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J. Phys. Chem. C 2008, 112, 17272–17280
Collisions of Polar and Nonpolar Gases with Hydrogen Bonding and Hydrocarbon Self-Assembled Monolayers Megan E. Bennett, William A. Alexander, Jessica W. Lu, Diego Troya, and John R. Morris* Department of Chemistry, Virginia Tech, Blacksburg Virginia 24060 ReceiVed: February 19, 2008; ReVised Manuscript ReceiVed: August 15, 2008
Molecular beam scattering experiments are used to explore collisions of 60 kJ/mol Ne, CD4, ND3, and D2O with long-chain CH3-, NH2-, and OH-terminated self-assembled monolayers (SAMs) created via the chemisorption of alkanethiols on gold. Time-of-flight measurements for the scattered gases reveal the extent of energy exchange and the propensity for a gas to thermally accommodate with the surface during a collision. Of the four gases studied, Ne transfers the least amount of translational energy into the monolayers and D2O the most. Neon atoms recoil from the OH-SAM with an average of 14.4 kJ/mol of energy, while D2O retains only 6.4 kJ/mol of its 60 kJ/mol incident energy when scattering from the same surface. Overall, the trend in final translational energies follows the order Ne > CD4 > ND3 > D2O for scattering from all three SAMs. The observed trend in the energy exchange is correlated with the gas-surface attractive forces, as determined by ab initio calculations. The thermal accommodation efficiencies of the four gases follow the opposite trend. Thermalization for the Ne atoms is nearly negligible for all three monolayers, whereas D2O and ND3 approach near complete accommodation on all of the monolayers studied. The overall energy exchange and thermal accommodation efficiencies also depend markedly on the terminal group of the SAM. For Ne scattering, the trend for the overall energy transfer follows: CH3- > NH2- ≈ OH-SAMs. In contrast, the overall D2O energy transfer is greater when colliding with the OH-SAM than the nonpolar CH3-SAM. Together, the results show that the extent of energy transfer depends on a balance between the rigidity of the surface, as affected by intrasurface hydrogen bonding, and the strength of the gas-surface attractive forces, as determined by intermolecular interactions. I. Introduction The dissolution of gas phase molecules into a hydrocarbon liquid or the uptake of a vaporous species onto a solid organic surface depends on the physical and chemical properties of the interface that govern the motions of the molecules as they collide. Pioneering molecular beam scattering experiments by Saecker and Nathanson have provided detailed insight into the roles of gas-surface collision energy, dipole-dipole interactions, and surface free energy in determining the outcome of collisions of atomic and small polyatomic gases on liquid organic surfaces.1 Their studies of Ne, CH4, NH3, and D2O impinging on liquid glycerol and squalane show that the molecules recoil from the surfaces with energies that separate into two channels corresponding to impulsive scattering and trapping desorption. The final energy distributions of molecules that scatter impulsively from the surfaces of the two liquids were found to depend largely on the kinematics of the system. However, the extent of trapping on the surfaces depends markedly on the gas-surface attractive forces and the rigidity of the liquid. For example, they found that high collision energy, ∼55 kJ/mol, D2O molecules trap much more readily on glycerol, a hydrogen bonding liquid, than on squalane, a pure hydrocarbon. In contrast, Ne appears to trap more efficiently on the hydrocarbon liquid than on the rigid hydrogen bonding surface of glycerol. Overall, these studies have correlated the enthalpies and free energies of solvation with the final energy distributions of the gases to show that the “like dissolves like” principle can be used to predict the relative energy transfer and extent of * Address correspondence to this author.
thermal equilibrium for gas-liquid collisions.1 Our studies are designed to explore whether similar principles can be extended to understand the dynamics of gases colliding with solid organic materials where the interfacial region is more static, solvation cannot occur, and the energetic degrees of freedom of the surface are altered significantly relative to those of a liquid. The following studies examine collisions of the nonpolar gases, Ne and CD4, and polar gases, ND3 and D2O, with model organic surfaces composed of ω-functionalized alkanethiol selfassembled monolayers (SAMs) on gold. Alkanethiol SAMs on gold provide the opportunity to study gas-surface scattering dynamics on well-organized, atomically flat systems where the chemical groups of interest are located precisely at the gas-surface interface. Model saturated hydrocarbon surfaces are created by using long-chain CH3-terminated SAMs and polar hydrogen bonding groups are placed at the interface by employing NH2- and OH-terminated SAMs. Previous studies with reflection-absorption infrared spectroscopy, scanningtunneling microscopy, and molecular dynamics simulations indicate that these monolayers are very similar in their packing density and chain orientation.2-7Long-chain hydrocarbon monolayers form the highly ordered c(4×2) superlattice of the basic (3×3) R30° unit mesh. Overall, the chains pack in an alltrans configuration with nearest neighbor spacing of 5 Å.8,9 The primary structural differences between long-chain (number of carbons >10) CH3-SAMs and the OH- or NH2-SAMs is likely in the order and tilt of the terminal groups. Simulations performed by Hase and co-workers10 and Klein and co-workers6,7,11 show that the terminal groups of the OH-SAM can form closely spaced regions as they associate into an extended
10.1021/jp801476f CCC: $40.75 2008 American Chemical Society Published on Web 10/11/2008
Collisions of Gases with SAMs
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TABLE 1: Selected Properties and Heats of Solvation of Gases Used in the Scattering Experiments gas
radius (Å)
electric dipole (D)
polarizability volume (Å3)
∆Hvap (kJ/mol)
∆Hsolv[in CH3OH] (kJ/mol)
∆Hsolv[alkanes] (kJ/mol)
Ne CH4 NH3 H2O
1.5 2.0 1.8 1.4
0 0 1.4 1.8
0.4 2.6 2.2 1.3
1.2 8.2 23 41
5.7 -3.7 -12 -47
6.5 -4.3 -6.9 -17
hydrogen bonding network that binds nearly every hydroxyl group within the monolayer. The gases studied in the following work are highlighted in Table 1. These incident gases were chosen due to their similar sizes and masses, while spanning a significant range of properties, thereby enabling the correlation of observed trends in the scattering dynamics to chemical or physical characteristics rather than to mass effects. Table 1 indicates many of the properties and solubility data that one may expect to help predict trends in the overall gas-surface scattering dynamics.12-14 The polarizabilities of the gases vary from 2.6 Å3 for CH4 to as low as 1.3 and 0.4 Å3 for H2O and Ne, respectively. The trend for dipole moment strength is just the opposite, as it ranges from 0 debye for Ne and CH4 to 1.8 debye for H2O. By using molecular beam scattering experiments of these gases on polar and nonpolar SAMs, we aim to develop an understanding for how the attractive and repulsive forces between the gases and solid organic surfaces affect energy transfer and the propensity for a gas to accommodate on a surface. Following the lead of early molecular beam experiments,15 a number of researchers have explored the dynamics of reactive and nonreactive gases scattering from a variety of organic selfassembled monolayers. Theoretical studies have provided significant insight into the dynamics controlling the outcome of collisions of rare gases with SAMs on gold. Molecular dynamics simulations by Hase and co-workers10,16-22 have shown that low-energy extended motions of monolayer alkane chains play a major role in dissipating the energy of a gas-surface collision, while the high-energy C-H motions play a minor role in the dynamics. They have further demonstrated that the final energy distribution for Ne scattering from a SAM is composed of two channels: an impulsive component and a Boltzmann component. The impulsive component is dominated by single-collision events and very short residence times. The Boltzmann component describes those atoms that scatter from the surface with a final energy distribution that can be described by a Maxwell-Boltzmann distribution. However, the attractive forces between Ne and a terminal CH3 group of a SAM appear to be insufficient to trap an incident atom before it recoils into the gas phase, and full thermal accommodation is not observed in these simulations.18-22 Complementary investigations by Troya and co-workers have demonstrated the importance of the gas-surface potential in determining the extent of energy exchange in rare gas collisions.23,24 In addition, they have shown that the extent of interfacial energy exchange decreases as the packing density of alkane chains increases due to restriction of the low-energy motions within a SAM and have performed extensive studies on the effect of mass in gas-surface energy exchange.25,26 Molecular dynamics simulations have also been coupled with experimental studies to reveal new scattering channels in collisions of high-energy Xe atoms with SAMs.27 Sibener and co-workers have, in addition to studying Ne and Ar collisions on saturated organic SAMs,17,19,28 shown that significant penetration into a monolayer can occur for energetic Xe atoms.17,19,27,28 Furthermore, Kandel et al. have recently followed the structural changes of an octanethiol monolayer upon Xe atom bombardment to find that high-energy atoms can
induce structural changes in the film that originate at defects and phase boundaries.29,30 In addition to the studies described above that focus on the dynamics of scattering from saturated organic SAMs, recent work has focused on how polar groups within a particular monolayer affect the energy exchange and thermal accommodation efficiencies in gas-surface collisions.10,31-36 Molecular beam scattering experiments have been employed to study the energy exchange for a variety of gases scattering from alcohol-, amine-, and carboxylic acid-terminated SAMs.27-31 The experiments suggest that intramonolayer hydrogen bonding plays a major role in determining the rigidity of the SAM by restricting the types of low-energy motions that govern energy transfer in saturated methyl-terminated monolayers. Even buried hydrogen bonding groups within a monolayer affect the relative rigidity of the film.32 The role of hydrogen bonding groups within organic films has also been explored by Scoles and co-workers, who performed Debye-Waller measurements using He atom scattering on single-crystal organic surfaces.37 These studies demonstrated that a hydrogen bonding surface is more rigid than a methyl-terminated surface. The molecular beam scattering experiments described below are designed to complement previous investigations into the dynamics of polar and nonpolar gases colliding with organic liquids. Specifically, we aim to explore whether the same principles that govern gas-surface energy transfer and trapping probabilities on liquids can be extended to describe collisions on solid organic surfaces. Overall, this work is focused on studying the interplay between surface rigidity, as provided by polar hydrogen bonding groups, that limits energy transfer into the material and deep gas-surface attractive interactions that may ultimately enhance the extent of energy transfer and thermal accommodation. II. Experimental Section SAM Preparation. The SAMs used in this study were prepared by spontaneous chemisorption of the alkanethiol of interest from a 1 mM ethanolic solution onto a clean Au surface. The substrates were prepared by Au evaporation onto Cr-coated glass slides (EMF Corp.) that were cleaned in a piranha solution (70/30 (v/v) mixture of H2SO4/H2O2) prior to use. We have created two classes of ω-functionalized SAMs for this study: monolayers terminated by nonpolar methyl groups and monolayers terminated by polar hydroxyl and amine groups. The CH3SAMs were created from hexadecanethiol (HS(CH2)15CH3) or undecanethiol (HS(CH2)10CH3). The time-of-flight molecular beam scattering data from these two surfaces were identical, except for a small even/odd effect that was reported earlier.38 The OH-SAM was created from 16-mercapto-1-hexadecanol (HS(CH2)16OH) or 11-mercapto-1-undecanol (HS(CH2)11OH). The time-of-flight distributions for scattering from these two monolayers were indistinguishable and, as also previously reported, we observed no effect on the even or odd character of the alkanethiols.34 These chemicals were purchased from Aldrich and used without further purification. The NH2-SAM was constructed of 11-aminoundecanethiol (HS(CH2)11NH2),
17274 J. Phys. Chem. C, Vol. 112, No. 44, 2008 which was purchased from Dojindo.27 For each monolayer, the clean gold slides were placed in the solutions for at least 48 h, rinsed with copious amounts of ethanol, dried under a stream of ultrahigh purity nitrogen, and then immediately transferred to the main UHV chamber (base pressure Ne. In addition, the average final energy changes across these three gases. The BC fractions and the final average translational energy for each gas-surface combination are summarized in Table 2. Gas-Surface Potential Energy Calculations. The extent of gas-surface energy exchange in interfacial collisions depends
Figure 3. P(Ef) distributions for (a) Ne, (b) CD4, (c) ND3, and (d) D2O scattering from CH3, NH2, and OH-SAMs. The dashed line in each figure represents the best fit for a Boltzmann distribution to the OH-SAM data at the temperature of the surface.
significantly on the rigidity and mass of the surface as well as the attractive forces between the impinging gas and the terminal groups of the surface. For the systems explored in this study, the attractive forces are expected to span a large range from the dispersion interactions that govern the attractions of the nonpolar gases with the CH3-SAM, to strong hydrogen bonding interactions between the polar gases and the polar terminal groups of the OH- and NH2-SAMs. Although the general trend in these attractive forces can be estimated from the polarity and polarizability of the gases, we present a more rigorous determination of the potential using ab initio calculations. Since it is impractical to map the entire potential energy surface (PES) for every possible molecular orientation and impact parameter, we have focused on characterization of the minimum-energy gas-surface potential wells for Ne, CH4, NH3, and H2O gases approaching a simple model of each SAM. The model SAMs are composed of a triad of molecules that are arranged to approximate the structure of the termini of three adjacent chains in an ideal CH3-, NH2-, or OH-SAM. Namely,
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TABLE 2: Summary of Results for 60 kJ/mol Gas Scattering from SAMs, Showing the BC Fractions and the Average Final Energy (kJ/mol) for Each Gas-Surface Pair Ne CD4 ND3 D2O
CH3-SAM (BC/〈Ef〉)
NH2-SAM (BC/〈Ef〉)
OH-SAM (BC/〈Ef〉)
0.39 ( 0.01/10.6 ( 0.5 0.70 ( 0.02/7.8 ( 0.3 0.78 ( 0.02/7.0 ( 0.3 0.84 ( 0.01/6.7 ( 0.3
0.30 ( 0.01/14.1 ( 0.5 0.60 ( 0.02/9.5 ( 0.4 0.72 ( 0.02/8.4 ( 0.3 0.79 ( 0.01/7.4 ( 0.3
0.22 ( 0.01/14.4 ( 0.5 0.53 ( 0.02/9.7 ( 0.4 0.78 ( 0.02/7.6 ( 0.3 0.89 ( 0.01/6.4 ( 0.3
TABLE 3: Energy of the Potential-Energy Minima of Model Gas/Surface Pairsa Ne CH4 NH3 H2O
CH3CH3a
CH3NH2a
CH3OHa
0.3 3.1 6.1 5.0
0.5 5.5 33.5 62.0
0.6 6.9 49.0 61.0
a Energies below the asymptote in kilojoules per mole for the lowest energy configuration for each gas interacting with a model SAM, as described in the text.
the internuclear axes are 5.0 Å away from each other, and the three C (and N or O) atoms form an angle of 60°. Geometries for each of the gas/surface-mimic pairs were optimized by using second-order Mo¨ller-Plesset perturbation theory (MP2) in combination with Dunning’s double-ζ correlation-consistent basis set augmented with diffuse functions (aug-cc-pVDZ). The geometries and depths of the potential wells for each system were subsequently corrected for basis-set superposition error (BSSE) by applying the counterpoise method.60 All calculations have been carried out with the Gaussian03 suite of programs.61 To preserve the 5.0 Å separation between chains and 60° arrangement, the heavy atoms of the “surface” molecules (both C atoms in the CH3-SAM mimic, C and O in the OH-SAM mimic, and C and N in the NH2-SAM mimic) are held fixed during the optimization. All other variables are optimized. Figure 4 illustrates the simplified model for calculating the minimum energy for an ammonia molecule approaching an NH2-SAM (a) and an OH-SAM (b). The structures exhibited in Figure 4 reveal that ammonia can establish three hydrogen bonds with either the NH2- or OH-SAM. The extensive hydrogen bonding can lead to significant potential energy wells. The counterpoise-corrected MP2/aug-cc-pVDZ minimum energy for the structure shown in Figure 4 is 33.5 kJ/mol for NH3 interacting with three methylamine molecules and is even greater, 49.0 kJ/mol, when it interacts with three methanol molecules. Similar calculations were performed for all 12 gas-surface combinations explored in the scattering experiments. The results are provided in Table 3. Analysis of the well depths for a particular surface mimic reveals that the attractive forces generally increase in the following order: Ne < CH4 < NH3 < H2O. In addition, the well depths for a particular gas approaching the three surfaces show the general trend of increasing in the order CH3-SAM < NH2-SAM < OH-SAM. However, Ne interacts relatively weakly with all three surfaces and its minimum energy is nearly independent of the surface terminal group. CH4 interacts slightly more strongly with all of the surfaces due to its increased polarizability. The well depths of the interactions with the NH2SAM and OH-SAM are only slightly larger than for the CH3SAM. The depths of the potential wells for NH3 increase with respect to CH4 for all of the surface mimics examined. The difference with CH4 is small for the CH3-SAM but noticeable for the polar surfaces, with increases of over 25 kJ/mol due to the presence of strong hydrogen bonds between NH3 and the surface mimics. Finally, H2O shows a well very similar to that
of NH3 for the CH3-SAM, but remarkably deeper wells for the polar surfaces. The deeper well for H2O relative to NH3, for polar surfaces, is also reflected by the enthalpy of solvation of these two gases in methanol as shown in Table 1. Overall, the calculations indicate that the hydrogen bonding ability of the H2O and NH3 molecules results in sharply enhanced interactions with the hydrogen bonding SAMs. Quantitatively, Figure 4 shows that H2O and NH3 can form up to three hydrogen bonds with the polar surfaces, leading to a very stable complex. Earlier calculations of the minimum-energy structure of H2O and a slightly different model of the OH-SAMs also showed the ability of a single water molecule to form three hydrogen bonds with the SAM.62 We note that caution should be exercised when using calculations of NH3 or H2O interacting with methylamine and methanol molecule triads to better understand the actual interactions of these gases with NH2- or OH-SAMs. For instance, the locations of the backbones of the surface-mimic molecules are fixed so that they are 5.0 Å away from each other. Although 5.0 Å is the average separation between the chains in a SAM grown on Au(111), in the actual SAMs there is a degree of mobility that lifts the 5.0 Å constraint. Therefore, gas-phase molecules approaching a SAM will likely not encounter chain termini separated by exactly 5.0 Å, and the interaction energies will be different from those reported here. Another point to consider is that it has been shown that OH-SAMs form extensive hydrogen bonding networks. Therefore, when a hydrogen bonding gas-phase molecule, such as ND3 or D2O, approaches a SAM possessing intrasurface hydrogen bonding, such as the NH2- or OH-SAM, it is expected that the approaching molecule will compete with surface molecules adjacent to the impact site for hydrogen bonds. This effect will probably impair the idealized structures depicted in panels a-d in Figure 4, where
Figure 4. Schematic representation of the minimum-energy potential wells in the interaction of ammonia with a triad of methylamine (a) and methanol (b) molecules and water interacting with the same triads (c and d). Numbers correspond to internuclear distances in angstroms and angles in degrees. The surface-mimic triad is arranged to match the minimum-energy conformation in a SAM (see text).
Collisions of Gases with SAMs no intrasurface hydrogen bonding is allowed. Furthermore, it should be noted that the structures and energies in Figure 4 and Table 3 correspond to the potential minima on the three surfaces. However, during actual collisions the system visits a variety of other regions of the highly multidimensional potential energy surfaces, which have not been described here. Despite these caveats, the PES calculations do highlight physically relevant trends that can serve as an approximation to the trends of the actual potentials. Below, the theoretical results are used to help construct an understanding of the dynamics that lead to the final energy distributions presented in Figure 3 and Table 2. IV. Discussion Gas-surface energy exchange and thermal accommodation have been explored for collisions of small polar and nonpolar molecules with ω-functionalized self-assembled monolayers. In general, we find that the final energy distributions are very broad, usually spanning over 20 kJ/mol in energy. The large distribution of final energies is due to the variety of possible impact parameters and molecular orientations for the impinging molecules. In addition to the broad nature of the final energy distributions, the data reveal that overall energy transfer is extensive for every gas studied. Figure 1b shows that, for Ne, the P(Ef) peaks at ∼6 kJ/mol and that virtually no atoms escape the surface (at 30° to the normal) with greater than 40 kJ/mol of energy. Previous experimental and theoretical studies have shown that the extensive energy transfer is due to the large number of low-energy modes available within the surface, such as chain wagging motions, torsions, and vibrations.19 However, the dynamics also depend markedly on the chemical and physical properties of the specific gas-surface interface. We find that the nonpolar gases, Ne and CD4, transfer more energy to the nonpolar CH3-terminated surface than to the polar, NH2and OH-SAMs. In contrast, the polar gases, D2O and ND3, have a larger tendency to thermally equilibrate with all of the surfaces studied. The following sections highlight the observed trends and provide a detailed discussion, augmented by potentialenergy calculations, of the properties that control the extent of energy exchange and thermal accommodation in these gas-surface collisions. Monolayer Rigidity: Ne and CD4 Scattering. Table 3 shows that the Ne PES minima for all three SAMs are relatively small and that they are all very similar, two characteristics that make Ne an excellent probe of monolayer rigidity, as influenced by the outermost functional groups. For such small potential energy wells, the dynamics are dominated by single collision events with residence times of only 0.75 to 1 ps for the initial conditions of our experiment.17-22 Such short interaction times for Ne limit the number of atoms along the alkane chains that can participate in the energy exchange event and the dynamics are governed by interactions with the terminal groups on these organic surfaces.17-22 In addition, the nearly identical (within 0.3 kJ/ mol) Ne PES minima for all three surfaces enables one to attribute differences in the scattering dynamics to differing intramonolayer properties of the surfaces, rather than the gas-surface forces. As described previously, the three SAMs employed in this work exhibit similar structures. For each SAM, the sulfur atoms are separated by 5 Å, the alkane chains are packed in an alltrans configuration, and the chains are tilted from the surface normal by 30°. However, the termini of the polar NH2- and OH-SAMs tilt toward one another to form hydrogen bonds that extend throughout the monolayer. Molecular dynamics simula-
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17277 tions show that the hydrogen bonding network is so extensive that nearly every OH group in an OH-SAM participates as a hydrogen bond donor and acceptor.6,7,11,10 Our results for Ne scattering suggest that this network has a significant effect on the final energy distributions for gas-surface collisions. Figure 3a shows that the dynamics for Ne scattering from the OH- and NH2-SAMs differ significantly from the nonhydrogen bonding CH3-SAM. Specifically, the 60 kJ/mol incident Ne atoms recoil from the CH3-SAM with an average of only 10.6 kJ/mol of energy, while Ne scatters from the polar SAMs with greater than 14 kJ/mol of final translational energy. This difference is likely the result of the hydrogen bonding network that renders the polar SAMs more rigid than the nonpolar CH3-SAM. These results are similar to prior observations for Ar scattering from a variety of ω-functionalized monolayers. Previous experimental studies suggested that the hydrogen bonding network that forms between alkane chains in OH-, NH2-, and COOH-SAMs creates a more rigid collision partner by restricting the types of low-energy extended motions that facilitate energy transfer on non-hydrogen bonding CH3or C(O)OCH3-SAMs.31-36 The role of intramonolayer hydrogen bonding in gas-surface collisions has also been explored through theoretical studies of Ar impinging on OH- and CH3SAMs.10 As observed for the Ne scattering data presented here, the molecular dynamics investigations demonstrate that the OHSAM is significantly more rigid than the nonpolar CH3-SAM.10 The overall dynamics governing CD4 scattering from the CH3-, NH2-, and OH-SAMs mirror the dynamics for Ne scattering. However, the overall energy transfer is much greater than that for Ne scattering. In fact, the P(Ef) distributions for scattering from each SAM display a maximum intensity close to 2.5 kJ/mol, the peak of a Boltzmann distribution at the temperature of the surface. The CD4/SAM potential energy minimum is deeper than the Ne/SAM potential. The greater gas-surface attraction leads to more efficient energy transfer and, most likely, a significant amount of trapping followed by full thermal accommodation on the surfaces, as indicated by the much larger BC fractions of CD4 with respect to Ne (see Table 2). Despite the larger potential and significant thermal accommodation for CD4 scattering on all surfaces, the effect of relative rigidity of the polar SAMs continues to play a significant role in the energy exchange for CD4. An interesting final observation about the polar SAMs is that, according to the results of the molecular-beam experiments, the NH2- and OH-SAMs have similar rigidities. For instance, the final translational energies of Ne scattering off these two surfaces are within experimental error bars of one another (14.1 ( 0.5 and 14.4 ( 0.5 kJ/mol for Ne/NH2-SAM and Ne/OH-SAM, respectively, Table 2). Given that the initial collision energy is the same in both experiments and that the difference in the potential energy surfaces seems to be very small (the well depth in the Ne-CH3NH2 system is only 0.15 kJ/mol shallower than that in the Ne-CH3OH system at the MP2/aug-cc-pVDZ level, see Table 3), the scattering results imply that the physical properties of the surfaces are very similar. Gas-Surface Attractive Forces: ND3 and D2O Scattering. The observed trends in the scattering dynamics for the polar gases, ND3 and D2O, are markedly different than the trends for Ne and CD4. We find that nearly the entire P(Ef) for the polar gases can be described by a Boltzmann distribution at the surface temperature. That is, very few molecules that collide with the NH2- or OH-SAMs escape without first thermally equilibrating on the surface. The PES calculations show that the large thermal components are due to the significant attractive forces between
17278 J. Phys. Chem. C, Vol. 112, No. 44, 2008 the polar gases and the functional groups at the interface. Large attractive forces enhance the overall energy exchange in two ways. First, during the approach to the surface, the molecules gain kinetic energy as they traverse the potential energy well. The PES calculations suggest that water molecules may gain as much as ∼60 kJ/mol if the potential energy minimum conformation is achieved. Even if the impinging molecules do not fall directly into the absolute minimum, the attractive character of the gas/surface interactions will result in an acceleration of the gases prior to collision. This acceleration results in a total kinetic energy immediately before impact that can be notably larger than the nominal energy of our beams. The molecules then collide with the surface to transfer some fraction of their total energy. Following collision, the molecules again traverse the potential energy well and decelerate as they recoil from the surface. The net result of the well is to enhance the fraction of incident energy, 60 kJ/mol, that is transferred to the surface. Second, the residence time on the surface for trapped molecules is exponentially proportional to the well depth. Longer residence times result in more collisions, which facilitate energy transfer and thermal accommodation. Our results indicate that these two effects are sufficient enough for ND3 and D2O collisions with the OH- and NH2-SAMs to result in near complete thermal accommodation for both surfaces. Furthermore, it appears that the rigid nature of the intramonolayer hydrogen bonding network of these SAMs is countered by the strong attractive forces. Molecules that would otherwise skip impulsively off the rigid surfaces are instead pulled into the well, where they dissipate their remaining excess energy through multiple collisions. Although attractive forces dominate the dynamics for the polar SAMs, the nonpolar CH3-SAM also produces final energy distributions that are composed primarily of low-energy molecules that appear thermal in nature. As for Ne scattering, the relatively large BC intensity for ND3 and D2O scattering from the CH3-SAM is due to the flexible nature of the chains within this monolayer that serve as an excellent energy sink for facile dissipation of the translational energy of the impinging gases. In addition, the BC fractions of ND3 and D2O are larger than those for CD4 and Ne because, as reported in Table 3, the polar molecules establish stronger interactions with the CH3-SAM than CD4 and Ne. Our observed trends in the scattering dynamics are very similar to those for previous molecular beam experiments involving collisions on polar and nonpolar liquid surfaces. Nathanson et al. reported that the final energy distributions for 55 kJ/mol NH3 and D2O scattering from liquid glycerol and squalane are both composed of large trapping-desorption components and that trapping is most likely for D2O impinging on the polar liquid, glycerol.1 Furthermore, their studies revealed that the opposite trend applies to the nonpolar gases. Specifically, the high-energy impulsive channel is much more prominent in the final energy distributions for Ne and CH4 scattering from the hydrogen bonding glycerol liquid than for these gases scattering from the pure hydrocarbon liquid.1 Our results suggest that, for both SAMs and liquid surfaces, the interfacial hydrogen bonding network that can form between polar surface groups creates a more rigid or glassy surface and makes thermal accommodation and trapping of nonpolar gases less likely than for collisions on nonpolar hydrocarbon surfaces. Impulsive Scattering. While the extent of overall energy transfer follows trends that can be understood based on minimum energy arguments and surface rigidity, the differences between the energy distributions of the impulsively scattered
Bennett et al.
Figure 5. P(Ef) distributions for the impulsive component (IC) for Ne and D2O scattering from the OH-SAM. The IC is determined by subtracting a Boltzmann distribution, fit to the low-energy portion of the data, from the overall P(Ef).
molecules do not follow such trends. Figure 5 provides a direct comparison between the P(Ef) for only the impulsively scattered D2O and Ne off the OH-SAM. Although these gases represent the extremes in terms of their polarity and hydrogen bonding ability, the final energy distributions are nearly identical for Ef > 12 kJ/mol (note that a normalization factor of 3.5 has been applied to the D2O distribution). This observation suggests that both gases experience similar dynamics for collisions that lead to the highest energy scattering. We find similar results for CD4 and ND3, suggesting the remarkable result that the dynamics for high-energy scattering depend little on the complexity of the gas. In addition, this result suggests that the potential energy surfaces that govern the dynamics of collisions leading to the highest final energies (i.e., single impulsive collision events) are very similar. If the potential energy wells were significantly different, simple kinematic descriptions of the dynamics show that the fractional energy transfer would be greatest for the molecule with the deepest well by a factor of (1 + V/Ei), where V is the well depth.45 Therefore, one may hypothesize that there exist particular orientations of the impinging D2O molecule that exhibit a similar potential energy as Ne when it approaches the OH-SAM. It may be that these approach geometries are responsible for the similar impulsive scattering distributions, for Ef > 12 kJ/mol, reported in Figure 5. Not unexpectedly from the highly directional nature of dipole-dipole interactions, calculations show that the H2O-OHSAM PES depends markedly on the orientation of the water molecule. Strong hydrogen bonding interactions and deep wells of ∼60 kJ/mol occur for the approach geometry depicted in Figure 4, but the PES for the water molecule orientated with its lone pair toward one of the oxygen atoms of the SAM mimic exhibits a very small potential energy well. We speculate that the orientation of the D2O molecules that scatter from the surface with the highest final energies are those that approach the OHSAM with an orientation that leads to weak attractions or even a repulsive interaction with the terminal groups of the monolayer. Therefore, incident water molecules that are oriented with the oxygen atom directed toward the O atom of the OH groups of the surface experience similar energy exchange as Ne atoms, because internal excitations may play a minor role in the energy exchange and the PES well depths for Ne and D2O, in this particular orientation, are similar. An important difference between Ne and D2O is that the latter can gain rovibrational energy during the collision. Previous studies have demonstrated the important role that internal
Collisions of Gases with SAMs excitation of a gas-phase molecule can play during interfacial energy exchange. Hase and co-workers and Nesbitt and coworkers have recently shown that translational-to-vibrational (TV) energy transfer is very inefficient in collisions with organic surfaces, but translational-to-rotational (T-R) energy transfer readily occurs.63-66 In fact, they find that for CO2 colliding with a liquid hydrocarbon, fluorocarbon, and model SAM, the molecules that do not trap on the surface scatter with rotational temperatures that can be as high as the final translational temperature. In the work presented here, if rotational excitation in D2O arises at the expense of translational energy, this effect should be reflected in differences between the final energy distributions for the polyatomic gases and Ne. However, Figure 5 shows very similar translational energy distributions for impulsively scattered D2O and Ne. Therefore, either rotational excitation is limited for impulsively scattered D2O, rotational excitation comes at the expense of energy transfer to the surface, or the molecules for which T-R energy transfer occurs are scattered to final angles not sampled by the fixed angle detector used in this work. Future experimental and molecular dynamics studies will probe additional scattering angles to learn about how the final translational and rotational energy distributions depend on the incident and final scattering angles. V. Summary Molecular beam studies of the energy exchange in collisions of Ne, CD4, ND3, and D2O with CH3-, NH2-, and OH-SAMs have been conducted to explore how polar and nonpolar gases interact with hydrogen bonding and pure hydrocarbon organic surfaces. The experiments show that the model OH-, NH2-, and CH3-terminated self-assembled monolayer surfaces are all efficient energy sinks for impinging gases. Greater than 50% of the incident translational energy of each gas (60 kJ/mol) is transferred into the low-energy modes of the monolayer for every system studied. The overall extent of energy transfer and full thermal accommodation depends on a balance between the rigidity of the SAM and the gas-surface attractive forces. The nonpolar gases, Ne and CD4, show that the NH2- and OH-SAMs are more rigid collision partners than the hydrocarbon CH3SAM. This result, in accord with previous rare gas scattering experiments and molecular dynamics simulations,10,31-36 is due to the intramonolayer hydrogen bonding network that forms between the surface OH groups. The network serves to anchor the terminus of each monolayer chain, thereby restricting many of the low-energy motions within the monolayer that are responsible for the more extensive energy transfer into the CH3SAM. However, the polar gases, ND3 and D2O, have a deep enough potential energy well to become trapped very efficiently on the OH- and NH2-SAMs and transfer essentially the totality of their collision energy, despite the rigidity of the monolayers. In addition to studying the energy transfer of small gases scattering from solid organic surfaces, these experiments further enable direct comparisons to the dynamics of the same gases when they collide with liquids. Previous experiments that probed collisions of Ne, CH4, NH3, and D2O with liquid glycerol, a hydrogen bonding liquid, and squalane, a liquid hydrocarbon,1 have revealed unique insight into the first steps of gas solvation into a liquid. These studies correlated the enthalpies and free energies of solvation with the final energy distributions of the gases to show that the “like dissolves like” principle can be used to predict the relative energy transfer and rate of approach to thermal equilibrium. The overall trends in the extent of energy exchange and the thermal accommodation fractions for the gas-liquid collisions are remarkably similar to the dynamics
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