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Mixing Behavior at the Solid/Liquid Interface: Binary Alcohol Monolayers on Graphite Loic Messe and Stuart M. Clarke* BP Institiute and Department of Chemistry, University of Cambridge, Madingley Rise, Madingley Road, Cambridge CB3 0HE, U.K.
Chucuan Dong and Robert K. Thomas Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, U.K.
Akira Inaba Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560, Japan
Maria D. Alba and Miguel A. Castro Instituto de Ciencia de Materiales de Sevilla, Avda. Americo Vespucio, Sevilla, Spain Received June 21, 2002. In Final Form: September 11, 2002 Differential scanning calorimetry and neutron diffraction have been used to characterize the mixing behavior of monolayers of binary linear alcohols adsorbed on a graphite surface from solution. Here we have investigated pairs of alcohols with alkyl chain lengths of C5-C11 and which differ in length by a single methylene group. If the shorter of the two alcohols is “odd”, then the monolayers show essentially good or ideal mixing. In contrast, if the shorter of the two alcohols is “even”, then a molecular compound is formed in the adsorbed layer.
Introduction Recently, we have presented data on the behavior of solid monolayers adsorbed from solutions of binary mixtures of linear alcohols which differ in chain length by two or more methylene groups.1,2 For the chain lengths investigated, it was generally found that these alcohols do not mix, but rather phase separate (eutectic behavior) on the surface. In this work we present a combination of calorimetry and neutron diffraction data on the phase behavior of adsorbed monolayers of binary mixtures of linear alcohols adsorbed on graphite that differ by only a single methylene group. The phase diagrams of these mixed adsorbed layers obtained by calorimetry indicates essentially two types of phase behavior depending on the alcohols being considered. If the shorter alcohol is “odd”, then essentially good or ideal mixing is observed. However, if the shorter alcohol is “even”, then molecular compound formation is indicated. Coherent neutron diffraction data are used to confirm the mixing behavior suggested by the calorimetry results. Experimental Section Calorimetry. A full description of use of differential scanning calorimetry (DSC) for the study of phase behavior in adsorbed layers has been given elsewhere.3 In outline, the composition * Address correspondence to this author. (1) Messe, L.; Clarke, S. M.; Inaba, A.; Arnold, T.; Dong, C. C.; Thomas, R. K. Langmuir 2002, 18, 4010-4013. (2) Clarke, S. M.; Messe, L.; Whitehead, C.; Inaba, A.; Arnold, T.; Thomas, R. K. Appl. Phys. A: Mater. Sci. Process., in press. (3) Clarke, S. M.; Inaba, A.; Arnold, T.; Thomas, R. K. J. Therm. Anal. Cal. 1999, 57, 641-651.
dependence of the monolayer melting point is used as an indicator of the adsorbed layer mixing behavior. The DSC measurements were performed on a Pyris 1 Power compensation system at the BP Institute, University of Cambridge, as discussed previously. The heating rate used was 10 °C/min.4 Neutron Diffraction. The apparatus and procedures for such experiments have been described elsewhere.5 The instrument, D20 at the ILL, Grenoble, France, was used for the neutron diffraction measurements. This instrument has a high incident flux and large multidetector. Scattering from crystalline twodimensional adsorbed layers gives rise to diffraction peaks6,7 with a characteristic saw-tooth line shape. At the higher coverages of interest here there is the solid monolayer coexisting with the bulk fluid phase of the adsorbate. The sharp peaks of the adsorbed layer are readily distinguished from the very broad peak characteristic of the fluid phase.8 For these diffraction experiments deuterated alcohols were used to minimize the incoherent scattering background that would have arisen from protonated samples. To optimize the sensitivity to the scattering from the adsorbed monolayers, the amount of bulk fluid was kept to a minimium. The mixing behavior in an adsorbed layer is evident from the position and intensities of peaks in the diffraction patterns as the monolayer composition is changed, as illustrated in Figure 1. Phase separation (Figure 1a) is indicated by the coexistence of diffraction peaks characteristic of each pure compound. As the (4) Castro, M. A.; Clarke, S. M.; Inaba, A.; Arnold, T.; Thomas, R. K. J. Phys. Chem. B 1998, 102, 10528-10534. (5) Castro, M. A.; Clarke, S. M.; Inaba, A.; Thomas, R. K. Physica B 1998, 241-243, 1086-1088. (6) Kjems, J. K.; Passell, L.; Taub, H.; Dash, J. G.; Novaco, A. D. Phys. Rev. B 1976, 13, 1446. (7) Warren, B. E. Phys. Rev. 1941, 59, 693. (8) Castro, M. A.; Clarke, S. M.; Inaba, A.; Arnold, T.; Thomas, R. K. Phys. Chem. Chem. Phys. 1999, 1, 5203-5207.
10.1021/la0205755 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/26/2002
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Figure 1. Schematic illustration of the peak position and intensity variation with monolayer composition expected for three types of mixing behavior: (a) phase separation, (b) ideal mixing, and (c) molecular compound formation. amount of each component is changed, the intensity of that peak changes accordingly. In contrast, good or ideal mixing (Figure 1b) is exemplified by a continuous shifting of the peak characteristic of one component to the position of the other with changing composition. Molecular compound formation (Figure 1c) is characterized by the appearance of a new peak at an intermediate position between peaks corresponding to the two pure components. Usually the molecular compound does not mix with either of the two pure materials. In this case the intensities will behave like phase-separated materials (cf. Figure 1a). On one side of the phase diagram there will be coexistence between pure compound A and the molecular compound. On the other side of the phase diagram, we expect to see the molecular compound coexisting with pure B. The relative intensities will depend on the surface composition and quantities of each phase. Examples of each of these types of phase behavior have been described for alkane mixtures adsorbed on graphite at submonolayer coverages.9 The adsorbent used was recompressed exfoliated graphite Papyex (Le Carbone Lorraine) described previously10 as a preferentially oriented power sample of graphite crystallites containing large, flat, and homogeneous exposed surfaces of the graphite basal plane. Our particular sample was characterized by adsorption of nitrogen and found to have a specific surface area of 31.6 m2 g-1. The deuterated alcohols were prepared by us by isotope exchange of the corresponding carboxylic acid and reduction to the alcohol. The deuteration levels are approximately >98%. The graphite substrates were outgassed under vacuum in an oven at 350 °C before known quantities of alcohol mixtures were added as liquid by microsyringe and annealed onto the surface at a temperature of approximately 40 °C below the bulk boiling point. For convenience, we express the amount of each alcohol adsorbed in terms of the number of equivalent monolayers adsorbed. This is estimated from the areas per molecule, taken from the work of Groszek11 and Morishige,12 and the specific surface area of the graphite. For the calorimetry measurements presented here the total coverage was approximately 40 monolayers. At this high coverage any pores in the graphite are essentially full.
Results Calorimetry. Differential scanning calorimetry thermograms from adsorbed monolayers have been reported elsewhere. Generally, the large endothermic transitions from the bulk adsorbates are readily distinguished from the high-temperature melting points of the solid adsorbed monolayers. Figure 2a-d present the composition dependence of the monolayer melting peaks identified in the DSC thermograms of several binary mixtures of (9) Inaba, A.; Clarke, S. M.; Arnold, T.; Thomas, R. K. Chem. Phys. Lett. 2002, 352, 57-62. (10) Coulomb, J. P.; Bianfait, M.; Thorel, P. J. Phys. (Paris) 1977, 38, C4-31. (11) Groszek, A. J. Proc. R. Soc. London 1970, A314, 473. (12) Morishige, K.; Kato, T. J. Chem. Phys. 1999, 111, 7095-7102.
alcohols differing by a single methylene group with chain lengths from C5 to C11. Figure 3 schematically illustrates the composition dependence expected for three different mixing behaviors: (a) phase separation, (b) ideal mixing, and (c) molecular compound formation (where the pure components and the molecular compound are insoluble). Previously, we have reported phase separation in binary alcohol monolayers that differ by two or more methylene groups.1 This phase separation is characterized in the DSC results by a pronounced minimium in the composition dependence of the monolayer melting points, illustrated in Figure 3a. In contrast, the mixing behavior of C7OH/C8OH (middle data Figure 2a) is much more similar to ideal mixing in the adsorbed monolayer (Figure 3b). The other combinations of alcohols with an “odd” shorter alcohol, C5OH/ C6OH and C9OH/C10OH in Figure 2a, also show similar good mixing behavior, although there is some deviation from completely ideal behavior. In Figure 2 we have presented monolayer melting points against bulk solution compositions, and these deviations could partly arise from preferential adsorption on the surface. Generally the longer alkyl chain molecule is expected to be preferentially adsorbed over a shorter molecule. It is also expected that the relative extent of preferential adsorption will be greater for two shorter molecules than two longer ones (differing by a single CH2 group); i.e., C5OH/C6OH should show more of an effect than C7OH/C8OH. If the surface is slightly richer in the longer component, then the monolayer melting point will be above that expected without preferential adsorption. In Figure 2a C5OH/C6OH and C7OH/C8OH show this expected behavior. However, it is clear that in Figure 2a C9OH/C10OH shows a small deviation in the opposite sense from that expected, suggesting that either there is some reversal in which shorter molecules are preferentially adsorbed or there is some nonideality in the monolayer mixing. However, it is clear that all these binary mixtures in Figure 2a essentially exhibit good mixing on the surface. The mixing behavior of C6OH/C7OH, and the other combinations with a shorter alcohol which is “even” (Figure 2b-d), show a completely different behavior from those with an “odd” shorter alkane. This behavior is more typical of the formation of a molecular compound (Figure 3c). On the basis of the position of the maximium in the phase diagram, the composition of the molecular compound is approximately 1:1, although preferential adsorption may lead to a different surface composition from the bulk. This behavior is extremely unusual in bulk materials although it has been reported previously in adsorbed monolayers.9
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Figure 2. Experimentally determined monolayer melting point variation from binary mixtures of alcohols which differ by a single methylene group, ∆n ) 1. (a) Shorter alcohol “odd” (top) pentanol/hexanol, heptanol/octanol, and (top) nonanol/decanol. Shorter alcohol “even” (b) hexanol/heptanol, (c) octanol/nonanol, and (d) decanol/undecanol. The three thermograms with shorter “odd” alkanes are shown separately to show the maximum in the middle of the phase diagram. Total coverage in all cases is approximately 40 layers.
Figure 3. Schematic illustration showing DSC phase behavior typical of (a) phase separation, (b) ideal mixing, and (c) molecular compound formation.
On the basis of the DSC data alone, we conclude that binary mixtures of linear alcohols form solid monolayers which coexist with the bulk solutions. In addition, we conclude that when the shorter alcohol is “odd”, then the behavior approximates ideal mixing. In contrast, when the shorter alcohol is “even”, then molecular compounds are formed. Neutron Diffraction. In Figure 4a we present variation in the monolayer (2,0) diffraction peak position, obtained from neutron diffraction, for mixtures of dheptanol and d-octanol for a range of solution compositions. On the basis of our DSC data, we expect approximately good mixing in this layer. The continuous progression of the peak from a position characteristic of pure heptanol to that of pure octanol, within the resolution of the experiment, does indeed suggest good mixing behavior.
Hence, we conclude that alcohol mixtures with the shorter alcohol “odd” essentially mix ideally. Figure 4b presents the variation of peak position and intensity for the (2,0) diffraction peaks from mixtures of C6OH and C7OH. These patterns were collected with solution mole fractions of Xheptanol of, on the hexanol-rich side of the phase diagram (bottom), 0, 0.2, 0.33, 0.5, 0.67, and 1 (top). It is clear from this figure that the mixing behavior is that expected for molecular compound formation (Figure 1c). The peak from pure hexanol (2θ ) 16.5) decreases in intensity and is replaced by a new peak at (2θ ) 14.5) with increasing amounts of heptanol. This new peak is clearly distinct from the pure heptanol peak (top pattern). The pattern with composition Xheptanol ) 0.67 is essentially only that of the molecular compound. The composition of the molecular compound is slightly larger
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pure alcohol monolayers, possibly containing two lines of hexanol and two lines of heptanol, would be at least uniaxially commensurate, 9 × x3ag. Such commensurability may favor formation of the molecular compound. Discussion and Conclusions
Figure 4. Experimentally determined peak positions for the (2,0) reflection from mixtures of heptanol and octanol. The continual shift of peak position with composition indicates good mixing in the monolayer. (b) Experimental neutron diffraction patterns from mixtures of hexanol and heptanol. These patterns were collected with solution mole fractions of Xheptanol of (bottom) 0, 0.2, 0.33, 0.5, 0.67, and 1 (top). The peaks from pure hexanol and heptanol are at 2θ ) 16.5 and 13.5, respectively. All patterns are taken at temperatures where the pure adsorbate in the absence of graphite is liquid.
than that deduced from the DSC data; however, given the small amount of adsorbed material in the diffraction experiments, we expect the DSC data to be more precise. These neutron diffraction results are therefore in excellent agreement with the calorimetry data given above. Here we have not attempted a detailed structural intepretation of our diffraction data. Structural determination of even single component monolayers is inherently difficult due to the limited number of monolayer reflections usually obtained. Identifying and determining the relative intensities of peaks from the molecular compound in the presence of peaks from the pure monolayers and from the bulk solution with which they coexist is even more difficult. However, the a parameters of the unit cells can be determined from the diffraction patterns: C6OH, 1.696 nm; C7OH, 2.076 nm; and the molecular compound, 1.919 nm. We note that the a parameter for the molecular compound does not appear to be a simple average of the two pure monolayers (1.886 nm). In addition, we note that the a parameter of the molecular compound is approximately 4.5 × x3ag, where ag (0.246 nm) is the graphite substrate lattice parameter. Hence, a unit cell doubled in the a direction relative to the
The major conclusion of this work is that binary mixtures of simple linear alcohols which differ in alkyl chain length by a single methylene group form solid adsorbed monolayers above the melting point of the bulk mixtures. We conclude that there are two broad categories of behavior such that if the shorter alcohol is “odd”, then essentially ideal mixing is observed. However, if the shorter alcohol is “even”, then there is clear evidence of molecular compound formation. This behavior contrasts with that observed for binary alcohol monolayers with molecules that differ by two or more methylene groups which do not mix to form a single phase on the surface but phase separate into patches of essentially pure components. It is clear that all these combinations of alcohols, which differ by a single methylene group, show good mixing in the monolayer. This is somewhat different from alkanes of similar alkyl lengths that phase separate. In the alkanes this is due to a difference in plane group of the monolayer crystals of “odd” and “even” members of the homogolous series.13,14 Monolayers of the longer alkanes (greater than about 12 carbons) have the same symmetry, and there is evidence for at least partial mixing. In contrast, recent X-ray diffraction measurements indicate that monolayers of “odd” and “even” alcohols have the same plane group.12,15,16 However, there is no evidence for molecular compound formation in adsorbed monolayers of alkanes coexisting with their liquids. Only at submonolayer coverages, where there is an expansion in the monolayer lattices and one might expect a relaxation of packing critera, are alkane molecular compounds formed.1 Based on the data presented here, molecular compound formation is the norm for binary mixtures of alcohols with an “even” shorter alcohol. Recent STM images of adsorbed layers of alcohols have also suggested molecular compound formation for triacontanol (C30) and “longer chain impurities”.17 In the reported structures there are alternating layers of shorter alcohol and longer alcohol molecules. This STM data are consistent with the DSC data presented here. For example, the composition of the molecular compound is also approximately 1:1, and the intermediate position of the diffraction peak from the molecular compound, compared to the pure hexanol and heptanol, is also consistent with a structure consisting of layers of alternating long and short molecules. Other workers have reported the formation of solid monolayers for alkane mixtures at the air-liquid interface.18-20 The reported behavior includes phase separation (13) Arnold, T.; Thomas, R. K.; Castro, M. A.; Clarke, S. M.; Messe, L.; Inaba, A. Phys. Chem. Chem. Phys. 2002, 4, 345-351. (14) Castro, M.; Clarke, S. M.; Inaba, A.; Thomas, R. K.; Arnold, T. Phys. Chem. Chem. Phys. 2001, 3, 3774-3777. (15) Morishige, K.; Takami, Y.; Yokota, Y. Phys. Rev. 1993, B48, 8277-8281. (16) Morishige, K.; Sakamoto, Y. J. Chem. Phys. 1995, 103, 2354. (17) Buchholz, S.; Rabe, J. P. Angew. Chem., Int. Ed. 1992, 31, 189191. (18) Wu, X. Z.; Ocko, B. M.; Tang, H.; Sirota, E. B.; Sinha, S. K.; Deutsch, M. Phys. Rev. Lett. 1995, 75, 1332-1335. (19) Doerr, A.; Wu, X. Z.; Ocko, B. M.; Sirota, E. B.; Gang, O.; Deutsch, M. Colls. Surf. A: Physicochem. Eng. Aspects 1997, 128, 63-74.
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or mixing depending upon the relative differences in alkyl chain lengths. However, there are likely to be significant differences between adsorption at the air-liquid interface and the solid-solution interface presented here. For example, the influence of any solid substrate periodicity is well-known to have profound effects when the periodicity of the substrate does not match that of the adsorbate.11,13,21
Acknowledgment. The authors thank The Leverhulme Trust (L.M.) and The Yamada Science Foundation (A.I.) for financial support and the Staff and scientists at ILL for beam time and technical assistance.
(20) Wu, X. Z.; Ocko, B. M.; Deutsch, M.; Sirota, E. B.; Sinha, S. K. Physica B 1996, 221, 261-266.
(21) Arnold, T.; Thomas, R. K.; Clarke, S. M.; Inaba, A. Phys. Chem. Chem. Phys. 2002, 4, 3430-3435.
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