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D2O Water Interaction with Textured Carboxylic Acid-Terminated Monolayer Surfaces Characterized by Temperature-Programmed Desorption and Molecular Dynamics Ronald L. Grimm, Douglas J. Tobias,* and John C. Hemminger* Department of Chemistry, UniVersity of California, IrVine, IrVine, California 92697-2025 ReceiVed: July 8, 2009; ReVised Manuscript ReceiVed: NoVember 18, 2009
We have used a combination of temperature-programmed desorption (TPD) experiments and molecular dynamics (MD) simulations to characterize interactions between water and carboxylic acid-terminated surfaces. In the TPD experiments, D2O water interacts with alkylthiol self-assembled monolayers (SAMs) comprised of 3-mercaptopropionic acid (C3 acid), 15-mercaptopentadecaonic acid (C15 acid), 16-mercaptohexadenoic acid (C16 acid), or two-component monolayers of these constituents. Water TPD spectra exhibit broad, firstorder desorption profiles with maximum desorption temperatures ranging from 168 K during desorption from the C16 acid to a maximum desorption temperature of 200 K when water desorbs from the C3 acid surface. For water desorption from the C15 acid and for the two-component surfaces, desorption traces adopt intermediate profiles between these two extremes. Desorption activation energies range from 42 and 50 kJ mol-1. In the MD studies, submonolayer concentrations of adsorbed water interact with slabs that simulate the one- and two-component SAMs employed in the TPD experiments as well as 4-mercaptobutanoic acid (C4 acid). The MD simulations are characterized by distributions of the water-carboxylic acid interaction energies and the orientation of the carbonyl groups. The water-carboxylic acid interaction energy distributions show excellent qualitative agreement with the desorption activation energy values determined from the TPD experiments. Analysis of the carbonyl group orientation from the MD simulations shows strong effects of SAM chain length and whether the SAM contains an odd or even number of carbon atoms. These “odd-even” and chain length effects support many of the results from the TPD experiments and the MD simulations. We discuss the effectiveness of employing MD simulations in concert with TPD studies as well as the atmospheric implications for the interaction of water with highly oxidized, multicomponent surfaces. Introduction Water on surfaces plays critical roles in biological and environmental chemistry, as well as catalysis.1-3 In the atmosphere, the presence of water on aerosol and other environmental surfaces determines which reactions occur, what active species are consumed, and the relative distributions of reaction products. Thus, characterizing the fundamental interactions of water with atmospherically relevant surfaces remains critically important. A century of investigations has provided significant insight into water interactions with environmental surfaces. Early studies by Langmuir estimate the adsorption of up to four monolayers of water on glass surfaces even at low relative humidities corresponding to arid conditions.4 Subsequent investigations using infrared spectroscopy, scanning probe microscopy, and photoelectron spectroscopy provide molecular-level insight into the structure and behavior of water on metals, metal oxides, and salts.5-8 Molecular dynamics (MD) and Monte Carlo calculations afford additional visualization of water interactions as well as adsorption isotherms.9-13 In particular, environmental organic surfaces represent a significant area of ongoing research for water interactions. Field studies note the presence of organic films on buildings,14 inorganic aerosol, fog, and rain.15 Organic surfaces are unique in that the surface is significantly altered by oxidation and photochemical reactions in the atmosphere.16 Such atmospheric processing may lead to changes in the water uptake and * Corresponding authors. E-mail addresses:
[email protected] (D.J.T);
[email protected] (J.C.H).
interaction behavior; however, reports diverge on the magnitude of these changes.17-19 Thus, water interactions with atmospherically relevant organic surfaces are an important avenue for ongoing research. Water interactions with highly oxidized organic surfaces represent a significant target for analysis. Numerous studies address water interactions with carboxylic acid-terminated surfaces,20-22 but questions remain regarding the interaction dependence on water concentration and acid surface morphology. Temperature-programmed desorption (TPD) experiments from this laboratory indicate that water desorbs from selfassembled monolayers (SAMs) of 6-mercaptohexanoic acid with a desorption activation energy of ∼50 kJ mol-1,20 similar to the 45 kJ mol-1 energy of water desorption from 16-mercaptohexadecanoic acid monolayers found by Dubois and coworkers.21 Water desorption from these acid surfaces is characterized by broad profiles that shift to lower peak temperatures with increasing initial water coverage. These profiles do not fit simple models attributed to Redhead that treat thermal desorption with an Arrhenius-like rate equation, a constant preexponential factor, ν, and constant energy of desorption, Ea.23 The application of such models requires that ν and Ea are instead functions of water coverage on the acid-terminated surface. Infrared spectroscopy of water adsorbed to oxidized organic surfaces demonstrates coverage-dependent behavior.24-26 Nuzzo and co-workers note the 110 K deposition of water as an amorphous solid that undergoes a transition to polycrystalline ice during desorption. Concomitant with initial water adsorption, a decrease in intercarboxylic acid hydrogen bonding and an
10.1021/jp9064642 2010 American Chemical Society Published on Web 01/06/2010
Interactions of Water and SAMs
Figure 1. Steric hindrance and van der Waals interactions limit the orientation of end groups in alkylthiol self-assembled monolayers. Rotation around the last C-C bond leads to a limited range of orientations with respect to the gold substrate for chains with an even number of total carbons, but a wider range of orientations for oddchained SAMs.
increase in water-acid hydrogen bond formation lead to a frequency shift in the carbonyl stretching vibration.24 Despite this evidence for water coverage-dependent interactions, functional forms do not exist to describe the interaction energy between the water adlayers and a carboxylic acid-terminated surface. Such analysis would have a profound impact for understanding and modeling how water interacts with oxidized environmental organic surfaces. Quantitatively understanding this coverage dependency in part motivates the present investigation. Understanding the effect of carboxylic acid orientation on the interaction with adsorbed water similarly motivates these studies. One-component alkane carboxylic acid crystals exhibit an “odd-even” effect as an oscillation in melting point and density as the length of the alkane chain increases between an odd or even number of carbon atoms.27 In well-packed SAMs, this odd-even effect manifests itself as unique vibrational spectra due to different orientation of the headgroups and subsequent hydrogen bond formation.10,28,29 Figure 1 represents these unique orientations for highly ordered alkane carboxylic acid SAMs in which sterics and van der Waals interactions maximize the number of trans configurations and minimize gauche defects in the alkane backbone. In Figure 1 and throughout this manuscript, “even” and “odd” designations are based on the total number of carbon atoms in a SAM molecule. In addition to one-component carboxylic acid-terminated SAM surfaces, this manuscript reports the orientation effects in twocomponent acid surfaces. These physically “textured” surfaces are subject to fewer steric constraints than a well-packed onecomponent surface comprised of the cartoon molecules in Figure 1. Thus the carboxylic acid groups are free to adopt a wider range of configurations to maximize hydrogen bonding. Indeed, STM studies of mixed 3-mercaptopropionic acid (C3 acid) and 11-mercaptoundecanoic acid (C11 acid) SAMs show regular patterns that the authors attribute to the C11 acid bending over and forming head-to-head dimers with adjacent C3 acid molecules.30 Thus odd-even and orientation effects play a wide role in the behavior of one- and two-component carboxylicacid terminated SAM surfaces, and it is reasonable to suggest these effects extend to interactions with adsorbates such as water. The exploration of these effects also motivates the present investigation. In the work described here, we employ a combination of temperature-programmed desorption (TPD) spectroscopy and molecular dynamics (MD) simulations to characterize interactions between water and carboxylic acid-terminated organic selfassembled monolayer surfaces. For the TPD experiments, ambient-pressure, solution-phase self-assembly forms carboxylic
J. Phys. Chem. C, Vol. 114, No. 3, 2010 1571 acid-terminated monolayers. These monolayers are either oneor two-component mixtures of 3-mercaptopropionic acid (C3 acid), 15-mercaptopentadecanoic acid (C15 acid), or 16-mercaptohexadecanoic acid (C16 acid). Following SAM formation, ultrahigh vacuum (UHV) TPD experiments probe the interaction of these surfaces with adsorbed water. To supplement the experimental investigations, molecular dynamics simulations model water interactions with acid-terminated surfaces. For the calculations, assembled surfaces represent one- or twocomponent SAMs comprised of the C3 acid, 4-mercaptobutanoic acid (C4 acid), C15 acid, or C16 acid. Both the experimental and computational results show strong odd-even and chain length effects in the interaction with adsorbed water. In the TPD studies, water exhibits the weakest interactions with the C16 acid surface and stronger interactions with the C15 acid and C3 acid surfaces. Desorption of water from two-component monolayer surfaces demonstrates a straightforward trend in the shift of the desorption traces between those of water desorbing from onecomponent surfaces that comprise the mixed monolayers. Interaction energy distributions generated from the molecular dynamics simulations similarly show comparably weak interactions between adsorbed water and a C16 acid surface but stronger interactions between water and a C3 acid surface. The excellent qualitative agreement between the experiments and computational simulations elucidates much of the nuanced behavior of water interactions with carboxylic acid-terminated surfaces. Experimental Section Thiol Surface Preparation. Methods for thiol surface preparation and temperature-programmed desorption follow previously descried procedures.20 Briefly, a 1 cm diameter single crystal Au(111) disk (MaTecK GmbH, Ju¨lich, Germany) serves as the substrate for alkylthiol self-assembly. In ultrahigh vacuum, argon ion sputtering is used to clean the sample at 1 kV and 5 × 10-5 torr of background argon followed by 5 min of annealing at 800 K. Auger electron spectroscopy (PHI model 10-155, single pass cylindrical mirror analyzer) is used to verify cleanliness of the Au(111) substrate. Following cleaning, the sample is transferred through a fast entry lock from UHV to the ambient laboratory for solution-based alkylthiol derivatization. In control experiments, short exposures to laboratory air and solutions without thiols do not lead to significant contamination of the Au(111) surface. To establish a carboxylic acid-terminated SAM, the gold crystal is placed in a thiol solution for approximately 12 h. Thiol solutions consist of single or two-component mixtures of 3-mercaptopropionic acid (g99%, Sigma Aldrich), 15-mercaptopentadecanoic acid (97%, Sigma Aldrich), and 16-mercaptohexadecanoic acid (99%, Sigma Aldrich) that were all used without further purification. All solutions were freshly prepared with a total thiol concentration of 10 mM in ethanol (Rossville Gold Shield, 200 proof, Hayward, CA). Following functionalization, copious ethanol rinsing and blowing with nitrogen (ultrahigh purity, Airgas) preceded sample reinsertion into vacuum through a fast entry lock on the UHV chamber. Temperature-Programmed Desorption. All desorption experiments are carried out in the UHV chamber with base pressures below 1 × 10-10 torr. The thiol-functionalized gold disk is cooled to ∼100 K before water vapor exposure. We employ D2O water vapor rather than H2O for desorption experiments due to its lower background concentration in the vacuum chamber. An effusive doser exposes the cooled gold surface to adlayers of D2O at exposure rates of ∼1 langmuir (1 L, where 1 langmuir ≡ 10-6 torr s-1) every 10 s. During
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desorption experiments, the sample was heated resistively at 10 K s-1. A K-type thermocouple inserted into the gold sample measures the sample temperature, while a quadrupole residual gas mass analyzer (UTI, Model 100C) monitors D2O desorption from the thiol-functionalized surface. Molecular Dynamics Simulations. A LabView-based program generates the starting coordinates for the thiol slabs employed in the molecular dynamics simulations. These slabs are based on the 3 × 3 R30° alkylthiol assembly on Au(111) with a 2.884 Å lattice constant. The slabs consist of 300 thiol molecules in an x-y periodic simulation box of dimensions x ) 86.52 Å, y ) 74.91 Å, and z ) 240.00 Å. For the cases of two-component thiol surfaces with randomly placed molecules, a 0 to 1 random number generator in the LabView program determines which of the two thiols is placed at each location relative to a specific threshold value set at 0.25, 0.5, and 0.75. Thus, five simulations are generated corresponding to surfaces of 100% short thiols, 75% short/25% long, 50% short/50% long, 25% short/75% long, and 100% long thiols for systems of the C3 acid and the C16 acid. Repeating this process for all twocomponent systems comprised of C3 acid, C4 acid, C15 acid, and C16 acid generated a total of 22 carboxylic acid-terminated slabs. These slabs explore the effect of chain length and the odd-even effect in SAMs and their interactions with an adlayer of water. Following an initial energy minimization, a 1 ns trajectory in the molecular dynamics program NAMD allows the thiol slabs to equilibrate in the absence of water. NAMD was developed by the Theoretical and Computational Biophysics Group in the Beckman Institute for Advanced Science and Technology at the University of Illinois at UrbanaChampaign.31 All simulations ultilize an adapted CHARMM27 force field to model the thiol molecules32 and the SHAKE algorithm to constrain covalent bonds to hydrogen.33 The particle mesh Ewald (PME) method34 truncates van der Waals interactions with a cutoff distance comparable to the slab dimensions. The addition of corrugation and chemisorption potentials simulates the interactions between the sulfur and carbon atoms in the thiol chain and an Au(111) substrate that is not expressed in the simulations.35 During the simulations, water molecules occasionally desorb from the carboxylic acid-terminated surface. A repulsive wall 75 Å above the thiol surface reflects these water molecules back down to the surface.13 All simulations run with a 1 fs time step and Langevin dynamics maintain the 300 K temperature. Snapshots are saved at 1 ps intervals for further analysis. After the 1 ns trajectory in the absence of water, 116 water molecules are added above each of the slabs and the simulations run for an additional 1 ns. This quantity of water is comparable to the adsorption of ∼0.18 monolayers of water, assuming a monolayer packing density of 1 × 1015 molecules cm-2.36,37 We use TIP3P water as it is optimized for interactions with molecules described by the CHARMM27 force field.38 Simulations are considered to be in equilibrium based on the ongoing stability of the number of water-water, water-acid, and acid-acid hydrogen bonds throughout a simulated trajectory. Hydrogen bond cutoffs are O · · · O distances less than 3.0 Å and O-H · · · O small angles less than 30°. By the hydrogen bond metric, the water-single component thiol trajectories equilibrate in less than 0.1 ns, and the water-two component thiol trajectories equilibrate in less than 0.5 ns. Thus, statistics on the water-acid interactions are collected over the last 0.5 ns of these 1 ns trajectories. In separate simulations, 29, 58, 116, 232, or 464 water molecules are added above one-component acid
Grimm et al.
Figure 2. TPD spectra for the adsorption of water of D2O water onto a C16 acid surface. Individual traces correspond to the adsorption of 0.1-1.5 L water as shown in the legend. Desorption traces are characterized by broad profiles that shift to lower peak temperatures with increasing initial water dose.
slabs, and the trajectories are similarly run to explore the effect of water coverage on the water-water and the water-acid interactions. The orientation of the carbonyl on the carboxylic acid and the nonbonded interaction energy quantify the water-acid interactions in these simulations. For the carbonyl orientation statistics, we calculate the angles of each carbonyl bond normal to the plane of the simulated Au(111) substrate. For the interaction energies, the NAMDEnergy procedure in VMD39 collects water-total water and water-total acid nonbonded energy values for each individual water molecule in each recorded snapshot in each of the simulations. The raw outputs are binned into histograms to represent the distribution of carbonyl angles and interaction energies throughout the equilibrium portion of the computed trajectories. These distributions quantify the impact of organic chain length, orientation, and surface texture on the interaction of water with carboxylic acidterminated monolayers. Results Desorption From One-Component Carboxylic AcidTerminated Surfaces. Figure 2 presents TPD spectra following the desorption of D2O water on a C16 acid surface. Successive traces indicate the desorption of 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, and 1.5 L water adlayers with the trace corresponding to the desorption of 1.0 L shown in bold. As shown in Figure 2, water desorption from this carboxylic acid-terminated surface exhibits broad profiles with half-height peak widths typically 50-60 K for initial coverages less than 1 L. Peak desorption temperatures are roughly 180 K for the 0.1 and 0.2 L initial coverages, and this desorption peak shifts to lower temperatures with increasing initial water dose. This characteristic desorption pattern for water adsorbed to carboxylic acid-terminated surfaces has been attributed to strong hydrogen bonding between the carboxylic acid surface and the water adlayer.20,21,40 The desorption pattern shown in Figure 2 contrasts with water desorption profiles from other organic surfaces such as methyl-,20,21,41 alcohol-,21,41 or methyl ester-terminated surfaces.21 Water desorption from those surfaces is characterized by narrow,
