Examination of ionic liquids and their interaction with molecules, when used as stationary phases in gas chromatography

Article abstract of DOI:10.1021/ac990443p

Stable room-temperature ionic liquids (RTILs) have been used as novel reaction solvents. They can solubilize complex polar molecules such as cyclodextrins and glycopeptides. Their wetting ability and viscosity allow them to be coated onto fused silica capillaries. Thus, 1-butyl-3- methylimidazolium hexafluorophosphate and the analogous chloride salt can be used as stationary phases for gas chromatography (GC). Using inverse GC, one can examine the nature of these ionic liquids via their interactions with a variety of compounds. The Rohrschneider-McReynolds constants were determined for both ionic liquids and a popular commercial polysiloxane stationary phase. Ionic liquid stationary phases seem to have a dual nature. They appear to act as a low-polarity stationary phase to nonpolar compounds. However, molecules with strong proton donor groups, in particular, are tenaciously retained. The nature of the anion can have a significant effect on both the solubilizing ability and the selectivity of ionic liquid stationary phases. It appears that the unusual properties of ionic liquids could make them beneficial in many areas of separation science.

Full text of DOI:10.1021/ac990443p

Anal. Chem. 1999, 71, 3873-3876
Examination of Ionic Liquids and Their Interaction
with Molecules, When Used as Stationary Phases
in Gas Chromatography
Daniel W. Armstrong,* Lingfeng He, and Yan-Song Liu
University of MissourisRolla, Department of Chemistry, 341 Schrenk Hall, Rolla, Missouri 65409
and polymeric substances. Many RTILs are immiscible with water
and nonpolar organic solvents. They have a liquid range of 300
°C and good thermal stability. The viscosity of RTILs can vary
considerably, but they have no effective vapor pressure. Finally,
they are very accessible, given their ease of preparation from
relatively inexpensive materials.3,5,11
Given their properties, room-temperature ionic liquids could
be used to advantage in a variety of separation methods. Recently
they were considered as a water-immiscible phase in liquid-liquid
extraction.11 Because of environmental concerns with volatile
organic carbon (VOC), the RTILs were considered attractive
alternatives (given their lack of vapor pressure). The distribution
ratios for a variety of analytes between RTIL and water were
approximately an order of magnitude less than the corresponding
octanol/ water partition coefficients.11 Although there was a rough
correlation between these systems, there was a clear polarity
difference.11
Our interest in RTILs initially was aroused by their unusual
combination of properties (i.e., volatility, viscosity, solubility, and
polarity). Our interest was further enhanced when it was found
that RTILs could solubilize a number of complex organic mol-
ecules of interest to the separation community. For example, the
solubility of some cyclodextrins and macrocyclic antibiotics is
summarized in Table 1. Also, we believed that gas-liquid
chromatography (GLC) could be an attractive way to evaluate
differences in various RTILs as well as their interactions with a
variety of molecules. In this initial work we examine two ionic
liquids by using them as stationary phases in gas-liquid chro-
matography. Also, they are evaluated as unusual stationary phases.
Stable room-temperature ionic liquids (RTILs) have been
used as novel reaction solvents. They can solubilize
complex polar molecules such as cyclodextrins and gly-
copeptides. Their wetting ability and viscosity allow them
to be coated onto fused silica capillaries. Thus, 1 -butyl-
3-methylimidazolium hexafluorophosphate and the analo-
gous chloride salt can be used as stationary phases for
gas chromatography (GC). Using inverse GC, one can
examine the nature of these ionic liquids via their interac-
tions with a variety of compounds. The Rohrschneider-
McReynolds constants were determined for both ionic
liquids and a popular commercial polysiloxane stationary
phase. Ionic liquid stationary phases seem to have a dual
nature. They appear to act as a low-polarity stationary
phase to nonpolar compounds. However, molecules with
strong proton donor groups, in particular, are tenaciously
retained. The nature of the anion can have a significant
effect on both the solubilizing ability and the selectivity of
ionic liquid stationary phases. It appears that the unusual
properties of ionic liquids could make them beneficial in
many areas of separation science.
Room-temperature ionic liquids that are air and moisture stable
have been subject to an increasing number of scientific investi-
gations.1-10 Their use as novel solvent systems for organic
synthesis has received a good deal of attention.2-9 Most recently,
polyether-based ionic liquids were shown to be viable solvents
for electrochemical studies.10 Room-temperature ionic liquids
(RTILs) resemble ionic melts of metallic salts in that, essentially,
every entity in the solution is an ion. RTILs have several properties
that could make them useful in a variety of chemical processes.
For example, they are good solvents for many organic, inorganic,
EXPERIMENTAL SECTION
Materials. 1-Methylimidazole, chlorobutane, hexafluorophos-
phoric acid, sodium tetrafluoroborate, squalane, anhydrous ethyl
ether, anhydrous dichloromethane, and all test solutes were
purchased from Aldrich (Milwaukee, WI) or Fluka Chemical Co.
(Ronkonkoma, NY). HPLC grade ethyl acetate was purchased
from Fisher (St. Louis, MO). Hexafluorophosphonic acid is a
corrosive, toxic solution and must be handled with care.
All untreated fused silica capillary tubing (0.25-mm i.d.) was
obtained from Supelco (Bellefonte, PA). DB-5 column (30 m ×
0.25-mm i.d., film thickness 0.25 µm), obtained from J & W
Scientific (Folson, CA), was cut into two pieces, each one of 15-m
length.
(1) Osteryoung, R. A. Molten Salt Chemistry: An Introduction and Selected
Application; Mamantov, G., Marassi, R., Eds.; NATO ASI Ser., Ser. C 1 9 8 7 ,
202, 329.
(2) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg. Chem. 1 9 8 2 ,
21, 1263.
(3) Wilkes, J. S.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1 9 9 2 , 965.
(4) Chauvin, Y.; Oliver-Bourbigou, H. CHEMTECH 1 9 9 5 , Sept. 26.
(5) Suarez, P. A. Z.; Dullius, J. E. L.; Einloft, S.; DeSouza, R. F.; Dupont, J.
Polyhedron 1 9 9 6 , 15, 1217.
(6) Adams, C. J.; Earle, M. J.; Roberts, G.; Seddon, K. R. Chem. Commun. 1 9 9 8 ,
2097.
(7) Earle, M. J.; McCormac, P. B.; Seddon, K. R. Chem. Commun. 1 9 9 8 , 2245.
(8) Dyson, P. J.; Ellis, D. J.; Parker, D. G.; Welton, T. Chem. Commun. 1 9 9 9 ,
25.
(9) Freemantle, M. Chem. Eng. News 1 9 9 9 , Jan. 4, 23.
(10) Dickinson, V, E.; Williams, M. E.; Hendrickson, S. M.; Masui, H.; Murray,
R. W. J. Am. Chem. Soc. 1 9 9 9 , 121, 613.
(11) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R.
D. Chem. Commun. 1 9 9 8 , 1765.
10.1021/ac990443p CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/31/1999
Analytical Chemistry, Vol. 71, No. 17, September 1, 1999 3873

Table 1. Approximate Solubility of Some Compounds in
[BuMIm][Cl], [BuMIm][PF6], and [BuMIm][BF4]a
Table 2. Kova´ tes Indices of the Five First Test Solutes
at 100 °C
solubility (%, w/ w)
2-pen-
nitro-
pyri-
dine
columna
squalane
DB-5
[BuMIm][Cl]
[BuMIm][PF6]
benzene
butanol tanone
propane
[BuMIm]-
[Cl]
[BuMIm]-
[PF6]
[BuMIm]-
[BF4]
compounds
656
682
730
763
600
666
1037
851
629
700
742
807
655
748
900
963
700
763
912
944
R-cyclodextrin
â-cyclodextrin
γ-cyclodextrin
2,3-dimethyl-â-CD
2,6-di-O-methyl-â-CD
permethyl-â-CD
avoparcin
rifamycin B
teicoplanine
vancomycin
30
21
30
12
3
34
8
3
<1
<1
<1
<1
28
<1
<1
<1
10
5
a
The DB-5 column has a stationary phase of (5%-phenyl)-methyl-
polysiloxane. [BuMIm][Cl] is 1-butyl-3-methylimidazolium chloride.
[BuMIm][PF6] is 1-butyl-3-methylimidazolium hexafluorophosphate.
16
<1
<1
5
<1
<1
<1
<1
<1
<1
∼10
∼15
Table 3. Rohrschneider-McReynolds Constants of the
Five First Test Solutes at 100 °C
a [BuMIm][Cl] is 1-butyl-3-methylimidazolium chloride. [BuMIm][PF6]
is 1-butyl-3-methylimidazolium hexafluorophosphate and [BuMIm][BF4]
is 1-butyl-3- methylimidazolium tetrafluoroborate.
columna
Xc
Yc
Zc
Uc
Sc
Avc
Squalane
DB-5
[BuMIm][Cl]
[BuMIm][PF6]
OV-22b
0
27
74
0
66
0
0
93
245
308
283b
0
0
64
216
218
215
71
113
178
191b
63
212
244
253b
437
251
188b
107
Methods. The synthesis of 1-butyl-3-methylimidazolium chlo-
ride ([BuMIm][Cl]), 1-butyl-3-methylimidazolium hexafluorophos-
phate ([BuMIm][PF6]), and 1-butyl-3-methylimidazolium tetraflu-
oroborate ([BuMIm][BF4]) were reported elsewhere.3,5,11 Briefly,
[BuMIm][Cl] was prepared by adding equal amounts (0.5 mol)
of 1-methylimidazole and chlorobutane to a round-bottomed flask
fitted with a reflux condenser and reacting them for 48-72 h at
70 °C. The resulting viscous liquid was allowed to cool to room
temperature and then was washed three times with 50-mL portions
of ethyl acetate. After the last washing, the remaining ethyl acetate
was removed by heating the liquid to 70 °C under vacuum.
[BuMIm][PF6] was prepared from [BuMIm][Cl] by slowly adding
hexafluorophosphoric acid (0.13 mol) to a solution of [BuMIm]-
[Cl] (0.1 mol) in 100 mL of water. After stirring for 12 h, the lower
liquid portion was washed with water until the washings were no
longer acidic. The ionic liquid was heated under vacuum at 70 °C
to remove any excess water. The yield was >80%.
Before coating, 15-m fused silica capillary tubing was subjected
to a sodium chloride pretreatment as reported by Huang et al.12
The capillary then was coated by the static method using a solution
of 0.15% (w/ v) of the stationary phase materials in dichlo-
romethane at 40 °C. Coated columns were flushed with dry Helium
gas for 60 min, then conditioned from 30-100 °C at 0.5 °C/ min.
After conditioning for 8-10 h, column efficiency was tested with
naphthalene at 100 °C. The [BuMIm][PF6] column had 1900
plates/ m, while the [BuMIm][Cl] column had 1700 plates/ m.
All test solutes were dissolved in ethyl ether. A Hewlett-
Packard model 5890 series II was used for all separations. Split
injection and flame ionization detection were utilized. The injector
and detector were held at 222 °C, and He was used as the carrier
gas. The Kova´ts retention indices and the Rohrschneider-
McReynolds constants were determined for these capillary col-
umns as reported previously.13
160b
a
The DB-5 column has a stationary phase of (5%-phenyl)-methyl-
polysiloxane. [BuMIm][Cl] is 1-butyl-3-methylimidazolium chloride.
[BuMIm][PF6] is 1-butyl-3-methylimidazolium hexafluorophosphate.
The OV-22 column has a stationary phase of phenylmethyldiphenyl-
polysiloxane (65% phenyl). b Obtained from ref 14. c X is benzene, Y is
butanol, Z is 2-pentanone, U is nitropropane, and S is pyridine. Av is
the average of the five values for each stationary phase.
mercial DB-5 bonded-phase column, and two columns containing
ionic liquid stationary phases. The largest Kova´ts index was for
butanol on the [BuMIm][Cl] column. The lowest values for all
five solutes were obtained on the squalane reference column
followed by the commercial DB-5 column (Table 2).
Table 3 gives the Rohrschneider-McReynolds constants of the
five reference analytes on five different columns. The data for the
first four stationary phases were generated in this study while
the values for OV-22 [phenyl methyl diphenylpolysiloxane (65%
phenyl)] were taken from the literature for the purpose of
comparison.14 The average of the five Rohrschneider-McReynolds
constants is sometimes used as an approximate polarity scale.
However, the individual constants must be considered in order
to evaluate different contributions to retention. The respective
constants (Table 3) are thought to measure: dispersive interac-
tions (X); proton donor and acceptor capabilities plus dipolar
interactions (Y); dipolar interactions plus weak proton acceptor,
but not proton donor capabilities (Z); dipolar interactions (U); and
strong proton acceptor (but not donor) capabilities (S).14 Unlike
many GLC stationary phases which have smaller variations in
these constants, the ionic liquids show considerable differences
(Table 3). In particular, proton donor and dipolar interactions
appear to be very strong, followed by proton acceptor capabilities.
Also, simply changing the nature of the anion has a significant
effect on the magnitude of the individual interactions but not on
the overall “average polarity” (Table 3).
RESULTS AND DISCUSSION
Table 2 lists the Kova´ts retention indexes of the first five
McReynolds solutes on a squalane reference column, a com-
Another interesting facet of the ionic liquids, when used as
GLC stationary phases, is that they appear to have a dual nature.
This is illustrated in Figure 1. Analytes that are relatively nonpolar
(12) Huang, A., Nan, N.; Chen, M.; Pu, X.; Tang, H.; Sun, Y. Acta Sci. Nat. Univ.
Pekin. 1 9 8 8 , 24, 425.
(13) Berthod, A.; Zhou, E. Y., Le, K.; Armstrong, D. W. Anal. Chem. 1 9 9 5 , 67,
849.
(14) McReynolds, W. O. J. Chromatogr. Sci. 1 9 7 0 , 8, 685.
3874 Analytical Chemistry, Vol. 71, No. 17, September 1, 1999

Table 4. Comparison of Retention Factors (k′) of Solutes Eluted from GLC Columns Containing Ionic Liquid,
Squalane, and (5%-Phenyl)-methylphenylsiloxane (DB-5) Stationary Phasesa
[BuMIm]- [BuMIm]-
[BuMIm]- [BuMIm]-
compounds
squalane DB-5
[Cl]
[PF6]
compounds
squalane DB-5
[Cl]
[PF6]
(A) Paraffins (100 °C)
(E) Alcohols (25-80 °C at 1 °C/ min)/ Diols (80 °C)
n-heptane
n-octane
n-nonane
n-decane
n-undecane
n-dodecane
n-dodecane
0.2
0.5
1.0
2.2
4.7
8.4
8.4
0.1
0.3
0.6
1.3
2.5
4.8
4.8
0.08
0.2
0.4
0.8
1.7
3.6
3.6
0.1
0.3
0.6
1.2
2.5
4.4
4.4
methanol
ethanol
-0.17
0.0
-0.17
-0.08
0.21
1.0
2.6
3.3
5.7
8.2
8.9
0.3
6.7
6.7
0.8
1.1
2.3
4.7
8.1
n-propanol
n-butanol
n-pentanol
2-hexanol
3-heptanol
n-heptanol
3-octanol
0.04
0.6
2.1
3.5
8.2
12.0
15.3
0.08
0.6
10.4
15.1
16.0
19.1
20.0
24.1
29.5
8.4
12.1
18.1
18.9
26.2
39.2
42.5
(B) Substituted Alkanes (25-80 °C at 1 °C/ min)
acetonitrile
propionitrile
1-chlorobutane
1-chloropentane
1-chlorohexane
1-chlorooctane
-0.1
0.2
0.9
2.8
6.3
0.08
0.3
0.7
2.1
4.5
1.1
6.7
0.5
1.7
4.5
2.7
3.3
0.7
2.2
5.5
glycol
1,3-butanediol
1,2-pentanediol
1.2
2.0
1.1
(F)Amines (100 °C)
16.5
10.9
16.7
18.1
N-butylamine
1,5-dimethylhexylamine
n-octylamine
0.08
1.3
1.8
0.09
1.0
2.0
3.1
1.3
1.6
2.0
2.8
3.3
5.4
0.04
0.5
1.4
0.2
5.1
0.1
1.1
6.3
(C) Aromatics (Nonionizable) (100 °C)
benzene
0.8
2.1
0.7
1.6
0.7
1.6
1.9
2.0
4.9
4.9
6.1
5.3
15.2
11.2
15.4
33.1
23.2
24.4
24.4
27.1
27.7
30.2
37.0
1.2
2.2
2.7
3.0
7.2
7.2
8.8
9.6
16.6
17.3
21.2
35.0
41.3
43.0
43.0
47.6
48.5
54.1
63.6
n-decylamine
11.9
15.2
12.6
6.5
12.7
11.0
17.5
56.3
0.6
p-dichlorobenzene
m-dichlorobenzene
toluene
p-xylene
m-xylene
o-xylene
nitrobenzene
m-dimethylbenzene
3-chloronitrobenzene
4-chloronitrobenzene
naphthalene
aniline
1.1
1.7
3.4
4.5
3.3
4.5
0.2
0.6
1.5
37.8
2.4
1.8
benzylmethylamine
benzylethylamine
benzylpropylamine
m-chloroaniline
p-chloroaniline
1-ethylpropylamine
diethyl amine
N,N-ethyldimethylamine
2.8
1.7
7.3
3.2
7.4
3.2
46.8
122.4
3.1
2.0
13.2
8.5
3.5
2.4
2.5
20.0
7.0
6.3
6.1
1.0
8.5
7.3
6.5
30.3
37.2
39.2
39.2
43.5
44.2
46.3
53.7
8.8
(G) Phenols (100 °C)
1,7-dimethylnaphthalene
1,3-dimethylnaphthalene
1,6-dimethylnaphthalene
1,4-dimethylnaphthalene
1,5-dimethylnaphthalene
1,2-dimethylnaphthalene
1,8-dimethylnaphthalene
18.5
18.5
18.8
20.6
20.9
22.4
25.2
m-nitrophenol
2,6-dimethylphenol
o-cresol
3.7
3.0
1.5
0.7
3.2
1.6
1.6
3.5
3.8
4.0
3.2
3.2
3.3
2.9
2.1
1.3
3.9
2.5
2.5
4.8
4.6
5.4
6.2
6.2
14.1
18.6
39.3
45.8
54.2
61.0
64.3
65.8
89.1
114.0
241.9
254.5
phenol
2,5-dimethylphenol
p-cresol
m-cresol
2,3-dimethylphenol
3,5-dimethylphenol
3,4-dimethylphenol
m-chlorophenol
p-chlorophenol
(D) Aldehydes (25 °C)/ Amides (100 °C)/ Esters
(30-60 °C at 0.5 °C/ min)/ Ketones (25-60 °C at 1 °C/ min)
2-methylbutyraldehyde
valeraldehyde
formamide
0.7
1.1
0.0
0.1
0.2
0.0
0.2
0.5
1.0
3.4
-0.1
0.2
0.9
1.1
1.7
1.0
1.5
0.2
0.3
0.3
0.08
0.4
0.8
1.4
3.8
0.0
0.4
1.1
1.6
1.9
0.8
1.3
1.9
3.1
19.7
12.3
4.3
0.5
0.9
1.1
2.0
4.6
0.9
1.7
3.0
5.4
3.9
N-methylformamide
N,N-dimethylformamide
methyl acetate
ethyl acetate
isopropyl acetate
n-propyl acetate
n-butyl acetate
acetone
2-butanone
2-pentanone
3-chloro-2-Butanone
4-methyl-2-Pentanone
27.3
1.0
0.1
0.3
0.4
0.9
2.4
0.2
0.5
1.1
3.2
1.7
(H) Carboxylic Acids (100 °C)
acetic acid
propionic acid
2-chloropropionic acid
dichloroacetic acid
benzoic acid
0.0
0.07
0.3
0.04
0.13
0.9
3.1
3.8
9.0
a
Whenever no value is reported, this indicates that the solute was not eluted under the indicated conditions.
and are not acidic nor basic, tend to separate on ionic liquid
stationary phases in much the same manner (and with similar
retention) as on relatively nonpolar stationary phases, such as
DB-5 (see compounds 1, 2, 3, 6, and 8 in Figure 1A and B).
Conversely, highly polar molecules and proton-donor molecules
(particularly weak acids) have very different retention behavior
(Figure 1B). These types of molecules are strongly retained by
the ionic liquid stationary phases (Figure 1) and in some cases
are not eluted under these test conditions (see the phenols,
carboxylic acids, and diols in Table 4). Some of the larger proton-
acceptor analytes (i.e., the amines) also show this type of behavior,
but not to the same extent as the acids. The strong proton donor
and acceptor effects observed for ionic liquids in this work support
a previous report by Elaiwi et al.15
Table 4 lists the retention factors for a variety of compounds
on GLC columns containing two different ionic liquid stationary
(15) Elaiwi, A.; Hitchcock, P. B.; Seddon, K. R.; Srinivasan, N.; Tan, Y.-M.; Welton,
T.; Zora, J. A. J. Chem. Soc., Dalton Trans. 1 9 9 5 , 3467.
Analytical Chemistry, Vol. 71, No. 17, September 1, 1999 3875

in the properties of various ionic liquids as well as how they
interact with molecules.
Molecules that do not contain good proton-donating or
-accepting groups (e.g., aliphatic and aromatic compounds, esters,
aldehydes, and ketones) are more strongly retained on the
[BuMIm][PF6] stationary phase. This is true even for the more
polar haloalkanes. However, all compounds with proton-donor
groups (e.g., alcohols, diols, phenols, and carboxylic acids) interact
much more strongly with the chloride-containing [BuMIm][Cl]
ionic liquid (Table 4). Amines also prefer the [BuMIm][Cl].
An examination of the Rohrschneider-McReynolds constants
(Table 3) confirms the experimental observations in Table 4. The
constant for proton-donor activity (i.e., the Y terms in Table 3)
was far and away the most dominant feature of [BuMIm][Cl].
Conversely, constants for dispersive and dipolar interactions (i.e.,
the X and U terms in Table 3) were more prominent for [BuMIm]-
[PF6]. Clearly, GLC can be a useful tool to evaluate and compare
ionic liquids. As indicated in this study, one can focus on the
nature of specific anions (or cations in other instances). Such
information could be useful in a number of areas other than
separations, where a knowledge of salt-organic interactions and/
or relative-ion behaviors are important.
The only compounds that were retained to a greater extent
on the less polar squalane and DB-5 columns than on the ionic
liquid columns were the unsubstituted alkanes, the smaller
chloroalkanes, and a few of the esters, aldehydes, and ketones.
Figure 1. Chromatograms comparing the retention and separation
of eight compounds on the same size GC columns (15 m × 0.25-
mm i.d.) and under identical conditions (isothermal @ 100 °C and a
pressure of 9 psi). Column A is a commercial DB-5 column, and
Column B utilizes the ionic liquid [BuMIm][PF6] as the stationary
phase. The test compounds are: 1, butyl acetate; 2, n-heptanol, 3,
p-dichlorobenzene, 4, o-cresol, 5, 2,5-dimethylphenol, 6, n-dodecane,
7, 4-chloroaniline, and 8, n-tridecane.
CONCLUSIONS
Room-temperature ionic liquids (RTILs) act as nonpolar
stationary phases when separating nonpolar analytes or somewhat
polar analytes that are not proton-donor or -acceptor molecules.
However, they act in the opposite manner (i.e., are highly
interactive and retentive) when used to separate molecules with
somewhat acidic or basic functional groups. Thus, molecules with
proton-donor or -acceptor characteristics tend to be spatially
resolved, as a group, from nonpolar analytes. The dual nature of
ionic liquid stationary phases also is evident from the Rohr-
schneider-McReynolds constants. Inverse GLC also is a good way
to examine the nature of different ionic liquids. It is apparent that
the chloride-containing ionic liquid interacted much more strongly
with proton-donor and -acceptor molecules. The hexafluorophos-
phate-containing ionic liquid tended to be somewhat less polar
and interacted more strongly with nonpolar solutes. GLC with
ionic liquids is an effective way to study differences in ions as
well to study interactions between ions and organic molecules.
RTILs often can solubilize complex macrocyclic molecules such
as cyclodextrins and their derivatives and macrocyclic antibiotics.
As a result of their novel properties, RTILs may be useful in many
separation methods.
phases as well as squalane and the DB-5 stationary phase. The
more nonpolar and nonionizable compounds are listed toward the
beginning of Table 4, while the ionizable and more polar
compounds are listed afterward. All four GLC columns tended to
have similar retention factors for the neutral, less polar com-
pounds. The relative retention on the ionic liquid stationary phases
increases with the more polar alcohols and formamids. This
increase is even more dramatic for the amines and particularly
the phenols and organic acids (Table 4). The observed retention
behavior corresponds well with that predicted by the previously
mentioned Rohrschneider-McReynolds constants. Also apparent
from the data in Table 4 is that the nature of the anionic portion
of the ionic liquids can have a profound effect on retention and
selectivity.
Effect of the Anion. The cationic portion of both ionic liquids
in this study consists of the 1-butyl-3-methylimidazolium ion (see
Experimental Section and Tables 2-4). Both ionic liquids have
approximately the same average polarity (see the “Average” in
Table 3). However, it is clear from the data in Table 4 that there
are significant differences in the chloride versus the hexafluoro-
phosphate ionic liquids. The data shows that some classes of
compounds interact more strongly with the hexafluorophosphate
ionic liquid, while other classes of compounds interact more
strongly with the chloride-containing ionic liquid (Table 4). Indeed,
it appears that GLC may be a very useful tool to study differences
ACKNOWLEDGMENT
Support by the National Institutes of Health (NIH R01
GM53825-04) is gratefully acknowledged.
Received for review April 27, 1999. Accepted June 24,
1999.
AC990443P
3876 Analytical Chemistry, Vol. 71, No. 17, September 1, 1999