Microscopic Wetting of Mixed Self-assembled Monolayers: A

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J. Phys. Chem. B 2009, 113, 4161–4169

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Microscopic Wetting of Mixed Self-assembled Monolayers: A Molecular Dynamics Study† Mila´n Szo¨ri,‡ Douglas J. Tobias,§ and Martina Roeselova´*,‡ Center for Biomolecules and Complex Molecular Systems, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, FlemingoVo na´m. 2, 16610 Prague 6, Czech Republic, and AirUCI EnVironmental Molecular Science Institute and Department of Chemistry, UniVersity of California, IrVine, California 92697, USA ReceiVed: August 18, 2008; ReVised Manuscript ReceiVed: December 22, 2008

Molecular dynamics simulations are used to study the evolution of the organization of water molecules on the flat surface of well-ordered self-assembled monolayers (SAMs) of eight-carbon alkanethiolate chains bound to a gold substrate, as the character of the surface is finely tuned from completely hydrophobic to completely hydrophilic, and as the level of hydration is increased from submonolayer to the equivalent of about two monolayers of water. The hydrophilicity of the SAM surfaces is increased by randomly replacing methylterminated alkanethiolate chains with carboxylic acid-terminated chains. We report on the evolution of the structure of the surfaces of the SAMs, both in the absence and presence of water, and the organization of water molecules and the extent of wetting of the surfaces, as the fraction of hydrophilic groups is increased. The results suggest that on the flat organic surfaces with a small fraction of the hydrophilic components the hydrophilic spots serve as nucleation sites, resulting in the growth of a larger number of (smaller) water droplets compared to the completely hydrophobic surface, whereas on the surfaces with a large fraction of the hydrophilic component the uptake of water proceeds via a water film growing, at first, over the hydrophilic domains and, eventually, bridging over the hydrophobic patches, and spreading out over the entire surface. We discuss the implications of these processes on the properties of the organic aerosols in the atmosphere. 1. Introduction The wetting characteristics of organic surfaces are fundamentally relevant to a wide and diverse range of biochemical, environmental, and technological phenomena, from molecular recognition and binding processes in biology,1 which inspire the design of biomimetic materials including biosensors,2 to the hygroscopicity of aerosol particles in the atmosphere,3-5 which has implications for the role of particles in affecting climate by enhancing or suppressing cloud formation.6 The detailed nature (geometry, strength, range, etc.) of the interactions between water molecules and surfaces depends sensitively on a myriad of properties of the surface, including the polarity of the chemical moieties present on the surface, the size and pattern of domains on surfaces that contain domains of differing degrees of polarity, and surface roughness, to name a few. Indeed, quantitative measures of the wetting behavior of a liquid on a surface, for example, contact angles, are routinely used to infer surface properties.7 The arrangement and interactions of water molecules near a surface also depend on the amount of water present. At one extreme, which is relevant in certain atmospheric settings, there is less than a monolayer of water molecules on the surface. At the other, which is representative of the biomolecular case, the surface is completely immersed in water. In the case of relatively low coverage, the qualitatively different behavior of water on hydrophobic and hydrophilic surfaces is immediately evident on the macroscopic scale. For example, on the flat surfaces of well-ordered self-assembled monolayers SAMs prepared from long-chain alkanes on solid †

Part of the special section “Aqueous Solutions and Their Interfaces”. * To whom correspondence should be addressed. E-mail: martina.roeselova@ uochb.cas.cz. ‡ Academy of Sciences of the Czech Republic. § University of California.

substrates, water forms well-defined droplets with experimentally measured contact angles >100° on hydrophobic, methylterminated SAMs,8,9 whereas water spreads out and forms films with contact angles on the order of 10° on hydrophilic, alcoholterminated SAMs.8,9 Molecular dynamics (MD) simulations have demonstrated that essentially the same wetting behavior persists down to the nanometer length scale.10 Temperature-programmed desorption (TPD) spectra of water on methyl-terminated SAMs display narrow peaks that are relatively insensitive to coverage (up to roughly a couple of monolayer equivalents) at ∼145 K, which is consistent with a desorption activation energy of ∼34 kJ/mol.11 In contrast, on a polar, carboxylic acidterminated SAM, the TPD peaks are much broader, shifted to higher temperature, and coverage dependent, consistent with desorption activation energies of ∼45-50 kJ/mol and indicating significantly stronger water-surface interactions on the polar versus the nonpolar surface as well as interaction strengths that vary with the amount of water present on the polar surface.11 The arrangement of water molecules near hydrophobic surfaces immersed in water is a subject of ongoing research and discussion. Theory predicts that, due to hydrogen bond depletion, drying occurs at extended, flat hydrophobic surfaces.12,13 MD and Monte Carlo simulations have confirmed this prediction by revealing a layer of reduced water density near flat hydrophobic surfaces, such as those formed by aligned, densely packed hydrocarbon chains (e.g., self-assembled monolayers),14,15 or liquid alkanes.16 Interpretations of specular X-ray and neutron reflectivity experiments, which probe density inhomogeneities in the direction normal to a surface, generally agree that there is a reduction of water density near extended hydrophobic surfaces, but estimates of the range of the depletion vary widely, in part because of the varying influence of dissolved gases, which is difficult to control,17 as well as ambiguities in molecular

10.1021/jp8074139 CCC: $40.75  2009 American Chemical Society Published on Web 02/25/2009

4162 J. Phys. Chem. B, Vol. 113, No. 13, 2009 models used to fit the data.18-20 In any case, the consensus that seems to be emerging from recent theoretical and experimental investigations is that the depletion layer at flat hydrophobic surfaces exists, and it has a thickness that is roughly the size of a water molecule.21 Hydrogen bonding with polar groups leads to significant changes in the organization of water molecules near flat, hydrophilic surfaces versus hydrophobic surfaces immersed in water. MD simulations have predicted that the range of density depletion decreases and the amplitude of density oscillations increases as the fraction of hydrophilic hydroxyl groups on the surface is increased,15 and that there is a substantial overlap in the densities of surface atoms and water at SAMs with carboxylic acid terminal groups.22 The picture derived experimentally from reflectivity data is ambiguous, in part due to model dependence of the data reduction. One recent X-ray reflectivity study has convincingly demonstrated water density oscillations near ultraflat mica surfaces,23 but other studies have found negligible effects on water density near soft hydrophilic organic monolayers, and either enhanced or reduced water density near the surface, depending on the model used to derive the density from the scattering data.24 Although a detailed description of the organization of semiinfinite regions of water near flat surfaces is an important element in the overall understanding of water-surface interactions, practical applications (e.g., in biophysics and atmospheric and geophysical chemistry) require knowledge of the effects of heterogeneity in surface composition (i.e., mixtures of hydrophobic and hydrophilic groups) and surface roughness. MD simulations have shown that the presence of hydrophilic groups on a flat hydrophobic surface, either in patches or uniformly distributed on a two-dimensional lattice, leads to an enhancement of the water density near the surface in comparison with a purely hydrophobic surface.15,25 A combined experimental and simulation study of rough hydrophobic SAM surfaces showed that the amount of water adsorbed from the vapor increases with increasing roughness and that water accumulates as microdroplets in concave regions of the surface.26 Hydration at simultaneously rough and chemically heterogeneous surfaces has been investigated in the context of biomolecular association using MD simulations. Dewetting was observed near a large hydrophobic patch on the surface of the protein melittin,27 but that case seems to be the exception, rather than the rule.28 The ability of aerosol particles in the atmosphere to take up water is a key factor determining the role of particles as cloud condensation nuclei (CCN), which is a large source of uncertainty in predicting climate change due to human activity.6 Atmospheric aerosols contain a substantial fraction of organic material from a variety of biogenic and anthropogenic sources.29 As aerosol particles are oxidized (“aged”) by trace gases in the atmosphere, their organic components tend to become progressively more hydrophilic.3,30 It is anticipated that organic particles will become more hygroscopic, and hence more effective CCN, as they are aged. In addition, the presence of water opens the door for additional heterogeneous chemical processes on the surfaces of the particles.4,5 Thus, a detailed description of the changes in the interactions of water with organic surfaces as they are increasingly oxidized, and as a function of the amount of water present, is required for a better understanding of fundamental atmospheric chemical processes. Changes in the organization and interactions of water on SAM surfaces as a function of increasing hydrophilic character, imparted by presumably random substitution of terminal methyl groups with hydroxyl or carboxylic acid groups, have been

Szo¨ri et al. characterized by contact angle measurements, infrared reflection-absorption spectroscopy, and TPD spectroscopy.31-33 These experiments revealed non-monotonic changes in the strength of the interaction of water molecules, and the morphology of ice formed on the surface as the hydrophilicity is changed. A molecular scale picture of the evolution of water behavior on surfaces as a function of surface hydrophobicity/ hydrophilicity and of hydration level is presently lacking. To fill an obvious void, we have performed a series of MD simulations aimed at investigating the evolution of the organization of water molecules on flat organic surfaces, specifically, SAMs bound to a gold substrate, as the character of the surface is finely tuned from completely hydrophobic to completely hydrophilic, and as the level of hydration is increased from submonolayer to the equivalent of about two layers (assuming uniform distribution over the area of the surfaces). The hydrophilicity is increased by randomly replacing methylterminated alkyl chains with carboxylic acid-terminated chains. The progression of the completely hydrophobic to completely hydrophilic character of the systems studied was designed to mimic the chemical aspects of the oxidative aging of atmospheric organic material, which results in an increasing presence of the hydrophilic functional groups on the surface of aerosol particles as they are processed in the atmosphere. In this paper we report on the evolution of the structure of the surfaces of the SAMs, both in the absence and presence of water, and the organization of water molecules and the extent of wetting of the surfaces, as the fraction of hydrophilic groups is increased. 2. Methods 2.1. Systems. Molecular dynamics simulations were performed for a number of SAM systems in the absence of water as well as in contact with varying amounts of water. The SAMs were composed of eight-carbon-long alkanethiolate chains, the sulfur atoms of which were chemisorbed to a gold (111) substrate. To vary the hydrophobic/hydrophilic character of the SAM surfaces in a systematic way, two components were used to construct the SAMs: hydrophobic S(CH2)7CH3 molecules and hydrophilic S(CH2)7COOH molecules. A total of seven different SAM surfaces were studied. The completely hydrophobic and completely hydrophilic surfaces were modeled using onecomponent SAMs consisting only of the CH3-terminated and COOH-terminated molecules, respectively. The mixed surfaces were prepared by randomly substituting a given fraction (10, 25, 50, 75, and 90%) of the CH3 endgroups by COOH groups. Each SAM consisted of 256 alkanethiolate chains, arranged in a well-ordered, defect-free array of 16 × 16 chains with a surface area of approximately 22 Å2 per molecule. The simulation box dimensions were x ) 80.16 Å, y ) 69.44 Å, and z ) 90.00 Å. The z-dimension was perpendicular to the SAM/vapor interface. The (virtual) gold surface was located at z ) 0 Å. The vertical (z-direction) thickness of the SAMs was approximately 13 Å. The CHARMM27 all-atom force field34 was adopted for the hydrocarbons. This force field has been successfully applied to molecular dynamics simulations of selfassembled monolayers of alkanethiolates on gold in previous studies.35,36 The gold atoms of the substrate were not explicitly treated; the substrate lattice was instead used to define the bonding sites for the sulfur atoms of the alkanethiolate chains. The interaction between the alkanethiolate chains and the gold substrate was modeled using the adsorption potential and surface corrugation potential, following the work of Mar and Klein.37 In the simulations of the carboxylic acid-terminated SAMs, the COOH endgroups were taken to be protonated. The

Microscopic Wetting of Mixed Self-assembled Monolayers confinement of the closely packed COOH groups into a planar array is expected to increase their pKa relative to a bulk solution. Indeed, a Gouy-Chapman model of a dense Langmuir monolayer of long-chain carboxylic acid molecules has predicted that the extent of dissociation is 0.004 on pure water at 28 °C, corresponding to a bulk pH of 5.5.38 Under the conditions of our simulations, we therefore do not expect any of the COOH groups to be ionized in systems with 256 or fewer chains. Molecular dynamics simulations of the above-described SAMs in the presence of water were performed in order to characterize the adsorption of water onto the surfaces of varying hydrophobic/hydrophilic character and the structure and dynamics of the surface-adsorbed water under the conditions relevant to the atmosphere. To this end, different amounts of water (33, 99, 232, 429, and 819 water molecules) were introduced into the simulation box, corresponding to a range between a submonolayer coverage to an equivalent of about 2 water layers adsorbed on the SAM surface. Two additional simulations were carried out with 1953 and 4312 water molecules on the fully hydrophobic (methyl terminated) and fully hydrophilic (carboxyl group terminated) SAMs for comparison of the SAM-water interfaces in bulk-like water to the systems at lower levels of hydration. Water was modeled using the SPC/E potential.39 A reflecting wall (repulsive part of a Lennard-Jones potential) was placed in the xy-plane at z ) 80 Å to prevent the evaporated water molecules from crossing over the periodic boundary of the simulation box in the z-direction. 2.2. Simulation Details. All simulations were performed with a constant number of molecules, volume, and temperature (NVT ensemble) using the NAMD 2.6 program.40 The average system temperature of 300 K was controlled using the Langevin thermostat with the damping coefficient set to 1 ps-1. Periodic boundary conditions were applied in all three dimensions. The cutoff for the van der Waals interactions and the real-space part of the electrostatic potential was set to 12 Å. The long-range electrostatic interactions were calculated using the smooth Particle Mesh Ewald technique.41 The equations of motion were integrated using the Verlet algorithm with a time step of 1.0 fs. All bonds involving hydrogen atoms were constrained using the SHAKE technique.42 After 500 ps equilibration, 500 ps simulations of the neat SAM systems were performed in order to obtain their structural characteristics. Then, a given number of water molecules were introduced into the simulation cell. The combination of 7 different SAM surfaces of increasing hydrophilicity with 5 different amounts of water resulted in the total of 35 simulations. The initial conditions for these 35 trajectories were prepared using the results of a separate 1 ns long simulation of the hydrophobic (CH3-terminated) SAM in contact with a 12 Å thick layer of 1953 water molecules. For comparison, a simulation of the hydrophilic (COOH-terminated) SAM with the same amount of water was also carried out. From the final snapshot of the simulation of 1953 water molecules on the hydrophobic SAM, 5 configurations with different amount of water in each case were created by removing all water molecules with the z-coordinate of the oxygen atom above a certain threshold. Selecting the threshold values of z ) 14, 15, 16, 17, and 19 Å (i.e., keeping only those water molecules that were within approximately 1, 2, 3, 4, and 6 Å of the SAM surface) resulted in obtaining the initial configurations for 33, 99, 232, 429, and 819 water molecules spread evenly over the hydrophobic SAM surface. Identical initial configurations were then used for each given amount of water on all the various SAM surfaces. Upon energy minimization to remove potential overlaps between water

J. Phys. Chem. B, Vol. 113, No. 13, 2009 4163 and SAM atoms, the solvated systems were propagated for 3 ns. Atomic coordinates were saved every picosecond. The analysis was performed by averaging over the final 500 ps of each trajectory. 3. Results and Discussion 3.1. Neat SAMs. The density profiles resulting from the simulations of the various neat SAM systems, in the absence of water, are presented in Figure 1. In the SAMs with the lower hydrophilic content (10 and 25%, panels b and c, respectively), the presence of the hydrophilic component represents only a minor perturbation to the structure compared to the completely hydrophobic SAM (panel a). Visual inspection of the 10% hydrophilic SAM reveals that the hydrophilic component is present mostly as individual COOH-terminated chains embedded in the hydrophobic matrix of the CH3-terminated alkanethiolates. In the 25% hydrophilic SAM, about one-third of the hydrophilic component is present as individual COOH-terminated chains separated from each other by the surrounding CH3-terminated chains, and the remaining two-thirds of the COOH-terminated molecules form small hydrophilic nanodomains consisting most typically of 2-3 chains. With an increasing hydrophilic content of the SAMs, the size of the hydrophilic domains grows. In the 50% hydrophilic SAM, the size of the hydrophilic/hydrophobic domains is on the order of 10-100 chains, and further increase of the hydrophilic fraction leads to a reversed surface pattern, with small hydrophobic domains being surrounded by the mostly hydrophilic SAM surface. The systematic change in the SAM composition, from hydrophobic to hydrophilic, is accompanied by broadening of the terminal carbon (C8) peak in the density profiles (panels d-g) due to intermolecular interactions between the COOH end groups, including an increasing occurrence of hydrogen bonds between the neighboring chains (see below). To a smaller extent, the structural changes induced by the presence of the COOH groups propagate from the end group region also inside the SAMs. As a result, the maxima of the carbon peaks in the density profiles of the more hydrophilic SAMs are shifted toward slightly larger z values compared to the hydrophobic SAM, thus indicating that the hydrocarbon chains in the more hydrophilic SAMs are, on average, somewhat more stretched than in the more hydrophobic SAM systems. As the fraction of the hydrophilic component of the SAM grows, hydrogen bonding between adjacent COOH end groups becomes increasingly possible. The hydrogen of the COOH group can form a hydrogen bond either to the hydroxyl oxygen or to the carbonyl oxygen of a neighboring COOH group. In addition, dimers involving two O-H · · · OdC hydrogen bonds between two neighboring COOH groups may also be formed. This internal hydrogen bonding between the chains constituting the SAM induces reorientation of the COOH end groups from their original, nonbonded conformation and, in turn, contributes to the above-discussed structural changes in the SAMs with varying fraction of the COOH-terminated component. Figure 2 shows evolution of the density profiles separately for the hydroxyl oxygen (O(H), panel a), carbonyl oxygen (dO, panel b), and hydroxyl hydrogen (H(O), panel c) of the neat SAMs (in the absence of water) as a function of the increasing COOH content, from the completely hydrophobic SAM (depicted in light green) to the completely hydrophilic SAM (depicted in blue). In the 10% hydrophilic SAM case, the density profile maximum of both O(H) and dO occurs at z ≈ 12.2 Å, whereas the density profile of H(O) exhibits a maximum at z ≈ 13 Å. This corresponds to a situation in which individual COOH-terminated chains with dangling O-H bonds are em-

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Figure 1. Atomic density profiles along the interface normal for the set of SAMs with an increasing fraction of the hydrophilic (COOHterminated) component: (a) 0, (b) 10, (c) 25, (d) 50, (e) 75, (f) 90, and (g) 100%. Color coding: S - orange, C - blue, O(H) - light green, dO - dark green.

bedded in the matrix of the CH3-terminated molecules of the SAM, with no partner in their vicinity to hydrogen bond with. Although the density profiles of both O(H) and dO become broader with the growing fraction of the COOH component in the SAM, the position of the maxima of the O(H) and dO density profiles changes only slightly (∆z < 0.5 Å). However, another maximum at z ≈ 12 Å develops in the H(O) density profile and becomes dominant as the COOH content in the SAM surface increases. This feature is a signature of the reorientation of the COOH groups as they become increasingly involved in mutual hydrogen bonding.

3.2. SAM-Water Interface for Hydrophobic and Hydrophilic SAMs. Before proceeding to the investigation of wetting of the mixed SAM surfaces, we first carried out simulations of approximately 12 and 24 Å thick water layers in contact with the completely hydrophobic and completely hydrophilic SAMs. The corresponding density profiles are shown in Figure 3. Previous simulations of water near both hydrophobic and hydrophilic surfaces have shown that the water reaches its bulk density at a distance of at least 10-12 Å from the surface.22,26 In the two systems with 1953 water molecules (12 Å thick water layer), this distance coincides with the location of the liquid/

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Figure 3. Density profiles of water in contact with hydrophobic, CH3terminated SAM (a) and hydrophilic, COOH-terminated SAM (b). Color coding: S - orange, C - blue, O(H) - light green, dO - dark green. Water (oxygen atom) densities are shown for 12 Å thick water layer (black dashed line) and 24 Å thick water layer (red line).

Figure 2. Atomic density profiles of (a) carbonyl oxygen (dO), (b) hydroxyl oxygen, and (c) hydroxyl hydrogen along the interface normal for the set of SAMs with an increasing fraction of the COOH-terminated component, from 10% (light green) to 100% (blue). The numbers in the legend indicate the percentage of the hydrophilic component in each SAM.

vapor interface. The systems with 4312 water molecules (24 Å thick water layer) support well-defined bulk water regions between z ) 20 Å and z ) 32 Å. The water density profiles near the SAM surfaces (up to z ) 22 Å) are remarkably similar in all four neat SAM systems. Thus, the interfacial structure of a thin film (few layers) of water is essentially identical to that of bulk water in contact with hydrophobic and hydrophilic SAM surfaces. The water density profiles display oscillations that suggest that roughly two layers of water are structured by interaction with both the hydrophobic and hydrophilic SAM surfaces. The water density next to the CH3-covered SAM surface (Figure 3a) exhibits a short-ranged (∼1.5 Å) near-surface depletion followed by a maximum. This behavior has been previously observed in simulations of water near solid-like and liquid hydrophobic surfaces.15,16,26 The first peak as well as the subsequent, lower maxima in the water

density profile correspond to layers of increased water density and are followed by regions in which the water density is lower than the bulk value. This layering of water has been rationalized in terms of the orientation of water molecules induced by the hydrophobic surface. Although the water density near the hydrophilic surface (bottom panel of Figure 3) is qualitatively similar to that next to the hydrophobic surface (top panel of Figure 3), there are some noteworthy, subtle differences. There is essentially no near-surface depletion of water on the hydrophilic surface; on the contrary, the water density profile overlaps with the density profiles of the oxygen atoms of the COOH groups, indicating strong interaction with the SAM surface due to hydrogen bonding. This is also reflected in the first peak of the water density profile next to the hydrophilic SAM, which is higher, more narrow, and located at a lower z value than the corresponding peak next to the hydrophobic SAM. The second structured layer of water near the hydrophilic surface is less pronounced than near the hydrophobic one. The water layering observed here is in contrast to the more pronounced layering of an extended slab of water near hydrophilic versus hydrophobic surfaces observed in simulations by Janecek and Netz.15 Because both the surfaces are smooth, with tightly packed chains of the same length and no surface defects present, there is no penetration of water inside either of the selfassembled monolayers, beyond the interaction of water with the COOH groups of the hydrophilic SAM and occasional penetration of individual water molecules between the CH3 endgroups of the hydrophobic SAM surface. In addition to examining the effect that the above two SAM surfaces have on the water in their vicinity, it is equally interesting to look at the effect the presence of water exhibits

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Figure 4. Atomic density profiles along the interface normal in the end group region of the hydrophobic, CH3-terminated SAM (a) without water and (b) in contact with a water layer, and the hydrophilic, COOH-terminated SAM (c) without water and (d) in contact with a water layer. Color coding: C - blue, O(H) - light green, dO - dark green, water oxygen - red.

on the SAMs. Figure 4 shows the comparison of the density profiles in the end group region of the completely hydrophobic SAM and completely hydrophilic SAM in the absence of water (top two panels) versus in contact with the 12 Å thick water layer (bottom two panels). The hydrophobic SAM remains unperturbed by the presence of water (compare panels a and b of Figure 4). On the contrary, in the case of the oxidized (COOH-terminated) SAM, water induces rather large structural changes that propagate inside the SAM. Comparison of panels c and d of Figure 4 reveals that the COOH end groups are much more ordered in the presence of water, as indicated by narrower and more symmetric density profiles of both oxygen atoms (carbonyl oxygen in dark green, hydroxyl oxygen in light green), as well as of the carbon atom of the COOH group when in contact with water (Figure 4d) as opposed to a bare SAM (Figure 4c). This can be rationalized in terms of formation of hydrogen bonds between water molecules and carboxylic acid end groups, resulting in conformational changes (reorientation of the COOH groups) and/or breaking some of the COOH-COOH hydrogen bonds. In addition, as a result of wetting of the hydrophilic SAM, the maxima of the density profiles corresponding to the two carbon atoms immediately below the COOH end group are shifted by ∼0.5 Å toward smaller z value, thus indicating somewhat less-extended hydrocarbon chains compared to the nonwetted SAM. 3.3. Wetting of Mixed SAMs. The main result of this work is summarized in Figure 5, which shows the final snapshots from the simulations of all the different SAM surfaces studied in contact with increasing amount of water. In Figure 5, each column of the matrix of snapshots corresponds to one particular SAM. From left to right, the character of the SAM surfaces was varied in a systematic way between a completely hydrophobic surface (100% CH3-terminated SAM chains) and a completely hydrophilic surface (100% COOH-terminated SAM chains). The corresponding number along the x-axis indicates

the fraction of the (randomly distributed) COOH-terminated chains in each SAM. The hydrophobic component of the SAM is depicted in green, and the hydrophilic component is in blue. The rows of the matrix of snapshots in Figure 5 correspond to the five different amounts of water studied, from 33 water molecules (submonolayer coverage) to 819 water molecules (approximately 2 water layers). On the completely hydrophobic surface, ball-shaped water droplets are formed at all hydration levels, minimizing the interaction of water with the surface and maximizing the water-water hydrogen bonding. With the increasing fraction of the hydrophilic (COOH-terminated) component of the surface, water remains increasingly in contact with the surface, forming H-bonds with the COOH groups as well as with other water molecules. At low water coverage, transient clustering of water molecules occurs. For larger amounts of water at the more hydrophobic surfaces, the clusters grow into three-dimensional “islands” (irregularly shaped droplets). At the more hydrophilic surfaces, water spreads over the surface, although transient domains with depletion of water molecules occur, with the exception of the completely hydrophilic surface that is wetted by 819 water molecules. A simple and convenient way to quantify the degree of wetting of the SAM surfaces as a function of their composition and of the water load is to evaluate the area of the free surface (i.e., the surface of the SAM that is not in contact with water) using the solvent-accessible surface area (SASA) technique. In this study, the solvent-accessible surface was calculated using the Lee and Richards algorithm43 with a probe radius of 1.4 Å. The results are summarized in Figure 6. In the upper panel (a), the total area of the free surface (not covered by water) is depicted for the different SAMs as a function of water coverage. For each number of water molecules, the largest area of the free, nonwetted surface is found on the hydrophobic SAM due to the formation of water droplets on this surface. The free

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Figure 5. Matrix of snapshots from the SAM simulations as a function of the fraction of COOH-terminated alkanthiols (along the x-axis) and number of water molecules wetting the SAMs (along the y-axis). The fraction of nonwetted surface area of each system (see text for definition) is indicated below the corresponding snapshot. Color coding: methyl-terminated alkanethiols - green, COOH-terminated alkanethiols - blue, water oxygen - red, water hydrogen - white.

surface area decreases with growing fraction of the hydrophilic component of the SAMs, which makes it possible for the water to spread more over the SAM surface. For each particular SAM, the area of the free surface decreases with the increasing water amount. This decrease of the free, nonwetted surface is rather small for the more hydrophobic SAMs (0-25% hydrophilic content), where adding more water results predominantly in three-dimensional growth of water droplets. For the more hydrophilic surfaces (75-100% hydrophilic content), however, a rapid decrease with increasing hydration of the free, nonwetted surface area is observed as a result of water spreading over the surface. In the intermediate case of 50% hydrophilic and 50% hydrophobic mixture, the initial decay follows that of the hydrophilic surfaces, whereas for larger water loads (n > 232 water molecules) the slope of the curve changes, indicating the onset of three-dimensional (3D) water growth. To quantify the degree of wetting of the SAM surfaces as a function of their composition and of the water load, it would be convenient to express the size of the free surface evaluated by the SASA procedure in each case as a fraction of the total surface area of the SAM. Since the SASA algorithm is sensitive to surface corrugation on a scale corresponding to the probe radius, the surface area of the SAM detected by the probe will be different from a simple geometrical area corresponding to the product of the x- and y-dimensions of the simulation cell (approximately 80 Å × 70 Å ) 5600 Å2). This number represents a lower limit of the SAM surface area; the SASA procedure yields an area larger than this limit due to its sensitivity to the surface corrugation on the molecular scale. The surface area measured by the probe will also differ for the different SAMs depending on their composition, as the presence of varying fraction of the COOH component will induce changes in the surface corrugation. Finally, the SASA procedure may in principle yield different surface area for the same SAM, depending on the amount of water wetting the SAM surface in

each particular case. When calculating the total surface area of a SAM that is in contact with water, the water molecules adsorbed on the SAM surface are ignored by the SASA algorithm, and the surface area is evaluated as if no water was covering the SAM surface. However, the interaction between the water molecules and the SAM may result in conformational changes of the SAM chains and, thus, the surface corrugation detected by the probe will be somewhat different for the same SAM surface in contact with different amounts of water. The above-discussed effects are demonstrated in the bottom panel (b) of Figure 6 where the total surface area of the various SAMs is depicted as a function of the number of water molecules present on the surface. This analysis shows that the total surface area of the SAMs detected by the probe decreases with the growing fraction of the hydrophilic component of the surface. In other words, the surfaces containing more COOH groups are effectively smoother. The effect is, however, rather weak compared to the effect the increasing water coverage has on the free surface area of the individual SAMs (Figure 6a). For the majority of the SAMs, the measured surface area is not sensitive to the presence of water and remains constant over the entire range of water coverage within the error bars of the calculation ((40 Å2). The only case in which the interaction with the increasing amount of water induced changes in the surface corrugation that are somewhat beyond the error bars of the calculation is the SAM with 10% of the COOH component. However, the change is rather small (∼100 Å2). Therefore, on the basis of the analysis summarized in Figure 6b, the total surface area for each SAM was calculated as the average of the values evaluated by the SASA algorithm for the different water amounts. The total surface area of each SAM was then used together with the data depicted in Figure 6a to calculate the fraction of the exposed, nonwetted SAM surface in each of the systems studied. The results are given in Figure 5 under each snapshot. Thus, by monitoring the free surface area using

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Figure 6. Results of the SASA analysis: (a) Total area of the free surface (i.e., not covered by water) for the different SAMs as a function of increasing water coverage. (b) Total SAM surface area as detected by the probe for the different SAMs when in contact with the increasing amount of water. The color coding corresponds to the change of the SAM surface from completely hydrophobic (green) to completely hydrophilic (blue). The numbers in the legend indicate the percentage of the hydrophilic component in each SAM.

the SASA algorithm, it is possible to quantify the degree of wetting of each SAM as a function of the water load. Such an analysis provides a useful tool for determining the hydrophobic/ hydrophilic character of a surface in a well-defined, quantitative way. 4. Conclusions We have performed a molecular dynamics study of flat, defect-free self-assembled monolayers of alkanethiolates on gold, the surface of which was tuned from completely hydrophobic to completely hydrophilic by randomly replacing an increasing fraction of methyl-terminated alkyl chains with carboxylic acid-terminated chains. The microscopic wetting characteristics of these SAM surfaces have been investigated for the level of hydration ranging from submonolayer to the equivalent of approximately two water layers. Using the solventaccessible surface area (SASA) calculation, we have determined the degree of wetting of the SAM surfaces as a function of the SAM composition and water load. In the low water coverage regime, the behavior of water in the extreme cases of the completely hydrophobic and hydrophilic surfaces is dramatically different. On the completely hydrophobic surface, water forms droplets from the very beginning of hydration, and the droplets grow in size as the water coverage

Szo¨ri et al. increases. On the other hand, in the case of the completely hydrophilic surface, the onset of hydration corresponds to individual water molecules and small water clusters being spread out over the surface; with increasing hydration the water clusters grow into islands, until the entire surface is eventually covered by a thin water film. On the mixed SAMs, the behavior of water depends both on the composition of the SAM and on the amount of water wetting the surface. In the case of the very low water coverage, water spreads out over all SAM surfaces, interacting with the (randomly distributed) hydrophilic component of the surfaces as individual water molecules or small water clusters. On the SAMs with a larger fraction of the hydrophilic component, as the level of hydration increases, water continues to spread out over the (irregularly shaped) hydrophilic domains, avoiding at first the hydrophobic patches, until the growing thickness of the water film makes it possible for the water to bridge over the hydrophobic domains and cover the entire surface. This is in stark contrast to the behavior of water on the SAMs with smaller fraction of the hydrophilic component, on which the hydrophilic spots serve as nucleation sites for the growth of water droplets while the rest of the surface remains water-free. The results of this investigation provides a microscopic perspective on the wetting properties of mixed hydrophobic/ hydrophilic organic surfaces that could be helpful in developing models describing the hygroscopicity of organic material in the atmosphere. Organic matter is ubiquitous in the atmosphere and is found in a variety of forms, including aerosol particles composed primarily of organic molecules, marine aerosols with aqueous cores and surfactant coatings, and organic thin films on dust particles and urban surfaces.3,29 Organic material is expected to become more hydrophilic as it ages and undergoes oxidative processing in the atmosphere.3,4,30,44 The present study provides molecular-scale insight into the changes in wetting characteristics of flat organic surfaces as the hydrophilicity is increased, and hence constitutes a first step toward elucidating the factors affecting changes in hygroscopicity that accompany the processing of organic material in the atmosphere. A previous study has investigated the role of surface roughness on the wetting behavior of purely hydrophobic surfaces.26 The next step is to include more of the complexity of organics in the atmosphere by exploring the interplay of roughness and mixed hydrophobicity/hydrophilicity. To this end, in ongoing work in our laboratories, we are extending the present study to SAMs composed of mixtures of alkanethiolates of mixed chain length and both polar and nonpolar terminal functional groups. Acknowledgment. This work was supported by the Czech Science Foundation (grant 203/06/1488), the Ministry of Education of the Czech Republic (grant LC512), and the National Science Foundation through the AirUCI Environmental Molecular Sciences Institute (grant CHE-0431312). The work in Prague was performed within the framework of the research project Z40550506. The authors are grateful to Barbara Finlayson-Pitts, John Hemminger, Samar Moussa, Theresa McIntire, Ron Grimm, and Lukasz Cwiklik for helpful discussions. References and Notes (1) Ball, P. Chem. ReV. 2008, 108, 74. (2) Cooper, M. A. Nature ReV. Drug Disc. 2002, 1, 515. (3) Ellison, G. B.; Tuck, A. F.; Vaida, V. J. Geophys. Res. 1999, 104, 11633.

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