Anisotropic ionic conductivities in lyotropic supramolecular liquid crystals

Article abstract of DOI:10.1039/b912472a

The designed aromatic amide discotic molecule with sulfonic acid groups at its periphery exhibits a hexagonal supramolecular columnar liquid crystalline phase, which leads to the achievement of anisotropic ionic conductivity through macroscopically aligni

Full text of DOI:10.1039/b912472a

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Anisotropic ionic conductivities in lyotropic supramolecular liquid
crystalsw
Youju Huang,a Yuanhua Cong,a Junjun Li,a Daoliang Wang,a Jingtuo Zhang,a Lu Xu,a
Weili Li,a Liangbin Li,*a Guoqiang Pana and Chuanlu Yang*b
Received (in Cambridge, UK) 24th June 2009, Accepted 22nd October 2009
First published as an Advance Article on the web 10th November 2009
DOI: 10.1039/b912472a
The designed aromatic amide discotic molecule with sulfonic
acid groups at its periphery exhibits a hexagonal supramolecular
columnar liquid crystalline phase, which leads to the achieve-
ment of anisotropic ionic conductivity through macroscopically
aligning the ionic channels.
shown in Part 1 of the ESIw). The discotic molecule exhibits a
hexagonal supramolecular columnar liquid crystalline phase in
the aqueous solution with a wide range of concentrations from
0.086 g mLÀ1 to 0.28 g mLÀ1 at room temperature.
The textures of the supramolecular liquid crystals were
microfibers as observed by Polarized Light Microscopy
(POM) (see S-Fig. 2 in the ESIw). The microfibers are
randomly cured without imposing any flow field (Fig. 1a),
but they are extremely easy to be aligned by flow (Fig. 1b).
Synchrotron-based small and wide angle X-ray scattering
(SAXS/WAXS) data provide essential evidence for the hexa-
gonal columnar phase. Neither birefringence from POM nor a
SAXS/WAXS signal was observed from the discotic molecule
solution with low concentration (lower than 0.086 g mLÀ1).
When the concentration is higher than 0.28 g mLÀ1, some
discotic molecules precipitate and form a heterogeneous
mixture of the hexagonal liquid crystalline phase and crystals
of the discotic molecule.
Ion transport through nanochannels or microchannels plays
an important role in cell membranes, biosensor/actuator,
molecular switches, biomedical materials, and energy materials
(proton and Li+ transport materials).1–7 Intense efforts have
been devoted to designing artificial ion channels in the past
decades. Combining the intrinsic molecular functions and
the anisotropic nature, supramolecular liquid crystals with
exquisite control over molecular structure are promising
candidates for preparing ordered ionic channels from the
molecular to macroscopic scale.8–12 For example, a columnar
liquid crystal self-organized by discotic molecules through
strong p–p stacking interactions is essentially a one-dimensional
electron conductor, while supramolecular liquid crystals
with dissociable ions are one or two dimensional ionic
conductors.13–16 The combination of fluidic properties and
orientation order of liquid crystals allows easy achievement of
macroscopic alignment of these conductive nanochannels.
Due to the targeting of technological application in the
bulk state, most reports focus on thermotropic rather than
lyotropic liquid crystals, though the latter share equal
importance.17–25 Indeed designing supramolecular lyotropic
liquid crystals can be a promising approach to create
macroscopically parallel proton nanochannels with superior
ionic conductivity like Nafion, 26,27 whose outstanding perfor-
mance stems from its parallel cylindrical water nanochannels.
Fig. 2A and B show a representative SAXS/WAXS pattern
with q as the x-axis from a sample with a concentration of
0.206 g mLÀ1. Here q = 4psiny/l is the module of the
scattering vector; l and 2y are the X-ray wavelength and
scattering angle, respectively. The major sharp scattering
peaks are located in the low q value region with a ratio of
1 : 31/2: 2 : 71/2 : 131/2. In the wide angle region, a single peak
with a large width is observed at a q value of 18.08 nmÀ1
,
which is orthogonal to the scattering peaks in the low q region
in the two-dimensional scattering pattern of the oriented
sample (S-Fig. 3, see ESIw). Thus we can conclude that the
structure is a lyotropic columnar liquid-crystalline phase with
hexagonal symmetry. The columns have a layered structure
with an average spacing of about 0.35 nm. Upon decreasing
the concentration of discotic molecules, the liquid crystals
Herein we report
a new organic discotic molecule
(Scheme 1) consisting of 10 phenyl rings joined symmetrically
with amide linkages based upon 1,3,5-benzenetricarbonyl
trichloride by a convergent synthetic approach, which is
functionalized at the periphery with sulfonic acid groups
(synthesis and characterization of the discotic molecule are
a National Synchrotron Radiation Lab and CAS Key Laboratory of
Soft Matter Chemistry, University of Science and Technology of
China, Hefei, China. E-mail: lbli@ustc.edu.cn;
Fax: 0086-551-5141078; Tel: 0086-551-3602081
b Department of Physics and Electrons, Ludong University, Yantai,
China. E-mail: yangchuanlu@263.net
w Electronic supplementary information (ESI) available: Materials,
measurements, synthesis of the discotic molecule, SAXS/WAXS
pattern, polarized optical photomicrographs of the discotic molecule
with different concentrations in water, computational details and the
measurement of ionic conductivities. See DOI: 10.1039/b912472a
Scheme 1 The chemical structure of the discotic molecule.
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molecules to form thermodynamically stable clusters with
finite size in the solution due to the interplay between the
attraction energy among discotic molecules and the Coulomb
interaction together with the contribution from the
translational entropy of counter ions. The clusters formed by
the multiple discotic molecules stack into columns of the
hexagonal liquid crystalline phase (S-Fig. 4c and d, see ESIw).
Based on the layer spacing of 0.35 nm from WAXS data,
which is close to the p–p stacking distance, the main driving
force for the formation of columnar structures can be p–p
interactions between the phenyl rings. This is reasonable as
hydrogen bonds can form between water and the amide
groups (see Part 5 and S-Fig. 8 of the ESIw).
Fig. 1 Polarized optical micrographs of the discotic molecule with a
concentration of 0.206 g mLÀ1 (a) randomly cured microfibers and
(b) aligned microfibers by flow field.
keep their hexagonal symmetry with an increase in the d
spacing calculated from the (100) peak position (see Fig. 2C
and D), which covers a range from 9.04 nm to 10.29 nm,
corresponding to a concentration range from 0.28 g mLÀ1 to
0.086 g mLÀ1. Two striking features were observed on the
discotic columnar liquid crystals: (i) The spacing of the (100)
plane is several times larger than the diameter of the discotic
molecule; (ii) The spacing of the (100) plane keeps relatively
constant in two concentration regions.
The formation of proton channels in the hexagonal liquid
crystalline phase is supported by experimental data (the X-ray
data and ionic conductivity) and computer simulation. The
SAXS/WAXS data show that the diameter of the columns in
the hexagonal liquid crystals is about three times that of the
discotic molecule, where each layer of the column contains
multiple molecules instead of one. Combining the symmetry of
sulfonic acid groups at the periphery of the discotic molecule,
it is easy to construct a molecular picture that the sulfonic acid
groups in the columns tend to self-organize into two regions.
One is at the outer surface of the column, while another locates
inside the column. Computer simulation shows that each layer
of the column is composed of 7 discotic molecules, which leads
us to propose a two-coaxial-proton-nanochannel model. The
inner nanochannel is circumambient at the periphery of the
interior discotic molecule, while the outer one surrounds
the columns of the hexagonal supramolecular liquid crystalline
phase (see Part 2 and S-Fig. 4b, c and d, of the ESIw). As the
outer channels are connected with each other, they are an open
ionic channel. Thanks to the novel layer structure of the
column with 7 discotic molecules, the inner channels were
isolated by the phenyl rings, which are relatively closed
ionic nanochannels. This novel structure is expected to have
anisotropic ionic conductivity after macroscopic alignment.
The anisotropic ionic conductivities with different
concentrations of the discotic molecule are confirmed by the
measurements on the samples with macroscopically oriented
ionic nanochannels, which are obtained by imposing a simple
shear. Illustrations of cells for the measurements of ionic
conductivity and the orientation of liquid crystals checked
by the POM are shown in S-Fig. 6 of the ESI.w The experi-
mental details are provided in S-Fig. 7 and Part 3 of the ESI.w
Fig. 3 illustrates the anisotropic ionic conductivities of the
columnar liquid crystalline phase as a function of concentration.
The ionic conductivities in the direction parallel to the
columnar axis (sJ) are significantly higher than those in the
direction perpendicular to the columnar axis (s>), while
the un-aligned samples have ionic conductivities that lie
between them. For example, the discotic molecule solution
exhibits conductivities of 8.5 Â 10À3 S cmÀ1 and 1.46 Â
The diameter of the discotic molecule is close to 3 nm
(S-Fig. 4a, see ESIw), while the intercolumnar distance in the
hexagonal supramolecular liquid crystalline phase covers a
range from 10.44 nm to 11.88 nm. Evidently the cross section
or the layer of the columns is constructed with multiple
molecules instead of a single discotic molecule. It is normal
for multiple fan-shaped molecules to form one disc, which
stacks into columns and further organizes into a liquid
crystalline phase. However disc-shaped molecules generally
form single molecular stacks. As far as the authors are aware
no exceptions have been reported because disc-shaped
molecules possess the right symmetry to stack into columns
directly. According to the earlier proposed theory28,29 of phase
equilibrium in the solution of associating polyelectrolyte and
charged colloid particles, it is possible for the charged discotic
Fig. 2 SAXS (A) and WAXS (B) patterns of the discotic molecule
solution with a concentration of 0.206 g mLÀ1. (C) The first peak of
the SAXS/WAXS pattern of the discotic molecule with different
10À3
S
cmÀ1 in parallel and perpendicular directions,
respectively, which lead to a ratio sJ/s> of 5.8 at a
concentration of 0.19 g mLÀ1. At low concentration where
no liquid crystal phase forms, no anisotropic ionic conductivity
is observed on the sheared samples, whose ionic conductivity
drops sharply. The anisotropic ionic conductivities of the
concentrations in water: (a) 0.280 g mLÀ1; (b) 0.256 g mLÀ1
(c) 0.221 g mLÀ1; (d) 0.206 g mLÀ1; (e) 0.176 g mLÀ1; (f) 0.158 g mLÀ1
(g) 0.114 g mLÀ1; (h) 0.108 g mLÀ1; (i) 0.087 g mLÀ1; (j) 0.086 g mLÀ1
;
;
.
(D) The plot of d spacing of the (100) peak of the SAXS pattern vs.
concentration.
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This journal is The Royal Society of Chemistry 2009
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NNSFC (20774091), Fund for one hundred talent scientist, the
‘NCET’ program, 973 program from MOST and the experi-
mental fund of NSRL.
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Fig. 3 Ionic conductivities of the liquid crystal phase (to the right of
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which exhibits a hexagonal supramolecular columnar liquid
crystalline phase in aqueous solution. Unexpectedly the
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of the hexagonal phase keeps relatively constant in two
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LB Li would like to thank Prof. Wim de Jeu (AMOLF) and
Prof. Stephen. J. Picken (Delft) for their great help in building
the soft matter group in Hefei. This work is supported by the
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