Investigation of the Adsorption of Alkanes on Hexagonal Boron Nitride

Apr 23, 2012 - Thomas Arnold,* Julia E. Parker, and Phi Macdonald. Diamond Light Source, Diamond House, Harwell Science & Innovation Campus, Chilton, ...
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Investigation of the Adsorption of Alkanes on Hexagonal Boron Nitride from Their Liquids and Binary Mixtures Thomas Arnold,* Julia E. Parker, and Phi Macdonald Diamond Light Source, Diamond House, Harwell Science & Innovation Campus, Chilton, Didcot, OX11 0DE, U.K.

ABSTRACT: We report a differential scanning calorimetry investigation of the adsorption of normal alkanes on the surface of hexagonal boron nitride. The alkanes investigated (pentane−hexadecane) all show small transitions above the bulk melting point. On the basis of similar results on graphite, we assign these transitions as the melting of solid monolayers adsorbed at the interface between the boron nitride and bulk-like fluid layers. However, in this case, the induced stabilization of the monolayer is significantly less than that for the same alkanes on graphite. The alkane monolayers melt approximately 5−8% above the bulk melting point (compared with about 10−14% on graphite). In addition, we have performed an extensive investigation of the behavior of this transition for binary mixtures of alkanes. These show similar behavior to that seen on graphite, which suggests that the rules developed for those systems are also applicable for boron nitride. A regular solution model has been used to determine parameters related to the excess enthalpy of mixing and preferential adsorption. These suggest that, although the preferential adsorption is almost exactly the same on both h-BN and graphite, there is a small, but significant, difference in the miscibility of alkanes when adsorbed on the two substrates. We suggest that there may be a structural reason for this behavior that is related to the small change in the substrate cell parameters and surface potential.

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INTRODUCTION The interaction of surfaces with physically adsorbed molecules is both of theoretical interest and relevant to a wide range of technological and industrial applications. Since the early 1970s, a very wide range of experimental techniques have been developed to study the structure and dynamics of monolayer and multilayer molecular films adsorbed on well-defined solid surfaces. Progress in this area of research over the past decade was recently reviewed.1,2 In this study, we use differential scanning calorimetry measurements to study a series of normal alkanes adsorbed on the surface of hexagonal boron nitride (hBN) powder. We are interested in how the chemical composition and the symmetry of a substrate affects the structures of adsorbed solid monolayers, and, consequently, the physical properties of the film, such as film growth, wetting, and melting (subjects of great theoretical interest). Despite the considerable differences in symmetry and chemical composition between the surfaces of graphite and magnesium oxide, short-chain alkane monolayers exhibit remarkably similar commensurate structures on both surfaces. For example, the structure of n-butane on both of these surfaces has a “herringbone” symmetry.3,4 This similarity might imply © 2012 American Chemical Society

that intermolecular interactions dominate over the molecule− substrate interaction. However, in both cases, the structure is commensurate despite the notable differences in the hexagonal versus square surface symmetry. This seems to lead to a small modification of the structure and illustrates the important role played by the molecule−substrate interaction. Unfortunately, the difference in these substrates is so large that it is difficult to identify the relative influences of surface symmetry and polarity without additional information. In that respect, the hexagonal form of boron nitride offers a particularly good opportunity for comparison since it can be obtained as high-quality h-BN powders that are suitable for thermodynamic measurements5 and has the same structure as graphite, but for small differences in the cell parameters. There have been relatively few studies of physisorption on hBN and, to our knowledge, none of the n-alkanes. There have been a number of thermodynamic and structural studies of simple species, such as noble gases,6−10 nitrogen,11,12and Received: January 19, 2012 Revised: April 18, 2012 Published: April 23, 2012 10599

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methyl-halides,13 and these seem to show similar structures and phase behavior to that seen for the same species on graphite. For example, reentrant layering and layer-by-layer melting phenomena are observed for argon films absorbed on both hBN and graphite.6,7,14,15 However, these studies do suggest that the corrugation of the surface potential of h-BN is weaker than that for graphite. For example, this is apparent in the fact that the tricritcal temperature of N2 on h-BN is some 20 K below the equivalent temperature on graphite.11 Despite this, these layers do still seem to melt above the bulk melting points when the monolayer is in the presence of bulk-like liquid layers. Like many other adsorbates on graphite,1,2,16,17 the normal alkanes (hexane and longer) show a small transition (about 10−14%) above the bulk melting point.18−22 This transition has been assigned to the melting of a single monolayer that is stabilized on the surface at layer completion and higher coverage. Diffraction measurements,23,24 recently confirmed by STM,25 show a distinct odd even effect in the structures. The odd alkanes (heptane, nonane, etc.) adopt a rectangularcentered structure in which the molecules align parallel to each other. However, the even alkanes (hexane, octane, etc.) form a herringbone structure. This difference in structures persists up to tetradecane, and thereafter, all the alkanes appear to show centered (parallel) structures in STM images. Interestingly, dodecane is seen to undergo a monolayer phase transition prior to melting that may be related to a transition between a herringbone and a parallel structure. Solid−solid transitions similar to the equivalent bulk rotator phases are also seen for the longer alkanes.20,21,26 Further, studies of alkane adsorption from binary mixtures have also been undertaken on graphite.18,19,27,28 These found that the mixing and/or phase separation within coadsorbed alkane monolayers was dependent on the symmetry of the structures adopted by the pure components and the relative difference in chain lengths. A quantitative parameter, the 2D coefficient of isomorphism, a derivative of the 3D equivalent,29,30 has been used to understand this mixing behavior within 2D monolayers.18,31

The result of this is that the composition on the surface is not generally the same as that on the bulk mixture. An equilibrium exists, which is defined by the following equilibrium constant, K34 K

AAdsorbed + BBulk ↔ ABulk + B Adsorbed K = XABulk XBAds /XAAdsXBBulk

where XAds A is the mole fraction of molecule A that is adsorbed and XBulk A is the mole fraction of molecule A in the bulk. Thus, if the surface has a preference for the adsorption of B over A, then the value of K will be greater than one. It is possible to absorb both components of binary mixtures of alkanes and other simple hydrocarbons. This occurs when the difference in molecular length of the components is small. Previous studies of binary mixtures adsorbed on graphite have shown that it is possible for the adsorbed two-dimensional monolayer to show either mixing or phase separation depending on the properties of the molecules in question. In this paper, we have compared our data to a simple model that uses regular solution theory and accounts for preferential adsorption. This specific analysis has been published in detail elsewhere.31,35 In short, the observed depression of freezing point is modeled using the regular solution theory36−38 and the preferential adsorption is manifested as a shift in the monolayer phase diagram parallel to the composition axis. The regular solution model has an interaction parameter, Ω, that is related to the excess enthalpy of mixing:36−38 ΔHex = ΩXA XB

In simple terms, a value of Ω of zero would recover the ideal mixing case and increasing values of Ω indicate increasingly nonideal mixing in the solid monolayers. Complete phase separation is seen when Ω ≥ 2RT, and as such, the ratio Ω/ 2RT gives a good relative account of the extent of mixing. In the models used here, the surface free energy of the solid monolayers is taken to be independent of the surface composition,39 and it is assumed that there is ideal mixing in the adsorbed liquid phase.

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PREFERENTIAL ADSORPTION AND REGULAR SOLUTION THEORY Here, we outline the thermodynamics of the simple models used to interpret our data. The theory is now well-established and does not require a detailed duplication here. Instead, we will give a brief outline of the concepts used and refer the reader to the literature for further details. A surface in the presence of a binary mixture may preferentially adsorb one of the components over the other. This effect has long been studied in detail for hydrocarbons on graphite,32,33 and in general, the longer molecule is adsorbed in preference to the shorter one. This has been explained on both energetic and entropic grounds. First, it was suggested that the interaction of the graphite surface with methylene groups within the alkane chain is stronger than that with methyl groups at the ends of the chains. Thus, it is energetically favorable for longer molecules to be adsorbed as this increases the relative proportion of methylene groups adsorbed. Second, it is entropically favorable to have more molecules in the bulk phase rather than the more ordered adsorbed phase. Thus, since a given surface area can be filled by less long molecules than short ones, the longer molecules are favored for adsorption.

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EXPERIMENTAL SECTION Differential Scanning Calorimetry. The procedure used in this work has been described elsewhere.40 The device used in these measurements was a PerkinElmer Diamond DSC. This device can measure the tiny energy changes in a sample as the sample is heated, at a fixed rate, relative to a reference. These tiny changes in heat can be used to identify transitions occurring in an adsorbed monolayer (see Figure 1). The bulk melting can be identified by large peaks in the DSC thermograms, which have an area that varies proportionally to the quantity of bulk material present in the sample. Additional small peaks corresponding to the melting of a solid monolayer occur at higher temperatures, do not scale with the amount of adsorbate material, and are not observed in the absence of the substrate. In some cases, there are several such peaks arising from the adsorbed material. The peak at the highest temperature can usually be assigned to the melting of a crystalline monolayer. This assignment has been confirmed for many systems using diffraction approaches,22−24,26,41 incoherent neutron scattering,19,42−44 and NMR.45−47 The other small peaks arising from the adsorbed material are more difficult to 10600

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alkane, we use the notation Cn, where n is the number of carbon atoms in a molecule CnH(2n+2). The four different grades of h-BN (HCM, HCP, HCPH, and HCPL) were obtained from Momentive Performance Materials Inc. These powders are similar to those used in an earlier volumetric isotherm study.5 Scanning electron microscopy images of the different powders as received were taken using an Hitachi TM1000 SEM and match reasonably well with those seen in the isotherm study and the suppliers' literature. The specific surface areas of the powders were quoted by the supplier and found to be reasonably consistent with test argon volumetric adsorption isotherms at ∼77K, using the “point-B” method.51 To clean the surface prior to the use of the h-BN powders, they were heated under vacuum (1 × 10−8 mbar) to 450−750 °C for between 12 and 48 h. After treatment, all powders were stored in sealed bottles but handled in air during sample preparation. There is evidence that this simple cleaning process is not sufficient to remove boric acid from the surface, and other approaches to cleaning, including washing in methanol or reaction with ammonia gas prior to heat treatment, have been used in the literature.5,52 These cleaning methods do have a noticeable effect on the quality of multilayer volumetric isotherms, but in general, we found that the details of the cleaning procedure had no noticeable effect on the quality of thermograms obtained from the DSC. This was specifically checked against some powder that had also been treated with ammonia (supplied by J. Z. Larese), though subsequently handled in air. For the bulk of the measurements presented here, we chose the HCM grade h-BN, since this gave some of the best results in the work of Wolfson et al.5 There were some small changes in the measured melting points of the monolayers when calibrated against the bulk melting points, which varied reproducibly, but unsystematically, by ±2 K. This may be due to effects of porosity of the samples, but for the sake of this study, we take this as our minimum error in the melting points. The melting enthalpies approximately scale with surface area of the different h-BN grades, but, on the whole, have a large error. This is, in part, due to a lack of sensitivity that results from the low surface area of the powder, but also because the powders are relatively “fluffy” and so were not very densely packed into the DSC sample pans.

Figure 1. DSC thermograms of (a) n-heptane and (b) n-dodecane on h-BN. The bulk melting is a large peak, but in both cases, the inset shows a close-up of the very small peak that corresponds to the monolayer melting. In particular, note that n-dodecane shows a second transition in the monolayer prior to the melting.

assign unambiguously without additional information. The transitions may be solid−solid phase transitions within the monolayer,20,21,24,42,48,49 or the freezing/melting of second and subsequent layers.50 In most cases, a heating scan rate of 10 K/min was used as a compromise between sensitivity and thermal lag. At faster scan rates, the peaks become larger and broader. If too fast, then the broad bulk peak (which is many times larger than the monolayer) may become so broad that it masks any small peaks close to it. Slower scan rates mean the peaks becomes smaller, which, in the case of the monolayer peaks, might mean that they are difficult to observe at all. The rate of 10 K/min is derived from experience but is fairly subjective and not necessarily the optimum rate. An indium calibration was taken prior to a series of runs, but in most cases, this melting point is a long way from the temperatures under investigation. However, since the samples are all measured in an excess of bulk solution, the bulk melting points have been used as an internal reference. Thus, for many of the binary mixtures, comparison of these transitions with previous data for the bulk phase diagram allows for a sample-by-sample calibration of the temperature, which is independent of the particular instrument calibration. Errors in the temperature are then minimal but are still of the order of 1−2 K. Samples. All the alkanes used in this study were purchased from Sigma-Aldrich (≥99.8%) and used without further purification. In the rest of this article, when referring to an

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RESULTS AND DISCUSSION Pure Alkanes. Figure 1a shows a DSC thermogram from heptane in the presence of h-BN (HCM grade). The amount of heptane corresponds to many times more than that required for one “monolayer”. (We estimate the amount of alkane taken to form a “complete” monolayer based on the area per molecule of 47 Å2, equivalent to the diffraction-determined area per molecule for heptane on graphite.)23 In addition to the very large peak from the “bulk” melting at 183 K, we can also see a very small peak at 196 K (see inset) that we assign to the melting transition of a monolayer on the surface of the powder. We assume that this transition, which is not present in the absence of h-BN powder, is exactly analogous to similar peaks seen on graphite. Similar peaks have been measured for each of the alkanes, hexane−hexadecane, and their transition temperatures are plotted in Figure 2. For comparison, the transition temperatures of the bulk melting and of the monolayer melting on graphite are also plotted. In general, on h-BN, the monolayer seemingly melts at a temperature approximately 10601

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mechanism on both surfaces is very similar, despite the reduced overall stabilization of the monolayer on h-BN. The measured enthalpy of melting is a bit more ambiguous than for graphite since the smaller surface area imposes a greater error in the measurement. This error means that a clear trend with chain length is not observable, though the enthalpy of melting is approximately proportional to the surface area (or mass) of the substrate. On average, the monolayer melting enthalpies observed for the alkanes on the HCM powder were approximately 0.015 J/g of powder with an error of ±20−30%. This error is very large but is not dissimilar to the error for the same alkanes on graphite, which was about ±10−20%. With such a large error, it is not surprising that any subtle trends are lost. To compare with graphite, we have converted to kJ mol−1, using the surface area of 1.65 m2 g−1 and the area per molecule known for these alkanes on graphite (which may be incorrect but are likely to be similar, given close packing considerations). The values are of the same order of magnitude at approximately 5 kJ mol−1 and, within the error of these measurements, are all about the same value (see Table 1). The resolution of these DSC measurements is, therefore, not able to detect any small substrate influence on the melting enthalpy. Binary Alkane Mixtures. The phase diagram of C11/C12 is shown in Figure 3, including the monolayer melting transitions for both graphite and h-BN. It is clear from this phase diagram that the variation of melting temperature of the monolayer with composition is very similar. In addition to this, we have measured a series of phase diagrams, Cn/C(n+1) for 6 ≤ n ≤ 11, Cn/C(n+2) for 6 ≤ n ≤ 9, C7/C10, and C7/C11. In each case, apart from a temperature offset, the composition variation of the monolayer melting temperature is similar to the equivalent variation on graphite. However, as we will see below, there are some small differences that may have an important consequence. We have attempted to fit the phase diagram data using the regular solution/preferential adsorption model outlined earlier. Figure 4 compares such a fit for the C11/C12 system with alternative calculations for ideal mixing and complete phase separation. It is clear from this figure that the regular solution model fits the experimental data (the same data as shown in Figure 3) reasonably well, whereas the alternatives are clearly incorrect. Figure 5 summarizes the monolayer phase diagrams

Figure 2. Variation of melting points for alkane monolayers adsorbed on h-BN and graphite compared to the bulk melting points.64

5% above the bulk melting point (for even chain length molecules) or 8% (for odd chain length molecules), compared with approximately 10% above the bulk (even) or 14% (odd) for alkanes on graphite18−21 (see Table 1). It is clear that the stabilization of the monolayer on h-BN is significantly less than that seen on graphite. On MgO, there is no such stabilization of the monolayer,53 and as such, there is an obvious substrate dependency to this behavior. Figure 1b shows a DSC thermogram of dodecane on h-BN. This clearly shows an additional small peak that is reminiscent of the solid−solid phase transition seen for dodecane on graphite.24 The consistency of this transition on the two surfaces is interesting. Dodecane does not show a rotator phase prior to melting in the bulk, and so this transition is an exception to other similar solid−solid transitions seen for longer alkanes on graphite, which can be correlated to their rotator phases.21 On graphite, this transition was assigned to a transition between a herringbone and a parallel structure,24 and its similarity on h-BN is perhaps an indication that the melting

Table 1. Melting Points and Enthalpies for Monolayer of Alkanes on h-BN reference bulk (average values from ref 64)

n-alkanes on h-BN 6 7 8 9 10 11 12 13 14 15 16

T2D m (K) (±2K)

−1 ΔH2D m (kJ mol ) (±30%)

184 196.2 227.4 239.7 256.4 267.6 278.5 289.3 298.0 307.1 314.7

1.5 3.2 4.9 4.3 6.3 6.6 5.4 5.5 5.0 2.1 4.5

T3D m (K) 178 182.6 216.3 219.5 243.3 247.4 263.5 268 278.7 283.0 291

± ± ± ± ± ± ± ± ± ± ±

1 0.4 0.3 0.5 0.6 0.8 0.3 1. 0.9 0.1 1

a ΔH3D m

(kJ mol−1)

12.8 14.05 20.5 21.9 28.73 28.8 36.75 36.15 45.07 43.77 51.5

± ± ± ± ± ± ± ± ± ± ±

0.4 0.05 0.4 0.3 0.05 0.3 0.15 0.01 0 0.01 2.4

ratio

n-alkanes on graphite20,40

ratio

3D T2D m /Tm

T2D m (K)

−1 ΔH2D m (kJ mol ) (±20%)

3D T2D m /Tm

1.02 1.07 1.05 1.09 1.05 1.08 1.06 1.08 1.07 1.09 1.08

187.8 206.8 238.9 251.3 268.3 280.7 291.1 303.4 317.4 322.6 333.0

5.5 4.1 6. 1 4.6 4.3 5.1 4.0 4.5 3.9 4.4

1.06 1.13 1.10 1.14 1.10 1.13 1.10 1.13 1.14 1.14 1.14

a

The bulk melting enthalpies quoted here are sums of the data of both melting and rotator phase transitions. Where more than one measurement is reported, the error represents the standard deviation of the reported values. 10602

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Figure 3. Phase diagram of the C11/C12 mixture, including transitions for both graphite and h-BN. For clarity, we have not included the monolayer solid−solid transition seen for the monolayer of dodecane. The lines drawn here are a guide to the eye. As far as we are aware, the detailed phase diagram of the bulk undecane/dodecane mixture has not been published. We have not made any assignments of this phase diagram, and the lines merely connect what may be equivalent transitions.

Figure 5. Monolayer phase diagrams for alkane mixtures (a) Cn/Cn+1 (including the data from Figures 3 and 4) and (b) Cn/Cn+2. The lines drawn are the regular solution fits for the liquidus of each system. The parameters for these fits are plotted in Figure 6.

equivalent data for the alkanes,35,54 carboxylic acids,55 and perfluoroalkanes35 on graphite. These fits are calculated from the melting temperatures and enthalpies of the pure components, and as such, the considerable error in the measured enthalpies can have a substantial influence. However, the dominating factor is the composition and depression of freezing point of the minimum in the phase diagram. Thus, we have varied the enthalpies substantially (and even used the graphite values) to determine an estimate of the error in the fitted values of K and Ω. Qualitatively, the fits for different enthalpies give an insignificant change in the parameters. For example, for the C11/C12 system, Ω/RT = 0.215 using the h-BN pure monolayer melting enthalpies, and Ω/RT = 0.158 using the graphite monolayer enthalpies. In both of these cases, the values indicate significant (nonideal) mixing. We can now compare the results with data from other cases. In each case, the variation in K with the ratio of chain lengths (difference in chain length/average chain length) is remarkably consistent (see Figure 6a). However, Figure 6b shows that the variation of Ω does seem to show a difference between the two substrates. For the alkanes on graphite, complete phase separation occurs when the chain length ratio reaches about

Figure 4. Comparison of possible fits of the experimental data obtained for the undecane/dodecane mixture. The regular solution model, as described in the text, is plotted (dark green line) and fits quite well with the measured data (dark green points). This model accounts for preferential adsorption and the excess enthalpy of mixing. For comparison, the case of ideal mixing (red) and complete phase separation (blue) are also plotted. These cases both account for preferential adsorption weighted toward the dodecane. For the example of complete phase separation, we have also plotted the calculation without accounting for the preferential adsorption (orange dotted line). The difference between the two (indicated by the black arrow) is a shift of the position of the calculated minimum relative to the composition axis.

for the n+1 and n+2 binary mixtures, and the parameters obtained from each fit are plotted in Figure 6, together with 10603

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incommensurate submonolayer coverage to fully commensurate multilayer coverage.23,24 It seems likely that there is some balance between the requirement for remaining commensurate and for maximizing van der Waals interactions by minimizing intermolecular separation. This could, perhaps, explain the observed increase in miscibility on h-BN. In the case of fluoroalkanes on graphite, the improved mixing was explained because low intramolecular forces and weak interactions with graphite mean that the monolayer crystal structures are more expansive (as shown by thermal expansion data from diffraction results35,56,57). This means that the structures can more easily incorporate longer chains. Analogously, a weaker interaction of alkanes with h-BN would mean that expansion of the crystal structures comes at a lower cost than on graphite. This, in turn, would mean a lower energy penalty in accepting longer alkane chains into the structure. As mentioned earlier, previous studies of adsorption on h-BN have suggested that the interaction potential is weaker than graphite.7,10,11 The Hamaker constant is a convenient way of quantifying the magnitude of the van der Waals forces involved.58 The Hamaker constant has been calculated for graphite59 (A = 23.8 × 10−20 J) and for MgO60 (A = 12.1 × 10−20 J) but, to our knowledge, has not been reported for h-BN. We have estimated this as A = 10 × 10−20 J (using the equation for the Hamaker constant,58 a function of the dielectric constant and refractive index61). This value is significantly less than that for graphite and similar to that of MgO. These values can be used to calculate the relative strength of the interaction between an alkane and the different surfaces. A for dodecane62 is 5 × 10−20 J; therefore, A(Gr−C12) = 1.1 × 10−19 J, A(BN−C12) = 7.07 × 10−20 J, and A(MgO−C12) = 7.78 × 10−20 J. These simple considerations do imply that alkanes have a weaker van der Waals interaction with h-BN than with graphite and, as such, support the reasoning used above to justify the increased miscibility on h-BN. However, this does not tell the whole story. As mentioned earlier, the alkanes on MgO do not show any stabilization of the monolayer solid above the bulk melting point,53 yet its Hamaker constant is of a similar magnitude to that for h-BN. Of course, the other factor in the stabilization of the monolayer solids is the presence of bulk-like liquid layers that effectively compress the film. The “effective pressure” exerted by these liquid-like layers act to decrease the mean square displacement of the molecules in the monolayer solid and, therefore, raise the melting temperature. In order for the monolayer melting points to vary between substrates, such a driving force must be inhibited by the interaction with the surface potential/corrugation of the different substrates. We know that the submonolayer structure of butane on MgO3 is fully commensurate, and as such, any compression at higher coverage may come at a significant energetic cost. For graphite, since there is a good match between the lattice spacing and the carbon−carbon distances in the alkane, the film can be compressed (uniaxially) to be fully commensurate.23,24 For hBN, some compression may be possible, but compression to the ideal near-neighbor distance seen on graphite would mean that the film is no longer commensurate with the larger h-BN lattice. The small energetic cost due to a loss of commensurateness may result in a lower melting point. It is worth noting that it has previously been suggested that compression of a film to commensurateness is harder than expansion to commensurateness because of the asymmetric

Figure 6. Regular solution fit parameters (a) Ω and (b) K, plotted as a function of the ratio of chain lengths for alkanes on h-BN (blue) and alkanes on graphite (red), and comparing these to similar data for acids on graphite (green) and perfluoroalkanes on graphite (black). Within the errors of these measurements, there is effectively no difference in the values of K, but there is a significant change visible in Ω.

0.3. However, on h-BN, mixing is seen to persist for longer, with complete phase separation occurring at a ratio of about 0.5. By comparison to two extremes (the carboxylic acids that phase separate very quickly, and the perfluoroalkanes that phase separate very slowly), we can see that the difference in slope between alkanes on the two substrates is a relatively significant one. The reason for the difference in the miscibility on the two substrates is related to small differences in the intermolecular and molecule−surface interactions. These may well result from the change in the cell parameters, which could mean that the monolayer structures are no longer commensurate with the substrate. The h-BN cell parameters are about 2% larger than those of graphite. For a monolayer to stay commensurate with this surface, then the area per molecule must expand by about 4%, which undoubtedly would come at some energetic cost. For comparison, we know that, on graphite, we see a compression of about 10% (uniaxially) on going from the 10604

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shape of the adsorption potential.11,63 (i.e., it is easier for a molecule that is smaller than a substrate lattice to expand to be commensurate than it is for a molecule that is larger than the lattice to compress to be commensurate.)

Performance Materials Inc. for supplying samples of their BN powders.

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CONCLUSIONS This work has suggested the formation of solid monolayers of n-alkanes that persist above the bulk melting point adsorbed on hexagonal boron nitride powders. Close comparison to equivalent monolayers on this isoelectronic substrate supports this assignment in the absence of structural data. These monolayers melt around 5−8% higher than the bulk melting point, significantly lower than the equivalent monolayers physisorbed on graphite. The similarity of the behavior of the binary mixtures of alkanes on the two substrates has two implications. First, it seems likely that the molecular structures on the two substrates are very similar, since the same odd−even effects are seen in the melting points (see Figure 2). Thus, we predict that the symmetry of structures on h-BN is likely to be the same as the “herringbone” and “parallel” structures seen on graphite. Second, the regular solution models fit the data reasonably well. These fits show that the preferential adsorption is not significantly influenced by the substrate and that the miscibility of alkanes coadsorbed on h-BN is greater than that on graphite. The reason for this could be related to the effects on commensurateness due to the change in the substrate cell parameters and the corrugation of the surface potential. To fully understand this behavior, a combination of theoretical (molecular dynamics and density functional theory calculations) and structural studies need to be undertaken. The structures could be determined using either diffraction or perhaps STM, although both of these techniques have some complications. For example, boron-10 is a very good neutron absorber, which obviously prevents the use of neutron scattering experiments, unless an isotopically enriched 11BN can be produced. We have already attempted to make some Xray diffraction measurements, but the low surface area and low density of the powders used in this study mean that it was very difficult to prepare samples with sufficient scattering from an adsorbed monolayer of low electron density molecules to be detected above the background, even with synchrotron radiation. Alternatively, as mentioned earlier, the structures of hexane, heptane, and octane on graphite have been observed using low-temperature STM,25 and in principle, if a suitable substrate could be found, this technique could be used to confirm the behavior seen here. Of course, this is not straightforward since, unlike graphite, h-BN is an insulator and so is not ideally suited to STM measurements.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

The authors would particularly like to thank S. M. Clarke and J. Z. Larese for their significant help and advice during the preparation of this document. We would also like to thank Diamond Light Source for financial support and Momentive 10605

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