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Langmuir 2004, 20, 2772-2780
Langmuir Monolayers of Bent-Core Molecules
Lu Zou,† Ji Wang,† Violeta J. Beleva,† Edgar E. Kooijman,†,⊥
Svetlana V. Primak,†,| Jens Risse,‡ Wolfgang Weissflog,‡ Antal Jákli,§ and
Elizabeth K. Mann*,†
Department of Physics, Kent State University, Kent, Ohio 44242-0001, University Halle
Wittenberg, Institute of Physical Chemistry, D-06108 Halle Saale, Germany, and Liquid
Crystal Institute, Kent State University, Kent, Ohio 44242-0001
Received November 21, 2003. In Final Form: January 20, 2004
A systematic study of five different, symmetric bent-core liquid crystals in Langmuir thin films at the
air/water interface is presented. Both the end chains (siloxane vs hydrocarbon) and the core (more or less
amphiphilic) are varied, to allow an exploration of different possible layer structures at the interface. The
characterization includes systematic surface pressure isotherms, Brewster angle microscopy, and surface
potential measurements. The properties of these layers are strongly dependent on the individual type of
molecule: the molecules with amphiphilic end chains lie quite flat on the surface, while the molecules with
hydrophobic end chains construct multilayer structures. In both cases, the three-dimensional collapse
structure is reversible.
Introduction
Bent-core or banana-shaped molecules exhibit a rich
variety of phases,including many liquid-crystalline ones.1
At minimum, a rank-three order parameter as well as the
vector and the nematic tensor order parameters are
necessary to completely describe this web of phases.2 A
dozen different liquid phases2 and five smectic phases3
have been suggested. Eight phases have been identified,
but most have not been fully characterized.1 Several of
these phases are smectic phases in which packing of the
bent molecules leads to polar ordering. Because of the
ordering, achiral bent-core molecules can demonstrate
chirality and (anti)ferroelectricity.4 The usefulness of bentcore molecules in scattering switching and in storage
devices has been demonstrated.5 It has also been suggested
that the unique properties of these molecules can make
them useful for electromechanical devices.6
The order in Langmuir monolayers, which are molecular
layers self-confined at the air/water interface, shows a
one-to-one correspondence to that in liquid crystal phases.7
Several molecules that form liquid crystals in bulk have
been shown to form stable Langmuir monolayers.8-11 The
richness of the phase diagram of bent-core molecules is
* Corresponding author: E-mail: [email protected]. Telephone: 330-672-9750. Fax: 330-672-2959.
† Department of Physics, Kent State University.
‡ University Halle Wittenberg.
§ Liquid Crystal Institute, Kent State University.
⊥ Present Address: Utrecht University, CBLE, Biochemistry of
Membranes, H.R. Kruytgebouw, Padualaan 8, 3584 CH Utrecht,
Netherlands.
| Present Address: Pacific Northwest National Laboratory,
EMSL, K8-98, Battelle Boulevard, P.O. Box 999, Richland, WA
99352.
(1) Pelzl, G.; Diele, S.; Weissflog, W. Adv. Mater 1999, 11, 707.
(2) Lubensky, T. C.; Radzihovsky, L. Phys. Rev. E. 2002, 66, 031704.
(3) Brand, H. R.; Cladis, P. E.; Pleiner, H. Eur. Phys J. B 1998, 6,
347. Roy, A.; Madhusudana, N. V.; Toledano, P.; Figueiredo Neto, A.
M. Phys. Rev. Lett. 1999, 82, 1466.
(4) Niori, T.; Sekine, T.; Watanabe, J.; Furukawa, T.; Takezoe, H. J.
Mater. Chem 1996, 6, 1231. Sekine, T.; Niori, T.; Sone, M.; Watanabe,
J.; Choi, S. W.; Takanishi, Y.; Takezoe, H. Jpn. J. Appl. Phys. 1997, 36,
6455. Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.;
Korblova, E.; Walba, D. M. Science 1997, 278, 1924.
(5) Jákli, A.; Krüerke, D.; Sawade, H.; Chien, L. C.; Heppke, G. Liq.
Cryst. 2002, 29, 377.
(6) Jákli, A.; Krüerke, D.; Nair, G. G. Phys. Rev. E 2003, 67, 051702.
(7) Knobler, C. M. Mol. Cryst. Liq. Cryst. 2001, 364, 133.
expected to carry over into the Langmuir monolayer. The
Langmuir layer can give insight into the molecular packing
within layers, particularly in the presence of an interface.
Bent-core molecules that show smectic ordering, even if
only at higher temperatures, may be expected to form
reversible collapsed layers. A stable Langmuir layer,
transferred to a solid interface, may form a natural
alignment layer for bent-core liquid crystals. It has been
very difficult to align bent-core molecules, and many of
the zero-field characteristics have had to be deduced from
quite inhomogeneous films. Studies of Langmuir layers
may thus help clear up structural questions about bentcore liquid crystals in two ways: directly, by what can be
deduced from the structure of the Langmuir layers
themselves, and indirectly, through the possibility of
providing a suitable alignment layer to produce more
homogeneous, thicker films.
The purpose of this work is the characterization of the
Langmuir mono- and multilayers for a range of bent-core
molecules, varying both the core and the end chains but
maintaining molecular symmetry, with identical end
chains on either end of the core. The characterization
includes systematic surface pressure isotherms, Brewster
angle microscopy, and surface potential measurements.
To our knowledge, the only work on Langmuir monolayers
of such molecules in the literature considers two distinct
cases. The first considers a single bent-core molecule,
similar to one of those we consider here but with longer
end chains.10 That work demonstrated that Langmuir and
Langmuir-Blodgett films could be formed from this
molecule. Surface pressure isotherms suggested films
more than one molecule thick at even the lowest measurable pressures. Details of the molecular distribution within
the layer were unclear. However, the authors demonstrated that the molecular orientation distribution function could be well-characterized by second harmonic
generation.10 The other work on Langmuir monolayers of
(8) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R.
Langmuir 1994, 10, 1251.
(9) De Mul, M. N. G.; Mann, J. A. Jr. Langmuir 1994, 10, 2311. De
Mul, M. N. G.; Mann, J. A. Jr. Langmuir 1998, 14, 2455.
(10) Kinoshita, Y.; Park, B.; Takezoe, H.; Niori, T.; Watanabe, J.
Langmuir 1998, 14, 6256.
(11) Xue, Q.; Yang, K.; Xiao, C.; Zhang, Q. Thin Solid Films 1999,
347, 263.
10.1021/la0361924 CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/03/2004
Langmuir Monolayers of Bent-Core Molecules
Langmuir, Vol. 20, No. 7, 2004 2773
Figure 1. Molecular formulas and space-filling models for the different bent-core molecules used in this work. (a) The bent-core
molecules with hydrocarbon end chains R1 and core substituent R2, as given in Table 1. (b, c) Formulas for the siloxane end-chain
molecules, Bc2-SiO and Bc3-SiO, respectively. (d-f) Space-filling models for Bc-H, Bc2-SiO, and Bc3-SiO, respectively. Black atoms
are oxygen. The models were produced with Spartan ′02, Wavefunction, Inc.; the minimum energy configurations are determined
for single isolated molecules with the semiempirical module.
bent-core molecules considers two different cores with very
short hydrophobic side chains.12 Langmuir-Blodgett film
of these molecules formed simple, well-aligned monolayers,
as demonstrated through X-ray reflectivity, secondharmonic generation, and molecular rectification.
A molecule which forms thermotropic liquid crystals
typically consists of two types of regions: (i) a core that
is sufficiently rigid that entropy minimization will align
molecules when dense packed and (ii) end chains that are
sufficiently long and flexible to fluidize dense-packed
states. In the bent-core case, the core may take a number
of different nonplanar chiral configurations, but when
isolated it is on average planar and bent.2
The interaction of such molecules with a water surface
depends on the details of the molecule. Benzene itself is
amphiphilic, spreading on water;13 any additional carboxyl
and amide groups in the core will make it more hydrophilic.
Groups that are more or less hydrophobic or hydrophilic
may be substituted at different positions on the core, which
may change the preferred orientation of the core on the
water surface. The hydrophobicity of the fluidizing chains
may also be varied.
In this work, we vary the hydrophobicity of both the
core and the end chains. Two molecules in which the
fluidizing chains are terminated with amphiphilic siloxane
groups and three molecules with pure hydrocarbon end
chains are considered. The siloxane molecules have cores
with different numbers of benzene rings. In both cases,
the amphiphilic siloxane chains might be expected to help
tether the molecule quite flat on the surface. The
hydrophobic hydrocarbon chains, on the other hand,
should lead to more upright molecules. These molecules
have different groups substituted at the inner angle of
the central benzene ring of the core, which can affect both
configuration and stability of the molecule at the surface,
as well as the way in which these molecules stack to form
a multilayer.
(12) Ashwell, G. J.; Amiri, M. A.; Mater, J. J. Mater. Chem. 2002, 10,
2181. Baldwin, J. W.; Amaresh, R. R.; Peterson, I. R.; Shumate, W. J.;
Cava, M. P.; Amiri, M. A.; Hamilton, R.; Ashwell, G. J.; Metzger, R. M.
J. Phys. Chem. B 2002, 106, 12158.
(13) Harkins, W. D. The Physical Chemistry of Surface Films;
Reinhold Publishing Corp.: New York, 1952.
Table 1. Identification and Bulk Phase Properties of
Bent Core Molecules Used
Figurea identifier
R1O
R2
H
phase sequence in bulk
1a,d
Bc-Hb
C8H17O
1a
1a
1b, e
Bc-NO2c
Bc-CH3d
Bc2-SiOe
1c, f
Bc3-SiOf
C9H19O
NO2
C8H17O
CH3
(CH3)3SiO(CH3)2Si(CH2)3O
(CH3)3SiO(CH3)2Cr 69 Ih
Si(CH2)3O
B4 139.7 B3 151.9 B2
173.9 Ig
Cr 116 B2 177 Ig
Cr 157 B5 163 B2 168 Ig
Cr 93 Ih
a Figures showing molecule structures. b Chemical name according to IUPAC nomenclature: 1,3-phenylene bis[4-(4-n-octyloxyphenyliminomethyl)benzoate]. c 2-Nitro-1,3-phenylene bis[4-(4n-nonyloxyphenyliminomethyl)benzoate]. d 2-Methyl-1,3-phenylene bis[4-(4-n-octyloxyphenyliminomethyl)benzoate]. e 4,4′-Bis[3(pentamethyldisiloxanyl)propoxy]benzophenone. f 1,3-Phenylene bis{4-n-[3-(pentamethyldisiloxanyl)propoxy]benzoate}. g From
ref 1. h From ref 14.
Experimental Methods
Five different molecules were used in this study, three
with hydrocarbon end chains1 (Figure 1a) and two with
siloxane end chains (Figure 1b, c).14 The bulk properties
for these molecules, with the nomenclature used to identify
the different molecules, are summarized in Table 1.
Chemical structures are shown in Figure 1. The preparation of the compounds Bc2-SiO and Bc3-SiO was performed
by a hydrosilylation reaction of the terminal unsaturated
starting materials 1,3-phenylene bis(4-allyloxybenzoate)
and 4,4′-diallyloxybenzophenone, respectively, with pentamethyldisiloxane in the presence of the platinum
catalyst (PtCl2(C5H5)2).14 The estimated purity of all the
bent-core molecules was >98%.
The molecules were deposited onto a pure water surface
with spreading solutions. Water was purified with a
Purelab Plus UV system (US Filter, resistivity 18.2 MΩ/
cm). Hexane (Fisher, OPTIMA grade) was the spreading
solution solvent for the molecules with siloxane end chains
(Bc2-SiO and Bc3-SiO), while chloroform (Aldrich, A.C.S,
HPLC grade) was the solvent for the molecules with
hydrocarbon end chains (Bc-H, Bc-NO2, and Bc-CH3),
(14) Risse, J. Ph.D. Thesis, Martin Luther University Halle-Wittenberg, 1999.
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Langmuir, Vol. 20, No. 7, 2004
Zou et al.
which did not dissolve in hexane. Typical concentrations
in the solvent were ∼1 mg/mL. Surface tension isotherms
were measured with the KSV (KSV, Finland) minitrough
system. The original commercial Teflon trough was
replaced with a homemade one of the same size, which
performed well in tests against leakage around the KSV
hydrophilic barriers.15 The surface pressure measurements were performed via the Wilhelmy method.
Surface potential measurements were performed with
the KSV SPOT1 surface potential meter, which works via
the vibrating plate capacitor method.16 The probe head
diameter was 17 mm. NaCl, 1 mM (Fisher, certified ACS
grade), was added to increase the conductivity of the pure
water solution, for the surface potential measurements
only. After 5 min for stabilization, the surface potential
drifted less than 3 mV over times comparable to the
experiment, under good conditions. All surface potential
increments ∆V are given with respect to the baseline on
pure water.
The layers were imaged with a Brewster angle microscope17 (BAM), which was assembled in our laboratory
with the standard design.17 Incident light at 668 nm (SDL
7470-P6), polarized (Glan-Tayler, Lambrecht MGTYE15)
in the plane of incidence, was reflected off the monolayer
at the Brewster Angle to a biconvex lens that focused the
image on a CCD camera (Panasonic GP-MF602). The field
of view was either 11 mm × 13 mm or 3.6 mm × 4.3 mm.
A diachroic sheet analyzer (Melles-Griot) before the final
lens checked for any in-plane optical anisotropy in the
monolayer. During the experiments, images were captured
directly from the camera output by a frame grabber.
Surface pressure measurements were recorded simultaneously using the KSV system.
The gray level in the CCD image depends on the
reflectivity R of the surface and with care can be used to
quantitatively compare R for different domains. Grey
levels G ranged from 0 for saturation to 255 for complete
darkness. We verified that Gr ) Gb - G was linear with
intensity over most of that range by checking that Gr ∝
(cos2(θp - θA)), as the angle of the second polarizer (the
analyzer, with angle θA) varies from parallel to perpendicular to the first polarizer (angle θp). Gb is the gray level
associated with the background light level. Since Gr is
linearly related to the light onto the deflector
Gr Iref
∝
)R
Iinc Iinc
(1)
In analyzing the reflectivity, we took Gb as the gray
level observed with a pure water surface and the same
Iinc, to maintain consistent experimental conditions. In
principle, pure water is rough and thus also reflects light
at the Brewster angle, which here is included in Gb.
However, typical values for Gr were much greater than
the variation in Gb, so this complication was ignored.
Reflectivities varied by a factor of 600 for different layers,
so that Iinc was adjusted during the different experiments
to maintain reasonable contrast without saturating the
camera.
(15) The tests used poly(dimethylsiloxane) (Polymer source # P530DMS, Mw 33, 500), which gives a stable, reproducible, reversible
monolayer with a strong tendency to leak around barriers. (Mann, E.
K. Langmuir 1991, 7, 1112. Mann, E. K. Doctoral Thesis, Paris VI,
1992.)
(16) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th
ed.; Wiley-Interscience: New York, 1997.
(17) Hénon, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. Hönig,
D.; Möbius, D. J. Phys. Chem. 1991, 95, 4590.
Figure 2. Thinnest layers observed on the water surface, at
very high average molecular areas σ, for three bent core
molecules: (a) Bc-H, σ ∼ 5.5 nm2/molecule; (b) Bc-NO2, σ ∼ 12
nm2/molecule; (c) Bc3-SiO, σ ∼ 4 nm2/molecule. Bars correspond
to 1 mm.
To check for reproducibility and to find any accessible
equilibrium or metastable states, all layers went through
at least two compression/decompression cycles, which
began from 10 min to 1 h after depositing a spreading
solution of the molecule to be studied. The waiting time
had no obvious effect, either on the isotherms or on the
texture of the monolayer. Typical compression speeds were
5 mm/min, which corresponded to molecular area changes
of ∼0.05 nm2/ (mol‚min) for the siloxane molecules and ∼
0.005 nm2/(mol‚min) for the hydrocarbon molecules. The
fully compressed state was held between 2 min and 2 h,
as was the decompressed state before recompression. In
some cases the compression was stopped at different values
of the surface pressure for ∼15 min each, to check for the
stability of the surface pressure. The stability varied and
will be discussed below.
All isotherms presented were done with fresh solutions;
aged solutions showed similar behavior but different
thicknesses after the compression/decompression cycling.
All the experiments were carried out at 17 °C and 50%
humidity.
Experimental Results
Brewster angle microscopy (BAM) allows the visualization of
the film and in particular any phase separation or threedimensional collapse. All the studied molecules phase separate
into different uniform layers at very low pressure. However, the
character of those layers is very different for the hydrocarbon
end-chain molecules compared to the siloxane end-chain molecules.
Hydrocarbon end-chain molecules give solid phases at zero
surface pressure. With very small amounts of material on the
surface (Figure 2a,b), BAM shows islands of uniform thickness
with sharp corners and jagged breaks, although the long edges
tend to curve. These solidlike islands move rapidly on the
surface: the surroundings are probably gaseous. As σ is
decreased, but the pressure remains unmeasurably small, we
see regions of at least three different reflectivities, again bounded
by irregular edges and showing cracks (Figure 3a); these have
reflectivity about 4 times greater than the islands observed at
high molecular areas σ (Figure 2a,b). In fact, the darkest regions
in Figure 3a are close in reflectivity to the bright regions seen
in Figure 2b. Further, the film at lower σ, very unlike that in
Figure 2, is nearly stationary, which also suggests that all phases
are very viscous.
In contrast, at high molecular areas σ, the siloxane end-chain
molecules form fluid domains with very low contrast and smooth
edges that relax quickly (Figure 2c). These fluid domains are
much less dense than the solids obtained with the hydrocarbon
molecule, as is evident both by the low contrast and because the
pressure increases at much higher areas per molecule than with
the hydrocarbon end chain. To quantify this, we can compare
coareas, determined in the usual way by extrapolating from the
Π-A curve to the baseline pressure.18 Under the assumption
that the pressure rises significantly when the molecules touch,
(18) Adam, N. K. The Physics and Chemistry of Surfaces, 3rd ed.;
Oxford University Press: London, 1941; p 47ff.
Langmuir Monolayers of Bent-Core Molecules
Langmuir, Vol. 20, No. 7, 2004 2775
Figure 3. Representative BAM images of Bc-NO2 layers on water at different molecular areas σ during successive compressions.
(a) First compression, σ ) 0.41 nm2/molecule. (b) First compression, σ ) 0.28 nm2/molecule. (c, d) First compression, σ ) 0.10
nm2/molecule. (e) Second decompression, σ ) 0.24 nm2/molecule. (f) Second decompression, σ ) 0.41 nm2/molecule. Bar corresponds
to 1 mm. Compression is symmetric from top and bottom in all images.
Table 2. Summary of Isotherm Properties
coareaa, σ0, nm2/molecule
molecule
1st compression
2nd compression
Bc-H
Bc-NO2
Bc-CH3
Bc2-SiO
Bc3-SiO
0.22 ( 0.01
0.3 ( 0.05
0.29 ( 0.03
1.1 ( 0.1
1.0 ( 0.1
0.12 ( 0.01
0.13 ( 0.01
0.14 ( 0.02
1.1 ( 0.1
0.9 ( 0.1
a Coareas are estimated as usual18 by extrapolating the isotherm
to the baseline; this provides a first estimate of the area/molecule.
The “error” value indicated the range of values found in different
experiments.
this gives a first estimation of the surface area taken up by the
molecule. The coareas of the siloxane end-chain molecules (given
in Table 2) are about 3-10 times the coareas of the hydrocarbon
end-chain molecules.
The BAM images of the three banana molecules with hydrocarbon end chains (Bc-H, Bc-NO2, and Bc-CH3) are similar. Figure
3 presents typical images of Bc-NO2, as examples. At the
beginning of the first compression (Figure 3a), we see reflectivities
(and thus phases) which are uniform over macroscopic areas,
∼0.5 mm and larger. However, at least three different types of
layer are seen, with the brightest as a minority phase. As we
compress, the darkest phase is reduced to thin lines. When the
islands of the brighter two phases are jammed together, the
pressure begins rising and the reflectivity increases. The laser
intensity must be decreased to avoid saturating the camera. A
semiquantitative analysis of image gray levels, normalized by
incident intensity (see above, with the experimental methods),
suggests that the reflectivities of the majority increase by up to
a factor of 10 during compression. At the end of the first
compression, a much brighter threadlike structure appears, while
the dark lines fade, as shown in Figure 3c,d). Finally, the other
two uniform phases become one uniform layer. When decompression begins, the whole layer immediately breaks along the
thread structure into several uniform sections, as shown in Figure
3e. The surface pressure drops very quickly to zero. As we
decompress, the fraction of darker phase increases, but the
reflectivity of the brighter layer changes little (Figure 3f). None
of these layers show evident in-plane optical anisotropy: the
whole surface dims in unison as the analyzer is turned from
parallel to perpendicular to the polarizer.
Relative proportions of the two brighter phases after deposition
are different for each different experiment but remain approximately constant both as the layer sits for up to an hour and
as the compression begins, until the pressure begins to increase
significantly. Naturally enough, the pressure rises at different
molecular areas for each experiment (Figure 4a,b gives representative isotherms for the three molecules), depending on the
relative proportion of the two brighter phases. In contrast, after
the first compression/decompression cycle, the brighter phase is
uniform; the second compression is much more reproducible (as
indicated in the spread of coareas given in Table 2), and a third
compression reproduces the second. However, if the compression
is stopped at any point, the pressure decreases slowly. Further,
during the decompression, the pressure decreases much faster
than the previous increase. Clearly the layer undergoes slow
rearrangement, as well as some three-dimensional collapse into
threadlike structures that are approximately parallel to the
compression barriers, but both rearrangement and collapse are
reversible, since the third compressions reproduce the second.
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Langmuir, Vol. 20, No. 7, 2004
Figure 4. Representative isotherms for Bc-H (solid line, 9),
Bc-NO2 (dash-line, b), and Bc-CH3 (dash-dot line, 2). (a)
Surface pressure π vs molecular area σ for the first compression.
(b) π versus σ for the second compression. (c) Surface potential
increment ∆V versus σ for the first compression. (d) ∆V versus
σ for the second compression.
All three hydrocarbon end-chain molecules follow this same
pattern, with one major difference: the reflectivities of the Bc-H
layers are about 4 times smaller than those of theBc-NO2 and
Bc-CH3 layers, which are alike within experimental uncertainty.
Figure 4c,d shows the surface potential isotherms for the first
and second compressions. In the first compression, all relative
Zou et al.
surface potentials reach values between 0.3 and 0.5 V as the
pressure starts to increase. As the surface pressure increases,
the surface potential increases further, by up to 30%. For the
second compression, the surface potential is 0.52 V for Bc-H, 0.6
V for Bc-NO2, and 0.4 V for Bc-CH3 in the nearly uniform layer
at nearly zero pressure, just before the surface pressure increases.
Note that in one case, the ∆V after decompression has decreased
to almost zero, with large fluctuations, before recompression.
The fluctuations correspond to dark and bright phase in
coexistence and the low value suggests that ∆V is about zero for
the dark phase. The reflectivity of the dark phase is also less
than before compression for all three hydrocarbon molecules,
becoming indistinguishable from the reflectivity of the pure water
surface.
Unlike for the three hydrocarbon-end-chains molecules, films
of the two siloxane end-chain molecules (Bc2-SiO and Bc3-SiO)
look different under the BAM as the pressure increases to the
point that the film collapses. In both cases, the film increases
uniformly in brightness as the pressure increases above ∼5 mN/
m. For Bc2-SiO, nonuniform bright domains appear, with
different shapes and sizes (Figure 5a). These domains are
unstable. After being left about 3 h, the size of each domain
enlarges as the domains break, as shown in Figure 5b. At
decompression, the domains break into snowflakelike images
(Figure 5c), which finally disappear.
Bc3-SiO shows different collapse morphology in different
experiments for the first compression. Parts a-c of Figure 6 are
presented as examples. Note the fairly compact domain in Figure
6c, which is a network that explodes upon decompression (Figure
6d). In the second compression, bow-shape domains, as shown
in Figure 6e, always appear. The angle of the bow is 122° ( 5°.
In another contrast with the behavior of the hydrocarbon endchain molecules, the isotherms (Figure 7a) for Bc2-SiO-3 and
Bc3-SiO are both stable and reproducible, at least up to about
5 mN/m. If we stop the compression in this range, the surface
pressure remains constant for at least 15 min. At higher
pressures, this is no longer true, as the isotherm difference for
the first and second compression, as well as the difference in the
images, suggests. For the second compression in both cases, the
pressure reaches a plateau after a typical nucleation hump. Much
higher pressures are reached in the first compression. With Bc3SiO on several experiments, the pressure reached a high plateau
of ∼15 mN/m, before suddenly decreasing, several minutes later
(even if compression was stopped); Figure 6b,c has examples of
domains that appear during such a plateau. In other experiments,
no such plateau is observed; Figure 6a is an example of domains
observed under these circumstances. Apparently, layers such as
those in Figure 6b,c are quite unstable with respect to other
collapsed structures. Both the isotherms and the images demonstrate that more than one metastable collapse structure is
available to the Bc3-SiO molecule. These structures are somewhat
different from those seen with the other siloxane molecule, but
with the apparent availability of several different metastable
collapse structures, even slight changes in the molecule may
well select a different structure.
During decompression, the surface pressure shows a pronounced shoulder as it decreases by about 50% (Figure 7b). This
shoulder is reproducible between the first and second decompression for Bc2-SiO. For Bc3-SiO, the shoulder smoothes out
with the second decompression. During the decompression, the
Figure 5. Representative BAM images of Bc2-SiO on water at different molecular areas σ during successive compressions. (a,
b) First compression, σ ) 0.58 nm2/molecule. (c) Second compression, σ ) 1.0 nm2/molecule. The dark backgrounds are uniform
dense layers. Bar corresponding to 1 mm. Compression is symmetric from top and bottom in all images.
Langmuir Monolayers of Bent-Core Molecules
Langmuir, Vol. 20, No. 7, 2004 2777
Figure 6. Representative BAM images of Bc3-SiO on water at different molecular areas σ during successive compressions. (a-c)
First compression, σ ) 0.35 nm2/molecule. (a) corresponds to a case with no high plateau in the isotherm; (b) and (c) correspond
to a high plateau; (d) decompression of (c); (e) second compression, σ ) 0.35 nm2/molecule. The dark backgrounds are uniform dense
layers. Bar corresponding to 1 mm. Compression is symmetric from top and bottom in all images.
Figure 7. Representative isotherms for Bc2-SiO (left side) and Bc3-SiO (right side) for the first (solid line) and second (dash line)
compression. (a, d) Surface pressure π versus molecular area σ, compression for Bc2-SiO and Bc3-SiO. (b, e) π versus σ, decompression.
(c, f) Surface potential increment ∆V vs σ, compression. The isotherms for Bc2-Si and Bc3-SiO are both stable for pauses >15 min,
as long as π < 5 mN/m.
islands both dispersed and left the field of view. Because the
field of view is so limited compared to the total area (151 mm2
compared to 8541 mm2 when decompression starts and 10 105
mm2 at the beginning of the shoulder), the absence of an image
is not definitive; that any remaining islands disappear at
pressures corresponding to the shoulder remains a reasonable
guess.
The surface potential isotherms are presented in Figure 7c.
For both compressions, the relative surface potential reaches
values of ∼0.3 for Bc2-SiO and ∼0.36 for Bc3-SiO when the surface
pressure just starts to increase. With the surface pressure
increases, the surface potential increases by up to 50%. The
surface potential increment returns to zero at the end of the
decompression.
2778
Langmuir, Vol. 20, No. 7, 2004
Figure 8. Top view: Schemata of possible close-packings of
a projection of Bc3-SiO onto a surface. (a) Layered structure,
showing the projections; (b) herringbone structure, with the
projections slightly reduced in size for clarity.
Discussion
Brewster angle microscopy immediately tells us that
all the molecules can form thin layers of constant
thickness, where this thickness can have more than one
value. The smallest such regions observed were ∼100 µm
across and the largest more than the maximum field of
view of the microscope, several millimeters, and probably
extending over much of the trough.
The surface pressure isotherms can give a first idea as
to the molecular configurations in these films. The
molecules with the hydrocarbon end groups and those
with the siloxane end groups behave very differently, so
we will consider these separately.
Dilute monolayers of the molecules with siloxane end
groups, Bc2-SiO and Bc3-SiO, phase separate into fluid
islands. Below σ ∼ 1.2 nm2/molecule, the islands become
continuous and the surface pressure starts, rising as the
layer is compressed. Because phase segregation corresponds to attractive interactions, such monolayers are
generally quite close-packed. Since both core and end
chains are amphiphilic, a first guess for the configuration
would be that the molecule lies flat on the surface, with
both parts in the immediate interfacial region. The collapse
pressure ∼10 mN/ m, near that observed for polysiloxanes,19 also suggests that the siloxane end groups remain
in contact with the water until collapse.
We can crudely estimate the area per molecule in such
a close-packed array by considering different projections
(see Figure 8a for a schematic example) of a space-filling
molecular model (as in Figure 1e,f) onto a surface. Copies
of a projection can then be packed in different ways to find
the close-packed molecular area σ0.18 In what follows, we
will compare the molecular area from different projections
and different packings to the experimentally observed σ0
(Table 2) derived from the σ where the pressure rises in
the isotherm. For both the siloxane molecules, the layer
is uniform as the pressure rises, so that this represents
a true molecular area. Using the projections will yield an
upper limit to a closed pack structure, since a threedimensional structure can always pack more tightly than
its projection. However, this limit will put constraints on
possible layer configurations.
Many configurations with respect to the surface are
possible, even keeping both core and end-groups quite
flat on the surface. Such flat configurations could be packed
as tightly as ∼1.6 nm2/molecule for both siloxane molecules. This is about 30% less dense than observed from
the isotherms, but quite reasonable considering the
limitations of the model, especially given the flexibility of
the molecules. Generally herringbone patterns (Figure
8b), with the two end-chains fitting into the bow-shaped
core, formed the tightest packings, with layers 20-30%
denser than a two-dimensional layered structure (Figure
8a), mainly because the end-chains are quite bulky in
(19) Fox, W.; Taylor, P. W.; Zisman, W. A. Ind. Eng. Chem. 1947, 39,
1401. Granick, S. Macromolecules 1985, 18, 1597. Mann, E. K.; Henon,
S.; Langevin, D.; Meunier, J. J. Phys. II France 1992, 2, 1683.
Zou et al.
Figure 9. Side-view schemata of possible layer structures of
the bent-core molecules with hydrocarbon end chains.
cross-section compared with the core. This is also why the
two different molecules gave similar close-packed areas
in these simplified models: the bulky siloxane groups fit
more compactly into the bend of the three-ring core than
the two-ring core. Given the limitations of these models,
the better agreement with the data of herringbone
structures is suggestive rather than definitive.
However, the very close agreement in coareas for the
two molecules with different core structure suggests that
perhaps the core is mostly off the surface. With both
siloxanes flat on the surface, projections give the molecular
area as low as ∼1.4 nm2; this is certainly also consistent
with the isotherm results, within the limitations of the
model.
These numbers should be compared with other possible
configurations. For an end-on configuration, with only
one of the siloxanes on the water and the rest of the
molecule in the air, close-packing corresponds to ∼0.6 nm2/
molecule. With both the siloxane ends either plunged into
the water (short siloxane oligomers are water-soluble) or
into the air, but the core flat on the surface, close packing
corresponds to ∼0.7 nm2/molecule. Since these values
should correspond to an upper limit for a true close-packed
layer, they are all too small to correspond to the observations.
The isotherm data thus strongly suggest that the
molecules pack quite flat on the surface, with both the
end groups and possibly the core in close contact with the
water. There are many detailed configurations of each
molecule that corresponds to this, including some with
the bend in the core nearly perpendicular to the surface
and others with the bend essentially parallel to the surface.
The isotherm data alone are not enough to distinguish
between these different configurations. However, the
images of collapsed structures in the Bc3-SiO show distinct
bow-like shapes, with an exterior angle of ∼122°, very
close to the expected angle at the bend in the core: these
images suggest, for the collapsed structures at least, a
distinct bend, parallel to the surface, in the core.
The hydrophobic hydrocarbon chains, on the other hand,
should lead to more upright configurations for that series
of bent-core molecules. Indeed, we find that the surface
pressure rises for these molecules only at much smaller
molecular areas: in fact, as much as 10 times smaller,
although the cores are more than two benzene rings larger.
All the cores are clearly not flat on the surface. An upper
limit for the monolayer closed-packed area can be found
as before, from projections of space-filling models (Figure
1d). The most likely monolayer configurations, with only
cores in contact with the water as schematized by the
lower rows of molecules in Figure 9, suggest close-packed
molecular areas σ ∼ 2 nm2/molecule, almost 20 times those
which are observed. End-on projections of these molecules
(as in the upper row of molecules in Figure 9c; here, the
molecules would very improbably have only one end of
one hydrocarbon end-chain in contact with the water)
give molecular areas σ ∼ 0.5 nm2/molecule. Even the
first compression suggests molecular areas half that
value.20
Langmuir Monolayers of Bent-Core Molecules
Using projections of the three-dimensional molecules
represents an upper limit for the molecular area, although
one which is surprisingly close to the observed molecular
areas in simple cases.18 A lower limit for the molecular
area σ in a monolayer configuration can be determined
assuming that the molecules can somehow pack to densely
fill all of space. From space-filling models, the approximate
volume of a molecule can be taken as ∼1.36 nm3. The
longest dimension of the molecule is 4 nm, so that the
upright configuration, which would have a single hydrocarbon chain end in contact with the water, leads to a
molecular area of σ ∼ 0.33 nm2/molecule. A more likely
configuration with the core in contact with the water and
the hydrocarbon chains in the air would give a thickness
∼1.3 nm or a molecular area σ ∼ 1 nm2. All the layers as
the pressure begins to rise, with the possible exception of
the very darkest one (similar to that observed in coexistence with a gas at very large molecular areas), are certainly
more than one molecule thick. The observed molecular
areas of ∼0.12 nm2 (at the second compression, where the
film was very uniform and the coarea was a good estimate
of the molecular area for that film) would, in this simplified
model, correspond to a film thickness ∼11 nm, or at least
three molecular lengths.
Indeed, because the cores are much bulkier than the
chains, one might expect some sort of interdigitated,
multilayer structure in order to form a dense hydrocarbon
layer while allowing some part of the amphiphilic core to
remain at the surface. Such structures have been observed
in Langmuir monolayers of the nCB series.9 The single
chain on one end of the nCBs also allowed a single layer
structure, with the cores tilted at the interface and a dense,
tilted hydrocarbon layer above it. It is difficult to imagine
such a structure for the double-chained molecules considered here. However, a more complicated layer structure,
with a first layer lying with the core flat on the water
surface and further layers with interdigitated hydrocarbon
chains and nearly upright configurations, is quite conceivable, Figure 9b,c, for example.
The images in Figure 2a,b and Figure 3a,f suggest that
in fact several such configurations are possible even at
zero pressure. An example of the relative reflectivity, as
estimated from the gray levels at different incident
intensities, of these different layers are given in Figure
10, for Bc-NO2. All reflectivities are given relative to that
of the layer observed at the smallest concentrations. If
the optical density of the layers remains constant, the
reflectivity is proportional to the square of the layer
thickness. If we further argue that the thicker layers after
a compression/decompression cycle are ∼11 nm, as argued
from the surface isotherms above, we would deduce that
the three different layers just before compression are ∼3.5,
∼4.8, and ∼5.5 nm thick. For this molecule, the layer
observed at very high molecular areas (Figure 2b) appears
similar to the thinnest of these three layers. These
configurations could correspond to a double layer, of the
general sort shown in Figure 9c, but the 11-nm configuration must be at least a triple layer. Also note that
fresh solutions and solutions more than 2 weeks old gave
different thicknesses after 2 compression/decompression
cycles, even small amount of impurities may allow the
system to explore different metastable layer structures.
(20) The layer is quite inhomogeneous in this first compression; the
dominant layer covers only ∼85% of the surface. However, the reflectivity
of the brighter minority layer is only ∼30% greater, and the darker
background covers ∼2% of the surface as the pressure rises, so that the
determined coarea should be within ∼20% of the molecular area for the
dominant phase. The layer is very homogeneous during the second
compression, so that the determined coarea should be within 10% of the
molecular area.
Langmuir, Vol. 20, No. 7, 2004 2779
Figure 10. Relative reflectivities, calculated from eq 1, of the
different layers observed at zero pressure for Bc-NO2. Thin
Layer: σ ∼ 5 nm2/molecule. 0 Cycle: σ ∼ 0.4 nm2/molecule
before 1st compression. 2 Cycles: σ ∼ 0.4 nm2/molecule, after
2 compression/decompression cycles. Fresh solutions showed
different thicknesses than aged solutions after 2 cycles.
The three hydrocarbon molecules have different groups
substituted at the inner angle of the central benzene ring
of the core, which can affect both configuration and
stability of the molecule at the surface, as well as the way
in which these molecules stack to form a multilayer.
Indeed, subtle differences appear. With the second compression, the Bc-H molecule shows the smallest area/
molecule, while the Bc-CH3 coarea is about 10% higher,
although these differences may be due to some residual
nonuniformity in the films rather than to true differences
in the uniform films.
The surface potential increment of the final uniform
layer at zero pressure varies from ∆V ) 0.4 V for the
molecule with the methyl group (Bc-CH3) to 0.5 V for the
unsubstituted molecule (Bc-H) to 0.6 for the molecule with
the nitro group (Bc-NO2). This suggests that the average
dipole density of Bc-NO2 is about 50% greater than that
of Bc-CH3. The average dipole moment of the molecules
are also different, estimated at 1.7 D for Bc-H, 1.9 D for
Bc-CH3, and 2.6 D for Bc-NO2,21 all in the direction
perpendicular to the long axis. The simplest model gives
the relation between molecular dipole moment µ and
surface potential drop ∆V as 22
∆V )
µ
.
σ0
(2)
A more complicated layered model, which can take into
account the variation of dielectric constant through the
interface from ) 80 in the water to ) 1 in air, would
be more realistic,22 but here the films are so thick that
most of the material should be in the interior of the film,
away from the water, and the simple model is a reasonable
place to start. Note that if all the molecules were aligned
with the molecular dipole moment facing up, as in Figure
9a, and ignoring the contribution from the water molecules
at the interface, we would expect a voltage drop of ∆V ∼
1.5 V. Our much lower values suggest a more complicated
layer, as in Figure 9c, where the bulk of the film is not
aligned with the long axis parallel to the surface; this is
consistent with the argument above on more physical
grounds. However, the ratio of the dipole moments for
Bc-NO2 and Bc-H is 1.5, which is also the ratio of the
corresponding surface potential drops for the dense
uniform layer. This agreement in the ratios would be
(21) As determined by Spartan ‘02, Wavefunction, Inc., with the spacefilling models as given in Figure 1d, which were determined with the
semiempirical module with the equilibrium geometry option.
(22) Taylor, D. M. Adv. Colloid Interface Sci. 2000, 87, 183.
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Langmuir, Vol. 20, No. 7, 2004
Zou et al.
expected from eq 2, given the very similar values for
molecular area σ, provided that the packing of the different
layers is quite similar. These surface potential results,
along with the very similar values for σ, suggest that the
packing is indeed quite similar for the different molecules.
There is only one major difference between the different
hydrocarbon end-chain layers: the reflectivities of the
Bc-H layers are about 4 times smaller than the corresponding layers of Bc-NO2 and Bc-CH3. If the optical
densities of the layers were the same, this would mean
that the Bc-H layer was half as thick as the other, which
would imply about twice the area per molecule σ. More
generally, a first approximation gives the Brewster angle
reflectivity of such a layer as proportional to the square
of the layer density 1/σ, rather than depending on the
thickness of the layer and the optical density separately:
this holds exactly if the optical density of the layer is
sufficiently close to that of water.23 However, from the
isotherms, the corresponding layers for the different
molecules clearly have roughly the same σ.
Some other explanation must hold for the large difference in reflectivity. Optical anisotropy can also affect the
reflectivity substantially.23,24 However, we observed no
in-layer optical anisotropy, so that any significant anisotropy is perpendicular to the layer. It would take
completely unrealistic anisotropies, of the order of the
refractive index of the layer, to yield a factor of 4 differences
in reflectivity for otherwise similar layers. We also note
that, whatever the dynamic response of the layer polarization to light, the average static dipole density is quite
similar for the three molecules, as revealed in the surface
potential differences.
A final possibility is that the optical density is significantly decreased in the Bc-H films by the inclusion of
enough air, while maintaining roughly the same σ. It is
easy to see that the inclusion of air in a film can make a
critical difference in reflectivity where inclusion of water
does not by considering an extreme case: if enough air
were included in a layer so that its average optical density
were that of water, the surface would not reflect p-polarized
light at the Brewster angle at all, however thick the film
became. We can estimate the amount of air needed in the
film to reduce the reflectivity of the film by a factor of 4,
without changing the amount of material in the film, with
the simple Maxwell-Garnett model,25 in which
- a
f - )φ
f + 2
+ 2a
(3)
where f is the effective dielectric constant of the film, is dielectric constant of a dense packed film, a ) 1 is the
dielectric constant of air inclusions, and φ is the volume
fraction of those inclusions (assumed in this model to be
spherical and much smaller than the wavelength of light).
We also assume that L(1 - φ) ) constant, to maintain the
total amount of material in the film a constant. If the
average refractive index of the Bc-NO2 and Bc-CH3 films
were near the bulk value of 1.5, the Bc-H would have to
contain 17% more air (and be about 17% thicker) to give
a reflectivity a quarter as large with the same amount of
(23) Mann, E. K.; Heinrich, L.; Voegel, J. C.; Schaaf, P. J. Chem.
Phys. 1996, 105, 6082. Mann, E. K.; Heinrich, L.; Voegel, J. C.; Schaaf,
P. Prog. Colloid Polym. Sci. 1832, 110, 296. Mann, E. K. Langmuir
2001, 17, 5872.
(24) Meunier, J. In Light Scattering by Liquid Surfaces and
Complementary Techniques; Langevin, D. Eds.; Marcel Dekker: New
York, 1992; p 333.
(25) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized
Light; North-Holland, 1977; p 359.
material. This is certainly quite conceivable, but more
direct evidence is necessary for any firm conclusions about
the origin of the much smaller reflectivity of Bc-H films
compared to the corresponding Bc-NO2 and Bc-CH3 films.
Conclusions
We have demonstrated that it is possible to make stable
Langmuir monolayers of a variety of different bent-core
molecules. The properties of these layers depend on the
molecule. The most obvious differences are between
molecules with end chains of different character: amphiphilic siloxane as compared to hydrophobic hydrocarbon chains. With amphiphilic chains, the molecules lie
quite flat on the surface, with both core and end chains
in direct contact with the air/water interface. With
hydrophobic chains, the molecules form a complex multilayer structure; surface potential and other results
suggest that these structures are quite similar with the
different cores. The length of the hydrophobic chains
probably plays a role in such structures. Other work,12
with somewhat different cores and very short hydrophobic
side chains, found a single layer, perhaps quite similar to
what we observed for the amphiphilic side chains. Different techniques would be required to determine the exact
structure of these layers, but we expect a first layer with
the core in contact with the surface and other layers with
more upright molecules to lead to dense packing of the
hydrocarbon chains. We note that at least three layers of
different discrete thickness are possible, two of them of
quite comparable thickness and one significantly thicker.
This final layer appears to be the most stable, in that it
forms irreversibly, at the expense of thinner layers, and
remains even at zero pressure. As the pressure on this
layer decreases, it simply breaks into pieces, with the
opening cracks indistinguishable from the bare water
surface. Upon being recompressed, the pressure rises, and
the film brightens by a factor of 2. Threads appear at very
high pressure. These may be similar in structure to those
observed in the bulk, where free-standing two-dimensional
films are not stable while free-standing threads are.6
Preliminary results suggest that the bent core molecules
with the siloxane side chains can form good alignment
layers for bent-core liquid crystals, which have been
difficult to align over macroscopic regions. The configuration of the cores on the surface may result in a periodic
modulation of the substrate that is compatible with the
bent-core liquid crystals. Effective alignment of bent-core
liquid crystals is important to studying their macroscopic
properties, including viscoelasticity, piezoelectric constant,
and electrostriction. Such alignment is crucial to their
eventual use in devices such as sensors, actuators, or
artificial muscles.
Acknowledgment. This material is based upon work
supported by the National Science Foundation under
Grant No. 9984304. The surface potential work was
supported by the Petroleum Research Fund, under grant
ACS PRF# 35293-G 7. We thank Julie Kim for verification
of the Bc2-SiO and Bc3-SiO structures by NMR and
acknowledge very helpful discussions with Mary Neubert
and with J. Adin Mann, Jr.
LA0361924