pubs.acs.org/Langmuir © 2010 American Chemical Society
Crystalline Structures of Alkylamide Monolayers Adsorbed on the Surface of Graphite Tej Bhinde, Stuart M. Clarke,* and Tamsin K. Phillips BP Institute and Department of Chemistry, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, United Kingdom
Thomas Arnold and Julia E. Parker Diamond Light Source Limited, Diamond House, Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom Received December 4, 2009. Revised Manuscript Received January 8, 2010 Synchrotron X-ray and neutron diffraction have been used to determine the two-dimensional crystalline structures of alkylamides adsorbed on graphite at submonolayer coverage. The calculated structures show that the plane of the carbon backbone of the amide molecules is parallel to the graphite substrate. The molecules form hydrogen-bonded dimers, and adjacent dimers form additional hydrogen bonds yielding extended chains. By presenting data from a number of members of the homologous series, we have identified that these chains pack in different arrangements depending on the number of carbons in the amide molecule. The amide monolayers are found to be very stable relative to other closely related alkyl species, a feature which is attributed to the extensive hydrogen bonding present in these systems. The characteristics of the hydrogen bonds have been determined and are found to be in close agreement with those present in the bulk materials.
Introduction Hydrogen-bonded systems have gained increasing importance due to their potential applications in supramolecular devices, liquid crystals, and molecular electronics.1-3 Ease of synthesis and control over the structure and properties of such assemblies provide a diverse range of applications for hydrogen-bonded systems. The N-H 3 3 3 O (amide) hydrogen bond is predominant in many natural and synthetic species such as DNA, nylon, etc., and can have an important role in the higher level structures of proteins. Primary alkylamides have applications in polymer systems where they are used as friction modifiers at the polymer/air interface.4 These amides have the ability to form two hydrogen bonds per amide headgroup, which can permit the formation of extensive hydrogen-bonded chains of molecules. Graphite is a convenient substrate for the study of physisorbed layers as the interactions with an adsorbate are essentially dominated by van der Waals forces. These weak interactions allow the influence of weak adsorbate-adsorbate interactions to *To whom correspondence should be addressed: e-mail
[email protected]. ac.uk; Tel þ44 1223 765706; Fax þ44 1223 765701.
(1) Salmon, L.; Donnadieu, B.; Bousseksou, A.; Tuchagues, J.-P. C. R. Acad. Sci., Ser. IIC 1999, 2(5-6), 305–309. (2) Gerrit Ten, B.; Olli, I. Chem. Rec. 2004, 4(4), 219–230. (3) Naoum, M. M.; Fahmi, A. A.; Alaasar, M. A. Mol. Cryst. Liq. Cryst. 2009, 506, 22–33. (4) Ramirez, M. X.; Hirt, D. E.; Wright, L. L. Nano Lett. 2002, 2(1), 9–12. (5) Cheah, N. P.; Messe, L.; Clarke, S. M. J. Phys. Chem. B 2004, 108, 4466– 4469. (6) Wang, G.; Lei, S.; De Feyter, S.; Feldman, R.; Parker, J. E.; Clarke, S. M. Langmuir 2008, 24(6), 2501–2508. (7) Espeau, P.; White, J. W. J. Chem. Soc., Faraday T rans. 1997, 93(17), 3197– 3200. (8) Lee, M.-a.; Alkhafaji, M. T.; Migone, A. D. Langmuir 1997, 13(10), 2791– 2794. (9) Thomy, A.; Duval, X.; Regnier, J. Surf. Sci. Rep. 1981, 1(1), 1–38. (10) Groszek, A. J. In Selective Adsorption at Graphite/Hydrocarbon Interfaces; Proceedings of the Royal Society of Chemistry; The Royal Society: London, 1970; pp 473-498.
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be expressed and investigated. These adsorbed layers have been studied using several approaches including calorimetry,5-8 adsorption isotherms,9-11 X-ray and neutron scattering,12-15 NMR,16 microscopy,17 and simulation studies.18 An overview of various systems and techniques used to study these physisorbed layers can be found in reviews published recently.19,20 We have recently reported a study on the formation of solid monolayers of alkylamides and their mixing behavior when adsorbed on graphite using differential scanning calorimetry (DSC).21 The work shows that the amide molecules adsorb to form considerably more stable solid monolayers than those formed by other alkyl molecules from their liquid, with the monolayer melting transition in the DSC thermograms observed to occur at temperatures up to 20% higher than the bulk melting points. The amides are found to adsorb preferentially to alkanes on graphite and were also found to mix surprisingly well on the surface. An understanding of the structure of the adsorbed layers is key to predicting the mixing behavior of different materials.22 (11) Suzanne, J.; Coulomb, J. P.; Bienfait, M. Surf. Sci. 1973, 40(2), 414. (12) Bickerstaffe, A. K.; Cheah, N. P.; Clarke, S. M.; Parker, J. E.; Perdigon, A.; Messe, L.; Inaba, A. J. Phys. Chem. B 2006, 110, 5570–5575. (13) Espeau, P.; Reynolds, P. A.; Dowling, T.; Cookson, D.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93(17), 3201–3208. (14) Morishige, K.; Takeuchi, A.; Kato, T. J. Phys. Chem. B 1998, 102(28), 5495–5499. (15) Herwig, K. W.; Wu, Z.; Dai, P.; Taub, H. J. Chem. Phys. 1997, 107(13), 5186–5196. (16) Alba, M. D.; Castro, M. A.; Clarke, S. M.; Perdigon, A. C. Solid State NMR 2003, 23(3), 174–181. (17) Giancarlo, L.; Cyr, D.; Muyskens, K.; Flynn, G. W. Langmuir 1998, 14, 1465–1471. (18) Hansen, F. Y.; Newton, J. C.; Taub, H. J. Chem. Phys. 1993, 98(5), 4128– 4141. (19) Inaba, A. Pure Appl. Chem. 2006, 78(5), 1025–1037. (20) Bruch, L. W.; Diehl, R. D.; Venables, J. A. Rev. Mod. Phys. 2007, 79(4), 1381–74. (21) Arnold, T.; Clarke, S. M. Langmuir 2007, 24(7), 3325–3335. (22) Clarke, S. M.; Messe, L.; Adams, J.; Inaba, A.; Arnold, T.; Thomas, R. K. Chem. Phys. Lett. 2003, 373, 480–485.
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Other workers have employed scanning tunneling microscopy (STM) for the study of amide monolayers on graphite and other closely related substrates. An STM image of steorylamide (C18 amide) adsorbed on graphite from phenyloctane solution was interpreted as indicating that the amide molecules can dimerize by forming hydrogen bonds, with additional interdimer hydrogen bonds also possible to yield extensive chains.23 A similar interpretation was made of an AFM image of oleamide, an unsaturated amide (C18 amide with a cis double bond), on graphite adsorbed from chloroform solution.24 However, these techniques cannot resolve the details of the hydrogen bonding directly. We have recently reported the structure of dodecanamide (C12 amide) adsorbed on graphite at submonolayer coverage determined by diffraction.25 The structure was found to contain hydrogen-bonded dimers which assemble to form extended chains in the monolayer, consistent with the bulk crystal structure and also with the previous microscopy work.23 Characteristics of the hydrogen bonds present in the layer were reported with a resolution greater than that obtainable by microscopy. The monolayer was found to be extremely stable even at submonolayer coverage which was attributed to the presence of extensive hydrogen bonding in the layer. In this work detailed crystal structure analysis of the adsorbed monolayers of saturated primary alkylamide molecules with alkyl chain lengths between C5 to C11 and C16 adsorbed on graphite is presented. This analysis has been performed combining synchrotron X-ray and neutron diffraction. Here we demonstrate that all these homologues form solid monolayers with extensive hydrogen bonding between the amide groups which lead to dimer formation. The dimers assemble into chains in the adsorbed layer. Significantly, we identify that the packing arrangement of these amide chains differs with different hydrocarbon chain length, resulting in different monolayer structures.
Experimental Methods The alkylamides used for this work were either purchased (from Sigma-Aldrich) or prepared from the corresponding carboxylic acid. The deuterated amides were prepared from the corresponding perdeuterated acid for the neutron scattering experiments to minimize the very strong incoherent scattering that would have arisen with protonated samples. In X-ray scattering both protonated and perdeuterated samples have very similar scattering, and hence protonated materials can be used. Because of the very high cost, only a limited number of deuterated amides were available for this work, which restricted the number of neutron measurements that could be made. The purities of all the amides were analyzed by elemental analysis and NMR (>98% pure). Synchrotron X-ray diffraction experiments were performed on the Materials Science beamline X04SA at the Swiss Light Source (SLS), Switzerland, with an incident beam wavelength of 1 A˚ (12.4 keV).26 Recompressed exfoliated graphite (Papyex from Le Carbone Lorraine) was outgassed under vacuum at 350 °C before use. The cleaned graphite was dosed with the appropriate amounts of amide under vacuum and annealed at 180 °C. While dosing the graphite, it is convenient to know the approximate number of equivalent monolayers adsorbed. This was estimated from the area per molecule of the corresponding carboxylic acid12 and the specific surface area of graphite (23) Takeuchi, H.; Kawauchi, S.; Ikai, A. Jpn. J. Appl. Phys. 1996, 35, 3754– 3758. (24) Miyashita, N.; Mohwald, H.; Kurth, D. G. Chem. Mater. 2007, 19(17), 4259–4262. (25) Bhinde, T.; Arnold, T.; Clarke, S. M. Prog. Colloid Polym. Sci., submitted 2009. (26) Gozzo, F.; Schmitt, B.; Bortolamedi, T.; Giannini, C.; Guagliardi, A.; Lange, M.; Meister, D.; Maden, D.; Willmott, P.; Patterson, B. D. J. Alloys Compd. 2004, 362(1-2), 206–217.
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(30.1 m2/g as measured by nitrogen adsorption isotherm). For X-ray diffraction experiments, the dosed graphite was cut into disks (2 mm diameter) and placed in a 3.5 mm glass capillary. The capillary was then sealed and placed in the beam while spinning gently to enhance powder averaging (∼60 rpm). A cryojet was used to achieve temperature control. In practice, the fragile glass capillaries were found to fracture on cooling, and this effectively limited the accessible temperature range to ∼200 K. The instrument D20 at the Institut Laue-Langevin (ILL), France, was used for the neutron diffraction experiments at a neutron wavelength of 2.42 A˚.27 A standard orange cryofurnace was used for temperature control. The samples contained in aluminum cans were able to be cooled to 10 K. The procedures used to obtain X-ray and neutron diffraction patterns from adsorbed layers have been described in detail elsewhere.12,28 A silicon standard (NIST 640c) was used to calibrate the incident synchrotron X-ray beam and to correct for any detector angle offset. The calibrated X-ray diffraction pattern from the bare graphite was used to correct any offset in the neutron data. While preparing the final diffraction pattern from the adsorbed monolayer for fitting, the graphite background was subtracted and the intense small-angle scattering from the graphite crystallites was removed as described previously.12,29 The procedure described earlier30 was used to determine the structure of the adsorbed layer. This essentially involved calculating a diffraction pattern based on the Warren/Kjems model31,32 and then comparing it with the experimental pattern until satisfactory agreement is obtained. The model includes the sawtooth line-shape profile characteristic to 2D layers and preferred orientation correction from the graphite substrate. The values of the preferred orientation parameters used while fitting were H0/H1 = 0.3-0.6 and δ = 12.7°.32 To account for the width of the diffraction peaks, the calculated pattern is convoluted with an instrument specific resolution function (Gaussian) and by fitting the size (L) of the 2D crystallites (L = 350 A˚).31 While fitting a limited number of peaks typical of two-dimensional diffraction patterns, it is desirable to limit the number of fitted variables and constrain the fitting procedure as much as possible. In these fits, the molecular bond lengths and angles have been taken to be those observed in the bulk crystal.33 The solution of diffraction patterns is also assisted by consideration of plane group symmetry12,34-36 and close-packing arguments.37 Here we have constrained the fitting to maintain favorable hydrogen bond geometry.38 With these constraints, the molecules essentially have no degrees of freedom when they form dimers and then a single hydrogen-bonded chain. However, as indicted below, there are two alternative chain packing arrangements possible, each with a different plane group symmetry. In addition, the fitting procedure initially focuses on the largest peaks in the diffraction patterns and aims to index these. Monolayer patterns are obtained by subtraction of two very intense patterns (substrate with monolayer less (27) Hansen, T. C.; Henry, P. F.; Fischer, H. E.; Torregrossa, J.; Convert, P. Meas. Sci. Technol. 2008, 19(3), 034001. (28) Clarke, S. M. The Structure and Properties of Adsorbed Layers by X-Ray and Neutron Scattering. D. Phil. Thesis, University of Oxford, Oxford, 1989. (29) Cullity, B. D., Stock, S. R. Elements of X-Ray diffraction, 3rd ed.; Prentice Hall: Englewood Cliffs, NJ, 2001. (30) Mowforth, C. W.; Rayment, T.; Thomas, R. K. J. Chem. Soc., Faraday Trans. 2 1986, 82, 1621–1634. (31) Warren, B. E. Phys. Rev. 1941, 9(69), 693–698. (32) Kjems, J. K.; Passell, L.; Taub, H.; Dash, J. G.; Novaco, A. D. Phys. Rev. B 1976, 13(4), 1446–1462. (33) Lide, D. R. CRC Handbook of Chemistry and Physics, 85th ed.; CRC Press: Boca Raton, FL, 2004. (34) Arnold, T.; Thomas, R. K.; Castro, M. A.; Clarke, S. M.; Messe, L.; Inaba, A. Phys. Chem. Chem. Phys. 2001, 4, 345–351. (35) Arnold, T.; Dong, C. C.; Thomas, R. K.; Castro, M. A.; Perdigon, A.; Clarke, S. M.; Inaba, A. Phys. Chem. Chem. Phys. 2002, 4, 3430–3435. (36) Hahn, T. Space Group Symmetry, 4th ed.; Kluwer Academic: London, 1995; Vol. A. (37) Kitaigorodskii, A. I. Molecular Crystals and Molecules; Academic Press: New York, 1973; Vol. 29. (38) Leiserowitz, L.; Schmidt, G. M. J. J. Chem. Soc. A 1969, 2372–2382.
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substrate alone) where the monolayer contribution is relatively small. Hence, there are inherent uncertainties in the monolayer diffraction pattern, and it is hard to be sure that weaker features in the monolayer pattern are genuine and not artifacts of the subtraction procedure. Diffraction by a combination of X-rays and neutrons is also useful in structure determination, as both highlight different parts of the molecule (neutrons are more sensitive to light elements than X-rays and can determine the location of the hydrogen atoms more readily). Hence, we also constrained our structural solution such that the same structure fits both X-ray and neutron data, when available. The details of the hydrogen-bonded geometry were refined once the coarse structural solution was achieved.
Results and Discussion Low Coverage Structure. The experimental and calculated X-ray diffraction patterns from saturated alkylamides, pentanamide (C5) to undecanamide (C11), on graphite at 0.5 monlayer coverage and at 300 K are shown in Figure 1. The neutron diffraction patterns for two amides hexanamide (C6) and hexadecanamide (C16) at 0.8 monolayer coverage and at 10 K are shown in Figure 2. Data in these figures show scattered X-ray and neutron intensity as a function of momentum transfer (scattering vector) q, given by q = (4π sin θ)/λ. The experimental patterns have sawtooth-shaped diffraction peaks characteristic of scattering from two-dimensional layers.31,32 These patterns thus confirm that all of the saturated alkylamides studied here do form solid adsorbed layers on the surface of graphite at these coverages and temperatures. Experiments were performed at two different coverages enable us to be sure that we are not too close to the monolayer completion coverage which may be expected to lead to monolayer compression and structural changes. Figure 3 illustrates representative structures that were found to fit the diffraction patterns for the amides in Figures 1 and 2. As discussed above, the molecules hydrogen bond to form dimers, and adjacent dimers form additional hydrogen bonds to yield an extensive chain. Depending on the number of carbons in the molecule, successive chains can pack in two different ways, giving structures with two possible plane groups: p2 (C6, C9, C10, C11), with two molecules in a unit cell, or pgg (C5, C7), with four molecules per unit cell. These essentially correspond to the extended chains packing in the same direction on the surface or alternative chains adopting a “herringbone” type arrangement on the surface. Parameters calculated for the fitted structures are shown in Table 1. In these results, molecular tilt represents the angle which the molecules make with the a-axis of the unit cell. The molecules are close-packed with packing angles between 21° and 24°, in reasonable agreement with the stacking angle (23°) found in closepacked structures of alkyl species.39 These calculated structures permit the determination of the area occupied per molecule in the unit cell. This helps in the determination of the coverage of the amides on graphite more accurately, shown in Table 1. The absence of any symmetric bulk peaks in the diffraction patterns (Figures 1 and 2) that are typical in higher coverage samples confirms that the coverage of the amides studied here was indeed submonolayer. The fitted diffraction patterns were not found to be sensitive to the rotation of the molecules about their long axes. The best fits for the structures were found to be when the hydrocarbon backbone was parallel to the graphite substrate with a precision of approximately (30°. The unit cell for nearly all the structures is either exactly rectangular (pgg symmetry dictates that the unit cell be exactly (39) Rice, S. Faraday Discuss. Chem. Soc. 1990, 89, 247.
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Figure 1. Experimental (light) and calculated (dark) X-ray diffraction patterns at 300 K from 0.5 monolayers of (a) amides with a p2 space group: hexanamide (C6) (bottom), nonanamide (C9), decanamide (C10), and undecanamide (C11) (top); (b) amides with a pgg space group: pentanamide (C5) (bottom) and heptanamide (C7) (top); and (c) octanamide (C8) adsorbed on the surface of graphite. For octanamide (C8) (c), fitted diffraction patterns resulting from structures with both space groups p2 (bottom) and pgg (top) are shown.
rectangular) or close to 90°. We note that the fits for both X-ray and neutron patterns for hexanamide (C6 amide) are reasonable, indicating that the proposed structure is consistent and that there is no significant difference in monolayer structure between coverages of 0.5 and 0.8. This appears to be so even though the two patterns are at different temperatures, suggesting minimal thermal expansion in the layer. Identical structures were also found to fit the X-ray and neutron data for dodecanamide (C12 amide).25 DOI: 10.1021/la904587u
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Bhinde et al. Table 1. Calculated Structural Parameters for Adsorbed Monolayers of Alkylamides on Graphite unit cell parametersa
amide 5 6 6 7 8b 8b 9 10 11 16
pattern fitted
a (A˚)
b (A˚)
ν (deg)
molecular tilt (deg)a
coverage (monolayers)
symmetry
X-ray 34.00 5.115 90.0 23.5 0.50 pgg X-ray 20.10 5.105 91.0 23.2 0.48 p2 neutron 20.10 5.105 91.0 23.2 0.78 p2 X-ray 42.65 5.170 90.0 23.5 0.55 pgg X-ray 49.67 5.210 90.0 21.5 0.52 pgg X-ray 25.00 5.130 98.0 24.5 0.52 p2 X-ray 26.35 5.130 94.0 22.2 0.49 p2 X-ray 29.70 5.150 93.0 24.5 0.52 p2 X-ray 31.00 5.168 91.0 22.2 0.53 p2 neutron 45.00 4.870 88.5 23.5 0.84 p2 a The uncertainty in the unit cell distances is estimated at 0.05 A˚ and in the tilt angle is about 1°. b Octanamide (C8), is discussed in the text.
Figure 2. Experimental (light) and calculated (dark) neutron diffraction patterns at 10 K from 0.8 monolayers of hexanamide (C6) (bottom) and hexadecanamide (C16) (top) adsorbed on the surface of graphite.
The lowest angle reflections in the diffraction patterns, viz. (1,0) or (2,0), correspond to the relatively longest interplanar spacings (d-spacings) in the unit cell. These d-spacings are plotted in Figure 4 as a function of alkyl chain length. They correspond to approximately twice the length of a straight chain amide molecule and confirm that the molecules adsorb with the long axis parallel to the graphite surface. From the figure, it is interesting to note the variation in the d-spacings with alkyl chain length depending on whether the number of carbons is odd or even. Even chain length amides have a structure with p2 symmetry. For the odd amides, the low members of the series (C5 and C7) have a unit cell with pgg symmetry, and the molecules “flip” to p2 symmetry as the chain length increases to C9 and C11. This odd-even variation has also been observed before for other organic species, such as the structures of carboxylic acids12 and alkanes34,35 and in the 2D melting points of amines.5 However, unlike other related species, such as alkanes, there does not seem to be a direct relation between the odd-even oscillation and the plane group of the layers for the amides;for example, all even alkane homologues have p2 and all odd alkane homologues have pgg symmetry. This may be because the difference between the two amide structures is more subtle than the alkanes: the amide p2 and pgg layers are very similar indeed, whereas the odd-even alkane monolayer structures have parallel or herringbone molecular arrangements. It may be subtle differences in the interactions with the graphite substrate that give rise to the symmetry differences in the amides. These amides may 8204 DOI: 10.1021/la904587u
Figure 3. Illustration of the calculated monolayer crystal structures for (a) decanamide (C10), with p2 (rotational) symmetry and (b) heptanamide (C7), with pgg (rotational þ glide plane) symmetry. The unit cell is shown with a solid box, where a and b are the unit cell lengths and ν is the angle between them.
Figure 4. Variation of the interplanar spacing along the long axis of the unit cell with alkyl chain length.
undergo a solid-solid transition such that there is a change from one symmetry to the other on heating; this needs further investigation. The scattering patterns of octanamide (C8) can be reasonably well fit by unit cells with both symmetries, pgg and p2, as illustrated in Figure 1c. The structure with p2 symmetry shows all the principal peaks in the correct positions with reasonably good intensities. On the other hand, the structure with pgg symmetry has a reasonably good fit to the experimental pattern; Langmuir 2010, 26(11), 8201–8206
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Article Table 2. Geometry of the Amide Hydrogen Bond in the Adsorbed Layera “intradimer” hydrogen bond values
chain length of the amide
distance (N 3 3 3 O) (A˚)
distance (H 3 3 3 O) (A˚)
“interdimer” hydrogen bond values
angle (N-H 3 3 3 O) (deg)
3.00 1.99 178.51 C6 2.95 1.93 175.60 C7 2.99 1.97 176.36 C8 (pgg) 3.05 2.06 164.26 C8 (p2) 2.93 1.91 172.73 C9 2.90 1.88 175.91 C10 2.87 1.85 175.30 C11 3.04 2.02 179.58 C16 a The uncertainty in the distances is about 0.05 A˚, and that for angles is about 5°.
Figure 5. Representative hydrogen bonds for the amides in the adsorbed layer.
however, it does not have all the principal peaks in quite the correct positions and also has an extra “shoulder” to the peak at 1.5 A˚-1. There, although these two symmetries could be considered to fit the data, the unit cell with p2 symmetry is slightly better. In optimizing these fits for octanamide, the carboncarbon bond lengths and angles were also refined allowing for slight variation in these values ((3%) as is seen in bulk structures.33 For the rest of the amides one structure was found to fit better than all others unambiguously. Other models and symmetries which did not have closed-packed structures and optimal hydrogen bonding were also considered for all the amides, but these did not fit the diffraction patterns. Geometry of the Hydrogen Bond. The energy of dimerization through hydrogen bonding in bulk amides in solution is between 10 and 20 kJ/mol.40,41 In contrast, van der Waals interaction energies between molecules in these types of adsorbed layers, reflected in the melting heats of alkanes on graphite,42 are about 7 kJ/mol. Thus, the ordering of the molecules on the surface is expected to be controlled by the stronger hydrogen bonds. In these amide layers there are two types of hydrogen bonds, “intradimer” and “interdimer” hydrogen bonds, as illustrated in Figure 5, i.e., within a single dimer and between adjacent dimers. Geometrical parameters for these hydrogen bonds have been calculated from the fitted structures and are shown in Table 2. In an ideal hydrogen bond, the N-H 3 3 3 O atoms are collinear (N-H 3 3 3 O angle = 180°).38 The N 3 3 3 O distance for various dimers in the bulk crystal falls in the range of 2.85-3.05 A˚43 and the N-H 3 3 3 O bond angles between 160° and 180°.44 From Table 2, it can be seen that all the intradimer hydrogen bonds are essentially linear, which makes the interaction of the molecules (40) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W.H. Freeman and Co.: San Francisco, 1960. (41) Vinogradov, S. N.; Linnell, R. H. Hydrogen Bonding; Van Nostrand Reinhold Co.: New York, 1971. (42) Messe, L.; Perdigon, A.; Clarke, S. M.; Inaba, A.; Arnold, T. Langmuir 2005, 21(11), 5085–5093. (43) Taylor, R.; Kennard, O.; Versichel, W. Acta Crystallogr. 1984, B40, 280– 288. (44) Gavezzotti, A.; Filippini, G. J. Phys. Chem. 1994, 98(18), 4831–4837.
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distance (N-O) (A˚)
distance (H-O) (A˚)
angle (N-H 3 3 3 O) (deg)
2.95 3.00 3.07 3.07 3.04 3.00 3.03 2.74
2.32 2.32 2.43 2.57 2.50 2.38 2.40 2.02
119.75 122.95 119.36 109.36 112.44 112.76 118.45 124.54
most favorable. The N 3 3 3 O distance for both types of hydrogen bond also compare very well with bulk values. The N-H 3 3 3 O angle in the interdimer bond is about 120°. This is a preferred geometry due to the position of the lone pairs on the oxygen atom, and a similar packing arrangement is seen in the bulk.38 It is interesting to see that the side-to-side packing (b-parameter) of the dimers does not change with increasing chain length, and the unit cell becomes longer only in the a-direction as the number of carbons is increased (Table 1). This implies that the hydrogen bond geometry for both types of hydrogen bonds is essentially the same as the chain length of the carbon atoms is changed. It is thus evident that the adsorbed layer is similar to a section of one of the bulk crystal planes. This could suggest that the amide bulk crystal could be expected to grow in a layer-by-layer fashion. Many adsorbed alcohols show a similar behavior, with the formation of multilayers close to the bulk melting point.45 The calculated structure of the amides here are in good qualitative agreement with the structure found for steoryl amide (C18) by STM.23 The STM structure reports the length of the unit cell (a-parameter) as ∼50 A˚ and the side-to-side packing distance (b-parameter) of about 4.8 A˚, which agree with the values reported here, although this diffraction work permits the calculation of the structure to a much greater resolution. From diffraction experiments, the melting temperature of the monolayer can be estimated. During melting, the sharp, intense peak due to the solid monolayer turns into a broad peak corresponding to the fluid46 as illustrated in Figure 6. The melting point of the monolayer for the saturated alkylamides studied here is much higher compared to the bulk melting point even at submonolayer coverage (Tmonolayer/Tbulk ∼ 1.14). These amides are the only systems studied so far that exhibit a solid monolayer that survives significantly above the bulk melting point at submonolayer coverage on graphite. The high stability of the monolayer is attributed to the extensive hydrogen bonding present in the system. One sample, C12 amide, was heated from 10 K until the melting point in the neutron experiments, and no structural changes were observed, as discussed previously.25 Any commensurate nature of the monolayer can only be inferred by a coincidence of lattice parameters of the monolayer with multiples of those of particular graphite directions (usually √ taken to be ag = 2.46 A˚ and 3ag = 4.26 A˚, where ag is the lattice parameter of the graphite). However, it should be noted that a simple rotation of the overlayer with respect to the graphite will mean that many more possible coincidences could be possible. Such analysis indicates that possible commensurate layers include (45) Findenegg, G. H. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1069–1078. (46) Warren, B. E. X-Ray Diffraction; Courier Dover Publications: New York, 1990.
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Figure 6. Melting of the monolayer of dodecanamide (C12 amide) at a coverage of 0.8 monolayers as observed by neutron diffraction. Note the change in the (2,0) reflection as the temperature increases. The falloff in the intensities at small angles is due to the Porod correction applied to the patterns.29 The monolayer melting point is estimated at 428 K.25 The bulk melting point reported previously is 375 K.21
√ √ √ the amides C5 (8 3ag by 2ag), C7 (10 3ag by 2ag), and C10 (7 3ag by 2ag).
Conclusions The two-dimensional submonolayer crystalline structures of saturated alkylamides, pentanamide (C5) to undecanamide (C11) and hexadecanamide (C16), have been calculated using synchrotron X-ray and neutron diffraction. All the amide molecules have been found to lie flat on the graphite surface. The positions of the
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molecules indicate the presence of extensive hydrogen bonds in the monolayer, in agreement with other works. A detailed analysis has revealed the symmetry of the monolayer structures and details of the hydrogen bonded structures to improved resolution. The molecules dimerize, and depending on the number of carbons in the alkyl chain, they adopt a plane group with either p2 symmetry or pgg symmetry. These structures are in agreement with the close-packing predictions of Kitaigorodskii,37 applicable to dimers with a p2 or center of inversion symmetry. There is good qualitative agreement with the adsorbed monolayer structures of a related homologue reported by STM23 and also with the bulk (3D) crystal structures.38 Here our structural analysis provides significantly more quantitative detail on the hydrogen-bonded structures, and by considering a number of members of the homologous series, we have identified new plane groups of the layers. We also identify evident presolidification of the layers even at submonolayer coverage again indicating that an extensive hydrogen-bonded network imparts exceptional stability to the amide monolayers. Acknowledgment. We thank the Nehru Trust for Cambridge University (TB) and Diamond Light Source for financial assistance. We also thank the staff and scientists at ILL, Thomas Hansen and Jacques Torregrossa, and at SLS, Fabia Gozzo and Antonio Cervellino, for beam time and technical assistance. This work is based on experiments performed at the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/ 2007-2013) under grant agreement no. 226716.
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