pubs.acs.org/Langmuir © 2010 American Chemical Society
Phase Behavior of Heptanamide Adsorbed on a Graphite Substrate Tej Bhinde, Tamsin K. Phillips, and Stuart M. Clarke* 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, Oxfordshire OX11 0DE, United Kingdom Received October 12, 2010. Revised Manuscript Received November 22, 2010 In this letter, the phase behavior of a saturated alkylamide, heptanamide (C7), adsorbed on the surface of graphite using synchrotron X-ray diffraction is presented. The diffraction patterns indicate that heptanamide undergoes a solid-solid phase transition in the monolayer at 330 K from pgg symmetry at lower temperatures to p2 symmetry at high temperatures. Other alkylamides with similar carbon chain lengths do not show this phase change, making the C7 homologue unusual.
Introduction Adsorbed monolayers underpin a variety of commercial and academic problems, including detergency, catalysis, and friction. However, their study at the solid-liquid interface is still a challenge because the adsorbed layer represents a very small percentage of the material that is present and it is inaccessible, buried between the much larger bulk phases. A variety of experimental and theoretical techniques have been used to study the surface behavior of organic molecules adsorbed on graphite,1-8 MgO,9,10 and BN11,12 among other substrates. An overview of these studies can be found in recent reviews.13,14 Of particular relevance here, scattering techniques have been used to investigate solid-solid phase changes in the monolayer of alkanes15 and fluoroalkanes16 adsorbed on graphite on increasing the temperature, the coverage, or both. Alkylamides are an important class of organic compounds that are used commercially as friction modifiers in polymer systems.17 The formation of stable solid monolayers of alkylamides adsorbed *To whom correspondence should be addressed. E-mail:
[email protected]. Tel: þ44 1223 765706. Fax: þ44 1223 765701.
(1) Wang, G.; Lei, S.; De Feyter, S.; Feldman, R.; Parker, J. E.; Clarke, S. M. Langmuir 2008, 24, 2501–2508. (2) Thomy, A.; Duval, X.; Regnier, J. Surf. Sci. Rep. 1981, 1, 1–38. (3) Suzanne, J.; Coulomb, J. P.; Bienfait, M. Surf. Sci. 1973, 40, 414. (4) Shrestha, P.; Migone, A. D. Phys. Rev. B 1996, 54, 17102. (5) Espeau, P.; Reynolds, P. A.; Dowling, T.; Cookson, D.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3201–3208. (6) Morishige, K.; Takeuchi, A.; Kato, T. J. Phys. Chem. B 1998, 102, 5495–5499. (7) Alba, M. D.; Castro, M. A.; Clarke, S. M.; Perdigon, A. C. Solid State NMR 2003, 23, 174–181. (8) Giancarlo, L.; Cyr, D.; Muyskens, K.; Flynn, G. W. Langmuir 1998, 14, 1465–1471. (9) Larese, J. Z.; Arnold, T.; Barbour, A.; Frazier, L. R. Langmuir 2009, 25, 4078–4083. (10) Arnold, T.; Chanaa, S.; Clarke, S. M.; Cook, R. E.; Larese, J. Z. Phys. Rev. B 2006, 74, 085421. (11) Morishige, K.; Komura, T. Langmuir 1998, 14, 4887–4890. (12) Diama, A.; Migone, A. D. Phys. Rev. B 1999, 60, 16103. (13) Bruch, L. W.; Diehl, R. D.; Venables, J. A. Rev. Mod. Phys. 2007, 79, 1381– 1374. (14) Inaba, A. Pure Appl. Chem. 2006, 78, 1025–1037. (15) Arnold, T.; Thomas, R. K.; Castro, M. A.; Clarke, S. M.; Messe, L.; Inaba, A. Phys. Chem. Chem. Phys. 2001, 4, 345–351. (16) Parker, J. E.; Clarke, S. M.; Perdigon, A. C.; Inaba, A. J. Phys. Chem. C 2009, 113, 21396–21405. (17) Ramirez, M. X.; Walters, K. B.; Hirt, D. E. J. Vinyl Additive Technol. 2005, 11, 9–12. (18) Arnold, T.; Clarke, S. M. Langmuir 2008, 24, 3325–3335.
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on graphite has been observed using differential scanning calorimetry (DSC)18 and scanning tunneling microscopy (STM).19 Recently, we determined the monolayer structures of saturated alkylamides with chain lengths of 5-12 carbon atoms adsorbed on the surface of graphite at room temperature (300 K) using X-ray and neutron diffraction.20,21 These structures showed extensive hydrogen-bonded chains of molecules packed in one of two arrangements with either p2 or pgg symmetry. This letter presents temperature-dependent X-ray diffraction data for the adsorbed layer of heptanamide (C7) on graphite at submonolayer coverage, which shows a phase change from pgg to p2 symmetry as the temperature is increased.
Experimental Section Heptanamide used in this work was prepared from heptanoic acid (Sigma-Aldrich, >99%) using the method outlined in ref 22. The purity of the prepared heptanamide (>99%) was analyzed by elemental analysis, 13C and 1H NMR, and liquid chromatographymass spectrometry. Synchrotron X-ray diffraction experiments were performed on Materials Science beamline X04SA at the Swiss Light Source (SLS), Switzerland, with an incident beam wavelength of 1.097 A˚ (11.27 keV).23 Recompressed exfoliated graphite sheets that were 1 mm thick (Papyex, Le Carbone Lorraine) were cleaned under vacuum at 350 °C before use. The graphite (with a mass, Mg, of 3.75 g) was dosed with heptanamide (with a mass, Mh, of 21.10 mg and a relative molecular mass, RMM, of 129.2 g/mol) and annealed at 180 °C. The coverage can be calculated from the area per molecule of heptanamide (Ah = 55.13 A˚2)20 and the specific surface area of graphite (Sg of 30.1 m2/g, by nitrogen adsorption isotherm) to be 0.48 equivalent monolayer as follows: coverage ¼
M h Ah N A S g M g RMM
Here, NA is Avogadro’s number. The experimental method for obtaining synchrotron X-ray diffraction patterns from adsorbed (19) Takeuchi, H.; Kawauchi, S.; Ikai, A. Jpn. J. Appl. Phys. 1996, 35, 3754–3758. (20) Bhinde, T.; Clarke, S. M.; Phillips, T. K.; Arnold, T.; Parker, J. E. Langmuir 2010, 26, 8201–8206. (21) Bhinde, T.; Arnold, T.; Clarke, S. M. Prog. Colloid Polym. Sci. 2010, 137, 5–8. (22) Kent, R. E.; McElvain, S. M. Org. Synth. Collect. 1955, 3, 490. (23) 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, 206–217.
Published on Web 12/03/2010
DOI: 10.1021/la1041053
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Letter
Bhinde et al. Table 1. Calculated Structural Parameters for the Adsorbed Monolayer of Heptanamide on Graphitea unit cell parametersb phase
a (A˚)
b (A˚)
ν (deg)
molecular tilt R (deg) b
pgg 42.65 5.170 90.0 23.50 p2 21.85 5.110 101.0 24.75 a The pgg phase (300 K) has four molecules per unit cell whereas the p2 phase (355 K) has two molecules per unit cell b The uncertainty in the unit cell distances is estimated to be 0.05 A˚ for b and 0.3 A˚ for a and that for the tilt angle is about 1°. All parameters are illustrated in Figure 3.
Figure 1. Experimental (light) and calculated (dark) synchrotron X-ray diffraction patterns from an adsorbed layer of C7 amide on graphite. The diffraction intensity is plotted as a function of the momentum scattering vector q=4π sin (θ)/λ, where θ is the scattering angle and λ is the incident beam wavelength. The patterns shown here are those from a pgg plane group at 300 K (bottom) and a p2 plane group at 355 K (top). The indexing of the principal reflections is indicated. monolayers has been explained in detail previously.20 A cryojet was used for temperature control. The sample was cooled to 250 K where the C7 adsorbate is solid and is heated in steps (10 K), with a few minutes of equilibration at each temperature before data collection. As in our previous studies24 after the graphite background was subtracted and the intense small-angle scattering from the graphite crystallites was removed, the structure was determined using well-established procedures.25 This involved the calculation of a diffraction pattern from various trial structures using the Warren/Kjems line shape26,27 and then comparing this pattern with the experimental one until a satisfactory match was obtained.
Results The diffraction patterns for the monolayer of heptanamide at 300 and 355 K are shown in Figure 1 together with the calculated patterns for the proposed pgg and p2 structures. The structures are shown in Figure 3, and the unit cell parameters are listed in Table 1. In Figure 1, two different experimental diffraction patterns for heptanamide are given. Data for the pgg phase shown in Figure 1 has been presented earlier.20 The structure of the p2 phase was determined using a similar procedure that involved the consideration of plane group symmetry28,29 while constraining the number of independently fitted variables and maintaining favorable hydrogen bond geometry.30 The fitted structure for this phase has an R value31 of 0.10. In these fits, the molecular bond lengths and angles have been taken to be those observed in the bulk crystal and the bond lengths were refined in accord with bulk systems ((3%)32 but by keeping the value of all C-C bond lengths the same to avoid an unreasonable number of fitted variables. (24) 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. (25) Mowforth, C. W.; Rayment, T.; Thomas, R. K. J. Chem. Soc., Faraday Trans. 2 1986, 82, 1621–1634. (26) Warren, B. E. Phys. Rev. 1941, 9, 693–698. (27) Kjems, J. K.; Passell, L.; Taub, H.; Dash, J. G.; Novaco, A. D. Phys. Rev. B 1976, 13, 1446–1462. (28) Hahn, T. Space Group Symmetry, 4th ed.; Kluwer Academic: London, 1995; Vol. A. (29) Arnold, T. The Adsorption of Alkanes from Their Liquids and Binary Mixtures. D. Phil Thesis, University of Oxford, Oxford, U.K., 2001. (30) Leiserowitz, L.; Schmidt, G. M. J. J. Chem. Soc. A 1969, 2372–2382. (31) Inaba, A.; Chihara, H.; Clarke, S. M.; Thomas, R. K. Mol. Phys. 1991, 72, 109–120. (32) Lide, D. R. CRC Handbook of Chemistry and Physics, 85th ed.; CRC Press: Boca Raton, FL, 2004.
16 DOI: 10.1021/la1041053
Data from the p2 phase (Figure 1) has a small feature at 0.750.8 A˚-1 that was present at all temperatures, even after the monolayer had completely melted. Hence, this is not attributed to the monolayer diffraction and has been removed for clarity. Figure 2 shows the diffraction patterns (0.80-1.60 A˚-1) measured for C7 amide at different temperatures. At lower temperatures (between 250 and 320 K), the patterns show that the monolayer structure possesses pgg symmetry. However, at a temperature of about 330 K and above, the patterns change to that for the p2-symmetry structure. Other alkylamides with similar chain lengths do not exhibit this temperature-dependent phase change in their 2D layers at submonolayer coverage. As the temperature approaches the monolayer melting point, the sharp diffraction peaks from the solid layer become increasingly broad until there remains effectively negligible intensity because of scattering from the liquid adsorbate.33 The monolayer melting temperature is estimated to be 420 ( 5 K from the diffraction patterns, which is considerably (∼17%) higher than the melting temperature of the bulk amide (358 K).18 This value is similar to the monolayer melting point of 415 K reported at high coverage (∼10-20 monolayers).18 It is rather unusual for a solid monolayer to exist at temperature so far above the bulk melting point at these submonolayer coverages. We have also observed this elevated monolayer melting point at submonolayer coverage for another alkylamide, dodecanamide (C12 amide),20 but to our knowledge, it has not previously been reported for other systems. The increased stability in the monolayer as compared to that in the bulk in these amides is attributed to extensive hydrogen bonding as explained below.
Discussion Both monolayer phases of heptanamide shown in Figure 3 are arranged such that two amide molecules interact via hydrogen bonds to form dimers,34 and adjacent dimers form additional pairs of hydrogen bonds to yield an extended chain. The hydrogenbonded chains can pack in two ways such that they are parallel to each other (p2 phase) or alternate in direction (pgg phase). Which of these two packing arrangements is adapted seems to be subtly dependent on the alkyl chain length. All of the alkylamides reported previously with even chain length possess p2 symmetry, as do those with odd chain lengths of C9 and longer.20 However, alkylamides C5 and the low-temperature form of C7 both possess pgg symmetry. In many alkyl species, packing arrangements for odd and even homologues are different: the difference is attributed to the packing of methyl groups at the end of the molecule. Carboxylic acids exhibit an odd-even effect where odd-chain-length members of the series investigated possess structures with pgg symmetry and even members have p2 symmetry.24 Interestingly, in the case of the alkanes on graphite, odd-chain-length (33) Warren, B. E. X-ray Diffraction; Dover Publications: New York, 1990. (34) Gilli, G.; Gilli, P. The Nature of the Hydrogen Bond; Oxford University Press: New York, 2009.
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Letter
Figure 2. Synchrotron X-ray diffraction patterns from 0.48 monolayer of C7 amide on graphite at different temperatures. The calculated patterns resulting from the pgg phase (far bottom) and the p2 phase (far top) are also shown. The vertical dotted lines indicate the reflections that are most easily distinguishable between the patterns of the two phases.
Figure 4. Illustration of the two types of hydrogen bonds present in the adsorbed layer of C7 amide.
Figure 3. Schematic structures of the (i) pgg (low-temperature) phase and the (ii) p2 (high-temperature) phase used to fit the diffraction patterns of C7 amide. The unit cells are shown by the solid boxes. Unit cell parameters (a, b, ν) for the structures are indicated, and the molecular tilt angle (R) is shown.
molecules have a cm plane group; the shorter even alkanes (chain length
