Gas chromatographic separation of structural isomers on cholesteric liquid crystal stationary phase materials

Article abstract of DOI:10.14233/ajchem.2013.13927

A new series of liquid crystalline polysiloxanes with a very wide range of mesomorphic temperatures had been prepared. They contained 4-cholesteryl-(10- undecen-1-yloxy)biphenyl-4′-carboxylate mesogenic and 4-biphenyl-4′- allyloxybenzoate mesogenic side groups displaying an enantiotropic cholesteric phases. The mesomorphic polysiloxane specimen with the widest temperature range was used as the stationary phase in a gas chromatography capillary column and it showed high column efficiency and made great promise for the separation of different structure isomer compounds.

Full text of DOI:10.14233/ajchem.2013.13927

Asian Journal of Chemistry; Vol. 25, No. 8 (2013), 4251-4255
http://dx.doi.org/10.14233/ajchem.2013.13927
Gas Chromatographic Separation of Structural Isomers on
Cholesteric Liquid Crystal Stationary Phase Materials
3,*
YI-WEN LEE1,2 and CHIH-HUNG LIN
1Department of Nursing, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan
2Department of Nursing, Chang Gung University of Science and Technology, 261 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan, Taiwan
3Center for General Education, Chang Gung University of Science and Technology, 261 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan, Taiwan
*Corresponding author: Fax: +886 3 2118866; Tel: +886 3 2118999; E-mail: chlin@gw.cgust.edu.tw
(Received: 18 April 2012;
Accepted: 7 February 2013)
AJC-12938
A new series of liquid crystalline polysiloxanes with a very wide range of mesomorphic temperatures had been prepared. They contained
4-cholesteryl-(10-undecen-1-yloxy)biphenyl-4′-carboxylate mesogenic and 4-biphenyl-4′-allyloxybenzoate mesogenic side groups
displaying an enantiotropic cholesteric phases. The mesomorphic polysiloxane specimen with the widest temperature range was used as
the stationary phase in a gas chromatography capillary column and it showed high column efficiency and made great promise for the
separation of different structure isomer compounds.
Key Words: Isomer, Polysiloxanes, Cholesteric, Liquid crystal, Capillary column.
Many liquid crystal polymers of nematic or smectic phase
had been reported as stationary phases8,9. Cholesteric liquid
crystals had also attracted particular interest in their unusual
helical supermolecular structure, which could be used as a
stationary phase in gas chromatography. Therefore, to take
advantage of all these properties and to prepare a gas chromato-
graphy stationary phase with optimum separation properties,
we selected polysiloxane as the polymer backbone and used
an 11-methyl unit as the spacer and modified this backbone
with a new series of cholesteric liquid crystalline polymers.
A cholesteric mesophase could be realized by changing
the composition of mesogens attached onto a polysiloxane
backbone. These materials tended to form cholesteric meso-
phases over a broad temperature range. For the preparation of
novel stationary phases in gas chromatography, we selected
the cholesteric mesophase polymers with the widest range of
temperatures10,11. This study selected nine kinds of isomers,
contained four kinds of volatile isomers, these isomers were
used for separation on liquid crystal phase column. The choles-
teric polysiloxane described herein had merit and had been
used to separate isomer compounds successfully.
INTRODUCTION
Liquid crystals had been used as stationary phases1-3 in
gas chromatography (GC) to separate a variety of compounds
including isomeric mixtures which could not be separated on
conventional stationary phase. Conventional stationary phase
separation of analytes is based on differences in vapour pressures
of the solutes and differences in solubility arising from specific
energetic interactions. A liquid crystal stationary phase sepa-
rates analytes based upon differences in solute molecular shape,
with the anisotropic packing behaviour of liquid crystalline
materials permitting the separation of isomers based on their
individual molecular geometries (length-to-breadth)4.
Finkelmann and Rehage5 were the first to synthesize
thermotropic side-chain liquid crystalline polymers. Since
then, nematic, smectic and cholesteric liquid crystalline
polymers and elastomeric liquid crystalline networks containing
a polysiloxane backbone6,7 had been synthesized. Polysiloxane
had been used as the backbone for the side-chain liquid crysta-
lline polymers because of its properties of low glass transition
temperature and high thermostability. Stationary phases based
on low molecular weight liquid crystals gain substantial
efficiency when they were attached onto flexible polymer
backbones. A flexible led group between the backbone and
mesogenic unit allowed the resulting polymers to retain liquid
crystalline properties. Polymeric stationary phases containing
liquid crystalline substituents were desirable for their high
thermal stability.
EXPERIMENTAL
1H NMR spectra were recorded on a Varian VXR-300 or
Bruker 300 MHz spectrometer. Thermal transitions and
thermodynamic parameters were determined by using a Seiko
SSC/5200 differential scanning calorimeter (DSC) equipped

4252 Lee et al.
Asian J. Chem.
with a liquid nitrogen cooling accessory. Heating and cooling
rates were 10 ºC/min. Thermal transition reports were collected
during the second heating and cooling scans. A SEIKO TG/
DTA 200 thermogravimetric analyzer (TGA) determined
thermal decomposition temperatures.A Nikon Microphot-FX
optical polarized microscope was equipped with a Mettler FP
82 hot stage and a FP 80 central processor was used to observe
the thermal transitions and to analyze the anisotropic textures.
Polymerization reactions were traced by using a Nicolet 520
FT-IR.
O
C
O
C
HO
+
CH3O
Cl
OH
NaOH/H2O
SOCl2
H
O
C
O
C
CH3O
O
OH
The gas chromatograph used was always a Hewlett
Packard 5890 Series II instrument equipped with a capillary
column, split injection system and a FID detector. The carrier
gas was N2.
O
C
O
C
CH3O
O
Cl
Synthesis of monomers M1-M2: The molecular structure
and the general synthetic procedures of these compounds were
shown in Schemes I-II and all the final products were examined
by the nuclear magnetic resonance spectrometer in order to
verify the correction of the molecular structure. The synthesis
of monomers 4-biphenyl 4′-allyloxybenzoate (M1) and [S]-
1-(2-naphthyl)ethyl 6-[4-(10-undecen-1-yl-oxy)biphenyl-4'-
carbonyloxy]-2-naphthoate (M2) were detailed synthetic
procedures for the intermediary compounds and monomers
were similar to those reported previously12. lH NMR chemical
shifts of M1 were 4.65(t,2H,-O-CH2-); 5.48 (m, 2H, =CH2);
HO
C
CH3
Pyridine/CH2Cl2
O
C
O
C
H
CH3O
O
O
C
OH
NH3/EtOH
O
H
HO
C
O
C
l
CH3
6.07(m, 1H, =CH-); 7.03-8.19 (m,13 aromatic protons). H
NMR chemical shifts of M2 were 1.24-1.77 (m, 17H, -CH2-
and -CH3), 1.98 (m, 2H, =CH-CH2-), 3.95 (t, 2H, -O-C-CH2-
C=), 4.78-4.95 (m, 2H, =CH2), 5.74 (d, 1H, =CH-), 6.32 (q,
1H, -C*H-),6.93-8.62 (m, 21 aromatic protons).
O
CH2
CH
(CH2)9
O
C
OH
SOCl2
Pyridine/CH2Cl2
O
C
O
C
H
CH2
CH
(CH2)9
O
O
O
O
C
CH2
CH CH2
Br
+
HO
C
OH
CH3
Scheme-II: Synthesis of monomers M2
KOH/EtOH
nitrogen for 2 h. After this reaction time, the FT-IR analysis
showed that the hydrosilation reaction was complete. The
polymers were separated and purified by several reprecipitations
from methylene chloride solution into methanol and then dried
under vacuum.
O
C
CH2
CH CH2
O
OH
SOCl2
Preparation of the column: A deactivated fused silica
capillary column with an I.D. 0.32 mm × 30 m (Restek) was
used. The capillary column was washed with methylene chlo-
ride (20 mL) before coating. The stationary phase (31.3 mg of
copolymer and 1 mg of dicumyl peroxide) was dissolved in
methylene chloride (10 mL). The solution was placed in a
rinsing reservoir and forced through the capillary column by
N2 gas. After filling, the column was sealed at one end and
was placed in a 40 ºC water bath. The column was then placed
under vacuum and the solvent evaporated, completing the static
coating procedure. The sealed end of the column was opened.
The column was cross-linked in a gas chromatography oven,
with an oven temperature program of 40-200 ºC at 4 ºC/min
and maintained at 200 ºC for 6 h. The column was cleaned
with 10 mL of methylene dichloride using N2 carrier gas. Fi-
nally, the column was installed on a gas chromatography ap-
paratus and conditioned at 200 ºC overnight.
O
CH2
CH CH2
O
C
Cl
+
HO
Pyridine/CH2Cl2
O
CH2
CH CH2
O
C
O
Scheme-I: Synthesis of monomers M1
Synthesis of copolysiloxanes P1-P3:All copolysiloxanes
P1-P3 were synthesized by the hydrosilylation of poly (methyl
hydrogen siloxane) with different ratios of both monomers in
the presence of a platinum divinyl tetramethyl disiloxane
catalyst. The reaction mixture was refluxed (75 ºC) under

Vol. 25, No. 8 (2013)
RESULTS AND DISCUSSION
Gas Chromatographic Separation of Structural Isomers 4253
column. The separation result was listed in Table-2. Most of
good separation of the nine kinds of isomers on the liquid
crystal phase column was achieved with the exception of
o-xylene, m-xylene, p-xylene and thymol and carvacrol. Table-2
contains the accompanying information that summarizes the
relative data of the retention time derived from optimized tempe-
rature programs, compounds, boiling points and separation
factor (α) for the isomers on liquid crystal phase column.
This liquid crystal phase column had the good separation
effect on ethylbenzene and o-xylene, but m-xylene and p-xylene
were actually difficult to separate, because their boiling points
were close (m-xylene = 139.1 ºC, p-xylene = 138.4 ºC) and
their molecular shapes were very similar. Trans-decalin and
cis-decalin isomer compounds had good separation results (α
= 1.261), as shown in Fig. 2, because their molecular structures
are obviously different.
We obtained three polymers (P1-P3). The thermal and
mesomorphic properties of polymers P1-P3 were measured
using differential scanning calorimeter and polarized optical
microphotograph. All the synthesized polymers in this study
showed liquid crystalline behaviour. The thermal transition
temperature and thermodynamic parameter of the polymers
were summarized in Table-1. Table-1 summarizes the thermal
transitions, ratio and structure of obtained polymers P1-P3,
which presented an enantiotropic cholesteric phase. Fig. 1
presented the polarized optical microphotograph of P2, showed
finger print texture of cholesteric phase at 102.3 ºC. Polymer
P1-P3 exhibited the wide range of temperatures for the choles-
teric mesophase. Polymer P3 showed the widest range of
temperatures for the cholesteric mesophase, a glass transition
at 53.2 ºC and a cholesteric to isotropic phase transition at
234.8 ºC upon DSC heating scan. In the cooling scan, the
isotropic to cholesteric phase transition was presented at
225.4 ºC. Polymer P3 showed a wide temperature range of
the cholesteric phase and the high thermal stability.All of these
factors indicated that polymer P3 was well suited to be a gas
chromatography stationary phase, thus we selected this new
material for all further studies. The capillary column coated
with P3 polymer was termed the liquid crystal phase column.
TABLE- 1
THERMAL TRANSITIONS AND RATIOS AND
STRUCTURES OF COPOLYMERS
Monomer feed ratio
M1/M2 (mol %)
Phase transitions C,
heating scan cooling scan
Polymer
P1
P2
P3
10/90
20/80
30/70
g 66.0 N*, 196.6 I, I 185 N*
g 54.1 N*, 213.2 I, I 209.4 N*
g 53.2 N*, 234.8 I, I 225.4 N*
Fig. 2. Chromatogram of the trans-decalin (1), cis-decalin (2) isomers
This study selected three kinds of isomers of polycyclic
aromatic compounds. These three kinds of isomers obtained a
good separation effect on the LCP column. The isomers of
C14H10 (anthracene and phenanthrene) had a separation factor
(α = 1.225). C16H10 (fluoranthene and pyrene) isomers had a
α = 1.142. C18H12 (triphenylene, benz[a]-anthracene and
chrysene) isomers had a good separation effect. The isomers
were difficult to separate by commercial capillary column
chromatography13. We found that the LCP column showed
better resolution for the above isomeric compounds (Table-2).
This molecular structure might especially confer on the bene-
ficial effects of the stationary phase for separation of compounds
with different L/B dimensions. The elution pattern of isomers
solutes in this study by the mesomorphic polysiloxane was
Fig. 1. Typical optical micrograph of polymer P2, finger print texture,
102.3 ºC, magnification 200x
The liquid crystal polymer in the cholesteric phase
especially had an unusual helical supermolecular structure.
This structure showed many similarities to the nematic phase,
but every layer showed a twist in the angle. This molecular
structure may especially confer on the beneficial effects of
the stationary phase for separation of compounds with different
length/breadth (L/B) dimensions. This study, selected nine
kinds of isomers and contained four kinds of volatile isomers,
these isomers were used for separation by liquid crystal phase

4254 Lee et al.
Asian J. Chem.
TABLE- 2
RETENTION TIMES OF ISOMERS ON THE LCP COLUMN, THEIR BOILING POINTS, AND THEIR SEPARATION FACTOR
Compounds
b.p. (ºC)
Operate temperature
C8H10
Retention time (min)
α
Ethylbenzene
136.3
144.4
139.1
138.4
4.249
4.634
4.634
4.634
1.000
1.345
1.345
1.345
o-Xylene
m-Xylene
p-Xylene
60 ºC isocratic
C10H18
trans-Decalin
cis-Decalin
187.3
195.7
5.373
6.154
1.000
1.261
30 ºC hold 5 min, then 60
ºC/min to 80 ºC
C14H10
170 ºC isocratic
C16H10
Phenanthrene
anthracene
340
342
6.641
7.798
1.000
1.225
Fluoranthene
pyrene
384
404
19.693
22.213
1.000
1.142
100ºC hold 3 min, then 20
ºC/min to 200 ºC
C18H12
210 ºC isocratic
C10H16
Triphenylene
Benz[a]-anthrancene
Chrysene
425
437.6
448
11.500
12.252
13.764
1.000
1.076
1.230
α-Pinene
β-Pinene
155
164
5.145
5.530
1.000
1.446
30 ºC hold 1 min, then 20
ºC/min to 120 ºC
C10H14O
Carvone
Thymol
Carvacrol
230
233
237
10.212
10.212
13.504
1.000
1.423
1.423
40 ºC hold 1 min, then 20
ºC/min to 100 ºC
C10H18O
Eucalyptol
Borneol
Geraniol
174
206
229
3.894
6.629
8.409
1.000
2.796
3.963
30 ºC hold 1 min, then 20
ºC/min to 120 ºC
C10H12O2
Eugenol
cis-Isoeugenol
255
266
5.681
7.483
1.000
1.546
40 ºC hold 5 min, then 20
ºC/min to 100 ºC
α: Separation factor.
consistent with the degree of their rod-like geometry, with the
more rod-like structures being retained longer. For example,
phenanthrene (L/B = 1.459) eluted earlier than anthracene (L/
B = 1.559), the retention time of fluoranthene (L/B = 1.142,
b.p. = 384 ºC) was relatively short, because boiling point of
fluoranthene was lower by 20 ºC than pyrene (L/B = 1.119,
b.p. = 404 ºC). The isomers of triphenylene (L/B = 1.120),
benz[a]-anthracene (L/B = 1.580) and chrysene (L/B = 1.720)
had the good separation effect on LCP column (Fig. 3). When
the separation of retention time of the triphenylene was shortest,
the benz[a]anthracene was the best and chrysene was the longest.
The result and the molecular length and breadth were compared
to related values, when length and breadth were compared to
the big value, the retention time would be longer.
In this study, we selected four kinds of volatile isomers.
These four kinds of isomers obtained a good separation effect
on LCP column. The isomers of C10H16 (α-pinene, β-pinene)
had a good separation effect on LCP column (α = 1.446).
Although they had a little different boiling points (α-pinene,
b.p. = 155 ºC; β-pinene, b.p. = 164 ºC) but their structures had
different duplet bond positions (α-pinene's duplet bond is in
six ring; β-pinene's duplet bond is outside six ring).
C10H14O (carvone, thymol, carvacrol) isomers, carvone
had the good separation effect, thymol and carvacrol's separa-
tion effect were not good. Because the separation effect was
Fig. 3. Chromatogram of the triphenylene (1), benz[a]anthracene (2),
chrysene (3) isomers

Vol. 25, No. 8 (2013)
Gas Chromatographic Separation of Structural Isomers 4255
influenced by the compound’s boiling point and the shape of
molecule, these isomers had slightly different boil points
(carvone b.p. = 230 ºC, thymol b.p. = 233 ºC, carvacrol b.p. =
237 ºC) and they had different structure arrangement. Carvone
molecular structure belonged to the non-plane six-ring
structure, but thymol and carvacrol belonged to the planar
molecule structures. Therefore, carvone compound had shorter
retention time than thymol and carvacrol did. Thymol and
carvacrol could not separate on LCP column.
which was a carbon composition, its' structure is like a spheroid.
Geraniol molecular structure was a straight chain molecule
and its' structure is like clavate molecule. Liquid crystal compound
as stationary phase materials in GC, especially had the good
separation effect on the shapes of different isomers. The
retention time of the clavate-shaped type of the molecule would
be longer than the retention time of the spherical type of the
molecule, therefore eucalyptol was eluted earlier than borneol
was and borneol was eluted earlier than geraniol was. C10H12O2
(eugenol, cis-isoeugenol) isomers had a good separation effect
on LCP column (α = 1.546). Thus, the LCP column showed
the good resolution of nine kinds of isomers.
In Fig. 4, eucalyptol, borneol and geraniol (C10H18O)
isomers had the good separation effect on LCP column.
Eucalyptol molecular structure was six-membered-ring and
included a bridge, which was a carbon and an oxygen atom
composition, therefore its' structure is like a spherical. Borneol
molecular structure was six-membered-ring and had a bridge,
Conclusion
The side-chain liquid crystalline polysiloxanes with wide
temperature ranges of cholesteric nematic phase had been
proven to be useful in separating many classes of compounds.
The obtained polymers showed high thermal stability and had
wide temperature ranges associated with the cholesteric liquid
crystal phase. These results might be shown due to the twisted
packing structure of the cholesteric mesophase exhibited by
the new stationary phases. Because the separation was based
on the molecular shape, isomers that had very similar intrinsic
properties could be separated by these kinds of mesomorphic
polymer stationary phases. The prepared column described
here displayed high column efficiency and made great promise
for the separation of a wide range of isomer compounds.
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Fig. 4. Chromatogram of the eucalyptol (1), borneol (2), geraniol (3)
isomers