Pentane Adsorbed on MgO(100) Surfaces: A Thermodynamic

Dec 15, 2014 - Diamond Light Source, Harwell Science and Innovation Campus, Chilton, Didcot, United Kingdom ... Neutron diffraction is used to establi...
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Pentane Adsorbed on MgO(100) Surfaces: A Thermodynamic, Neutron, and Modeling Study Richard E. Cook,† Thomas Arnold,‡ Nicholas Strange,† Mark Telling,¶ and J. Z. Larese*,† †

Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States Diamond Light Source, Harwell Science and Innovation Campus, Chilton, Didcot, United Kingdom ¶ Rutherford Appleton Laboratory, ISIS Spallation Neutron Source, Chilton, Didcot, United Kingdom ‡

ABSTRACT: Thermodynamic measurements using high-resolution volumetric adsorption isotherms were performed on n-pentane films physisorbed on MgO(100) surfaces between 181 K and 244 K. The isotherms show two distinct adsorption steps before the saturated vapor pressure is reached. The heat of adsorption is found to be 33.7 ± 0.3 kJ mol−1 for the first layer and 32.9 ± 0.3 kJ mol−1 for the second layer. Evolution of the two-dimensional compressibility, as a function of temperature, suggests that a phase transition occurs at 185.5 ± 1 K in the second layer. Neutron diffraction is used to establish that the melting of the pentane monolayer takes place between 101 K and 105 K. Computer modeling studies indicate that the pentane molecules adsorb with the molecular axis parallel to the substrate plane. These results suggest that the monolayer forms a solid with a rectangular unit cell, consistent with the neutron diffraction measurements.

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crystal type), exhibits single facet (100) exposure,16 and is readily applicable to computational modeling and studies using neutron diffraction and inelastic scattering. n-Pentane is commonly used as a solvent, because it is the most volatile hydrocarbon that is a liquid at room temperature and, hence, evaporates easily. In addition to use as a solvent, the ease with which pentane can be isomerized or used in the formation of branched-chain hydrocarbons illustrates its overall usefulness. Furthermore, a recent report suggested that pentane, when used in combination with nanocrystalline MgO, enhances the decomposition of toxic chemicals.17 The results of the thermodynamic investigation described below may provide enough information about the solvent−solid interaction energy to shed some light on the process(es) associated with the toxic chemical remediation. However, we emphasize that our intention, in this study, is to ultimately use the information described below to aid in the development of understanding the interaction potential as a function of odd/ even alternation and chain length of the alkane series and MgO(100).

INTRODUCTION The synthesis and characterization of nanometer-scale materials is an area of great scientific and technological interest. Metal oxides,1 carbonaceous archetypes2 (nanotubes, nanohorns, etc.), porous silicas,3 and metal-organic framework materials (MOFs),4 among other materials,5−7 currently represent significant components of nanomaterial research, because of their widespread use in optoelectronics, sensors, molecular adsorbents, fuel cells, separation chemistry, and catalysis. Understanding the nature of the interaction between adsorbed molecules and the surfaces of these materials is a necessity for developing synthetic methods to produce materials with specific functional (i.e., physical, chemical, and mechanical) properties. Providing the input to develop accurate, robust interaction potentials for producing materials by design is the goal of researchers in this area, and multipronged investigations that combine synthetic, thermodynamic, and microscopic studies with computer modeling are becoming more widespread. Many of these are important materials in the chemical industry, because they are used as catalysts and catalyst supports for the formation and separation of hydrocarbons.8 For example, MgO is used for the oxidative coupling of methane to form C2 hydrocarbons or to produce formaldehyde from methane oxidation.9 The study described here is part of an ongoing investigation of the thermodynamic and microscopic adsorption properties of small hydrocarbons on the MgO(100) surface.10−15 The thermodynamic values derived from a sequence of volumetric adsorption isotherms can be used to aid in the development of realistic potential energy surfaces, which describe how the molecules interact with the substrate. MgO is employed in our studies because it serves as a prototypical metal oxide; MgO is simple in structure (rocksalt © 2014 American Chemical Society

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EXPERIMENTAL SECTION The MgO powders used in these studies were synthesized via thermal decomposition of an intermediate magnesium carbide compound and subsequent reaction with a controlled oxygen flow using an inductance furnace; details of the preparation and treatment can be found elsewhere.13,18 Transmission electron Received: September 9, 2014 Revised: December 13, 2014 Published: December 15, 2014 332

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RESULTS AND DISCUSSION More than 30 isotherms were collected in the interval 181 K ≤ T ≤ 244 K and used to determine the adsorption thermodynamics of n-pentane on MgO(100). Figure 1 shows a typical n-pentane isotherm at 184 K. Several different MgO samples were utilized in this study; the initial methane isotherm13 was used to normalize the difference in adsorption area. By comparing the monolayer capacity23 of methane and npentane on the same MgO sample, one can determine the average area occupied by a n-pentane molecule (the area occupied per molecule or APM). This quantity is found to be 42.95 Å2 (compared to that of methane, 17.74 Å2; see ref 10). Using the set of isotherms, several thermodynamic quantities (e.g., differential enthalpy, differential entropy, and heat of adsorption) were calculated. One of our analysis procedures used the Clausius−Clapeyron equation, cast in the form

microscopy (TEM) analysis indicates that the MgO powders, with a specific area of 10 m2 g−1, are comprised of cubic particles with a mean size of 150−250 nm.18 Before use, the powders are heat treated in vacuo (∼10−7 Torr) at 950 °C for ∼36 h. It has been shown that heat treatment at this temperature homogenizes the surface structure of the MgO; i.e., it ensures that all of the exposed facets are the (100) equilibrium surface.19 The inset in Figure 1 shows a TEM

A(n) (1) T (n) (n) to determine the A and B parameters by plotting the location of the nth layer step versus inverse temperature. Figure 2 shows the Clausius−Clapeyron plot for n-pentane on MgO. ln(p) = B(n) −

Figure 1. A typical volumetric adsorption isotherm (circles) plotted against the numerical derivative (line) at 184 K. The secondary y-axis of the left panel is a factor of 6 greater than that of the right panel. Inset shows a TEM image of a 200-nm MgO cube.

image of a typical MgO nanoparticle, illustrating the cubic crystal habitat and the attendant (100) surface exposure. After the heat treatment, the MgO is loaded into a a thin-walled Al (neutron20) or an oxygen-free high conductivity (OFHC) Cu (adsorption) cell and sealed with an indium wire gasket inside a glovebox filled with ultrahigh-purity-grade argon. This procedure is necessary because exposure of the MgO powder to atmospheric moisture increases the likelihood of dissolution and hydroxylation of the MgO(100) surface. The surface hydroxylation is more noticeable in commercially produced MgO, because of the greater number of surface defects, which are primarily oxygen vacancies. The OFHC cell is then mounted onto a helium closed-cycle refrigerator (APD cryogenics). The temperature of the sample cell is regulated within 3 mK of the setpoint using a Neocera LTC-10 equipped with a silicon diode sensor (Scientific Instruments). The absolute temperature of each isotherm is checked by comparing the thermometer reading with the temperature value calculated using the saturated vapor pressure (SVP) with a semiempirical Antoine equation (n-pentane SVP parameters obtained from the NIST online database21). The automated isotherm apparatus used in these experiments is described in detail elsewhere.22 In this study, the temperature of the calibrated volume is controlled above ambient (at ∼320 K) to reduce the effects of room-temperature fluctuation during the experiment. Before each experiment, the MgO loaded sample cell is evacuated at room temperature for a minimum of 12 h, reaching a base pressure of ∼10−7 Torr. A methane (UHP grade, Matheson) isotherm was performed on each sample to determine the surface area and verify sample quality.11,13 About 200 data points are recorded for each isotherm, which takes ∼12 h to complete. The n-pentane liquid (99+% anhydrous, Aldrich) was prepared for use through a series of freeze−thaw distillation cycles.

Figure 2. Clausius−Clapeyron plot of the two adsorbed layers ((red, ●) first layer, (blue, ■) second layer), compared with the saturated vapor pressure of bulk pentane ((green, ◆) SVP).

Data plotted in Figure 2 uses the numerical derivative, (Δnads/ Δp)T, of the isotherm (see Figure 1) to locate the step position. Larher24 has shown that the parameters A(n) and B(n) can be used to calculate the differential enthalpy, ΔH(n), and the differential entropy, ΔS(n), ΔH (n) = −R(A(n) − A(∞))

(2)

ΔS(n) = −R(B(n) − B(∞))

(3) −1

−1

where R is the universal gas constant (R = 8.314 J K mol ). Table 1 displays A(n), B(n), and the resulting thermodynamic values. The values ΔH(n) and ΔS(n) represent the difference between the bulk value (n = ∞) and the value associated with the nth layer. As the quantity of gas admitted to the sample cell approaches the SVP, ΔH(n) and ΔS(n) each approach zero. The convergence of H(n) and S(n) to the bulk values and the observation of two adsorption steps in the isotherms (even in the derivative plot) indicate that only two discrete pentane 333

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Table 1. Thermodynamic Values Calculated from Isotherm Data n

A(n)

B(n)

Qads(n) [kJ mol−1]

ΔH(n)[kJ mol−1]

ΔS(n) [J K−1 mol−1]

1 2 ∞

4056 3955 3878

17.15 19.54 19.72

33.72 ± 0.3 32.88 ± 0.3 32.24 ± 0.3

−1.4774 ± 0.2 −0.6410 ± 0.075

21.39 ± 1 1.530 ± 0.3

layers form. The average heat of adsorption for the nth layer, Q(n) ads, can be determined using (n) Q ads = RA(n)

(4)

(n)

Q(n) ads

where A is the value found in eq 1. approaches the heat of vaporization of the bulk system if the temperature exceeds the bulk triple point. This is understandable, since the effects of the MgO(100) surface are significantly diminished. At low coverage, the heat of adsorption can be used to probe the interaction energy of isolated pentane molecules with the surface. A linear fit of the low coverage adsorption data (see Figure 1), nads vs p, can be used to determine the heat of adsorption using Henry’s law: ⎛ Q0 ⎞ K (T ) = K * exp⎜ ⎟ ⎝ RT ⎠

Figure 4. Isosteric heat of adsorption at 190 K. The dashed vertical lines denote the peaks in the numerical derivative for the first and second layers, respectively.

(5)

where K(T) is the Henry’s law constant, K* is a constant of integration, and Q0 is the low-coverage heat of adsorption.25 In the Henry’s Law regime, it is assumed that the molecular density is low enough to ignore molecule−molecule interaction. Figure 3 shows a linear plot of the Henry’s Law data. This fit

1.2 × 10−5 mol, identified by the dashed vertical lines, correspond to the completion of the first and second layers, respectively. The data in Figure 4 is limited to the coverage region just beyond the second layer completion. However, we expect Qst to approach the bulk enthalpy as nads approaches infinity (i.e., as the pressure approaches the saturated vapor pressure). The isosteric heat is difficult to determine accurately because of the extrapolation required to determine the difference in pressure at the same surface coverage. For example, the isosteric heat values shown in Figure 4 at 190 K for the monolayer (bilayer) steps are found to be ∼3% (14%) higher (lower) than the average Qst values given for the entire monolayer (bilayer) region in Table 1. We note that the isosteric heat shown in Figure 4 corresponds to a specific path through the phase diagram but we reiterate that the value should ultimately converge to the heat of vaporization (∼32 kJ/ mol), consistent with the value of Qst shown in Table 1. With these difficulties in mind, we find this agreement to be satisfactory. To examine the interaction of the film with the substrate more quantitatively, the Accelrys Materials Studio Package was employed. The relative binding energy of a single pentane molecule with a model MgO(100) surface (supercell of 4 × 4 × 2 MgO unit cells) was investigated using the centralized forcefield model, COMPASS26 and the Forcite27 module. The procedure for determining the minimum energy configuration (MEC) for an isolated molecule (i.e., the “optimum adsorption site and orientation”) was as follows. Initially, the center of mass of the molecule was positioned at one of the high coordination sites of the substrate (e.g., atop the Mg2+ and O2− and at the hollow (saddle) and bridge sites identified in Figure 5) with the molecular axis parallel to the MgO(100) plane, and then the MEC code was executed. The potential energy landscape was explored by orienting the molecular axis at 10 equally spaced directions (i.e., ±5° apart) within a 45° sector on the (100) surface (as designated by φ in Figure 5). The results of this exercise found the minimum energy to be

Figure 3. Natural logarithm of the Henry’s Law constant plotted as a function of inverse temperature to determine low-coverage heat of adsorption from the slope of the linear fit.

yields a low-coverage heat of adsorption, Q0, of 32.6 ± 1.5 kJ mol−1, consistent with the values obtained from the Clausius− Clapeyron plot. The isosteric heat of adsorption, Qst, is the energy required to adsorb a molecule from the bulk vapor (at infinity) into the surface film (at a particular surface coverage) and can be determined from Q st = RT 2

δ(ln p) δT

θ

(6)

It is assumed that the partial differential in this expression can be approximated numerically using the difference between two isotherms with a small difference in temperature.13 Figure 4 is a representative example of Qst for pentane adsorption on MgO(100) at 190 K. The two features at 3.7 × 10−6 mol and 334

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molecules in the unit cell suggested by the diffraction result. While not a perfect match (the diffraction analysis used rigid molecules), the good agreement between the structural analysis and MEC approach suggests that this strategy will be a valuable route for solving the solid-state structure of the pentane monolayer solid once a more-extensive diffraction measurement is performed. Recently, we investigated the binding energy of an isolated alkane molecule as a function of molecular orientation for odd members of the alkane series (e.g., heptane) and found that the preferred configuration is when the molecular axis is parallel to the MgO(100) plane.29 Those modeling studies clearly showed that, for odd members of the alkane series, when the molecular axis is parallel but the molecular plane is perpendicular to the surface plane, additional consideration must be given to the number of carbon atoms closest to the surface. For example, in this study, the orientation of an isolated pentane molecule with the lowest energy (tightest binding) occurs when the plane of the molecule is parallel to the MgO(100) surface and the molecular axis is oriented along the surface ⟨11⟩ direction. The next lowest energy is when a molecule is oriented with the molecular plane and axis parallel to the MgO(100) surface but the molecular axis points along the ⟨10⟩ direction. The next lowest two states are found to be when the plane of the molecule is perpendicular to the MgO(100) surface, the molecular axis is pointed along the ⟨11⟩ direction, and three carbon atoms are equidistant from the surface. Finally, the states that are least favorable (most weakly bound) are those with the plane of the molecule perpendicular to the surface, the molecular axis along the ⟨11⟩ direction, and two carbon atoms equidistant from the surface. Figure 6 illustrates these molecular configurations for an isolated molecule and the binding energies are summarized in Table 2.

Figure 5. Depiction of four adsorption sites on the MgO(100) surface (i.e., atop over Mg2+ and O2−, hollow and bridge) over which the molecule’s center of mass is positioned prior to MEC execution. Then, the angle φ represents the 45° sector utilized at each site for examining the potential energy surface.

approximately −48 kJ mol−1, with the four orientations shown in Figure 6. This energy can be compared with the

Figure 6. Results of MEC calculations for an isolated pentane molecule on the MgO(100) surface using the Accelrys modeling package. Panels (a) and (b) illustrate the plane of the molecule parallel to the surface in the ⟨10⟩ and ⟨11⟩ directions, respectively, while panels (c) and (d) show the molecular plane perpendicular to the surface with the molecular axis in the ⟨11⟩ direction in both cases. The structure in panel (c) places three carbons in contact with the surface, while the structure in panel (d) has two carbons in contact with the surface. The binding energies of these orientations are provided in Table 2.

Table 2. Calculated Binding Energies for Parallel (C5) and Perpendicular (C2 and C3) Molecular Plane Configurations Relative to the MgO(100) Surface with Molecular Axis Orientations in the ⟨10⟩ and ⟨11⟩ Directions surface orientation ⟨11⟩ ⟨10⟩ ⟨11⟩ ⟨10⟩

experimentally obtained Henry’s Law value of 32.6 kJ mol−1. The force field calculation is not expected to provide us with a quantitatively accurate minimum energy, but it should serve as a useful guide in developing an appropriate scaling factor for the alkane series in the future. We note that this MEC was performed for an isolated molecule, and it is important to recognize that molecule−molecule interactions also play a role in the hydrocarbon/metal oxide adsorption thermodynamics. Hence, we will use these simple computational exercises to identify trends for describing the alkane interaction with MgO, especially in terms of solving the solid structures that form and in understanding the evolution of the wetting properties. For example, this method was used to examine the structure of the butane (C4H10) monolayer solid on MgO(100) by comparing the modeling results to our previously published structural determination.28 There, we found that the butane monolayer structure determined using neutron diffraction compares well to the unit cell obtained using MEC and the Forcite modeling package if we initially placed the center of mass of the

⟨11⟩ ⟨10⟩

total energy [kJ mol−1] C5 System −101.671 −100.741 C3 System −99.2477 −96.0429 C2 System −92.6479 −92.2874

nonbonding energy [kJ mol−1] −48.4633 −47.7353 −45.7730 −42.4802 −39.3589 −39.2292

For a condensed phase, we employed the same Forcite and Compass force field combination to perform simple molecular dynamics (MD) simulations on a condensed monolayer solid of pentane on MgO(100) surface. Figure 7 illustrates two snapshots of the energy equilibrated solid structures that we identified. The starting configurations in the MD simulation were chosen based on our earlier investigations of heptane, butane, and methane on MgO(100), as a well as studies of the alkanes on graphite30 and hexagonal BN basal planes31 and references therein. While these two solid structures appear to be stable, we note that they are likely only local minimum configurations. Based on previous studies, we know that several density-dependent solid phases can exist (see, for example, 335

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Figure 8. Two-dimensional compressibility (K2D) versus the chemical potential determined from an isotherm recorded at 193 K. The inset is a sample of the Gaussian fit to a second layer K2D peak at 193 K (with a coverage of 0.01 mmol, or 2 layers) used to determine the full-width at half maximum (fwhm) subsequently plotted in Figure 9.

μ2D = μ3D = kBT ln p Figure 7. Snapshots of two low-energy configurations of a monolayer pentane solid adsorbed on the MgO(100) surface, as a result of a preliminary MD simulation using the Forcite package and the Compass force field from Accelrys as described in the text. Both panels illustrate the solid structures that result when the molecular axis is oriented parallel to the MgO(100) plane. The top (bottom) panel shows a centered rectangular (herringbone) solid with the plane of the molecule parallel (perpendicular) to the surface. Note that, in the right panel, three of the carbons in the molecular axis are closer to the surface, as noted in the text and in ref 25.

(8)

where kB is the Boltzmann’s constant. Larher24 suggested that changes in the fwhm of K2D could be used to locate phase transitions. The rationale in using this method for K2D to locate phase transitions is a reflection of the fact that the threedimensional compressibility of solids, liquids and gases are very different. Figure 9 displays the behavior of the fwhm of K2D, as a function of temperature.

ethylene on graphite32,33). We employed the molecular dynamics package to gain some additional insight into possible solid monolayer pentane structures. Additional neutron diffraction work will be necessary before we can confidently assign a monolayer solid structure, as we have in the butane, ethane, and methane on the MgO(100) case. The adsorption thermodynamic series presented here can also be employed to construct a phase diagram and identify the location of potential phase transitions. The two-dimensional compressibility, K2D (analogous to the three-dimensional quantity), can be used for this purpose.24 One formulation of the two-dimensional compressibility, described earlier by Dash,34 is given by K 2D =

Sp NAkBTNads

2

dNads dp

Figure 9. Temperature dependence of the fwhm of the K2D for the second adsorbed layer peaks calculated using the Gaussian fits, as noted in the text. The reduced chemical potential (μ − μ0) where these peaks appear is ∼115 ± 0.5 K below 184 K and ∼125 ± 5 K above 184 K. The two-dimensional critical temperature determined from this data for the second layer is observed at 185.5 ± 1 K. The inset at the top left illustrates the linear fit for the low-temperature fwhm data.

(7)

where S is the total surface area of the sample, p is the equilibrium vapor pressure, NA is Avogadro’s number, kB is Boltzmann’s constant, and Nads is the number of adsorbed moles. Figure 8 displays the typical K2D behavior for pentane on MgO(100) at 193 K. Once again, a numerical approximation to the derivative is used, but in order to consistently gauge the width (and height) of the compressibility feature as a function of temperature, a Gaussian was fit to the data, as illustrated by the inset shown in Figure 8. The abscissa is expressed as the difference in chemical potential between the film and bulk (i.e., μ − μ0). Assuming that the three-dimensional vapor is in thermal equilibrium with the two-dimensional film and that the gas behaves ideally, then

Note how the width remains relatively constant until the temperature exceeds ∼185 K. We use the sharp change in the fwhm in the neighborhood of 185.5 K to identify T2t at 185.5 ± 1 K as the temperature location of a potential phase transition within the second pentane layer. Because the formation of the first layer takes place at very low pressures, it is not possible to precisely evaluate the fwhm in the monolayer regime. In addition, identifying the microscopic change in state associated 336

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range shown in Figure 10a was chosen because only 15 min were needed to record this pattern at each temperature. At lower temperatures, two asymmetrically shaped diffraction peaks appear in the NPD trace. The location in Q of these two prominent diffraction peaks is consistent with the rectangular unit cells pictured in Figure 7. We note that the rectangular cell is consistent with the structures found for pentane and other linear n-alkane monolayer films adsorbed on graphite,30,36 hBN,31 and MgO.1−4 Unfortunately, we cannot draw a definitive conclusion about the orientation or disposition of the molecules within the unit cell, because of the limited Q-range. Warren was the first to note that the diffraction from twodimensional (2D) layered systems exhibits asymmetric profiles that look resemble the kerfs on a saw blade.37,38 When a solid melts (and the long-range order disappears), a broadening and decrease in the peak intensity of the diffraction peaks takes place. Above 100 K, a noticeable change in the diffraction patterns occurs. In order to track the variations of these traces with temperature more quantitatively, the diffracted intensity (derived from the difference plots) was numerically integrated over the entire Q window (1.5 Å−1 < Q < 1.75 Å−1) shown in Figure 10a. Clearly, there is a dramatic decrease in the integrated intensity between 101 K and 105 K; a direct consequence of the loss of long-range order as the film melts and a broadening and shifting in position of the diffraction peaks. As a rule of thumb, monolayer films typically melt at ∼70% of the bulk melting point (143.4 K for pentane). Earlier inelastic neutron measurements probing the dynamics of the pentane monolayer film, as a function of temperature,39 and the results displayed in Figure 10b clearly indicate that the melting of monolayer pentane takes place near 100 K and that the phase transition identified using K2D is one associated with the fluid phase (see Figure 9).

in this temperature regime is more difficult. For assistance in this task, we turn to diffraction. Neutron powder diffraction (NPD) was used to identify the thermodynamic status (e.g., solid or liquid) of the monolayer pentane film, as a function of temperature. NPD lends itself to studies of this type, because thermodynamically equilibrated monolayer films of deuterated n-pentane (C5D12) can be prepared in situ (pressures of 0.1−100 Torr) without the difficulties associated with ultrahigh vacuum techniques. Diffraction scans as a function of temperature are commonly used to locate phase transitions. The neutron diffraction patterns were collected using the OSIRIS time-of-flight spectrometer35 in the diffraction mode at the ISIS neutron facility. To recover the diffraction signal from the surface film, the diffraction pattern of the substrate and sample can (without the adsorbate) is recorded first and then subtracted from the pattern obtained after adsorbing the monolayer on the surface. Figure 10a displays the changes in the NPD pattern from the monolayer pentane film, as a function of temperature. The Q-

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CONCLUSION

The thermodynamic properties of pentane films adsorbed on MgO(100) substrates between 181 K and 244 K were determined. It was determined that, at 184 K, the area occupied per molecule (APM) is ∼43 Å2. This compares with the APM value for hexane on MgO(100) of ∼87 Å2 and ∼22 Å2 for ethane on MgO(100).11,12 The average heat of adsorption for the monolayer pentane film using a Clausius− Clapeyron-type analysis is found to be 33.72 kJ mol−1. This value compares to the 17.80 kJ mol−1 determined in the ethane on MgO(100) study.12 Using the isothermal compressibility, two phase transitions were observed. While the absolute value of the pressure (PSVP(180 K) ≅ 0.5 Torr) makes it challenging to precisely locate the monolayer transition temperature, the transition associated with the second layer was determined to be 185.5 ± 1 K. From the temperature dependence of the neutron diffraction scans, it was found that the melting of the monolayer solid occurs near 105 K. This neutron diffraction result, along with the convergence of the isosteric heat of adsorption to the bulk heat of vaporization at higher coverage suggests that the second layer transition at 185.5 ± 1 K takes place from a liquid phase and may be associated with a transition from liquid to a hypercritical fluid or vapor phase.34 This study brings us one step closer to our ultimate goal of contributing to the development of potential energy surfaces for the C1−C10 alkanes on the MgO(100) surface. In the future, we plan to perform additional neutron diffraction and vibrational spectroscopy measurements to more fully character-

Figure 10. (a) Neutron diffraction difference pattern for a 0.9 monolayer (∼0.06 mmol) fully deuterated n-pentane layer plotted as a function temperature (used because deuterium scatters neutrons coherently). For ease of inspection, the traces are uniformly displaced vertically from each other by the same amount. (b) Plot of the temperature dependence of the numerically integrated intensity within the Q window (1.5 Å−1 < Q < 1.75 Å−1) for the diffraction patterns displayed in panel (a). 337

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ize the solid structure and dynamics of n-pentane films on MgO.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to express our thanks to Andi Barbour, Sami Chanaa, Lillian Frazier, Paige Landry, Peter Yaron, and Stuart Clarke for helpful discussions and assistance with some of the measurements. This work was performed with the support of the Joint Institute for Neutron Science (partial graduate fellowship for R.C.), the University of Tennessee, Knoxville, the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy (under Contract No. DE-AC0500OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC) and the National Science Foundation (under Grant No. DMR-0412231). J.Z.L. gratefully acknowledges the access to neutron beamtime at the ISIS facility of the Rutherford Appleton Laboratory.

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