Hydration Structures on γ-Alumina Surfaces With and Without Electrolytes Probed by Atomistic Molecular Dynamics Simulations

A wide range of systems, both engineered and natural, feature aqueous electrolyte solutions at interfaces. In this study, the structure and dynamics of water at the two prevalent crystallographic terminations of gamma-alumina, [110] and [100], and the influence of salts—sodium chloride, ammonium acetate, barium acetate, and barium nitrate on such properties—were investigated using equilibrium molecular dynamics simulations. The resulting interfacial phenomena were quantified from simulation trajectories via atomic density profiles, angle probability distributions, residence times, 2-D density distributions within the hydration layers, and hydrogen bond density profiles. Analysis and interpretation of the results are supported by simulation snapshots. Taken together, our results show stronger interaction and closer association of water with the [110] surface, compared to [100], while ion-induced disruption of interfacial water structure was more prevalent at the [100] surface. For the latter, a stronger association of cations is observed, namely sodium and ammonium, and ion adsorption appears determined by their size. The differences in surface–water interactions between the two terminations are linked to their respective surface features and distributions of surface groups, with atomistic-scale roughness of the [110] surface promoting closer association of interfacial water. The results highlight the fundamental role of surface characteristics in determining surface–water interactions, and the resulting effects on ion–surface and ion–water interactions. Since the two terminations of gamma-alumina considered represent interfaces of significance to numerous industrial applications, the results provide insights relevant for catalyst preparation and adsorption-based water treatment, among other applications.

. Radial distribution functions: for surface groups, and surface groups to water; comparison with abinitio data.
4 Figure S3. Atomic density profiles, spanning the complete thickness of the water layer above the γ-alumina [110] and [100] surfaces. Aqueous phase: pure water.
13 Figure S10. Surface density distributions of ions, within first and second interfacial hydration layers of γalumina [110].
14 Figure S11. Surface density distributions of ions, within first and second interfacial hydration layers of γalumina [100].

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Section S19. Crystallographic information file (CIF) contents for the γ-alumina unit cell model of Digne et al.
The smaller of the two systems investigated in the manuscript is the γ-alumina [100] surface (surface dimensions: 44.696 * 25.239 * 16.136 Å in x, y and z dimensions, respectively). To determine the extent of any system size effects, a simulation for this surface, with pure water as the aqueous phase, was repeated, but doubling the size of the surface in the y-dimension, yielding a bigger surface size of 44.696 * 50.478 * 16.136 Å. Comparison of results for the 'original' and 'bigger' surface sizes are shown below, in terms of atomic density profiles of water, perpendicular to the surface, and water residence times at the interface. Density profiles of water, oxygen and hydrogen atoms, are shown in panels A) and B), respectively. Water residence times within the first and second interfacial hydration layers are shown in panels C) and D), respectively. Radial distribution functions (RDFs) obtained for γ-alumina surfaces, compared with selected ab-initio data 1 . Ab-inito data points obtained through DigitizeIt software. Panels A, B: O (μ1-OH, μ3-OH, surface H2O (μ1-H2O) groups) vs. H (liquid water), for γ-alumina [110] and [100] surfaces, respectively. Ab-initio data obtained from Figure 5c, Wakou et al. 1 MD results appear to pick up the main features of the ab-initio data. An exception is for the first peak at 1Å, representing transient protonation of surface OH groups (μ1-OH and chemisorbed H2O). At distances of 1.75Å (2Å for MD), bonding occurs between these OH groups and water hydrogens. Panels C, D: RDFs between all oxygen and hydrogen atoms of [110] and [100] alumina surface groups, respectively; OH pairs within μ1-OH, μ3-OH and surface H2O groups (μ1-H2O). Ab-initio data obtained from Figure S4  Atomic density profiles of (pure) water perpendicular to γ-alumina surfaces, spanning the complete thickness of the water layer (~50-55 Å); pale red and grey indicate water oxygen and hydrogen atoms, respectively. Profiles over the thickness considered in the main text (15 Å) are shown alongside (in red and black, for water oxygen and hydrogen, respectively). Panel A: atomic density profiles, [110] surface. Panel B: atomic density profiles, [100] surface. Simulation box dimensions and the amount of water incorporated is sufficient to ensure that 'bulk-like' water conditions are established in the middle of the layer, away from the influence of the two interfacial regions; solid-liquid and liquidvacuum, respectively. Simulation snapshots from the γ-alumina [110] surface, showing water within the first and second hydration layers. Selected surface atoms from the aerial view are labelled for position reference. Dynamic bonds are shown in the snapshots, colour-coded to illustrate the transition from the first to the second hydration layer. Green; interaction between the surface and water molecules in the first hydration layer. Pale yellow; interaction between water molecules in the first and second hydration layers (i.e., the transition between layers). Cyan; interaction of water molecules in the second hydration layer with alumina surface groups. Lengths of the dynamic bonds shown range from 1.7 to 1.9 Å.

FIGURE S5.
To provide an indication of uncertainty for the computed average water residence times, the analysis was repeated, starting from different time origins within the simulation trajectory output. The full simulation trajectory utilised for the present work comprises 10,000 frames (each frame = 400 fs duration). Average residence times presented in the main text were computed between frames 5000-10,000. Results below show the same analysis, starting from frame zero (to 5000), and frames 2500-7500.   A) and B) -first interfacial hydration layer; water oxygen and hydrogen density distributions, respectively. Unit cells (white borderlines) are shown for clarity. White = hydrogen, red (where visible) = oxygen atoms part of an OH group, pale pink = free surface oxygen atoms.
Scale bar is applicable to both graphs.

FIGURE S7a.
The [100] γ-alumina surface, superimposed onto surface density distribution graphs of interfacial water (taken from main text Fig. 6, row A). For clarity, only the surface atoms of [100] γ-alumina are shown. Diagrams A) and B) -first interfacial hydration layer; water oxygen and hydrogen density distributions, respectively. Unit cells (white borderlines) are shown for clarity. OH groups are shown in initial configuration (bond vectors aligned in the direction normal to the surface). See simulation snapshot, showing a single unit cell surface, for representative orientations (indicated with red arrows). White = hydrogen, red (where visible) = oxygen atoms part of an OH group, pale pink = free surface oxygen atoms. Scale bar is applicable to both graphs.

FIGURE S7b.
The [100] γ-alumina surface, superimposed onto surface density distribution graphs of interfacial water (taken from main text Fig. 6, row A). For clarity, only the surface atoms of [100] γ-alumina are shown. Diagrams A) and B) -second interfacial hydration layer; water oxygen and hydrogen density distributions, respectively. Unit cells (white borderlines) are shown for clarity. OH groups are shown in initial configuration (bond vectors aligned in the direction normal to the surface). See simulation snapshot, showing a single unit cell surface, for representative orientations (indicated with red arrows). White = hydrogen, red (where visible) = oxygen atoms part of an OH group, pale pink = free surface oxygen atoms. Scale bar is applicable to both graphs.
Planar density distributions of water over the γ-alumina [110] surface. Columns 1 and 2: first hydration layer, water oxygen and hydrogen (OW and HW), respectively. Columns 3 and 4: second hydration layer, water oxygen and hydrogen (OW and HW), respectively. Rows A), B), C), D) and E) are for pure water, 1M aqueous solution of sodium chloride, 1M aqueous solution of ammonium acetate, 0.3M aqueous solution of barium nitrate, and 1M aqueous solution of barium acetate, respectively. The scale bar is applicable to all graphs in the figure. S13 FIGURE S10.

Surface density distributions of ions, within first and second interfacial hydration layers of γ-alumina [110]
. Labels, 1) and 2), are used to indicate ion density distributions within the first and second hydration layers, respectively. Scale-bar density units: 1/Å 3 . Panels A, B, C and D correspond to aqueous systems of 1 molar sodium chloride, ammonium acetate, barium acetate and 0.3 molar barium nitrate, respectively. Feature enlargements are outlined in white; placed in blank areas. S14 FIGURE S11.
Surface density distributions of ions, within first and second interfacial hydration layers of γ-alumina [100]. Labels, 1) and 2), refer to ion density distributions within first and second hydration layers, respectively. Density units: 1/Å 3 . Panel A: sodium chloride (1 molar). Panels B, C: ammonium acetate (1 molar), barium nitrate (0.3 molar). Panel D: barium acetate (1 molar). Feature enlargements are outlined in white; placed in blank areas. FIGURE S12. [100] γ-alumina surface, aqueous phase: barium acetate (1 molar). Panel A) aerial view (initial configuration with unit cells shown, for clarity). Selected atoms are labelled for position reference. A region of complex ion association, as seen during the simulation, is superimposed; an enlargement, minus surface atoms, is provided for clarity. This region of ion accumulation, in the second hydration layer, is visible in Fig. S10 (Panel D) and appears to be responsible for interfacial water displacement within this layer, as seen in Figure 10, row E (main text). Panel B) simulation snapshots. Atoms of the reference plane are shown in faded colours, to aid visualisation. The dynamic bonds depicted between bariums and acetate carboxyl oxygens are of 2.8 Å length. This equates to the distance to the first peak of the radial distribution function for Ba -carboxyl oxygens, in aqueous solution (see Drecun et al., 2021; supplementary information, Figure S2) 2 . S16 Figure S13.
Planar density distributions of water over the γ-alumina [100] surface. Columns 1 and 2: first hydration layer, water oxygen and hydrogen (OW and HW), respectively. Columns 3 and 4: second hydration layer, water oxygen and hydrogen (OW and HW), respectively. Rows A), B), C), D) and E) are for pure water, 1M aqueous solution of sodium chloride, 1M aqueous solution of ammonium acetate, 0.3M aqueous solution of barium nitrate, and 1M aqueous solution of barium acetate, respectively. The scale bar is applicable to all graphs in the figure. S17 FIGURE S14. [100] γ-alumina surface, aqueous phase: sodium chloride (1 molar). Panel A) aerial view of surface (initial configuration with unit cells shown, for clarity). An example adsorption site of Na+ (yellow, label 4), as seen during the simulation, is superimposed. Nearest neighbours of the sodium ion at this site -oxygen atoms, from two Al4O1H surface groups, and an Al5O1H2 group -are labelled 1), 3) and 2), respectively. Panel B) simulation snapshots showing sodium ion interaction at the adsorption site and orientation of alumina surface groups, with a water molecule (stick representation) appearing in the first hydration layer that appears to 'stabilise' the configuration (label 5). Atoms of the plane below (reference plane) are shown in faded colours, to aid visualisation. Atom coordinates from the snapshots (x, y, z): 1) 5. 66, 4.58, 19.62 2) 8.38, 1.86, 19.55 3) 11.14, 4.58, 19.65 4) 8.19, 4.17, 19.78 = sodium ion 5) 8. 70, 6.48, 20.25 = water (oxygen atom) In Panel A, another observed adsorption site of sodium is shown (label 6), where the same configuration, with a 'stabilising' water molecule, is observed. S18 FIGURE S15. [100] γ-alumina surface, aqueous phase: ammonium acetate (1 molar). Panel A) aerial view of surface (initial configuration with unit cells shown, for clarity). An example adsorption site of ammonium (label 6), as seen during the simulation, is superimposed. Nearest neighbours of the ammonium ion at this siteoxygen atoms, from four Al4O1H surface groups, and an Al5O1H2 group -are labelled 1-4, and 5, respectively. Panel B) simulation snapshots showing ammonium ion interaction at the adsorption site and orientation of alumina surface groups, with a water molecule (stick representation) appearing in the first hydration layer that appears to 'stabilise' the configuration (label 7). Atoms of the reference plane are shown in faded colours, to aid visualisation. Atom coordinates from the snapshots (x, y, z): 1) 5.75, 13.08, 19. 59 2 #--eof--eof--eof--eof--eof--eof--eof--eof--eof--eof--eof--eof--eof--eof--eof--#