Crystalline films of interdigitated structures formed via amidinium-carboxylate interactions at the air-water interface

Article abstract of DOI:10.1021/ja002597o

Electrostatic interactions between amidinium and carboxylates were used for the construction of interdigitated architectures at the air-solution interface. Spreading the water-insoluble amphiphile p-pentadecylbenzoic acid (A) on an aqueous solution of p-m

Full text of DOI:10.1021/ja002597o

J. Am. Chem. Soc. 2001, 123, 3771-3783
3771
Crystalline Films of Interdigitated Structures Formed via
Amidinium-Carboxylate Interactions at the Air-Water Interface
Ivan Kuzmenko,| Maik Kindermann,§ Kristian Kjaer,† Paul B. Howes,† Jens Als-Nielsen,‡
Rony Granek,| Gunther v. Kiedrowski,*,§ Leslie Leiserowitz,*,| and Meir Lahav*,|
Contribution from the Department of Materials and Interfaces, The Weizmann Institute of Science,
76100 RehoVot, Israel, Organic Chemistry 1, Ruhr-UniVersity Bochum, D-44780 Bochum, Germany,
Materials Research Department, Risø National Laboratory, DK 4000, Roskilde, Denmark, and
Niels Bohr Institute, H. C. Ørsted Laboratory, DK 2100, Copenhagen, Denmark
ReceiVed July 14, 2000
Abstract: Electrostatic interactions between amidinium and carboxylates were used for the construction of
interdigitated architectures at the air-solution interface. Spreading the water-insoluble amphiphile p-
pentadecylbenzoic acid (A) on an aqueous solution of p-methylbenzamidinium (B) ions results in an intercalation
of the water-soluble base between the acidic headgroups of the water-insoluble amphiphile to form an amorphous
A-B-A-B monolayer according to grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity
measurements. Upon compression the monolayer transforms into a crystalline film composed of three bilayers
with interdigitated hydrocarbon chains, and a top layer whose chains are disordered. Water-insoluble
p-heptadecylbenzamidinium spread on an aqueous solution of benzoic acid displays a surface pressure-area
isotherm similar to that obtained from the above system. A mechanism that accounts for the formation of
these films is presented. Deposition of p-heptadecylbenzamidinium and p-pentadecylbenzoic acid amphiphiles
in a 1:1 ratio on pure water led to the formation of a crystalline monolayer phase but which is partially disordered.
Over an aqueous solution containing a 1:1 mixture of benzamidinium and benzoic acid no measurable binding
of these solute molecules to the polar headgroups of the 1:1 mixed monolayer could be detected by X-ray
reflectivity or GIXD.
Introduction
Scheme 1
Here we have focused on two aspects of the assembly of
molecules into two-dimensional (2D) and 3D arrays. The first
is linked to the design of interdigitated films. As already
demonstrated, long-chain acid molecules (pentadecyl R-mandelic
acid), when spread on aqueous solutions containing a water-
soluble amine (R-phenylethylamine), form a 1:1 acid-base
monolayer on the solution that is amorphous owing to the poor
packing between the hydrocarbon chains. Upon compression,
such a film transforms into a crystalline interdigitated bilayer
and a top layer containing disordered hydrocarbon chains.1 The
process of interdigitation at the interface is remarkably sensitive
to the molecular and chiral composition of the acid and base
components, suggesting further elaboration. In the present study,
we take advantage of electrostatic interactions between ami-
dinium and carboxylic acid functions (Scheme 1) as the starting
building blocks. These structures are of importance in biological
systems containing arginine-active sites.2-5 Simple chemical
systems based on these interactions were extensively studied
in aqueous and in nonaqueous solutions.6-12 Strong hydrogen
bonds of the type N-H‚‚‚O-C between amidinium (or guani-
(5) Roderick, S. L.; Banszak, L. J. J. Biol. Chem. 1986, 261, 9461.
(6) Andres, J.; Moliner, V.; Krechl, J.; Silla, E. J. Chem. Soc., Perkin
Trans. 2 1995, 1551.
(7) Krechl, J.; Smrckova, S.; Pavlikova, F.; Kuthan, J. Collect. Czech.
Chem. Commun. 1989, 54, 2415.
| The Weizmann Institute of Science.
§ Ruhr-University Bochum.
† Risø National Laboratory.
‡ Niels Bohr Institute.
(1) Kuzmenko, I.; Buller, R.; Bouwman, W. G.; Kjaer, K.; Als-Nielsen,
J.; Lahav, M.; Leiserowitz, L. Science 1996, 274, 2046.
(2) Adams, J. M.; Buehner, M.; Chandrasekhar, K.; Ford, G. C.; Hackert,
M. L.; Liljas, A.; Rossman, M. G.; Smiley, J. E.; Allison, W. S.; Everse,
J.; Kaplan, N. O.; Taylor, S. S. Proc. Natl. Acad. Sci. U.S.A. 1973, 70,
1968.
(3) Eventoff, W.; Rossmann, M. G.; Taylor, S. S.; Torff, H. J.; Meyer,
H.; Keil, W.; Kiltz, H. H. Proc. Natl. Acad. Sci. U.S.A. 1977, 74, 2677.
(4) Christianson, D. W.; Lipscomb, W. N. Proc. Natl. Acad. Sci. U.S.A.
1986, 83, 7568.
(8) Krechl, J.; Bohm, S.; Smrckova, S.; Kuthan, J. Collect. Czech. Chem.
Commun. 1989, 54, 673.
(9) Krechl, J.; Bohm, S.; Smrckova, S.; Kuthan, J. Collect. Czech. Chem.
Commun. 1989, 54, 673.
(10) Krechl, J.; Smrckova, S.; Kuthan, J. Collect. Czech. Chem. Commun.
1990, 55, 460.
(11) Krechl, J.; Smrckova, S.; Ludwig, M.; Kuthan, J. Collect. Czech.
Chem. Commun. 1990, 55, 469.
(12) Krechl, J.; Smrckova, S. Collect. Czech. Chem. Commun. 1989, 30,
5315.
10.1021/ja002597o CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/29/2001

3772 J. Am. Chem. Soc., Vol. 123, No. 16, 2001
Kuzmenko et al.
spectra are reported as broad (br), singlet (s), doublet (d), triplet (t),
quartet (q), and multiplet (m). Thin-layer chromatography (TLC) was
performed on Macherey-Nagel polygram silica plates, and column
chromatography was performed on ICN silica gel 32-63.
Scheme 2
Hexadecyltriphenylphosphoniumbromide, 1. To a suspension of
triphenylphosphane (25.0 g, 95 mmol) in 180 mL of acetonitrile was
added 1-bromohexadecane (29.0 g, 98 mmol). The mixture was refluxed
overnight, the solvent removed in vacuo, and the resulting yellow oil
treated several times with dry diethyl ether to yield a white precipitate
(38.8 g, 80%).
4-(Heptadec-1-enyl)benzonitrile, 2. A solution of n-hexadecyl-
triphenylphosphoniumbromide (38.8 g, 68 mmol) in 120 mL of dry
THF was treated with n-butyllithium (2.5 M in n-hexane, 28 mL, 70
mmol), resulting in a bright orange solution. The mixture was stirred
for 1 h, and p-cyanobenzaldehyde (9.18 g, 70 mmol), dissolved in 50
mL of THF, was added dropwise over a period of 2 h. After stirring
for 15 h at room temperature the reaction mixture was quenched with
12 mL of acetone, stirred for an additional hour, and filtered over a
short column of neutral aluminum oxide to remove all triphenylphos-
phaneoxide. The solvent was evaporated and the solid residue recrystal-
1
lized from methanol, yielding 2 (14.04 g 60%): mp 51 °C. H NMR
(250 MHz, CDCl3): δ ) 0.88 (t, J ) 7.4 Hz, 3H, CH3), 1.17-1.39
(m, 24H, 12-H-24-H), 1.45 (q, J ) 7.3 Hz, 2H, 11-H), 2.18-2.35 (m,
Ar-CHdCH-CH2), 5.81 (dt, J ) 11.6 Hz, J ) 7.5 Hz, 1H, Ar-
CHdCH), 6.35-6.43 (m, 1H, Ar-CHdCH), 7.31-7.42 (m, 2H, 3-H,
5-H), 7.53-7.64 (m, 2H, 2-H, 6-H).
4-Heptadecylbezonitrile, 3. 4-(Heptadec-1-enyl)benzonitrile (8.0 g,
23.6 mmol) was hydrogenated overnight under normal pressure in 100
mL of a solvent mixture (ethyl acetate:methanol, 1:1) containing 200
mg of Pd/C (10%) as the hydrogenation catalyst under normal pressure.
The catalyst was removed by filtration over Celite, and the solvent
was evaporated. The residue was purified by chromatography on silica
gel (cyclohexane:ethyl acetate, 20:1) to yield 3 as a white solid (6.25
dinium) and carboxylic moieties existing in nonpolar solutions
were employed for the design of supramolecular architec-
tures13,14 and in artificial self-replicating systems that mimic
natural molecular self-replication and might shed light on the
origin of evolution of living systems.15,16 The strong electrostatic
interactions between carboxylate and amidinium moieties would
thus appear suitable to guarantee the formation of a mixed
monolayer of the -A-B-A-B-type, where A is the amphiphilic
acid moiety and B the water-soluble base (or vice versa). The
hydrocarbon chains bound to the A headgroups in the extended
-A-B-A-B-type rows are poorly packed so that such a monolayer
should be amorphous but, upon compression, transform into a
crystalline interdigitated multilayer (Scheme 2, right and left).
Our second goal was to examine the potential replicating
properties of the system at the air-water interface as a result
of binding of the water-soluble amidinium and carboxylic
components to the polar headgroups of the preformed mixed
monolayer containing the same chemical functions, as depicted
in Scheme 2 (middle). The water-soluble amidinium and benzoic
acid molecules in the subphase, if bound to the mixed monolayer
interface in an ordered way, may be designed to react with each
other to form a polymer system and thus initiate a self-
replicating system in a manner akin to self-replicating systems
in bulk solutions.15
1
g, 78%): mp 58 °C. H NMR (250 MHz, CDCl3): δ ) 0.89 (t, J )
6.5 Hz, 3H, CH3), 1.25 (m, 28H, 9-H-23-H), 1.60 (q, J ) 7.0 Hz, 2H,
8-H), 2.65 (t, J ) 8.0 Hz, 2H, Ar-CH2), 7.25-7.28 (m, 2H, 3-H, 5-H),
7.53-7.57 (m, 2H, 2-H, 6-H).
4-Heptadecyl-benzamidiniumchloride, 4. A 2 M solution of
trimethylaluminum in toluene (7.33 mL, 14.6 mmol) was added slowly
at 50 °C under an argon atmosphere to a vigorously stirred suspension
of ammonium chloride (0.815 g, 15.2 mmol) in 15 mL of dry toluene
(distilled from Na). After addition, the mixture was allowed to warm
to room temperature and stirred for 2 h until the gas evolution (methane)
has ceased. A solution of 4-heptadecylbenzonitrile (3.0 g, 8.8 mmol)
in 10 mL of dry toluene was added, and the mixture was kept 18 h at
80 °C for 18 h. After cooling, the reaction mixture was slowly poured
into a slurry of 8 g of silica gel in 50 mL of chloroform and stirred for
10 min. The silica was filtered off and washed several times with
methanol. The filtrate and wash were combined, and the solvent was
evaporated. The solid residue was extracted with hot acetone and water,
leaving the pure product as a colorless solid (2.24 g, 65%): mp 138
1
°C. H NMR (250 MHz, DMSO-[d6]): δ ) 0.83 (t, 3H, CH3), 1.1-
1.4 (m, 30H, (CH2)15), 2.61-2.68 (t, 2H, J ) 7.9 Hz, Ar-CH2), 7.41
(d, J ) 9 Hz, 2H, 3-H, 5-H), 7.83 (d, J ) 9 Hz, 2H, 2-H, 6-H), 9.20
(s, 2H, amidinium (syn)), 9.34 (s, 2H, amidinium (anti)). MS (EI, 70
eV): m/z (%) ) 359(100) [M+], 160(9), 154(20), 147(40), 134(31),
107 (10).
p-Methylbenzamidine, 5. p-Methylbenzonitrile (5.0 g, 42.6 mmol)
was dissolved in 50 mL of dry toluene (distilled from Na), treated with
a suspension of sodium amide in toluene (50%, 17 mL, 213 mmol),
and refluxed for 18 h under argon atmosphere. After cooling to room
temperature the reaction mixture was quenched carefully with 5 mL
of water and evaporated to dryness. The solid residue was recrystallized
from small amounts of acetone to provide a pale yellow solid (1.85 g,
32%): mp 68 °C. 1H NMR (250 MHz, CDCl3): δ ) 2.31 (s, 3H, CH3),
5.62 (s (br), 3H, amidinium), 7.12 (d, J ) 9 Hz, 2H, 3-H, 5-H), 7.43
(d, J ) 9 Hz, 2H, 2-H, 6-H).
2. Experimental Section
2.1. Synthesis. General Procedures. 1H NMR spectra were recorded
on a DPX-200 Bruker spectrometer (200 MHz) in CDCl3 and DMSO-
[d6] using TMS as an internal standard. Multiplicities in 1H NMR
(13) Echaverren, A.; Galan, A.; Lehn, J.-M.; de Mendoza, J. J. Am. Chem.
Soc. 1989, 111, 4994.
(14) Mueller, G.; Riede, J.; Schmidtchen, F. P. Angew. Chem., Int. Ed.
Engl. 1988, 27, 1516.
(15) Terfort, A.; von Kiedrowski, G. Angew. Chem., Int. Ed. Engl. 1992,
31, 654.
(16) Hoffmann, S. Angew. Chem., Int. Ed. Engl. 1992, 31, 1013.
p-Methyl-benzamidinium-tetrafluoroborate, 6. A solution of p-
methylbenzamidine (1.0 g, 7.45 mmol) in 10 mL of diethyl ether was
treated with tetrafluoroboric acid (54%, 1.02 mL, 7.45 mmol) and
sonicated for 10 min. The precipitate was filtered off and suspended

Films Formed Via Amidinium-Carboxylate Interactions
J. Am. Chem. Soc., Vol. 123, No. 16, 2001 3773
1
in 20 mL of acetone, sonicated, and filtered again (1.63 g, 98%). H
NMR (250 MHz, DMSO-[d6]): δ ) 2.39 (s, 3H, CH3), 7.43 (d, J ) 9
Hz, 2H, 3-H, 5-H), 7.0 (d, J ) 9 Hz, 2H, 2-H, 6-H), 8.76 (s, 2H,
amidinium (syn)), 9.20 (s, 2H, amidinium (anti)).
a measure of the thickness of the crystalline film, T ) 0.9(2π)/∆qz.
More detailed descriptions of the GIXD method used for monolayer
structure determination are given elsewhere.17,18
Diluted chloroform solutions of the amphiphilic compounds were
spread on Millipore water or on aqueous solutions containing the water-
soluble components at ∼15 °C and then cooled to the temperature of
5 °C, at which the GIXD and X-ray reflectivity measurements were
performed.
2.3. Single-Crystal X-ray Diffraction Studies. Pentadecylbenzoate
and p-Methylbenzylamidinium. p-Methylbenzamidine and penta-
decylbenzoic acid in a 1:1 ratio were mixed in ethyl acetate, and the
resulting salt was filtered and redissolved in analytical ethanol. Slow
cooling of saturated solutions yielded flat crystalline needles. A suitable
single crystal was mounted on an AFC5 Rigaku diffractometer equipped
with the rotating anode tube, and the intensity data were measured with
the specimen crystal at ambient temperature (17 °C). The crystal
structure was solved by direct methods and refined with use of SHELX-
97 software.19 The experimental X-ray data are summarized in
Table 4.
n-Tetradecyltriphenylphosphoniumbromide, 7. 1-Bromotetradec-
ane (34.5 g, 125 mmol) was added to a solution of triphenylphosphane
(32.6 g, 125 mmol) in 200 mL of dry acetonitrile. The mixture was
refluxed overnight, the solvent removed in vacuo, and the resulting
yellow oil treated several times with dry diethyl ether, yielding a white
precipitate (53.1 g, 78%) that was used without further purification.
4-(Pentadec-1-enyl)-methylbenzoate, 8. A solution of n-tetradec-
yltriphenylphosphoniumbromide (16.4 g, 30 mmol) in 100 mL of dry
THF was treated with n-buthyl-lithium (2.5 M in n-hexane, 14.4 mL,
36 mmol) resulting in a bright orange solution. The mixture was stirred
for 1 h before methyl(4-formyl)benzoate (5.0 g, 30 mmol) dissolved
in 20 mL of THF was added drop-by-drop over 1 h. After stirring for
15 h at room temperature the reaction mixture was quenched with 12
mL of acetone, stirred for an additional hour, and filtered over a short
column of neutral aluminum oxide to remove all triphenylphosphane-
oxide. The solvent was evaporated, and the solid residue was chro-
matographed on silica gel (cyclohexane:ethyl acetate, 5:1) to yield 8
p-Methylbenzamidinium Benzoate. Needlelike colorless crystals
were grown from water solution by slow evaporation. Structural data
and experimental details of single-crystal X-ray diffraction measure-
ments are summarized in Table 4.
1
(5.88 g 52%). H NMR (250 MHz, CDCl3): δ ) 0.9 (t, 3H, CH3),
1.2-1.5 (m, 26H, (CH2)13), 2.3-2.5 (m, 2H, Ar-CHdCH-CH2), 3.91
(s, 3H, CH3), 5.78 (dt, J ) 11 Hz, J ) 7.5 Hz, 1H, Ar-CHdCH),
6.36-6.51 (m, 1H, Ar-CHdCH), 7.25-7.44 (m, 2H, 3-H, 5-H), 7.9-
8.1 (m, 2H, 2-H, 6-H).
3. Results
3.1. Surface Pressure-Molecular Area (π-Α) Isotherms.
The π-Α isotherms of several amphiphilic systems incorporat-
ing carboxylic acid and amidinium functions were measured at
the air-water (or air-aqueous solution) interface. The surfac-
tants employed were p-pentadecylbenzoic acid, C15H31-C6H4-
CO2H (labeled C15-benzoic acid), and p-heptadecylbenzami-
dinium C17H35-C6H4-CN2H4Cl (labeled C17-benzamidinium).
Benzamidinium and p-methylbenzamidinium (X-C6H4-CN2H3,
X ) H, CH3 respectively), sodium benzoate, C6H5-CO2Na were
the water-soluble components.
4-Pentadecylmethylbenzoate, 9. 4-(Tetradec-1-enyl)methylbenzoate
(3.5 g, 10.1 mmol) was hydrogenated under normal H2-pressure
overnight in 100 mL of a solvent mixture (ethyl acetate:methanol, 1:1)
containing 200 mg of Pd/C (10%) as the hydrogenation catalyst. The
catalyst was removed by filtration over Celite, and the solvent was
evaporated. The crude product was purified by chromatography on silica
gel (cyclohexane:ethyl acetate, 40:1) to yield 9 as a white solid (3.3 g,
95%): mp 94 °C. 1H NMR (250 MHz, CDCl3): δ ) 0.9 (t, 3H, CH3),
1.2-1.5 (m, 26H, (CH2)13), 1.6-1.78 (m, 2H, CH2) 2.7 (t, 2H, Ar-
CH2), 3.91 (s, 3H, CH3), 7.25 (d, J ) 8 Hz, 2H, 3-H, 5-H), 7.95 (d,
J ) 8 Hz, 2H, 2-H, 6-H).
The π-Α isotherms of C15-benzoic acid, spread on Millipore
water at 5 °C and at 20 °C, indicate a limiting area per molecule
of 5-7 Å2 which is about 5 times smaller than that usually
occupied by amphiphilic molecules each bearing a single
hydrocarbon chain (Figure 1a, solid line). This observation
suggested formation of a multilayer film. Addition of 1.8 mM
of KOH into the water subphase yields a limiting area per
molecule of 21-22 Å2 (Figure 1a, long-dashed line) that clearly
corresponds to the formation of a monolayer of potassium C15-
benzoate.
According to π-A isotherm measurements, C17-benzami-
dinium forms a monolayer on the water surface, (Figure 1b,
solid line), with a limiting area per molecule of 23-24 Å2.
When C15-benzoic acid is spread on a solution containing
water-soluble C6H4-CN2H3 or CH3-C6H4-CN2H3, the iso-
therm curve becomes expanded with a large kink at A ≈ 30 Å2
preceding the plateau region, followed by a second increase of
the surface pressure at ∼10 Å2 (Figure 1a, short-dashed line).
The amphiphile C17-benzamidiniumchloride spread on an aque-
ous subphase containing C6H5-CO2Na yields a similar isotherm
(Figure 1b, dashed line). Their shapes resemble the π-Α
isotherm of the long-chain mandelic acid on phenylethylamine
solution.1
4-Pentadecylbenzoic Acid, 10. 4-Pentadecylmethylbenzoate (3.0 g,
8.6 mmol) was dissolved in 50 mL of a 5 N solution of KOH in water/
methanol, 1:10 (v/v), and stirred at room temperature for 2 h. The
reaction mixture was acidified with HCl, and the white precipitate was
filtered off, washed with water, and recrystallized from ethanol to give
1
10 (1.78 g, 5.3 mmol, 62%) as a white solid: mp 92 °C. H NMR
(250 MHz, CDCl3): δ ) 0.8 (t, 3H, CH3), 1.1-1.4 (m, 26H, (CH2)13),
1.51-1.62 (m, 2H, CH2) 2.7 (t, 2H, Ar-CH2), 7.2 (d, J ) 8 Hz, 2H,
3-H, 5-H), 7.85 (d, J ) 8 Hz, 2H, 2-H, 6-H).
2.2. Grazing Incidence X-ray Diffraction (GIXD). The GIXD
experiments on the Langmuir films were performed on the liquid-
surface diffractometer at the synchrotron undulator beamline BW1,
HASYLAB (Hamburg Synchrotron laboratory), DESY. The dimensions
of the footprint of the incoming X-ray beam on the liquid surface were
approximately 5 × 50 mm2. The synchrotron white beam was
monochromated with a beryllium (002) crystal to the wavelength of
1.304 Å, and the incident angle R was adjusted to 0.85Rc, where Rc ≈
0.14°. The scattered intensity was collected by means of a vertical
position-sensitive detector, PSD (OED-100-M, Braun, Garching, Ger-
many), which intercepts photons over the range 0 e qz e 0.9 Å-1
(qz is the vertical component of the X-ray scattering vector). The
measurements were performed by scanning across the horizontal
component of the scattering vector, qxy, and simultaneously resolving
qz with the PSD. The diffraction data are represented in three ways:
as contour plots I(qxy,qz); as a pattern that shows the Bragg peak intensity
profiles I(qxy) obtained by integrating the whole qz intensity for any qxy
along the measured range, and finally as Bragg rod profiles that show
the scattered intensity I(qz) in channels along the PSD, integrated over
the whole range in qxy for a Bragg peak.
The influence of the concentration of C6H5-CO2K, present
within the aqueous subphase, on the degree of expansion of
the π-A isotherm of C17-benzamidinium was studied before
the plateau region (40 Å2 < A < 100 Å2). There was no
The qxy positions of the Bragg peaks yield the lattice repeat distance
d ) 2π/qxy, which may be indexed by the two Miller indices h,k to
yield the unit cell. The full width at half-maximum (fwhm) in qxy units
of a Bragg peak yields the 2D crystalline coherence length associated
with the h,k reflection. The fwhm of the Bragg rod profile ) ∆qz gives
(17) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav,
M.; Leiserowitz, L. Phys. Rep. 1994, 246, 251.
(18) Majewski, J. Chem. Eur. J. 1995, 1, 304.
(19) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.

3774 J. Am. Chem. Soc., Vol. 123, No. 16, 2001
Kuzmenko et al.
Figure 1. Surface pressure-molecular area isotherms of various systems involving interactions between benzamidinium and benzoate moieties, as
described in the inset of each graph and in the text. GIXD and XR measurements were performed in the points marked by arrows.
observable change in the isotherm over a broad range in
concentration (3 × 10-5 to 8 × 10-3 M), suggesting that the
water-soluble and water-insoluble components always form a
1:1 salt at the air-solution interface (Figure 1c).
The 1:1 mixture of C15-benzoic acid and C17-benzamidinium
spread on Millipore water forms a stable monolayer, according
to the π-A isotherm measurements, with a limiting area per
molecule of 24 Å2 (Figure 1d, solid line). On aqueous solutions
of benzamidinium (or methylbenzamidinium) benzoate, the
isotherm is generally more expanded. the degree of which
depends on the concentration of the water-soluble components
(Figure 1d, dashed line).
3.2. Grazing incidence X-ray diffraction (GIXD) measure-
ments. C15-benzoic Acid on Pure Water. The measured GIXD
pattern of the acid spread on pure water for a calculated area
per molecule of 25 Å2 contains three reflections with strongly
modulated Bragg rods, corresponding to a multilayer with in-
the π-A isotherm (5 Å2) and the unit cell area Axy ) 35 Å2.
Given the bilayer thickness T of 30.5 Å and an estimate of the
length of the C15-benzoic acid molecule, Lm ≈ 52 Å, we derive
the tilt angle t of the chain from the layer normal [t ) arc cos-
(T/Lm)] to be ∼54°. This value corresponds to a cross-sectional
hydrocarbon chain area Axy‚cos t of 20 Å2.
The packing motif of the C15-benzoic acid at the air-water
interface is not similar to the 3D packing motif of analogous
p-heptylbenzoic acid C7H15-C6H4-CO2H, in which the chains
are interdigitated.20 Furthermore, the multilayer formation of
C15-benzoic acid at the air-water interface is in contrast to the
monolayer assembly of the similar compound 4-(octadecycloxy)-
benzoic acid (C16H33-O-C6H4-CO2H)21 and the regular fatty
acids.22-25 A spontaneous multilayer crystalline formation at
the air-water interface has been observed previously for R-ω
alkane diols,26 diacids,27 simple alkanes,28 and short-chain
R-amino acids.29
plane cell dimensions a ) 6.3 Å, b ) 5.6 Å, γ ) 100°, Axy
)
34.7 Å2 (Figure 2a). The thickness of the film T is ∼90 Å, as
estimated from the average full width at half-maxima (fwhm),
∆qz ) 0.06 Å-1, of the Bragg rod modulations (T ) 0.9(2π/
∆qz)). The average ∆qz difference between the neighboring
maxima of the Bragg rod modulations corresponds to a spacing
of 30.5 Å. On the assumption that the acid molecules form
centrosymmetric hydrogen-bonded cyclic dimers, the spacing
of 30.5 Å is that of a bilayer. The total number of bilayers,
which constitute the film of thickness 90 Å, is therefore equal
to three. About the same total number of bilayers is suggested
by the ratio between the extrapolated area per molecule from
(20) Blake, A. J.; Fallis, I. A.; Parsons, S.; Schroder, M.; Bruce, D. W.
Acta Crystallogr., Sect. C 1995, 51, 2666.
(21) Weissbuch, I.; Berkovic, G.; Yam, R.; Als-Nielsen, J.; Kjaer, K.;
Lahav, M.; Leiserowitz, L. J. Phys. Chem. 1995, 99, 6036.
(22) Peterson, I. R. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1417.
(23) Riviere, S.; Henon, S.; Meunier, J.; Schwartz, D. K.; Tsao, M. W.;
Knobler, C. M. J. Chem. Phys. 1994, 101, 10045.
(24) Peterson, I. R.; Kenn, R. M. Langmuir 1994, 10, 4645.
(25) Kuzmenko, I.; Kaganer, V. M.; Leiserowitz, L. Langmuir 1998, 14,
3882.
(26) Popovitz-Biro, R.; Majewski, J.; Margulis, L.; Cohen, S.; Leiser-
owitz, L.; Lahav, M. AdV. Mater. 1994, 6, 956.
(27) Weissbuch, I.; Guo, S.; Cohen, S.; Edgar, R.; Howes, P.; Kjaer,
K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. AdV. Mater. 1998, 10, 117.

Films Formed Via Amidinium-Carboxylate Interactions
J. Am. Chem. Soc., Vol. 123, No. 16, 2001 3775
Figure 2. GIXD patterns of (a) C15-benzoic acid, (b) C17-benzamidinium, both on pure Millipore water.
C17-benzamidinium Chloride on Pure Water. The am-
phiphile was spread on Millipore water for an area per molecule
A of 35 Å2, and the film was compressed until A ) 25 Å2, at
which point the surface pressure had risen to π ) 1 mN/m.
The GIXD pattern contains a very broad peak (Figure 2b),
reflecting a high degree of molecular disorder. We attribute the
lack of crystalline order as arising from repulsive interactions
between positively charged headgroups.
structure of the same molecular composition (a ) 9.58 Å, b )
5.73 Å, γ ) 90°, Axy ) 54.9 Å2, vide infra), and therefore should
have a similar packing motif. The interlayer distance d in the
film ) 26.4 Å as calculated from the average ∆qz difference of
the Bragg rod modulations. The unit cell volume in the film
(dab sin γ) ) 1449 Å3, is about equal to the corresponding
volume of the 3D crystal structure, (abc)/2 ) 1438 Å3. The
full thickness of the film T ≈ 80 Å, as extracted from the fwhm
of the Bragg rods, is compatible with 6+ crystalline layers,
where the plus sign denotes a partially crystalline top layer
incorporating disordered hydrocarbon chains as depicted in
Figure 4a. This thickness is just over 3 times that observed for
R-pentadecylmandelic acid on R-phenylethylamine,1 that con-
tains one interdigitated bilayer and a top layer with disordered
chains.
Pentadecylbenzoic Acid on p-Methylbenzylamidinium
Solution. The amphiphile C15-benzoic acid was spread on
aqueous solution containing p-methylbenzamidinium (0.008 M)
prepared by neutralizing p-methylbenzamidinium chloride with
an equimolar amount of potassium hydroxide. The GIXD
patterns were measured at three points along the π-A iso-
therm: one before the kink at the beginning of the plateau region
and the two others along the plateau, as marked by arrows in
Figure 1a. In the region before the kink, no Bragg peaks were
observed, indicating an amorphous monolayer. The compression
to the plateau region gave rise to a diffraction signal. The GIXD
pattern measured at A ) 30 and 20 Å2 contains seven reflections
(Figure 3), five of which are relatively intense, and indexed
with an oblique unit cell a ) 10.06 Å, b ) 5.50 Å, γ ) 83.2°,
Axy ) 54.9 Å2. The two remaining peaks (at qxy ) 1.19 and
1.44 Å-1) of near threshold intensity belong to another crystal-
line phase, because all seven peaks could be expressed in terms
of one unit lattice. This minor crystalline phase will not be
further discussed. The unit cell dimensions of the major phase
almost match the intralayer cell dimensions of the 3D crystal
1:1 Monolayer Mixture of Heptadecylbenzamidinium and
Pentadecylbenzoic Acid on Pure Water and on Solution
Containing p-Methylbenzamidinium Benzoate. A 1:1 mixture
of the two amphiphiles was spread on Millipore water and on
a saturated solution of p-methylbenzamidinium benzoate. The
GIXD measurements of the monolayer on pure water were
performed at two points along the isotherm, at A ) 24 Å2, π )
1 mN/m, and at A ) 22 Å2, π ) 10 mN/m. GIXD scans on the
solution were made at 24 Å2 (π ) 20 mN/m). The diffraction
pattern in each case contains two peaks corresponding to a
rectangular crystalline lattice of plane symmetry p1g1 (Table
1, Figure 5a) in which the hydrocarbon chain axes are tilted
from the vertical along the b-axis. On water at A ) 24 Å2 the
rectangular unit cell dimensions are a ) 4.93 Å, b ) 9.42 Å.
The molecular chains are tilted from the layer normal by 29°.
X-ray structure factor calculations with a refined molecular
(28) Weinbach, S. P.; Weissbuch, I.; Kjaer, K.; Bouwman, W. G.;
Nielsen, J. A.; Lahav, M.; Leiserowitz, L. AdV. Mater. 1995, 7, 857.
(29) Weissbuch, I.; Berfeld, M.; Bouwman, W.; Kjaer, K.; Als-Nielsen,
J.; Lahav, M.; Leiserowitz, L. J. Am. Chem. Soc. 1997, 119, 933.

3776 J. Am. Chem. Soc., Vol. 123, No. 16, 2001
Kuzmenko et al.
Figure 3. GIXD pattern of C15-benzoic acid spread on p-methylbenzamidine (0.008 M), measured at ∼20 Å2
model yielded Bragg rod intensity profiles that fit well to the
observed data (Figure 5d,e). On the saturated solution, the
molecules are tilted by 24° from the layer normal in a cell of
dimensions a ) 4.90 Å, b ) 9.06 Å, according to the GIXD
pattern (Figure 5c). The decrease in chain tilt from 29° to 24°,
reflected in the decrease in length of the b axis, can be explained
by the increased surface pressure due to the presence of surface-
active p-methylbenzamidinium benzoate. A GIXD pattern of
the monolayer film on pure water studied at a higher surface
pressure π ) 10 mN/m (Table 1, Figure 5b) indicates a
molecular tilt angle and lattice cell dimensions more akin to
those found on the saturated solution.
Here we describe the 1:1 mixed monolayer structure derived
from the GIXD data. Three-dimensional crystal structures
incorporating amidinium and carboxylic functional groups
generally exhibit the linear hydrogen-bonding array shown in
Scheme 1b, but with a translation repeat of 9.8 Å twice that of
the a axis corresponding to the (1,0) spacing of 4.9 Å observed
in the mixed monolayer phase. This observation suggests that
the hydrogen-bonding array is parallel to the a-direction. This
array cannot be parallel to the b-axis, or even the diagonal a +
b, since the b-axis is reduced in length upon compression (9.4
to 9.0 Å), whereas the a-axis remains unchanged but, however,
is half of the hydrogen-bonding repeat distance. To overcome

Films Formed Via Amidinium-Carboxylate Interactions
J. Am. Chem. Soc., Vol. 123, No. 16, 2001 3777
Figure 4. (a) Proposed model packing arrangement of the interdigitated film of p-methylbenzamidinium C15-benzoate as formed at the air-
solution interface, according to the GIXD data, (b,c) Two views (along a and b) of the 3D structure of p-methylbenzamidinium C15-benzoate.
Table 1. 2D Crystallographic Dataa from GIXD of the Monolayer
of (1:1) C17-benzamidinium and C15-pentadecylbenzoate on Water
and on Saturated Solution of p-Methylbenzamidinium Benzoate
Table 2. Parametersa Fitted to the Observed X-ray Reflectivity
Data of p-Pentadecylbenzoic Acid on an Aqueous Solution of
p-Methylbenzamidinium (0.008 M), Assuming a Three-Box Model
Am
π
I(0,2)/I(1,1)
a
b
t
Axy
ao
bo
Ao
Am
π
N1
L1
N2
L2
N3
L3
σ
t
Ao
On Water:
85
64
55 12
1
9
50 1.3 97 1.8 113
50 1.5 97 2.7 113
50 2.0 97 2.9 113
5.8 3.5 72 26
7.8 3.4 65 27
8.9 3.5 61 27
24
1
10
0.37
0.48
4.93 9.42 29.7 23.2 4.93 8.18 20.1
4.91 9.11 24.0 22.4 4.91 8.32 20.4
44 20b 50 2.4 97 4.0 113 10.0 3.8 57 24
On Solution:
a Am - Nominal area per molecule (in Å2). π - Surface pressure (in
mN/m). Ni and Li (i ) 1,3) are the number of electrons and box length
(in Å). σ - The surface roughness parameter (in Å2). t - Formal value
of tilt angle (deg) of the C15-hydrocarbon chain, assuming it is fully
stretched and has a length of 18.5 Å, as shown in Figure 7(left). Ao -
cross-sectional area (Å2) occupied by one hydrocarbon chain as
calculated based on the molecular tilt t and the corresponding Am. b This
value corresponds to the maximum value right after the monolayer
compression. The pressure was slowly decreasing along the reflectivity
measurement due to monolayer relaxation reaching 12 mN/m.
24 20
0.48
4.90 9.06 24.1 22.2 4.90 8.27 20.3
a π- Surface pressure (in mN/m). Am- Nominal area per molecule at
the water surface. Axy - Unit cell area. Ao - Molecular chain cross-
sectional area (Å2). I(0,2)/I(1,1) - intensity ratio of the (0,2) and (1,1)
reflections. a,b- Unit cell dimensions. ao,bo - Unit cell dimensions as
projected down the molecular chain axis (Å). Molecular chain tilt -
t (deg).
this inconsistency, we propose that the hydrogen-bonding arrays
with a translation repeat of 9.8 Å are aligned parallel to a, but
randomly occupying two possible positions along a differing
by (9.8/2 Å. (Figure 6). These stacking faults along b, smear
out the difference between the acid and the amidinium molecules
in the diffraction pattern to yield an apparent “repeat” distance
along a of 4.9 Å. The molecular disorder is also expressed in
the relatively low density of the packing of the hydrocarbon
chains; the cross-sectional area of the chains are equal to
0.5(ab cos 29°) ) 20.2 Å2, compared with 18.7 Å2 for the
standard herringbone chain packing. This result suggests a
mesomorphous state of the monolayer.
3.3. Specular X-ray Reflectivity Measurements. Penta-
decylbenzoic Acid on Aqueous Subphase Containing p-
Methylbenzamidinium. The proposed model of the interdig-
itation process (Scheme 2) implies, as a prerequisite, an
amorphous mixed monolayer composed of long-chain am-
phiphilic acid or base molecules and the corresponding water-
soluble counterpart. This model agrees with the isotherm
behavior as well as with the GIXD data (no observed diffraction
peaks before the plateau region), but direct experimental
evidence was provided by specular X-ray reflectivity measure-
ments.
measurements Figure 7(a-d) and the treatment thereof are
summarized in Table 2. To fit the experimental curves a fixed
molecular area and a three-box model was used with a fixed
number of electrons in each box. The model tested incorporates
a 1:1 complex of C15-benzoic acid and p-methylbenzamidinium.
Only the lengths of the three boxes and the surface roughness
parameter were refined. The results of the fitting support our
interpretation of the expanded region of the π-A isotherm. In
this region the molecular chains are highly tilted and disordered
due to the difference in the cross-sectional area between a
hydrocarbon chain (18-20 Å2) and the sum of the areas
occupied by the benzamidinium and benzoic acid headgroups
(∼40 Å2), as shown in Figure 7(top, left). According to the
reflectivity analysis, there is a gradual increase in film thickness
upon compression, as a result of a corresponding decrease in
molecular tilt. We note that there is a clear-cut linear dependence
between the thickness L3 of the disordered hydrocarbon chains
and the molecular area A (Figure 8). The estimated cross-
sectional area per hydrocarbon chain of 24-27 Å2 is about 15-
30% larger than the maximum cross-sectional area observed in
mesomorphous monolayer phases incorporating hydrocarbon
chains.25 Such loose packing would account for the lack of a
diffraction signal before the plateau region. Once the minimum
possible area occupied by the two headgroups is reached (∼40
Å2), further compression forces the monolayer to buckle and
The X-ray reflectivity curves of C15-benzoic acid deposited
on aqueous 0.008 M solution of p-methylbenzamidinium were
measured at four points along the π-A isotherm before the
plateau region indicated in Figure 1a. The results of these

3778 J. Am. Chem. Soc., Vol. 123, No. 16, 2001
Kuzmenko et al.
Figure 5. (a-c) GIXD patterns (contour plots) of the mixed monolayer C17-benzamidinium C15-benzoate on water and on solution containing
water-soluble benzamidinium benzoate; (d) observed and calculated intensity distribution along the Bragg rods; (e) model packing arrangement
corresponding to the calculated curves, presented in (d)
Table 3. Parametersa Fitted to the Observed X-ray Reflectivity
interlayer interdigitation to occur. A detailed description of the
Data of a 1:1 Mixture of Heptadecylbenzamidinium and
Pentadecylbenzoic Acid on Pure Water and on a Saturated Aqueous
Solution of p-Methylbenzamidinium Benzoate
buckling process leading to interdigitation is given in the
Discussion.
1:1 Mixture of Heptadecylbenzamidinium and Penta-
decylbenzoic Acid on Pure Water and on Saturated Aqueous
Solution of p-Methylbenzamidinium Benzoate. A three-box
model was used to calculate a fit to the observed and calculated
specular X-ray reflectivity curves from the mixed monolayer
on pure water and on the solution subphase containing p-
methylbenzamidinium benzoate (see Table 3 and Figure 7e,f).
The results of this analysis indicate no detectable binding of
the solute molecules to the preformed monolayer (Scheme 2).
3.4. Single-Crystal X-ray Diffraction Studies. Pentadecyl-
benzoate:p-Methylbenzamidinium. The 3D crystal structure
consists of interdigitated bilayers (Figure 4b,c), as in the 3D
crystal structure of R-pentadecylmandelate:R-phenylethylamine.1
The present structure displays packing features, characteristic
of two reported crystal structures that incorporate only ami-
dinium and carboxylic donor and acceptor functional groups
(vide infra). In these two structures the amidinium and car-
boxylic moieties form heterocyclic hydrogen-bonded dimers
(Scheme 1a) interlinked by N-H‚‚‚O bonds (2.8 Å) to form a
continuous array (Scheme 1b) with a translation repeat of 9.6-
Am coV
π
N1 L1 N2 L2
N3
L3
σ
t
Ao
On Water
23 100
1
23 2.3 40 4.1 129 18.0 2.8 31 20
On Solution
23
95 20 23 3.1 40 4.0 113 19.5 3.0 22 21
a The same type of parameters as in Table 2, but for coV which is
the amphiphilic percentage surface coverage. Molecular tilt t was
calculated based on the length of the fully stretched C17-chain ) 21 Å
and L3 layer thickness; similar values for t were extracted from the
GIXD data (see Table 1).
9.9 Å. This motif is also manifest in the present structure (a )
9.58 Å). The carboxylic and amidinium moieties belonging to
the same hydrogen-bonding row form hydrogen-bonding dimers
with two neighboring rows along the b-axis (Figure 4c), leading
to a hydrogen-bonding bilayer network in the ab plane. The
dihedral angle between the planes of carboxylate and amidinium
neighboring species within a row is close to 90°. An analogous
motif has been observed in the crystal structure of primary
amides defined as the “shallow” glide motif.30

Films Formed Via Amidinium-Carboxylate Interactions
J. Am. Chem. Soc., Vol. 123, No. 16, 2001 3779
Table 4. Crystallographic Data of the 3D Crystal Structures
p-methylbenzyl-
amidinium:
p-pentadecylbenzoate
p-methylbenz-
amidinium
benzoate
empirical formula
formula weight, g
temperature, °C
wavelength, Å
crystal system
space group
unit cell dimensions:
a, Å
C30H46N2O2
466.71(3)
15
0.71073
monoclinic
P21/n
C15H16N2O2
256.30(2)
“
“
orthorhombic
Pba2
9.580(2)
5.734(1)
51.99(1)
92.04(2)
2854.4(3)
4
1.086
0.07
1024.0
18.411(3)
15.954(4)
9.794(2)
90
2876.8(6)
8
b, Å
c, Å
â, deg
volume, Å3
Z
density (calc), g/cm3
absorp. coeff., mm-1
F(000)
1.184
0.08
1088.0
0.3 × 0.3 × 0.6
50
0 < h < 21,
0 < k < 18,
0 < l < 11
2885
crystal size, mm3
2θmax (deg)
index ranges
0.3 × 0.1 × 0.08
60
0 < h < 13,
0 < k < 5,
-73 < l < 73
16173
7066
full-matrix
reflections collected
unique reflections
refinement method
2679
“
Least-Squares on F2
none
weighting scheme
absorp. correction
data/parameters
goodness-of-fit on F 2
R (F0 > 4σ(F0))
largest F (e/Å3)
“
“
Figure 6. Schematic representation of the molecular disorder in the
mixed monolayer C17-benzamidinium C15-benzoate on water and on
solution containing water-soluble benzamidinium benzoate
none
1948/307
1.549
1593/336
0.894
0.0717
0.1019
p-Methylbenzamidinium:Benzoate. The analysis of the 3D
crystal structure of this compound indicates the N-H‚‚‚O
interactions as given in Scheme 1. However, the amidinium and
carboxylic moieties form a channel-like structure rather than
the bilayer arrangement, as observed for p-methylbenzami-
dinium:C15-benzoate. The channel incorporates crystallographic
2-fold axis, but a distorted noncrystallographic 42 symmetry
(Figure 9). Each four-sided channel is constructed from two
symmetry-independent cyclic hydrogen-bonded dimers and two
more dimers generated by the 2-fold symmetry axis.
peak and hole
0.35 and -0.31
0.16 and -0.15
interdigitated bilayers and a top layer that exposes disordered
hydrocarbon chains (Figure 4c).
We now address the question how such crystalline multilayers
are formed on compression. An economic way to release stress
upon compression of Langmuir monolayers may be achieved
by a buckling process yielding a corrugated film according to
experimental31-34 and theoretical studies.35,36 In the Appendix
we describe a simple model for the monolayer buckling. The
model assumes buckling into a one-dimensional ordered array
of inverted arcs, which are connected at line defects (Figure
10). It contains two basic energy parameters: one is the bending
constant κ that determines the energy cost for bending the
monolayer, the other is the line energy ꢀ of the defect, which is
negatiVe by virtue of the acid-base headgroup attraction. Given
these parameters the critical pressure for buckling is predicted
4. Discussion
Deposition of p-pentadecylbenzoic acid on water containing
p-methylbenzamidinium yields an amorphous 1:1 acid-base
monolayer at the air-water interface. Strong acid-base interac-
tions at the interface result in intercalation of the water-soluble
component between the polar headgroups of the amphiphile in
the monolayer. According to the X-reflectivity results the
monolayer almost doubles in thickness upon reduction in surface
molecular area from 85 to 44 Å2. At the latter area the aromatic
rings are aligned almost upright and the hydrocarbon chain
occupies a thickness of almost 10 Å2. This value formally
corresponds to highly tilted linear hydrocarbon chains that are
poorly packed because of their large surface area of about 40
Å2 per repeat unit.
Further compression of the film resulted in an overshooting
of the isotherm at A = 30 Å2 followed by a decrease in surface
pressure to a plateau region. Analysis of the GIXD measure-
ments performed along the plateau region of the π-A isotherm
indicates a transition from the amorphous monolayer to a
crystalline multilayer film. The derived unit cell dimensions of
the multilayer film are very similar to that of the corresponding
macroscopic 3D crystal. The multilayer film comprises three
to be π* = σo - /4ꢀ2/κ where σo is the air-water surface
3
tension. The pressure-area isotherm in the buckling region is
predicted to be concave down (Figure 12) that is consistent with
the shape of the upward part of the measured overshoot (Figure
1a), from which we derive the buckling transition point. This
estimate for the transition point appears to give a reasonable
value for ꢀ. The calculated wavelength of the buckled structure
is 50-100 Å. This value is close to the period of about 200 Å
(31) Saint-Jalmes, A.; Gallet, F. Eur. Phys. J. B. 1998, 2, 489; Saint-
Jalmes, A.; Graner, F.; Gallet, F. Europhys. Lett. 1994, 28, 565.
(32) Bourdieu, L.; Daillant, J.; Chatenay, D.; Braslau, A.; Colson, D.
Phys. ReV. Lett. 1994, 72, 1502.
(33) Fontaine, P.; Daillant, J.; Guenoun, P.; Alba, M.; Braslau, A.; Mays,
J. W.; Petit, J.-M.; Rieutord, F. J. Phys. II Fr. 1997, 7, 401.
(34) Lip, M. M.; Lee, K. Y. C.; Takamoto, D. Y.; Zasadzinski, J. A.;
Waring, A. J. Phys. ReV. Lett. 1998, 81, 1650.
(35) Milner, S. T.; Joanny, J.-F.; Pincus, P. Europhys. Lett. 1989, 9, 495.
(36) Hu, J.-G.; Granek, R. J. Phys. II Fr. 1996, 6, 999.
(30) Leiserowitz, L.; Hagler, A. T. Proc. R. Soc. London 1983, A388,
133.

3780 J. Am. Chem. Soc., Vol. 123, No. 16, 2001
Kuzmenko et al.
Figure 7. X-ray reflectivity measured and calculated intensity profiles for: (a-d) C15-benzoate on p-methylbenzamidinium solution at different
compression ratios and for (e,f) C17-benzamidinium C15-benzoate on pure water and on saturated solution of benzamidinium benzoate
Figure 8. Linear relation between the thickness L3 of the disordered
hydrocarbon chains and the molecular area A of the monolayer model
structure of C15-benzoic acid and p-methylbenzamidinium in a molar
1:1 ratio (see Table 2).
Figure 9. 3D Packing arrangement of p-methylbenzamidinium ben-
zoate viewed along the c-axis.
ethylamine, leading to a “finger”, to explain the formation of
the crystalline 2+ layers composed of an interdigitated crystal-
established by a GIXD experiment on a monolayer film of
arachidic acid on CdCl2 solution at high pH that exhibited a
corrugated structure.37
(37) Fradin, C.; Braslau, A.; Luzet, D.; Alba, M.; Gourir, C.; Daillant,
J.; Grubel, G.; Vignaud, G.; Legrand, J. F.; Lal, J.; Petit, J. M.; Rietord, F.
Physica B 1998, 248, 310.
In a previous study1 we had invoked the presence for a
buckled monolayer of pentadecyl-mandelic acid and phenyl-

Films Formed Via Amidinium-Carboxylate Interactions
J. Am. Chem. Soc., Vol. 123, No. 16, 2001 3781
long-chain zwitterionic amino acids induce binding thereto of
a layer of glycine or of glutamine molecules by hydrogen bonds,
according to epitaxial crystallization and X-ray reflectivity
studies. In the present system we tend to account for the absence
of binding because the hydrogen-bonding patterns of the
monolayer template and that of the 3D crystal structure of the
p-methylbenzamidinium benzoate are distinctly different, unlike
those of the amino acids.
Figure 10. Schematic illustration of the buckled monolayer according
to the model described in the Appendix. The line-defects connecting
the arcs, denoted by short-dashed lines, are in the direction perpendicular
to the plane of the page.
To achieve binding at the air-solution interface appropriate
for replication other types of molecular interactions should be
considered. Experiments involving binding between an am-
phiphilic functionalized nucleic acid base pair and its water-
soluble complement at the air-water interface has been
reported.40 Therefore, we propose to use a mixture of long-
chain nucleic acid base pairs as a template for the binding of
the complementary water-soluble base pairs at the air-water
interface.
Scheme 3
Acknowledgment. We thank the German Israeli Foundation
(GIF); Research Grant Marvin Reinstein; the G. M. J. Schmidt
Minerva Centre of Supramolecular Architectures, the Danish
Foundation for Natural Sciences (DanSync Program), and the
European Community (TMR Contract ERBFM-GECT950059)
for their support We are grateful to HASYLAB at DESY for
X-ray synchrotron beamtime.
Appendix: A Simple Model for Monolayer Buckling
Here we present a simple model that includes the main driving
force for buckling in this system. Models for buckling in other
monolayer systems are discussed elsewhere.36 Let us consider
a one-dimensional buckling array, which, in its cross section,
is simply a collection of inverse arcs (see Figure 10). The
buckles are thus connected at singular line defects. We shall
assume that the acid-base attraction is the one that controls
the line-defect energy, and thus it will be assumed attractive.
Even though the microscopic structure of this defect is loosely
defined, it is simply viewed as a portion of a bilayerssimilar
in its microscopic structure to the one in the final (multilayer)
stateswhich is only a single headgroup wide (but macroscopi-
cally long).
Let ꢀ be the maximum strength of the line energy (energy
per unit length) of the defect, that is, the energy per unit length
when the headgroups are at most close proximity; let σ be the
surface tension σ ) σo - π, where σo is the water-air surface
tension and π the external pressure; let κ be the bending modulus
(units of energy).38 With these parameters, simple dimensional
analysis dictates that the critical tension σ* for buckling into
the array described above should scale as σ* ≈ ꢀ2/κ. This will
be confirmed below.
line bilayer and a top layer whose hydrocarbon chains are
disordered. For the formation of the 4+ or 6+ layers as found
in the compressed film of p-pentadecylbenzoic acid and
p-methylbenzamidinium, a model may be invoked in which
individual domains comprise “fingers” of 2+ layers as shown
in Scheme 3. Further compression bringing such fingers into
proximity can lead to a 4+ layer system as depicted in Scheme
3, and by a similar route to the 6+ layer.
We define r as the radius of curvature of each arc and R as
the arc opening-angle (Figure 10). The period of the buckling
structure is thus given by λ ) 2r sin(R/2), and its amplitude is
U ) r- r cos(R/2). We now calculate the Helmholtz free-energy
F of the system whose size is L2 (L is the linear dimension).
The second goal of the present work was to use the
amphiphilic polar headgroups of the crystalline mixed acid-
base monolayer as a template for induced complementary
binding of the water-soluble acid and amidinium ions at the
air-solution interface via hydrogen bonds. However, the X-ray
reflectivity studies presented here did not indicate binding of
the solute molecules to the monolayer film. It has been
established that in aqueous solution there is no observed binding
between the carboxylate and amidinium moieties.7 On the other
hand we might have expected binding to occur at the air-
solution interface by virtue of cooperative hydrogen bonding
as indeed observed in the 3D crystal structure of the analogous
interdigitated system. For example, crystalline monolayers of
Consider first the Helfrich bending free-energy.38 Here we shall
2
neglect the spontaneous curvature Co since we expect κCo
ꢀ2/κ. The bending free-energy of one stripe is thus38
,
(38) Helfrich, W. F. Z. Naturforsch. 1973, 28c, 693.
(39) Ben-Shaul, A. In Structure and Dynamics of Membranes; Lipowsky,
R., Sackmann, E., Eds.; Elsevier: Amsterdam, 1995; Chapter 7 and
references therein.
(40) Ariga, K.; Kunitake, T. Acc. Chem. Res. 1998, 31, 371-378.

3782 J. Am. Chem. Soc., Vol. 123, No. 16, 2001
Kuzmenko et al.
Figure 11. Buckling wavelength λ and buckling amplitude U against
the ratio of surface tension σ ) σo - π to critical tension σ*, which is
a measure of the relative distance from the buckling transition. Both λ
and U are measured in units of κ/ꢀ, the ratio of bending modulus to
line energy.
Figure 12. Trough pressure π against trough area A for the buckled
monolayer. The pressure is normalized to (π - π*)/(σo - π*), where
π* is the buckling critical pressure and σo is the air-water surface
tension, as a measure of the relative distance from the buckling
transition. The area is normalized to A/A* where A* is the critical trough
area. The plot does not depend on the model energy parameters.
κ
ꢀbend
)
RrL
(1)
2r2
We shall assume that for all angles R < π ) 180° the deviation
of the line energy from its maximum value ꢀ (at R ) π ) 180°)
is determined by the increase in distance between the opposing
acid and base headgroups, and this deviation will be calculated
within the harmonic approximation. Therefore, the dependence
of line energy on R is
Using these results, we can calculate the predicted pressure
area isotherm, if we assume that the area per molecule on the
buckled surface does not change during compression. Even
though this cannot be exactly so, it is expected to be a good
approximation. Therefore the change in the projected area (per
molecule), which is the area measured by the Langmuir trough,
is purely due to buckling. Using σ ) σo - π, where σo is the
air-water surface tension, we find the following relation
ꢀline ) -ꢀL sin2 (R/2)
(2)
The free-energy density f ) F/L2 is thus
π - π*
σo - π*
4
) 1 - σ˜(R)
3
(7)
R κ
2 r
- ꢀ sin2(R/2) + σRr
f )
(3)
2r sin(R/2)
The ratio of projected area A to the real area, which (as discussed
above) is assumed to be equal to the critical area Α*, is related
to R by
The free-energy eq 1 is now minimized with respect to R and
r. The buckling transition is defined as the value of σ (or π) at
which R changes from zero to a positive value. Minimizing over
r we obtain the relation
2 sin(R/2)
A
A*
)
(8)
R
κ
ꢀ
R
sin R
r )
(4)
2
Using these relations we plot in Figure 12 (π - π*)/(σo - π*)
against A/A*. This plot is universal (within the model assump-
tions), that is, it is independent of κ and ꢀ. Note that the overall
shape of the isotherm is concaVe down, although it is, in
fact, linear close to criticality, (π - π*)/(σo - π*) =
(37/5)(1 - A/A*)
Since direct evidence for buckling in this system is yet to be
obtained, we have to rely on the experimental π - A isotherm.
In principle, a singular change in the slope has to be observed
at the buckling transition if it is second-order. This, however,
may be a too small change to be observed. Nonetheless, the
above calculation also shows that the shape of the curve in the
buckling region has to be concave down, whereas for a flat
monolayer it should always remain convex, for example, as in
the case of an ideal gas (π ) kBT/A). This means that the
buckling transition can be assumed to occur if there is an
inflection point in the isotherm, that is, when ∂π/∂A reaches a
minimum, signifying a transition from convex to concave shape.
For the isotherm of the C15-benzoic acid on benzamidinium
solution (see Figure 1a), it is possible (even though somewhat
difficult) to detect such an inflection point (before the turndown
Minimizing over R and using eq 4 we find
sin3(R/2)(2 sin(R/2) - 3R cos(R/2))
κσ
ꢀ2
≡ σ˜(R); σ˜(R) )
2R2(R cot(R/2) - 2)
(5)
Plotting σ˜(R) against R, we find that it is a monotonically
decreasing function starting at a value σ˜(0) ) 3/4. Thus the
critical tension for buckling is
3 ꢀ2
4 κ
σ* )
(6)
obeying the expected scaling. The buckling transition is a
second-order one. To find how R and r, and hence λ and U,
change as σ is decreased below σ*, we need to solve eq 5
numerically. In Figure 11 we plot the resulting λ and U, in units
of κ/ꢀ, for 0 < σ/σ* < 1. It is seen that λ remains in the range
4 - 5.5 × κ/ꢀ, whereas U increases from zero to about 2κ/ꢀ.

Films Formed Via Amidinium-Carboxylate Interactions
J. Am. Chem. Soc., Vol. 123, No. 16, 2001 3783
of the overshoot) at about π′ ) 27 dyn/cm, slightly above the
monolayer-trilayer coexistence πc ) 23 dyn/cm.
is a reasonable number. It is, however, difficult to get an
independent estimate for this energy, since it involves accurate
counting of hydrogen bonds formed between acid and base
groups in the bilayer configuration and the number of hydrogen
bonds broken in dehydration. With these estimates, the basic
scale κ/ꢀ is estimated to be about 10-20 Å. This implies, for
instance, that the wavelength λ is in the range 50-100 Å, which
is quite small. It is just sufficiently longer than the size of an
headgroup to make our continuum approach valid.
By using calculations of Ben-Shaul and co-workers,39 the
bending constant due to alkane-tail conformations is roughly
10kBT/n/10)3.2 (ao/32)-7.5, where n is the number of CH2 units
and ao is the area per headgroup measured in Å2. Thus, for a
typical area in the range of ao ) 35 - 40 A2 and n ) 15, we
estimate κ in the range 7 - 18kBT. Using eq 6 for σ*, this
implies ꢀ = 4 ( 1 × 10-6. Since the linear size of a single
headgroup consisting of an acid-base pair is about 10 Å, we
find that the net attraction between two such headgroups is about
4 ( 1 × 10-13 erg = 10 ( 2 kBT (= 6 ( 1 kcal mol-1), which
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