Selectivity of stationary phases based on pyridinium ionic liquids for capillary gas chromatography

Article abstract of DOI:10.1134/S0036024414040268

A number of capillary columns with stationary liquid phases based on mono- and dication pyridinium ionic liquids (ILs) were prepared. Their polarity was evaluated using McReynolds system and the selectivity was estimated from intermolecular interactions. The parameters of intermolecular interactions were obtained from retention data using the (Abraham) model of the linear free energy relationship. The dependences of intermolecular interactions on the structure of the cation in the ILs under study were revealed. The results were compared with the data for the traditional phases (HP-5, ZB-WAX). Examples of separation of mixtures of oxygen-containing compounds on the phases under study are given.

Full text of DOI:10.1134/S0036024414040268

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2014, Vol. 88, No. 4, pp. 717–721. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © M.V. Shashkov, V.N. Sidelnikov, P.A. Zaikin, 2014, published in Zhurnal Fizicheskoi Khimii, 2014, Vol. 88, No. 4, pp. 714–719.
PHYSICAL CHEMISTRY OF SEPARATION PROCESSES.
CHROMATOGRAPHY
Selectivity of Stationary Phases Based on Pyridinium Ionic Liquids
for Capillary Gas Chromatography
,
b
,c
M. V. Shashkova , V. N. Sidelnikova, and P. A. Zaikinb
a Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia
b Novosibirsk State University, Novosibirsk, Russia
c Vorozhtsov Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia
eꢀmail: shashkov@catalysis.ru
Received May 22, 2013
Abstract—A number of capillary columns with stationary liquid phases based on monoꢀ and dication pyriꢀ
dinium ionic liquids (ILs) were prepared. Their polarity was evaluated using McReynolds system and the
selectivity was estimated from intermolecular interactions. The parameters of intermolecular interactions
were obtained from retention data using the (Abraham) model of the linear free energy relationship. The
dependences of intermolecular interactions on the structure of the cation in the ILs under study were
revealed. The results were compared with the data for the traditional phases (HPꢀ5, ZBꢀWAX). Examples of
separation of mixtures of oxygenꢀcontaining compounds on the phases under study are given.
Keywords: ionic liquids, gas chromatography, highly polar stationary phases, Abraham model.
DOI: 10.1134/S0036024414040268
INTRODUCTION
compounds cannot be determined. Therefore, a
more detailed evaluation of the separating ability of
phases is required, which would be based on determiꢀ
nation of the contribution of each basic type of interꢀ
molecular interaction. An example is the model of
the linear free energy relationship (LFER) developed
by Abraham [11].
The model is based on the assumption that the free
energy of the interaction between the substance and
the phase depends linearly on the contribution of each
individual type of interaction. The logarithm of the
distribution constant (KL) is used instead of the energy
at a given temperature. Thus the basic equation of the
model is [11, 12]
Ionic liquids (ILs) are a new class of chromatoꢀ
graphic stationary liquid phases (SLPs) for capillary
chromatography. They are characterized by a set of
attractive properties potentially useful for preparing
chromatographic columns [1]. The major advantage
of ILs over other types of SLP lies in their high thermal
stability combined with high polarity [1, 2].
Phases based on ammonium, imidazolium, and
phosphonium ILs are currently best defined [3–5].
However, other classes of ILs can also be used for preꢀ
paring chromatographic columns [5, 6]. One of such
examples are piridinium ILs, whose thermal stability is
comparable to that of imidazolium ILs [7, 8]. At the
same time, the pyridinium ring offers more opportuniꢀ
ties for creating structures with different properties
than the imidazolium ring [9]. Therefore, investigaꢀ
tion of phases based on pyridinium ILs is of interest
from the viewpoint of creating columns with higher
selectivity with respect to various classes of polar comꢀ
pounds.
(1)
where KL is the coefficient of analyte distribution
between the mobile and liquid phases, and is the conꢀ
log KL = c + eE + sS + aA + bB + lL,
c
stant of the model. The coefficients and L
E,
S,
A
,
B,
(called the descriptors) are the contributions of anaꢀ
lytes to intermolecular interactions. They are deterꢀ
mined experimentally and can be found in the literaꢀ
The phase polarity is one of the characteristics of ture for many compounds [13, 14]. The coefficients
the separating ability of a column in gasꢀliquid chroꢀ and are the parameters that describe the conꢀ
matography. The traditional (McReynolds [10]) sysꢀ tribution of the liquid phase to the intermolecular
tem for polarity evaluation fails to reflect the individꢀ interactions. The and interactions are
ual contribution of each type of specific interaction described by the parameter; the dipolarityꢀpolarizꢀ
to analyte retention and to determine the phase ability by ; the contribution of a phase to the formaꢀ
selectivity with respect to a definite class of subꢀ tion of a hydrogen bond by the and parameters (
stances. As a result, the selectivity of a set of phases in is responsible for the forces in which the phase is
a definite range of polarity (generally 70–100 in the hydrogen acceptor, and for the forces in which it is
e,
s,
a,
b,
l
π
–
π
n–
π
e
s
a
b
a
b
case of ILs) with respect to different classes of polar hydrogen donor); and the dispersion interaction by
l.
717

718
SHASHKOV et al.
R3
R1
N+
R5
R2
R4
R5
R2
R2
R3
R4
R3
N+
N+
R1
R5
R1
NTf2–
(NTf2–
)
2
R4
bis4MPyC9: R3 = Me, R1245 = H
bis2MPyC9: R1 = Me, R2345 = H
2MPy: R1 = Me, R2345 = H
3MPy: R2 = Me, R1345 = H
4MPy: R3 = Me, R1245 = H
3.5MPy: R24 = Me, R135 = H
2.6MPy: R15 = Me, R234 = H
2.4.6MPy: R135 = Me, R24 = H
NTf2–
N+
bis3.5MPyC9: R24 = Me, R135 = H
C6H13
Hex4MPy
Anion
N+
N+
O
O
N−
S
S
(NTf2–
)
2
F3C
CF3
O O
NTf2–
bis4MPyC6
Fig. 1. Structures of ILs and notation.
This set of parameters is used for classifying the staꢀ the first time; the exceptions were 3MPy (SigmaꢀAldꢀ
tionary phases according to the type of intermolecular
interaction and comparing the phases with one
another [11].
rich) and bis4MPyC6 and Hex4MPy, which were
mentioned in [15, 16]. NTf2 was used as an anion in all
ILs (Fig. 1).
Here, the Abraham model was used to evaluate
the retention ability of new SLPs based on pyridinꢀ
ium ILs. An analysis of the results allowed us to reveal
the dependence of various types of intermolecular
interactions between the phases and the substances
being separated on the cation structure in the pyriꢀ
dinium ILs.
Procedure for the synthesis of 4MPy, 3.5MPy,
2MPy, and Hex4MPy. A mixture of 1ꢀbromopropane
(1ꢀbromohexane in the case of Hex4MPy) and the
corresponding methylpyridine in a molar ratio of
1.25 : 1 was heated at 120°C for 2 h. After the heating
was completed, the mixture was washed three times
with ethylacetate. For anion exchange, the obtained
alkylpyridinium bromides were dissolved in water, and
lithium bis(trifluoromethanesulfonyl)imide was
added. After stratification, the IL was separated by
centrifuging, washed with deionized water, and dried
in vacuum.
EXPERIMENTAL
Capillary columns based on ILs presented in Fig. 1
were prepared. The phases were applied by the highꢀ
pressure static procedure described in [6]. A fused silꢀ
ica capillary with a diameter of 0.22 mm and a length
of 20 m was used; the calculated film thickness in the
Procedure for the synthesis of 2,6MPy and
2,4,6MPy. A mixture of pyridine (7 mmol) and 1ꢀbroꢀ
mopropane (10 mmol) in a hermetically closed 10 mL
prepared columns was 0.1–0.15 m. The reference
µ
columns were HPꢀ5 (Agilent) and ZBꢀWax (Phenomꢀ
enex).
The reagents used included 2ꢀ and 4ꢀmethylꢀ; 2,6ꢀ
and 3,5ꢀdimethylꢀ; 2,4,6ꢀtrimethylpyridines; 1ꢀbroꢀ
mopropane; 1ꢀbromohexane; 1,6ꢀdibromohexane;
1,9ꢀdibromononane (Alfa Aesar), tertꢀbutanol; 3ꢀ
ampule was heated with vigorous stirring at 160°С for
1 h in a microwave system. Purification and anion
exchange were performed by the procedure described
above.
Procedure for the synthesis of dication ILs
(bis4MPyC9, bis2MPyC9, bis3,5MPyC9, and
bis4MPyC6). A mixture of appropriate methylpyriꢀ
dine and 1,9ꢀdibromononane (1,6ꢀdibromohexane) in
a molar ratio of 3 : 1 was heated while stirring at 120°С
for 6 h. Purification and anion exchange were perꢀ
formed as described above. The resulting ILs were
methylꢀ1ꢀpropylpyridinium
bis(trifluoromethaneꢀ
sulfonyl)imide (SigmaꢀAldrich); and lithium bis(trifꢀ
luoromethanesulfonyl)imide. Syntheses with microꢀ
wave heating were performed on an Anton Paar
Monowave 300 unit with smooth power control and
temperature control with a pyrometric detector. The
majority of ILs shown in Fig. 1 were synthesized for characterized by 1H NMR and HPLC/MS.
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SELECTIVITY OF STATIONARY PHASES BASED ON PYRIDINIUM IONIC LIQUIDS
719
Parameters of the Abraham model (e, s, a, b, and l) and McReynolds polarities (P) for columns based on ILs and HPꢀ5
and ZBꢀWAX phases
Compound
e
s
a
b
l
r2
P
Monocation ILs
2MPy
0.06
0.03
0.08
0.19
0.16
0.39
0.35
1.62
1.60
1.52
1.66
1.43
1.60
1.14
1.19
0.20
0.20
0.13
0.46
0.39
0.59
1.54
0.31
0.31
0.29
0.38
0.36
0.43
0.43
0.998
0.998
0.990
0.999
0.999
0.998
0.999
89
86
84
85
76
81
80
3MPy
1.15
1.03
1.41
1.13
1.38
1.02
4MPy
2.6MPy
3.5MPy
2.4.6MPy
Hex4Mpy
Dication ILs
bis2MPyC9
bis4MPyC9
bis3.5MPyC9
bis4MPyC6
0.30
0.20
0.38
0.16
1.62
1.61
1.42
1.69
1.17
1.26
1.15
1.26
0.35
0.25
0.75
0.16
0.35
0.32
0.41
0.29
0.999
0.998
0.999
0.997
92
91
80
100
Reference columns
HPꢀ5
–0.01
0.004
0.29
0.32
0.31
1.53
1.51
0.19
0.2
0.00
0.00
0.00
0.00
0.515
0.52
0.49
0.50
0.9999
1.000
0.998
1.000
8
HPꢀ5[23]
ZBꢀWAX
DBꢀFFAP [23]
2.39
2.21
51
0.23
Note: r is the correlation coefficient for n = 15 test compounds.
2
The McReynolds polarity (P) was measured by methyl esters C9, С10, and С11; phenol and oꢀ and pꢀ
chromatographing the McReynolds mixture of the test cresols; 2,6ꢀ and 3,5ꢀdimethylphenols; and 2ꢀethylꢀ
substances (benzene, butanol, pentanoneꢀ2, nitroproꢀ hexanoic acid.
paneꢀ1, and pyridine) at 120°C. The Kovats indices
were measured from the retention of the normal
hydrocarbons С10–С16. The polarity was determined
from the scale used in [17], where the polarity of
squalane was set at zero and that of the SPBꢀIL100
(Supelco) phase at 100.
RESULTS AND DISCUSSION
Earlier, it was reported that the use of imidazolium
ILs allowed the creation of thermally stable columns
and good results in separation of complex mixtures of
polar compounds from various chemical classes [1, 2,
19, 20]. As to pyridinium ILs, the literature gave only
several examples of their use in gas chromatography
[21, 22]. As mentioned above, the McReynolds system
for polarity evaluation can give a preliminary concluꢀ
sion about the separating properties of phases with
respect to definite classes of compounds. The table (its
last column) lists McReynolds polarities for the ILs
under study and for ZBꢀWAX (Carbowax 20M) and
HPꢀ5 (polysiloxaneꢀ5%ꢀphenyl) reference columns.
The polarity is 80–95 for the majority of the tabulated
ILs. The highest polarity (100) is inherent in the
bis4MPyC6 IL. These phases can therefore be conꢀ
ventionally classified as highly polar, but their selectivꢀ
ity is unknown.
The parameters of the Abraham model were calcuꢀ
lated by the procedure described in [18]. For this purꢀ
pose, 15 test substances (benzene, toluene, benzaldeꢀ
hyde, nitropropaneꢀ1, pentanoneꢀ2, cyclohexanone,
butanolꢀ1, octanolꢀ1, pyridine, pyrrole, phenol, and
alkanes С12–С15), whose descriptors (
) are found in [18], were chromatographed at 120
The parameters of the model ( , and ) were calꢀ
E, S, A, B, and
L
°C.
e,
s,
a,
b
l
culated by the multiple linear regression procedure
using Microsoft Excel 2003 software.
The polarity and retention characteristics were
measured on an Agilent 7890 chromatograph with a
flameꢀionization detector. The detector and injector
temperature was 250
gas. The oven temperature was 120
ture was chromatographed according to the following
temperature program: 90
to 160 , and 160 for 15 min. The composition of affords a more comprehensive evaluation of the sepaꢀ
the test mixture: nꢀalkanes С12, С14, С15, and С16; aliꢀ rating abilities of phases [11]. The table lists the
phatic primary alcohols С8, С10, and С11; fatty acid parameters ( , and ) of the model correspondꢀ
°C
; helium was used as a carrier
°C. The test mixꢀ
A description of selectivity according to the type of
°C for 3 min, then 6 K/min intermolecular interaction using the Abraham model
°C
°C
e,
s,
a,
b
l
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720
SHASHKOV et al.
the IL molecule. Similar regularities were also
observed for imidazolium ILs [4, 18].
The parameter (hydrogen bond with substances
with a mobile hydrogen atom) changes within wide
limits and is one of the main types of specific interacꢀ
tion characteristic of IL along with the dipolarity–
bis4MPyC6
10
15
2
13
9
1
12
7
а
14
4
3
8
5
16
6
5
11
polarizability (s) [4, 18]. If we compare the a parameꢀ
ter of ILs with that of the ZBꢀWAX polar phase (polyꢀ
ethyleneglycol), it is substantially lower for the ILs
under study. This is explained by the fact that polyethꢀ
yleneglycol contains oxygen atoms with a free electron
10
20
bis2MPyC9
30
40
15
8
+
10
12
13
11
14
2
pair involved in Hꢀbonding. The highest value of a was
6
+ 9
16
observed for 2,4,6MPy and 2,6MPy, i.e., for phases in
which two methyl groups lie at positions closest to
nitrogen.
4
3
7
1
The highest
b parameter (the hydrogen bond with
5
10
15
20
25 t, min
substances that are electron pair donors) is characterꢀ
istic of Hex4MPy and bis3,5MPyC9 ILs. In the series
of monocation ILs, the b parameter increases substanꢀ
tially with the number of methyl groups in the ring and
their proximity to the heterocyclic nitrogen atom. This
can be explained by the fact that the methyl hydrogen
atoms in pyridinium compounds exhibit acidic propꢀ
erties to some extent [9]. For ILs under study, the
parameter is not zero, in contrast to that of the majorꢀ
ity of known SLPs [4, 23].
Fig. 2. Chromatograms of separation of a test mixture of
alkanes and oxygenꢀcontaining compounds on phases with
pyridinium ILs: (1) С , (2) C , (3) C , (4) C , (5)
C COOMe, (
12 14 15 16
) C COOMe, (
6
7
) C OH, (
8
) C COOMe,
9
9
10
11
8
11
(
) C OH, (10) C OH, (11) 2ꢀethylhexanoic acid, (12
)
)
10
2,6ꢀdimethylphenol, (13
)
o
ꢀcresol, (14) phenol, (15
p
ꢀcresol, and (16) 3,5ꢀdimethylphenol. Column parameꢀ
ters: bis2MPyC9 20 m 0.22 mm 0.15 m, bis4MPyC6
25 m 0.22 mm 0.15 m.
b
×
×
µ
×
×
µ
The
e
parameter for
π
–
π
and
n
– interactions
π
ing to the contributions of the individual types of
interaction characteristics of the phases based on the
pyridinium ILs under study.
increases in proportion to the number of methyl
groups and depends slightly on their position in the
ring. The e parameter also increases with the length of
substituent of alkyl link in dication ILs. All this is
explained by the donor effect of the alkyl substituents
in the aromatic ring. This tendency was also observed
for imidazolium ILs [4, 18].
The table gives the parameters (our values and
those taken from [23]) for the phases based on ZBꢀ
MAX and HPꢀ5 for comparison. Let us analyze the
tabulated parameters corresponding to different types
of interaction. Note that the measured parameters of
HPꢀ5 and ZBꢀWAX are close to the corresponding litꢀ
erature values for phases of this type (HPꢀ5 and DBꢀ
FFAP, respectively).
Thus at close McReynolds parameters, ILs with
different positions of substituents can exhibit different
predominant types of interaction and hence have difꢀ
ferent selectivities with respect to different classes of
substances. Based on this principle one can choose
phases for particular analytical problems.
Let us consider the l parameter corresponding to
dispersion interactions. It is lower than for less polar
phases (HPꢀ5 and ZBꢀWAX) for all ILs. This is quite
understandable in view of poor retention of nonpolar
The tendencies observed in this study can be demꢀ
onstrated using as an example the separation of mixꢀ
tures of oxygenꢀcontaining compounds. Figure 2
shows the chromatograms of the separation of the test
mixture on columns with phases based on bis2MPyC9
and bis4MPyC6. The separation of polar compounds
observed for the bis4MPyC6 phase is better than for
bis2MPyC9. On column with bis4MPyC6 alcohols are
retained much better than methyl ethers of fatty acids.
This confirms the data of the table about the high
degree of dipole–dipole interactions for this IL (alcoꢀ
hols are more polar than methyl ethers). At the same
substances on ILs. The l parameter is proportional to
the number of methyl groups in the pyridinium ring
and to the length of the alkyl substituent (or link) in
the 1st position of the pyridinium ring. This increase in
the degree of dispersion interactions at higher numꢀ
bers of hydrocarbon fragments in the structure of the
stationary phase is characteristic not only of ILs, but
also of any stationary phases [4, 18, 23].
The dipole–dipole interaction parameter (s)
depends on the position of heterocyclic nitrogen atom. time, bis2MPyC9 showed better retention of hydroꢀ
The highest value is characteristic of 2,6MPy. The carbons, which is confirmed by the higher parameter
l
degree of dipole–dipole interactions decreases as the for it. Note the good quality of the peaks of phenols,
length of the alkyl substituents or links (in dication acid, and alcohols for the bis2MPyC9 phase, which
ILs) increases. This is explained by the increased size can be a problem for both traditional [24, 25] and
of the cation and hence decreased dipole moment of ILꢀbased phases [19].
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SELECTIVITY OF STATIONARY PHASES BASED ON PYRIDINIUM IONIC LIQUIDS
721
9. J. Joule and K. Mills, Heterocyclic Chemistry (5th ed.,
CONCLUSIONS
Wiley, 2010; Mir, Moscow, 2004).
Due to the combination of their physicochemical
properties, ionic liquids can be considered a good
alternative to polar polysiloxane and polyethyleneglyꢀ
col phases. Today there are several SLPs based on imiꢀ
dazolium and phosphonium ILs, which afford good
results in the separation of polar substances. The pyriꢀ
dinium ILs are also promising materials for preparing
qualitative capillary columns because they show good
selectivity with respect to various classes of polar comꢀ
pounds. By varying the position and length of alkyl
substituents in the structure of the cation of pyridinꢀ
ium ILs, one can regulate the occurrence of definite
types of dominant intermolecular interactions. This
makes it possible to produce columns in a wide range
of polarity and selectivity with respect to classes of
compounds to be determined.
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