Langmuir 2008, 24, 2501-2508
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Behavior of Binary Alcohol Mixtures Adsorbed on Graphite Using Calorimetry and Scanning Tunneling Microscopy Guojie Wang, Shengbin Lei, and Steven De Feyter DiVision of Molecular and Nanomaterials and INPACsInstitute for Nanoscale Physics and Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200 F, B-3001 HeVerlee, Belgium
Rebecca Feldman, Julia E. Parker, and Stuart M. Clarke* Department of Chemistry and BP Institute, Cambridge UniVersity, Madingley Rise, Madingley Road, Cambridge CB2 1EW, United Kingdom ReceiVed October 18, 2007. In Final Form: NoVember 15, 2007 The mixing behavior of binary combinations of linear alcohols adsorbed from their liquids is studied by calorimetry and scanning tunneling microscopy (STM). In particular, we consider combinations of primary alcohols that differ by a single methylene group. Where the shorter alcohol has an odd number of carbon atoms, the combination is found to mix, essentially, ideally on the surface. However, for combinations where the shorter alcohol has an even number of carbon atoms, we find that there is molecular complex formation for shorter members but ideal mixing for longer (n > 12) homologues. This extends previous work in this area by the determination of the limits of surface molecular complex formation. We also exploit STM to address this unexpected complex formation.
Introduction study1,2
In a recent we reported the unusual mixing behavior of binary mixtures of primary linear alcohols adsorbed on graphite from their liquid mixtures. In short, it was reported that for mixtures of molecules differing by two or more methylene groups in alkyl chain length the different species phase separated on the surface. Interestingly, when the two alcohols differed by a single methylene group, two types of behavior were found depending upon which of the two molecules had an odd number of carbon atoms in the alkyl chain (“odd” members) or an even number of carbons (“even” members). If the shorter member was odd (C5OH/C6OH, C7OH/C8OH, and C9OH/C10OH), then the surface mixing behavior was essentially idealsindicated by a simple linear variation in monolayer melting point with composition in the surface phase diagram. However, if the shorter alcohol was even (C6OH/C7OH, C8OH/C9OH, and C10OH/ C11OH), then a molecular complex appeared to form with a 1:1 composition of the two alcoholssindicated by a maximum in the monolayer melting point variation with composition at the 1:1 composition. The previous work also reported a neutron diffraction study that did indeed support the formation of a molecular complex. However, diffraction from these buried monolayers is extremely difficult, and it was not possible to determine the structural origin of the difference in behavior. In this work we extend the alcohol systems that have been investigated to both longer and shorter alcohols. We find that shorter combinations with an even alcohol as the shorter member still exhibit molecular complex formation. However, we also find that combinations of longer alcohols (e.g., C12OH/C13OH and above) do not show the molecular complex formation seen for shorter members. Here we also endeavor to exploit scanning tunneling microscopy (STM) as a means of addressing the structural origins of * To whom correspondence should be addressed. E-mail: stuart@ bpi.cam.ac.uk. (1) Messe, L.; Clarke, S. M.; Inaba, A.; Arnold, T.; Dong, C. C.; Thomas, R. K. Langmuir 2002, 18, 4010-4013. (2) Messe, L.; Clarke, S. M.; Inaba, A.; Dong, C. C.; Thomas, R. K.; Castro, M. A.; Alba, M. Langmuir 2002, 18, 9429-9433.
this interesting behavior. STM has been found to be very effective in the identification of the ordered structures of physisorbed molecules, including alkanes, alcohols, acids,3-5 and more complex multifunctional systems6-9 at the liquid-solid interface. The images obtained from alkylated species often have a characteristic lamella structure with the alkyl chains of adjacent molecules packing side by side. In some cases the internal structure of the lamellae can also be determined down to atomic resolution and the imaging of the hydrogen atoms of the adsorbate. Structures of alcohols adsorbed on graphite have been extensively studied by STM.10-13 In one of the earliest reports Rabe3,4 considered C18OH, C24OH, and C30OH, which were all reported to form a herringbone arrangement of molecules, as illustrated in Figure 1A. More recent work has suggested that longer homologues, C22OH,14 C24OH,15 C26OH,14 and C30OH,14,16,17 adsorbed from phenyloctane solution, show a parallel arrangement of molecules, illustrated in Figure 1B. Interestingly, C22OH is reported to form both herringbone and parallel arrangements, as is a coexistence of these two, depending (3) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424-427. (4) Rabe, J. P. Ultramicroscopy 1992, 42-44, 41-54. (5) Hibino, M.; Sumi, A.; Tsuchiya, H.; Hatta, I. J. Phys. Chem. B 1998, 102, 4544-4547. (6) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290-4302. (7) Plass, K. E.; Grzesiak, A. L.; Matzger, A. J. Acc. Chem. Res. 2007, 40, 287-293. (8) Wan, L. J. Acc. Chem. Res. 2006, 39, 334-342. (9) Hermann, B. A.; Scherer, L. J.; Housecroft, C. E.; Constable, E. C. AdV. Funct. Mater. 2006, 16, 221-235. (10) Wintgens, D.; Yablon, D. G.; Flynn, G. W. J. Phys. Chem. B 2003, 107, 173-179. (11) Le Poulennec, C.; Cousty, J.; Xie, Z. X.; Mioskowski, C. Surf. Sci. 2000, 448, 93-100. (12) Gunning, A. P.; Kirby, A. R.; Mallard, X.; Morris, V. J. J. Chem. Soc., Faraday Trans. 1994, 90, 2551-2554. (13) Elbel, N.; Roth, W.; Gunther, E.; Vonseggern, H. Surf. Sci. 1994, 303, 424-432. (14) Gunning, A. P.; Kirby, A. R.; Mallard, X.; Morris, V. J. J. Chem. Soc., Faraday Trans. 1994, 90, 2551-2554. (15) Buchholz, S.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1992, 31, 189191. (16) DeFeyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139-150. (17) Cyr, D. M.; venkataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. J. Phys. Chem. 1996, 100, 13737-13759.
10.1021/la703240y CCC: $40.75 © 2008 American Chemical Society Published on Web 02/01/2008
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Figure 1. Schematic illustrations of the structures of linear alcohol monolayer structures determined by STM.
on temperature.14 These different structures are attributed to the different packing requirements of hydrogen bonding and the alkyl chains. However, another short homologue, dodecanol,18 adsorbed from its liquid, has also been reported to form a herringbone structure at low temperatures (above the bulk melting point) and a parallel structure at higher temperatures, the phase change at 308-313 K. A study of the adsorption of pure decanol/ graphite19 reports that the molecules are in the parallel arrangement. Yet another study shows that monolayers of pure decanol, dodecanol in phenyloctane, and tetradecanol in phenyloctane adsorbed on graphite surface arrange in a herringbone structure.20 Although many of these adsorbed layers have lamellae that follow the direction of the underlying graphite, the periodicity and appearance of Moire fringes suggest that they are incommensurate. Binary mixtures of alcohols adsorbed on graphite have been reported, but the focus is on rather long homologues, for example, the binary mixture C18OH/C30OH, which exhibits preferential adsorption of C30OH.14 Binary mixtures of C18OH and C24OH, adsorbed from phenyl octane,21 have also indicated the preferential adsorption of the longer homologue but did show coadsorption giving mixed double lamellae containing one molecule of each alcohol in a stoichiometry of 1:1, as illustrated in Figure 1D. The authors comment that coadsorption in the zigzag herringbone array cannot occur due to packing difficulties, hence the adoption of this parallel arrangement. Binary mixtures of alcohols and alkanes have also been studied and indicate extensive phase separation in the mixed layer (e.g., C30/C30OH mixture22 from phenyloctane showing extensive phase separation of the components). In this study, we find that adsorbed monolayers of binary mixtures of C9OH and C10OH, and C10OH and C11OH, form patterns distinctly different from those of the pure systems. There are also reports of X-ray diffraction studies of the adsorption of pure alcohols from the liquids,23-25 although the details of the structures are reported to differ from the results of the STM studies. For example, the alcohols C9OH, C10OH, (18) Yeo, Y. H.; McGonigal, G. C.; Thomson, D. J. Langmuir 1993, 9, 649651. (19) Yackoboski, K.; Yeo, Y. H.; McGonigal, G. C.; Thomson, D. J. Ultramicroscopy 1992, 42-44, 963-967. (20) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978-5995. (21) Elbel, N.; Roth, W.; Gunther, E.; Seggern, H. v. Surf. Sci. 1994, 303, 424-432. (22) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608-6619. (23) Morishige, K.; Kato, T. J. Chem. Phys. 1999, 111, 7095-7102. (24) Morishige, K.; Sakamoto, Y. J. Chem. Phys. 1995, 103, 2354-2360. (25) Morishige, K.; Takami, Y.; Yokota, Y. Phys. ReV. 1993, B48, 82778281.
C11OH, C12OH, and C18OH all exhibit incommensurate, herringbone structures but with a periodicity equal to that of the STM data. Differential scanning calorimetry (DSC) also indicates that alcohols adsorb to form solid monolayers from their liquids to graphite. In addition, the monolayers also exhibit other thermodynamic transitions that could arise from solid-solid phase transitions within the layer or from multilayers. Experimental Section Differential scanning calorimetry measurements were made on a Pyris 1 DSC instrument, at the BP Institute, University of Cambridge, as described previously.2 The heating/cooling rate in this work was generally 10 °C/min, representing a compromise of temperature precision and sensitivity. A rate of 40 °C/min was also used where required by particularly weak features. The device was calibrated with indium and heptane. All chemicals were purchased from Aldrich, with a purity of >98%, and used without further purification. The graphite used was Papyex, a recompressed, exfoliated graphite, commercially available from Le Carbone with a specific surface area of approximately 30 m2 g-1, determined by nitrogen adsorption. A typical DSC sample consists of approximately 10 mg of the adsorbate on 20 mg of graphite. The alcohols were added to the DSC pans individually, and the amount was determined by mass. All samples were annealed at elevated temperature, prior to measurement. DSC is a dynamic technique, so the temperature of a feature, such as the melting point, can be identified as the “onset” of the peak rather than the peak maximum. Experimentally, for the very weak monolayer peaks discussed here, it is often the peak maximum that is the easier quantity to measure. In addition, for the small monolayer peaks these two measurements differ by a relatively small constant amount of approximately 2°. In this work approximately 35-40 equiv of monolayers of adsorbate has been added, an estimate based on the specific surface area of the graphite and the area per molecule. STM samples were prepared by placing a 5 µL droplet of the neat alcohols or mixtures onto a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface (GE Advanced Ceramics). Samples containing neat nonanol or a mixture of nonanol and decanol were kept at about -10 °C for 1/2 h prior to imaging at room temperature to favor adsorption. The samples of neat decanol, undecanol, and the mixture of decanol and undecanol were prepared and measured at room temperature. 1-Nonanol (98%), 1-decanol (99%), and 1-undecanol (99%) were purchased from Aldrich and used without further purification. Binary mixtures were prepared by mixing nonanol and decanol, and decanol and undecanol, with a 1:1 molar ratio. The STM experiments were carried out using a PicoSPM (Molecular Imaging, Arizona), in the constant-current mode: the color coding in the STM graphs reflects differences in height. Pt/Ir STM tips were prepared by mechanical cutting from Pt/Ir wire
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Figure 2. DSC thermograms of pure pentanol (top) and pentanol in the presence of graphite (bottom). The large endothermic peak in each trace is due to melting of the bulk pentanol. The smaller peak (expanded in the inset) in the bottom trace is due to the melting of the solid monolayer of pentanol adsorbed on the graphite.
(80%/20%, diameter 0.2 mm). For analysis purposes, the imaging of a molecular layer was immediately followed by recording at a lower bias voltage the graphite lattice, under otherwise identical experimental conditions. Drift effects were corrected for using Scanning Probe Image Processor (SPIP) software (Image Metrology ApS). To reduce noise, the images were filtered. The filtering procedure does not change the molecular features, yet enhances the visual presentation.
Results Calorimetry Results. Figure 2 presents DSC thermograms for the melting behavior of pentanol in the pure state and adsorbed on graphite. The melting of pure pentanol exhibits a single large peak corresponding to the bulk melting point of pentanol, in good agreement with the literature. Significantly, the thermogram in the presence of graphite shows an additional peak at approximately -60 °C corresponding to the melting of the solid pentanol monolayer adsorbed on the graphite. Graphite alone shows no transitions at all in this temperature range. Other closely related simple alkyl species, including alkanes, alcohols, and carboxylic acids,23,24,26-31 adsorbing on graphite all show a similar DSC transition that diffraction studies have been able to confirm arise from the melting of a 2D solid layer. We therefore conclude that pentanol forms a solid monolayer in coexistence with the liquid alcohol. This is particularly interesting because, although many alkanes form a solid monolayer on graphite that coexists with the liquid, the pentane monolayer is anomalous. Pentane does form a solid monolayer, but its melting point is the same as the bulk melting point, so the transition cannot be observed separately in the DSC thermograms.16 In this work we will be (26) Arnold, T. The adsorption of alkanes from their liquids and binary mixtures. Ph.D. Thesis, Oxford University, Oxford, U.K., 2001. (27) 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. (28) Arnold, T.; Thomas, R. K.; Castro, M. A.; Clarke, S. M.; Messe, L.; Inaba, A. Phys. Chem. Chem. Phys. 2002, 4, 345-351. (29) Bickerstaffe, A. K.; Cheah, N. P.; Clarke, S. M.; Parker, J.; Perdigon, A.; Messe, L.; Inaba, A. J. Phys. Chem. 2006, B110, 5570-5575. (30) Messe, L.; Perdigon, A.; Clarke, S. M.; Inaba, A.; Castro, M. A. J. Colloid Interface Sci. 2002, 266, 19-27. (31) Taub, H. In The Time Domain in Surface and Structural Dynamics; Long, G. J., Grandjean, F., Eds.; Kluwer: Dordrecht, The Netherlands, 1988; Vol. 228.
considering the mixing of solid alcohol monolayers adsorbed from their liquid mixture. Adsorption from Phenyloctane Solution. We have already established that both linear alcohols and phenylalkanes, with an alkyl chain of eight carbons or longer, form solid monolayers when adsorbed from their pure liquids.1,2,30,32 Figure 3a presents the variation in the monolayer melting point with composition for the binary mixture of octadecanol and phenyloctane. The variation in monolayer melting point can be modeled to identify the preferential adsorption of the two species. For heptadecanol/ phenyloctane and octadecanol/phenyloctane we can see that the heptadecanol and octadecanol are strongly preferentially adsorbed. It is only at compositions around 0.1 mole fraction that a contribution from the phenyloctane becomes significant on the surface. The extent of preferential adsorption can be quantified using the equilibrium constant, K, between the two states (i) with only the alcohol adsorbed and the phenyloctane in solution and (ii) with only phenyloctane adsorbed and the alcohol in solution.33 A K of unity indicates that both components can compete equally for the surface. A high value of K indicates strong preferential adsorption for the alcohol (as seen here). It is also possible to model the surface behavior assuming ideal mixing in the adsorbed layer, but the extent of preferential adsorption is essentially unchanged. For phase diagrams of the form in Figure 3, with strong preferential adsorption and without any pronounced minimum, which indicates nonideality of mixing, it is not possible to unambiguously identify the mixing behavior. Figure 3b presents similar data for heptadecanol and phenyloctane on graphite and illustrates similar trends that the long alcohol is strongly preferentially adsorbed, and only at low mole fractions of alcohol does the phenyloctane become significant on the surface. The equilibrium constant has been determined as K ) 12. Binary Mixtures of Alcohols. Figure 4a presents the monolayer phase diagram for binary mixtures of octadecanol/heptadecanol adsorbed on graphite from the liquid. The data show a smooth temperature variation, suggesting good, essentially ideal mixing. (32) Castro, M.; Clarke, S. M.; Inaba, A.; Perdigon, A.; Prestidge, A.; Thomas, R. K. Stud. Surf. Sci. Catal. 2001, 132, 873-876. (33) Messe, L.; Perdigon, A.; Clarke, S. M.; Inaba, A.; Arnold, T. Langmuir 2005, 21, 5085-5093.
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Figure 3. (a) Monolayer melting point variation with composition for octadecanol and phenyloctane adsorbed on graphite. The dashed line is the theoretical liquidus curve calculated assuming complete phase separation in the solid monolayer. The solid line is this theoretical curve adjusted to take preferential adsorption of octadecanol into account, with K ) 15. (b) Similar data for heptadecanol and phenyloctane.
This behavior is similar to that reported previously for shorter binary alcohol mixtures where the shorter member has an odd number of carbon atoms. Hence, we conclude that C17OH/ C18OH exhibits the same behavior as their shorter homologues. Figure 4b presents the monolayer phase diagram of C18OH/ C19OH adsorbed on graphite. This phase diagram also appears to exhibit a smooth variation in monolayer melting point that suggests good, almost ideal mixing in the solid monolayer. Importantly, this behavior is completely different from that observed with shorter homologues which exhibit surface molecular compound formation. Molecular compound formation has a characteristic maximum at a composition of 0.5; this is not observed in the C18OH/C19OH combination. Parts c and d of Figure 4 present the monolayer phase diagram of other binary alcohol mixtures adsorbed on graphite. In each case the shorter homologue is even. These phase diagrams also appear to exhibit a smooth variation in monolayer melting point that suggests good, almost ideal mixing in the solid monolayer. The dashed lines in Figure 4c,d indicate the expected depression of the freezing point of the monolayer melting point if the surface molecular compound is formed. We can conclude that the differences are significant and that these alcohol combinations essentially mix ideally. This behavior is entirely different from that observed with shorter homologues which show molecular compound formation. The next shorter binary pair, below C12OH/C13OH, with an even shorter member is C10OH/C11OH. This has been reported previously and exhibits molecular compound formation. Hence, we conclude that the change in behavior from short and long homologues occurs between C10OH/C11OH and C12OH/ C13OH. We have also extended the study to consider shorter combinations than previously studied to include butanol/pentanol, C4OH/
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C5OH. The experimental phase diagram is given in Figure 5. These data exhibit a pronounced minimum as we change composition from the pure alcohol monolayers, essentially showing complete phase separation in the layer. The middle composition region deviates significantly from this depression and shows a slight rise indicative of molecular complex formation and a rather weak maximum. Hence, we conclude that this combination also forms a molecular compound in a fashion similar to that of the other combinations C6OH/C7OH, C8OH/C9OH, and C10OH/C11OH. At this stage it is difficult to be sure as to the origin of the change in behavior between the shorter homologues which form a molecular complex and those longer species that do not and why this seems to occur at C10OH/C11OH. This effect could possibly arise from a change in monolayer crystal structures. A different monolayer crystal structure can be evident from an odd/even variation in monolayer melting point. The variation of the monolayer melting point for bulk and monolayer alcohols is given in Figure 6, where a strong odd/even variation is evident. However, this odd/even variation persists beyond C10OH/ C11OH. The last obvious odd/even variation is for C12OH/ C13OH, suggesting that it is not the underlying origin of the effect. In addition, X-ray diffraction data, and the STM data shown below, indicate that odd and even homologues adopt isomorphous monolayer crystal structures. A maximum in the phase diagram can also have other possible causes including the related dystetic behavior. In this case associated molecules become separated on melting, which gives rise to a maximum in the temperature variation of the monolayer melting point. According to the most simple theories, the gradient at a dystetic point should be zero. However, in most of the alcohol cases the gradient appears to be nonzero and represents a discontinuity; this also suggests that the complex that forms is retained in the melt. However, the degree of association and ideal mixing in the solid and melt can result in a number of effects on the phase diagram. STM Results. Parts A-C of Figure 7 show representative STM images for 1-nonanol, 1-decanol, and 1-undecanol monolayers on highly ordered pyrolytic graphite. From the highresolution images, the herringbone packing is clearly observed for each of the alcohols. For nonanol, the distance between two adjacent molecules along the long lamella axis, b, is 0.50 ( 0.02 nm, the mean width, a, of a lamella is 2.44 ( 0.05 nm, and, φal, the angle between unit cell vector b and the alkyl chain measures 62 ( 4°. Similar results are obtained for decanol and undecanol except for the unit cell vector a, which is 2.68 ( 0.05 and 2.96 ( 0.05 nm for decanol and undecanol, respectively. The parameters for decanol are in accordance with the results reported by Claypool et al.20 All parameters are summarized in Table 1. φg expresses the orientation of the monolayer with respect to the graphite substrate and refers to the angle between one of the main symmetry axes of graphite, i.e., the 〈-1,-1,2,0〉 direction, and unit cell vector b. The angle φg is close to zero (-3 ( 2°, -2 ( 2°, and 0 ( 2° for nonanol, decanol, and undecanol, respectively): the lamellae follow the direction of the underlying graphite. The structural data obtained here agree well with previous reports of alcohol monolayers studied by X-ray diffraction data23,25 (Table 1). It is known that the OH groups in alcohol monolayers on HOPG exhibit a reduced tunneling probability compared to the rest of the alkyl chain. Therefore, the darker troughs running parallel to the lamellae are attributed to the location of the OH groups,15,20,22,34 which are marked with black arrows in Figure 7A-C. The angle between two adjacent alcohols is about 120°.
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Figure 4. Phase diagram showing the monolayer melting transition for binary mixtures of (a) heptadecanol and octadecanol, (b) octadecanol and nonadecanol, (c) tetradecanol and pentadecanol, and (d) dodecanol and tridecanol adsorbed on graphite. The black lines are the solidus (lower line) and liquidus (upper line) curves calculated assuming ideal mixing in the solid monolayer and the liquid.
Figure 5. Phase diagram showing the monolayer melting transition for binary mixtures of butanol and pentanol adsorbed on graphite. The lines are the theoretical liquidus curves calculated assuming complete immiscibility in the solid monolayer.
These data are in agreement with the model suggesting that alcohols, like alkanes, are adsorbed in a manner that the bonds in the trans carbon-carbon skeleton are oriented in a plane parallel to the graphite surface. OH groups are located within the carbon plane of the alcohol overlayer. Molecules in adjacent lamellae are staggered by half of the molecular width. Each molecule is involved in two hydrogen bonds with its neighboring molecules, and the intermolecular hydrogen bonding produces a V-shaped dimer, resulting in a herringbone packing arrangement, as illustrated in Figure 7D. (34) Zhang, H. M.; Yan, J. W.; Xie, Z. X.; Mao, B. W.; Xu, X. Chem.sEur. J. 2006, 12, 4006-4013.
Figure 6. Variation in alcohol (9) bulk and (b) monolayer melting point with alkyl chain length.
Parts A and B of Figure 8 show representative STM images of monolayers formed from solutions containing a 1:1 mixture of 1-nonanol and 1-decanol (C9OH/C10OH) and 1-decanol and 1-undecanol (C10OH/C11OH), respectively, on highly ordered pyrolytic graphite. Interestingly, a double-sized V-shaped herringbone packing arrangement, in other words, an alternating herringbone packing of dimers, is observed. Lamellae are composed of four rows of molecules. The different packing for the mixtures compared to the pure alcohol systems clearly indicates that, for the mixtures, both alcohols are indeed simultaneously adsorbed: there is no apparent preferential adsorption of one of the alcohols. However, due to the small difference in alkyl chain length, it is difficult to differentiate the short from the long molecules in the mixtures. For C9OH/C10OH, the mean width, a, of the lamellae is 5.28 ( 0.05 nm (Figure 8A), the distance between two adjacent molecules along unit cell
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Figure 7. High-resolution STM images of monolayers formed by self-assembly of alkanols on HOPG: (A) C9OH (1-nonanol, Iset ) 1.0 nA, Vbias ) 0.5 V), (B) C10OH (1-decanol, Iset ) 1.0 nA, Vbias ) 0.6 V), (C) C11OH (1-undecanol, Iset ) 1.0 nA, Vbias ) 0.6 V). The scale bar is 1 nm. (D) Schematic diagram depicting an alkanol herringbone packing on graphite, in which the angle between two alkanols is about 120°. Black arrows mark the troughs which are composed of alkanol groups. Table 1. Unit Cell and Other Characteristic Parameters of Monolayers of Alcohols (C9OH, C10OH, C11OH) As Determined by STM and X-ray Diffraction: Unit Cell Vector a (Perpendicular Distance between Two Parallel Lamella Axes), Unit Cell Vector b (Distance between Two Adjacent Molecules along the Lamella Axis), Oal (Angle between the Lamella Axis and Alkyl Chain), Og (Angle between the Symmetry Axis of Graphite and Unit Cell Vector b)a parameters by STM
a
parameters by X-ray diffraction
molecule
a (nm)
b (nm)
φal (deg)
φg (deg)
a (nm)
b (nm)
φal (deg)
C9OH C10OH C11OH
2.44 ( 0.05 2.68 ( 0.05 2.96 ( 0.05
0.50 ( 0.02 0.50 ( 0.02 0.52 ( 0.02
62 ( 4 66 ( 4 64 ( 4
-3 ( 2 -2 ( 2 0(2
2.51 2.65 2.95
0.517
63.2 64.8
X-ray diffraction data taken from refs 23 and 25.
vector b is 0.50 ( 0.02 nm, and the angle φal between unit cell vector b and the alkyl chains is 64 ( 4°; these values are similar for C10OH/C11OH (Figure 8B), except for the lamella width, which is significantly larger, 5.60 ( 0.05 nm (Table 2). Similar to that of the pure alcohols, φg is close to 0°: the lamellae follow the direction of the underlying graphite. Note that the neighboring molecules on each side of the dark troughs run parallel, marked
with black arrows. Since the darker troughs are attributed to the location of the OH functional groups,15,20,22,34 we may conclude that adjacent alcohols interacting by hydrogen bonding run parallel, resulting in a model as shown in Figure 8C. As already indicated, it is difficult to identify the different alcohols with certainty in the mixtures due to their similar alkyl chain lengths. DSC data indicate that the mixture of C9OH/
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Figure 8. High-resolution STM images of self-assembled monolayers of alkanols on an HOPG surface: (A) mixture of 1-nonanol and 1-decanol (C9OH/C10OH) with a molar ratio of 1:1 (Iset ) 1.1 nA, Vbias ) 0.3 V), (B) mixture of 1-decanol and 1-undecanol (C10OH/C11OH) with a molar ratio of 1:1 (Iset ) 1.0 nA, Vbias ) 0.4 V). The scale bar is 1 nm. (c) Schematic diagram depicting an alternating herringbone packing of the mixtures on graphite. Black arrows mark the troughs which are composed of alkanol groups. Table 2. Unit Cell Parameters and Other Characteristics of Monolayers Formed by C9OH/C10OH and C10OH/C11OH Mixtures As Indicated by STM: Unit Cell Vector a (Perpendicular Distance between Two Parallel Lamella Axes), Unit Cell Vector b (Distance between Two Adjacent Molecules along the Lamella Axis), Oal (Angle between the Lamella Axis and Alkyl Chain), Og (Angle between the Symmetry Axis of Graphite and Unit Cell Vector b) mixture
a (nm)
b (nm)
φal (deg)
φg (deg)
C9OH/C10OH C10OH/C11OH
5.30 ( 0.05 5.60 ( 0.05
0.50 ( 0.02 0.54 ( 0.02
64 ( 4 60 ( 4
0(2 0(2
C10OH forms an ideally mixing solid monolayer and C10OH/ C11OH forms a molecular compound. To harmonize the DSC and STM observations, it is proposed that C9OH and C10OH appear randomly in the lamellae, yet C10OH and C11OH are dispersed in the lamellae in a way that one row is composed of C10OH and one row of C11OH, although we are unable to observe this directly. The difference in mixing behavior may also be indicated in the new lamella spacing of the 1:1 binary mixtures.
We note that the sum of the C9OH/C10OH pure lamella spacing is 5.12 ( 0.07 nm. The actual value in the mixture is 5.30 ( 0.05 nm. Hence, the actual mixed lattice seems to be slightly expanded relative to that of the pure materials. In contrast the total of the lamella spacings of the C10OH/C11OH materials is 5.64 ( 0.07 nm. The experimentally determined value from the actual mixture is 5.60 ( 0.05 nm. This combination has a mixed lattice parameter that shows a packing comparable to that of the two pure materials. Such a behavior might also occur with molecule compound formation. Interestingly, we see no evidence for the previously reported structures of binary alcohol mixtures illustrated in Figure 1C,D, although that may be due to the similarity of the alkyl chain lengths of the two species employed here.
Conclusions We conclude that mixtures of linear alcohols which differ in alkyl chain length by a single carbon atom exhibit an odd/even variation. Where the shorter member is odd, ideal mixing is observed. Where the shorter member is even, molecular compound formation is observed but only for combinations with
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C10OH and shorter. Longer combinations exhibit essentially ideal mixing, independent of parity. The limit of this effect may arise from an odd/even variation of the pure alcohol monolayer crystal structures, although this is not indicated by either X-ray diffraction or STM data. The limit may arise from some subtle variations of commensurability with alkyl chain length; however, all species appear to be similarly commensurate with the underlying substrate. STM data indicate that binary mixtures of C9OH/C10OH and C10OH/C11OH form different monolayer structures on graphite compared to the alcohols: for the pure alcohols, the monolayers show a herringbone packing structure in which the lamellae are composed of two rows of molecules. For the mixtures, a new alternating herringbone packing structure is formed, in which the lamellae are composed of four rows of molecules. To rationalize both the DSC and STM data, we propose that C9OH
Wang et al.
and C10OH are dispersed randomly in the mixed lamella structures, while C10OH and C11OH form separate rows in the monolayer mixture, but we are unable to observe this directly due to the similarity of the alkyl chain lengths of the species. However, the difference in mixing behavior observed in calorimetry seems also to be expressed in the relative densities of the two mixtures. The C9OH/C10OH mixture shows a slight expansion relative to the pure materials, contrasting with the C10OH/C11OH combination, where the mixed lattice is essentially of the same density as the pure materials. Acknowledgment. We thank the EPSRC (S.M.C., J.E.P.), the Katholieke Universiteit Leuven (K. U. Leuven) via Grant IDO/02/014, and the Fund for Scientific ResearchsFlanders (FWO) for financial support. LA703240Y
