Hydration Structure of the Barite (001)âWater Interface: Comparison of

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Hydration Structure of the Barite (001)-Water Interface: Comparison of X-ray Reflectivity with Molecular Dynamics Simulations Jacquelyn N. Bracco, Sang Soo Lee, Joanne E. Stubbs, Peter J Eng, Frank Heberling, Paul Fenter, and Andrew G. Stack J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017

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The Journal of Physical Chemistry

Hydration Structure of the Barite (001)-Water Interface: Comparison of X-ray Reflectivity with Molecular Dynamics Simulations Jacquelyn N. Bracco1*, Sang Soo Lee1, Joanne E. Stubbs2, Peter J. Eng2,3, Frank Heberling4, Paul Fenter1, Andrew G. Stack5 1

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL, USA 2

Center for Advanced Radiation Sources, University of Chicago, Chicago, IL, USA 3

James Franck Institute, University of Chicago, Chicago, IL, USA

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Institut für Nukleare Entsorgung, Karlsruher Institut für Technologie, Karlsruhe, Germany 5

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

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Corresponding Author: Phone (630)252-6292; fax (630)252-9570; email [email protected]

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ACS Paragon Plus Environment


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Abstract The three-dimensional structure of the barite (001)-water interface was studied using in situ specular and non-specular X-ray reflectivity (XR). Displacements of the barium and sulfate ions in the surface of a barite crystal and the interfacial water structure were defined in the analyses. The largest relaxations (0.13 Å lateral and 0.08 Å vertical) were observed for the barium and sulfate ions in the topmost unit cell layer, which diminished rapidly with depth. The best fit structure identified four distinct adsorbed species, which in comparison with molecular dynamics (MD) simulations, reveals that they are associated with positions of adsorbed water, each of which coordinates one or two surface ions (either barium, sulfate, or both). These water molecules also adsorb in positions consistent with those of bariums and sulfates in the bulk crystal lattice. These results demonstrate the importance of combining high resolution XR with MD simulations to fully describe the atomic structure of the hydrated mineral surface. The agreement between the results indicates both the uniqueness of the structural model obtained from the XR analysis and the accuracy of the force field used in the simulations.

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1. Introduction Barite is the most common barium-containing mineral at Earth’s surface, and its structure and reactivity have numerous environmental and industrial implications, including those involving the problems of barite scale formation in oil pipelines during offshore drilling/hydraulic fracking1 and long term sequestration of radium,2-4 and other contaminants such as chromate.5,6 As such, barite has been the focus of a number of experimental and computational studies directed towards better understanding the barite-water interface, including a prior X-ray reflectivity (XR) study7 and computational simulations.8-13 Natural barite crystals exhibit variable morphologies based on the reactivity and growth rates of the specific surfaces, among which the (001) and (210) surfaces are the most common.14-17 The growth rates of these surfaces in aqueous solutions have been found to vary with different saturation states and the presence of electrolytes and impurities.18 Barite has an AB-type crystal structure with a unit cell that consists of four BaSO4 units in which the barium and sulfate ions are bonded together ionically (Figs. 1 and S1, Supporting Information). At the (001) surface, the barite structure can be separated into two sublayers (A vs. A’), each with two barium and two sulfate ions, that are related through a screw axis. Each sublayer is referred to as a monolayer in atomic force microscopy (AFM) based literature, with a step height of ~3.6 Å,19-21 and contains one barium and one sulfate in a high position along the [001] direction and one barium and one sulfate in a low position (Figs. 1a, 1b, and S1). Formation of the barite (001) surface breaks three bonds to the topmost barium ion (high position - Bahigh) and one bond to the second topmost barium ion (low position - Balow), leaving these two ions undercoordinated with respect to their bulk coordination environments. Adsorption of water 3



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can mitigate this undercoordination in aqueous solutions and humid air.22 Both the distribution and type of these undercoordinated ligands (e.g., by their charged states) are expected to determine the structure and density of adsorbed water layers,23 which may deviate from the 2D packing density of ~10 H2O/nm2, calculated based on the density of bulk water (ρ3D = 33 H2O/nm3 where ρ2D = ρ3D2/3). A previous study using in situ specular XR observed a surface hydration layer with a density of 5.6 H2O/nm2, roughly half of the expected 2D packing density.7 The results were interpreted to arise from a partial hydration of the surface ligands.7 However, the hydration structure on each functional group (i.e., Ba2+ or SO42-) could not be determined separately because specular XR measurements only probe the vertical interfacial structure. Therefore, it is possible that the previous understanding of the barite (001)-water structure may be oversimplified and needs to be reevaluated. A combination of specular and non-specular measurements can determine both the vertical and lateral hydration structure at the interface.

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Figure 1: (a) View of the (001) barite surface from the best fit model with adsorbed water (blue), barium (black), sulfur (yellow), and sulfate oxygen (red). Labels 1-4 are the four adsorbed water positions derived from the CTR analyses. In the top half unit cell, Slow is a sulfate ion in the low position, Balow is a barium ion in the low position, Shigh is a sulfate in the high position, and Bahigh is a barium in the high position. Hydrogen atoms on adsorbed water species are not shown. The unit cell is denoted by the red box. (b) Surface normal view (looking down the [001]) of the top barite layer and adsorbed waters. (c) Differences in the A and A’ termination. The two surfaces are related to each other through a screw axis.

In this paper we present a three-dimensional (3D) structure of the barite (001)-water interface using in situ specular and non-specular X-ray crystal truncation rod (CTR) 5



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measurements. The results are compared with MD simulations to determine the number and positions of adsorbed water species. The benefits of directly comparing the predicted structure from MD simulations and the best-fit structure from XR studies are two-fold: First, the results can be used to determine if the force fields utilized in the computational simulations accurately predict the interfacial structure, which in turn can be used to predict chemical reactivity, and second, the results can be used to evaluate different XR “best-fit” structures to determine the most physically realistic model.24 These results are compared with the hydration structure for calcite, another sparingly-soluble AB-type mineral, to explore the differences in hydration between cations and anions.

2. Methods 2.1. Sample Preparation Optically clear barite samples from Sichuan, China containing minor impurities were used.20 Prior to the start of the experiments, barite (001) surfaces were cleaved with a razor blade, mounted on a sample puck using CrystalBond adhesive,25 and transferred to either a barite saturated solution (SI=0; SI=log({Ba2+}{SO42-}/Ksp), Ksp=10-9.98 at 25°C where {} denotes the activity of a given species) or a barite growth solution (SI=1) within 1-3 minutes. The barite saturated solution (pH ~ 5.6) was prepared by reacting powdered barite with deionized water (resistivity = 18.2 MΩ·cm) for more than a month in contact with the ambient atmosphere. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis has confirmed this is a sufficient time for the solution to reach equilibrium. The barite growth solution (Ionic Strength = 0.01 M) was prepared by combining sodium sulfate, barium nitrate, and sodium nitrate stock solutions of 99.99% purity or higher (trace metal basis). The molarities of the 6


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barium nitrate and sodium sulfate stock solutions were determined using ICP-OES. The analysis of Na+ was used for determining SO42- concentrations. PHREEQC was used to calculate solution speciation using the minteq.v4 database.26 An uneven mixture of A and A' terminations (Fig. 1c) was present on the as-cleaved sample surface which can arise during cleaving of the crystal.27 Differences in the (1,0) and (1,0) non-specular XR signals for the as-cleaved surface reflect an uneven admixture of these two terminations due to the presence of a 21 screw axis along [001] (Fig. S2a). We found that samples grown in SI=1, {Ba2+}/{SO42-}=1 solution for 4.5-24 hours had a 50:50 mix of terminations. CTRs were measured to confirm that the percentages of the terminations were equivalent (Fig. S2b). This preparation step is important because it minimizes systematic errors in the CTR data analysis due to the unknown variability in the coverage of two terminations which may not be uniform across the mineral surface. AFM images (Fig. S3 in the Supporting Information) of reacted samples after the growth process indicate that the surface was predominately covered by large terrace regions ≥1 µm wide. Large scale defect features such as macrosteps or growth hillocks were also present but their areal coverage was small, less than 10%. 2.2. Atomic Force Microscopy The AFM images were collected with an Asylum Research MFP-3D operating in alternating current (AC) mode in air to determine the sample roughness and morphology. The probes utilized were Si Olympus OMCL-AC cantilevers with a tip radius of ~ 20 nm. Images were collected at 5 × 5 µm2, 10 × 10 µm2, and 20 × 20 µm2 in multiple locations on the sample surfaces to check for uniformity. The Asylum Research AFM software was used to determine the

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surface roughness and check for X-ray beam damage, if any, after prolonged exposure to the Xrays. 2.3. X-Ray Reflectivity Specular and non-specular XR measurements were performed in situ at the GSECARS beamline 13-ID-C of the Advanced Photon Source at Argonne National Laboratory. Measurements were made using a Newport kappa six (4+2) circle diffractometer and 16 keV Xrays (X-ray wavelength of λ = 0.7748 Å) using a cryogenically-cooled Si (111) double crystal monochromator. The beam was collimated using two 1-m long, Rh-coated Si mirrors in Kirkpatrick-Baez geometry. The final beam size of 0.1 × 1 mm2 (horizontal x vertical) was defined using slits. XR measurements were collected using a Pilatus 100K pixel array detector (Dectris, Inc.). The XR signal was measured as a function of momentum transfer, Q, whose magnitude is related to the scattering angle (2θ) of reflection, |Q| = (4π/λ)sin(2θ/2) and whose projection on the surface can be described using the barite Bragg indices, HKL, through the relation: [Qx, Qy, Qz]= [H(2π/a), K(2π/b), L(2π/c)], where a, b, and c are the unit cell lengths along the x, y, and z directions.28 The Q-dependent variation in reflectivity can be described as a crystal truncation rod (CTR), which is a rod of weak intensity oriented normal to the surface (as indicated by the continuous index, L) and that connects the Bragg peaks in reciprocal space.22, 29 The specular CTR (also referred to hereafter as the 00L rod) is sensitive to the interfacial structure perpendicular to the barite (001) surface, but it is insensitive to the lateral ordering. Non-specular rods are sensitive to structure that is ordered both verticality and laterally with respect to the surface. The combination of these two measurement geometries probes the full three-dimensional interfacial structure.

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Specular reflectivity data were collected to a Qmax ≈ 5.5 Å-1, corresponding to a vertical spatial resolution of ~ 0.6 Å (resolution ~ π/Qmax). Non-specular reflectivity was collected to Qmax ≈ 1.4 Å-1 in x (along the [100] direction) and 2.3 Å-1 in y (along the [010] direction), corresponding to a lateral resolution of ~ 1.2 Å (π/Q∗max, Q∗max ≈ 2.7 Å-1). These resolutions are simple estimates of the length scale over which the structure can be uniquely determined, which is necessary for distinguishing adsorbed species from one another. For example, two species separated by a distance shorter than the resolution could instead be modelled as a single but spatially distributed species within the structural model.22 The XR measurements were performed in a ‘thin film’ cell.30 In this configuration a layer of water with a thickness of ~ 40 µm was held in place at the sample surface by an 8 µm-thick Kapton film (DuPont). Because the solution volume is small, the composition of the solution can change due to any reaction between the solution and surface.31 The Kapton film is also permeable to water and atmospheric gases making it susceptible to changes in concentration due to evaporation. To minimize changes in solution composition, the cell was flushed with solution every two hours. During solution exchanges, the cell was “puffed up” to expose the sample to a larger volume of fresh solution with the same composition. After flushing, this new solution was allowed to equilibrate with the sample for ~ 10 minutes before the Kapton film was collapsed into the thin film geometry. To limit evaporation of the solution through the Kapton (which would lead to a supersaturated solution), the thin-film cell was placed in a secondary Mylar hood containing humid helium. Sample stability was evaluated by measuring the same rods (including the entire 00L rod and sections of either the 10L or 22L rods) repeatedly over time. As can be seen in Fig. S4, the differences in the specular CTR collected before and after measurements of the non-specular rods are generally within the statistical uncertainties, suggesting minimal 9



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sample evolution with time. Nevertheless, we imposed a minimum error of 2% on the reflectivity to minimize the effects of systematic errors, such as the background subtraction near the Bragg peaks, and differences in intensities of the symmetry equivalent rods. Overall, 10 CTRs were measured with a total of 1254 data points, excluding the duplicates. Two of the rods, the (10L) and (-10L) were symmetry equivalents. 2.4. CTR Data Analyses The CTR data were analyzed using structural models which consist of a bulk barite structure, a liquid water profile, and an interfacial structure composed of barium, sulfate, and adsorbed water molecules. The X-ray reflectivity is expressed as, ሬሬറ)|B(Q୸ )|2|Ftot(Q ሬറ)||2 R = (reλ/AUC)2/[sin(αi)sin(αe)]T(Q where re is the radius of an electron, λ is the X-ray wavelength, AUC is the area of the unit cell, αi ሬറ) is X-ray transmission through the thin film and αe are incident and exit angles respectively, T(Q ሬറ)| is of water and Kapton, B(Q୸ ) is the roughness factor (B(Q୸ ) = (1-β)/(1- βe௜୕౰ ௖ )),29 and |Ftot(Q the total structure factor modulus. Ftot is expressed as Ftot = FUCFCTR+Fsurf+Fwater where FUC is the structure factor summed over atoms within a single bulk unit cell, Fsurf is the structure factor summed over all atoms near the surface which are displaced from bulk lattice positions, and Fwater is the structure factor from the fluid, including adsorbed and bulk water species. FCTR is the CTR structure factor of a semi-infinite crystal.22, 29 A roughness parameter, β, was included to account for the loss of signal due to surface roughness corresponding to an associated rms width of (c/2)(β1/2/(1-β)),29 where c is the (001) layer spacing. The barite samples were analyzed using single crystal XRD (unit cell: a = 8.88050 Å, b = 5.45270 Å, and c = 7.15380 Å; space group – Pnma) the results of which can be found in the Supporting Information. These results are similar to those found by Colville and Staudhammer.32 10


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The (001) plane has an AUC of 48.423 Å2, and exposes four different surface functional groups33: a sulfate in a low (Slow) and a high position (Shigh) and a barium in a low (Balow) and a high position (Bahigh) (Fig. 1). To determine which functional groups were most likely terminating the surface, the measured CTRs were compared with CTRs calculated from models terminated by all the different combinations of surface functional groups. Surfaces that were terminated by a barium in the low position showed significant differences between measured and calculated CTRs (Fig. S5 in the Supporting Information), and therefore were excluded from consideration.34 These differences are due to termination interference arising from a missing symmetry center, which leads to a node in the unit cell form factor.34 Surfaces that were terminated by either Bahigh and Shigh, or Bahigh and Slow (i.e., one with a missing Shigh) showed generally good agreement with the data. These models were then considered in the full model-dependent optimization. Both vertical and horizontal displacements were allowed for barium ions and sulfate groups within the top two unit cells of the crystal. The sulfate groups were allowed to move as rigid structures without any distortion of the S-O bond distances. The initial analysis included both vertical relaxations along the [001] direction (zԦ) and lateral relaxations along the [100] and [010] directions (xሬԦ and y ሬԦ). The magnitude of displacements along the y ሬԦ direction was negligible presumably because any displacements in this direction would break the mirror plane symmetry of the bulk crystal (XRD analysis available in the Supporting Information). Therefore, the relaxations along the y ሬԦ direction were excluded, and only those in the ሬxԦ and zԦ directions were considered in subsequent models. Angular displacements of the sulfate groups were included to incorporate their rotation about the y ሬԦ direction, which does not break the crystal symmetry.7 Fits to models which allowed for rotation and relaxation which broke the mirror plane symmetry were of similar quality to the best fit model presented here. Based on 11



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observations in the MD simulations, the rms-widths of the surface ions were allowed to vary both laterally and vertically. The increase in the rms-widths for the sulfurs were calculated by averaging the increase in rms-width for the top and bottom oxygens in the sulfate. Solution species adsorbed on the barite surface (e.g., water molecules) were described by their x, y, and z positions, occupancies, and lateral and vertical rms-widths (e.g. due to vibrations and/or static disorder). We assumed that the lateral rms-widths were the same in x and y for simplicity. The minimum rms-widths were fixed to 0.1 Å to prevent unphysically small values. The models initially assumed that all adsorbed species are water molecules, and the identity of the adsorbed species was estimated on the basis of the occupancy. For example, an occupancy value larger than one can be understood as a result of partial substitution of heavy element Ba at the same site (see Results for details). Since X-rays are not sensitive to the positions of hydrogen atoms, the water species could be H2O, OH-, or H3O+. To investigate adsorption of sulfate ions, we included sulfate ions as an adsorbed species, optimized the structure, and compared the quality of fit with the models that had no adsorbed sulfates. The structure of water above these adsorbed species was expressed using a multi-layered, laterally disordered water model.35-36 This water model was described with a first-layer z position (z0), a layer spacing (dwater), an rms-width (uwater = [u02 + (n-1)ubar2]1/2 for a given layer index, n, a first layer vibrational amplitude, (u0), and a parameter that increases with increasing distance from the surface (ubar). The occupancy of each water layer was fixed to ensure that the bulk fluid density was the same as that of water (~33 H2O/nm3). The parameters were optimized using least-squares fitting using χ2 (= 1/NpΣi(|Ri-Rc|/σi)2) as a goodness-of-fit where Np is the number of data points, Ri and Rc are the measured and calculated intensities for the ith data point, and σi is the measured uncertainty of the ith data point. 12


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Multiple models were tested by varying the number of water molecules as well as their initial locations at the surface. The best-fit model was determined based on both physical constraints (e.g., steric effects) and the smallest χ2. A total of 79 parameters were included for the analyses of 10 CTRs with a total of 1254 data points. The quality of the fit was also evaluated using the R-factor defined as R-factor = 1/NpΣi|Ri-Rc|/Ri. The specific barite surface termination was further explored by full barite-water interfacial structural models, including the displacements of the barite species, adsorption of water to the surface and the bulk water density. In this, we found that the barite structure terminated by Bahigh and Slow had a χ2 value which was twice that of the case for a surface terminated by a Bahigh and Shigh. Furthermore, the positions and densities of the adsorbed molecules on the Bahigh and Slow terminations were approximately the same as that for the missing sulfate ion, indicating the surface is terminated by Bahigh and Shigh. Consequently, the stoichiometric barite surface terminated by Bahigh and Shigh was used in the final optimization and reported below.

2.5. Molecular Dynamics Simulations Molecular dynamics simulations were carried out using the “alternate” MSXX barite forcefield37 with the modifications detailed in Stack8 and Stack et al.38 and a flexible threecentered water model (F3C39) which is a nonpolarizable, simple point charge water model. Simulations were equilibrated for 1 ns and were then followed by production runs of 10 ns with a 1 fs timestep at constant particle number, volume, and temperature (NVT) at 300 K using LAMMPS.40 The simulation cell was 34.4475 Å × 28.1442 Å × 57.2552 Å and contained 320 BaSO4 formula units in a slab eight monolayers thick, oriented parallel to (001), with 925 water 13



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molecules. The density of bulk-like water was measured at 0.0973 atoms/Å3 (33.4 H2O/nm3, 0.970 g/cm3) and periodic boundary conditions were imposed on the simulation. The average number of water molecules adsorbed to different sites on the barite surface was determined as in Stack.38 Briefly, all barium and sulfate surface sites in the simulation were first classified as either high or low (40 each of barium high, barium low, sulfate high, and sulfate low). Pair distribution functions with respect to surface bariums and sulfates sites were made. The minimum probability for the first shell water was used as a cut-off distance for determining if a water was bound to a specific site. These were 3.65 and 4.25 Å for barium and sulfate sites, respectively, with little discernable difference in minimum position for high or low sites. (These cut-offs are slightly different from the 3.60 and 4.00 Å found previously,35 due to the presence of condensed phase liquid rather than a few adsorbed monolayers). The number of oxygens on water within those distances to each site was averaged over 10 ns production runs to obtain the coordination number. The locations and rms-widths of the water adsorption positions were determined as follows: A 3D probability histogram of the position of the oxygens on water molecules within 5 Å of each of type of surface site was made over the 10 ns production run. Next, the position of maximum probability for a water adsorbed to a site in a specific location was determined relative to the surface site. The full width at half maximum (FWHM) was used to create a probability isosurface whose size, shape and center in x, y, and z was used to generate the position and rms-width of the adsorbed water. Measurements of isosurface widths were made using VisIt.41 2.6. Comparison between MD Simulations and XR Data To compare the MD simulations to the XR data, the method of Fenter et al.24 was followed in which the predicted structure factor from the atom positions in the MD simulations is 14


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compared with the reflectivity data. Due to differences in the size of the unit cells for the MD and XR results, the results from the simulations were rescaled to enable a comparison. Two cases were compared. The first was one where the full interfacial structure (crystal + adsorbed species) from the MD simulations was used. In the second case, the interfacial barium and sulfate ions were allowed to relax and the interfacial water profile from the MD simulations was used. In both cases, the structure factor was calculated using the same bulk crystal structure as that utilized in the XR model fitting. This comparison was performed only for the specular data (i.e., 00L rod). 3. Results 3.1. Atomic Force Microscopy Barite (001) surfaces were characterized using atomic force microscopy (AFM) both prior to and after XR measurements. The unreacted barite surfaces displayed typical cleavage morphologies (Fig. S3a). The barite samples that were grown in SI = 1.0 solution had similar features to the unreacted ones, although small growth hillocks were present at macrosteps (Fig. S3b). Samples exposed to the X-ray beam appeared similar to unexposed samples (Fig. S3c), indicating that the beam effect on the surface morphology was negligible. 3.2. XR Analysis Three CTRs for the barite (001) surface in contact with barite saturated solution are shown in Fig. 2 (00L: Fig. 2a, 22L: Fig. 2b, 01L: Fig. 2c), while the other 7 CTRs and normalized CTRs can be found in Fig. S6 (specular reflectivity) and Fig. S7 (non-specular reflectivity). Relaxation of surface ions and water hydration gives rise to the asymmetry and bumps between the Bragg peak positions which can be observed in many of the rods (particularly near the L=5 for the 00L and L=2 and 4 for the 01L CTR). These features can be 15



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observed more easily after the normalization of the reflectivity signals by dividing out the generic CTR shape of each rod (Figs. S6 and S7).

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Figure 2: X-ray reflectivity data (circles), the bulk termination with no adsorbed species or relaxation (green lines), and the corresponding best-fit model (blue lines) for a surface which had 17



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been allowed to relax. (a) shows the specular (00L) rod and (b) and (c) show the (22L) and (01L) rods, two of the nine non-specular rods (Fig. S7) collected. The Bragg peaks for the (00L) rod are 002, 004, and 006. The Bragg peaks for the (22L) rod are 221, 222, 223, 224, and 225. The Bragg peaks for the (01L) rod are 011, 013, and 015.

The best fit model (with quality of fit χ2 = 1.73 and R-factor = 0.087) quantitatively reproduces the intensity variations observed in all 10 CTRs (Figs. 2, S6, and S7). The vertical root-mean square roughness σ is ~2 Å and was calculated as (c/2)(β/(1-β))1/2 where β = 0.084 ± 0.003 and c/2 = 3.5769 Å is the barite (001) half unit cell spacing (Table S1). A non-zero roughness factor is consistent with the AFM observations of steps and growth hillocks on the surface that leads to diffuse scattering outside of the detector window. The average surface domain size, D// (e.g. average step spacing), based on the lateral width of the reflected beam at the first midzone is ~ 800 nm, which is also similar to the AFM observations (Fig. S3). This was measured at Qz = 1.55 Å-1 (L = 1.31 rlu) by measuring the width of the line scan (∆2θ) on the detector at a given Qz and is based on the relation D// ~ 4π/(Qz ×∆2θ).7, 22 Vertical structural relaxations were small but significant up to two unit cells into the surface (Fig. 3; Table S2). Lateral relaxations were statistically significant through the third monolayer for barium atoms and the fourth monolayer for sulfate groups. The vertical displacements of sulfate exhibited oscillatory behavior near the surface. The maximum displacements were for sulfate ions: δx = 0.13 (± 0.03) Å and δz = 0.08 (± 0.01) Å (Fig. 3). The vertical displacements of barium and sulfates often had opposing directions (i.e. expansion of one and contraction of the other within the same half unit cell). Especially, at the topmost layer, this led to a slight contraction of Ba–Ba distances and a slight expansion of the SO4 – SO4 18


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distances, and a decrease in the atomic corrugation as the barium and sulfate ions moved closer to each other in height. The rotation angle for the topmost sulfate group (Shigh) was 8.7 ± 0.8° and the angles for all subsequent sulfates were small or negligible (Slow: 0.03 ± 0.8°; top sulfate of second layer: 0.71 ± 0.08°; bottom sulfate of second layer: 0.29 ± 0.3°). An increase in the rmswidth for the surface ions was allowed based on observations from the MD simulations. An increase in the lateral rms-width was significant for Shigh, Bahigh, and Balow, but not Slow (Table S2). An increase in the vertical rms-width was significant for Balow, the sulfur and bottom oxygens on Shigh, and the sulfur and top oxygens on Slow. The increase in rms-width decreased with increasing distance from the surface.

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Figure 3: Displacement of barium and sulfates ions in x (displacement along the [100] direction) and z (displacement along the [001] direction) as a function of distance from the surface, which is defined as the position of the unrelaxed top surface barium. The vertical displacements exhibit oscillatory behavior, with the displacement of Ba ions gradually decreasing and displacement of sulfate ions changing from position to negative back to positive. 20


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The relaxation of surface atoms leads to changes in the bond distances between the Ba ions and oxygens in sulfate groups. The Ba-O bond distances for Bahigh in the top half unit cell range from 2.71 (± 0.02) to 3.29 (± 0.02) Å, with an average value of 2.89 (± 0.01) Å. The Ba-O bond distances for Balow in the top half unit cell range from 2.74 (± 0.01) to 3.49 (± 0.02) Å, with an average value of 2.91 (± 0.01) Å. In contrast, the bulk Ba-O distance ranges from 2.82 to 3.32 Å, with the average value of 2.95 Å.42 This comparison shows that the bond distances for Bahigh and Balow on average decrease due to relaxation of the barium into the crystal. Bahigh in the second half unit cell has the average Ba-O distance of 2.92 (± 0.01) Å, indicating that the Ba-O coordination becomes more bulk-like with increasing depth into the crystal. A number of possible structures were evaluated for the hydration structure at the barite interface, including models with three, four, five, and six distinct adsorbed water species. These water species can either be pure water or water with dissolved barium or sulfate. The structures which contained more than four distinct water species included water molecules which were located closer to each other than our resolution limit. Inclusion of adsorbed sulfate also created an overlap between the sulfate ion and one of the water species. Finally, cases where there were fewer than four adsorbed waters had higher χ2 values than the one for the four adsorbed water species. The best fit structure (χ2 = 1.73) has four distinct water positions which are strongly correlated with the locations of surface functional groups (Table 1, Figs. 1 and 4). Water 1 (Fig. 4a) sits roughly on top of Slow, Water 2 (Fig. 4b) is located on top of Balow, Water 3 (Fig. 4c) is located next to Bahigh and Shigh, and Water 4 (Fig. 4d) sits on top of Shigh. Views of these sites are available in the Supporting Information (Figs. S8-S11). Two water species (Water 1 and Water 2) 21



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adsorb at distances of 3.09 Å and 3.19 Å (Fig. 1) respectively from the topmost and second topmost barium atoms (Bahigh and Balow, respectively). These distances are slightly larger than both the bulk Ba-O distance in barite and the Ba-O distance in the primary shell of a hydrated Ba ion.43 Water 1, located close to Bahigh, is also positioned close to two oxygens in Slow and Water 2 (Table S3). It has an occupancy of 0.92 ± 0.05 Weq/AUC, where 1 Weq corresponds to the number of electrons of one water molecule (10),44 and is laterally and vertically well constrained (Table 2). Water 2 is located close to Balow, four oxygens in the surface sulfate groups (two oxygens in two Slow and two oxygens in two Shigh), Water 1, and Water 4. It has an occupancy of 1.67 ± 0.09 Weq/AUC and is laterally diffuse but vertically well constrained. Water 3 is located closest to Bahigh, two oxygens in two Slow, and two Water 4s. It has an occupancy of 0.887 ± 0.1 Weq/AUC and is vertically well constrained but laterally diffuse. Finally, Water 4 is closest to two oxygens in Shigh and two Water 3s, has an occupancy of 0.864 ± 0.08 Weq/AUC, and is vertically well constrained but laterally diffuse. A weakly ordered, layered water profile is located at z ≥ 5 Å, and the layer density oscillations dampen out by 12 Å. The surface-normal electron density profile and views of the model surface can be found in Figs. 1 and 5, respectively.

Table 1: A comparison of the positions for the four water locations. Water number 1 2 3 4

Nearest surface site Slow Balow Bahigh Shigh

x

y

z

0.725 (±0.06) Å 2.65 (±0.06) Å 5.40 (±0.2) Å 4.93 (±0.07) Å

1.46 (±0.03) Å 3.91 (±0.05) Å 3.89 (±0.1) Å 1.38 (±0.05) Å

2.41 (±0.02) Å 1.86 (±0.02) Å 2.42 (±0.04) Å 3.52 (±0.03) Å

*z = 0 is defined as the unrelaxed position of the high barium and x=0, y=0 is defined as the edge of the unit cell nearest the low sulfate.

22


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Figure 4: The nearest neighbor environment for each of the four water positions on the barite surface shown looking down onto the (001) plane. Adsorbed waters are shown in blue, oxygens on sulfate are red, sulfurs are yellow, and bariums are grey. The nearest neighbor ions are shown as large spheres and the surrounding environment is shown as small spheres. A) Water 1 (z = 2.41 Å) has a nearest neighbor environment of one water and two oxygens in one sulfate, b) Water 2 (z = 1.86 Å) has a nearest neighbor environment of one Balow, 4 oxygens in four different sulfates and two oxygens in water, c) Water 3 (z = 2.42 Å) has a nearest neighbor environment of two oxygens in two sulfates, three waters, and a Bahigh, and d) Water 4 (z = 3.52 Å) has a nearest neighbor environment of two oxygens in one sulfate and two waters. Additional

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views and a table of atom-atom distances are available in the Supporting Information (Figs. S8S11 and Table S3).

Table 2: A comparison of the rms-widths, the occupancies, and the z positions for the waters coordinating the four sites. Occupancies (Weq/AUC) XR MD 0.919 1.13 (±0.05)

XR 2.41 (±0.02) Å

0.19 Å

1.67 (±0.09)

0.67

1.86 (±0.02) Å

1.70 Å

0.1* Å

0.17 Å

0.887 (±0.1)

1.75

2.42 (±0.04) Å

2.54, 2.28 Å

0.1* Å

0.21 Å 0.19 Å

0.864 (±0.08)

1.61

3.52 (±0.03) Å

3.81 Å

Water number

Nearest surface site

XR

MD

XR

MD

1

Slow

0.1* Å

0.47 Å

0.1* Å

0.20 Å

0.24, 0.40 Å

0.1* Å

0.88, 0.38 Å 0.43, 0.54 Å 0.21, 0.34 Å

2

Balow

3

Bahigh

4

Shigh

U//, lateral

1.17 (±0.04) Å 1.33 (±0.12) Å 0.73 (±0.08) Å

U⊥, vertical

*Values noted as * were fixed during optimization.

24


z position MD 2.69 Å

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Figure 5: The surface-normal electron density profile for the best-fit structure (blue) and the MD simulations (red). Both profiles include resolution broadening by 0.6 Å full width at half maximum to represent the finite resolution of the experiments. This minimizes features which are not possible to resolve under our experimental conditions. The adsorbed water molecules are represented by the peaks around 2-5 Å from the surface.

While the occupancies of Waters 1, 3, and 4 are consistent with the expected value from adsorption of water (one Weq/AUC), the value for Water 2 is higher (1.67 Weq/AUC). This excess electron density could be explained by the presence of either two water molecules located closer to each other than our resolution or more electron dense species, such as dissolved barium or sulfate. Since Water 2 is within 3.5 Å of Bahigh it is unlikely that the excess electron density is from dissolved barium. These four adsorbed water species comprise a well-defined primary hydration layer at the barite (001) surface (Fig. 5). This primary hydration layer has two sublayers. The first sublayer includes Waters 1-3, which extend to ~3 Å from the surface. The other sublayer includes Water 4 as well as a fraction of laterally-disordered water, and extends to ~4 Å from the surface. This primary hydration layer is followed by the secondary hydration layer in which water molecules are weakly ordered with respect to the surface. The total occupancy of this primary hydration layer is ~5.8 Weq/AUC (Table 1; Fig. 5) corresponding to a 2D density of 12.0 H2O/nm2. Of this, ~75% corresponds to Waters 1–4, i.e., adsorbed water molecules with a strong positional correlation with the surface functional groups. This 2D density is similar to those reported on other minerals such as calcite (10.9 H2O/nm2) and muscovite (10 H2O/nm2). The larger value of 12 H2O/nm2 than what would be expected based on 25



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the bulk density of water (ρ2D = ρw2/3 = 10 H2O/nm2) for a flat layer is consistent with corrugation of the hydration layer structure.23 This result indicates that water molecules in the primary hydration layer is a distinct fluid phase whose organization is controlled by the structure and chemistry of the solid surface rather than a continuation of the hydrogen bonding networks from bulk water.23 3.3. MD simulations The results from the MD simulations indicate that waters are present at z = 1.70, 2.28, 2.69, and 3.81 Å (defined as the z distance from Bahigh) which form a single space filling hydration layer with a 2D density of 10.5 H2O/nm2. Similar to the results from the XR data, water in the MD simulations adsorbs to all four sites at the surface: Bahigh, Balow, Shigh, and Slow (Fig. 6). Pair distribution functions from the MD simulations for oxygen of water molecules adsorbed in the barium and sulfate surface sites are available in the Supporting Information (Fig. S12). Balow and Slow are coordinated by 0.67 and 1.13 water molecules, which were both laterally and vertically well constrained (Figs. 6a and 6b). In contrast, there are 1.61 water molecules coordinating Bahigh, which were laterally diffuse. Finally there are 1.75 water molecules coordinating Shigh, indicating one of the water molecules coordinating this sulfate may also spend time in the bulk water position. Regarding the non-integer coordination numbers observed in the MD, no adsorbed ions, such as Ba2+, are present which could create these, thus the results represent an average coordination by water for the site. At any given time step, zero to three coordinated water molecules were observed depending on the site. These results also suggest that the water molecules are not bonding to a specific ion on the surface, but rather multiple sites on the surface, which is corroborated by the XR model results.

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Figure 6: Probability isosurfaces as viewed along the a) [010], b) [100], and c) [001]. Bariums are green, sulfurs are yellow, and oxygens in sulfates are red. The areas with a high probability of water molecules are dark blue and the areas with a lower probability of water are shown in light blue. In image b) the top oxygens on the high sulfates have split into two regions of high probability.

Figs. 6b and 6c demonstrate that there is a significant increase in the lateral rms-width for Shigh, which leads to two distinct probability regions for the topmost oxygen for Shigh. This arises from rotation of the sulfate between two distinct positions. There was also significant relaxation of the bariums and sulfates at the surface (Fig. S13). Displacements were most significant for the topmost sulfate (0.044 Å) and second and third topmost bariums (0.071 and -0.050 Å,

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respectively). The bariums displayed significant oscillations in vertical displacement near the surface, whereas such oscillations were less significant for sulfates. 4. Discussion 4.1. Hydration Structure of Barite The model derived from the specular and non-specular reflectivity indicates the presence of a primary hydration layer on a slightly relaxed barite (001) surface. This hydration layer consists of four distinct waters which coordinate the barium and sulfate ions at the surface (Table S3 in the Supporting Information). The water molecules adsorb in similar positions to the bulk sulfate and barium lattice positions. Water 1 adsorbs in the same position as the missing low barium, whereas Water 4 adsorbs in the same position as the missing high barium (Fig. 1). Waters 2 and 4 adsorb in similar projected positions as the missing oxygens in the low sulfate of the second monolayer. Similar results have been observed for calcite,25, 45 suggesting this may be a general trend for sparingly soluble AB-type minerals. Water 2 completes the coordination shell for Balow, while it is also within 3.0 – 3.4 Å of four oxygens on Shigh and Slow. This suggests that the water may be weakly coordinating the anionic species in addition to the cationic species. Barium atoms are 12-fold coordinated in the bulk barite structure. Formation of the (001) surface breaks three Ba-O bonds to Bahigh and one to Balow. Adsorption of water replaces roughly one Ba-O bond for each of these sites, completing the coordination shell for Balow, but leaving Bahigh with two fewer bonds than a bulk barium ion. In solution, barium ions are likely coordinated by 8.146 - 9.747 oxygen atoms.43 This suggests that Bahigh may have a coordination environment intermediate between a barium ion in the bulk lattice and that hydrated in solution. Bond valence sums were calculated for the surface bariums to corroborate this observation using the method of Brown (1981).48 Here, s = [Rbd/R0]-N, where s is the strength of the bond, Rbd is the 28


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bond distance, R0 = 2.297 Å and is the empirical bond distance, and N = 7.0 is a fitted constant. The bond valence sum (BVS) is the summation of the strengths of each of the bonds coordinating the bariums. For the topmost barium, BVS = 1.89 valence units (vu), though BVS = 1.77 vu if the adsorbed water is excluded. For the second barium, BVS = 2.17 vu (2.07 vu if the adsorbed water is excluded), but if the occupancy of the water is included, BVS ~ 2.27 vu. These are smaller than the results for the bulk structure (BVS = 2.29 vu). Since the average strength of a Ba-O bond in barite is ~0.2, the results for the Bahigh are consistent with adsorbed water replacing one of the missing Ba-O bonds, but not the other two missing bonds. The results for the Balow suggest that while water replaces some of the missing coordination, the ion is still slightly undercoordinated as compared to a bulk barium. These results corroborate MD simulations in which the coordination shell is predicted to be completed by waters for Balow, but not Bahigh, leaving Bahigh undercoordinated as compared to bulk bariums8. Stack8 suggested that this undercoordination of Bahigh may occur due to steric hindrances that limit water access to sites at the interface. That is, water molecules coordinating other surface sites repel some of the water necessary to complete the coordination shell. The sulfate coordination geometry in aqueous solutions is not well described compared to Ba2+. A number of experimental and computational methods revealed that sulfate tends to be surrounded by 6-14 nearest neighbor waters with a S-Ow distance of 3.7 – 3.9 Å and Os–Ow distances of 2.8 - 2.9 Å. 49 Molecular Dynamics simulations using a MD force field developed by Jang et al.37 with the F3C water model39 predicted a coordination number of 16,8 larger than most studies. Large angle X-ray scattering experiments each sulfate is coordinated with an average of 12 water molecules in water. The number of neighbors is smaller in the barite lattice. A sulfate is surrounded by five bariums and three oxygens from other sulfates within 3.75 Å. For Slow, one 29



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nearest neighbor barium ion is lost during the cleaving process, which is replaced by Water 1. In the case of Shigh, three nearest neighbor bariums and two nearest neighbor oxygens (defined as those not part of the sulfate tetrahedron) are broken during the cleaving process. Waters 3 and 4, with a combined occupancy of 1.75 waters, coordinate Shigh, but do not complete the coordination shell, leaving it undercoordinated by 2-3 nearest neighbor oxygens. However, most of the bond valence sum for the sulfates arises from the four oxygens comprising the sulfate tetrahedra (5.68 vu), rather than the other oxygens, which only contribute ~ 0.1 vu total. 4.2. Correlation between adsorbed water molecules The adsorbed water molecules have strong positional correlations with the barite surface functional groups as well as other adsorbed water molecules (Fig. 4). Water 1 sits in the middle line of two top oxygens in Slow and is also close to two Water 2 (Figs. 4 and S8). Water 2 is located on top of Balow and sits in the trapezoidal pocket defined by four oxygens, each of which is in one of the four sulfates surrounding Balow (Fig. S9). It also has three adsorbed water molecules (two Water 1 and one Water 3 forming a right-triangular shape), as nearest neighbors (Figs. 4 and S9). This leads to Water 2 having a total of 8 species as nearest neighbors. This number of neighbors is higher than the typical number of neighbors for a water molecule in liquid water (i.e., 3 to 4 connected by hydrogen-bonded network), indicating that the coordination geometry is defined mostly by the crystallographic arrangement of the atoms at the surface (e.g., closest packing controlled by the steric effect) rather than formation of chemical bonds. This water species may include more multiple adsorbed states of water, whose distribution could not be resolved given the spatial resolution of our X-ray measurements. The occupancy greater than 1 as well as wide lateral distribution width of this water species (~1.2 Å rms width compared to ~0.1 Å for Water 1, Table 2) is consistent with this interpretation. Water 30


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3 is located close to Bahigh but with about 1 Å of lateral offset from the Bahigh along the x direction (Figs. 4 and S10). This lateral offset is likely controlled by the location of three neighboring atoms located slightly below Water 3: two top oxygens in two adjacent Shigh and Water 2 which form a triangle on which Water 3 sits (Fig. S10). This Water 3 is located also close to a pair of Water 4 which adsorbs ~1 Å above Water 3. This Water 4 sits in the centerline of the two top oxygens in two Shigh, and, with Water 3, has a total of 4 nearest neighbor species (Figs. 4 and S11). Overall, these comparisons show a clear interdependence among adsorbed water molecules in terms of their location at the barite–water interface. 4.3. Comparison with MD results There are a number of key similarities and differences between the MD results and bestfit XR model. Both the MD and XR results show significant relaxations of barium and sulfate ions from their bulk lattice positions. The displacements are most extensive for the ions in the topmost monolayer and diminish rapidly with depth. The major discrepancies between the MD and XR arise from the displacement magnitudes for Bahigh (-0.07 and ~0 Å for the XR and MD results respectively) and Slow (-0.08 and -0.01 Å for the XR and MD results respectively). The MD simulations also predict significant rotations of all sulfate groups in the crystal whereas the XR results reveal that the rotation is limited to the topmost layer. As a result, minor differences are observed between the MD and XR-derived electron density plots, in particular the presence of additional electron-density peaks between any of two adjacent Ba doublets from the MD simulations (Fig. 5). Both the MD and XR results demonstrate that waters coordinate each of the four types of surface functional groups in a half unit cell, Bahigh, Balow, Shigh, and Slow. The heights of the first adsorbed water from the surface (defined as the position of Bahigh) are similar (1.86 Å for XR 31



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and 1.70 Å for MD), so are the total numbers of waters within the primary hydration layer (~5.8 for XR fit and ~5.2 for MD). The distances from the waters to the individual surfaces sites are also similar for one of the waters coordinating Bahigh (XR: 3.09 Å; MD: 2.81 Å) and the water coordinating Balow (XR: 3.19 Å; MD: 2.98 Å). However, the distances from the waters to the sulfurs in the sulfate sites and one of the waters coordinating Bahigh are dissimilar. For Bahigh, in the MD simulations the second closest water is 3.53 Å away, whereas in the XR best fit model, the second closest water is 4.33 Å away. This latter water would be most likely included as part of the bulk water in the XR best fit model. For Shigh the distance to Water 3 is 3.69 Å in the XR model (defined as the center of the sulfate), whereas it is 4.42 Å in the MD simulations. For Slow, the difference is less pronounced (XR: 3.59 Å; MD: 3.97 Å). While the bond distances for the Bahigh and Shigh may have some differences, the MD simulations predict that one of the waters coordinating Bahigh and the waters coordinating Shigh will be laterally diffuse, which is also found in the XR best fit model (Table 2). The 2D packing densities are also similar (12.0 H2O/nm2 for the XR fit versus 10.5 H2O/nm2 for the MD results). Additional analysis of the distances between nearest neighbor species in the XR best fit model can be found in Table S3 of the Supporting Information. The differences between the XR and MD results lead to the discrepancies observed in the electron density profiles. The water species with the larger occupancies in the MD simulations are farther from the surface (Waters 3 and 4), whereas in the XR results, the waters closest to the surface (Waters 1 and 2) have the larger occupancies. A difference in the MD and XR methods is the high sensitivity of MD simulations to the dynamics of water molecules at the barite surface compared to the XR data. Quasielastic neutron scattering measurements38 revealed that the resident time for waters adsorbed to Bahigh was 40 times shorter than the other adsorbed water, 32


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suggesting that those adsorbed waters continuously exchange with bulk water. The XR results are not sensitive to jump diffusion of waters from one adsorbed site to another adsorbed site, which occurs between Slow and one of the adsorbed waters on Bahigh in the MD simulations. The water adsorbed on Bahigh in the XR model has a large rms width which may be a result of this behavior, though the water on Slow is laterally well constrained. Finally, the MD simulations are sensitive to rotation of Shigh, which leads to the ion spending time in two distinct positions. The XR model includes larger rms widths for Shigh in addition to rotation and relaxation of the molecule, but the model is too simplistic to fully replicate the bimodal configuration of the sulfate groups observed using MD. We can calculate the reflectivity signal directly from the MD simulations to quantify the similarity (and the difference) between two approaches. A calculation using the structure factor for the water and relaxed barite surface from the MD simulations combined with the unrelaxed barite substrate provided poor agreement with the specular XR data (χ2 = 138; Fig. 7a). The goodness of fit improved significantly (χ2 = from 138 to 5.46) when only the water structure was adopted from the MD simulations while relaxation of the atoms at the barite surface was optimized using a regular model-fit procedure. The calculated curve mostly reproduces the measured reflectivity signals (Fig. 7b).

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Figure 7: The measured specular reflectivity signal and the MD-derived signal. In (a), the MD signal was generated using both the MD simulated barite surface and interfacial water structure whereas in (b), the MD signal was generated using the MD simulated water and allowing the barium and sulfates atoms to move from their positions optimized in a model dependent XR fit.

Despite this good agreement between MD and XR data, there is some inconsistency between the simulated and measured barite-water interfacial structure. The primary differences in the XR and MD simulations arise from the differences in electron density for the adsorbed species. The XR-derived profile shows a discrete layering of adsorbed water which is less 34


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prominent in the MD-derived profile (Fig. 5). This could be in part because the MD simulations do not include the possibility of adsorbed barium or sulfate ions at the surface, which could lead to a higher electron density near the surface. Furthermore, the optimized structure using the MD simulated water profile includes significant displacements at the surface, up to 0.5 Å for the top two sulfates, larger than the results reported previously.7 In spite of these differences, there is a generally good agreement between the XR best fit model and the MD simulations, particularly in the number of water molecules, the coordination of different barium and sulfates on the surface, and the overall 2D packing density. The good agreement between MD and XR results indicates the accuracy of the force field used in the simulations. The barite-water force field used here is the “alternate” one developed in Stack.8 It was calibrated against structural and thermodynamic data, including a bulk barite lattice energy estimate, barite bond lengths, and surface energies (both hydrated and in vacuo). The water model, F3C, was calibrated against the heat of vaporization, diffusion, and structure.39 Interaction of barium and sulfate with water was calibrated and compared against the experimentally-determined hydration enthalpies, coordination numbers, and bond lengths. Most recently it has been compared to quasi-elastic neutron scattering measurements of the dynamics of water at the barite-water interface, where it performed well at the room temperature simulations used here, but progressively worse at lower temperatures.38 The reference data and MD model performance are listed in Table 1 of Stack,8 but particularly relevant data here are the following factors. For bulk water, the first shell peak for oxygen-oxygen distances between water is at 277 pm, consistent with other water models.39,50 The water coordination number for the aqueous barium is 8.8 with a bond length of 278 pm and a residence time for first shell water of 208 ps. These are all close to experimental estimates, e.g., experimental estimates of aqueous 35



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barium ion coordination numbers determined previously range from 8.146 to 9.7.47 For the aqueous sulfate ion, the coordination number given by the MD is 16 with a bond length of 381 pm and a residence time of first shell water of 5.8 ps. While bond lengths for aqueous sulfate fall within previous estimates, the coordination number for this MD model is higher than most other estimates, which range from 6-14.8, 49, 51-52 In comparing these data to the CTR fit parameters in Table 2, it seems that the MD model tends to predict higher occupancies of water bound to surface bariums and sulfates. The differences in the occupancies could be due to the MD model overestimating coordination of barium and sulfate by water, but it is difficult to say unequivocally that the MD model predicts coordination numbers that are too high given the large uncertainty in the actual values for the coordination numbers of aqueous barium and sulfate.

4.4. Comparison with previous XR results The current XR analyses significantly improve our understanding of the structure of the barite – water interface compared with observations from the previous studies.7 A significantly larger number of specular data were measured at a higher precision, providing more rigorous constraints on the model parameters (e.g., with smaller uncertainties of estimation). In addition, specular reflectivity is combined here with non-specular reflectivity data, providing additional information on lateral ordering of the adsorbed species. The following observations are comparable: the direction of the vertical displacements of Ba and sulfate groups in the barite surface, which in both studies resulted in reduction of the atomic corrugation at the surface; the number of monolayers of vertical relaxations into the crystal; and the height of the water molecules adsorbed on Balow. However, there are also clear differences. The previous results reported significantly larger vertical relaxations for Shigh and Slow than our results: (-0.418 and 36


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0.352 Å for the previous study vs. 0.053 and -0.083 Å for our current results), leading to significant distortions in the electron density in the barite surface. The total number of adsorbed water species also differs significantly, 2.7 Weq/Auc vs. the 5.8 Weq/Auc reported here, which leads to a significantly different conclusion with regards to the lateral density of water at the surface. Our results are consistent with the expectation that interfacial hydration layers should have a 2D density of ~10 H2O/nm2, whereas the previous results implied that waters only saturate the broken Ba-O bonds. Overall these results suggest that the previously reported barite structure does not provide an accurate representation of the full barite-water interfacial structure, possibly due to the smaller number of data points (~39) and the lack of non-specular reflectivity results to constrain the lateral interfacial structure.

5. Conclusions This study presents the first assessment of the three-dimensional structure of the barite (001)-water interface from high resolution X-ray reflectivity measurements performed with high precision. The best-fit model for our results demonstrates that barite lattice displacements extend as deep as two unit cells into the crystal in the lateral direction and one unit cell deep in the vertical direction. This relaxed surface is covered with the primary hydration layer which contains four distinct adsorbed water species. Each of these adsorbed water coordinates one of four surface ions (Balow, Bahigh, Slow, and Shigh). The positions of these waters are related to the positions of the barium and sulfates in the bulk crystal structure, leading to the waters partially or fully completing the coordination geometry of the surface ions. Non-specular reflectivity demonstrates that undercoordination at the surface leads to a templating effect for adsorption of the water molecules on barite. These waters also have a complex coordination environment, 37



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demonstrating some of the waters strictly coordinate sulfate ions whereas others simultaneously coordinate sulfates and bariums. These results are similar to the hydration structure predicted by MD simulations, although the distortion of sulfate molecules at the surface in the MD simulations is less consistent with our XR results. Further research comparing computational simulations and XR results in three dimensions may shed light on where the simulations can be improved to obtain a better agreement with the XR results. Supporting Information Figures S1-S13, Tables S1-S3, and the XRD analysis of the barite (BaSO4.cif). Acknowledgements The authors are grateful to Radu Custelcean (ORNL) for performing the bulk XRD on the barite. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. CTR measurements were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR-1128799) and Department of Energy- GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. References 1. Frenier, W. W.; Ziauddin, M. Formation, Removal and Inhibition of Inorganic Scale in Oilfield Environment; Society of Petroleum Engineers: Richardson, TX, 2008. 2. Curti, E.; Fujiwara, K.; Iijima, K.; Tits, J.; Cuesta, C.; Kitamura, A.; Glaus, M. A.; Mueller, W. Radium Uptake During Barite Recrystallization at 23 +/- 2 Degrees C as a Function of Solution Composition: An Experimental Ba-133 and Ra-226 Tracer Study. Geochim. Cosmochim. Acta 2010, 74, 3553-3570. 38


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