Phase Behavior and Molecular Packing of Octadecyl Phenols and

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 Würthwein,† and Hans J. Schaf̈ er*,† †

Organisch-Chemisches Institut der Westfälischen Wilhelms-Universität, Correns-Str. 40, 48149 Münster, Germany Physikalisches Institut der Westfälischen Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany § Max-Planck-Institut für Kolloid- und Grenzflächenforschung, Abteilung Grenzflächen, 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-(3,4dihydroxyphenyl)-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.

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INTRODUCTION

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-alkylaniline, 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 At ambient pressure, phenol molecules form in a 3D-crystal helical hydrogen bonded chains .31 In crystals obtained at high

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 interactions,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 alignment.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 © 2014 American Chemical Society

Received: November 9, 2013 Revised: January 3, 2014 Published: January 8, 2014 5780

dx.doi.org/10.1021/la404340h | Langmuir 2014, 30, 5780−5789

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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. 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, Göttingen, 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. 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 outof-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

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 2,5dimethylphenol 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-phenylScheme 1. Structures of the Investigated Amphiphiles 1−7

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RESULTS Synthesis. 1-Phenyloctadecane (1a) is synthesized by Friedel−Crafts acylation of benzene and subsequent hydrogenation of the aryl ketone. The aryl ethers 2a, 3, and 4 are prepared analogously to 1a from the corresponding mono-, 1,2di-, 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, IR, 1H NMR, 13C NMR, and MS data and elemental analyses 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

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.

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EXPERIMENTAL SECTION

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. 5781


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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 loworder 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 > Qdxy, where Qnxy 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 Åscale. 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). 5782


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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.

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DISCUSSION

Phenyloctadecane (1a), n-Docosane (1b), and Docosanoic 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-phenyloctadecane (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. GIXDinvestigations 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 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

Figure 4. Contour plots of the diffracted X-ray intensities, corrected for polarization, effective area, and Lorentz factor, as function of the inplane 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 alltrans 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. 5783


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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

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

3

5 20

0.5178 0.5125

0.6339 0.6078

113.9 113.8

48.1 44.3

0.300 0.285

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. 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 (twophase 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/Aisotherm 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 measurements. 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

Figure 5. 3D plot (left) and contour plot (right) of the corrected Xray 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.

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. 5784


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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 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

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 paralleldisplaced 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 paralleldisplaced 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. 5785


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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 arrangement) 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 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 significantly 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-tocenter 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 OHgroups 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

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. 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 crosssection 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). Similar to phenol 5a, the best fit for the alkyl and aryl crosssection of phenol 6 and phenol ether 3 into the experimentally 5786


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corresponds to side b of the grid and for the parallel-displaced arrangement the aryl−aryl center-to-center distance corresponds 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.

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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)-octadecane (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. 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-trihydroxyphenyl)-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

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 distance (d) center-to-center (nm) no. 1 2 3 4 5 6 7 8 9

compd a

5a 6b 3 benzene tetramere benzene tetramerh benzene dimeri p-cresol dimerj benzene 2D crystalj p-cresol 2D crystalj

edge-to-face 0.4886 0.5688 0.612c 0.63 (0.59)f

parallel-displaced 0.5642 0.4954 0.514d 0.26 (0.31)f

−14.78g −11.8g

0.5042; 0.4831 0.49−0.5

ΔE (kcal mol−1)

0.35−0.40

−2.7 to −2.9 −4.41

0.4723 0.4540

0.5728

−15.92k

0.4541

0.5770

−29.83k

d corresponds to b and a in grid of 5a at 10 mN·m−1 (Table 1). bd corresponds to b and a in grid of 6 at 10 mN·m−1 (Table 1). cd: average center-to-center distance for edge-to-face arranged cross sections in Figure 6f. dd: average center-to-center distance for paralleldisplaced arranged cross sections in Figure 6f. eReference 55. fCalcd d in ref 55. gEnergy per four benzene rings.55,56 hReference 56. i References 35−37. jM06L/6-311G(d,p). kEnergy of a dimer, embedded in a periodic system. a

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(10) Colquhoun, H. M.; Zhu, Z.; Williams, D. J.; Drew, M. G. B.; Cardin, C. J.; Gan, Y.; Crawford, A. G.; Marder, T. B. Induced-Fit Binding of π-Electron-Donor Substrates to Macrocyclic Aromatic Ether Imide Sulfones: A Versatile Approach to Molecular Assembly. Chem.Eur. J. 2010, 16, 907−918. (11) Fang, L.; Park, J. Y.; Ma, H.; Jen, A. K.-Y.; Salmeron, M. Atomic Force Microscopy Study of the Mechanical and Electrical Properties of Monolayer Films of Molecules with Aromatic End Groups. Langmuir 2007, 23, 11522−11525. (12) Mehdi, A.; Reye, C.; Corriu, R. From Molecular Chemistry to Hybrid Nanomaterials. Design and Functionalization. Chem. Soc. Rev. 2011, 40, 563−574. (13) Eidenschink, L. A.; Kier, B. L.; Andersen, N. H. Determinants of Fold Stabilizing Aromatic-Aromatic Interactions in Short Peptides. Adv. Exp. Med. Biol. 2009, 611, 73−74. (14) Bhattacharyya, R.; Samanta, U.; Chakrabarti, P. Aromatic− Aromatic Interactions in and around α-Helices. Protein Eng. 2002, 15, 91−100. (15) Wheeler, S. E. Local Nature of Substituent Effects in Stacking Interactions. J. Am. Chem. Soc. 2011, 133, 10262−10274. (16) Kannan, N.; Vishveshwara, S. Aromatic Clusters: A Determinant of Thermal Stability of Thermophilic Proteins. Protein Eng. 2000, 13, 753−761. (17) Meyer, E. A.; Castellano, R. K.; Diederich, F. Interactions with Aromatic Rings in Chemical and Biological Recognition. Angew. Chem., Int. Ed. 2003, 42, 1210−1250. (18) Ibarra, C.; Nieslanik, B. S.; Atkins, W. M. Contribution of Aromatic−Aromatic Interactions to the Anomalous pKa of Tyrosine-9 and the C-Terminal Dynamics of Glutathione S-Transferase A1-1. Biochemistry 2001, 40, 10614−10624. (19) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966; Chapter 3. (20) Hoenig, D.; Moebius, D. Direct Visualization of Monolayers at the Air−Water Interface by Brewster Angle Microscopy. J. Phys. Chem. 1991, 95, 4590−4592. (21) Hénon, S.; Meunier, J. Microscope at the Brewster Angle: Direct Observation of First-Order Phase Transitions in Monolayers. Rev. Sci. Instrum. 1991, 62, 936−939. (22) Knobler, C. M. Seeing Phenomena in Flatland: Studies of Monolayers by Fluorescence Microscopy. Science 1990, 249, 870−874. (23) Knobler, C. M. Recent Developments in the Study of Monolayers at the Air-Water Interface. Adv. Chem. Phys. 1990, 77, 397−449. (24) Jacquemain, D.; Grayer Wolf, S.; Leveiller, F.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. Two-Dimensional Crystallography of Amphiphilic Molecules at the Air−Water Interface. Angew. Chem., Int. Ed. 1992, 31, 130−152. (25) Leveiller, F.; Jacquemain, D.; Leiserowitz, L.; Kjaer, K.; AlsNielsen, J. Toward a Determination at near Atomic Resolution of Two-Dimensional Crystal Structures of Amphiphilic Molecules on the Water Surface: A Study Based on Grazing Incidence Synchrotron Xray Diffraction and Lattice Energy Calculations. J. Phys. Chem. 1992, 96, 10380−10389. (26) Majewski, J.; Popovitz-Biro, R.; Bouwman, W. G.; Kjaer, K.; AlsNielsen, J.; Lahav, M.; Leiserowitz, L. The Structural Properties of Uncompressed Crystalline Monolayers of Alcohols CnH2n+1 OH (n = 13−31) on Water and Their Role as Ice Nucleators. Chem.Eur. J. 1995, 1, 304−311. (27) Pignat, J.; Daillant, J.; Leiserowitz, L.; Perrot, F. Grazing Incidence X-ray Diffraction on Langmuir Films: Toward Atomic Resolution. J. Phys.Chem. B 2006, 110, 22178−22184. (28) Adam, N. K. The Structure of Thin Films. Part IV. Benzene Derivatives. A Condition of Stability in Monomolecular Films. Proc. R. Soc. London, Ser. A 1923, 103, 676−687. (29) Giles, C. H.; Neustadter, E. L. Researches on Monolayers. Part I. Molecular Areas and Orientation at Water Surfaces of Aromatic AzoCompounds Containing Long Alkyl Chains. J. Chem. Soc. 1952, 918− 923.

pressures of the polyphenols 6 and 7 are much higher than those of the polyethers 3 and 4 due to stronger H-bonds. Thus, phenol 5a and less pronounced phenol 6 and phenol ether 3 offer a unique possibility to fix aryl rings in a distinct arrangement and distance by hydrogen bonding to the water surface, van der Waals bonding between the alkyl chains, and aryl−aryl interactions. Such compounds could be useful to study steric and polar interactions of aryl rings at the molecular level and to improve the stability of monolayers formed by noncovalent interactions of amphiphiles.

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ASSOCIATED CONTENT

S Supporting Information *

(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 measurements and a quantum-chemically modeled two-dimensional crystalline benzene monolayer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 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. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft as a contribution from the Sonderforschungsbereich SFB 424. We thank HASYLAB at DESY, Hamburg, Germany, for beam time and providing excellent facilities and support.

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