Langmuir Films of Chiral Molecules on Mercury - ACS Publications

Mar 3, 2009 - liquid mercury as a function of surface coverage by surface tensiometry ... For both cases, the standing-up molecules are tilted by 44°...
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Langmuir Films of Chiral Molecules on Mercury )

L. Tamam,†,§ T. Menahem,‡,§ Y. Mastai,‡,§ E. Sloutskin,†,§ S. Yefet,†,§ and M. Deutsch*,†,§,

Physics Department, ‡Chemistry Department, §The Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel. Present address: SEAS, McKay 525, Harvard University, Cambridge, Massachusetts 02138 )

†

Received December 14, 2008. Revised Manuscript Received January 26, 2009 Homo- and heterochiral Langmuir films of a chiral derivative of stearic acid are studied in situ on the surface of liquid mercury as a function of surface coverage by surface tensiometry and surface-specific synchrotron X-ray diffraction and reflectivity. A transition from a phase of surface-parallel molecules to a phase of standing-up molecules is found. The former shows no surface-parallel long-range order. The standing-up phase of both homochiral and heterochiral compositions exhibit long-range order. However, the former has an oblique unit cell with parallel molecular planes, and the later has a centered rectangular unit cell with a herringbone molecular packing. For both cases, the standing-up molecules are tilted by 44 from the surface normal and pack at a density of 19.5 A˚2/molecule in the plane normal to the molecular long axis. Important differences are found, and discussed, between this behavior and that of a Langmuir film of the nonchiral stearic acid on mercury.

I.

Introduction

Chirality, or “handedness”, which relates two objects by mirror symmetry, is an intriguing property of many types of objects. It has profound effects in physics, chemistry, and biology, ranging from parity violation for weak forces1 to the almost exclusive use of left-handed amino acids by all life forms on earth.2 It has long been pointed out that at the molecular level chirality can be studied best in two dimensions, where only a restricted set of symmetry operations are possible.3,4 Hence, there has been increased activity over recent years in the study of chiral surfaces and monolayers and, in particular, Langmuir films. The advent of synchrotron sources and the subsequent development of techniques and facilities for studying liquid surfaces and their overlayers by X-ray methods5-7 provided a new powerful tool for studying in situ, and with submolecular resolution, the structure of liquid-supported Langmuir films (LFs) in general8,9 and of chiral LFs in particular.10,11 *Corresponding author. E-mail: [email protected]. (1) Sozzi, M. S. Discrete Symmetries and CP Violation; Oxford University Press: Oxford, U.K., 2008.Zel’dovich, Ya. B.; Saakyan, D. B.; Sobel’man, I. I. Pis’ma Zh. Eksp. Teor. Fiz. 1977, 25, 106. Zel’dovich, Ya. B.; Saakyan, D. B.; Sobel’man, I. I. J. Exp. Theor. Phys. Lett. 1977, 25, 94. (2) Miller, S. W.; Orgel, L. E. The Origin of Life on Earth; Prentice-Hall: New York, 1974. Fox, S. W.; Dose, K. Molecular Evolution and the Origin of Life; Dekker: New York, 1977.Kondepudi, D. K.; Nelson, G. W. Nature 1985, 314, 438. Weissbuch, I.; Leiserowitz, L.; Lahav, M. Top. Curr. Chem. 2005, 259, 123. (3) Flapan, E. When Topology Meets Chemistry: A Topological Look at Molecular Chirality; Cambridge University Press: Cambridge, U.K., 2000. (4) Kuzmenko, I.; Rapaport, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. Chem. Rev. 2001, 101, 1659. (5) Pershan, P. S.; Als-Nielsen, J. Phys. Rev. Lett. 1984, 52, 759. (6) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246, 252. (7) Als-Nielsen, J.; Christensen, F.; Pershan, P. S. Phys. Rev. Lett. 1982, 48, 1107. (8) Wolf, S. G..; et al. Nature 1987, 328, 63. Kjaer, K..; et al. Phys. Rev. Lett. 1987, 58, 2224. Dutta, P..; et al. Phys. Rev. Lett. 1987, 58, 2228. :: (9) Kaganer, V. M.; Mohwald, H.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779. (10) Nassoy, P.; Goldmann, M.; Bouloussa, O.; Rondelez, F. Phys. Rev. Lett. 1995, 75, 457. (11) Weissbuch, I.; Berfeld, M.; Bouwman, W.; Kjaer, K.; Nielsen, J. A.; Lahav, M.; Leiserowitz, L. J. Am. Chem. Soc. 1997, 119, 933.

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Considerable work has been carried out by X-ray methods on LFs of chiral molecules on aqueous subphases,4,11-13 exploring the dependence of the structure on alkyl chain length, molecular architecture, subphase composition, ionic strength, and so forth. Many aspects of these studies have been thoroughly discussed in several excellent recent reviews.4,13-16 In general, a binary mixture of molecules of opposite chirality tends to form a uniform mixture, a racemate. However, if the chiral interactions dominate, then phase separation into homochirally pure domains will occur. In chiral molecules having long alkyl tails, the crystalline order in homochiral and heterochiral LFs was found to be different. Homochiral domains form oblique unit cells, with a parallel alignment of the molecular planes. Heterochiral domains form rectangular unit cells, with herringbone packing of the molecular planes. However, the packing is strongly influenced by the presence of additional structures and interactions, either among the tails (by including interacting moieties) or among the headgroups (through additives to the aqueous subphase). It is quite difficult to predict whether a given chiral molecule would interact more favorably with its twin or with its mirror image. A theoretical molecular model by Andelman, deGennes, and co-workers17 indicates that for the molecular pair distance prevailing in our system a heterochiral phase should be expected for a racemic mixture when van der Waals interactions dominate and a separation into homochiral phases should be expected when electrostatic interactions dominate. The mixing entropy, which drives toward a racemate, being much smaller than the free-energy difference between the resolved and racemic phases, was (12) Eliash, R.; Weissbuch, I.; Weygand, M. J.; Kjaer, K.; Leiserowitz, L.; Lahav, M. J. Phys. Chem. B 2004, 108, 7228. (13) Nandi, N.; Vollhardt, D. Chem. Rev. 2003, 103, 4033. (14) Ariga, K.; Nakanishi, T.; Hill, J. P. Soft Matter 2006, 2, 465. (15) Nandi, N.; Vollhardt, D. Curr. Opin. Colloid Interface Sci. 2008, 13, 40. (16) Weissbuch, I.; Leiserowitz, L.; Lahav, M. Curr. Opin. Colloid Interface Sci. 2008, 13, 12. (17) Andelman, D. J. Am. Chem. Soc. 1989, 111, 6536. Andelman, D.; deGennes, P. G. C. R. Acad. Sci. 1988, 307, 323. Andelman, D.; Orland, H. J. Am. Chem. Soc. 1993, 115, 12322.

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neglected in those studies. However, although this “tripod model” is conceptually very clear, it is necessarily rather simple and may not capture the full essence of the gamut of interactions and factors influencing the ordering of real chiral molecules in an LF.4 The structural studies of LFs of chiral molecules on aqueous solutions of various solute molecules show that the suphase-headgroup interactions influence the LFs’ structure, sometimes dominantly. Also, the strong hydrophobic repulsion of the hydrocarbon tail from, and the hydrophilic attraction of the headgroup to, the aqueous subphase has the dominant effect of orienting amphiphilic chiral molecules roughly normal to the surface at all coverages. Moreover, the hydration shells formed around the water-immersed headgroups mask electrostatic interactions. Thus, the structure-determining balance created between the various intermolecular interactions is different from that which would be created without a hydration shell. Because all in situ X-ray structure measurements on LFs of chiral molecules to date have been carried out on an aqueous subphase, we have undertaken measurements of these structures on a nonaqueous subphase having considerably different interactions with the various parts of the chiral molecules. The study reported here, of LFs of stearic acid comprising a chiral serine moiety on the surface of liquid mercury, is the first to address the structure of such LFs on nonaqueous liquid subphases by X-ray methods. Serine was selected because of its role in many biochemical processes (e.g., its function in polar clamps and serine zippers in membrane proteins).18 The molecules studied, denoted C18-serine, are shown in Figure 1A,B. Whereas the alkyl tails of the C18-serine and stearic acid (shown in Figure 1C,D) are identical, the serine headgroup is bulkier than the simple carboxylic headgroup of stearic acid. This may lower the tendency of C18-serine to pack closely in-plane and to stack in surface-parallel layers, which are features that are found to exist for stearic acid films on mercury.19,20 Also, the orientation of the carboxylic moiety within the plane of the alkyl tail is different in the two molecules. This may inhibit the formation of molecular dimers, the basic structural unit of the stearic acid monolayer on mercury.19,20 As we show below, these expectations, based on the molecular shape, are indeed confirmed by the measurements. Mercury has been introduced in previous studies of our group19-25 as a novel, very advantageous subphase for X-ray studies of the structure of LFs in general on nonaqueous liquid subphases. Our initial studies have been inspired by the seminal work of Smith,26 where, however, only macroscopic methods such as surface tension, surface potential, and (18) Adamian, L.; Liang, J. Proteins 2002, 47, 209. (19) Kraack, H.; Ocko, B. M.; Pershan, P. S.; Sloutskin, E.; Tamam, L.; Deutsch, M. Langmuir 2004, 20, 5375. (20) Kraack, H.; Ocko, B. M.; Pershan, P. S.; Deutsch, M. Science 2002, 298, 1404. Kraack, H.; Deutsch, M.; Ocko, B. M.; Pershan, P. S. Nucl. Instrum. Methods, B 2003, 200, 363. (21) Kraack, H.; Ocko, B. M.; Pershan, P. S.; Sloutskin, E.; Deutsch, M. J. Chem. Phys. 2003, 119, 10339. (22) Kraack, H.; Ocko, B. M.; Pershan, P. S.; Sloutskin, E.; Tamam, L.; Deutsch, M. Langmuir 2004, 20, 5386. (23) Ocko, B. M.; Kraack, H.; Pershan, P. S.; Sloutskin, E.; Tamam, L.; Deutsch, M. Phys. Rev. Lett. 2004, 94, 017802. (24) Tamam, L.; Kraack, H.; Sloutskin, E.; Ocko, B. M.; Pershan, P. S.; Ulman, A.; Deutsch, M. J. Phys. Chem. B 2005, 109, 12534. (25) Tamam, L.; Kraack, H.; Sloutskin, E.; Ocko, B. M.; Pershan, P. S.; Ofer, E.; Deutsch, M. J. Phys. Chem. C 2007, 111, 2573. Tamam, L.; Kraack, H.; Sloutskin, E.; Ocko, B. M.; Pershan, P. S.; Ofer, E.; Deutsch, M. J. Phys. Chem. C 2007, 111, 2580. (26) Smith, T. Adv. Colloid Interface Sci. 1972, 3, 161.

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Figure 1. (A) Side and (B) top views of the C18-serine molecules. (C, D) The same for stearic acid. Note that the C18 alkyl tail is truncated. ellipsometry were employed, rather than the angstrom-resolution X-ray methods used by us, which were still unavailable at that time. Beyond its obviously different interactions with the LFs and the ability to form new, lying-down phases not observed on water, mercury has several practical advantages over water. The high surface energy of mercury, ∼500 mN/m, as compared to that of water, 72 mN/m, provides a supersmooth substrate (with roughness amplitudes of only e1.3 A˚) of macroscopic lateral dimensions, which allows the surfacespecific X-ray methods employed here to measure out to a wavevector transfer of 3 A˚-1, which is 4- to 5-fold larger than that measurable on water. This yields a commensurately better spatial resolution of e 1 A˚. The high surface tension also enhances the spreadability of LFs and allows the spreading of molecules that are not readily, or not at all, spreadable on water, as well as water-soluble molecules. The study reported here employs surface tensiometry measurements to determine the pressure-area isotherm of the LF and surface-specific X-ray diffraction techniques to determine the surface normal and the surface parallel structure of the LFs as a function of surface coverage. The experimental methods used here are described in the next section, followed by two sections describing and discussing the results obtained for LFs comprising homochiral and heterochiral molecules.

II.

Experiment

The experimental methods were discussed in detail in previous publications,21,24,25 and only a brief summary is given here. The thermodynamics of the LFs was studied by surface tension measurements. The surface normal and parallel structure were studied by X-ray reflectivity (XR), grazing incidence diffraction (GID), and Bragg rod (BR) measurements using a synchrotronbased liquid surface diffractometer.7,27-29 All measurements were made in a sealed, temperature-controlled Langmuir trough, allowing both surface tension and X-ray measurements. (27) Sanyal, M. K.; Sinha, S. K.; Huang, K. G.; Ocko, B. M. Phys. Rev. Lett. 1991, 66, 628. (28) Penfold, J. Rep. Prog. Phys. 2001, 64, 777. (29) Braslau, A.; Pershan, P. S.; Swislow, G.; Ocko, B. M.; Als-Nielsen, J. Phys. Rev. A 1988, 38, 2457.

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(30) Lapidot, Y.; Rappoport, S.; Wolman, Y. J. Lipid Res. 1967, 8, 142. (31) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966.

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)

)

1. Stearic Acid N-Hydroxy Succinimide Ester (SHE). Stearic acid (8.534 g, 30 mmoles) was added to a solution of N-hydroxysuccinimide (3.452 g, 30 mmoles) in dry ethyl acetate (130 mL). A solution of dicyclohexylcarbodiimide (6.19 g, 30 mmoles) in dry ethyl acetate was added, and the reaction was left stirring overnight at room temperature. Dicyclohexylurea was removed by filtration, and the filtrate was evaporated. The obtained crystals were recrystallized from ethanol, yielding 8.5 g (75%) of pure N-hydroxysuccinimide ester of stearic acid. Melting point 93 C. 1H NMR (ppm) (300 MHz) (CDCl3): 0.87 (3H, t), 1.27 (28H, m), 1.67 (2H, t), 2.57 (2H, t), 2.81 (4H, m). Elemental analysis (%): H, 10.23; C, 69.29; N, 3.67. 2. N-Stearic (L or D) Serine (C18-Serine). A solution of stearic acid N-hydroxy succinimide ester (5 g, 13 mmoles) in tetrahydrofuran (130 mL) was added to a solution of L- or D-serine (1.36 g, 13 mmoles) and sodium bicarbonate (1.09 g, 13 mmoles) in water (130 mL). The reaction was left stirring overnight at a temperature of 40 C. The solution was acidified to pH 2 with 1 M hydrochloric acid and the tetrahydrofuran was evaporated. The product was filtered and recrystallized first from acetone and then from ethanol-water (4:1) to give 4.33 g (90% yield) of N-stearoyl L-serine with a melting point of 103 C. 1H NMR (ppm) (300 MHz, DMSO): 0.87(3H, t), 1.27 (28H, m), 1.49 (2H, t), 2.12 (2H, t), 3.6 (2H,dq), 4.24 (1H, dd), 7.88 (1H, d). Elemental analysis (%): H, 11.547; C, 66.454; N, 4.083. The measured specific optical rotations of the synthesized C18-serine were [R]25 D = 20.84 and -26.32 for the L and D enantiomers, respectively. Triply distilled, 99.999% pure (Merck) or quadruply distilled, 99.9995% pure (Bethlehem) mercury was used as the subphase. Spreading solutions had a (3-8)  10-4 molarity in HPLCgrade, 99.9% pure methanol (Aldrich). The pure solvent left no surface-tension-detectable residue upon evaporation. B. Isotherm Measurements. 1. Langmuir Trough. The KelF trough, with dimensions of 175  65  3 mm3, has an ∼0.3-mm-thick bottom to allow good thermal contact with the underlying brass baseplate, the temperature of which was controlled to (0.1 C by a commercial water circulator. The trough is mounted in a hermetically sealed, helium-filled (for X-ray measurements) or nitrogen-filled (for isotherm measurements) aluminum box having Kapton windows for the X-ray entrance and exit. Surface tension is measured by an amalgamated Pt Wilhelmy plate26,31 hanging from a balance with an LVDT transducer. 2. Isotherms. The surface thermodynamics of the LFs is studied by pressure-area (π-A) isotherms. The surface pressure, π = σHg - σfilm, is the surface tension difference of the bare (σHg = 490 mN/m) and film-covered (σfilm) surfaces. A is the nominal area per molecule, obtained by dividing the number of deposited molecules by the surface area of the trough. The areal density of the molecules was increased by stepwise depositing 4-8 μL of the spreading solution onto the surface rather than by the conventional method of barrier compression because a wellsealing movable barrier is difficult to construct for a mercury subphase.26 C. X-ray Measurements. The structure of the deposited films was studied at several coverages using surface-specific X-ray techniques. Measurements were carried out using the Harvard/BNL liquid surface spectrometer at beamline X22B, NSLS, Brookhaven National Laboratory, at a wavelength of λ = 1.507 ( 0.001 A˚. The trough box was mounted on an active vibration isolation unit attached to the spectrometer. This

arrangement effectively eliminates the pickup of vibrations from the environment.32 We now briefly summarize the X-ray methods used. For further details, see refs 5-7, 9, and 33. 1. Surface-Normal Structure. This is accessible by measurements of X-ray reflectivity (XR),33,34 the intensity ratio R(qz) of the specularly reflected to incident X-rays. Here, qz = (4π/λ)sin R is the surface-normal scattering vector, and R is the grazing angle of incidence. R(qz) provides information on the LF’s surface-normal electron density profile, thickness, and surface roughness.33 2. Surface-Parallel Structure. This is investigated by grazing incidence diffraction (GID), where the detector is rotated from the specular reflection plane by an angle 2θd, yielding a surface-parallel scattering vector q = (2λ/π)(cos2 R + cos2 β - 2 cos R cos β cos θd)1/2. β is the grazing angle of exit of the detected X-rays. To obtain the full 2D intensity distribution I(q , qz), we employ a linear-position-sensitive detector, aligned normal to the liquid surface, which covers the range 0 < qz < 0.75 A˚-1. The large qz range required the 2θd scans to be carried out in sections, scanning first a 2θd range with a detector angle of β = R and repeating the scan with β ≈ 8 . R. Combining the two allowed covered the qz range up to 1.6 A˚-1. 3. Bragg Rods. The surface-parallel crystalline packing and the molecular tilt and its azimuthal direction of molecules within the ordered part of the LF can all be obtained from I(q , qz). In particular, the molecule tilt magnitude and its azimuthal direction are obtained from the qz intensity distributions at the 2θd positions of the GID peaks, known as Bragg rods (BRs).9,33 )

A. Materials and Samples. The molecules employed in this study were synthesized in-house according to the following procedure.30

Article

III.

Results

A. Isotherms. Figure 2 shows the measured room-temperature (T = 24 C) π-A isotherms of the LFs of racemic C18-serine and the resolved L- (left-handed) and D- (righthanded) C18-serine on mercury. The curves are almost identical except near the collapse region at ∼40 A˚2/molecule. As A is reduced from >400 A˚2/molecule, four regions are observed in the isotherms: (a) a flat, nearly zero region for A > 210 A˚2/molecule, (b) a fast rise at A ≈ 150 A˚2/molecule, (c) a plateau at from A ≈ 150 A˚2/molecule down to A ≈ 60 A˚2/molecule, and (d) a fast rise for A j 60 A˚2/molecule down to film collapse at ∼40 A˚2/molecule. On the basis of our previous studies of mercury-supported LFs of alcohols,22 fatty acids,20 alkyl thiols,23,35 and biphenyl thiols,24 these four regions can be identified as (a) a 2D gas of surfaceparallel-oriented (lying-down) molecules, (b) a single condensed layer of lying-down molecules, (c) a coexistence of lying-down and roughly surface-normal (standing-up) molecules, and (d) a close-packed monolayer of standing-up molecules. The X-ray measurements discussed below confirm the general features of this interpretation and add much detail on the structure of the LF in each of these regions. The fast-rising parts of the isotherms at 150 e A e 300 A˚2/ molecule are very well fitted (dashed line for D-C18-serine) by the Volmer equation19 of an ideal 2D gas of finite-area (A0) molecules, π(A - A0) = kBT, where kB is the Boltzmann (32) Barton, S. W.; Thomas, B.n.; Novak, F.; Weber, P. M.; Harris, J.; Dolmer, P.; Bloch, J. M.; Rice, S. A. Nature 1986, 321, 685. Lu, B. C.; Rice, S. A. J. Chem. Phys. 1978, 68, 5558. Bosio, L.; Oumezine, M. J. Chem. Phys. 1984, 80, 959. Bosio, L.; Cortes, R.; Folcher, G.; Froment, M. J. Electrochem. Soc. 1992, 139, 2110. (33) Deutsch, M.; Ocko, B. M. In Encyclopedia of Applied Physics, Trigg, , G. L., Ed.; VCH: New York,1998; Vol. 23, p 479. Als-Nielsen, J.; McMorrow, D. Elements of Modern X-Ray Physics; Wiley: New York, 2001. (34) Lekner, J. Theory of Reflection, Martinus Nijhoff: Dordrecht, The Netherlands, 1987.Abeles, F. Ann. Phys. 1950, 5, 596. (35) Kraack, H.; Tamam, L.; Sloutskin, E.; Pershan, P. S.; Deutsch, M.; Ocko, B. M. Langmuir 2007, 23, 7571.

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constant. The fits yield exclusion areas of A0 = 145 ( 5 A˚2/ molecule for both the LFs of the racemic and the resolved molecules. This value is very close to the sum of fitted area of the stearic acid,19 115 ( 8 A˚2/molecule, and the calculated area of the serine group,36 24 A˚2/molecule. This implies that the molecules are oriented parallel to the surface for A J 160 A˚2/molecule, and the fast rise in π at 160 A˚2/molecule is associated with the formation of a densely packed singlelayer (SL) phase of flat-lying molecules. The plateau observed for 60 j A j 150 A˚2/molecule indicates a coexistence region between two phases.9 The flat, long plateau observed here is different from the two-region structure observed for the corresponding A range (110-40 A˚2/molecule) in stearic acid,19 shown as diamonds in the same Figure. There, the sloping, 60 j A j 110 A˚2/molecule region of stearic acid corresponds to a bilayer of lying-down molecules, whereas the plateau at A < 60 A˚2/molecule is a coexistence region between the lying-down and standing-up phases. The fact that for our samples only a single plateau is observed below the bend at A ≈ 150 A˚2/molecule implies that the bilayer phase of lying-down molecules observed for stearic acid is missing here and the plateau extending from the bend at A ≈ 150 A˚2/molecule down is due to coexistence between a single layer of lying-down molecules and a monolayer of standing-up molecules. Moreover, the absence in our samples of the additional plateaus observed in the equallength tetracosanoic acid, shown by triangles, indicates that other multilayers also do not form in our samples. The isotherm of our samples is similar, therefore, to those of thiols on mercury,35 where only a single lying-down phase, converting upon decreasing A to a single standing-up phase, was observed. For thiols, this behavior was caused by the strong chemisorption of the thiol headgroup onto the mercury surface, manifested also in the fast increase in the surface pressure, Δπ ≈ 20 mN/m, prior to collapse of the film. For our samples, Δπ ≈ 10 mN/m is about half that of the thiols, similar to those of fatty acids.19 The similarity in Δπ in the lowest A region, where the molecules are standing up, implies similar Hg-headgroup interactions for our samples and for stearic acid. Because the balance between this interaction and that between a lyingdown molecule and the subphase determines the transition from lying-down to standing-up molecules, the fact that we are seeing no indications for multiple-layer plateaus must be due to other reason. A possible reason for this behavior is the conformation of the C18-serine molecules, which has a CH2OH moiety protruding normal to the molecular plane, inhibiting the stacking of molecules. To confirm the conclusions derived from the isotherms and detect possible lateral order within the film, we have carried out XR, GID, and BR X-ray measurements, which are discussed in the next two subsections. B. Surface Structure: X-ray Measurements. In this section, we describe the results obtained from X-ray measurements on LFs of hetero- and homochiral C18-serine. To explore both the low- and high-coverage regions of the isotherms, the XR measurements were carried out in three regions: at the onset of the plateau, A = 121 ( 5 A˚2/ molecule; in the middle of the plateau, A = 80 A˚2/molecule; (36) Schade, B.; Fuhrhop, J. H.; Hubert, V.; Weber, M.; Luger, P. Acta Crystallogr., Part C 1997, 53, 107. Molecular dimension calculations were made using the cif file RESHOM and the Mercury software, both from the Cambridge Crystallographic Data Center, U.K.

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Figure 2. Measured π-A isotherms of LFs of racemic C18-serine (0), D-C18-serine (4), and L-C18-serine (O). For comparison, the mercury-supported isotherms of stearic acid ()) and tetracosanoic acid (3) are also shown. Fits to the Volmer equation (see the text) are shown as dashed lines. Arrows mark the coverages where X-ray measurements were made. Curves are y shifted by 10 mN/m from each other for clarity. and in the fast rising regions of each isotherm, A = 40 ( 5 A˚2/molecule. The three regions are marked by vertical arrows in Figure 2. 1. Surface-Normal Structure. Figure 3 shows a selection of the Fresnel-normalized XR curves measured at room temperature for homo- and heterochiral C18-serine at the indicated coverages. Figure 3a plots the measured XR (O) divided by the Fresnel reflectivity of an ideally flat and abrupt surface,33 along with their fits (-) to the corresponding electron density profile models shown in lines in Figure 3b. The inset in Figure 3a shows the same curves for the bare mercury surface. The peak at qz = 2.2 A˚-1 is due to the surface-induced layering at the mercury surface, which can be seen in the real-space density profiles of Figure 3b at z > 0. This is also the cause for the rise at the high-qz end of the R/RF curves in Figure 3a. A rough estimate of the structure in the surface-normal direction can be obtained from the period of the modulation, Δqz, in the R/RF curves, which is related to the layer thickness, D, of the corresponding films by Δqz = 2π/D. The amplitude of the modulations depends on the electron density differences among the gas phase, the surface layer, and the bulk and also on interfacial roughness.37 For the three high-density XR curves, A = 40 ( 1 A˚2/molecule and the two medium-density curves A = 80 A˚2/molecule, Δqz ≈ 0.3 A˚-1, which yields D ≈ 21 A˚. The main difference between the high- and medium-surface-density curves is in the amplitudes of the modulations of R/RF. The modulation periods of the three lower-density curves, A = 121 ( 5 A˚2/ molecule are roughly the same, indicating layer thicknesses to be about D ≈ 21 A˚ in all three cases. These estimates are, however, rather rough, particulary for the low-coverage lying-down phase where the maximum-minimum contrast in the R/RF curves is weak. A more accurate determination (37) Ocko, B. M.; Wu, X. Z; Sirota, E. B.; Sinha, S. K.; Gang, O.; Deutsch, M. Phys. Rev. E 1997, 55, 3164.

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Figure 3. (a) Measured Fresnel-normalized X-ray reflectivity curves of hetero- and homochiral LFs of C18-serine on mercury (O), with box model fits (-). Curves are shifted vertically by 0.35 for clarity. (Inset) Bare Hg surface. (b) Surface-normal electron density profiles derived from the fits in plot (a). The Hg surface is at z = 0, with z > 0 pointing into the liquid. The bare mercury is shown by dashed lines. Curves are shifted by 1 for clarity. Numbers are the nominal areas per molecule. of the structure requires detailed modeling, which we now discuss. We use a multislab model, employed successfully in our previous studies.19-23 The model consists of eight slabs, two of which are used for the organic layer, one representing the standing-up alkyl chains and a second representing the lyingdown molecules (including the serine group). The remaining six are required to mimic the oscillatory electron-density profile of the layered mercury surface.38 Each box has a width, a constant electron density, and a roughness at its interfaces with the adjacent boxes. The electron density for each of the two slabs of the organic layer was calculated by dividing the number of relevant electrons by the corresponding volume obtained from the GID measurements of the standing-up phase (see next section) and taking into account the fitted value of the relative surface coverage by each of the phases at the given A. Following an extensive set of fits and varying different combinations of model parameters, we have finally employed a fixed 5.1 A˚ thickness for the lyingdown slab in all of the fits presented here. The thickness of the slab representing the standing-up fraction of the molecules was allowed to vary freely in the fits. Following previous studies,19-23 six slabs of fixed, equal 1.3 A˚ width and a fixed interfacial roughness of 0.7 A˚, were used to describe the near-surface oscillatory density profile38 of mercury. Only the electron densities of these slabs, common to all R/RF value in this study, were allowed to vary in the fits, with the exception of the first slab, the electron density of which was fixed at 5.5 e/A˚3. Finally, the fits to R/RF were performed using a matrix method implementation of the (38) Magnussen, O. M.; Ocko, B. M.; Regan, M. J.; Penanen, M. J.; Deutsch, M.; Pershan, P. S. Phys. Rev. Lett. 1995, 74, 4444. DiMasi, E.; Tostmann, H.; Ocko, B. M.; Pershan, P. S.; Deutsch, M. Phys. Rev. B 1995, 58, R13419.

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Parratt formalism,39 which is well suited to modeling XRs of multiple-interface surfaces, such as our mercury-supported LFs. The fits are shown in solid lines in Figure 3a, and the corresponding electron density profiles are shown in Figure 3b, where the mercury/organic layer interface is at z = 0 A˚ and the Langmuir film resides at negative z values. The profiles provide an overview of the phase sequence, showing an increase in the fractional surface coverage by the standing-up layer with decreasing A. Even at the lowest coverage (A = 121 ( 5 A˚2/molecule) of the C18-serine, the XR curves exhibit short-period oscillations, indicating the existence of a thick layer at the surface. In stearic acid, thiols, and alcohols, oscillations of similar periods were attributed to a single layer of standing-up molecules.22,24,35 The model fits yield a common thicknesses of 21.4 ( 0.1 A˚, covering 5% (racemic and L-C18-serine) and 10% (L-C18-serine) of the surface area, in coexistence with the lying-down phase. Comparing this thickness with the calculated36 29 A˚ length of an extended D-C18-serine molecule leads to the conclusion that the layer consists of standing-up molecules that are strongly tilted from the surface normal: θ = arccos(21.5/29) ≈ 42.1. These conclusions are strongly supported by the BR data presented below. This molecular tilt is close to those observed for LFs of alkyl esters of several R-amino acids at the air-water interface.12 For L- and D-C18-serine at a coverage of A = 80 A˚2/ molecule, the contrast is more pronounced; the fit yields a coverage of 20% of the total area by standing-up molecules, with the same layer thickness. Moreover, even for the highest coverage of the C18-serine, A = 40 ( 1 A˚2/molecule, the thickness of the layer of standing-up molecules (39) Parratt, L. G. Phys. Rev. 1954, 95, 359.

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Table 1. Peak Positions (qr, qz), Peak Index (h, k), Number of Molecules in the Unit Cell (N), Lattice Constants (a, b), Angle between Lattice Vectors (γ), the Surface-Parallel and Chain-Normal Molecule Areas (A, A^) for the Homochiral and Heterochiral C18Serine in the Standing-Up Phasea

homo

γ

q (A˚-1)

qz (A˚-1)

(h, k)

N

a (A˚)

b (A˚)

(deg)

A (A˚2)

A^ (A˚2)

1.17 1.32 1.5

1.1 0.43 0.74

(01) (10) (11)

1

4.95

5.57

106.2

26.5

19.3

)

type

1.145 1.08 (02) 1.39 0.54 (11) 2 4.96 10.92 90 27.1 19.5 a The uncertainties in the peak positions are Δqz = (0.015 A˚-1 and Δq = (0.01 A˚-1. )

hetero

)

does not change. However, the fraction of standing-up molecules inside the LF increases to (44 ( 2)% at the highest compression. 2. Surface-Parallel Structure. In contrast to octadecanoic acid on mercury, where the lying-down phase shows 1D smecticlike order at room temperature,19,20 no GID peaks were found for the lying-down phases of either the racemic C18-serine or the homochiral L- and D-C18-serine, implying the absence of long-range order. For the standingup phase, different order is found for the racemate C18-serine, which exhibits a body-centered 2D unit cell, and the resolved D-C18-serine and L-C18-serine, which show an oblique unit cell. The GID patterns at the two high coverages studied here (40 and 80 A˚2/molecule) are identical and independent of temperature, both for the racemate and for the resolved compounds. We now discuss these structures in more detail. a. Resolved L- and D-C18-Serine. The GID results for the standing-up phases of resolved L- and D-C18-serine at a nominal coverage of 40 ( 1 A˚2/molecule and T = 25 C are summarized in Table 1. They show the same qz and qr peak positions. As shown in Figure 4a, this phase exhibits three GID peaks, which are the hallmarks of an oblique unit cell. For such a cell, the three lowest-order GID peaks are related by qaz = qbz + qcz, where peak a is the one with the largest qz and qnz = qn cos Ψn tan t, with n = a, b, c. Ψn is the angle between the reciprocal lattice vector and the projection of the molecular long axis onto the surface plane. t is the angle between the molecular long axis and the surface normal.9,40 These relations were indeed found to hold for the three GID peaks observed here. The three peaks are indexed as (01), (10), and (11) in an oblique unit cell with the dimensions given in Table 1. The structure has translational symmetry only. The molecules tilt toward their next nearest neighbors (NNN), as obtained from the BR’s peak positions,9 at an angle of 44. This yields a unit cell in the plane perpendicular to the molecular long axis, with lattice parameters of a^= 4.95 A˚ and b^ = 7.80 A˚ and a molecular area of 19.3 A˚2, which is close to the molecular area measured20 for stearic acid, A^ = 19.6 ( 0.2 A˚2/molecule. The BR fits37 reveal a molecular length of ∼29 A˚, which is very close to the calculated length of the extended molecule,41 implying that the serine and alkyl moieties are collinear and the molecule tilts as a rigid, rod-like, unit. (40) Wiegart, L.; Struth, B. Physica B 2005, 357, 126. (41) Kolesov, B. A.; Boldyreva, E. V. J. Phys. Chem. B 2007, 111, 14387.

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Figure 4. GID and BR patterns for the standing-up phase of at a coverage of 40 A˚2/molecule and T = 25 C. (a) Measured GID (symbols) and a fit to three Lorentzians and a constant background (line). The peaks are indexed in an oblique unit cell (Table 1). (b-d) Measured BRs corresponding to the three GID peaks (symbols) and a fit (lines) by the model discussed in the text. The BR fits yield a tilt angle of θ = 44 from the surface normal in the next-nearest-neighbor direction.

D-C18-serine

The GID peaks’ fwhm σ is related to the coherence length ξ in the direction of the diffraction vector through the Debye-Scherrer formula42 ξ ≈ 0.9  2π/σ. σ is determined from the measured value, σmes, and the experimental resolupffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi tion width, σres, as σ ¼ ðσ2mes -σ 2res Þ. The resolution in our setup σres = 0.017 A˚-1 is determined by Soller slits. Because σres is very close to the measured σmes ≈ 0.019 A˚-1, only a lower limit can be given for the crystalline coherence length: ξ > 1000 A˚. b. Racemic C18-Serine. The GID pattern for the standing-up racemic C18-serine at a nominal coverage of A = 40 A˚2/molecule in the fast-rising, low-A region of the isotherm and T = 25 C is shown in Figure 5, with values summarized in Table 1. This phase exhibits two GID peaks, which can be indexed9 as the (02) and (11) diffraction peaks of a body-centered rectangular unit cell with dimensions listed in Table 1. As for the resolved compounds, the molecules tilt toward their next nearest neighbors (NNN), as evidenced by the BR’s qz peak positions, qz(02) ≈ 2  qz(11) 6¼ 0 . The position of the (11) peak’s qz maximum yields a molecular tilt of θ = 44 through tan θ = qz(02)/qr(02).9 The unit cell dimensions in the plane perpendicular to the molecular long axis are therefore a^ = 4.96 A˚ and b^ = 7.86 A˚, and the molecular area is 19.5 A˚2, slightly larger than that of the resolved compounds and close to the A^ = (19.5 ( 0.1) A˚2/molecule measured for stearic acid.20 The crystalline coherence length is ξ > 1000 A˚, as for the homochiral compounds. As expected from the XR measurements, which do not show a variation in the layer thicknesses for 43 < A < 120 A˚2/molecule, the unit cell is found to be almost independent of coverage. The only variation is a decrease in the intensity (42) Guinier, A. X-ray Diffraction; Freeman: San Francisco, 1968.

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Figure 5. GID and BR patterns for standing-up phase of the racemic C18-serine at a coverage of 40 A˚2/molecule and T = 25 C. (a) Equal-intensity contour plot of the GID pattern. Measured (O) GID (b) and BR (c, d) scans and model fits (-). The BR fits yield a tilt angle of θ = 44 from the surface normal, in the next-nearestneighbor direction, and the GID peaks are indexed in the bodycentered rectangular unit cell shown in (e). of the GID peaks at lower surface coverages, in accord with the lower fraction of standing-up molecules at these coverages. Note also that the LFs are found to consist of rather coarse 2D crystalline grains so that the intensity variations of the peaks of a measured GID pattern cannot be assigned to the structure factor but rather to the different sizes of the domains, giving rise to the individual reflections. This precludes carrying out a full refinement of the structure and, consequently, determining the molecular plane orientation, the molecules positions within the unit cell, etc. The similarity between the structures of the racemic C18-serine at the two coverages implies that the serine moiety, rather than the stearic tail, dominates the structure because in conventional stearic acid LFs on mercury, dominated by the stearic tails, different coverages exhibit different surface structure.20 Finally, same as for the resolved compounds, the BR fits37 shown in Figure 5c,d yield a molecular length of 29 A˚, commensurate with the length of an extended molecule.

IV.

Discussion and Conclusions

To illuminate the role of the chiral interaction between like and unlike enantiomers, we discuss the present C18-serine results in comparison with those obtained for the nonchiral stearic (C18) acid on mercury and with those obtained for similar molecules on a different subphase, namely, water. Although bulk micellar aggregates of C18-serine have been studied by circular dichroism measurements,43 no X-ray (43) Shinitzky, M.; Haimovitz, R. J. Am. Chem. Soc. 1993, 115, 12545.

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measurements for LFs of these molecules on water are available in the literature. The comparison with LFs on water will include, therefore, related, though not identical, molecules.12 We focus first on the characteristics of the lying-down phase of C18-serine. Lying-down phases have not been hitherto observed in LFs of chain molecules on water, even at low surface coverages. Rather, island phases of standing-up molecules were sometimes observed at low coverages.44,45 On mercury, however, the strong surface attraction of the alkyl tails yields extended A ranges of lying-down phases. At low coverage, the lying-down phase of a stearic acid LF on mercury19,20 is a monolayer. As the coverage is increased, the monolayer converts over a long coexistence A-range to a stack of two layers of lying-down molecules. A 1D smecticlike in-plane order is found in both of these phases at room temperature, with a transition to 2D crystalline order at T j 10 C. The basic structural unit in these phases is a dimer in which both carboxyl groups reside at the dimer’s center. By contrast, the lying-down phase of both the homochiral and the heterochiral C18-serine studied here shows no long-range order in any direction, and no double layer of lying-down molecules was observed. A comparison of the molecular conformations of the fatty acid and the C18-serine, shown in Figure 1a,c, immediately reveals the likely reason for the lack of the double layer: surface-normal steric hindrance due to the CH2OH moiety of the C18-serine, which protrudes above the flat-lying plane of the molecular backbone and thus prevents stacking of these molecules. The reason for the lack of in-plane order in the lying-down phase is more subtle. We suggest here a probable explanation, though others may also be possible. The emergence of order in the lying-down phases of stearic acid LFs is related to the formation of the dimers. Other end-functionalized molecules (e.g., alkyl thiols and alcohols) exhibit neither dimers nor inplane order in the lying-down phases of their mercury-supported LFs.22,23,35 The dimers in stearic acid LFs likely form by two surface-parallel hydrogen bonds, connecting the two oxygens of the carboxylic headgroups of the molecular pair,46 as shown in Figure 6. The extra ∼2.5 A˚ length found for the dimer19,20 over the combined lengths of two stearic acid molecules is in line with this interpretation. The resultant dimer is almost linear, with a rather small shift between the backbones of the two molecules of the dimer. The exact value of the shift depends on the hydrogen bond length (taken here to be ∼2.5 A˚ 47) and the azimuthal orientation of the carboxylic headgroup. The small shift renders the surface parallel packing of such dimers almost as unrestricted as that of a fully linear molecule. By contrast, the line connecting the centers of the two carboxylic oxygens is azimuthally rotated by 20-25 closer to the molecular long axis in C18-serine than in stearic acid, rendering a hypothetical C18-serine dimer much less linear and imposing significantly higher restrictions on their surface-parallel packing. Thus, although dimers may form in the lying-down phase of C18-serine, their self-assembly into an in-plane-ordered structure does not occur.

(44) Kurtz, R. E.; Toney, M. F.; Pople, J. A.; Lin, B.; Meron, M.; Majewski, J.; Lange, A.; Fuller, G. G. Langmuir 2008, 24, 14005. (45) Wolf, S. G.; Leiserowitz, L.; Lahav, M.; Deutsch, M.; Kjaer, K.; AlsNielsen, J. Nature 1987, 328, 63. (46) The additional inclusion of a single mercury atom in this bond may also be possible.19,20 (47) Desiraju, G. R.; Steiner, T. In The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: Oxford, U.K., 1999. Libowitzky, E. Monatsh. Chem. 1999, 130, 1047.

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Figure 6. Top view of the suggested structure of the fatty acid (A) and C18-serine (B) dimers in the lying-down phases. The dotted lines denote the hydrogen bonds and correspond to a length of ∼2.5 A˚. Note the large shift between the backbones of the two molecules for C18-serine. For a discussion, see the text. The interaction of the C18-serine molecule with the underlying mercury also plays a role in the in-plane-ordered formation in the lying-down phase. For fatty acid LFs on mercury, it was suggested that each dimer also includes a mercury ion bound to the two carboxylic headgroups.19,20 In our molecules, the serine moiety has an additional hydroxyl group, which adds a strong dipolar interaction with the underlying mercury subphase.26 It is not clear how this dipolar interaction would influence the in-plane ordering of the LF, but one cannot exclude the possibility that it may interfere with the ordering of the already-nonlinear dimers or even hinder the dimer formation. Further work is needed to elucidate this issue. For the standing-up phase of C18-serine, coverage- and temperature-independent layer thicknesses and GID patterns were found within the ranges studied here (8 ( 3 C e T e 25 C and A = 80 ( 1 and 40 ( 1 A˚2/molecule) for both resolved and racemic LFs. This indicates constant molecular tilt and unit cell dimensions. Although the tilt direction (NNN) and magnitude (44) are the same for the heteroand homochiral LFs, the corresponding unit cells differ: the former has a body-centered rectangular cell, and the later has an oblique cell (γ = 106.2). This behavior differs from that of stearic acid on mercury,19 where although the tilt direction is the same (i.e., NNN) its magnitude decreases continuously from 30 to 19, with decreasing A. Moreover, whereas for stearic acid the tilted body-centered rectangular phase converts to an untilted hexagonal rotator phase at the highest coverage, no such transition is found for C18-serine. The molecular area in the plane perpendicular to the long axis of the tilted molecules is independent of the tilt and equals A^ = 19.6 ( 0.2 A˚2/molecule for all molecules discussed here: the stearic acid and the homochiral and heterochiral C18-serine. Very close tail-normal molecular areas were also found for the LFs of chiral molecules on water.12 This is surprising because the protruding CH2OH moiety should have led to a larger area than that of stearic acid. The fact that this does not occur may imply a conformational change allowing closer packing of the C18-serine molecules. Indeed, the molecular area found here is still ∼1 A˚2/molecule larger than that found for the highest-density herringbone-packed alkyl chains, ∼18.5 A˚2/ molecule.9,19,20 5118

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Previous X-ray measurements of mercury-supported LFs of fatty acids,19 alcohols,22 and alkyl thiols 23,35 imply that the low A^ values found here are characteristic of a herringbonepffiffiffi packing motif. Indeed, the ratio b^/a^ = 1.58 < 1.73 = 3 found here for the rectangular-cell standing-up phase of the heterochiral LFs48 was shown to be the characteristic signature of herringbone packing in water-supported LFs for a wide range of molecules (Figure 13 in ref 9). The present large crystalline coherence length of the heterochiral LF, ξ > 1000 A˚, is also typical of the herringbone-packed crystalline CS phase of LFs on mercury22,35 and on water,9 whereas rotatorlike phases exhibit shorter coherence lengths of only a few hundred angstroms.9 We note, however, that the oblique phase, while crystalline, is not herringbone-packed but rather packs with parallel molecular planes. Nevertheless, it also exhibits a high packing density and a long coherence length very close to those of herringbone packing. These properties seem, therefore, to characterize crystalline structure rather than exclusively herringbone-packed phases. Regrettably, a full refinement of the GID pattern, which would have allowed a determination of the exact positions and molecular plane orientations of the molecules in the unit cell, could not be carried out because of the coarse-grained 2D powder nature of the LFs studied here, which significantly varied the observed GID peak intensities over the small accessible azimuthal rotation range. The different in-plane order of the standing-up phases of the hetero- and the homochiral LFs, where the first shows a rectangular unit cell with tilted molecules having both translational and glide symmetries and the second shows a tilted oblique unit cell having only translational symmetry, were also found in LFs of alkyl-tailed chiral molecules on water,4,11,12 albeit in molecules different from the C18-serine studied here. In those LFs, the homo/hetero difference in the structure of the unit cells was assigned to a delicate balance between two different tendencies. The alkyl tails tend to pack with a (dense) herringbone motif, promoting a centered rectangular unit cell comprising two glide-related molecules. The amino headgroups tend to form a (dense) network of hydrogen bonds, promoting an oblique unit cell comprising a single molecule only and all molecular planes parallel to each other. Thus, the homochiral LF’s structure is dominated by the headgroups’ interaction, whereas that of the raecemic LF is dominated by the tails’ interaction. However, the different distance dependence of the tails’ van der Waals interaction and the headgroup’s hydrogen bonding interaction render the balance between them sensitively distance- and orientation-dependent.13,49 Steric hindrance plays a major role in determining the packing motif, as do additional possible interactions between molecules. For example, an additional hydrogen bond between amide groups placed along the alkyl tails of R-amino acids effectively suppressed the racemate LF’s rectangular phase and induced phase separation into enantiomerically pure domains exhibiting oblique unit cells.11 Hydrogen bonding may even lead to chiral supramolecular structures from achiral molecules, as found in LFs of aliphatic barbituric acid derivatives on water where spiral structures of both clockwise and anticlockwise directions were observed.50 (48) Note that for the homochiral phase the lattice constants in Table 11 refer to a single-molecule oblique unit cell (i.e., N = 1). An equivalent nonrectangular (γ = 80.4) unit cell with two molecules is also possible.11 (49) Harris, A. B.; Kamien, R. D.; Lubensky, T. C. Rev. Mod. Phys. 1999, 71, 1745. (50) Huang, X.; Li, C.; Jiang, S.; Wang, X.; Zhang, B.; Liu, M. J. Am. Chem. Soc. 2004, 126, 1322.

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These structures were concluded to form by intermolecular hydrogen bonding, and it appears that the assembly could involve the formation of a bent-core supramolecular structure. The headgroup-subphase interaction may also influence the structure. For example, adding solute ions to the aqueous subphase may induce phase separation or a change in the unit cell structure.4,13,16 It is therefore conceivable that a totaly different subphase (i.e., mercury instead of water) may induce a significant change in the packing of the racemate and homochiral LFs, particularly because the screening of the charged headgroups by the water molecules, which reduces their electrostatic interaction as compared to that of the bare headgroups, does not exist in LFs on mercury. In fact, however, rather small variations are found when comparing the results obtained here with those obtained on other chiral LFs on water, as presented in Table 1 of ref 11 and Table 2 of ref 12. All measured and derived quantities are close to each other, particularly when considering the fact that the molecules are only similar but not identical. Small differences are found in the tilt angle and the angle between the unit cell vectors γ: 44 and 106 for C18-serine versus ∼37 and ∼113 for others. The only significant difference is in the crystalline coherence lengths, which are 2 to 7 fold larger here than for the water-supported molecules. This may result from the stronger lateral interaction between the unscreened headgroups and also from the 3- to 4-fold-lower amplitudes, as compared to that of water, of the thermal capillary waves decorating the surface of the monolayer-covered mercury. This reduction results from the monolayer-covered mercury’s 10-fold-higher surface tension, ∼450 mN/m, as compared to that of the monolayer-covered water, ∼45 mN/m.

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Finally, this is the first angstrom-resolution study of the structure of an LF of chiral molecules on a nonaqueous nonpolar liquid. The study focuses on one molecule only, C18-serine. Extending this research to other types of chiral molecules may lead to a deeper understanding of the impact that different intermolecular interactions have on the structure. An obvious extension of this work may be the variation of the relative strength of the van der Waals interactions by varying the length of the alkyl chains. The watersupported LFs of other chiral molecules reveal that a 29-carbon-long alkyl tail completely suppresses the oblique standing-up phase of the homochiral compound, whereas for very short chains (∼8 carbons) no in-plane order is found.11 The investigation of molecules with more then one chiral center may enhance the structural role of chirality. Increasing the interaction with the mercury subphase, particularly in the standing-up phase, by including a thiol moiety should also provide another “tuning knob”. The study of the lying-down phases, not hitherto observed experimentally on aqueous subphases, of chiral molecules with different chain lengths and hydrogen bonding capabilities is of particular importance. These extensions of this first angstrom-resolution study of chiral molecules on mercury should lead to a better understanding of the structural role and influence of chirality. Acknowledgment. We thank B. M. Ocko (Brookhaven National Laboratory) for extensive discussions and advice on and help with the measurements. Support by the U.S.Israel Binational Science Foundation, Jerusalem, is gratefully acknowledged, as is beam time at beamline X22B, NSLS, Brookhaven National Laboratory. NSLS is supported by DOE contract No. DE-AC02-98CH10886.

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