J. Phys. Chem. C 2008, 112, 2109-2115
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Dynamics of Water Adsorption onto a Calcite Surface as a Function of Relative Humidity Asif Rahaman, Vicki H. Grassian, and Claudio J. Margulis* Department of Chemistry, UniVersity of Iowa, Iowa City, Iowa 52242 ReceiVed: September 20, 2007; In Final Form: October 22, 2007
We have developed a molecular dynamics scheme in order to understand the dynamics of water adsorption on calcite surfaces as a function of relative humidity. In contrast to previous studies where either a monolayer or bulk water was assumed to cover the surface, we observe the formation of two to three prominent layers of water depending on the relative humidity. Due to the fact that these simulations are at room temperature, the distribution of water molecules on the surface is inhomogeneous and nonuniform. Our simulation results agree well with recent grazing incidence X-ray diffraction studies. The free energy of adsorption of a single water molecule onto the bare calcite (101h4) surface is predicted to be -10.6 kcal/mol at room temperature while the enthalpy for the same process is -21.3 kcal/mol. The time scale for the bare calcite surface to become in dynamic equilibrium with water vapor at 100% relative humidity is determined to be close to 6 ns, and the adsorption follows a BET (Brunauer-Emmet-Teller)-like isotherm, in that multilayers form. From our orientational distribution functions, we are able to determine the existence of three binding modes for water. The mobility of water adsorbed on the calcite surface is greater in directions parallel to the surface. Motion perpendicular to the suface is slower. The diffusivity of water significantly increases with increasing relative humidity.
1. Introduction Carbonate minerals, in the form of mineral dust aerosol, are reactive components of the Earth’s atmosphere.1 Carbonate mineral dust can readily react with trace atmospheric gases including nitrogen oxides, sulfur dioxide, and organic acid as they are transported through the atmosphere. These heterogeneous reactions are significantly enhanced, both in the rate and extent of reaction, in the presence of water vapor as the relative humidity (RH) increases due to the presence of increasing amounts of adsorbed water.2-5 In general, water can adsorb on the surface of mineral dust particles and therefore play an important role in heterogeneous atmospheric reactions. Adsorbed water can affect particle reactivity by (i) assisting in ion mobility on the surface; (ii) providing a medium for reactions; (iii) serving as a reactant in surface-catalyzed hydrolysis reactions; and (iv) in some cases, blocking surface sites for reactions to occur. Recent experimental studies suggest that water adsorption is not uniformly distributed on the surface of these particles. Water may preferentially cluster on the surface leaving bare surface sites. For example, in a study of ozone uptake on hematite particle surfaces, reactivity data suggested that at 23% RH about 12% of the hematite surface is not covered with water despite the calculated water coverage of two monolayers determined from BET (Brunauer-EmmetTeller) analysis.6 Further evidence for a nonuniform distribution of adsorbed water molecules on surfaces include an infrared study of water uptake by nitrated R-Al2O2 particles which shows spectroscopic evidence for both oxide-coordinated and watersolvated surface nitrate over a wide range of relative humidities, suggesting that the water coverage is nonuniform at RH values corresponding to multilayer coverages.7,8 In another vibrational spectroscopic study using sum frequency generation (SFG), a surface-sensitive nonlinear spectroscopic technique, was used * To whom correspondence should be addressed.
to detect isolated, non-hydrogen-bonded surface hydroxyl sites on R-Al2O3 at even higher RH, 54%, where the expected water coverage is three monolayers.9 Similar evidence for nonuniform adsorbed water layers was found on silica surfaces.10 Water uptake on another oxide, MgO, appears to yield threedimensional islands prior to coalescence and eventual surface saturation.11,12 Even though some of the nonuniformity in the water coverage of particles may be induced by imperfections on the surface, it is important as a first step to clearly understand from a molecular perspective the dynamics of adsorption as it happens at room temperature on a perfect crystal surface plane. In this work, we use molecular dynamics (MD) simulations to study the dynamics of water adsorption onto calcite, an abundant carbonate mineral. From our results we are able to derive time scales, length scales, and structural and dynamical information associated with the adsorption process. The (101h4) plane of calcite has been subjected to many experimental and theoretical investigations, as it is the most stable crystal plane of calcite.13,14 Many studies have been aimed at understanding crystal growth, dissolution, and structure of calcite and its polymorphs. To determine the surface structures of calcite, several studies have been carried out using a variety of surfacesensitive techniques.15 Using X-ray photoelectron spectroscopy and low-energy electron diffraction, Stipp and Hochella showed immediate formation of hydration species on a calcite surface prepared under various atmospheres and aqueous solutions.16 Kendall and Martin show similar hydration patches on the calcite surface with atomic force microscopy.17 Grazing incidence X-ray diffraction studies of the interfacial structure of the (101h4) plane of calcite showed a large shift in surface carbonate groups under dry conditions.18 Under humid conditions a laterally ordered monolayer of water was formed on the surface, and the truncated coordination site was filled by water molecules which formed hydrogen bonds with the surface carbonate groups.18
10.1021/jp077594d CCC: $40.75 © 2008 American Chemical Society Published on Web 01/17/2008
2110 J. Phys. Chem. C, Vol. 112, No. 6, 2008 Infrared studies of adsorption of water and ammonia on calcite particle surfaces as a function of heating temperature have been carried out by Neagle and Rochester.19 Theses studies suggest that the dissociative adsorption of water leads to isolated hydroxyl groups at the Ca+2 sites and surface bicarbonate anions. However, ab initio studies predicted that if a hydroxyl group was placed just above each of the surface calcium ions and a hydrogen atom was added to each surface carbonate group (to the surface oxygen atom),20 upon minimization the added surface hydrogen atom would displace to form a bond with the surface hydroxyl group generating water. This suggests that associative adsorption of water on the calcite surface is favorable, in contrast to experimental findings which appear to indicate the presence of hydroxyl species such as OH, H2O, and H3O+.19,21 The issue of dissociative versus associative adsorption of water on the calcite surface is still controversial, and our classical simulations do not attempt to resolve it. In this Article, we only focus on the nondissociative scenario. The energetics of adsorption of water onto the CaCO3 (101h4) surface have previously been investigated using MD simulations by de Leeuw and co-workers.22 Their study appears to indicate that the physisorption of water on all calcite surface planes is energetically favorable and, in agreement with experimental observations, the (101h4) surface is the most stable for adsorption. de Leeuw and co-workers observed bulk ordering with rotated carbonate groups on the surface layer, in agreement with experiments.18 The free energy of the adsorption of water and metal ions (magnesium, calcium, and strontium) onto the (101h4) calcite surface has also been previously determined using MD simulations.23 These simulations predicted the free energy of adsorption of water to be small compared with the calculated enthalpy, which implies a large entropic contribution. Furthermore, an ab initio surface phase diagram of the calcite surface has also been constructed using density functional theory. This study suggests that nonstoichiometric surfaces play an important role in the chemistry of calcite at high RH.24 The main drawback of previous MD or quantum chemical calculations on this system is that the interface was treated either as a monolayer or a bulk water-calcite interface. This leaves aside all dynamical processes associated with the adsorption and desorption of water on the surface. It is clear that if a system containing one or more monolayers of water on top of the calcite surface is studied at constant temperature and external pressure, a constant process of evaporation will occur until only the bare surface is left unless water is introduced into the system in the form of vapor. Studies in which this process is not taken into account are actually looking at a nonequilibrium situation in which a monolayer or a number of layers are not in equilibrium with the corresponding vapor but instead are in an atmosphere with no water vapor. In this study, we explicitly include the vapor pressure in the dynamics and observe the formation of a solid-liquid-vapor interface. From this, we derive the time scales on which this occurs and the persistence length scale for the surface water interactions. We also derive the equilibrium number of water molecules on the surface as a function of RH and are able to follow the dynamical processes of formation of “defects” or bare surface patches. To our knowledge, so far no theoretical investigation has been performed to understand the dynamical process of adsorption and corresponding thermodynamic equilibrium as a function of RH in this system. For this purpose, it is essential to have a model of water that will capture both the bulk liquid properties and the gas-phase dimerization energies. We use a polarizable model to correctly describe both gas-phase and liquid-phase limits.
Rahaman et al. 2. Simulation Methods 2.1. Potential Energy Surface. In this work, the total potential energy surface (V) for the adsorption of water onto the calcite surface is written as
V ) Vsurface + Vwater + Vsurface-water
(1)
Here, Vsurface, Vwater, and Vsurface-water are the corresponding potential energy terms for the bare calcite surface, the waterwater interaction term, and the potential of interaction between water and calcite, respectively. Several model potential functions are available for calcite and its polymorphs. de Leeuw et al.25 used a model potential which is based on the Born model for solids.26 In this model they included the polarizability of ions via the shell model of Dick and Overhauser,27 and other potential parameters were obtained from Pavese et al.28 Cygan et al. have developed the CLAYFF model for aqueous Ca2+ ions and octahedral CaO.29 Hwang et al. developed force-field parameters for calcite in order to study corrosion-inhibitor squeeze treatments. These authors optimized their parameters to reproduce vibrational frequencies, cell parameters, densities, and the compressibility of the calcite crystal.30 A different atomistic model was also developed by Dove et al. which includes short-range repulsive interactions, long-range Coulombic interactions, and two-, three- and fourbody potential energy terms.31 This model correctly reproduces the phonon frequencies, structure, and elastic constant for calcite as well as the lattice energy. We have chosen for our simulations the model developed by Dove and co-workers31 for Vsurface. As mentioned in the Introduction, previous simulations have used fixed charge models such as SPC or TIP4P in order to describe water. These models are only parametrized to reproduce bulk water properties and not the gas phase or interfacial conditions. The dipole moment of water dramatically changes as it goes from the gas phase to the bulk solution. Because our simulations attempt to describe the vapor-solid interface, we need a polarizable potential that can reproduce both the gasphase dimerization energies and bulk liquid properties. We have chosen the polarizable POL3 model32 of water since it well reproduces both limits. In our calculations, the interaction potential between carbonate ions, Ca2+ ions, and water molecules consists of short- and longrange terms expressed in terms of Lennard-Jones-type functions and Coulombic interactions. Dove et al. represented the shortrange repulsive interactions using only the Born-Mayer potential.31 Since we used the AMBER33 suite of programs for our MD simulations, we re-fitted Dove’s potential to the functional forms available in AMBER. All parameters used in our simulations are given in Table 1. Charges were kept the same as those reported by Dove et al.31 2.2. Molecular Dynamics Procedure. A calcite crystal with an exposed (101h4) plane was built using the crystal building software GDIS (Sean Fleming, http://gdis.sourceforge.net) with dimensions of 99.6, 99.11, and 10.63 Å. This surface, which is large compared with the ones used in previous studies, insures that finite size effects are kept to a minimum. The system contained four layers of calcium and carbonate ions with a total of 9600 atoms. In order to guarantee that the size of this crystal was sufficient and that no distortion of the lattice would occur, we used the AMBER suite33 to minimize the energy of the crystal while applying periodic boundary conditions. The optimized structure of the calcite crystal was very similar to that of the original, confirming that the system is large enough to keep its bulk-like structure and that the potential parameters for the crystal are accurate.
Water Adsorption onto a Calcite Surface
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TABLE 1: Table of Potential Energy Parameters atom type
description
charges
Ca CH OR HW OW
calcium ion carbonate carbon carbonate oxygen water hydrogen water oxygen
1.64203 1.04085 -0.894293
Bond Stretching: Kb(R-R0)2 i
j
Kb
R0
CH HW HW
OR OW HW
567.0 320.0 553.0
1.2826 1.0000 1.6330
Angle Bending: Kθ(θ-θ0)2 i
j
k
Kθ
θ0
OR
CH
OR
76.94993
120 Figure 1. Free-energy profile for the adsorption of a water molecule onto a bare calcite surface.
Improper Torsion: Kφ/2 [1 - cos(nφ - γ)] i
j
k
l
Kφ
γ
n
OR
OR
CH
OR
10.2
180
2
Nonbonded Terms atom type
Ri
i
Ca CH OR HW OW
1.7131 1.9080 1.6612 0.0000 1.7980
0.04598 0.08600 0.21000 0.00000 0.15600
As mentioned earlier in the Introduction, earlier studies only considered the oversimplified situation of either a single water monolayer or a bulk liquid-solid interface. At room temperature, a study of monolayers or multiple layers will run into problems due to naturally occurring evaporation. The equilibrium at the interface is dynamical and therefore in our study we considered the system in equilibrium with its vapor. The relative humidity (RH) is defined as
RH )
P × 100 (%) P°
(2)
where P° is the saturated vapor pressure of water at 300 K and 1 atm and P is the actual water pressure at the same temperature and pressure. In order to establish the correct solid-wetting layer-vapor interface, we introduced water molecules in the gas phase with velocities drawn from a Maxwell distribution and the condition that the z component (the one perpendicular to the calcite surface) is negative (this insures that newly introduced vaporphase water molecules are initially moving toward the surface in the correct direction for collision). Water molecules are introduced at a frequency dictated by the theoretical rate of collisions with the surface. For our surface size at 100% RH, the rate of collision is 0.33 ps-1. For 75 and 50% RH, the rates of collisions are 0.22 and 0.17 ps-1, respectively. This type of MD simulation is not totally new. Previously, Margulis et al. have used this technique to simulate ionic aggregates in the vapor phase.34 Any evaporating water molecule which is more than 30 Å from the surface is deleted from the system. In order to establish equilibrium conditions at particular RH levels, long simulations were carried out at constant temperature and volume using the algorithm developed by Berendsen et al.35 with temperature rescaling occurring every 0.5 ps. A time step of 1 fs was used for integration of the equations of motion in all our simulations. Vapor molecules introduced in the system
not only had random velocities but also random rotational orientations with respect to the surface. As the simulation proceeded, we monitored the number of water molecules in the system to determine the adsorption isotherm at points corresponding to different RH levels. Simulations were carried out until the rates of insertion and deletion of water were the same. At this point the number of water molecules fluctuated around a fixed equilibrium value. In the case of the simulations at lower RH, since thermodynamic equilibrium is independent of initial conditions, to reduce computational cost we choose to start our studies from the equilibrated system at 100% RH and lower the collision frequency to the corresponding values at 75 and 50% RH, respectively. A correct calculation of the diffusion constants for adsorbed water molecules must be carried out in the NVE ensemble. We therefore used final equilibrated configurations obtained from the 100, 75, and 50% RH simulations and carried out NVE MD simulations for 100 ps. During this short time, no appreciable evaporation occurred in the system. From these short trajectories we computed the ensemble averaged mean square displacements (MSD) of water molecules parallel and perpendicular to the calcite surface. Corresponding diffusion coefficients were established. 3. Results and Discussion We have used the weighted histogram method (WHAM)36-38 to determine the free energy of adsorption of a single water molecule onto a bare calcite surface. In order to do this, a water molecule was placed randomly at different positions on the bare surface and the geometry was minimized. Starting from the lowest energy configuration, MD simulations were run in which the water molecule was allowed to move freely along the x and y directions, i.e., parallel to the surface, but was constrained with a restraining potential in the z direction. The average over this calculation results in the free-energy profile shown in Figure 1. The free energy of adsorption of polarizable water on the bare calcite surface is estimated to be -10.6 kcal/mol. In order to compare with previous studies, we also computed the energy difference at 0 K between an energy-minimized water molecule on the calcite surface and an identical water molecule 10 Å above the surface. The energy difference computed in this way is -21.3 kcal/mol. References 20 and 39 report this value to be -22.4 kcal/mol. Our values are of comparable magnitude,
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Figure 2. Number of water molecules on the calcite surface as a function of simulation time at different RHs.
and the differences stem from the fact that we are using a polarizable force field and a different potential for the calcite surface. 3.1. Adsorption Isotherms. The time-dependent adsorption uptake calculated from our MD simulations is given in Figure 2a for the case of 100% RH. In our simulation, the initially bare calcite surface accumulates water as time progresses. When a dynamical equilibrium is established, the average number of water molecules entering and leaving the system is the same. The water build up on the surface is evident from Figure 2a. The number of water molecules reaches a plateau at about 6 ns. This time scale is relevant and is simulation-size independent since the rate of collisions is surface-area dependent and for a larger surface more collisions per unit time would occur, leading to the same equilibration time. The period of 6 ns is therefore a good estimate of the time it takes for a perfectly dry calcite surface with no defects to establish a solidwater vapor equilibrium interface after a sudden change to 100% RH. In order to study systems at 75 and 50% RH levels, we used the final configuration obtained from our 100% RH study and reduced the collision frequencies to 0.22 and 0.17 ps-1, respectively, which are the appropriate values at these RH levels. Partial water desorption occurred, leading to a new dynamical equilibrium. The number of water molecules as a function of time for systems coexisting with a vapor at 75 and 50% RHs is given in Figure 2b. In going from no coverage to full coverage or from 100% RH to 50% RH, the process occurs on a time scale of several nanoseconds.
Figure 3. Equilibrium configurations of adsorbed water at 100, 75, and 50% RHs. As the RH decreased, the number of adsorbed water molecules decreased. Only the water molecules are shown in Figures (a)-(c).
Figure 3 shows the typical thermal equilibrium configurations of water on the calcite surface at different RH levels. One of the obvious features that can be derived from these pictures is that the calcite surface is never fully covered by water. Even in a perfect crystal face at room temperature, there are fluctuations that lead to the presence of water vacancies (black holes seen in the above images (a)-(c)). These vacancies are dynamical and constantly appear and disappear throughout the simulation. At or close to 100% RH, three prominent layers of adsorbed water can be observed in our simulations. This is in agreement with earlier experimental findings suggesting isotherms to be
Water Adsorption onto a Calcite Surface
Figure 4. z-density distribution function of adsorbed water molecules perpendicular to the calcite surface and corresponding integrals.
Figure 5. Distribution of the angle formed between the resultant vector of water and the z axis at various RHs. Green + signs correspond to calcium ions, gray and red sticks represent carbonate ions, and water molecules are represented as white and red sticks. Figure 5b shows the definition of the angle θ. Arrows show typical orientations corresponding to the peaks in the distribution depicted in Figure 5a.
BET-like in that multilayers form. As we move away from the surface, water molecules on the third layer appear to be more similar in nature to those found at the bulk liquid-vapor interface. Liquid water in contact with its vapor will neither evaporate nor condense. Although we expect that the calculated vapor pressure of the POL3 model might be slightly dif-
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Figure 6. MSDs along z axis and along x-y plane at different RHs.
erent from that measured for water, the coverage obtained from our studies should be at least qualitatively accurate. In fact, if we increase the vapor pressure above 100% RH to a value of 135%, we find a constant increase in the number of water molecules on the surface and the slope in the curve of number of water molecules vs time is steep (graph not shown). This suggests the formation of an infinite liquid phase on top of the surface. 3.2. z Distribution of Adsorbed Water Molecules. Figure 4 shows the normalized density distribution F(z)/F0 in the cases of 100, 75, and 50% RH. Three maxima at 2.1, 3.2, and 4.6 Å above the surface can be observed. This result is consistent with grazing incidence X-ray diffraction experiments, indicating that water oxygen distances from the surface appear at 2.2 ( 0.1, 3.5 ( 0.2, and 4.6 Å. Closer inspection of our trajectories shows the presence of two types of adsorption modes: one in which water oxygens are bound to the calcium ion, corresponding to the 2.1 Å peak (first layer), and another in which one of the hydrogen atoms of water forms H bonds with surface oxygen atoms, corresponding to the 3.2 Å peak (second layer). Similar findings have previously been reported in theoretical studies.23 The third layer appearing at 4.6 Å is less structured, as can be appreciated from Figures 5a and 4. Integration of the water density distributions in Figure 4 indicates that for all RH studied, the number of molecules above the surface is almost identical up to about 3 Å. It is only above this distance that noticeable differences can be observed. Detailed analysis shows that within the first layer (density peak at about 2.1 Å) two different orientational conformations can be detected (see Figure 5a,b), one conformation in which the two hydrogens are pointing up away from the surface at 20° with respect to the z axis (normal to the surface) and another in which one of the water hydrogens is pointing down toward a carbonate ion at 65° with respect to the z axis. Parts a and b of Figure 5 clearly show that there are three preferential angular orientations for water on the calcite surface. The third orientation at 125° corresponds to water molecules in which an OH bond is pointing straight down toward a carbonate ion; this is the predominant conformation for water molecules in what we have defined above as the second layer. 3.3. Diffusion Coefficients. Diffusion coefficients for adsorbed water on the surface were determined from their corresponding MSD using the Einstein relations. In studying diffusion, we considered two different cases; the diffusion along
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TABLE 2: Linear Fits to the MSDs in the Interval of 5-10 ps slope of MSD (Å2/ps) perpendicular to the surface along z axis parallel to the surface along x-y plane
100% RH
75% RH
50% RH
4.1 × 10-2 ( 1.0 × 10-5 2.2 ×10-1 ( 2.1 × 10-4
1.2 × 10-2 ( 5.3 × 10-5 1.1 ×10-1 ( 4.3 × 10-4
1.1 × 10-2 ( 3.2 × 10-5 9.5 × 10-2 ( 1.9 × 10-4
TABLE 3: Table of Calculated Propertiesa relative humidity (RH) properties
100%
free energy (kcal/mol) no. of waters adsorbed height of first H2O layer diffusion coefficient (z)(m2/s) diffusion coefficient (x,y)(m2/s) a
75%
50%
1181 830 765 2.1 2.0 2.0 2.05 × 10-10 6.03 × 10-11 5.40 × 10-11
0% 10.8 NA NA NA
5.50 × 10-10 2.81 × 10-10 2.38 × 10-10 NA
NA indicates “not applicable” for the current simulations.
the z axis in the direction perpendicular to the surface and that in the x-y plane. Diffusion coefficients for water oxygen atoms were calculated using the following expressions:
〈(x(t) - x(0))2 + (y(t) - y(0))2〉 tf∞ 4t
D(x, y) ) lim
〈(z(t) - z(0))2〉 tf∞ 2t
D(z) ) lim
(3)
(4)
The MSDs perpendicular and parallel to the surface at different RHs are plotted in Figure 6. Linear fits to these functions give corresponding diffusion coefficients listed in Tables 2 and 3. Clearly, the slope of the MSD parallel to the surface decreases as the RH is lowered from 100 to 50%. This is because in the case with lower RH, most water molecules are in the first and second layers, which are more tightly bound to the surface. The diffusion coefficient perpendicular to the surface is 2-5 times smaller depending on the RH; this makes sense, since motion in this direction requires climbing the steep potential of mean force. 4. Conclusions In this Article, we have applied a new approach to study the dynamics of the formation of a solid-liquid-vapor interface between water and calcite. A back-of-the envelope calculation on the collision frequency of vapor molecules impinging on the surface tells us that any computational equilibrium study at room temperature must include the effect of water vapor since otherwise continuous evaporation will occur on a time scale compatible with the computer simulation. From our simulations we know that the time for the formation of an interface or for readjustment to a new environment with a different RH is several nanoseconds. In agreement with experiments, we predict the adsorption to be BET-like in that multilayers form and that at thermodynamic equilibrium, the first few water layers on the surface appear to be clearly structured. Our studies coincide well with recent X-ray experiments and explain the different modes of binding of water onto calcite. We are also able to extract density probability distributions as a function of the distance to the surface. It is interesting that the different peaks of our angular distributions can be qualitatively associated with molecules at different distances from the surface. This information is interesting since it cannot be obtained from experimental data such as grazing angle X-ray diffraction. Diffusion calculations show that water mobility on the surface at 100% RH is
greatly enhanced when compared with the situation at lower RH. At lower RH water molecules appear to be much more tightly bound to the surface; this can be derived from the fact that diffusion constants are low and that angular distribution peaks are narrow. This clearly indicates that molecules ocupy well-defined locations and orientations on the surface. Diffusion perpendicular to the surface is considerably hindered when compared with diffusion parallel to the surface. Water molecules moving perpendicular to the surface must have large initial velocities in order to climb the steep potential of mean force. Independent of the RH, all our simulations show that even in the case of a perfect surface, the water distribution on the surface is nonuniform. “Dry” patches constantly appear and disappear, and we would expect this to be much more pronounced in a surface with defects. The understanding that at room temperature parts of the surface are constantly being populated and vacated with adsorbed water is important and absent from previous studies. Experiments clearly show that reactivity in these and other surfaces has characteristics of both solution-like and drylike environments. Studies on larger surfaces including defects such as terraces and edges should be carried out to see how these play a role in nucleation and dry patches formation. We plan to undertake some of these challenges in the future. Acknowledgment. This research was funded by the University of Iowa and by Grant No. 05-2182 from the Roy J. Carver Charitable Trust awarded to C.J.M. A.R. and C.J.M. would like to thank Dr. Dale Swenson (U. of Iowa) and Prof. George Kaminski from Central Michigan University for many helpful discussions. This material is also based upon work supported by the National Science Foundation under Grant No. CHE0503854 (V.H.G.). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. References and Notes (1) Usher, C. R.; Michel, A. E.; Grassian, V. H. Chem. ReV. 2003, 103, 4883-4940. (2) Baltrusaitis, J.; Usher, C. R.; Grassian, V. H. Phys. Chem. Chem. Phys. 2007, 9, 3011. (3) Al-Hosney, H. A.; Grassian, V. H. Phys. Chem. Chem. Phys. 2005, 7, 1266-1276. (4) Prince, A. P.; Grassian, V. H.; Kleiber, P.; Young, M. A. Phys. Chem. Chem. Phys. 2007, 9, 622. (5) Santschi, C.; Rossi, M. J. J. Phys. Chem. A 2006, 110, 67896802. (6) Mogili, P. K.; Kleiber, D. P.; Young, M. A.; Grassian, V. H. Phys. Chem. Chem. Phys. 2006, 110, 13799-13807. (7) Goodman, A. L.; Bernard, E. T.; Grassian, V. H. J. Phys. Chem. 2001, 105, 6443. (8) Baltrusaitis, J.; Schuttlefield, J.; Jensen, J. H.; Grassian, V. H. Phys. Chem. Chem. Phys. 2007, 9, 4970-4980. (9) Liu, D.; Ma, G.; Xu, M.; Allen, H. C. Chem. ReV. 2005, 39, 206. (10) Xu, M.; Liu, D.; Allen, H. C. EnViron. Sci. Technol. 2006, 40, 1566. (11) Ewing, G. E. Chem. ReV. 2006, 106, 1511. (12) Foster, M.; D’Agostino, M.; Passno, D. Surf. Sci. 2005, 590, 31. (13) Blanchard, D. L.; Baer, D. R. Surf. Sci. 1992, 276, 27-39. (14) Didymus, J. M.; Oliver, P.; Mann, S.; Devries, A. L.; Hauschka, P. V.; Westbroek, P. J. Chem. Soc., Faraday Trans. 1993, 89, 2891-2900. (15) Liang, Y.; Lea, A. S.; Baer, D. R.; Engelhard, M. H. Surf. Sci. 1996, 351, 172-182. (16) Stipp, S. L.; Hochella, M. F. Geochim. Cosmochim. Acta 1991, 55, 1723-1736. (17) Kendall, T. A.; Martin, S. T. J. Phys. Chem. A 2007, 111, 505-514.
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