Interaction of Water with the Gypsum (010) Surface: Structure and

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Interaction of Water with the Gypsum (010) Surface: Structure and Dynamics from Nonlinear Vibrational Spectroscopy and Ab Initio Molecular Dynamics Jaciara C. C. Santos,†,§ Fabio R. Negreiros,‡,§,⊥ Luana S. Pedroza,‡ Gustavo M. Dalpian,‡ and Paulo B. Miranda*,† †

Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, São Carlos, São Paulo 13560-970, Brazil Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André, São Paulo 09210-580, Brazil

‡

J. Am. Chem. Soc. Downloaded from pubs.acs.org by LA TROBE UNIV on 12/03/18. For personal use only.

S Supporting Information *

ABSTRACT: Water−mineral interfaces are important for several environmental, industrial, biological, and geological processes. Gypsum, CaSO 4·2H 2 O, is a widespread mineral of high technological, medical, and environmental relevance, but little is known about its surface structure and its interaction with water. A molecular-level understanding of gypsum/water interface is given here by a combined experimental/theoretical study. We investigate the structure and dynamics of water adsorbed from vapor on the gypsum (010) single-crystal surface at room temperature, combining sum-frequency generation (SFG) vibrational spectroscopy experiments and ab initio molecular dynamics (AIMD) simulations. The SFG spectra of gypsum at low relative humidity (RH) show an anisotropic arrangement of structural water molecules and the presence of dangling OH groups. The AIMD simulations allow a detailed assignment of the SFG spectra and show that the cleaved (010) surface rearranges to have only 25% of the OH groups pointing away from the surface. At higher RHs, the first adsorbed water layer binds to these OH groups and forms an anisotropic arrangement, but with the amount of free OH groups significantly suppressed and without any significant diffusion. Upon adsorption of a second water layer, although the topmost layer of molecules is more disordered and dynamic than the previous one, its structure is still influenced by the gypsum surface underneath because it has a much reduced amount of free OH groups with respect to the free surface of water, and a slower surface diffusion with respect to bulk water. The theoretical results corroborate the experimental ones and provide an accurate atomic characterization of the surface structure.

1. INTRODUCTION The peculiar properties of confined and interfacial water have been the subject of intense research for many years1−5 due to its importance in several geological, chemical, and physical processes. In particular, water−mineral interfaces are important for a variety of environmental, industrial, biological, and geological processes,6 such as mineral flotation,7 oil recovery,8,9 and cloud nucleation.10,11 Furthermore, a thin film of water covers most surfaces under ambient conditions, and understanding its properties is of great practical importance as, for example, they are known to affect triboelectricity generation and dissipation,12 with important consequences for thunderstorm generation13 and technological processes.14,15 Although adsorbed water has been the subject of extensive studies for a long time,15−17 only recent experimental techniques have the selectivity and sensitivity to study these interfaces to the last surface atomic layers. The traditional techniques employed to study interfacial water layers included ellipsometry,18,19 vibrational spectroscopy,20−22 and a combination of neutron23 and X-ray24−26 diffraction, reflectivity, and © XXXX American Chemical Society

scattering. More recently, other techniques such as scanning probe microscopies,27−29 photoelectron spectroscopies,30−32 and sum-frequency generation (SFG) vibrational spectroscopy33−37 have also contributed to our further understanding of such interfaces at the molecular level. From a theoretical point of view, ab initio molecular dynamics yields a more detailed atomic and electronic description of the water structure,38,39 and it has provided remarkable insights about adsorbed water layers.21,35,40−42 The interaction of calcium sulfate minerals with water is particularly important for industrial, medical, and biological processes. For example, it has been used in medicine as a bone graft substitute for over a century, because it mimics the mineral phase of bone and is resorbed at a rate similar to that of bone formation.43,44 Calcium sulfate has three crystalline phases with different hydration levels: a dihydrate (gypsum, CaSO4·2H2O), a hemihydrate (bassanite, CaSO4·H2O), and Received: September 12, 2018

A

DOI: 10.1021/jacs.8b09907 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society the anhydrous form (anhydrite, CaSO4).45,46 Gypsum is one of the most abundant naturally occurring minerals on Earth and can also be found on Mars.47 Upon different calcination processes, part of the structural water of gypsum is removed, yielding the hemihydrate commonly known as “Plaster of Paris”.48 This hemihydrate powder can be rehydrated to gypsum, usually leading to a low-cost microcrystalline and porous material (“plaster”) that is extensively used as a commercial construction material, with a global production of ∼100 billion kg per year.49 Although this form of gypsum is widely used, it is a relatively fragile material. Recently, a new hydration and compaction method has been demonstrated that yields high-strength gypsum parts, with mechanical performance similar to that of concrete.50−52 This extraordinary improvement has been attributed to the presence of confined water within the crystallites, serving as a “molecular glue” among the densely packed gypsum microcrystals.50−52 This may suggest that the interaction of water with gypsum could be particularly strong, and more detailed studies of water adsorption on gypsum surfaces (or water/gypsum interfaces) are important to verify this hypothesis. Gypsum has a monoclinic crystal structure consisting of bilayers of Ca2+ cations and tetrahedral SO42− anionic groups, which are separated by a bilayer of water molecules and are stacked along the b-axis. The adjacent bilayers of calcium sulfate are linked through weak hydrogen bonding, allowing the perfect cleavage of (010) faces (see Figure 1a). In the ac plane, the structure is also anisotropic, and the gypsum crystal is therefore optically biaxial. Water molecules in gypsum are

coordinated to a Ca2+ ion and are asymmetric, with two nonequivalent hydrogen bonds to sulfate groups and also different interatomic O−H bond lengths.53,54 Although there is vast literature on the bulk structure and properties of gypsum and related compounds,45,46,51,55−59 much less is known about its surface structure and its interaction with water. Most of the previous studies on gypsum surfaces were performed using atomic force microscopy (AFM) techniques. It was found60 that the cleaved gypsum (010) surface has two water molecules per unit cell and they remain rigid on the surface after cleavage, each probably with the oxygen coordinated to a Ca2+ ion and stabilized by making hydrogen bonds to the surface oxygen of sulfate groups. Each Ca2+ ion is also coordinated to two water molecules. Such a structure is consistent with an unreconstructed cleaved surface (ideal bulk termination), shown in Figure 1a. However, the AFM images cannot image the water H atom, so that the surface H-bonding network cannot be determined from these experiments. In agreement with this report, our calculations (see Figure S1) also show that there is a large energy stabilization of the (010) surface including two water molecules per unit cell with respect to the surface without structural water (desorbed upon cleavage), indicating that the surface structure is also hydrated as in the bulk crystal. Another AFM study by Finot et al.61 on cleaved (010) gypsum face under different relative humidities (RH) observed that the (010) face is highly reactive and there is a formation of “clusters/precipitates” in the range of RH 10−35%. Because AFM is not sensitive to chemical composition, this study could determine the nature of these “clusters”. However, they demonstrate that these “clusters/precipitates” on the surface were induced by the AFM tip, probably due to a thin water film dragged by the tip while scanning the surface. Later, they investigated the surface forces between different gypsum microcrystals in air using AFM.62 The study was conducted under various conditions of relative humidity, and it was observed that in all studied interfaces, increasing humidity increases the adhesion forces between the surfaces. Furthermore, the adhesion forces between faces of different structures are greater than those between similar faces. More recently, Jordan and Astilleros studied the hydration processes of gypsum under different conditions of temperature and humidity using hydrothermal atomic force microscopy (HAFM).63 They again observed the formation of similar “clusters/precipitates” in gypsum (010), but these were observed when increasing temperature. Given the importance of this material and the relatively few studies of its interaction with water, here we use a combination of SFG spectroscopy and ab initio molecular dynamics (AIMD) simulations to investigate the structure, H-bonding, and dynamics of the cleaved gypsum (010) surface with and without adsorbed water layers. SFG vibrational spectroscopy of OH stretch modes (which are highly sensitive to H-bonding) has submonolayer sensitivity, probes qualitatively the hydrogen-bonding network at the surface, and is also very sensitive to the orientation of the moieties contributing to the SFG spectrum. The calculated vibrational spectra from the AIMD simulations were directly compared to the SFG experiments, validating the simulations, which in turn provided additional insights into the molecular structure and dynamics that would be difficult to infer only from the SFG spectra. This study might provide insights on the potential properties of gypsum for ice or cloud nucleation, and on the role played by water in

Figure 1. (a) Left/right: A top/side view of the unreconstructed CaSO4·2H2O (010) surface (“frozen” bulk termination upon cleaving the gypsum (010) surface). In the top view, only the outermost layers are shown for visualization, and the side view is along the a axis. In (b), the same views are shown for the optimized surface. The water molecule that shows a major reorientation is highlighted by the green oval. Ca/S/H atoms are in cyan/yellow/white, respectively. Oxygen atoms of SO4 are in pink and of H2O are in red. B


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interpretation for the PPP polarization is more complex, because it has contributions from several χ(2) elements. Sample Preparation. In this work, a large transparent crystal of natural gypsum (selenite) from China was purchased and submitted to X-ray diffraction for structural confirmation. The SFG measurements were performed in a closed chamber equipped with a calcium fluoride (CaF2) window for optical input and output. The relative humidity (RH) in the chamber was measured with a hygrometer (Testo 605-H1) and varied by adjusting the flow of N2 gas bubbling through a container with ultrapure water. The samples were cleaved inside the experimental chamber under nitrogen flow before SFG measurements, obtaining a clean (010) surface free from atmospheric contaminants. To avoid interference upon reflection, the gypsum thickness was about 7 mm. The experiments were carried out in different azimuthal angles of the (010) gypsum face with respect to the plane of incidence. The reference direction (ϕ = 0°) was the c-axis in Comodi’s cell53 (used throughout this work), as illustrated in Figures 1 and 2. We have also verified that laser heating can be

the exceptional mechanical strength presented by gypsum produced by the hydration and compaction method.50,51

2. MATERIALS AND METHODS SFG Spectroscopy. Sum-frequency generation (SFG) spectroscopy is unique for its ability to obtain the vibrational spectrum of surfaces (or interfaces) even in the presence of resonant bulk absorption. Unlike other surface/interface-sensitive techniques whose selectivity is determined by the penetration depth of incoming field, its surface sensitivity originates from the selection rule of the secondorder nonlinear optical process, which is forbidden under the electricdipole approximation in the bulk of media with inversion symmetry.33,34 Thus, in this study, the contribution of structural water within the bulk gypsum crystal is suppressed, and all of the (few) adsorbed water layers will contribute to the SFG spectra, because neither will have a completely isotropic arrangement, as in bulk water. In SFG spectroscopy, pulsed visible (ωVIS) and tunable midinfrared (ωIR) beams are overlapped spatially and temporally producing a third beam with the sum-frequency (ωSFG = ωIR + ωVIS). The SFG intensity is proportional to the intensity of the incident beams and the square of the effective second-order nonlinear susceptibility χ(2) eff of the interface. Scanning the IR frequency through the vibrational resonances of surface molecules yields the vibrational spectrum of the interface. For the analysis of the SFG spectrum, χ(2) eff may be expressed as a superposition of a resonant contribution (sum of Lorentzian resonances with amplitudes Aq, resonant frequencies ωq, and widths Γq) interfering with a nonresonant background, χ(2) NR: (2) (2) χeff (ωIR ) = χR(2) + χNR =

∑ q

Aq ωIR − ωq + i Γq

(2) + χNR

(1)

The resonant terms are proportional to the surface density Ns of interfacial molecules, but are also very sensitive to their degree of orientational order. By controlling the input polarizations and detecting a specific output polarization, we can probe specific tensor (2) that are related to the molecular second-order elements χR,ijk hyperpolarizability tensor α(2) R,ξηζ by a coordinate transformation from the molecular reference frame (ξ,η,ζ) to the laboratory frame (i,j,k), as shown in eq 2, where the angular brackets denote an average over the molecular orientational distribution. Therefore, the average orientation of molecular groups can be deduced. A detailed description of this technique64,65 and of the orientational analysis66,67 can be found in the literature.

χR,(2)ijk = N

Figure 2. (a) Scheme of the SFG geometry in the laboratory frame (x,y,z). (b) Gypsum sample used in this work, showing a large cleaved (010) surface and the a and c axes, and a cleaved thin film from the (010) face using an adhesive tape. (c) Definition of the azimuthal angle ϕ of the crystal (c axis) with respect to the plane of incidence (x axis).

̂ j ̂ · η )( ̂ k ·̂ ζ )̂ ⟩αR,(2)ξηζ ∑ ⟨(i ·̂ ξ )( ξ ,η,ζ

(2)

neglected in our experiments by reducing the laser repetition rate to 2 Hz at high RH (∼90%) and observing no noticeable change in the SFG spectra. Ab Initio Simulations. All calculations were performed at the Density Functional Theory (DFT)68 level with the free CP2K software.69 We have adopted the Perdew−Burke−Ernzerhof (PBE)70 exchange-correlation approximation, with Grimme’s VDW corrections71 and the pseudopotentials of Goedecker, Teter, and Hutter72−74 with 1/6/6/10 electrons for H/O/S/Ca, respectively. Molecularly optimized basis functions DZVP59 and cutoffs of 500 Ry/ 50 Ry for the finest grid/relative grid for Gaussian mapping were used. Ab initio Molecular Dynamics (AIMD) were performed at 300 K for 40 ps with a NVE ensemble, after 10 ps of equilibration time using a velocity rescaling thermostat. A time step of 0.5 fs was adopted. The simulated gypsum structure is composed of 192 atoms in a monoclinic (super)cell, as illustrated in Figure 1a, and with the following optimized lattice parameters: a = 12.74 Å, b = 15.10 Å, c = 11.42 Å, β = 115°, where β is the angle between the a and c axes. This supercell is twice as large as the Comodi’s cell53 in the ac plane. The vibrational spectra were evaluated by the Fourier transform of the velocity autocorrelation using the Travis code.75−77 To simulate the gypsum (010)/water interface, we increased the supercell’s dimension in the b direction and added different amounts

The SFG spectra were obtained with a commercial spectrometer (Ekspla, Lithuania) equipped with a mode-locked Nd3+:YAG laser producing 32 mJ pulses of 30 ps duration at 1064 nm with a repetition rate of 20 Hz. These pulses pump an optical parametric amplifier and difference-frequency generation unit to generate tunable mid-infrared (IR) pulses of energy ranging from 100 to 200 μJ in the OH stretch region (2800−3800 cm−1). A nonlinear crystal is used to generate visible pulses at 532 nm (∼600 μJ). The IR and green pulses are then overlapped spatially and temporally on the sample interface, and the sum-frequency output is detected with a photomultiplier after spatial and spectral filtering, with each data point being an average of 100 laser shots, with a resolution of 3 cm−1. The spot sizes and incidence angles for the IR and visible beams are (0.50 mm, 55°) and (1.00 mm, 60°), respectively. The spectra were normalized using the reference signal from a z-cut quartz crystal. Molecular orientation was probed by analyzing the SFG measurements under SSP, SPS, and PPP polarization combinations (letters indicate the polarizations of SFG, VIS, and IR beams, respectively). For stretch modes of single OH bonds, the SSP polarization is more sensitive to groups with a significant projection of the IR transition dipole along the interface normal, whereas SPS is more sensitive to those groups with a significant projection of the IR dipole parallel to the interface. The C


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Journal of the American Chemical Society of water and/or vacuum. Four coverages are considered: 1 ML, which is the cleaved gypsum structure keeping one-half of the structural water bilayer hydrogen-bonded to the CaSO4 plane and with 11 Å of vacuum (illustrated in Figure 1a); 2 and 3 ML, including 13 Å of vacuum each; and 6 ML water without vacuum, periodically repeated in Z, which mimics the liquid water/gypsum interface. An illustration of the supercell and the different coverages is shown in Figure S2. Inside the gypsum structure there is a large amount of structural water that has a significant contribution to the high frequency window of the vibrational spectrum, between 3350 and 3600 cm−1. These water molecules, however, are not very sensitive to the surface coverage (see Figure S3), having an almost constant background contribution to the total vibrational spectrum in all cases. In addition, the SFG technique is selective to surface species. Therefore, all simulated vibrational spectra shown in this work include only the surface water molecules.

3. RESULTS AND DISCUSSION The “Dry” Surface. Because gypsum has water in its crystalline structure, we need to distinguish the contribution to the SFG spectra due to adsorbed water and surface structural water from an eventual bulk contribution (e.g., electric quadrupole).78 For that purpose, we have performed SFG measurements in the transmission geometry and also using isotopic substitution for the adsorbed water (D2O), which are described in the Supporting Information. These experiments demonstrate that there is significant bulk contribution only in the transmission geometry, and for the reflected SFG signal reported in this Article, the surface contribution from structural and adsorbed water dominates. We first studied the structure of the gypsum/vapor interface under nitrogen atmosphere at low relative humidity (RH 0.1%). In this case, we are probing only the outermost structural water molecules on the gypsum (010) surface, designated here as 1 ML coverage. Cleaving it at low humidity should yield a surface terminated by structural water, as this water is strongly bonded (see the Supporting Information) and to be removed would need heating to high temperatures.46,48 A first guess for its structure would be a half water bilayer of the crystal structure (shown in Figure 1a),60 with one free OH group per molecule pointing away from the surface and the other OH group with a more in-plane (slightly down) orientation, H-bonded to sulfate groups. Figure 3a shows the SFG spectra with SSP polarization for this interface acquired at different azimuthal angles (ϕ, Figure 2b). The sharp peak at 3672 cm−1 is assigned to the dangling OH groups of the gypsum (010) structural water, while the broad bands from 3100 to 3400 cm−1 are due to H-bonded OH groups. Fitting those spectra to eq 1 with frequencies and line widths as common parameters yields four resonances listed in Table 1, where it can be seen that the broad band at low frequencies comprises two peaks at 3228 and 3330 cm−1, and another weak contribution can be seen at 3575 cm−1, which overlaps partially with the free OH peak. We also observe low intensity bands at 2840 and 2918 cm−1, more evident for ϕ = 60°, 120°, and 150°. Although the natural crystal has a high degree of purity, it may also have some contaminants such as calcium carbonate, clay, anhydrite, etc. It is likely that these peaks arise from combination and overtone bands from calcium carbonate species in the samples.59 All contributions vary with azimuthal angle as shown in Figure 3b, which displays the amplitudes for the peak at 3672 cm−1 (free OH) and at 3228 cm−1 (Hbonded OH). The dangling OH peak has a maximum intensity at ϕ = ∼30° and minimum at ϕ = ∼120°, while the broad

Figure 3. (a) SFG spectra for the gypsum (010) surface cleaved under N2 atmosphere, RH 0.1%, with SSP polarization combination. The spectra were recorded at different azimuthal angles ϕ. (b) The fitted amplitudes A (see eq 1) were plotted versus azimuthal angle for the peaks at 3672 cm−1 (free OH) and 3228 cm−1 (H-bonded OH).

Table 1. Experimental and Calculated Vibrational Resonances for the 1 ML Water Coverage on Gypsum (010) and Their Corresponding Theoretical/Experimental Ratioa frequency

a

peak

calculated

experimental

ratio

δ2 δ4 δ3 δ1

3180 3310 3680 3750

3228 3330 3574 3672

0.98 0.99 1.03 1.02

All frequencies in cm−1.

bands attributed to H-bonded OH have approximately the opposite azimuthal dependence. They also present a distinct polarization dependence, which is different for H-bonded and free OH peaks, as shown in Figure S11. From these results, it D


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Journal of the American Chemical Society can be inferred that the arrangement of the structural water at the gypsum (010) surface is highly anisotropic. Although the SFG spectra of Figure 3a are in qualitative agreement with the ideally terminated bulk structure shown in Figure 1a, it is difficult to assign the observed peaks (except for the free OH peak at 3672 cm−1) and to infer the surface structure from the spectra. For that reason, we resorted to AIMD simulations. We first performed a geometry optimization of the bulk-terminated (010) surface illustrated in Figure 1a. The result of such optimization, shown in Figure 1b, gives a considerably restructured surface, whose density profile is displayed in Figure S6. In the nonrelaxed water terminated gypsum (010) surface (Figure 1a), one-half of the OH bonds point outward from the surface, while the other one-half point toward the sulfate oxygen atoms. Relaxation reduces the surface energy by making one-half of these molecules nearly planar, increasing the number of water−water hydrogen bonds at the surface layer. This relaxed structure of Figure 1b has only one-quarter of the OH groups pointing away from the surface (free OH groups). In Figure 4 we show the top and side views of a representative snapshot of the 1 ML AIMD run, together

Table 2. Average Hydrogen-Bond Lengths and Angles (OH−O) for Each Different OH Bond, As Illustrated in Figure 4, with Their Respective Standard Deviations bond α1 α2 α3 α4

average value (2.8 (1.7 (2.4 (1.8

± ± ± ±

0.3) 0.1) 0.3) 0.1)

Å/angle Å/angle Å/angle Å/angle

OH1−O OH2−O OH3−O OH4−O

→ → → →

(58 ± 4)° (170 ± 10)° (150 ± 10)° (170 ± 10)°

nonequivalent water molecules on the surface. As expected, there is a direct relation between the hydrogen-bond length and the vibrational frequency. In Figure 5a we quantify the distribution of the polar angle between each distinct OH bond and the surface normal.

Figure 4. For the 1 ML coverage, the total vibrational spectrum is the sum of four different contributions from four distinct OH bonds (δ1−δ4), illustrated in blue, cyan, magenta, and green. To illustrate these bonds, top (010) and side views of the optimized structure are shown on the left. The distances from each distinct hydrogen to its closest neighboring oxygen atom are also illustrated (α1−α4), and their values are reported in Table 2.

with the vibrational spectrum decomposed into contributions from each OH bond. The full vibrational spectra for all simulated structures are presented in Figure S7. We first notice that during the dynamics, the water is ordered, with the strongly adsorbed water molecules moving around their equilibrium positions. Indeed, one can distinguish four different types of OH bonds: a free OH pointing nearly up, in the direction of the surface normal (green δ1); a strongly bonded OH, pointing in the direction of a surface oxygen (blue δ2); a weakly bonded OH pointing in the direction of a neighbor oxygen water molecule (pink δ3); and a strongly bonded OH, pointing in the direction of a surface oxygen (cyan δ4). The average values of their hydrogen bond angles and lengths are shown in Table 2. The calculations provide a clear assignment of the vibrational spectrum to localized vibrations of the four uncoupled OH groups of the two

Figure 5. Probability distribution of OH group orientation normalized by the number of OH bonds as a function of the polar θ (left) and azimuthal φ (right) angles (defined above the top panel) for the 1 ML (a,b), 2 ML (c,d), and 3 ML (e,f) coverages. The height of the outermost oxygen layer of gypsum (Gn in Figure S2) was set as Z = 0. A white dashed line separates each ML of water. The assigned peaks δn are explained in the text.

Integrating along Z each of the three distinct regions, we find that approximately 1/4 of the OH groups are free and nearly point up (θ1 ≅ 24°), while 1/2 are strongly H-bonded and point slightly down (θ2 = θ4 ≅ 106°), and 1/4 of the groups are very weakly bonded and point slightly up (θ3 ≅ 73°). Similarly, the azimuthal angular distribution is shown in Figure 5b, and the contribution of each OH group is again highlighted, with their average azimuthal angles listed in E


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spectra, as their intensities are lower and the modes are strongly overlapping, which leads to larger uncertainties in obtaining the mode amplitudes from the fits to eq 1.79 This excellent agreement between the SFG spectra and their analyses with the calculated spectrum and structure obtained from the AIMD is a strong validation of the simulation methodology. Therefore, the AIMD simulations can be used to provide further insights into the surface structure and water adsorption that could not be obtained only from the experiments, in particular for the case of water adsorbed from vapor that will be discussed below. The “Wet” Surface. For a detailed study of water adsorption from vapor on the gypsum (010) surface by SFG spectroscopy, we have chosen the azimuthal orientations ϕ = 30° and ϕ = 120° using the polarization combinations SSP, SPS, and PPP. As seen in Figure 3a, for these orientations the SFG spectra are dominated either by the free OH (ϕ = 30°) or by the H-bonded OH (ϕ = 120°) peaks of the structural water. Because in SFG spectroscopy there is interference between neighboring peaks, this choice makes the spectral analyses simpler. Furthermore, by probing the polarization dependence of the SFG spectra, we hope to gain additional insight on changes in orientation for the adsorbed water. Selected spectra are shown in Figure 6 for increasing relative humidities (RH).

Table 3. It is important to notice that an equivalent configuration rotated by 180°, with the two δ1 and δ3 peaks Table 3. Average OH Group Azimuthal Angle with Respect to the [001] Direction, Obtained from the AIMD Simulation and from Fitting the Azimuthal Dependence of the SFG Amplitude of Figure 3b OH group

experimental

MD

H-bonded (δ2, δ4) free (δ1)

27 ± 3° 110 ± 3°

9 ± 8° 108 ± 11°

pointing in the opposite φ orientations, is degenerate so that the surface has a global C2 azimuthal symmetry. The SFG spectra for the gypsum (010) surface cleaved under N2 (Figure 3a) exhibit four main contributions, in excellent qualitative and quantitative agreement with the spectrum for 1 ML coverage from AIMD simulation, illustrated in Figure 4, with four distinct OH bonds (δ1−δ4). Table 1 displays a comparison between the experimental and calculated frequencies for the four resonances, where an excellent agreement between the calculated and experimental frequencies within 3% can be observed. Given the detailed orientational analysis of the calculated structure shown in Figure 5a,b, we can make further comparisons between the SFG spectra and the MD results. The line shape of the SFG spectra is very sensitive to the relative sign of the mode amplitudes in eq 1, which are related to the relative polar orientation of each OH group (up/down). If we analyze the spectra where all peaks have appreciable amplitudes (e.g., ϕ = 150° in Figure 3a), we note that the two peaks with lower frequencies (corresponding to δ2−δ4) appear with strong destructive interference between them and two peaks with higher frequencies (corresponding to δ1−δ3) also exhibit destructive interference between them. Such destructive interference occurs when the peak amplitudes have the same sign, implying that amplitudes for δ1 and δ3 have the same sign, and that for δ2 has the same sign as for δ4. By fitting the spectra with different relative signs between (δ1,δ3) and (δ2,δ4) amplitudes, the small but significant changes in the line shape (see Figure S8) allow us to conclude that amplitudes for (δ1,δ3) have opposite sign with respect to those for (δ2,δ4). Indeed, the AIMD simulation shows that δ1 and δ3 are pointing up, while δ2 and δ4 are pointing down. Another favorable comparison between the SFG spectra and the simulated structure comes from the azimuthal orientation of the OH groups. The mean orientation of the free and Hbonded OH groups in the gypsum (010) surface can be obtained from fitting the azimuthal dependence of the mode amplitudes shown in Figure 3b to a model for the orientation dependence of the SFG amplitude of individual OH groups. The model shows that for SSP polarization combination, the SFG signal is minimum when the OH group is parallel to the incidence plane (details in the Supporting Information). Therefore, the mean azimuthal orientations of the OH groups determined from Figure 3b are listed in Table 3, together with those obtained from the AIMD simulation that are shown in Figure 5b. The azimuthal angles are in excellent agreement for the free OH group, and a reasonable one for the H-bonded OH groups. This difference could be due to imperfect simulation of water with the AIMD functional used in this work, but most likely results from a poor determination of the H-bonded OH group orientation from the experimental

Figure 6. SFG spectra for the gypsum (010) surface with increasing relative humidity (RH) at azimuthal orientations (a) ϕ = 30° and (b) ϕ = 120°. The polarization combination is indicated in each panel.

Initially the measurements were performed at RH 0.1% under nitrogen atmosphere, after which we have gradually increased the RH inside the chamber by adjusting the N2 gas flow bubbling in a container with ultrapure water. The SFG spectra for both azimuthal angles exhibit significant changes in SSP polarization (and also SPS, for ϕ = 120°) when the RH is increased to ∼20% and ∼50%, but beyond RH ≈ 50% there are no appreciable changes in the spectra for all polarization combinations. It is difficult to quantify the amount of adsorbed water from the SFG spectra, because χ(2) is proportional both to the surface density of molecules N and to a factor depending on the average molecular orientation (see eq 2). However, the significant changes in the spectra up to RH ≈ 50% suggest the formation of a monolayer of adsorbed water around that value of RH. Multilayer adsorption in equilibrium with vapor occurs F


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Journal of the American Chemical Society only very close to saturation,36,80,81 and perhaps a slight change in the SPS and PPP spectra at ϕ = 30° from RH 50% to >90% indicates initial multilayer formation. The SFG spectrum for ϕ = 30° and SSP polarization is dominated by the free OH peak at low RH. As the RH increases, there are no changes in the H-bonded OH stretch region, while there is a marked decrease and shift to lower frequencies of the free OH peak at 3672 cm−1. The same suppression of the free OH peak occurs for the SPS and PPP polarization combinations, where the relatively small peak at low RH vanishes at RH ≈ 50%. This implies that the adsorbed water monolayer H-bonds to the dangling OH groups of the structural water (δ1), suppressing their contribution. Additionally, this adsorbed water layer is not contributing strongly to the free OH stretches (only a broad and weak band above 3600 cm−1 remains in Figure 6a, SSP), implying that it does not have a significant fraction of OH groups pointing toward the vapor phase. The H-bonded OH stretches below 3600 cm−1 appear prominently in the SFG spectra for ϕ = 120° with SSP and SPS polarizations, and for ϕ = 30° with PPP polarization. They can be separated into three major bands at approximately 3220, 3340, and 3500 cm−1. The first two, below 3400 cm−1, are associated with molecules that are more strongly H-bonded, while the peak at 3500 cm−1 is due to water molecules that make weaker H-bonds, usually associated with a more disordered H-bonding network.33,36 When the RH increases, there is a change in their relative intensities, with an increase of the higher frequency peak at ∼3500 cm−1 for all polarizations where it was originally present, and a decrease of the lower frequency peaks (below 3400 cm−1). For ϕ = 120° with PPP polarization, at low RH there are only weak contributions from the higher frequency peaks (δ1 and δ3, above 3600 cm−1), which are suppressed upon increasing the RH and give rise to the peak at ∼3500 cm−1 that is due to weakly H-bonded molecules. Again, all of these changes are more pronounced from RH 0.1% to ∼50%, which we interpret as the formation of a nearly complete water monolayer on the gypsum (010) surface. They suggest that the H-bonding network of the adsorbed water (2 ML) is not as strong as for the structural water binding to the gypsum (010) surface (1 ML). Another interesting observation is the distinct polarization dependence of these OH stretches, both at low RH (see Figure S11 for a better comparison) and also for higher RH. For example, at high RH the H-bonded peaks appear significantly only with PPP polarization for ϕ = 30°, while for ϕ = 120° they show up in all polarization combinations, but with the peak at ∼3500 cm−1 vanishing in SPS polarization, while the lower frequency peaks are more noticeable in SSP and SPS polarization combinations. This implies that stronger Hbonded groups tend to be aligned a bit more in-plane, while weakly H-bonded groups are somewhat more upright. From these results, it can also be inferred that the arrangement of the adsorbed water is also anisotropic, as the spectra at high RH are very different for ϕ = 30° and ϕ = 120°. Indeed, Figure S12 shows the SFG spectra for gypsum (010) at high relative humidity (RH ≈ 98%) recorded with SSP polarization as a function of the azimuthal angle ϕ, demonstrating that the adsorbed water is clearly anisotropic, with the H-bonded OH bands showing a minimum at ϕ ≈ 40° (which is the average direction of these OH groups) and the nearly free OH bands showing a minimum at ϕ ≈ 120°. Similar surface-induced anisotropy of adsorbed ethanol from vapor on the sapphire

(11̅02) surface was also observed by SFG spectroscopy.81 Therefore, the distinct polarization dependence for each OH peak at high RH and their anisotropy indicate that they result from OH groups that have different molecular orientations and H-bonding strengths. This could not occur if the adsorbed water layer were liquid-like, with high orientational and positional diffusion, suggesting that the adsorbed water layer maintains a good orientational order. Further insights on this structure can be obtained from AIMD simulations with larger water coverages. Because there is no accurate definition of the size of a water monolayer, we have performed MD simulations adding 3 and 6 Å-thick slabs containing 12 and 25 additional water molecules, which correspond to the 2 and 3 ML cases, respectively. We noted that, during the AIMD simulation for these two coverages, there is a layering of water at the gypsum/water interface, with densities near 2.5 g/cm3 in the first and second water monolayers. In the third layer, however, we already obtain a density close to 1 g/cm3, as expected at 300 K (see Figure S6 for further details). Therefore, the experimental condition of high relative humidity can be well simulated using only two monolayers (structural and adsorbed water layers), with any additional water showing liquid-like properties. The simulated vibrational spectrum for the 2 ML coverage is shown in Figure 7e, together with snapshots of the AIMD run in Figure 7a and b. The largest contribution to the spectrum comes from a broad peak centered around 3400 cm−1 due to H-bonded water molecules, while the smaller narrow peak near 3700 cm−1 refers to free OH groups still present in the system. From the simulation we can separate the contribution of each layer to the total spectrum, which is also shown in Figure 7e. This reveals that the first layer (structural water) is still strongly bound to the surface, with a more red-shifted spectrum. However, the free OH peaks from groups pointing up in the 1 ML case have now disappeared, because they are now binding to the second layer (adsorbed water). In turn, this second layer shows higher vibrational frequencies, blue-shifted by about 100 cm−1 as compared to the structural layer, and it is solely responsible for the contribution to the higher frequency peak (free OH groups). As compared to the SFG spectra at high RH (∼50%), the calculated spectrum presents the same relative increase of the higher-frequency contributions from the adsorbed water, as in the experimental spectra, and the free OH peak is considerably suppressed but still present in the calculated spectrum of Figure 7e, in agreement with the experimental spectrum (Figure 6, ϕ = 30°, SSP polarization). In Figure 7f we show the contribution of the second layer to the vibrational spectrum for the 2 ML simulation, separated in four representative types of OH bonds that could be easily identified throughout the simulation, as illustrated in Figure 7a and in the angular distribution of the second layer OH groups (Figure 7d): β1, pointing down, bound to the structural water or sulfate groups below; β2, the other OH group of the molecule containing β1, on average pointing parallel to the surface layer and binding to neighboring water molecules; and β3 and β4, also pointing in the plane and H-bonding to neighboring molecules, but occasionally pointing slightly up and becoming nearly free. The polar and azimuthal angular distributions from the simulations are shown in Figure 5c and d, and in polar versus azimuthal plots for the first and second water layers in Figure 7c and d, respectively. They confirm the scenario described above, where we observe a very ordered G


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nearly free OH groups responsible for the broad above 3600 cm−1. The latter is then attributed to the contributions of β4, while they become nearly free and point slightly upward. In this geometry, we expect stronger contributions to SSP or PPP spectra, with minimum intensity at φ4 ≈ 150°, close to the experimental ϕ0 ≈ 120°, as observed in Figure 6. Indeed, Figure 7f shows that the weak free OH contribution comes from water molecules in the second layer that do not have an OH binding to the first layer (β3 + β4). The contributions of β2 and of more in-plane β3 and β4, while binding to neighboring water molecules, give rise to the H-bonded bands below 3600 cm−1 in the experimental spectrum, with (β3 + β4) contributing to the more red-shifted OH stretches (at 3220 and 3340 cm−1 in the experimental spectrum). They should appear in all polarizations (including SPS) due to their more in-plane orientation, and with a minimum intensity at φ ≈ −130°, close to what is observed experimentally in Figure S12 (ϕ0 ≈ 40° or ϕ0 ≈ −140°) and in agreement with Figure 6. The water molecules with OH binding to the surface (β1 + β2) are assigned to the band at ∼3500 cm−1, because Figure 7f shows that it yields a more blue-shifted contribution to the vibrational spectrum, and it should give a vanishing contribution to SPS spectra because of its more downward orientation, in agreement with the polarization dependence of Figure 6. The angular plots of Figure 7c and d provide a physical understanding of the reason for the anisotropic adsorption of water on the gypsum (010) surface. The OH groups of the first layer pointing upward are clearly anisotropic, with azimuthal angles of φ ≈ −80° and +100° (close to the a axis direction). Indeed, the snapshot of Figure 7a (view along the a axis) shows that the OH groups pointing upward appear with very little tilt sideways, while in Figure 7b (view along the c axis), they are considerably tilted sideways. The OH groups of the second layer pointing downward (β1) are also anisotropic, with a preferred azimuthal direction of φ ≈ −40°. The azimuthal order of these OH groups that make the connection between the surface and the adsorbed layer then induces the azimuthal ordering of the in-plane water molecules due to H-bonding correlations, as shown in Figure 7d, where the orientations of β2, β3, and β4 are clustered around φ ≈ −130°. For further insights on the interaction between the adsorbed water and the gypsum (010) surface, we will discuss below the water dynamics at the surface. Figure 8a compares the diffusion of water molecules in the first and second layers during 30 ps of the 2 ML simulation. It

Figure 7. (a and b) A snapshot of the AIMD run for the 2 ML coverage shown along two different directions (a and c, respectively), with a line separating the first ordered layer (structural water) and the second more diffuse layer (adsorbed water). The angular distributions for the OH groups in the first and the second layers are shown in (c) and (d), respectively. The total vibrational spectrum for the 2 ML coverage and for the second water layer alone is shown in (e) and (f), respectively, where we highlight the contributions of each water layer and of the OH bonds in the second layer labeled as β1, β2, β3, and β4 in (a).

structural first layer followed by a more diffuse second layer, but still with significant orientational ordering. These results from the AIMD simulations are in agreement with the marked polarization dependence seen in the experimental SFG spectra, different for H-bonded and free OH groups and also for different azimuthal angles ϕ, indicating that each OH group maintains a good orientational order. In fact, the anisotropic adsorbed water layer revealed by the azimuthal dependence of the SFG spectra at high RH (Figure S12 and Table S1) indicates a strong interaction with the underling structural water (first layer), in accordance with the AIMD simulations, where Figure 7d shows that β1 binds to the first layer with an average azimuthal orientation of φ ≈ −40°, while β2, β3, and β4 point on average along φ ≈ −130° with a broad, but not uniform distribution. These azimuthal orientations agree with those obtained by fitting the anisotropy of the SFG spectra in Figure S12, where we get ϕ0 ≈ 40° (corresponding to φ ≈ −140° or φ ≈ 40°) for the H-bonded OH groups responsible for the bands below 3600 cm−1, and ϕ0 ≈ 120° (corresponding to φ ≈ −60° or φ ≈ 120°) for the

Figure 8. (a) Sampled positions of water oxygen atoms (XY projection on the (010) plane) are shown for the last 30 ps of the 2 ML simulation, with the first/second water monolayers in blue/red, respectively, as illustrated in Figure 7a. (b) The same plot is shown for the 3 ML case, with the third monolayer contribution in green. H


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comprehensive comparison with SFG spectroscopy, because the latter is highly sensitive to molecular orientation. For the “dry” gypsum (010) surface at low humidity, the peaks observed in the SFG spectra arise from uncoupled vibrations of OH groups with different H-bonding and orientation (single OH oscillators). The cleaved surface (with structural water) is different from a “frozen” halfinterlayer of bulk structural water, with only 25% of free OH groups, instead of the 50% expected for a “frozen” cleaved surface. These free OH groups are slightly tilted away from normal, leading to an anisotropic surface hydration layer, with the experimental and simulated tilt directions in good quantitative agreement. When the surface is covered by one water layer, it shows a considerably lower fraction of free OH groups, and the adsorbed water contributes with blue-shifted H-bonded OH stretch modes. However, the adsorbed water keeps an ordered structure that is azimuthally anisotropic, without any significant surface diffusion, due to a strong interaction with the underling azimuthally anisotropic first layer (structural water). Upon adsorption of a second water layer, although the last layer of molecules is more disordered and dynamic than the previous one, its structure is still influenced by the gypsum surface underneath, because it still has a much reduced amount of free OH groups with respect to the free surface of water. This strong interaction of the adsorbed water with the gypsum surface may put in firmer basis the hypothesis that confined water within gypsum microcrystallites could be the reason for the large increase in mechanical strength of gypsum parts fabricated by the wet compaction method,50,51 which initially motivated our work. The gypsum (010) surface induced an anisotropic arrangement of adsorbed water molecules at room temperature. This is very unusual when compared to other mineral surfaces studied,15,25,41,83 in particular to water adsorption on the mica surface,36,40 an anisotropic mineral with a structure similar to that of gypsum, but whose structural water in its cleavage plane is buried within the surface, whereas the gypsum structural water is exposed after cleavage. Ice films formed at mineral surfaces are of widespread occurrence in nature and take part in important atmospheric processes.84 The formation of ice films was reported in different mineral surfaces but with an isotropic arrangement, different from this anisotropic film adsorbed on the gypsum (010) surface. Anisotropic adsorption of water layers was reported on the rutile TiO2 (110) mineral surface21 and on the Pt (111) metal surface,85 but both studies were conducted at low temperatures (∼160 K). Only recently did another SFG study identify surface-induced anisotropic ethanol adsorption on the sapphire (11̅02) surface at room temperature.81 Therefore, due to the significant environmental and technological relevance of gypsum, we hope that such detailed information obtained herein about the interaction of gypsum with water adsorbed from vapor at room temperature could be useful to the understanding of several physico- and geochemical processes.

can be seen that the mean square displacement of water molecules in the structural first layer (evaluated as 0.90 Å) is considerably smaller than that for the second layer (4.24 Å). However, the self-diffusion coefficient for both layers is close to zero because the water molecules are strongly attached to their “sites”, even for the second layer where only a few molecules actually diffuse for a short period of time. Therefore, even though both the calculated and the experimental vibrational spectra suggest that adsorbed water in the second layer is not as strongly H-bonded as that in the first layer, their dynamics is still much slower than for bulk water. This strong interaction of the adsorbed water with the gypsum surface correlates well with the proposed hypothesis that confined water within gypsum microcrystallites may be the reason for the large increase in mechanical strength of gypsum parts fabricated by the wet compaction method.50,51 Considering now the simulations for the higher 3 ML coverage, Figure 5e and f shows the resulting polar and azimuthal angular distributions. It is clear that the third added layer shows a less ordered structural pattern than the first two, as it can also be seen in the diffusion pattern in Figure 8b. More quantitatively, the water molecules in the third layer have a self-diffusion coefficient of 1.5 × 10−9 m2/s, which is already within the order of magnitude of the bulk water coefficient (D = 2.3 × 10−9 m2/s at 25 °C).82 In addition, we notice more clearly that the δ3 peak of the first structural layer (Figure 5e) progressively disappears, indicating that water molecules that were parallel to the surface gradually return to their bulk orientation, where each water molecule has only one OH bond nearly along the surface normal. In the calculated vibrational spectrum (shown in Figure S7), we see an absolute contribution from the free OH groups similar to that in the 2 ML case, showing that the last layer of adsorbed water still maintains a low amount of free OH groups. Although the third layer of molecules is more disordered and dynamic than the second, its structure is still influenced by the gypsum surface underneath. Obviously, there is an increase of the H-bonded contribution from the molecules within the liquid-like film, so that the normalized spectral profile is similar to the 2 ML case, but with a relatively smaller importance of the free OH peak. The H-bonded part of the vibrational spectrum has also a slightly increased contribution of more strongly H-bonded molecules around 3200 cm−1 with respect to the 2 ML simulation. Finally, MD runs were also performed in the infinite 3D cell, where 6 ML of water interact on both sides with the gypsum surface as illustrated in Figure S2c. The vibrational spectrum is also shown in Figure S7, where we can observe that it is similar to the 3 ML case where, but without the high frequency peak (free OH) present.

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CONCLUSIONS The excellent qualitative agreement between what could be inferred from the experimental SFG spectra and the calculated structure and spectra gives strong support to the theoretical methodology employed and validates the AIMD simulation results. In turn, the AIMD simulations provide an exquisite level of detail about the structure of the gypsum (010) surface and the adsorbed water layers, which could not be obtained experimentally. We should also highlight that we have introduced an innovative visualization method for the whole angular distribution of the interfacial water molecules (Figures 5 and 7), which is rich in detail and allows a more

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b09907. I


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Figure S1, total energy calculation and structure for the hydrated and dehydrated gypsum (010) surface; Figure S2, snapshots of optimized structures for bulk gypsum and the gypsum (010) with 1, 2, and 6 ML coverages; Figure S3, vibrational spectrum of confined “bulk” structural water for all simulated structures; Figures S4 and S5, SFG data showing H−D exchange of surface water and analysis of surface versus bulk contribution for the SFG spectra in reflection and transmission; Figure S6, density versus height profiles from AIMD simulations; Figure S7, vibrational spectra of adsorbed water for different coverages from AIMD simulations; Figure S8, with fitting of the SFG spectra for ϕ = 300° and 330° for the gypsum (010) surface and discussion of up/down orientation of different OH groups at RH = 0.1%; Figures S9 and S10, including a molecular orientation model for the SFG amplitude of OH groups; Figure S11, polarization dependence of the SFG spectra for the gypsum (010) surface at low RH; and Figure S12, anisotropy of the adsorbed water on the gypsum (010) surface at RH ≈ 98% (PDF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Gustavo M. Dalpian: 0000-0001-5561-354X Paulo B. Miranda: 0000-0002-2890-0268 Present Address ⊥

F.R.N., Departamento de Quı ́mica Teórica y Computacional, Facultad de Ciencias Quı ́micas, Universidad Nacional de Córdoba, 5000, Cordoba, Argentina. Author Contributions §

J.C.C.S. and F.R.N. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Brazilian agencies FAPESP (grants 11/199242, 14/01595-0, 14/14271-9, and 18/13753-0) and CNPq for financial support. J.C.C.S. acknowledges a Ph.D. fellowship from CAPES-Finance Code 001. Computer simulations were performed at CENAPAD-SP and on the Santos Dumont supercomputer at LNCC. P.B.M. acknowledges interesting discussions with Prof. Milton F. de Souza during the initial stages of this work.

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Interaction of Water with the Gypsum (010) Surface: Structure and

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