %). When
saturated with water, [4MOPYR]+[N(CN)2]- shows a water
content of 54.9 %. All other ILs studied here have lower water
contents (0.10, 0.20 and 0.25) % for air-saturated [4MOPYR]+-
[Tf2N]-, [4MOPYR]+[BETI]-, and [4MOPYR]+[AsF6]-, re-
spectively.
The water content of air-saturated [4MOPYR]+[BETI]- (0.20
%) at 25 °C and RH of 49 % is twice that for [4MOPYR]+-
[Tf2N]- (0.10 %), even though [4MOPYR]+[BETI]- is less
soluble in water than [4MOPYR]+[Tf2N]-. These water contents
w are unexpectedly high as compared to that for [4MOPYR]+-
[TfO]- (0.23 %), a much less hydrophobic IL than [4MOPYR]+-
[Tf2N]- or [4MOPYR]+[BETI]-.
When air-saturated [4MOPYR]+[BETI]- is in contact with
liquid water, the water content increases to 0.52 %. Water
content of water-saturated [4MOPYR]+[Tf2N]- is 0.73 %.
When the methyl group on the octylpyridinium cation changes
from the ortho to para position, the water content decreases for
ILs containing [Tf2N]- anions. For air-saturated [2MOPYR]+-
[Tf2N]- and [4MOPYR]+[Tf2N]-, the water content decreases
from (0.31 to 0.10) %. The influence of the position of the
methyl group on water content of water-saturated [2MOPYR]+-
[Tf2N]- and [4MOPYR]+[Tf2N]- is much smaller: the water
content decreases from (0.78 to 0.73) %.
The higher water content of [2MOPYR]+[Tf2N]-, as com-
pared to that for [4MOPYR]+[Tf2N]-, is consistent with results
obtained for the water solubilities of [2MOPYR]+[Tf2N]- and
[4MOPYR]+[Tf2N]-, suggesting that the interaction between
water and 1-octyl-2-methylpyridinium cation is stronger than
that between water and 1-octyl-4-methylpyridinium cation.
The few data available for the water contents of ILs
containing imidazolium cations17,19 are similar to those obtained
in this study. The water content of water-saturated [BMIM]+-
[Tf2N]- is close to that of [OPYR]+[Tf2N]-, [2MOPYR]+[Tf2N]-
or [4MOPYR]+[Tf2N]-.
Water contents for [4MOPYR]+[AsF6]- and [3MOPYR]+-
[BF4]- or [4MOPYR]+[BF4]- were higher than those for
[OPYR]+[Tf2N]-, [2MOPYR]+[Tf2N]-, or [4MOPYR]+[Tf2N]-
ILs, in agreement with previous measurements indicating that
ILs containing fluorinated spherical anions such as [PF6]- have
water content higher than those of other ILs containing [Tf2N]-
anions.19 Considering that results obtained for ILs containing
imidazolium are similar to those for pyridinium cations, the
water content of an IL appears to be mostly influenced by the
anion.
Water Content. Table 3 gives water contents for “air-
saturated” and “water-saturated” ILs for [2MOPYR]+[Tf2N]-,
[4MOPYR]+[Tf2N]-, [4MOPYR]+[BETI]-, [4MOPYR]+[TfO]-,
[4MOPYR]+[N(CN)2]-, and [4MOPYR]+[AsF6]-.
In agreement with the high solubilities in water obtained for
[3MOPYR]+[BF4]-, [4MOPYR]+[BF4]-, [4MOPYR]+[TfO]-,
and [4MOPYR]+[N(CN)2]-, water contents for these four ILs
are the highest measured in this study. Water contents for water-

Journal of Chemical and Engineering Data, Vol. 52, No. 3, 2007 839
Table 4. Viscosities η for Selected Ionic Liquids from (0 to 75) °C
t/°C
an alternative to ILs containing imidazolium for application in
extraction of heavy metal cations from wastewaters.
η/mPa‚s
0
25
37.5
50
62.5
75
Literature Cited
[3MOPYR]+[BF4]-
[4MOPYR]+[BF4]-
[OPYR]+[Tf2N]-
7410
6120
476
919
a
332
460
77.1
122
105
365
345
149
194
42.2
64.0
a
77.3
96.1
26.8
37.2
37.6
90.8
87.3
44.3
52.8
18.5
22.0
a
29.3
33.1
13.3
15.1
15.9
34.3
37.9
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[2MOPYR]+[Tf2N]-
[4MOPYR]+[Tf2N]-
[4MOPYR]+[TfO]-
[OMIM]+[BF4]-
a
a
a
a
a
a
a Not measured.
Viscosity. Table 4 shows viscosities for [3MOPYR]+[BF4]-,
[4MOPYR]+[BF4]-, [4MOPYR]+[TfO]-, [OPYR]+[Tf2N]-,
[2MOPYR]+[ Tf2N]-, and [4MOPYR]+[Tf2N]-. A plot of
viscosities is shown in Figure 2. Viscosities measured for
[OMIM]+[BF4]- are also shown in Table 4.
Two groups appear in the plot. The first group of ILs
with lower viscosities includes, as expected, ILs containing
[Tf2N]- anions: [OPYR]+[Tf2N]-, [2MOPYR]+[Tf2N]-, and
[4MOPYR]+-[Tf2N]-.
The lowest viscosities are those for [OPYR]+[Tf2N]-, which
shows a viscosity of 77.1 mPa‚s at 25 °C. The absence of a
methyl group on the pyridinium explains the lower viscosity as
compared to those for [2MOPHR]+[Tf2N]- or [4MOPYR]+-
[Tf2N]-. Viscosities for [4MOPYR]+[Tf2N]- are close to those
for [3MOPYR]+[Tf2N]- reported by Crosthwaite et al.27
(11) Wilkes, J. S. Properties of ionic liquid solvents for catalysis. J. Mol.
Catal. A: Chem. 2004, 214, 11-17.
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The position of the methyl group on the pyridinium cation
has little influence on the viscosity of IL containing the [Tf2N]-
anion. The second group of ILs with higher viscosities are those
containing [3MOPYR]+[BF4]-, [4MOPYR]+[BF4]-, and [4MO-
PYR]+[TfO]-.
The difference between the viscosities for [3MOPYR]+[BF4]-
and [4MOPYR]+[BF4]- are higher than that observed for
[3MOPYR]+[Tf2N]- and [4MOPYR]+[Tf2N]-. For example,
at 25 °C, viscosities for [3MOPYR]+[BF4]- and [4MOPYR]+-
[BF4]-, respectively, are (332 and 460) mPa‚s. For [3MOPYR]+-
[Tf2N]- and [4MOPYR]+[ Tf2N]-, the viscosities are 112 mPa‚
s27 and 105 mPa‚s. The position of the methyl group influences
the viscosities of ILs containing [BF4]- anions.
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based on room-temperature ionic liquids containing monoaza-
substituted crown ethers. Anal. Chem. 2004, 76, 2773-2779.
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Influence of solvent structural variations on the mechanism of
facilitated ion transfer into room-temperature ionic liquids. Green
Chem. 2003, 5, 682-685.
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2004, 39, 2155-2169.
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Graetzel, M. Hydrophobic, highly conductive ambient-temperature
molten salts. Inorg. Chem. 1996, 35, 1168-1178.
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crystals. J. Chem. Soc. Dalton Trans. 1999, 2133-2139.
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Broker, G. A.; Rogers, R. D. Characterization and comparison of
hydrophilic and hydrophobic room temperature ionic liquids incor-
porating the imidazolium cation. Green Chem. 2001, 3, 156-164.
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3-methylimidazolium ionic liquids. ACS Symp. Ser. 2002, No. 819,
34-49.
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M. Physicochemical properties and structures of room temperature
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The viscosities measured here are similar to those obtained
for ILs containing 1-octyl-3-methylimidazolium cations. At 25
°C, viscosities for [OMIM]+[BF4]- and [3MOPYR]+-[BF4]-
are (345 or 332) mPa‚s, respectively. It appears that viscosities
of ILs are mainly influenced by the nature of the anions.
Nevertheless, cation structure such as the presence and position
of a methyl group on the pyridinium cations also has some
influence on viscosity.
Conclusion
ILs containing octylpyridinium-type cations have some phys-
icochemical properties similar to and others significantly
different from those that contain 1-methyl-3-octylimidazolium
cation. Melting points are more influenced by the nature of the
anion than those for their imidazolium homologues. Glass
transition temperatures are (10 to 20) °C higher. Thermal
stabilities are similar to those for ILs containing [OMIM]+
cations. Viscosities for ILs containing octylpyridinium-type
cations are close to those containing [OMIM]+ cations. But most
important, all ILs containing octylpyridinium-type cations are
nearly water immiscible. Solubilities in water are an order of
magnitude lower than those for their imidazolium homologues.
ILs containing octylpyridinium cations, therefore, may provide
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Received for review October 6, 2006. Accepted December 15, 2006.
The authors are grateful to the Office for Basic Sciences, U.S.
Department of Energy and TEKES for financial support.
JE060440R