Liquid chromatography and characterization of ether-functionalized imidazolium ionic liquids on mixed-mode reversed-phase/cation exchange stationary phase

Article abstract of DOI:10.1016/j.crci.2010.04.014

A new series [C_nO_mmim]Cl of imidazolium cation-based ionic liquids (ILs), with an ether functional group on the alkyl side-chain, has been prepared. The possibility of analyzing the ionic liquids by high performance liquid chromatography (HPLC) was investigated on mixed-mode reversed/cation exchange stationary phase with the aqueous-acetonitrile mobile phase. Elution parameters, such as retention factor, selectivity and column efficiency, were studied as functions of mobile phase composition and pH. The ILs were characterized by elemental analysis, and infrared, UV and ¹H, ¹³C NMR spectroscopy.

Full text of DOI:10.1016/j.crci.2010.04.014

C. R. Chimie 13 (2010) 1335–1340
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´
Full paper/Memoire
Liquid chromatography and characterization of ether-functionalized
imidazolium ionic liquids on mixed-mode reversed-phase/cation
exchange stationary phase
ˇ
*
ˇ
ˇ
Je¸Iena Leicunaite, Igors K¸Iimenkovs, Jorens Kviesis , Dzintars Zacs, Juris Pauls Kreismanis
¯
Faculty of Chemistry, University of Latvia, Kr. Valdemara street 48, LV-1013 Riga, Latvia
A R T I C L E I N F O
A B S T R A C T
Article history:
A new series [CnOmmim]Cl of imidazolium cation-based ionic liquids (ILs), with an ether
functional group on the alkyl side-chain, has been prepared. The possibility of analyzing
the ionic liquids by high performance liquid chromatography (HPLC) was investigated on
mixed-mode reversed/cation exchange stationary phase with the aqueous-acetonitrile
mobile phase. Elution parameters, such as retention factor, selectivity and column
efficiency, were studied as functions of mobile phase composition and pH. The ILs were
characterized by elemental analysis, and infrared, UV and 1H, 13C NMR spectroscopy.
Received 15 January 2009
Accepted after revision 12 April 2010
Keywords:
Imidazolium ionic liquids
Ether-functionalized
HPLC
´
ß 2010 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Mixed-mode stationary phase
Mobile phase composition
1. Introduction
asymmetric aldol reaction. Catalytic properties of imida-
zolium ILs have been reviewed recently [3].
The choice of ionic liquids (IL) available to synthetic
chemists widens every day. New ILs are often obtained by
combining a given cation with different anions, so that the
properties of the obtained ILs correspond to certain
requirements. It is considered necessary that the synthe-
sized ILs can be reused several times and that their
degradation products are not toxic [1].
Catalytic reactions make up a significant part of green
chemistry processes. ILs show great potential as solvents
and catalysts for these processes, due to their unique
physicochemical properties. Corma et al. used imidazo-
lium IL with a covalently linked palladium complex for
catalysis of Heck and Suzuki–Miyaura cross-coupling. They
also demonstrated stability over reuse for this catalyst [2].
Imidazolium ILs have been used as catalysts also in a great
variety of other processes, such as hydroformylation,
asymmetric hydrogenation, ring-closing metathesis and
Imidazolium-based ILs, particularly chlorides, have also
been recognized as perspective biopolymer solvents with
ability to dissolve cellulose [4]. Imidazolium ILs bearing
alkoxy [5], hydroxyl, and glycidyl groups [6] in the side-
chain have been synthesized. Polar substituents in the
chain attached to N-1 in the imidazolium ring and chain
length determine physicochemical properties of the
molecule and at the same time solvent properties of the
IL. Relevant information on physicochemical properties of
the IL can be obtained by HPLC [7]. This method allows
finding the most appropriate conditions for purity
determination of the IL and identification of impurities
by their sorption parameters.
Recently some ILs have been separated by reversed-
phase (RP) [8] and ion exchange (IE) chromatography [9].
The described results of analysis of 1-alkyl- and 1-aryl- 3-
methyl imidazolium IL by RP chromatography testify that
it is possible to ensure satisfactory indicators of peak
tailing and system efficiency by choosing a sorbent with a
low content of free silanol groups. Unfortunately, in such a
case, poor repeatability of retention factors is observed,
which can be improved by stabilizing pH of the mobile
* Corresponding author.
E-mail address: cations@inbox.lv (J. Kviesis).
1631-0748/$ – see front matter ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2010.04.014

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J. Leicunaite et al. / C. R. Chimie 13 (2010) 1335–1340
¯
1336
phase. In order to be able to maintain the preferred limits
of retention factors, it has been suggested to use
acetonitrile instead of methanol as mobile phase modifier
[10]. Adjustment of separation parameters, keeping the
content of mobile phase modifier unchanged, becomes
possible by adding salts at different concentrations.
However, decreasing the determination limit of the IL 20
times has to be considered a disadvantage [11]. Phosphate
buffer solution can be added to the mobile phase to
separate hydrophobic 1-alkyl-3-methyl imidazolium IL on
RP [12] under the above-mentioned IE conditions. By
changing concentration of the buffer solution, it is possible
to adjust significantly the retention factor values of the IL.
It must be taken into consideration that by increasing
acetonitrile content in the buffer solution the elution order
of compounds under IE conditions is changing, which is
attributed to transition from RP to direct phase sorption
mechanism. It should be remarked that it is very difficult to
separate hydrophilic cations of IL with relatively short
alkyl chains in the described RP and IE systems. Even
though such separation has been accomplished by using a
polar RP sorbent, adequate column efficiency and asym-
metry factor (As) have not been achieved [13]. Peak
asymmetry is attributed to the fact that the sorbent surface
is heterogeneous and sorption of a low molecular IL mainly
occurs by IE mechanism. Due to highly charged points on
sorbent surface, sorption and desorption of highly charged
sorbates are encumbered more than sorption and desorp-
tion of low charged sorbates.
thermostatted column compartment (CTO-10ASVP) and
data acquisition and analysis were performed by the
Shimadzu LC Solution Chromatography Data System
Operation. Separations were carried out on a Mixed-Mode
RP-C18/Cation 100A column (Alltech; 250 mm  4.6 mm
I.D., 5 mm particle size). The pH of the buffer was measured
in aqueous solutions before the addition of acetonitrile.
The pH of the solution was measured using a calibrated
glass electrode on a Metrohm 780 pH meter at 295.0(Æ0.10)
K. The mobile phases were filtered using a Millipore (Bedford,
MA, USA) 0.45 mm filter before use in HPLC. All HPLC
experiments were run at 1.0 mLÁminÀ1 and the temperature
of 30 8C. The detection wavelength was set at 210 nm. Each
value was measured in triplicate and averaged. Five-
microlitre sample volumes were injected in each experiment.
The system dead time was measured by injecting water into
the system. Column was pre-equilibrated with mobile phase
before running the actual isocratic experiments.
NMR spectra (1H, 13C) were recorded on a Varian600
instrument and are referenced to residual protons in the
solvent (1H), the solvent carbon-13 signal (13C) in CDCl3
at 25 8C.
Infrared (IR) spectra were collected on Avatar 330 FT-IR
spectrometer. Samples for IR were dissolved in CHCl3.
Elemental analysis (C, H, N analyzer) of each synthe-
sized IL was performed on Carlo Erba 1108 CHN Elemental
Analyser.
The absorption spectra were recorded on Uvikon-930
spectrophotometer by using a 1 cm quartz cell.
Other studies have concluded that mixed-mode phases
allow more variables for fine-tuning separations than are
available in classical RP-HPLC. For example, the use of a RP/
cation exchange column has been showing good peak
shape for ionized bases and significant column efficiency in
comparison to analysis on a C18 single-mode column [14].
In this work, we studied performance of a mixed-mode
phase (RP 18/Cation 100A) for the determination of sorption
parameters of ether-functionalized imidazolium IL. The
effect of mobile phase composition and pH of the buffer
solution on the separation of IL cations is demonstrated.
2.2. Chemicals and reagents
For preparation of the mobile phase, HPLC grade
acetonitrile (Petrochem) was used, as well as deionized
water purified using
a Milli-Q system (Millipore),
NaH2PO4, 85% solution of HPLC grade orthophosphoric
acid. The materials used in IL synthesis are as follows: 2-
ethoxyethyl chloride (Aldrich 99.0%, redistilled), 2-chlor-
oethyl phenyl ether (Fluka 97.0%, redistilled), 2-chlor-
oethyl vinyl ether (Aldrich 99.0%, redistilled), triethylene
glycol monochlorohydrin (Aldrich 96.0%, redistilled), 2-
ethoxyethyl chloride (Aldrich 98.0%, redistilled), chlor-
oethanol (Aldrich 98.0%, redistilled), 2-chloroethylmethyl
ether (Aldrich 98.0%, redistilled), 1-chloro-2-(2-methox-
yethoxy)ethane (Aldrich 98.0%, redistilled), and 1-methy-
limidazole (Fluka 99.0%, dried over KOH and redistilled).
Structures of analyzed IL and their main properties are
given in Table 1.
2. Experimental
2.1. Equipment
HPLC was performed with a Shimadzu LC20AD series
system comprising a diode-array detector (SPDM20A), a
vacuum degasser (DGU-20A3), an autosampler (SIL-20A), a
Table 1
Structure of room temperature ionic liquids of the series [CnOmmim][Cl].
Retry
Structure
R
Formula of ionic liquid
Log P [15]
1
2
3
4
5
6
7
8
H
[C2OHmim][Cl]
[C3Omim][Cl]
À4.16
À3.58
À1.80
À2.99
À3.24
À3.74
À4.49
À3.91
CH3
C6H5
[C2OBzmim][Cl]
[C2OAllymim][Cl]
[C4Omim][Cl]
CHCH2
CH2CH3
(CH2)2OCH3
(CH2)2O(CH2)2OH
(CH2)2O(CH2)2OCH3
[C5O2mim][Cl]
[C6O2OHmim][Cl]
[C7O3mim][Cl]

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J. Leicunaite et al. / C. R. Chimie 13 (2010) 1335–1340
1337
¯
2.3. Preparation of ionic liquids
2H), 3.94 (s, 3H), 3.76 (t, J = 7.2 Hz, 2H), 3.60 (t, J = 6.6 Hz,
2H), 3.54 (t, J = 6.6 Hz, 2H), 3.47 (m, 4H); 13C NMR
2.3.1. 1-[2-(2-ethoxy)ethyl]-3-methyl imidazolium chloride
2-ethoxyethyl chloride (8.2 mL, 0.075 moL) and 1-
methylimidazole (4 mL, 0.050 moL) were stirred for 60 h
at 80 8C until two phases formed. The top phase, containing
unreacted starting material, was decanted and ethyl
acetate (50 mL) was added with thorough mixing. The
bottom phase was washed with ethyl acetate (4 Â 25 mL),
heated at 80 8C and stirred under vacuum (0.5 mmHg) for
2 d. The product [C4Omim][Cl] was obtained as a colourless
(150 MHz, CDCl3, d ppm): 137.38, 123.09, 122.99, 72.46,
69.94, 69.67, 68.45, 60.57, 49.18, 36.23; IR (CHCl3, cmÀ1):
3279.9, 3146.4, 3074.7, 2868.6, 1572.8, 1453.7, 1377.1,
1290.2, 1171.6; UV spectrum (nm): lmax (H2O) = 211.4;
elemental analysis calcd (%) for C10H19N2O3Cl (250.73): C
47.91, H 7.64, N 11.17; found: C 47.70, H 7.80, N 11.10.
2.3.5. 1-{2-[2-(2-methoxy)ethoxy]ethoxy}ethyl-3-methyl
imidazolium chloride
liquid (9.34 g, 98%). 1H NMR (600 MHz, CDCl3,
d
ppm):
2-[2-(2-methoxyethoxy) ethoxy]ethyl chloride (12.54 g,
0.075 moL) and 1-methylimidazole (4 mL, 0.050 moL) were
used, and the product [C7O3mim][Cl] was obtained as a
colourless liquid (11.25 g, 85%); 1H NMR (600 MHz, CDCl3,
10.21 (s, 1H), 7.57 (t, J = 1.7 Hz, 1H), 7.46 (t, J = 1.7 Hz, 1H),
4.42 (t, J = 7.2 Hz, 2H), 3.96 (s, 3H), 3.64 (t, J = 7.2 Hz, 2H),
3.35 (q, J = 10.2 Hz, 2H), 1.0 (t, J = 7 Hz, 3H); 13C NMR
(150 MHz, CDCl3,
d
ppm): 137.58, 123.16, 123.04, 68.22,
d ppm): 10.22 (s, 1H), 7.68 (t, J = 2.7 Hz, 1H), 7.49 (t,
66.62, 49.75, 36.43, 14.86; IR (CHCl3, cmÀ1): 3381.3,
3140.7, 2954.0, 2857.4, 2359.6, 1571.9, 1456.5, 1377.1,
1294.0, 1172.8; UV spectrum (nm): lmax (H2O) = 211.4;
elemental analysis calcd (%) for C8H15N2OCl (190.67): C
50.39, H 7.93, N 14.69; found: C 50.35, H 7.90, N 14.72.
J = 2.7 Hz, 1H), 4.52 (t, J = 7.2 Hz, 2H), 3.99 (s, 3H), 3.78 (t,
J = 7.2 Hz, 2H), 3.56 (m, 6H), 3.45 (t, J = 3.6 Hz, 2H), 3.27 (s,
3H); 13C NMR (150 MHz, CDCl3,
d ppm): 137.57, 123.37,
122.72, 71.58, 70.08, 70.01, 70.01, 68.83, 59.63, 49.35, 36.23;
IR (CHCl3, cmÀ1): 3389.1, 3146.9, 3077.0, 2901.3, 1572.9,
1456.4, 1377.1, 1290.3, 1171.6; UV spectrum (nm): lmax
(H2O) = 211.8; elemental analysis calcd (%) for
2.3.2. 3-methyl-1-(2-phenoxyethyl) imidazolium chloride
2-chloroethyl phenyl ether (10.4 mL, 0.075 moL) and 1-
methylimidazole (4 mL, 0.050 moL) were used, and the
product [C2OBzmim][Cl] was obtained as a colourless
C11H21N2O3Cl (264.75): C 49.90, H 8.00, N 10.58; found: C
49.73, H 8.05, N 10.48.
The products [C2OHmim][Cl], [C3Omim][Cl] and
[C5O2mim][Cl] are known compounds and spectral data
are in accordance with the corresponding literature values
[16].
liquid (11.90 g, 99.7%). 1H NMR (600 MHz, CDCl3,
d ppm):
10.48 (s, 1H), 7.66 (t, J = 2.7 Hz, 1H), 7.49 (t, J = 2.7 Hz, 1H),
7.19 (m, 2H, ArH), 6.90 (t, J = 2.4 Hz, 1H, ArH), 6.82 (m, 2H,
ArH), 4.81 (t, J = 7.2 Hz, 2H), 4.31 (t, J = 7.2 Hz, 2H), 3.99 (s,
3H); 13C NMR (150 MHz, CDCl3,
d
ppm): 157.30, 137.96,
3. Results and discussion
129.53, 123.12, 122.96, 121.68, 114.31, 66.09, 50.37,
36.43; IR (CHCl3, cmÀ1): 3381.3, 3083.9, 2963.2, 2854.4,
2360.2, 1599.1, 1492.2, 1462.9, 1377.1, 1294.5, 1174.0; UV
spectrum (nm): lmax (H2O) = 212.4; elemental analysis
calcd. (%) for C12H15N2OCl (238.72): C 60.38, H 6.33, N
11.74; found: C 60.31, H 6.58, N 11.64.
The Alltech mixed-mode RP-C18/cation exchange col-
umn that we used for separation of ILs has C18 groups and
carboxyl groups. Depending on elution conditions and
properties of the solutes used, it offers multimode retention
mechanisms,includingRP, cationexchange,andhydrophilic
interaction. These interactions can be controlled by
changing pH of the mobile phase. In acidic medium
carboxylic, groups of the sorbent are not ionized and
retention occurs mainly by the RP mechanism. Then,
compound retention is governed by hydrophobic interac-
tions, which can be weakened by increasing the concentra-
tion of the organic component in the mobile phase. In basic
medium carboxylic, groups of the sorbent are ionized, and
both cation exchange and hydrophobic interaction contrib-
ute to retention. In highly organic mobile phase, the mixed-
mode column can serve as a hydrophilic interaction liquid
chromatography (HILIC) column.
The retention behaviour of imidazolium IL was studied
by varying the percentage of acetonitrile in the mobile
phase at constant sodium dihydrogen phosphate concen-
tration (0.01 M) and pH 6.5. In Fig. 1, the retention factor of
the imidazolium IL is plotted as a function of acetonitrile
content in the mobile phase. As shown in Fig. 1, the k-
wMeCN plot exhibits three different series of curves.
ILs usually possess a strongly polar (charged) group
along with a strongly hydrophobic alkyl group. However,
this is not the case. Ether groups in IL 1-8 are not as
hydrophobic as simple alkyl groups. These groups are
capable of interacting not only with a non-polar sorbent,
2.3.3. 3-methyl-1-[2-(vinyloxy)ethyl] imidazolium chloride
2-chloroethyl vinyl ether (7.6 mL, 0.075 moL) and 1-
methylimidazole (4 mL, 0.050 moL) were used, and the
product [C2OAllymim][Cl] was obtained as a colourless
liquid (9.40 g, 99.7%). 1H NMR (600 MHz, CDCl3,
d ppm):
10.28 (s, 1H), 7.57 (s, 1H), 7.56 (s, 1H), 6.28 (dd, J1 = 14.3 Hz,
J2 = 6.6 Hz, 1H), 4.64 (t, J = 3.8 Hz, 2H), 4.11 (dd, J1 = 2.5 Hz,
J2 = 14.3 Hz, 1H), 3.98 (m, 6H); 13C NMR (150 MHz, CDCl3,
d
ppm): 150.16, 137.60, 122.93, 123.11, 88.10, 65.66,
48.72, 36.34; IR (CHCl3, cmÀ1): 3381.7, 3144.2, 2959.9,
2359.9, 1573.3, 1460.4, 1377.1, 1285.9, 1169.6; UV
spectrum (nm): lmax (H2O) = 211.9; elemental analysis
calcd (%) for C8H13N2OCl (188.66): C 50.93, H 6.95, N 14.85;
found: C 50.69, H 7.03, N 14.58.
2.3.4. 1-{2-[2-(2-hydroxyethoxy)ethoxy]ethyl}-3-methyl
imidazolium chloride
Triethylene glycol monochlorohydrin (10.9 mL,
0.075 moL) and 1-methylimidazole (4 mL, 0.050 moL)
were used, and the product [C6O2OHmim][Cl] was
obtained as a colourless liquid (12.43 g, 99.1%). 1H NMR
(600 MHz, CDCl3,
d ppm): 9.75 (s, 1H), 7.72 (t, J = 2.7 Hz,
1H), 7.54 (t, J = 2.7 Hz, 1H), 4.60 (bs, 1H), 4.44 (t, J = 7.2 Hz,

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J. Leicunaite et al. / C. R. Chimie 13 (2010) 1335–1340
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When the mobile phase contains more than 30%
acetonitrile, the ILs are eluted in the order of increasing
lipophilicity coefficients (log P) (Table 1). Log P values were
calculated using Marvin and Calculator Plugin Demo [15].
This software uses a sum of group contributions model that
has been compiled based on experimentally determined
log P values of different molecules. These results demon-
strate that all the ILs follow the reverse phase mechanism
of sorption and their retention factors remain constant at
acetonitrile content above 30%. If acetonitrile content is
under 30%, elution of imidazolium ILs does no longer
occurs in the order of increasing lipophilicity factors.
Obviously, this model is too simplified for complex
molecules.
A hydroxyl group in the side-chain of imidazolium ring
in compounds 1 and 7 lowers retention more than one or
two ethoxy groups at an acetonitrile content of both 50%
and 10%. The retention-lowering effect of the hydroxyl
group is probably caused by direct hydrogen bond
Fig. 1. Influence of acetonitrile in mobile phase on retention factors (k) for
1-alkyl ether-3-methylimidazolium ionic liquid (pH = 6.5). The chemical
structure (Table 1).
but also with a polar mobile phase. So, there is dynamic
equilibrium that is easily shifted by changing mobile phase
composition. It is evident that polyether groups are
capable of stronger interaction with the sorbent at low
acetonitrile content. It is most prominent for compound 8
that has the longest polyether chain. At low acetonitrile
content, compounds 4 and 5 that contain ether groups with
fewer carbon atoms are eluted just before compound 8.
Compounds 6 and 2 that contain substituents terminating
in the shortest ether group and the rather hydrophilic
compounds 1 and 7 have even lower retention times.
Increasing of acetonitrile content results in reduction of
retention times for all solutes, but also changes order of
elution for several IL. Most markedly retention time is
decreased for compound 8, for which hydrophobic
interaction at low acetonitrile content in the mobile phase
may be considered most significant. In this acetonitrile
concentration range and pH value, the retention factor of
compound 3 was not obtained (k > 50) because it was not
eluted from the column.
Fig. 3. Dependence of the number of theoretical plates N and asymmetry
factor As on acetonitrile content in the mobile phase (pH = 5.0) (A) and on
pH
of
buffer
solution
(
w
MeCN = 20%)
(B)
of
1-{2-[2-(2-
Fig. 2. Effect of pH of the mobile phase on retention factor
MeCN = 10%). The chemical structure (Table 1).
k
methoxy)ethoxy]ethoxy}ethyl-3-methyl imidazolium cation.
(w

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J. Leicunaite et al. / C. R. Chimie 13 (2010) 1335–1340
¯
1339
Table 2
Separation selectivity
(
a)
and resolution (Rs) (the chromatographic
conditions were the same as in Fig. 4).
Ionic liquid
Isocratic
Gradient
a
Rs
a
Rs
[C2OHmim][Cl]
[C6O2OHmim][Cl]
[C3Omim][Cl]
1.32
1.24
1.13
1.02
1.13
1.05
5.75
0.56
0.50
0.34
0.05
0.32
0.12
8.01
2.05
1.06
1.16
1.10
1.12
1.09
1.22
4.52
0.52
1.65
1.32
1.49
1.25
3.13
[C5C2mim][Cl]
[C4Omim][Cl]
[C2OAllymim][Cl]
[C7O3mim][Cl]
[C2OBzmim][Cl]
asymmetry factor (As) was calculated as the ratio of widths
of the rear and front sides of the peak at 10% peak height.
In Fig. 3.A, N and As are plotted as functions of
acetonitrile content in the mobile phase. In general,
addition of acetonitrile provides more efficient analysis
(higher number of theoretical plates N), particularly within
w
MeCN range of 30–50%. The most significant increase in N is
observed for acetonitrile content 10–30%, if the pH is 5. As
N increases, As values for the IL decrease.
Fig. 4. Chromatograms of separation of ether-functionalized
imidazolium ionic liquids. A. Isocratic elution, mobile phase 10%
CH3CN/10 mM NaH2PO4/H3PO4, pH 5.0. B. Gradient elution. The
concentration ratio of acetonitrile to buffer solution at the gradient
changed: 0 min, 0%, CH3CN; 3–6 min, 2% CH3CN; 12–20 min, 5% CH3CN;
20–40 min, 10% CH3CN. Column: Alltech Mixed-Mode RP-C18/Cation
The relationship observed between pH of the mobile
phase and peak asymmetry factor or number of theoretical
plates is displayed in Fig. 3.B. At higher pH N decreases and
peak tailing is observed. The lowest As can be achieved by
use of acidic eluents.
100A column (250 mm  4.6 mm I.D., 5
mm). Sample concentration in
mobile phase: 0.5 mgÁmLÀ1. 1 – [C2OHmim][Cl], 2 – [C6O2OHmim][Cl],
The selectivity of systems is demonstrated in separation
3
– [C3Omim][Cl], 4 – [C5O2mim][Cl], 5 – [C4Omim][Cl], 6 –
of eight imidazolium IL, as shown in Fig. 4. Selectivity (a)
[C2OAllymim][Cl], 7 – [C7O3mim][Cl], 8 – [C2OBzmim][Cl].
and resolution (Rs) values for the IL under isocratic and
gradient conditions are shown in Table 2. Under isocratic
conditions satisfactory separation was not achieved
(Fig. 4A), and gradient elution was preferred. By using
acetonitrile as the organic modifier on mixed-mode
stationary phase, partial separation of all analyzed IL
was achieved by gradient elution (Fig. 4B). Compounds 2
and 3 could not be completely separated.
As mentioned before, at high acetonitrile content and in
an acidic medium k becomes smaller, and so does the
difference between the analyzed IL. This makes use of
acidic eluents with a large percentage of acetonitrile
impractical for separation of the analyzed IL. So, the choice
of proper chromatographic conditions presents a compro-
mise between resolution, reasonable retention times and
acceptable peak shape. Taking into account all chro-
formation to the eluent constituents. It is necessary to note
that change of sorption character of alkyl imidazolium IL
was also observed in an earlier study in which a C18 single-
mode column was used, but this effect was not marked
using IE column [9].
The pH of the mobile phase containing 0.01 M sodium
dihydrogen phosphate in acetonitrile-water was varied by
adding sodium hydroxide or phosphoric acid. In Fig. 2 the k
values of imidazolium IL are presented as a function of
eluent pH. Fig. 2 shows that k does not increase up to pH 3–
4. It can be presumed that at low pH carboxylic groups in
the stationary phase remain protonated, and their
interaction with positively charged imidazolium rings is
weak. Large increase in retention for IL was observed in pH
range from 4.0 to 6.5. In this case, it is likely that IL are
retained due to IE. Mobile phase pH seems to exert a
greater influence on k values of the more lipophilic IL,
because the curve corresponding to compound 3 is the
steepest.
matographic properties of the investigated compounds (a,
Rs, k, N, T) under discussed conditions, it can be seen that
the best chromatographic results can be obtained by use of
systems with pH = 5.0–6.0 and
wMeCN = 10–30%.
Efficiency of the C18/cation exchange column was
studied and characterized by the number of theoretical
plates (N) as a function of mobile phase composition and pH
under otherwise identical chromatographic conditions. The
4. Conclusions
A complementary set of ether-functionalized imidazo-
lium IL have been successfully prepared from 1-methyli-

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[3] L. Sang-gi, Chem. Commun 10 (2006) 1049.
midazole and the corresponding chlorides. An HPLC
method for separation and determination of ILs using a
RP/cation exchange type column has been developed. This
column has given strong retention of IL cations at pH close
to neutral. The IL eluted from the column in order of
increasing lipophilicity factor, if the contents of acetoni-
trile in the eluent was higher than 30%. Partial separation
of the studied IL has been achieved under gradient
conditions using a buffered eluent that contained acetoni-
trile.
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