Influence of Hydrogen Bonding on the Structure of the (001

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Influence of Hydrogen Bonding on the Structure of the (001) Corundum−Water Interface. Density Functional Theory Calculations and Monte Carlo Simulations Jiří Janeček,†,‡ Roland R. Netz,*,⊥ Mathias Flörsheimer,§ Reinhardt Klenze,∥ Bernd Schimmelpfennig,∥ and Robert Polly∥ †

ENSTA ParisTech, UCP, 828 Bd des Maréchaux, 91762 Palaiseau Cedex, France Mines ParisTech, CTP, 35 rue Saint Honoré, 77305 Fontainebleau Cedex, France ⊥ Physik Department, Freie Universität Berlin, Arnimallee 14, 14195 Berlin-Dahlem, Germany § Hochschule RheinMain, Fachbereich Ingenieurwissenschaften, Am Brückweg 26, 65428 Rüsselsheim, Germany ∥ Karlsruher Institut für Technologie (KIT), Campus Nord, Institut für Nukleare Entsorgung (INE), Postfach 3640, 76021 Karlsruhe, Germany ‡

S Supporting Information *

ABSTRACT: Density functional theory calculations and classical Monte Carlo simulations are applied to study the behavior of water in contact with a hydroxylated corundum (001) surface. Using DFT with periodic boundary conditions at T = 0 K, we systematically study the influence of the number of water molecules on the surface geometry and on the structure of the contact water layer. Only little effect of the thickness of the water layer on the geometry of the surface hydroxyl groups is observed. On the other hand, the molecules in the second layer have strong influence on the arrangement of water molecules in direct contact with the solid surface. In order to mimic macroscopic systems at room temperature, we perform inhomogeneous MC simulations of model corundum surface in contact with the water phase modeled by SPC/E model. The water molecules are classified according to their hydrogen-bonded partners into several groups. It is found that the preferential orientation of interfacial water molecules is primarily determined by the type of their hydrogen bonding. The hydroxyl groups at the corundum surface can serve as hydrogen bond donor or acceptor, depending on their orientation. No surface hydroxyls are found to coordinate two water molecules at the same time. On the other hand, water molecules coordinated by two different surface groups appear in MC simulations.

1. INTRODUCTION Various functional groups at mineral surfaces control the sorption of solutes as well as the water ordering in the vicinity of the surface.1−4 The affinity of a molecule to the surface is determined not only by its direct interactions with the surface, but also by the mutual interactions between the solute and water and by the interactions between water molecules and the mineral surface. In this work we investigate the properties of the hydroxylated (001) corundum surface in contact with liquid water. Corundum has favorable optical properties and therefore is often studied by spectroscopic methods. It is also isomorphic with hematite, and the structure of its surface is similar to many important clay minerals, so it is a suitable model for more complex systems. Studies of gibbsite (form of aluminum hydroxide) proved that surface hydroxyls coordinated by two Al atoms are less prone to be charged compared to singly coordinated OH groups;5,6 one can thus assume also that the (001) surface of hydroxylated corundum (which contains only doubly coordinated hydroxyls) should be rather resistant to protonation/dissociation in a wide pH range. Thanks to these © 2014 American Chemical Society

favorable characteristics, the surface properties of corundum were extensively studied both experimentally7−18 and theoretically.19−35 Crystal truncation rod diffraction (CTR) and X-ray reflectivity (XR) methods were used to determine the water density profile above the surface and provided information on the interlayer spacing.9−12 Electron energy loss spectroscopy was employed to estimate the amount of hydroxyl groups at the corundum surface.7,8 There were major efforts applying sum frequency (SF) spectroscopy to the OH vibrational region of the corundum (001)/water interface.13−15 This nonlinear optical technique is sensitive to polar ordering and can provide information about the preferential orientation of water molecules and hydroxyl groups at the mineral surface. Valuable information can also be extracted from charging16,17 and zetapotential measurements.18 Received: October 9, 2013 Revised: February 25, 2014 Published: February 26, 2014 2722

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Apart from previous DFT studies20,24,25 and SF experiments,13−15 the coverage of a (001) corundum surface by a monolayer of hydroxyls is supported by electron energy loss spectroscopy.7 The preferred orientation of the OH surface groups follows from SF experiments13 and DFT calculations. For gibbsite (aluminum hydroxide with similar structure of (001) plane), two distinct orientations of OH groups were also found in bulk phase by vibrational spectroscopy;17 preservation of the two distinct orientations also for the groups at the surface is supported by the interpretation of titration experiments using the MUSIC model.5,16 It is known that the separation between the two outermost Al-layers and the O-layer is strongly reduced in the case of an Al-terminated (001) corundum surface; however, MC simulations,33 DFT calculations,24,26 as well as CTR experiments9 show that hydroxylation reduces the deformations and the separation between the outermost aluminum and oxygen layers is much closer to the bulk values than to those for relaxed Al-terminated surface.

The interaction between water and the corundum (001) surface was subject to several density functional theory (DFT) studies either using the cluster model19,20,22,26 or with periodic boundary conditions to mimic an infinite planar interface.23−25 All these DFT studies were limited to a relatively small number of adsorbed water molecules, corresponding at most to one monolayer.25,27 Classical atomistic interfacial simulations are applied in particular for systems containing simple or welldefined surfaces (e.g., diamond, graphite, or self-assembled monolayers), while studies concerning complex minerals are rare because the surface is typically affected by the presence of water.30,31 Several studies investigated the structure and stability of different crystal faces of corundum.32−35 Recently, Argyris and co-workers used molecular dynamics to compare the properties of Al-terminated and hydroxylated corrundum crystals in contact with water.28,29 In this work we first analyze the effect of the thickness of the water layer on the structure of a solvated (001) corundum surface using zero-temperature DFT calculations with periodic boundary conditions. Information about the arrangement of surface hydroxyl groups obtained from the DFT calculations is then used to construct a simplified corundum model for finitetemperature MC simulations. The MC simulations are applied to study the arrangement and hydrogen bonding (HB) of water molecules in the vicinity of the corundum surface. In order to gain deeper insight into the structure of interfacial water layers, we analyze the obtained results in terms of hydrated species,36,37 i.e., we distinguish the water molecules according to their hydrogen-bonded partners and determine the properties of interest separately for every group. Both approaches used in this work are described in the next section and their detailed description is provided in the Supporting Information. In Sections 3.1 and 3.2 we discuss the main achievements of both theoretical approaches, followed by a short summary.

3. RESULTS AND DISCUSSION 3.1. DFT with Periodic Boundary Conditions. In this section we present the results of DFT calculations of a hydroxylated corundum surface in contact with different numbers of water molecules. Geometric properties of aluminol groups and of water molecules discussed in this section are presented in Tables S3−S5 (in Supporting Information). In the first step of our DFT calculations we optimized the unit cell for bulk corundum. The unit cell parameters of the hexagonal cell were found to be a = 4.82 Å and c = 13.17 Å. These values compare well to the experimental lattice constants of a = 4.76 Å and c = 12.99 Å as well as to former theoretical reports.23−25 3.1.1. Hydroxylated Corundum (001) Surface in Contact with Vacuum. In the case of free hydroxylated corundum (001) surface three different inclinations of aluminol groups are observed: τOH = 89°, τOH = 19°, and τOH = 32°. The inclination (tilt angle) of an OH group is defined as the angle between the O−H bond and the surface normal (τOH = 0° corresponds to an O−H bond pointing directly toward the water phase, τOH = 90° to an O−H bond in the plane of the interface). The four groups with τOH = 89° can then be considered as flat groups, the remaining eight as steep. The three groups with different inclinations (τOH = 19°, 32°, and 89°) are distributed regularly in the surface, and thus all groups with the same value of inclination are parallel, but we should note that structures not showing this high symmetry have only marginally higher electronic energies (ΔE ≈ 10 kJ/mol per unit cell). Hence at room temperature, all these structures are populated. Thanks to the formation of a quasi-hydrogen bond between flat aluminol (as a proton donor) and one of the two steep groups lying in the same surface-cell, the bond length in the flat groups, rOH = 0.98 Å, is slightly higher compared to that for the two types of steep groups, rOH = 0.97 Å. The difference is comparable to the statistical uncertainty of these values; however, it appears systematically for all studied systems and it was reported previously by Ranea et al.25 In the classical view, the shorter OH bond in steep groups can be expected because of the attraction of partially positive hydrogen atoms by the partially negatively charged layer of oxygen atoms. 3.1.2. Corundum (001) Surface in Contact with One Monolayer of Water. For the calculation assuming one monolayer of water above the surface we added eight water molecules on top of the hydroxylated lattice containing 2 × 2 surface cells; this corresponds to a surface density σ = 8.7 nm−2, approximately 80% of the surface density observed by XR.12 Similarly as in a previous study,25 the water molecules form a

2. METHODS In this section, the principal features of the two employed techniques are described together with a short justification of the MC model for corundum surface. For a detailed description of both methods see the Supporting Information. We perform DFT calculations with periodic boundary conditions (PBC) for a hydroxylated corundum (001) surface in contact with 0, 8, and 26 water molecules (per unit simulation cell). These numbers of water molecules enable the formation of 0, 1, and 3 water layers above the surface. The calculations are done using the Vienna Ab Initio Simulation Package (VASP).38 The stoichiometry of the corundum lattice in these calculations in the direction normal to the surface plane can be expressed as Al−4 × (O3−Al−Al)−(OH)3. In lateral dimensions the unit cell contains 4 Al2O3 units per one layer, which corresponds to 12 hydroxyl groups at the surface. The surface hydroxyls are called also aluminols. In the model of a corundum surface used in MC simulations, we consider surface hydroxyls of two types 2/3 oriented in the direction normal to the surface (called steep groups) and 1/3 lying in the plane of the surface (flat groups); this ratio (exactly) and the orienations (roughly) correspond to the DFT results. For atomic charges we used the values resulting from previous DFT claculations.20 The thickness of the lattice is the same as in the case of DFT calculations (four stoichiometric layers + Al- and OH-layers at the ends); however, the lateral dimensions are 32.9795 Å × 28.5602 Å, with 144 surface aluminols, 96 steep and 48 flat. Between two such corundum lattices a block of 2000 SPC/E water molecules is placed and MC simulations in NPzAxyT ensemble are carried out at T = 300 K and T = 325 K. 2723

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of eight), i.e., only 25% of steep aluminols serve as HB donor, while all (four) flat aluminol groups form HBs with water molecules of the first layer. Two of the water molecules of the first layer are located at separation dz > 3.0 Å above the surface and do not form hydrogen bonds with any aluminol group. The average distance from the surface for the S-bonded species, dz = 2.73 Å, becomes closer to that for F-bonded species, dz = 2.72 Å . We again have not observed any doubly coordinated species. The OH bond lengths in the F-bonded water molecules are rOH = 1.00 ± 0.01 for both OH bonds and the bond angle is α = 106 ± 5°. For the S-bonded species we found rOH = 0.99 ± 0.01 and α = 108 ± 5°. The preferential orientation of both types of surface bonded water species remains similar to the case of only 8 water molecules above the surfaceboth OH bonds of the S-bonded water species are roughly parallel to the surface; the donor bond to flat groups is oriented toward this group. This gives a qualitative picture of the orientation of the dipole moments of both types of surface bonded water species (see Figure S3). The bond lengths and angles in both species slightly differ for the water molecules of the first layer, due to the additional interaction with the second water layer. The second layer forms at a separation z2 = 5.99 ± 1.00 Å, which is in good agreement with MC results as well as with the experimental results of Catalano.12 The large broadness of the second layer (and an inspection of Figure S1) indicates significantly less order in the second layer. 3.2. Monte Carlo Simulations. In this part we discuss the results obtained by MC simualtions of a block of 2000 water molecules at finite temperature placed between two model corundum lattices. 3.2.1. Wetting Properties. Using the same approach as in refs 39,40, we found the wetting coefficient (the cosine of the contact angle) to be k = +0.8 ± 0.3 at T = 300 K and k = +0.5 ± 0.1 at T = 325 K. The first value corresponds to a contact angle in rather wide range 0° ≤ θ ≤ 60°, and the latter to 53° ≤ θ ≤ 66°, much closer to the experimental values 43° for receding and 82° for advancing angle (at T = 293.15 K).41 Such a strong temperature dependency of the contact angle is interesting. We assume that the behavior of the system at T = 300 K might be affected by the crystallization of the hydration water layer induced by relatively high partial charges in the surface groups and also by the spatial confinement. This assumption is also supported by strong oscillations in the density profile for water oxygen sites in the central part of the simulation box at T = 300 K. In a previous study of model hydroxylated surfaces, the wetting coeffient was found to be strongly dependent on the inclination of the surface hydroxyls.40 While for OH groups with τOH ≈ 45° the surface can become superhydrophilic (k ≫ 1), a surface containing only steep (or only flat groups) can bewith the same surface density of hydroxyl groupseven hydrophobic (k < 0). An optimally inclined OH group can serve as a HB donor as well as as an acceptor simultaneously; steep or flat groups can fulfill only one of these functions. From this point of view, a relatively low wetting coefficient (with respect to high surface density of hydroxyls) is understandable. A slightly unexpected decay of the hydrophilic character with increasing hydroxylation of (001) corundum surface (i.e., from Al-terminated to completely hydroxylated) was reported also in measurements of Gentleman and Ruud.8 In contrast, Argyris et al.28 found both surfaces studied in their MD simulationsAlterminated as well as hydroxylatedto be superhydrophilic (θ

highly regular hexagonal arrangement. We again observe two distinct inclinations of steep groups; nevertheless, these two values are closer together. The value τOH ≈ 7° is found for groups forming a HB to water molecules, while the larger value τOH ≈ 17° corresponds to OH groups that are not hydrogenbonded to water. The tilt angles of the flat aluminols remain almost unchanged (τ = 93°). All the flat groups serve as HB acceptor, while only 50% of the steep donate a HB to a water molecule, similarly to observations in the MC simulations. The ratio 2:1 between steep and flat aluminols remains unchanged, in agreement with previous reports.20,22,24,25 The bond lengths in steep groups forming a HB with water molecules (rOH = 0.98 Å) are slightly longer compared to those which remain free (rOH = 0.97 Å). The length of the OH bond in flat groups is again larger, rOH = 0.99 Å. The distance from the surface is dz = 2.71 Å for water molecules bonded to flat aluminols (“F-bonded”) and dz = 2.81 Å for those located above steep groups (“S-bonded”). Ranea et al.25 reported only one single distance from the surface (dz = 2.70 Å) which is very close to our first value. The S-bonded water molecules are oriented with both OH bonds almost parallel to the surface. The F-bonded water molecules have one OH bond pointing toward the surface and the second one roughly parallel to the surface, in agreement with intuitive expectations. Donating a proton to a flat aluminol leads to elongation of the OH bond length in water molecule to rOH = 1.01 Å, while the second OH bond has a length rOH = 0.99 Å. The bond angle in these water molecules is αHOH = 102.7°. For S-bonded water molecules, the two OH bonds are equivalent with a length rOH = 0.99 Å and the bond angle is αHOH = 108.2°. For an isolated water molecule we found bond length rOH = 0.97 Å and angle αHOH = 104.5°. Although there is only one layer of water molecules present at the surface, all water molecules form four hydrogen bonds, one with the surface groups and three with neighboring water molecules. Hydrogen bonding with molecules of the same layer is the reason for the large differences in the bond angle in differently coordinated water species. We should also note that we do not find any water molecules bonded to two different surface hydroxyls, in contradiction with the results of our MC simulations. 3.1.3. Corundum (001) Surface in Contact with Three Water Layers. To see the effect of additional water layers, we added 18 more water molecules to the converged and optimized structure with eight water molecules. Subsequently, we generated more initial structures by moving one or two water molecules from the first layer to the ”bulk” phase (represented by the 18 added molecules). As in the previous case, the bond length in steep groups is affected by whether they form a hydrogen bond with a water molecule (rOH = 0.99 Å) or not (rOH = 0.97 Å). The tilt angle also has different values for hydrogen bonded (10° ≤ τOH ≤ 17°) and free (15° ≤ τOH ≤ 19°) steep aluminols. The increasing number of water molecules thus leads to convergence of the two originally discrete values, and the angular intervals for bonded and nonbonded steep aluminols overlap. Flat groups have a bond length of rOH = 0.99 Å and a tilt angle in the range 88° ≤ τOH ≤ 91°. The regular hexagonal arrangement of the water molecules (which was observed in calculations with eight water molecules) is slightly disrupted by the collective effects of the additional water layers. The number of water molecules coordinated by steep aluminol groups is reduced to two (out 2724

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molecules bonded to surface groups can also differ in the number of remaining HB partners (neighboring water molecules)that is why we speak of different familiesfamily of S-bonded species (S-family), F-family, and FS-family. The density profiles for the three families of surface bonded species are shown in Figure 2 together with the total water density

= 0°). The difference follows from our simplified corundum model, which contains only ideally steep and flat aluminols, while their model has wide distribution of inclination angles for OH bonds. 3.2.2. Density Profiles. The density profile of the water oxygen sites has a distinctive first maximum corresponding to the layer of water molecules adsorbed at the surface (i.e., hydrogen bonded to surface aluminols). The positions of the first two hydration layers are in good agreement with the experimental and DFT results (see Tables S3 and S4 of SI). We compare the density profile obtained by X-ray reflectivity (XR)12 with our MC results for T = 300 K in Figure 1.

Figure 1. Comparison of oxygen density profile obtained by MC simulations (solid black curve) with the density profile determined by means of XR measurements by Catalano12 (dashed red curve); the dotted line shows the convoluted MC density profile in order to mimic the profile based on electron density. In this figure (as well as in Figures 2 and 4, z = 0 corresponds to the positions of the oxygen atoms of the surface aluminols and the water phase is located at z > 0. Figure 2. Density profiles for different families of surface bonded species and for the three most populated water species nonbonded to surface for T = 300 K (upper part) and T = 325 K (lower part). Different lines correspond to different hydrated species according to the inserted legend; the dotted curves show the density profile for water oxygen sites.

Although the position of the first maximum agrees well between the MC- and XR- results, the width and height of the peak differ significantly. Moreover, the second layer seems to be split into two peaks and a local minimum appears in MC profile at z ≈ 6 Å where the XR profile has maximum. These differences reflect the fact that the scattering of X-rays happens on electrons, while the presented simulated density profile is based on the position of the oxygen nuclei. The dotted curve in Figure 1 shows the convoluted nuclear density profile by approximating the water molecule as a sphere of diameter d = 2.38 Å (the position of first maximum at oxygen−oxygen radial distribution function for bulk SPC/E water) with uniform electron distribution within this sphere. Despite there being more sophisticated convolution methods (see, e.g., ref 42), this simple approach demonstrates qualitative accordance between the results of MC simulations and XR experiments. By integration the oxygen density profile between the solid phase (z = 0) and the first minimum, we find approximately 104 water molecules in the adsorption layer (per surface), which corresponds to a surface density of σ = 10.9 nm−2 (and σ = 10.5 nm−2 at T = 325 K). This value is slightly smaller than the value obtained from the XR density profile (σ = 12 nm−2). The surface density in the DFT calculations described in the previous section was σ = 8.7 nm−2. 3.2.3. Hydrogen Bonding. By analysis of the hydrogen bonds we found three types of water species forming HBs with surface aluminolsdonor to flat groups, acceptors from steep groups, and water molecules donating a proton to flat groups and accepting a proton from steep groups simultaneously. The

profile (dotted curve) and with the density profiles for fourfold hydrated water molecules (denoted as ”4W”) and partially hydrated water moleculeswith one missing acceptor (”free H”, thin dashed line) and with missing donor (having free electron pair, ”free EP”, dash−dotted line). In the upper plot, which shows the results for T = 300 K, one can see that oscillations of the oxygen density profile are stronger and survive for larger separations than at T = 325 K (lower part). This layering is an indication of possible freezing of the system at lower temperature discussed before. The surface densities and net dipoles for particular members of these three families are listed in Table S5 (SI). Similar to the case of water molecules in bulk phase, the surface bonded molecules have a strong tendency to be fully coordinated by four hydrogen-bond partners. The positions of the density maxima for the three families of surface bonded water species indicate that donation of a proton to a flat hydroxyl group requires close approach of the water molecule to the surface compared to accepting a proton from a steep aluminol. Different distances from the surface for different surface bonded water molecules were observed also in our DFT calculations with 8 water molecules, however, involving a higher number of water molecules in DFT leads to a closer approach 2725

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of the S-acceptors toward the corundum lattice (in contrast to the MC results). Almost 85% of the water molecules in the first layer form a hydrogen bond to one or two surface hydroxyl groups. From the point of view of the surface hydroxyls, almost 55% of the steep groups serve as a hydrogen bond donor for a water molecule (including also the species doubly coordinated by surface hydroxyls, i.e., the FS-family). More than 85% of the flat OH groups accept a hydrogen atom donated by a water molecule. The number of bonded steep aluminols in DFT calculations is 25%, while all flat groups accept HB. We mention again that in the DFT calculations no water species coordinated by two surface aluminols (i.e., FS-family) were observed and the appearance of these species in the MC simulations might be an artifact due to the simplifications of our MC model, probably from the neglect of the inclination of the steep group from the surface normal (which in DFT is 10° ≤ τOH ≤ 19°) or from ignoring the electronic polarization effects related to HB formation (affecting the partial charges on neighboring aluminols). 3.2.4. Preferential Orientation. The distributions of cos τOH for the S-, F-, and FS-families are plotted in Figure 3. For the

Figure 4. Z-projection of the molecular dipole for different coordinated species. The dashed (thick black) line shows this property for all water molecules irrespective of coordination. The schematic pictures of water molecules show the typical orientation of HB-donor to flat surface group (green) and of HB acceptor from steep group (red) with the molecular dipole (dashed arrow). For the (green) Fbonded molecule also the projection on z-axis is shown; in the case of (red) S-bonded water molecule, the molecular dipole is parallel to zaxis and the z-component is equal to the magnitude of dipole.

the second layer the contribution of these species becomes dominant but considerably smaller compared to the surface bonded species. The average z-component of molecular dipole for species in the bulk phase shows observable oscillations with a period approximately 3 Å up to z ≈ 20 Å. The overall zcomponent of the dipole moment (irrespective of the coordination) is shown as a thick black dashed curve. The dipolar density can be integrated to obtain the surface dipole densities for given species. As for the surface bonded species, at T = 300 K we find for the S-family PSz = −4.75 D/ nm2, for the F-family PFz = +4.19 D/nm2, and for the doubly 2 S/F/FS bonded species PFS = −4.61/+4.06/ z = +2.08 D/nm (Pz 1.89 D/nm2 for temperature T = 325 K). Values for particular members of these families are listed in Table S5 (SI). The dipolar densities for species not bonded to the surface oscillate and the resulting net dipoles are affected by high uncertainty. For the fourfold coordinated water molecules we find P4W = z 2 +0.1 ± 0.2 D/nm2 (P4W = +0.3 ± 0.2 D/nm at T = 325 K). z The net-dipole due to water molecules with missing donor is PfreeEP = −0.1 ± 0.1 D/nm2, and due to those with one z hydrogen site, free PfreeH = −0.2 ± 0.1 D/nm2 (PfreeEP = −0.1 ± z z freeH 2 0.1 D/nm and Pz = −0.1 ± 0.1 D/nm2 at T = 325 K). The positive net-dipole (solid-directed) due to F- and FSbonded water molecules is almost compensated by net-dipole of S-family which is of opposite sign. Nevertheless, the total net-dipole (∑P ≈ +1.3 D/nm2) is larger by an order of magnitude compared to the net-dipoles of partially hydrated water molecules from the bulk liquid. With respect to the spatial distribution of flat OH groups at the corundum surface, we can assume a rather homogeneous distribution of the positive and negative contributions to the net dipole. Our MC simulations thus support the assumption that the polar water ordering at the noncharged corundum (001) surface is dominated by hydrogen bonding rather than by electrostatics.13 We notice that the water ordering is crucial for adsorption of ions at the surface.4,6 In addition, only the vibration modes with

Figure 3. Orientation distribution of the water OH bonds for the three families of surface bonded species as obtained in MC simulations. The open symbols are for T = 300 K; the solid (small) for T = 325 K. The inserted scheme shows the definition of angle τOH. The schematic picture shows the definition of angles τOH; the definition of τOH for surface aluminol groups is analogous.

species donating a proton to a flat aluminol (both F- and FSfamilies), one of the OH bonds points toward the solid surface while the second one is oriented to the water phase with an angle around 20° to the plane of interface. In the case of the Sfamily, the distribution curve has only a single maximum around cos τOH = 0 which corresponds to the OH bond lying in the plane of the interface (this does not, however, mean that both OH bonds lie in this plane at the same time). In Figure 4 we show the course of the average projection of the molecular dipole on the z-axis for the water species of interest; the data are presented as the dipolar density per unit volume, in order to reflect the population of different species. In agreement with the orientational distribution (Figure 3), the contribution due to S-bonded family is negative (solid red curve), while those due to F- (dotted blue) and FS-families (dashed green) are positive. The dipolar projections for species not forming HBs with surface aluminols (i.e., for ”4W”, ”free H”, and ”free EP” in Figure 4) are vanishing in the region of the first maximum because of low concentration of these species. In 2726

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rather marginally; the most important is widening of the distribution of inclination angles in the surface hydroxyl groups. Despite gross simplifications in the present MC model of the corundum surface, some simulated quantities, like the wetting coefficient and density profile of water molecules, are in satisfactory agreement with experimental data. Both techniques indicate that most of the molecules in the first layer form hydrogen bonds to surface aluminol groups. Contrary to the DFT calculations, MC simulations indicate the existence of water molecules which have two hydrogen bonds to surface groups; however, these double-bonded species are in many aspects similar to water molecules forming (single) hydrogen bond to flat aluminols. Both techniques indicate that a large fraction of steep surface aluminol groups (50−75%) do not participate in hydrogen bond formation with water molecules. This is also supported by the SF spectra.

nonzero change of dipole moment with respect to the plane of the interface are active in SF spectra. Three main peaks are observed in SF spectra of aquatic interfaces ν̃ ≈ 3200 cm−1 (”ice-like”), ν̃ ≈ 3450 cm−1 (”liquid-like”), and ν̃ ≈ 3700 cm−1 (”dangling H”). The rather vague term ”ice-like” can be viewed at the molecular level as fourfold coordinated water molecules, while ”liquid-like” can represent molecules with perturbed hydrogen bonding. The dangling hydrogen is caused by the outermost water molecules which expose their free hydrogen atom out of the liquid phase (in the case of water/air or water/nonpolar interfaces); in the case of water/corundum system, this band is attributed to (steep) aluminol groups which do not form (donor) hydrogen bonds to water molecules. Because of small dependence of its intensity on pH (for pH = 9), Zhang et al.14 concluded that the aluminols do not form donor bonds with water molecules and can serve only as acceptors. Contrary to this, Braunschweig et al.15 observed the dangling bond only at the surface with nanocorrugations, while on smooth surface, it disappeared (nevertheless, this might be a consequence of strong conditions during the annealing). Our MC model is in agreement with Zhang et al. in the sense that it considers steep aluminols which do not form HB with water molecule (approximately one-half). On the other hand, we would assume a large difference in intensity of the dangling peak between corundum/air and corundum/water interfaces. We should also note that water molecules which donate HB to flat group while the second hydrogen remains nonbonded (denoted as ”XFww” in Table S5) have rather high net-dipole, and the free OH bond can also contribute to the peak at ν̃ ≈ 3700 cm−1. This might be supported by observation of an additional peak at ν̃ ≈ 3570 cm−1 with inclination τOH = 40° by Flörsheimer et al.13 Zhang et al.14 attributed the ”liquid-like” component to water molecules forming (donor) hydrogen bonds to aluminol groups and the ”ice-like” to the fully coordinated molecules of the second layer; rather low intensity of the ν̃ ≈ 3200 cm−1 peak (for neutral surface) is in agreement with our observation of low net-dipole of fourfold coordinated water molecules (denoted as ”4W” in Figure 4). We can only speculate about the effect of pH on the water arrangement, but in the most naive view, the flat groups are protonated at low pH and steep groups deprotonated at high pH. This will lead to a change of fractions of donor-acting and acceptor-acting water molecules in the hydration layer and modification of intensity and orienation of the ”liquid-like” peak. Similar models which assume few distinguished types of water molecules were proposed to interpret the results of different experiments related to mineral surfaces.18,43,44 One can assume that, in these cases also, the different types of water follow from different hydrogen bonding partners and the hydrated species concept can provide alternative insight.

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ASSOCIATED CONTENT

S Supporting Information *

Description of simulation techniques and detailed tables with obtained results. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Access to the MetaCentrum computing facilities provided under the research intent MSM6383917201 is (highly) appreciated. J.J. would like to acknowledge the support by Carnot Mines Institute and by Mines Telecom Institute and also thank dr. Patrice Paricaud (ENSTA ParisTech) and dr. Christoph Coquelet (Mines ParisTech) for providing hospitality and general support.

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REFERENCES

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4. CONCLUSION The corundum (001)−water interface was investigated using two different computational techniques, quantum chemical density functional theory and classical Monte Carlo simulations. The results provided by the DFT calculations with a varying amount of water molecules demonstrate the influence of the water molecules in the second hydration layer on the arrangement of the molecules in direct contact with the corundum surface. The structure of hydroxylated corundum (001) surface is affected by the presence of water molecules 2727

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