Phase behavior and molecular packing of octadecyl phenols and their methyl ethers at the air/water interface

Article abstract of DOI:10.1021/la404340h

Noncovalent molecular interactions, such as hydrogen bonding and van der Waals forces, play an important role in self-assembling to supramolecular structures. To study these forces, we chose monolayers at the air/water interface to limit the possible arrangements of the interacting molecules. Furthermore, monolayers provide useful tools to understand and study interactions between molecules in a controlled and fundamental way. The phase behavior and molecular packing of the phenols 1-(4-hydroxyphenyl)-octadecane (5a), 1-(3,4-dihydroxyphenyl)-octadecane (6), and 1-(2,3,4-trihydroxyphenyl)- octadecane (3) and their methyl ethers in monolayers at the air/water interface have been examined by π/A isotherms, Brewster angle microscopy (BAM), grazing incidence X-ray diffraction (GIXD) measurements, and density functional theory (DFT) calculations. The phenols are synthesized by Friedel-Crafts acylation of methoxybenzenes, hydrogenation of the resulting aryl ketones, and cleavage of the aryl methyl ethers. In the π/A isotherms and in BAM, the phenols show patches of the solid condensed phase at large molecular areas and the monolayers collapse at high pressures. Furthermore, the dimensions of the unit cell obtained by GIXD measurements are compatible with an arrangement of the phenyl rings that allows one aryl ring to interact with four adjacent phenyl rings in an edge-to-face arrangement, which leads to a significant binding energy. The experimental data are in good agreement with DFT calculations of 2D crystalline benzene and p-cresol arrangements. The enhanced monolayer stability of phenol 5a can be explained by hydrogen bonds of the hydroxyl group with water and van der Waals forces between the alkyl chains and aryl-aryl interactions.

Full text of DOI:10.1021/la404340h

Article
pubs.acs.org/Langmuir
Phase Behavior and Molecular Packing of Octadecyl Phenols and
their Methyl Ethers at the Air/Water Interface
Miroslawa Peikert,† Xiadong Chen,‡,∥ Lifeng Chi,‡ Gerald Brezesinski,§ Simon Janich,†
Ernst-Ulrich Wurthwein,† and Hans J. Schafer*,†
̈
̈
†Organisch-Chemisches Institut der Westfalischen Wilhelms-Universitat, Correns-Str. 40, 48149 Munster, Germany
̈
̈
̈
‡Physikalisches Institut der Westfalischen Wilhelms-Universitat, Wilhelm-Klemm-Str. 10, 48149 Munster, Germany
̈
̈
̈
§Max-Planck-Institut fur Kolloid- und Grenzflachenforschung, Abteilung Grenzflachen, Wissenschaftspark Golm, 14476 Potsdam,
̈
̈
̈
Germany
S
* Supporting Information
ABSTRACT: Noncovalent molecular interactions, such as
hydrogen bonding and van der Waals forces, play an important
role in self-assembling to supramolecular structures. To study
these forces, we chose monolayers at the air/water interface to
limit the possible arrangements of the interacting molecules.
Furthermore, monolayers provide useful tools to understand
and study interactions between molecules in a controlled and
fundamental way. The phase behavior and molecular packing
of the phenols 1-(4-hydroxyphenyl)-octadecane (5a), 1-(3,4-
dihydroxyphenyl)-octadecane (6), and 1-(2,3,4-trihydroxy-
phenyl)-octadecane (3) and their methyl ethers in monolayers
at the air/water interface have been examined by π/A
isotherms, Brewster angle microscopy (BAM), grazing
incidence X-ray diffraction (GIXD) measurements, and density functional theory (DFT) calculations. The phenols are
synthesized by Friedel−Crafts acylation of methoxybenzenes, hydrogenation of the resulting aryl ketones, and cleavage of the aryl
methyl ethers. In the π/A isotherms and in BAM, the phenols show patches of the solid condensed phase at large molecular areas
and the monolayers collapse at high pressures. Furthermore, the dimensions of the unit cell obtained by GIXD measurements are
compatible with an arrangement of the phenyl rings that allows one aryl ring to interact with four adjacent phenyl rings in an
edge-to-face arrangement, which leads to a significant binding energy. The experimental data are in good agreement with DFT
calculations of 2D crystalline benzene and p-cresol arrangements. The enhanced monolayer stability of phenol 5a can be
explained by hydrogen bonds of the hydroxyl group with water and van der Waals forces between the alkyl chains and aryl−aryl
interactions.
monolayer structure at the air/water interface has been
obtained by grazing incidence X-ray diffraction (GIXD)
measurements combined with computer calculations.24−27
Investigations of the monolayer of 4-alkylphenol, 4-alkylani-
line, and 4-alkylacetanilide belong to the first applications of the
Langmuir balance.28 To our knowledge, there are only few
further studies on the behavior of amphiphiles at the air water/
interface, where a phenyl group is inserted between a polar
methoxy or hydroxy headgroup and the alkyl chain. They
concern aromatic azo-compounds containing long alkyl
chains29 and the packing of 4-octadecylphenol in Langmuir−
Blodgett layers.30
INTRODUCTION
■
Noncovalent bonds are essential for self-assembling of
supramolecular structures.1,2 They include hydrogen bonding,3
ion pairing,4,5 dipole−dipole interactions,6 hydrophobic inter-
actions,7−9 aryl−aryl interactions,10,11 and van der Waals
interactions.2,12 Hydrogen bonds are especially important due
to their variable strength and their structure defining align-
ment.3 The aryl−aryl interaction plays an important role in
folding,13,14 in stabilization,15,16 and in recognition processes of
proteins.17,18 To study the combination of hydrogen bonding,
aryl−aryl interaction and van der Waals forces, we chose
monolayers at the air/water interface to limit the arrangements
of the interacting molecules.
Information on noncovalent interactions in amphiphilic
assemblies at the air/water interface has been gained from π/
A isotherms measured with the Langmuir film balance19 and
the combination with Brewster angle microscopy (BAM)20,21
or fluorescence microscopy.22,23 Near atomic resolution of the
At ambient pressure, phenol molecules form in a 3D-crystal
helical hydrogen bonded chains .31 In crystals obtained at high
Received: November 9, 2013
Revised: January 3, 2014
Published: January 8, 2014
© 2014 American Chemical Society
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The surface pressure was measured with a Wilhelmy system on
Millipore purified water as subphase at a temperature of 20 °C. For the
preparation of the monolayers, the compounds were dissolved in
trichloromethane, p.a. (1−2 mg mL−1) and put on the subphase with a
gastight syringe (100 μL). After waiting for 15 min to let the solvent
evaporate the amphiphiles were compressed with a rate of 80 cm2·
min−1, if not stated otherwise.
pressure (0.16 GPa) phenol produces H-bonded linear chains,
where the aryl rings above and below the chain of H-bonds
adopt a coplanar arrangement.32 In m-cresol, 2,3- and 2,5-
dimethylphenol the phenols are linked by hydrogen bonds to
produce in most cases parallel chains.33,34 Parallel arrangements
of benzene rings are of interest, because an attraction between
the benzene rings arranged in this fashion has been
calculated.35−37
To provide further information on the interaction of phenyl
groups in monolayers and on hydrogen bonding of the hydroxy
group in phenols and of the methoxy group in aryl methyl
ethers to water, we investigated π/A isotherms, applied
Brewster angle microscopy (BAM) and GIXD measurements
combined with computer calculations to compounds 1−7
(Scheme 1). The hydroxy- and methoxy-substituted 1-phenyl-
Brewster Angle Microscopy. Film balance measurements and
the Brewster angle microscopy were obtained with different troughs
and in different experiments. To allow an unequivocal comparison of
these data the same x-axis with units in area per molecule [nm2] is
used. Brewster angle microscopy was performed on a Langmuir film
balance of type 601 BAM (Nima, Coventry, U.K.) with a trough area
of 750 cm2 and compression rates of 10−100 cm2·min−1 combined
with a BAM2+ (Nanofilm Technologie GmbH, Gottingen, Germany)
̈
with a Nd:YAG laser as light source. The p-polarized laser beam is
reflected at the air/water interface at the Brewster angle. The reflected
beam is recorded with a charge-coupled device camera, and the data
were digitally saved. The objective (10-fold magnification) provides a
diffraction-limited lateral resolution of approximately 2 μm. The size of
the BAM images in this paper is 430 × 537 μm2.
Scheme 1. Structures of the Investigated Amphiphiles 1−7
Grazing Incidence X-ray Diffraction (GIXD). The lateral
structures in condensed monolayers at the air/water interface were
investigated using grazing incidence X-ray diffraction measurements at
the BW1 beamline, HASYLAB, DESY (Hamburg, Germany). The
Langmuir film balance was thermostatted (20 °C) and placed into a
hermetically closed container filled with moistened helium. The
Langmuir trough was equipped with a single movable barrier and a
Wilhelmy plate for monitoring the lateral pressure. At BW1, a
monochromatic X-ray beam (λ = 0.1304 nm) strikes the water surface
at a grazing incidence angle αi = 0.85αc (αc = 0.13°) and illuminates
roughly 2 × 50 mm2 of the monolayer surface. During a diffraction
experiment, the trough was laterally moved to avoid sample damage by
the strong X-ray beam. A linear position-sensitive detector (PSD,
OEM-100-M, Braun, Garching, Germany) measured the diffracted
signal and was rotated to scan the in-plane Qxy component of the
scattering vector. The vertical channels of the PSD measured the out-
of-plane Qz component of the scattering vector between 0.0 and 1.0
Å−1. The diffraction data consist of Bragg peaks at diagnostic Qxy
values. The in-plane lattice repeat distances d of the ordered structures
in the monolayer were calculated from the Bragg peak positions: d =
2π (Qxy)−1. To access the extent of the crystalline order in the
monolayer, the in-plane coherence length Lxy, was approximated from
the full-width at half-maximum (fwhm) of the Bragg peaks using Lxy
∼
0.9(2π) (fwhm(Qxy))−1. The diffracted intensity normal to the
interface was integrated over the Qxy window of the diffraction peak
to calculate the corresponding Bragg rod. The thickness of the
monolayer was estimated from the fwhm of the Bragg rod using
0.9(2π)(fwhm(Qz))−1. Experimental details are described in the
literature.38−42
RESULTS
■
Synthesis. 1-Phenyloctadecane (1a) is synthesized by
Friedel−Crafts acylation of benzene and subsequent hydro-
genation of the aryl ketone. The aryl ethers 2a, 3, and 4 are
prepared analogously to 1a from the corresponding mono-, 1,2-
di-, and 1,2,3-trimethoxybenzene in 76%, 76%, and 71% yield,
respectively. The phenols 5a, 6, and 7 are obtained from the
aryl methyl ethers 2a, 3, and 4 by cleavage with boron
tribromide in a respective yield of 95%, 96%, and 76%. For
detailed experimental procedures, see Supporting Information,
part A. Isolation, purification, yields, melting points, Rf values,
octadecanes 2−7 are compared with 1-phenyloctadecane (1a).
The methoxy group can act only as hydrogen bond acceptor,
whereas the hydroxy group can be both a hydrogen bond donor
and acceptor. The behavior of 1a, 2a, and 5a is also compared
with that of the corresponding alkanes with a similar molecule
length but without a phenyl ring, namely, the hydrocarbon 1b,
the ether 2b, and the alcohol 5b, to study the influence of the
aryl group.
1
IR, H NMR, 13C NMR, and MS data and elemental analyses
EXPERIMENTAL SECTION
■
are given there. These data and gas chromatography indicate a
purity of >99% for the compounds.
π/A Isotherms and Brewster Angle Microscopy (BAM).
The monolayers of the hydrocarbon 1a, the phenols and
phenol methyl ethers 2−7, and aliphatic analogues were studied
Preparation. Preparation of the phenyloctadecanes 1a, 2a, 3, 4, 5a,
6, and 7 is reported in the Supporting Information, part A.
Film Balance Measurements. π/A isotherms at the air/water
interface were obtained with a Langmuir film balance type 622 (Nima,
Coventry, U.K.) with a trough area of 1300 cm2.
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by π/A isotherms and BAM. For details of film balance
measurements and the Brewster angle microscopy, see
Experimental Section. Figure 1 shows the π/A isotherms of
At low surface pressure, compound 5a exhibits three low-
order diffraction peaks characteristic for an oblique chain
lattice. Compression leads to a transition to a condensed phase
with a centered rectangular lattice of the chains. The degenerate
Bragg peak above the horizon and the nondegenerate one at
zero Qz are indicative of the L2 phase. The chains are tilted in
the direction toward nearest neighbors (NN) along the short
axis of the in-plane unit cell. The lattice is distorted from
hexagonal packing in NN direction (Qnxy > Qxdy, where Qxny is the
maximum position of the nondegenerate peak). With increasing
pressure, the degenerated peak moves to slightly larger Qxy and
slightly smaller Qz values, indicating that the tilt angle of the
chains decreases only marginally (see Table 1).
The introduction of a second OH-group (phenol 6) leads to
a strong increase of the chain tilt and to a different phase
structure. This phase (Ov) is characterized by an intensity
distribution with two diffraction peaks at nonzero Qz values
with Qnz = 2Qdz. The chains are tilted in the direction of the
next-nearest neighbors (NNN). As in the case of compound 5a,
the tilt angle decreases only slightly upon compression. In both
cases, the cross-sectional area of the chains around 0.2 nm2 is
indicative for a rotator phase. However, plotting the lattice
distortion as a function of sin2 of the tilt angle (Figure S2 in
Supporting Information, part B) shows that the extrapolated
value for zero tilt deviates distinctly from zero (−0.0712). This
gives a hint that also the packing (in this case of the phenol
rings, which cannot rotate freely) has a certain influence on the
chain lattice distortion.43
Figure 5 shows a 3D surface plot together with the
corresponding contour plot of a monolayer of compound 7
taken at 40 mN·m−1. The scattering intensity is distributed
continuously over a certain Qxy and Qz range. Such intensity
distribution has been explained by a monolayer structure with a
defined chain tilt but an undefined tilt direction.43 Assuming
that the intensity at the largest Qz value characterizes the
nondegenerate Bragg peak of a NNN-tilted phase, we were able
to estimate the tilt angle at different lateral pressures (36.2° at
10 mN·m−1, 31.8° at 25 mN·m−1, 26.1° at 40 mN·m−1).
Compound 3 forms an oblique phase with strongly tilted
chains (a contour plot is shown in Figure S1, Supporting
Figure 1. π/A isotherm of 1a, 1b, and 1c on high purity water as
subphase at 20 °C, film balance type 622 (Nima, trough area 1300
cm2), compression rate = 80 cm2·min−1.
the arylalkane 1a, the alkane 1b, and docosanoic acid (1c).
Figure 2 displays the π/A isotherms of aryl methyl ether 2a at
different compression rates together with the corresponding
BAM images.
π/A isotherms of 5a at different compression rates together
with the corresponding BAM images are presented in Figure 3.
Grazing Incidence X-ray Diffraction (GIXD). GIXD was
applied to elucidate the two-dimensional symmetry of the
molecular in-plane structures of selected monolayers on the Å-
scale. GIXD is sensitive to the condensed parts of the
monolayer, while the liquid-expanded phase contributes to
the background scattering only. All diffraction studies were
performed on water at 20 °C at different lateral pressures
between 1 and 40 mN·m−1.
Figure 4 shows selected contour plots of the corrected X-ray
diffraction intensities as a function of the in-plane scattering
vector component Qxy and the out-of-plane scattering vector
component Qz of the phenols 5a and 6 obtained at 1, 20, and
40 mN·m−1.
Figure 2. Left: π/A isotherms of 2a at different compression rates. Right: BAM images of 2a, which refer to the isotherm taken with the compression
rate of 10 cm2·min−1. A, B are taken at 0.5 nm2· molecule−1. Encircled in B are 3D-structures recognized as brighter regions with higher reflectivity.
The pictures in A, B as well as those in C, D and in E, F were taken at slightly different times to document the heterogeneous distribution of the
structures; C, D were taken at 0.25 nm2·molecule−1 and E, F at about 0.1 nm2·molecule−1. Conditions: high purity water as subphase at 20 °C, film
balance Type 622 (Nima, trough area 1300 cm2).
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Figure 3. Left: π/A isotherms of 5a at different compression rates. Right: BAM images of 5a, which refer to the isotherm taken with the compression
rate of 10 cm2·min−1. A−D were taken at 0.25−0.5 nm2 molecule−1, and E, F taken at the area marked by the arrows. For conditions, see Figure 1.
Further results are presented in Figures S1−S15 in the
Supporting Information; they are discussed in connection with
the results in Figures 1−6 and Tables 1 and 2.
DISCUSSION
■
Phenyloctadecane (1a), n-Docosane (1b), and Doco-
sanoic Acid (1c). The π/A isotherm of n-docosane (1b)
shows down to an area of 0.025 nm2·molecule−1 no visible
increase of the film pressure (Figure 1). For 1-phenyl-
octadecane (1a), a slight increase can be seen starting at 0.2
nm2·molecule−1. In contrast to that, the film pressure of
docosanoic acid (1c) shows a steep increase at 0.25 nm2·
molecule−1. If n-docosane (C22H46) would form a stable
monolayer at the air/water interface, the cross-section area in
the solid condensed state (SC) would be around 0.2 nm2·
molecule−1, and at this area an increase of the pressure would
be expected.45 However, similar to the case of n-tetracosane
(C24H50),46 the molecules of n-docosane (1b) probably form
multilayers already during the spreading process. GIXD-
investigations of n-tetracosane at the air/water interface show
that this hydrocarbon aggregates at an area of 0.1 nm2·
molecule−1 and a film pressure of 0 mN·m−1 to form a stack of
more than 20 layers.46
The slow rise in the π/A isotherm of 1-phenyloctadecane
(1a) between an area of 0.21 and 0.07 nm2·molecule−1 to a
pressure of only 8.5 mN·m−1 also indicates the formation of
multilayers. The small pressure increase, compared to 1b, could
be due to a weak interaction between the phenyl ring and
water. For benzene/water the calculated interactions are
stronger (−0.96 to −1.04 kcal·mol−1) than those between
cyclohexane/water (−0.38 to −0.49 kcal·mol−1).47
Figure 4. Contour plots of the diffracted X-ray intensities, corrected
for polarization, effective area, and Lorentz factor, as function of the in-
plane and out-of-plane scattering vector components Qxy and Qz,
respectively, for 5a (top) and 6 (bottom) at lateral pressures of 1 mN·
m−1 (left), 20 mN·m−1 (center), and 40 mN·m−1 (right). For 5a the
phase sequence, oblique−L2 can be seen. The transition occurs
between 1 and 10 mN·m−1. The monolayer of compound 6 forms the
Ov phase in the whole pressure region investigated.
Information, part B). Again, the tilt angle decreases only slightly
with compression (see Table 1).
Further information about the two-dimensional packing
features can be obtained from the thickness of the monolayers.
The thickness of the diffracting layer can be estimated using Lz
∼ 0.9·2π (fwhm(Qz))−1. The hydrocarbon chains are in the all-
trans conformation so that a molecule length of 2.29 nm can be
expected because the maximum length of a stretched alkyl chain
with n CH2 groups amounts to lmax = (n·0.126 + 0.15) nm.44 In
the case of the monolayers investigated, the measured
fwhm(Qz) amounts to approximately 2.6 nm−1 (the vertical
instrumental resolution is negligible in this case) giving a
thickness of 2.17 nm. This shows that the diffraction signal
arises only from the alkyl chains and not from the aryl rings.
The steep rise of the π/A isotherm of docosanoic acid (1c)
(Figure 1) with a high collapse pressure indicates a transition
from a 2D gas-analogous phase to a condensed phase based on
strong interactions between the molecules (hydrogen bonds
between carboxyl groups and between carboxyl groups and
water as well as van der Waals interactions between the long
alkyl chains). Additionally, the slope of the isotherm changes at
∼30 mN·m−1 due to a second-order transition into an
incompressible phase with nontilted molecules. It is important
to note that the condensed phases of fatty acids are
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Table 1. Lattice Parameters a, b, and γ of the Unit Cell, Tilt of the Chains from the Surface Normal, and in-Plane Area Axy
compd
pressure (mN·m−1
)
a (nm)
b (nm)
γ (deg)
tilt (deg)
Axy (nm2)
5a
1
10
20
30
40
0.5641
0.5642
0.5613
0.5596
0.5588
0.4937
0.4886
0.4875
0.4854
0.4833
110.2
125.3
125.1
125.2
125.3
28.1
26.2
26.2
25.8
25.2
0.228
0.225
0.224
0.222
0.220
6
3
1
10
20
30
40
0.4980
0.4954
0.4947
0.4945
0.4930
0.5818
0.5688
0.5563
0.5493
0.5422
115.3
115.8
116.4
116.8
117.0
39.1
37.4
35.0
33.8
32.5
0.262
0.254
0.246
0.243
0.238
5
0.5178
0.5125
0.6339
0.6078
113.9
113.8
48.1
44.3
0.300
0.285
20
Irregular, porous 2D and 3D structures are already visible at 0.5
nm2·molecule−1. The latter ones can be recognized as brighter
regions with higher reflectivity (encircled in Figure 2 B). At
lower areas (0.25 nm2·molecule−1), the size of the 2D and 3D
structures increases, the 3D structures do that to a larger extent.
At 0.1 nm2·molecule−1, the 3D structures cover nearly
completely the water surface. The formation of 3D structures
explains the smaller area required for the aryl ether 2a in the
condensed phase. Contrary to the case of 2a, the aliphatic ether
2b shows only 2D structures in the BAM images (not
displayed). The different behavior could be due to a mesomeric
delocalization of the lone electron pair at the oxygen atom into
the aromatic ring in 2a, which would weaken the hydrogen
bond of water to the ether group.
Figure 5. 3D plot (left) and contour plot (right) of the corrected X-
ray intensities as function of the in-plane and out-of-plane scattering
vector components Qxy and Qz, respectively, for 7. Note that the
diffracted intensity is distributed along a characteristic arc in reciprocal
space.
The aryl ether 3 exhibits a steep pressure increase at 0.34
nm2·molecule−1 (Figure S5, Supporting Information, part C)
indicating that the monolayer is compressed without any
pressure increase until the LC or SC phase is reached (two-
phase coexistence region between gas-analog and condensed
phases), from which the collapse occurs at around 30 mN·m−1
(Figure 5, Supporting Information, part C). This shows that the
binding of 3 to the water surface is much stronger than that of
2a. The overall basicity of the two methoxy groups in 3 is
higher than that of one in 2a as in 3 less electron density is
delocalized to the benzene ring per methoxy group. The π/A-
isotherm of the aryl ether 4 increases less steeply (lift-off point
at 0.39 nm2·molecule−1, Figure S5), which points to a higher
compressibility of the layer. One would expect that the collapse
pressure of the aryl ether 4 increases compared to that of 3,
because of the additional methoxy group, which, however, is
not the case. An explanation for both observations could be: the
additional methoxy group enlarges both the width and the
thickness of the aryl ring; the latter expands due to out of plane
conformations of the methoxy groups. This should weaken the
attraction between the phenyl rings and between the alkyl
chains and lead to a less densely packed LC phase compared to
that of 3. Furthermore, the third methoxy group in 4 has
probably a larger distance to the water surface than the two
other ones, which lessens the strength of the hydrogen bond.
The molecular area of 3, determined from the π/A isotherm,
agrees well with the one obtained from the GIXD measure-
ments. The extremely large tilt angle of nearly 50° at low
pressure (Table 1) is due to the large mismatch between the
area requirement of the aryl group and the alkyl chain. The tilt
angle decreases on compression until the limiting area of
thermodynamically metastable (the equilibrium spreading
pressure of zero indicates that condensed monolayers are at
all pressures metastable compared to the stable 3D crystalline
state).
Docosyl Methyl Ether (2b) and Methoxyphenyl
Octadecanes 2−4. The docosyl methyl ether (2b) (Figure
S4, Supporting Information, part C) shows a similar π/A
isotherm as docosanoic acid (1c) (Figure 1) and the docosyl
ethyl ether.48 However, the behavior of the aryl ether 2a is
different from that of 1-phenyloctadecane (1a) and of docosyl
methyl ether (2b). The small lift-off area of 0.08 nm2·
molecule−1 in the π/A isotherm of 2a (Figure 2) indicates
that a significant amount of 2a is lost from the air/water
interface during compression. This could be due to a partial
dissolution of 2a in the subphase or due to the formation of a
multilayer extending into the air. The distribution coefficient of
2a in n-octanol and water amounts to log Pn‑octanol/water
=
11.63,49 which indicates that 2a is insoluble in water and makes
the second assumption more probable.
The lift-off area of 2a depends on the compression rate,
which was varied from 50 to 10 cm2·min−1 (Figure 2). The
lower the rate the smaller is the apparent molecular area, what
is in accord with the formation of a larger portion of 3D
structures (areas with multilayers seen as brighter regions in
BAM images). Some 3D crystallites are already formed during
the spreading process and act as seeds for further crystallization.
The growth process is time-dependent explaining the
appearance of larger 3D structures at slower compression.
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approximately 0.27−0.28 nm2·molecule−1 for the packing of the
aryl rings is reached. The possible arrangement of the aryl rings
in the 2D crystal of 3 is discussed together with that of the
phenols 5a and 6 below.
Hydroxyphenyloctadecanes 5−7 and Docosanol (5b).
The π/A isotherm of 5a rises steeply at 0.24 nm2·molecule−1 to
a collapse pressure of 58 mN·m−1 (Figure 3 and Figure S6
Supporting Information, part C). The low compressibility of
the monolayer is also seen in the low pressure dependence of
the in-plane area Axy (Table 1) observed by GIXD (only 3.5%
between 1 and 40 mN·m−1). The isotherms of phenol 5a are
independent of the compression rate (Figure 3) in contrast to
those of the ether 2a (Figure 2, left). The 2D structures in the
BAM images of 5a appear already at large areas (Figure 3,
right) in the two-phase coexistence region between gas-analog
and condensed phases. The fissured structures of aryl ether 5a
have some similarity with these of aryl ether 2a (Figure 2,
right). However, they cover at comparable compression rates
larger areas and show no multilayer formation (3D structures),
which would be indicated by brighter regions. A further
difference is that their molecular area of 0.23 nm2·molecule−1 is
independent from the compression rate.
The different collapse behavior of the phenol 5a and the aryl
ether 2a appears to be partly due to the stronger hydrogen
bond of phenol 5a, whose hydroxy group is both a hydrogen
bond donor and acceptor, whereas the methoxy group in ether
2a is only a hydrogen bond acceptor.
The phenol 5a and the corresponding aliphatic alcohol 5b
(Figure S6, Supporting Information, part C)26,50 behave
differently at the air/water interface. The first and expected
difference is the in-plane area in the LC phase: 0.23 nm2·
molecule−1 for 5a and 0.20 nm2·molecule−1 for 5b at 10 mN·
m
−1 (determined from the isotherms in Figure S6). The larger
Figure 6. Cross sections of the alkyl-chains and aryl groups of the
phenols 5a and 6 and the phenol ether 3 fitted into the experimentally
observed grids. (a) Cross-section of the alkyl-chains of 5a in a parallel-
displaced arrangement fitted into the experimentally observed grid of
phenol 5a at 10 mN·m−1. Dimensions of the cross-sectional area of the
alkyl chain as rectangle: longer side = 0.502 nm, shorter side = 0.424
nm, and in-plane area = 0.213 nm2. (b) Cross-section of the aryl
groups of 5a in an edge-to-face arrangement in the grid of phenol 5a
measured at 10 mN·m−1. Dimensions of the cross-section of the aryl
group as rectangle are longer side = 0.67 nm, shorter side = 0.35 nm,
and in-plane-area = 0.231 nm2. In the drawing, the numbering of the
atoms is 3(5)-H, 3(5)-C, 4-C, 5(3)-C, and 5(3)-H, shown from left to
right. The circles have the corresponding van der Waals radii corrected
for the tilt. (c) Cross-section of the alkyl-chains of 6 in a parallel-
displaced arrangement fitted into the experimentally observed grid of
phenol 6 at 10 mN·m−1. Dimensions of the cross-section of the alkyl
chain as rectangle: longer side = 0.570 nm, shorter side = 0.424 nm,
and in-plane area: 0.242 nm2. (d) Cross-section of the aryl group of 6
in an edge-to-face arrangement fitted into the experimentally observed
grid of phenol 6 at 10 mN·m−1. Dimensions of the cross-section of the
aryl group as rectangle: width = 0.666 nm, thickness = 0.35 nm, and
in-plane area = 0.233 nm2. (e) Cross-section of the alkyl group of
phenol ether 3 in a parallel-displaced arrangement fitted into the
experimentally observed grid of phenol 3 at 20 mN·m−1. Dimensions
of the cross-section of the alkyl group as rectangle: length = 0.630 nm,
width = 0.425 nm, and in-plane area = 0.267 nm2. Circles have the van
der Waals radii of H-atoms, center points are grid points, other point
are C1 of the alkyl group. (f) Cross-section of the aryl group of phenol
ether 3 in an edge-to-face arrangement fitted into the experimentally
observed grid of phenol ether 3 at 20 mN·m−1. Dimensions of the
cross-section of the aryl group as rectangle: length = 0.768 nm, width
= 0.384 nm, and in-plane area = 0.295 nm2. Shown are the van der
Waals area of the methoxy groups; center is the grid point; the other
point is C4 of the aryl group.
molecular area of 5a compared to 5b can be attributed to the
larger cross-section area of the benzene ring compared to this
of the alkyl group in 5b. The second difference is the missing
LC to SC transition in the isotherm of 5a compared to that of
5b (Figure S6, see Supporting Information, part C). In
monolayers of aliphatic alcohols with a shorter chain length,
the transition from a NNN-tilted L2′ to the nontilted
orthorhombic S phase has been observed during compression
at ambient temperature.51,52 The transition from a phase with a
tilt angle above 20° at low pressure into the nontilted phase
explains the larger compressibility of 5b of around 10%. The tilt
angle of 5a changes only by 3°, when the pressure is increased
from 1 to 40 mN·m−1. The nontilted phase cannot be achieved
due to the large area mismatch between the aryl ring and the
aliphatic chain. The condensed phase of 5a with an
orthorhombic packing of the chains is the L2 phase, which
can appear in a monolayer of 5b only at higher temperatures.
An attraction between the aryl rings (see below) and the fixed
conformation of the aryl ring might be responsible for the
restricted changes in the tilt angle and the in-plane area
observed for 5a. The monolayer structure is determined by the
packing of the aryl rings, which does not change much on
compression.
The packing of phenol 5a has also been investigated by
electron diffraction after the transfer of Langmuir−Blodgett
layers of the phenol onto a solid substrate.30 The centered
rectangular unit cell is similar to the one observed on water
(unit cell dimensions of 0.535 and 0.825 nm and an area of
0.219 nm2·molecule−1 on solid support, compared with 0.564
and 0.796 nm and 0.225 nm2·molecule−1 on water). Due to
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monolayer−substrate interactions, the transfer can induce
structural changes in the monolayer.53,54 The proposed
model of the chain arrangement in rows30 forming furrows
cannot be directly supported nor excluded by the presented
data.
The alcohol 5b and the phenol 5a show different
compression−expansion cycles (Figures S10 and S11, see
Supporting Information, part C). Alcohol 5b forms 3D
structures at the collapse, which only partially dissolve on
expansion, whereas those of the phenol 5a totally dissolve upon
expansion, which supports the high stability of the SC phase of
5a.
observed unit cell has been obtained being shown in Figure
6c−f.
Edge-to-face arrangements are reported for the alkyl chains
of fatty acids and fatty alcohols,25,26 and a parallel-displaced
arrangement is found for the alkyl chains of α-amino acids.38
The central aryl group in phenol 5a (Figure 6b), phenol 6
(Figure 6d), and phenol ether 3 (Figure 6f) is surrounded by
two parallel-displaced aryl groups and four aryl groups with an
edge-to-face arrangement. Computations and experimental data
on nonbonding energies and distances of similar arrangements
in benzene dimers and a benzene tetramer have been reported.
For benzene dimers, parallel-displaced sandwich structures and
T-shape arrangements (special case of an edge-to-face arrange-
ment) are described. Geometrical parameters and bond
energies have been calculated for the T-shape structure of
acene tetramers (Table 2, no. 4).55 In a tetramer, where the
benzene rings are connected in a rigid framework, the X-ray
structure shows the four aryl rings in a nearly square
arrangement with the individual rings in T-shape orientations
(Table 2, no. 5).56 For benzene dimers distances and
nonbonding energies have been obtained from structure
computations (Table 2, no. 6).35−37
With increasing number of the hydroxyl groups in the
phenols 6 and 7, the molecular area in the LC phase increases
(Figure S7, see Supporting Information, part C).
The area Axy of the ether 3 is 16% larger (at 20 mN·m−1)
than that of phenol 6 (Table 1), which is due to the larger size
of the methyl group compared to the hydrogen atom. The
collapse pressures of the polyphenols 6 and 7 (55−65 mN/m)
(Figure S7) are higher than those of the polyethers 3 and 4
(30−35 mN/m) (Figure S5) as a consequence of the stronger
hydrogen bonds.
The tilt angles of compounds 3, 5a, 6, and 7 are plotted
versus the lateral pressure in Figure S3 (Supporting
Information, part B). Comparing the tilt angles of 5a and 6,
which increase with the rising number of OH-groups (Figure
S3, Supporting Information, part B), one would expect for
polyphenol 7, which has a third OH-group attached to the ring,
a tilt angle around 50°. Obviously, such a strongly tilted
structure is energetically not favorable. Therefore, the
molecules arrange in a way that they do not exhibit a defined
tilt direction therewith reducing the tilt angle.
Diphenol 6 shows similar compression−expansion cycles as
phenol 5a (Figures S11 and S12, see the Supporting
Information, part C). The 3D structures formed during the
collapse of the monolayer of diphenol 6 dissolve totally upon
expansion. The compression−expansion curve of triphenol 7
(Figure S13, see the Supporting Information, part C) shows a
partially irreversible transition of the monolayer into 3D
structures or micelles.
Arrangement of Phenols 5a and 6 and Phenol Ether 3
in the Monolayer at the Air/Water Interface Based on
GIXD Measurements. Taking literature values of bond
lengths and bond angles for C−C and C−H bonds and the
van der Waals radius of hydrogen, the cross sections of the alkyl
chain and the aryl ring were drawn in scale as rectangles and
circles, respectively. These cross sections were then arranged in
the unit cell obtained by the GIXD measurements. The Bragg
peaks obtained by GIXD arise only from the chain packing of
the alkyl group. However, the alkyl group and the aryl group
must fit into the same grid as they are covalently connected and
therefore their unit cells must have the same dimensions (for
details, see the Supporting Information, part D).
Applying the experimental data a, b, and γ for the unit cell of
phenol 5a at a pressure of 10 mN·m−1 (Table 1), the cross-
section area of the alkyl and aryl part of phenol 5a has been
fitted into the unit cell (for details of the fitting, see the
Supporting Information, part D). The best fit for the connected
alkyl and aryl groups is a parallel-displaced arrangement for the
alkyl chains (Figure 6a) and an edge-to-face arrangement for
the aryl groups (Figure 6b).
Quantum Chemical Calculations in Packed 2D
Crystalline Systems. Since these previously reported
calculations are limited to isolated benzene clusters and
hence neglect potential cooperative effects in closely packed
two-dimensionally crystalline systems, we performed additional
quantum-chemical DFT calculations for such monolayers. A
periodic system consisting of two molecules of either benzene
or p-cresol in an edge-to-face arrangement in the unit cell was
modeled and fully optimized for the gas phase with the
Gaussian program package,57 using the robust M06L functional
by Truhlar and Zhao58 to take noncovalent dispersion
interactions important for aryl−aryl contacts into account.
For benzene and p-cresol (Table 2, no. 8 and 9, respectively),
the results show that an edge-to-face arrangement of the
molecules is strongly favored (Figure 7 for p-cresol); for the
arrangement of benzene, see Figure S15, Supporting
Information, part D. The binding energies for the calculated
dimers, embedded in extensive periodic systems, are signifi-
cantly larger than for isolated dimers of comparable geometry
(Table 2, no. 6 and 7), clearly indicating the importance of
cooperative effects for such systems. Although the calculated
gas-phase model lacks the surrounding conditions of the
experimentally studied aryl monolayers between water and air
phases, the obtained geometries are in good agreement with the
experimental results for 5a, especially regarding the center-to-
center distances of the molecules. The angle enclosed by two
molecules in an edge-to-face arrangement, measuring 47.8° for
benzene and 49.8° for p-cresol, is slightly more acute than the
approximately 60° observed for 5a in the experiments. The tilt
angle of the p-cresol molecules amounts to 28.3° in the
computationally obtained geometry, which is close to the
experimental values (Table 1). Interestingly, there are only
minor differences in the structural parameters of the 2D
benzene and p-cresol structures. The aryl−aryl interactions
appear to be the governing factor, since no significant
intermolecular interactions between the hydroxyl groups and
the methyl substituents in p-cresol are observed: while the OH-
groups point to each other, the oxygen−oxygen distances of at
least 3.3 Å allow only for weak hydrogen bonds. Similarly, the
orientation of the methyl groups indicates mutual, but only
quite weak interaction. More important for the significantly
Similar to phenol 5a, the best fit for the alkyl and aryl cross-
section of phenol 6 and phenol ether 3 into the experimentally
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corresponds to side b of the grid and for the parallel-displaced
arrangement the aryl−aryl center-to-center distance corre-
sponds to side a of the grid.
The hydroxyl group in the phenol enhances the stability of
the monolayer due to hydrogen bonds to the water surface.
Additionally, the van der Waals forces between the octadecyl
chains contribute to the stability of the monolayer. Finally, the
aryl rings assemble in such a way that one aryl ring can form
edge-to-face arrangements with four adjacent aryl rings.
Calculations with p-cresol as model compound lead to a
binding energy of nearly 30 kcal·mol−1 for two p-cresol
molecules embedded in a 2D crystal. The sum of these
attractive forces explains the high stability of the SC phase of 5a
that forms patches at large molecular areas and leads to a
monolayer with a high collapse pressure.
CONCLUSION
■
For octadecylphenols and their methyl ethers, the phase
behavior at the air/water interface has been studied by π/A
isotherms, compression−expansion cycles, Brewster angle
microscopy, GIXD measurements, fitting of phenols 5a and 6
and phenol ether 3 into their unit cells, and DFT calculations.
1-Phenyloctadecane (1a) forms a monolayer, which at low
pressure collapses to a multilayer. This finding indicates that
the phenyl ring develops only weak hydrogen bonds to the
water molecules. The aryl ether 1-(4-methoxyphenyl)-octade-
cane (2a) behaves differently from the aryl alkane 1a and the
aliphatic ether 1-methoxydocosane (2b). The π/A isotherms of
2a and the BAM images indicate that the aryl ether forms 3D
structures at an area of 0.5 nm2·molecule−1. The different
behavior between the aryl ether 2a and the aliphatic ether 2b,
which forms a stable monolayer, could be due to a lower
basicity of the ether oxygen in 2a caused by a delocalization of
the lone electron pair at the oxygen atom into the aromatic
ring, which would weaken the hydrogen bond to water. The
diether 3 shows a more stable monolayer, while the third
methoxy group in 4 does not lead to a further increase of the
collapse pressure.
Figure 7. Section of the quantum-chemically modeled two-dimensiona
crystalline p-cresol monolayer (M06L/6-311G(d,p)). The methyl
substituents point toward the viewer. The black frame indicates the
unit cell.
stronger binding energies in p-cresol compared to benzene is
the pronounced dipole moment of the former.
For the edge-to-face arrangement in phenol 5a and 6 (Table
2, no. 1 and 2), the aryl−aryl center-to-center distance
Table 2. Distances of Cross Sections for the Arrangement of
Phenol 5a and 6 and Phenol Ether 3 in the Experimentally
Determined Grid and Reported Calculated Values for
Benzene Dimers and Tetramers with the Corresponding
Binding Energies in Comparison to Data for Modeled
Benzene and p-Cresol Monolayers
The phenol 1-(4-hydroxyphenyl)-octadecane (5a) forms an
oblique phase at low pressure, which transforms into a
rectangular phase of NN-tilted chains on compression. The
patches of the SC phase can be already seen in the two-phase
coexistence region at large molecular areas. In contrast, the
aliphatic alcohol 1-docosanol (5b) shows a phase transition
into a nontilted phase indicated by a change of the slope in the
isotherm (second order phase transition). In the SC phase, one
phenol molecule can interact with six other phenol molecules in
four edge-to-face and two parallel-displaced arrangements.
DFT-calculations of p-cresol as model compound for phenol 5a
result in a binding energy of −29.83 kcal·mol−1 for two
molecules embedded in a 2D crystal. This strong interaction
between the aryl rings in 5a seems to be responsible for the
only marginal changes in the monolayer packing on
compression, which leads to only slight changes in the tilt
angle and area per molecule. The different behavior of phenol
5a and aryl ether 2a appears to be due to stronger hydrogen
bonds of 5a to water, because the phenol 5a is both a hydrogen
bond donor and an acceptor, whereas the ether 2a is only a
hydrogen bond acceptor. The increasing number of OH-groups
in 1-(3,4-dihydroxyphenyl)-octadecane (6) and 1-(2,3,4-trihy-
droxyphenyl)-octadecane (7) leads to a larger area required by
the phenol ring, causing much larger tilt angles of the chains to
optimize their van der Waals interactions. The collapse
distance (d) center-to-center (nm)
no.
compd
a
edge-to-face
parallel-displaced ΔE (kcal mol−1
)
1
2
3
4
5a
0.4886
0.5688
0.5642
0.4954
b
6
c
d
3
0.612
0.514
f
f
g
benzene
tetramer
0.63 (0.59)
0.26 (0.31)
−14.78
e
g
5
6
7
8
9
benzene
tetramer
0.5042; 0.4831
0.49−0.5
0.4723
−11.8
h
benzene
0.35−0.40
−2.7 to −2.9
−4.41
i
dimer
p-cresol
dimer
j
k
benzene 2D
0.4540
0.5728
0.5770
−15.92
j
crystal
k
p-cresol 2D
0.4541
−29.83
j
crystal
a
b
d corresponds to b and a in grid of 5a at 10 mN·m−1 (Table 1). d
c
corresponds to b and a in grid of 6 at 10 mN·m−1 (Table 1). d:
average center-to-center distance for edge-to-face arranged cross
d
sections in Figure 6f. d: average center-to-center distance for parallel-
e
f
displaced arranged cross sections in Figure 6f. Reference 55. Calcd d
in ref 55. Energy per four benzene rings.55,56 Reference 56.
g
h
i
j
k
References 35−37. M06L/6-311G(d,p). Energy of a dimer,
embedded in a periodic system.
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ASSOCIATED CONTENT
* Supporting Information
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■
S
(A) Experimental part: Synthesis of the 1-Aryloctadecanes 1a,
2a, 3, 4, 5a, 6, and 7. (B) Unit-cell distortion and tilt angle
versus pressure for 3, 5a, 6, and 7. (C) π/A Isotherms and
compression and expansion cycles for 3, 4, 5b, 5a, 6, and 7. (D)
Arrangement of phenols 5a and 6 and phenol ether 3 in the
monolayer at the air/water interface based on GIXD measure-
ments and a quantum-chemically modeled two-dimensional
crystalline benzene monolayer. This material is available free of
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AUTHOR INFORMATION
Corresponding Author
*E-mail: schafeh@uni-muenster.de. Tel.: +49(0)25077128.
Fax: 49(0)2518336523.
■
Present Address
∥X.C.: School of Materials Science and Engineering, Nanyang
Technological University, 50 Nanyang Avenue, Singapore
639798.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This work was supported by the Deutsche Forschungsgemein-
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