The Mixing Behavior of Alkanes Adsorbed on Hexagonal Boron

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The Mixing Behavior of Alkanes Adsorbed on Hexagonal Boron Nitride Matthew Forster, Julia E. Parker, Akira Inaba, Claire A. Murray, Nicholas A Strange, John Z. Larese, and Thomas Arnold J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07701 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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The Journal of Physical Chemistry

The Mixing Behavior of Alkanes Adsorbed on Hexagonal Boron Nitride Matthew Forster1, Julia E. Parker1, Akira Inaba2, Claire A. Murray1, Nicholas A. Strange3, John Z. Larese3 and Thomas Arnold1,* 1

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Diamond Light Source, Diamond House, Harwell Campus, Chilton, Didcot, OX11 0DE, UK

Research Center for Structural Thermodynamics Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, JAPAN 3

Depatment of Chemistry, University of Tennessee, Knoxville, TN, USA

*Address correspondence to this author: [email protected], tel: +44 1235 778543

Abstract

In this work we report the mixing behavior of a series of normal-alkanes adsorbed on the surface of hexagonal boron nitride (h-BN) using X-ray powder diffraction. We have investigated a range of simple binary mixtures which are indicative of a rich phase behavior with examples of complete mixing, partial mixing and phase separation. On graphite surfaces, the mixing behavior is strongly influenced by the structure of the pure components; the odd-even effect seen in the pure structures, which favors either a “herringbone” or “parallel” structure, influences the miscibility of alkanes within the monolayer. On hBN a more complex phase behavior is observed with partial mixing or phase separation depending upon the exact composition of the monolayer. In particular, we see improved miscibility for certain mixtures containing n-decane which we associate with the fact that pure n-decane has been observed with both ACS Paragon Plus Environment

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herringbone and parallel structures on h-BN. This difference between these two very similar substrates is a sensitive indicator of the subtle interplay between surface-molecule and molecule-molecule interactions that govern the phase behavior of these systems.

Introduction. The interaction of physisorbed molecular mixtures on surfaces is of fundamental scientific and technological importance due to the fact that many interfacial physical properties are dominated by weak van-der-Waals forces. Despite this significance our understanding of how these forces influence phenomena such as wetting, melting, preferential adsorption, and two-dimensional (2-D) crystallization remains incomplete. These physical properties of absorbed molecular films are intimately linked to the structure adopted at the interface. For adsorbed multi-component mixtures, the phase behavior can be thought of in terms of a two-dimensional analogue of common three dimensional phenomena such as ideal or non-ideal mixing and phase separation. However, in the surface adsorption case this phase behavior depends not only on the intermolecular interactions but also upon the relative magnitude of the molecule-substrate potential which, at present, are not well understood. Molecular physisorption has been extensively studied on a wide range of surfaces using many different techniques. Some examples include studies of adsorption on single crystals, quasi-crystals as well as on powder substrates

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. This latter category of materials has allowed the use of bulk rather

than surface specific techniques and has largely concentrated on powdered Graphite materials. A large body of literature exists containing detailed information on the structural, thermodynamic and physical properties of such adsorbed molecular films and this is well summarized in the recent reviews by Bruch et al. and others14-16. The adsorption of n-alkanes on graphite has attracted particular interest both due to their technological relevance and the fact that they provide a series of systematically varying molecules, which therefore allows the subtle effects of molecule-substrate and intermolecular interactions to be probed. These studies reveal that the n-alkanes form stable, ordered monolayer solids on graphite surfaces which can adopt either a ‘parallel’ or ‘herringbone’ molecular arrangement depending upon ACS Paragon Plus Environment

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chain length17-19. Far less is known about molecular physisorption on non-graphitic materials. Some studies have now been conducted using magnesium oxide (MgO) or hexagonal boron nitride (h-BN) as a substrate. For MgO, it was shown that the adsorption of butane leads to very similar monolayer structures to those identified for graphite despite considerable differences in surface symmetry and chemical composition20. Various other short-chain n-alkanes have also now been studied on MgO8, 21-28 although in most cases their detailed structures are yet to be published. Hexagonal boron nitride (h-BN) is a material that is structurally closely related to graphite (often referred to as white graphite). To date most adsorption studies on h-BN that have employed thermodynamic or X-ray scattering techniques have focussed on relatively simple molecules29-36. Structural studies using neutrons have not been performed because the abundant isotopic form of boron is a strong neutron absorber thus limiting its use in neutron scattering studies. Similarly STM studies on pure BN substrates are not possible because it is an electrical insulator. Recently, we have used differential scanning calorimetry to systematically investigate the thermodynamic properties of pure and mixed films of n-alkanes on h-BN37. These results were compared with the literature from the same systems on graphite38-47. It is well established that surfaces will preferentially adsorb the longer molecule from a binary mixture48 (this principal is commonly used to form monolayers of hydrocarbons dissolved in uncompetitive solvents for imaging with STM). However, when molecules are of a similar length it is possible to co-adsorb both components, and once adsorbed, the mixtures can show phase separation or (ideal or non-ideal) mixing. Our recent calorimetry study suggested that binary mixtures of alkanes on h-BN mix more easily than the equivalent mixtures on graphite. In order to understand why this might be we have performed a structural study of the pure alkanes; hexane to hexadecane (C6H12C16H34, hereafter abbreviated to Cn indicating the number of carbon atoms within the alkane backbone)49. These results predominantly show structures that are very similar to those previously reported for graphite17-19,

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implications for the phase behavior of the binary mixtures. One such difference between the structures for the alkanes on these substrates is that the alkane monolayers are uni-axially incommensurate at subACS Paragon Plus Environment

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monolayer coverage on graphite, whereas they are fully commensurate at sub-monolayer coverage on hBN49. So the change in surface potential and substrate cell parameters evidently has an impact on packing within the monolayer. This is particularly so for decane, for which we identified a complex coverage and temperature dependent phase behavior that is not observed on graphite, although it is reminiscent of behavior seen for longer molecules on that substrate49. In general, all of the odd-alkanes (i.e. CnHn+2 where n is odd) on both substrates show a structure in which molecules pack with their principal axes parallel to each other, whereas the short even alkanes (i.e. where n is even) adopt a herringbone molecular arrangement. An illustration of both the herringbone and parallel structures formed from alkane adsorption on h-BN is shown in Figure 1. However, as the length of the carbon chain of the even molecules increases there is a transition between these two structures so that the longest molecules all show the parallel molecular arrangement. This transition is found to occur at decane on h-BN while on graphite the transition is seen for dodecane (C12) or tetradecane (C14) depending on the exact experimental conditions18, 52. Interestingly, diffraction data for decane (C10) on h-BN shows peaks related to both parallel and herringbone structures, depending on coverage and temperature49. At low coverage (θ ~ 0.6) both structures are present, however, at higher coverage (θ ~ 0.9) only the parallel structure is observed. We have now begun a more detailed study on the coverage dependence of decane on h-BN in order to better characterize this system. However, we note that the coexistence of parallel and herringbone structures may be kinetic in origin. The complex phase behavior presented by decane on h-BN is, nonetheless, indicative of a delicate balance that determines which structure is the most thermodynamically stable. It is known that on graphite the symmetry of the structure adopted by the pure components has an important role to play in whether a binary mixture is miscible40. Thus, in this study we examine whether the apparent miscibility of alkanes on h-BN is related to the ease with which decane (C10) molecules can switch between structures. To do this we make a direct comparison with the same systems studied on graphite and to avoid complications of preferential adsorption we have restricted this work to the sub-monolayer regime.

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Figure 1 Examples of the two structure types, the herringbone structure of hexane (left) and the parallel structure of heptane (right)49. In each case the unit cell is shown as a red box. The molecules are shown with their C-C zig-zag in the plane of the substrate, although the X-ray data is not sensitive to this orientation. Here, we describe the mixing behavior of binary mixtures of alkanes adsorbed on the surface of h-BN as determined by x-ray powder diffraction. Specifically, we determine the phase behavior for mixtures of Cn/Cn+1 (for n = 7, 8 and 9) and Cn/Cn+2 (for n = 7 and 8). These mixtures represent examples of the following key combinations. The C7/C8 and C8/C9 systems are mixtures of herringbone and parallel favoring alkanes, with the shorter molecule preferring either the parallel or herringbone structure respectively. For C7/C9 system both molecules prefer the parallel arrangement and for C8/C10 both can adopt the herringbone structure. Finally the behavior of the C9/C10 and C8/C10 systems may be affected by the ability of C10 to adopt both structures. Experimental All the alkanes used in this study were purchased from Sigma-Aldrich (≥99.8%) and used without further purification. The h-BN powder was obtained from Momentive Performance Materials Inc. and has a quoted specific surface area of 15.56 m2g-1. We have previously provided a comprehensive characterization of this powder

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as used in our studies. This characterization includes the effect of

powders with different specific surface areas, the average crystallite size and any preferential orientation of crystalline planes arising from compressing the powders into pellets. We also discussed the substrate cleaning (referring back to an earlier study36 which directly assessed such issues) and the corrections that we have applied to the data. For the sake of brevity this information is not repeated here and we refer the reader to this study for details.


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For clarity we briefly outline our sample preparation method. To ensure the highest possible density of scatterers in the beam the powder was compressed into 3mm diameter pellets using a standard pellet press (0.5 tons). The h-BN powder was then cleaned by washing in methanol for approx. 12 hours followed by heating at 300°C under a vacuum of approx. 10-7 mbar. Several grams of the cleaned pellets were then placed in a sealed glass vial to which known quantities of alkane mixtures, equivalent to a surface coverage of 0.8 monolayers, were added as a liquid by micro-syringe. The sealed vials were annealed at 60°C and allowed to cool slowly in order to create a homogenous monolayer. We note that the error in the calculated surface coverage is significant due to evaporation losses and uneven spreading of the alkane monolayer on a relatively large amount of powder. However, our previous studies into melting of alkane monolayers on h-BN show that our calculation of coverage is reasonable49. The X-ray diffraction patterns were collected on beamline I11 at Diamond Light Source at 12keV53 (1.032490Å and 1.032219Å for two different occasions) using the Mythen detector. This detector offers the best compromise between resolution and flux for these measurements. The zero-point error of this detector and the wavelength were calibrated using a silicon standard (NIST 640c). The samples were contained in 3.5mm capillaries and cooled using a standard cryostream. The large size of the capillaries maximizes the number of surface scatterers in the relatively large beam (0.7mm (v) x 2.5mm (h)) at the cost of a reduction in resolution. Diffraction peaks arising from the formation of 2D alkane monolayers on a powder substrate are readily distinguished, following subtraction of a reference clean h-BN sample pattern, by their characteristic “saw-tooth” lineshape54, 55. We have used the intensity of these peaks as an approximation of the relative amounts of a particular structure, by comparison to the peaks observed for the pure components. This approach assumes that the structures do not change substantially for the mixed monolayers. We note that the intensity of the peaks is related to both the amount of a crystal and its structure factor, and may be affected by a number of different factors including disorder within the crystal resulting from the incorporation of solute molecules into a solvent lattice. However, the


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contribution from the amount of a particular crystal structure is likely to be the dominating factor and, therefore, the use of peak heights as an approximation to the amount of a structure is a useful one. Results Complete Mixing, C7/C9: Figure 2a presents X-ray diffraction patterns collected for the binary C7/C9 mixtures adsorbed on the surface of h-BN powder. The diffraction patterns across the entire composition range (including the pure components) consist of just two prominent peaks, occurring at a momentum transfer (q) of approx. 0.5 and 1.45Å-1. These peaks were previously49 indexed to (2,0) and (1,1) reflections, respectively. Note that the (4,0) peak is also visible but relatively weak (especially in the mixed systems) and so not as useful in our analysis here. The structures proposed for both of the pure monolayers show the parallel molecular arrangement and are close to fully commensurate with the h-BN substrate (see figure 1). The gradual transition of the experimental patterns between these two extremes suggests complete mixing of the components. Figures 2b and c show plots of the position and the peak intensity of the two peaks as a function of composition. Both peaks show a gradual movement with composition. The (2,0) peak is entirely defined by the a-direction of the unit cell and is closely related to the length of the molecule. Increasing the fraction of C9 in the mixture leads to a shift to lower momentum transfer and indicates an expansion of the unit cell along this direction. A similar but smaller shift is seen for the (1,1) reflection. The position of this peak is also sensitive to the b-spacing of the unit cell (approximately 4.34Å for both molecules and commensurate with the underlying substrate). Since the indices of these two peaks are so simply related (with a rectangular unit cell) we can directly determine how the drift in the positions of the (2,0) and (1,1) peaks affects the lattice parameters.


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Here q is the momentum transfer of the peak concerned, h and k are the Miller indices and a and b are the lattice parameters of the unit cell. Thus, the a-parameter is simply determined from the position of the (2,0) peak since k = 0. We can then substitute these values into the above equation and use the position of the (1,1) peak to calculate b, where h = k = 1. Both a and b are also plotted in figure 2b, which shows that the shift in the position of the (1,1) peak is almost entirely accounted for by the expansion of the a-parameter with increasing C9 composition, while b remains fixed at approximately the commensurate spacing of 4.34Å. For the most part the expansion of a is linearly proportional to the composition, and as such this mixture approximately obeys Vegard’s law56, 57. Interestingly, however, at low concentrations of C7 in C9 (0.8 ≤ Mole fraction (XC9) ≤ 1) we see that the lattice parameter is completely unaffected by the presence of solute molecules, presumably because it is easier to accommodate the shorter C7 molecule within the C9 parallel structure.


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Figure 2 (a) X-ray diffraction patterns collected for mixtures of heptane and nonane (C7/C9) at compositions of XC9 = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1. The arrows indicate the (2,0) (black) and (1,1) (red) reflections. Plots of (b) the position of these peaks (open circles), cell parameters (filled circles, a = black and b = red) and (c) the relative intensities shown as a function of monolayer composition.

Figure 2c shows a plot of the relative intensities of the same peaks as a function of composition. It can be seen that there is a loss of intensity (and corresponding broadening) of both peaks in the composition range 0.4 ≤ XC9 ≤ 0.8, which clearly suggests some disorder in these mixed structures. From this we infer that the mixing in this system is effectively random and that the peaks arise from the average unit cell. Phase Separation and Partial Mixing, C7/C8: Figure 3a presents X-ray diffraction patterns collected for the binary mixtures of C7/C8 adsorbed on the surface of h-BN powder. Unlike C7/C9, the pure components in this case are found to adopt ACS Paragon Plus Environment

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structures with different symmetry 49. As explained above, C7 monolayers adopt the parallel structure, while C8 show a herringbone packing arrangement. This contrast in preferred packing arrangement has significant implications for the ability of the molecules to mix on the h-BN surface.

Figure 3 (a) X-ray diffraction patterns collected for mixtures of heptane and octane (C7/C8) at compositions of XC8= 0, 0.2, 0.4, 0.6, 0.8 and 1. The arrows indicate the (2,0) (black) and (1,1) (red) reflections for the parallel structure of pure heptane, and the (2,0) (green), (1,1) (magenta), (2,1) (dark blue) and (3,1) (cyan) reflections for the herringbone structure of pure octane. Plots of (b) the position of these peaks and (c) the relative intensities are also shown as a function of monolayer composition.

At low C8 concentration, 0 ≤ XC8 ≤ 0.4, we observe two prominent peaks in the diffraction patterns at approximately q ≈ 0.55 and 1.47Å-1 which are indexed to the (2,0) and (1,1) reflections and associated with the parallel structure. As above we have not shown the (4,0) peak since it is weak and therefore not useful for our analysis. Thus, at low C8 concentration we only observe the parallel molecular ACS Paragon Plus Environment

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arrangement at the surface which must contain both C7 and C8 molecules. However, at XC8= 0.4 additional peaks are also observed in the diffraction pattern, occurring at q ≈ 0.57, 1.28, 1.37 and 1.52. These peaks are associated with a herringbone structure and can be indexed to the (2,0), (1,1), (2,1) and (3,1) reflections, respectively. The coexistence of these peaks clearly suggests that this system displays a degree of phase separation in which separate domains of parallel and herringbone structures are formed on the surface. Finally, for 0.6 ≤ XC8 ≤ 1 the peaks associated with the parallel structure are no longer observed. As above we can again monitor the position and intensity of each of these peaks with composition. In this case we see that the positions of the herringbone peaks are relatively invariant as are their intensities up until the coexistence regime. This suggests that partial mixing occurs but the average unit cell is not modified by the presence of solute molecules (see Figure 3b). Instead the presence of increasing quantities of solute destabilizes the crystal until there is enough to phase separate. Interestingly, the composition of XC8 ≈ 0.4 at which the transition between structure types occurs is weighted in favor of the herringbone structure adopted by the longer C8 molecules. This would suggest that it is easier for the C7 molecules to fit within this structure than vice versa. For 0 ≤ XC8 ≤ 0.2 we again see a slight drift in the position of the peaks, consistent with the behavior seen in the C7/C9 system in which the average unit cell expands in the a-direction to incorporate the longer molecules. In the coexistence regime the position of peaks do not move substantially, which suggests that instead of modifying the structure a change in composition simply changes the relative proportions of the coexisting phases. Partial Mixing and Phase Separation C8/C9: The C8/C9 system is similar to that of C7/C8 in that the pure components adopt structures of differing symmetry. However, here the roles are reversed since it is now the shorter molecule that prefers the herringbone structure. This change has a notable influence on the phase behavior. Figure 4a shows the diffraction patterns collected for the C8/C9 mixtures. In a similar fashion to the two earlier examples, at both ends of the composition range, 0 ≤ XC9 ≤ 0.1 and 0.4 ≤ XC9 ≤ 1, we see changes to the diffraction patterns that are consistent with mixing in which a solute alkane molecule is incorporated into the ACS Paragon Plus Environment

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solvent lattice. As with the C7/C9 system, we can calculate the cell parameters for the parallel structure as a function of composition for 0.4 ≤ XC9 ≤ 1. Again we see that the a-parameter shows linear expansion that is consistent with Vegard’s law, while the b-parameter remains fixed within error at 4.34Å ± 0.01Å (see Figure 4b).

Figure 4 (a) X-ray diffraction patterns collected for mixtures of octane/nonane (C8/C9) at compositions of XC9= 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.6, 0.8 and 1. The coloured arrows indicate the principal diffraction features plotted in (b) and (c). For the pure components these peaks can be indexed to the (2,0) (black) and (1,1) (red) peaks for nonane and the (2,0) (green), (2,1) (dark blue) and (3,1) (cyan) peaks for octane. Also shown are plots of (b) the positions (open circles), cell parameters (filled circles, a = black and b = red) and (c) the relative intensities of these peaks as a function of monolayer composition. The lines shown are a guide to the eye.

For the mixtures 0.15 ≤ XC9 ≤ 0.35 the diffraction patterns show a coexistence of peaks associated with both the herringbone and parallel structures and again the cell parameters do not change substantially over this range. As above, this indicates phase separation in this composition range. We ACS Paragon Plus Environment

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can see further evidence in the relative intensities of the peaks, which vary approximately in proportion to the relative quantities of the two structure types (see Figure 4c). The key difference for the C8/C9 system is the extent over which mixing occurs. The transition between structure types is further towards the shorter molecule; XC9 ≈ 0.25 for the C8/C9 system compared to XC8 ≈ 0.4 for the C7/C8 system. This is an indication that when the parallel molecular arrangement is preferred by the longer molecule it is more accommodating for shorter solute molecules and so this structure persists further towards the short molecule side of the phase diagram. Mixing and Phase separation C9/C10: Decane (C10) represents a special case for alkanes adsorbed on h-BN since decane can adopt both parallel and herringbone structures, depending on coverage / temperature. This suggests that the driving force selecting one structure over the other is relatively weak. We expect this to have a direct effect on the structure and miscibility observed for this system. Figure 5a shows the diffraction patterns measured for C9/C10 mixtures. For mixtures of 0 ≤ XC10 ≤ 0.4, we only observe the reflections associated with the parallel structure. As for the systems above, we find that these peaks move approximately in accordance with Vegard’s law (Figure 5b). Again we can extract the cell parameters, which show a constant value of b and a linear expansion of a. This behavior is consistent with mixing, with both molecules able to adopt the parallel molecular arrangement. This is notably different to the C7/C8 system (i.e. the most equivalent system for comparison) in which phase separation occurs by XC8 = 0.2.


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Figure 5 (a) X-ray diffraction patterns collected for mixtures of nonane/decane (C9/C10) at compositions of XC10= 0, 0.1, 0.25, 0.3, 0.4, 0.5, 0.6, 0.8, 0.9 and 1. The colored arrows indicate the principal diffraction features plotted in (b) and (c). For the pure components these peaks can be indexed to the (2,0) (black) and (1,1) (red) peaks for nonane or decane and the (2,0) (green), (2,1) (dark blue) and (3,1) (cyan) peaks for decane. Also shown are plots of (b) the positions (open circles), cell parameters (filled circles, a = black and b = red) and (c) the relative intensities of these peaks as a function of monolayer composition. The lines shown are a guide to the eye.

As the ratio of C9/C10 reaches 1:1 we start to observe peaks associated with the herringbone structure in addition to the peaks from the parallel structure. Thus, for 0.5 ≤ XC10 ≤ 0.9 there is a co-existence of the parallel and herringbone structures. We cannot attribute this coexistence entirely to phase separation, however, because pure decane also shows both structures in this coverage regime. In fact, although the herringbone peaks remain in roughly fixed positions, we continue to see the expansion of the parallel unit cell as the proportion of decane increases. This behavior suggests that the parallel structure contains ACS Paragon Plus Environment

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both C9 and C10 over the entire composition range. In contrast, the herringbone structure either contains only C10, or a mixture in which any C9 solute molecules present do not modify the structure. This interpretation is supported by the increase in the relative intensity of the herringbone peaks and corresponding reduction of the intensity of parallel peak as the concentration of C10 increases up to XC10 ≈ 0.9, i.e. as the proportion of C10 increases the proportion of molecules in the parallel arrangement decreases (Figure 5c). Interestingly, for 0.9 ≤ XC10 ≤ 1, we see slightly different behavior. The expansion of the parallel unit cell stops while the relative intensity of the peaks increases as the proportion of C9 decreases. Meanwhile the herringbone peaks show the opposite behavior. Unfortunately we cannot be sure why this is because the intensities are closely related to the overall order within the crystal (which depends on the number of impurities, i.e. solute C9 molecules) as well as the coverage and proportion of the two structures. It does seem, however, that in this coverage regime the solute molecules are being incorporated into the two C10 structure types without significantly affecting them. As with our study of pure decane on h-BN, we must also qualify this conclusion, since it is likely that the coexistence of the two decane phases has a kinetic contribution. Although we have taken considerable care to ensure that these systems are frozen slowly, this may not have been slow enough to ensure that the lowest energy structure is adopted throughout the sample. For such a subtle driving force the required equilibration time to achieve this low energy structure may be extremely long and beyond a practical timescale for this study. Formation of a Solid Solution C8/C10: The diffraction patterns collected for C8/C10 mixtures are shown in Figure 6a. At low C10 concentration, 0 ≤ XC10 ≤ 0.25, we observe only the peaks associated with the herringbone structure of pure C8. At the other extreme 0.55 ≤ XC10 ≤ 0.8 we see only peaks that are associated with the herringbone structure of pure C10. The positions of these peaks are approximately fixed over their respective composition ranges (Figure 6b), so we conclude that in each case the solute molecules are incorporated into the solvent structure without significantly altering it. It is notable that the C10


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herringbone structure is stable over a wider composition range than the C8, presumably because it is easier for this structure to incorporate shorter molecules than the other way around.

Figure 6 (a) X-ray diffraction patterns collected for mixtures of octane/decane (C8/C10) at compositions of XC10= 0, 0.12, 0.25, 0.3, 0.4, 0.45, 0.55, 0.7, 0.8, 0.9 and 1 The colored arrows indicate the principal diffraction features plotted in (b) and (c). For the pure components these peaks can be indexed to the (2,0) (black) and (1,1) (red), (2,1) (green) and (3,1) (dark blue) peaks for the herringbone structure of either pure octane or decane and the (2,0) (cyan) and (1, 1) (pink) peaks for the parallel structure of pure decane. The purple arrow indicates the (2,0) peak for a proposed 1:1 solid solution. Also shown are plots of (b) the positions and (c) the relative intensities of these peaks as a function of monolayer composition. The lines shown are a guide to the eye. Interestingly, for 0.9 ≤ XC10 ≤ 1, we see the peaks associated with a coexistence with the pure C10 parallel structure. The fact that these peaks are not present for XC10 ≤ 0.8, does suggest that the C8 molecules destabilize this parallel structure relative to the herringbone structure. For 0.35 ≤ XC10 ≤ 0.55 we cannot see many well-defined diffraction peaks, indicating significant disorder in this regime. ACS Paragon Plus Environment

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Crucially, it is possible to resolve a small peak at around 0.54Å-1, shown with a purple arrow in figure 6a, which is distinct from and around half way between the (2,0) peaks of either pure herringbone structure. By comparison to similar behavior seen for the C8/C10 system on graphite, we assign this peak to the (2, 0) peak from a composite structure of a 1:1 solid solution. Unfortunately we cannot unambiguously make this assignment due to the absence of well-defined peaks at higher momentum transfer. It is not clear why the ordering within such a solid solution is so poor, but as mentioned above it may be related to the cooling rate used in these measurements. The equilibration time required to facilitate ordering in this a mixed crystal must be relatively long by the standards of normal first-order phase transitions and is similar to the same system on graphite (see below). A comparison with Graphite The structure and phase behavior of some of the binary alkane mixtures presented in this work may be readily compared with results for the same mixtures adsorbed on graphite (C7/C9, C8/C9, C9/C10 and C8/C10)40, 42, 58. The phase behavior on graphite for the mixtures not involving C10 is qualitatively very similar (see Figure 7). This is particularly evident for the C8/C9 system 42, 58 (c.f. Figures 4, 7 and 8) for which we see very similar behavior (allowing for the shift in cell parameters due to the expansion of the unit cells between h-BN and graphite). In both cases for approximately 0.2 ≤ XC9 ≤ 0.4, we see a coexistence of a mixed crystal with the parallel molecular arrangement and the C8 herringbone structure. Above XC9 ≈ 0.4 the parallel structure is exclusively observed and while the b cell parameter remains fixed, the a cell parameter increases with increasing C9 composition, in line with Vegard’s law. The only real difference between the two substrates (other than the absolute values of the cell parameters) is that the parallel structure perhaps persist to lower C9 compositions on graphite.


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Figure 7 X-ray diffraction data for C8/C9, C9/C10 and C8/C10 on graphite (reproduced from40, 58).

In contrast, the systems involving C10 show some interesting and more significant differences between these two substrates. For the C9/C10 system we see broadly similar behavior for XC10 < 0.5 (see Figure 8). On both substrates at very low C10 composition the structure of C9 is initially unaffected by the presence of the longer molecule. As the C10 content increases, 0.2 ≤ XC10 ≤ 0.4, we see a variation of the a cell parameter that indicates mixing within a parallel structure, again, in line with ACS Paragon Plus Environment

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Vegard’s law. However at compositions XC10 ≥ 0.4 we see a difference between the two substrates. On graphite the a cell parameter from the parallel structure ceases to change with composition while peaks associated with the herringbone structure appear and become more prominent as the parallel structure disappears at XC10 ≈ 0.8. However, on h-BN the mixed parallel structure continues to follow Vegard’s law up until XC10 ≈ 0.6-0.8, even while coexistence with a herringbone structure occurs for XC10 ≥ 0.5. Importantly, and in a marked difference to the behavior on graphite, the parallel structure persists over the entire composition range for h-BN (c.f. Figures 3, 5, 7 and 8). Thus, unlike on graphite, the ability of n-decane to adopt both structure types on h-BN directly improves the miscibility of these alkanes.

Figure 8. A comparison of the cell parameters (a, filled circles and b, open circles) for the parallel structures in each of the systems (a) C7/C9, (b) C8/C9 and (c) C9/C10 on graphite (black) and h-BN (blue). The lines are a guide to the eye. Note that the b-parameter is significantly larger for graphite because unlike on h-BN it is not commensurate with the substrate at this sub-monolayer coverage. For comparison the theoretical commensurate distances for parallel structures are shown as green (graphite) or red (h-BN) crosses for each of the pure components. For the C8/C10 system (Figures 6 and 7), because C8 prefers the herringbone structure, there is no particular advantage for the decane to adopt the parallel structure. Thus the phase behavior on both substrates is again broadly similar, with a molecular compound occurring for 1:1 mixtures. Only at high C10 concentration (XC10 ≥ 0.9) does the parallel structure appear on h-BN (and not at all on graphite) Discussion and Conclusion


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X-ray diffraction was used to investigate the structure of adsorbed films of mixed n-alkanes on h-BN at submonolayer coverage. Comparison of these measurements with the same mixtures on the graphite basal plane suggests that similar phase behavior takes place on both materials. For example, on both substrates we see phase separation between the odd and even chain length alkanes (see Figure 9a) driven by their preference to adopt either parallel or herringbone structure types. Similarly, we also observe complete mixing for odd-odd mixtures on both substrates (see Figure 9b).

Figure 9. A schematic representation of the observed phase behavior discussed in the text: (a) Phase separation in heptane-octane is generally observed for short-chain odd-even mixtures. (b) Complete mixing in heptane-nonane is seen for odd-odd mixtures. (c) Partial mixing of shorter odd molecules into the herringbone lattice of a longer even alkane is possible and illustrated for nonane in decane. Less likely is the converse of a longer odd molecule in a shorter even lattice. For decane on h-BN, however, the decane molecules can easily mix into the parallel structure shown in (d), in the same way as for two odd alkanes (b). For even-even mixtures phase separation is observed, the pure structures coexisting with a solid solution (e & f). The similarities observed between h-BN and graphite substrates suggest that intermolecular interactions are the principal driving force behind the observed phase behavior. On the other hand, the data presented here indicates that on h-BN, n-decane (C10) can adopt both parallel and herringbone adsorbed (solid) structures leading to enhanced miscibility of binary mixtures involving C10. Thus, C10 is found to have significantly better miscibility with C9 molecules when adsorbed on h-BN than on graphite (see Figures 8, 9b and 9e). This behavior is noteworthy because the (solid) structures formed by ACS Paragon Plus Environment

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C10 on graphite at sub-monolayer coverage are uniaxially incommensurate while on h-BN they are in registry with the surface (commensurate). This is illustrated in Figure 8, where the theoretical commensurate unit cell parameters are indicated and can be compared with the experimental peak positions. Thus the influence of the substrate on this behavior is somewhat counter-intuitive. We might expect that the expanded incommensurate system (graphite) would show less of a constraint on the miscibility of the alkanes, but actually this system seems to be dominated by the intermolecular forces that favor the odd-even effect. On h-BN this intermolecular effect is partially offset by the energy gain from being commensurate with the substrate which means that the driving force away from the parallel structure is weaker. The effect is certainly a subtle one because when we consider the even-even mixture of octane and decane (see Figures 9c and 9f), the miscibility of the preferred herringbone structures is again similar to that observed on graphite. The evidence presented above indicates that the mixing occurs more favorably for the parallel structure, wherein Vegard’s law is generally obeyed. Deviations from Vegard’s law due to the compressibility that are seen for longer molecules on graphite10 are not apparent here. To fully understand this behavior will require further experiments to understand how the coverage modifies the phase behavior and computational studies to help explain the qualitative description outlined above. There have been a number of simulation studies of alkane interactions with graphite in recent years5,

59-68

, but relatively few of these have considered such subtle substrate effects as those

observed experimentally in this work. We hope that the results presented herein will stimulate work in this area and allow for a more complete understanding of the behavior observed. Acknowledgements. The authors would like to thank Diamond Light Source for beamtime on beamline I11 (experiment number EE8994) and for financial support.


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The Mixing Behavior of Alkanes Adsorbed on Hexagonal Boron

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