Stabilizing bolaform amphiphile interfacial assemblies by introducing mesogenic groups

Article abstract of DOI:10.1002/chem.200390215

We describe the synthesis and characterization of the mesogen-bearing bolaform amphiphile 4,4′-dihydroxybiphenylbis(11-pyridinium-N-yl-undecanoic ester) dibromide (BP-10) and its solid/liquid interfacial self-assembly. Cylindrical micelles are directly observed by atomic force microscopy (AFM) at the interface between mica and the aqueous solution above the critical micelle concentration (cmc). In situ and ex situ AFM studies indicate that the cylindrical micelles are stable both at the mica/solution interface and in the dry state. The enhanced stability of the micellar structures enables a detailed investigation of their self-assembly behavior and supramolecular structures at the interface. The adsorption model proposed here is supported by the variation of the interfacial self-assemblies on changing the solution concentration and substrate temperature.

Full text of DOI:10.1002/chem.200390215

FULL PAPER
Stabilizing Bolaform Amphiphile Interfacial Assemblies by Introducing
Mesogenic Groups
Mingfeng Wang, Dengli Qiu, Bo Zou, Tao Wu, and Xi Zhang*[a]
Abstract: We describe the synthesis and
characterization of the mesogen-bearing
bolaform amphiphile 4,4'-dihydroxybi-
phenylbis(11-pyridinium-N-yl-undeca-
noic ester) dibromide (BP-10) and its
solid/liquid interfacial self-assembly. Cy-
lindrical micelles are directly observed
by atomic force microscopy (AFM) at
the interface between mica and the
aqueous solution above the critical mi-
celle concentration (cmc). In situ and ex
situ AFM studies indicate that the
cylindrical micelles are stable both at
the mica/solution interface and in the
dry state. The enhanced stability of the
micellar structures enables a detailed
investigation of their self-assembly be-
havior and supramolecular structures at
the interface. The adsorption model
proposed here is supported by the
variation of the interfacial self-assem-
blies on changing the solution concen-
tration and substrate temperature.
Keywords: amphiphiles
self-assembly supramolecular
chemistry
¥ micelles
¥
¥
By introducing mesogenic groups into amphiphiles, we
obtained stable nanostructures with different morphologies
at the mica/solution interface.
Introduction
Supramolecular self-assembly confined to interfaces has
attracted increasing interest from chemists, physicists, and
biologists.[1, 2] Conventional surfactants such as hexadecyltri-
methylammonium bromide (CTAB) can form adsorbed
spherical, cylindrical, or hemicylindrical micelles as well as
Here we present the synthesis and characterization of a
bolaform amphiphile (BP-10) that contains a cationic pyridi-
nium head group at each end of the alkyl chain and a biphenyl
mesogenic unit. The structures of the assemblies at the
interface were investigated by means of in situ and ex situ
atomic force microscopy (AFM). We anticipate that the
increased stability of the adsorbed micelles by the introduc-
tion of the mesogenic unit will enable detailed studies of their
supramolecular structures and interfacial behavior. Knowl-
edge of the interfacial self-assembly of amphiphiles is
important not only for better understanding of self-assembly
but also for potential applications in materials science, such as
the template synthesis of functional nanomaterials, and
surface patterning on the nanometer scale.
bilayer structures at the solid/liquid interface.[3 Such self-
6]
assembled structures are labile, however, and can be easily
influenced by the external environment, which prevents the
thorough identification of the supramolecular structures of
the interfacial micelles.[6] Both basic research and applications
in materials science require the design and synthesis of
building blocks suitable for constructing stable supramolec-
ular assemblies.
A stable supramolecular assembly requires a strong asso-
ciation among its assembling units through different forms of
intermolecular forces. The amphiphilic lipid assemblies can be
made more rigid by using noncovalent approaches that lead to
hydrogen bonding or an increase of the van der Waals
interaction through p p stacking. The assembly can also be
strengthened by covalent modifications such as surface cross-
linking, coating, and internal polymerization.[7] We have
focused on the confined supramolecular nanostructures of
mesogen-bearing amphiphiles at the solid/liquid interface.[8]
Results and Discussion
Design, synthesis, and self-organization in solution: Bolaform
amphiphiles are molecules that contain two hydrophilic
moieties connected by a hydrophobic chain.[9] The synthetic
route to the mesogen-bearing bolaform amphiphile BP-10 is
shown in Scheme 1. Both the mesogenic biphenyl group and
the long flexible hydrocarbon-chain spacer may enhance the
intermolecular interaction among the amphiphiles and allow a
more stable supramolecular organization.
[a] Prof. Dr. X. Zhang, M. Wang, D. Qiu, Dr. B. Zou, T. Wu
Key Lab of Supramolecular Structure & Materials
College of Chemistry, Jilin University
Changchun 130023 (P. R. China)
The dependence of the conductivity of the solution on
concentration of BP-10 is shown in Figure 1. The plot shows
Fax :(‡86)431-8980729
E-mail: xi@jlu.edu.cn
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Scheme 1. Synthesis of BP-10.
Figure 1. Dependence of the solution conductivity of BP-10 on concen-
tration; the cmc is 1.2 Â 10À3 m.
Figure 2. A) In situ AFM images (5 mm  5 mm) of BP-10 adsorbed at the
mica/solution interface. The solution concentration is 4.0 Â 10À3 m (ꢀ3 Â
cmc). The arrows indicate looplike self-closed cylindrical micelles. B)
Schematic of the loop-like self-closed cylindrical micelles.
that the critical micelle concentration (cmc) occurs at 1.2 Â
10À3 m; above this concentration, the amphiphiles self-organ-
ize into micelles.
The mesogen-bearing bolaform amphiphile BP-10 should
tend to form various organized supramolecular structures, for
example lamellar structures and cylindrical micelles, in the
solution above the cmc. These possibilities are partially
attributed to the molecular structure of BP-10, which
comprises a single alkyl chain containing a rigid biphenyl
unit and two ionic pyridinium head groups.[10]
namically more stable assemblies than the cylindrical micelles
with open ends.
After the mica was incubated for 30 min in the solution with
the same concentration, it was removed from the solution,
immediately dip-rinsed in pure water, and then dried over-
night at room temperature. Ex situ AFM reveals that the
adsorbed micellar structures persist even in the dry state
(Figure 3). It should be noted that the density of the adsorbed
cylindrical micelles in the dry state is greater than that
observed by in situ AFM, which can be attributed to the
capillary forces among micelles during the solvent evapora-
tion in the drying process. The measured width of the micellar
structures with ex situ AFM is approximately 40 nm, which is
almost the same as the width measured by in situ AFM. There
should be some effect of the tip sample convolution that may
influence the measured width of the micellar structures. In our
experiment, for the in situ AFM measurements we used a
sharpened Si3N4 cantilever with a tip radius of about 20 nm for
the tapping mode in fluid. For the ex situ AFM measurments,
we used an Si cantilever with a tip radius of about 50 nm for
the tapping mode in air. Comparing the AFM images
obtained with these two modes, we find that tip curvature
has little influence in our case.
AFM in situ and ex situ characterization of interfacial
micelles: The adsorbed micelles of BP-10 were directly
observed by in situ AFM at the interface between mica and
a 4.0 Â 10À3 m (ꢀ3 Â cmc) solution of BP-10. As shown in
Figure 2A, BP-10 forms cylindrical micelles extending over
micrometers. These micellar structures are rather monodis-
perse, with an average width of 40 nm as measured from the
section analysis. There is no change even after prolonging the
incubating time up to 24 h. Interestingly, large loops with
different sizes formed, as indicated with arrows in Figure 2A.
We believe the looplike assemblies are formed by the self-
closure of the cylindrical micelles. Hydrophilic and hydro-
phobic parts of BP-10 are completely segregated in this kind
of supramolecular organization (Figure 2B), suggesting that
the looplike self-closed cylindrical micelles are thermody-
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X. Zhang et al.
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suggesting monolayer deposition if the tilted orientation of
BP-10 adsorbed on mica is taken into consideration. For a
solution concentration of 1.0 Â 10À3 m, slightly lower than the
cmc, the interfacial cylindrical micelles have already begun to
appear (Figure 4B). This effect can be rationalized by the
possibility that the concentration of BP-10 near the mica
surface is higher than in the bulk because of the electrostatic
surface potential.[6a] For the concentrations of 1.0 Â 10À3 m and
2.0 Â 10À3 m, some defects embedded in the adsorbed assem-
blies are always observed in addition to the cylindrical
micelles (Figure 4B and C, respectively). The depth of the
defects against the monolayer islands is approximately 3 nm,
which also corresponds to the deposition of a monolayer of
BP-10. For the concentration of 4.0 Â 10À3 m, cylindrical
micelles covering at least 5 mm  5 mm without defects can
be observed (Figure 3). In general, the coverage of the mica
surface and the density of the cylindrical micelles both
increase as the concentration increases from 1.0 Â 10À3 m to
4.0 Â 10À3 m.
Figure 3. Ex situ AFM images (5 mm  5 mm) of BP-10 adsorbed at the
mica/solution interface. The solution concentration is 4.0 Â 10À3 m (ꢀ3 Â
cmc). The micellar structures remained after the dip-washing and drying
process, suggesting that such supramolecular self-assemblies are robust.
Based on the AFM images, we conclude that there is a
coadsorption of cylindrical micelles and monomers at the
interface between mica and the solution above the cmc. The
ratio of cylindrical micelles to monomers increases as the
concentration of the solution increases; that is, the higher the
concentration, the more cylindrical micelles are formed in the
solution and thus adsorbed at the interface. The adsorption
model proposed here is consistent with the adsorption process
Usually, the micelles formed by conventional surfactants
are labile at the solid/liquid interface. For example, cylindrical
micelles of CTAB at the mica/solution interface can evolve
into a flat bilayer sheet within 24 h. The adsorbed second layer
is removed in the drying process.[6] In contrast, the adsorbed
cylindrical micelles of BP-10 are stable both at the mica/
solution interface and in the dry state, however, suggesting
that these supramolecular assemblies are robust. Their
enhanced stability can be partially attributed to the strong
association within the cylindrical micelles and the strength-
ened intermolecular interaction due to the introduction of the
mesogenic unit into the amphiphile.
‡
and mechanism of CTA XÀ proposed by Chen et al.[4]
In situ temperature-dpendent AFM studies: To further study
the stability of the micellar structures of BP-10 at the
interface, we investigated the change of the interfacial
assemblies on increasing the temperature of the substrate.
The interfacial structural transformation was monitored by in
situ temperature-dependent AFM. Figure 5A shows the dry-
state structure at room temperature of BP-10 which was
adsorbed from a solution of 4.0 Â 10À3m onto the mica
substrate. There are some defects embedded in the adsorbed
monolayer and cylindrical micelles. The width of the cylin-
drical micelle is approximately 40 nm (outlined by the
rectangle in Figure 5A). No obvious changes are observed
until the substrate is heated to 508C. With the gradual
Concentration effect: To substantiate the adsorption of the
micellar structures, we studied the adsorption of BP-10 at
different bulk solution concentrations by ex situ AFM. For a
solution concentration of 2.0 Â 10À4 m, far below the cmc, we
observed only flat sheet structures and random domains with
different diameters (Figure 4A). The section analysis gives a
height of approximately 3 nm for the flat sheet structures,
Figure 4. Ex situ AFM images of the adsorbed self-assemblies at different solution concentrations: A) 2.0 Â 10À4 m; B) 1.0 Â 10À3 m; C) 2.0 Â 10À3 m (the
image size is 5 mm  5 mm in each case and each image is recorded in the height mode). The density of the cylindrical micelles increases as the concentration
increases from 1.0 Â 10À3 m to 2.0 Â 10À3 m. The dark regions in B) and C) are adsorption defects, the depth of which is (3 Æ 0.1) nm, corresponding to a
monolayer deposition of BP-10.
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Mesogen-Stabilized Amphiphiles
1876 1880
neighboring micellar structures
combine to give a wider one on
increasing the temperature.
Conclusion
We have synthesized a meso-
gen-bearing bolaform amphi-
phile that contains two cationic
pyridinium head groups and a
biphenyl mesogenic unit. By
introducing the mesogenic unit
into the bolaform amphiphile,
we have succeeded in obtaining
stable supramolecular assem-
blies at the mica/solution inter-
face. In situ and ex situ AFM
studies provide direct evidence
for the interfacial self-assembly
of cylindrical micelles and mon-
omers. The formation of the
cylindrical micelles depends on
the solution concentration. For
the concentrations above the
cmc, the density of the cylindri-
cal micelles increases as the
concentration increases. In situ
temperature-dependent AFM
provides detailed information
about the diffusion and integra-
tion process of the cylindrical
micelles on the mica surface.
Due to the stability of the
cylindrical micelles, AFM has
been effectively used to inves-
tigate the morphology, the su-
pramoledular structure, and the
interfacial behavior of such assemblies. The stable cylindrical
micelles are also promising for use in template synthesis and
assembly of further functional supramolecular structures and
nanomaterials.
Figure 5. Ex situ AFM images showing the structural transformation of the adsorbed micellar structures on
changing the substrate temperature: A) 258C; B) 558C; C) 598C; D) 708C (the image size is 3 mm  3 mm in each
case and each image is recorded in the height mode). The rectangular blocks indicate a single cylindrical micelle
that becomes wider and wider on increasing the substrate temperature. The arrows indicate the defects embedded
in the monolayer islands. These defects are enlarged during the heating process but do not change further after
they extend over the fringes of cylindrical micelles. The circles show the defects embedded among the micellar
structures. These defects change little during the heating process.
increase of the substrate temperature from 508C, the cylin-
drical micelles become wider and wider. After the substrate is
heated to 708C and held at this temperature for 1 h, the single
cylindrical micelle becomes as wide as 70 80 nm (see
rectangle in Figure 5D). We believe these changes are the
result of the lateral diffusion of the cylindrical micelles on
mica, which is caused by the enhanced mobility on heating the
substrate. In addition, the height difference between the white
and gray regions is around 0.5 nm, which also supports the
model of cylindrical micelles.
We note that the defects embedded in the monolayer
islands (shown with arrows in Figure 5) are enlarged on
heating the substrate. The defects do not change further after
they extend over the fringes of the cylindrical micelles. But the
defects embedded among the cylindrical micelles (shown as
circles in Figure 5) change little during the heating process,
indicating that in spite of lateral diffusion the cylindrical
micelles do not collapse to the substrate to form a monolayer
structure. We conclude from these results that the monolayer
islands around the defects are not as stable as the cylindrical
micelles. From Figure 5, it is also clearly observed that
Experimental Section
Materials: 4,4'-Dihydroxybiphenyl (97% purity) and 11-bromoundecanoic
acid (99% purity) were purchased from Aldrich. DMF, THF, and triethyl-
amine were all distilled and dried before use.
Preparation of 11-bromoundecanoic acid chloride: A mixture of 11-
bromoundecanoic acid (2.86 g, 10.8 mmol) and thionyl chloride (10 mL,
137 mmol) in THF (50 mL), and several drops of DMF as catalyst were
refluxed for 4 h. When the mixture was cooled, the solvent and excess
thionyl chloride were evaporated in vacuum.
Synthesis of 4,4'-dihydroxybiphenylbis(11-bromoundecanoic ester): The
freshly prepared 11-bromoundecanoic acid chloride (10.8 mmol) was
added dropwise to
a stirred solution of 4,4'-dihydroxybiphenyl (1 g,
5.4 mmol) and triethylamine (1.6 mL) in THF (10 mL). The mixture was
refluxed for 2 h and then stirred at room temperature overnight. The
precipitated triethylamine hydrochloride was filtered. The filtrate was
extracted with diethyl ether. The organic layer was washed with 0.1m HCl,
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X. Zhang et al.
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saturated aqueous NaHCO3, and water. After the solution had been dried
with MgSO4, the solvent was removed in vacuum. The crude product was
purified by column chromatography (silica gel, dichloromethane) and
recrystallized from 1:1 (v/v) dichloromethane/cyclohexane to obtain color-
less needlelike crystals of 4,4'-dihydroxybiphenylbis(11-bromoundecanoic
ester). Yield: 45%; 1HNMR (400 MHz, CDCl3): d ˆ 7.56 7.54 (d, 4H; Ph-
H), 7.15 7.13 (d, 4H, Ph-H), 3.43 3.39 (t, 4H, Br-CH2), 2.60 2.56 (t, 4H, -
OCOCH2), 1.89 1.84 (m, 4H; Br-CH2CH2), 1.81 1.73 (m, 4H; -OCO-
CH2CH2), 1.56 1.32 ppm (m, 24H; -CH2CH2(CH2)6CH2CH2-).
Acknowledgements
This project is supported by the Major State Basic Research Development
Program (Grant No. G2000078102), National Natural Science Foundation
of China, and a key project of Ministry of Education, P. R. China. We
gratefully acknowledge F. Shi for the synthesis of the amphiphile BP-10. We
also gratefully acknowledge Barbara Whitesides from Harvard University
for carefully polishing the English of this paper. We also thank the one of
the referees for helpful comments.
Synthesis of 4,4'-dihydroxybiphenylbis(11-pyridinium-N-yl-undecanoic
ester) dibromide (BP-10): Pyridine (6 mL, 75 mmol) was added to the
stirred solution of 4,4'-dihydroxybiphenylbis(11-bromoundecanoic ester)
(0.5 g, 0.73 mmol) in CHCl3 (10 mL). The mixture was stirred and refluxed
for 24 h (under a highly purified nitrogen atmosphere). After the mixture
had been allowed to cool to room temperature, it was added dropwise to
benzene (100 mL). A white solid precipitated. The crude product was
redissolved in acetonitrile and precipitated twice in diethyl ether. Yield:
85%; m.p. 1678C; 1HNMR (400 MHz, DMSO): d ˆ 9.10 (t, 4H; Py-H),
8.62 8.58 (d, 2H; Py-H), 8.18 8.14 (t, 4H; Py-H), 7.70 7.68 (d, 4H; Ph-
H), 7.21 7.19 (d, 4H; Ph-H), 4.62 4.58 (t, 4H; Py-CH2), 2.61 2.57 (t, 4H;
-OCOCH2), 1.91 (m, 4H; Py-CH2CH2), 1.66 1.63 (m, 4H; -OCO-
CH2CH2), 1.28 ppm (m, 24H; -CH2CH2(CH2)6CH2CH2-).
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Sample preparation and AFM characterization: Muscotive mica (PLANO
W. Plannet GmbH, Germany) was freshly cleaved before immersion in the
aqueous solution of the amphiphile at an appropriate concentration. AFM
images were recorded in situ and ex situ at the solid/liquid interface by
using commercial instruments Nanoscope IIIa AFM Multimode (Digital
Instrument, CA) at room temperature. Sharpened Si3N4 cantilevers of
ꢀ20 nm tip radius were used for the tapping mode in fluid; Si cantilevers of
ꢀ50 nm tip radius were used for the tapping mode in air. All cantilevers
were purchased from Park Scientific, CA. Before the in situ AFM images
were recorded, an aqueous solution of the amphiphile with an appropriate
concentration was injected into the liquid cell and allowed to equilibrate
for at least 1 h. The solution was held within the liquid cell by an O-ring. In
situ AFM images were obtained with a tapping mode in fluid. Before the ex
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aqueous solution of BP-10 for 30 min, then taken out from the solution,
immediately dip-rinsed in Milli-Q water, and dried overnight at room
temperature in a desiccator with P2O5. Ex situ AFM images were obtained
with tapping mode in air. In the in situ temperature-dependent AFM
experiment, the mica substrate with the adsorbed sample was glued onto a
stainless steel disk and then heated in air using the temperature heater of
the AFM (MultiMode). The images were captured with tapping mode in air
at a desired temperature equilibrated for at least 5 min.
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Received: September 30, 2002
Revised: January 21, 2003 [F4461]
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